A Study of Zinc-Nickel as an Alternate Coating to Cadmium 

for Electrical Connector Shells Used in Aerospace Applications 

By 

Odunayo Ogundiran 

A Thesis Submitted to the Graduate 

Faculty of Rensselaer Polytechnic Institute 

In partial fulfillment of the 

Requirements for the degree of 

Master of Science in Mechanical Engineering 

Approved: 

_________________________________________ 

Ernesto Gutierrez-Miravete: Thesis Adviser 

Rensselaer Polytechnic Institute 

Hartford, CT 

April, 2011 
ii 

© Copyright 2011 

by 

Odunayo Ogundiran 

All Rights Reserved 
iii 

TABLE OF CONTENTS 

LIST OF KEYWORDS .................................................................................................... vi

LIST OF SYMBOLS ....................................................................................................... vii

LIST OF TABLES .......................................................................................................... viii

LIST OF FIGURES .......................................................................................................... ix

ACKNOWLEDGMENT .................................................................................................. xi

ABSTRACT .................................................................................................................... xii

1. Introduction .................................................................................................................. 1

1.1 Overview of Circular Connectors. ..................................................................... 1

1.2 Corrosion on Circular Connectors ..................................................................... 2

1.3 Electromagnetic Interference Shielding ............................................................. 3

1.4 Project Background ............................................................................................ 5

1.5 Overview of Project Goals ................................................................................. 5

2. Coatings for Circular Connectors ................................................................................ 7

2.1 Existing finishes and their applications ............................................................. 7

2.1.1 Class A finish: Cadmium over nickel. Light Gold Color ...................... 7

2.1.2 Class F & G finish: Electroless Nickel .................................................. 7

2.1.3 Class W finish: Cadmium over nickel. Olive Drab Color ..................... 8

2.1.4 Class K finish: Passivate. ....................................................................... 8

2.1.5 Class L finish: Electrodeposited Nickel ................................................. 9

2.2 Currently identified alternate finishes to Cadmium ........................................... 9

2.2.1 Class P finish: Electrodeposited Aluminum. ......................................... 9

2.2.2 Class T finish: Nickel fluorocarbon polymer. ........................................ 9

2.2.3 Class Z finish: Zinc Nickel .................................................................... 9

2.3 How Circular Connectors Corrode .................................................................. 10

2.4 Cadmium Protection ........................................................................................ 11

2.5 Zinc Nickel finish ............................................................................................. 12
iv 

2.5.1 Electrodeposition of zinc nickel alloy Coating Deposits on Circular 

Connectors ........................................................................................... 16

2.5.2 Properties of Cadmium and zinc nickel of Interest in this Application 17

3. Experimental .............................................................................................................. 19

3.1 Test Unit Identification .................................................................................... 19

3.1.1 Test Unit Set-up ................................................................................... 20

3.1.2 Mounting bracket ................................................................................. 31

3.2 Shield Braid Termination. ................................................................................ 32

3.3 Salt Spray Test ................................................................................................. 33

3.3.1 Test Apparatus ..................................................................................... 33

3.3.2 Salt Composition .................................................................................. 34

3.3.3 Procedure.............................................................................................. 35

3.4 Electrical Continuity Test ................................................................................ 36

3.4.1 Equipment ............................................................................................ 36

3.4.2 Procedure.............................................................................................. 36

3.5 Coupling Torque Test ...................................................................................... 38

3.5.1 Equipment ............................................................................................ 38

3.5.2 Procedures ............................................................................................ 38

3.6 Visual Inspection .............................................................................................. 38

3.7 Test Sequence................................................................................................... 39

4. Results and Discussion .............................................................................................. 41

4.1 Pre-Salt Fog Exposure Readings ...................................................................... 41

4.1.1 Coupling Torque at 0 Hours ................................................................. 41

4.1.2 Electrical Continuity Readings at 0 Hours ........................................... 41

4.2 Post-Salt Fog Exposure Readings .................................................................... 43

4.2.1 Electrical Continuity Readings at 48 Hours ......................................... 43

4.2.2 Electrical Continuity Readings at 96 Hours ......................................... 45
v 

4.2.3 Electrical Continuity Readings at 500 Hours ....................................... 47

4.2.4 Coupling Torque at 500 hours.............................................................. 50

4.3 Discussion ........................................................................................................ 51

4.3.1 Recommendations for Additional Testing ........................................... 56

5. Conclusion ................................................................................................................. 57

6. References .................................................................................................................. 59

7. Appendix .................................................................................................................... 60

7.1 Test Unit 1 ........................................................................................................ 60

7.2 Test Unit 2 ........................................................................................................ 62

7.3 Test Unit 3 ........................................................................................................ 64

7.4 Test Unit 4 ........................................................................................................ 66

7.5 Test Unit 5 ........................................................................................................ 68

7.6 Test Unit 6 ........................................................................................................ 70

7.7 Test Unit 7 ........................................................................................................ 71

7.8 Test Unit 8 ........................................................................................................ 73

7.9 Test Unit 9 ........................................................................................................ 75

7.10 Mounting Bracket Detailed Design .................................................................. 77

 
vi 

LIST OF KEYWORDS 

Circular Connector Part of a harness assembly used to terminate wires and provide 

connection between harnesses or a harness and avionics 

equipment on an aircraft. 

Receptacle Type of connector that is attached to a bulkhead of an aircraft or 

avionic equipment on an aircraft. 

Backshell A connector accessory that provides protection to the wire 

terminations on the connector 

Strain Relief A connector accessory mounted to the rear of the backshell to 

keep the harness secure thus preventing the wires from being 

pulled out of the connectors 

Finish A protective layer of coating meant for corrosion resistance of the 

substrate to which it is applied. 

Bulkhead: The sheet metal structure of an aircraft to which receptacles are 

mounted. 

Zinc Nickel finish Newly identified alternate finish to cadmium for circular 

connectors 

Cadmium finish Widely used finish on electrical connectors today. 

Plug Type of connector that is free moving and mates with a receptacle. 

Harness An assembly of a group of wires that transmits data or current 

bound together by string ties, clamps or electrical tape. 
vii 

LIST OF SYMBOLS 

Tn/Zn - Tin plated finish on Zinc Nickel finish 

Tn/Cd - Tin plated finish on Cadmium finish 

Cd/Zn - Cadmium finish on Zinc Nickel finish 

Zn/Zn - Zinc Nickel finish on Zinc Nickel finish 

Cd/Cd - Cadmium finish on Cadmium finish 

En/Zn - Electroless Nickel finish on Zinc Nickel finish 

CdC/Zn - Composite Cadmium finish on Zinc Nickel finish 

EnC/Zn - Composite Electroless Nickel finish on Zinc Nickel finish 

SS/Zn - Stainless steel passivate finish on Zinc Nickel finish 

Zn - Zinc Nickel finish 

Cd - Cadmium finish 

En - Electroless Nickel finish 

SS - Stainless steel passivate finish 

Sn - Tin 

Co - Cobalt 

Ni - Nickel 

NaCl - Sodium Choride 

Al - Aluminum 

C - Composite 
viii 

LIST OF TABLES 

Table 1: Summary of Properties of Cadmium, Zinc Nickel and Pure Zinc Coatings [4].

 ......................................................................................................................................... 18

Table 2: Matrix of existing materials and finishes .......................................................... 20

Table 3: Test Unit Finish and Material Matrix (Al – Aluminum. C – Composite) ......... 21

Table 4: Test Unit 1 Parts List ......................................................................................... 22

Table 5: Test Unit 2 Parts List ......................................................................................... 23

Table 6: Test Unit 3 Parts List ......................................................................................... 24

Table 7: Test Unit 4 Parts List ......................................................................................... 25

Table 8: Test Unit 5 Parts List ......................................................................................... 26

Table 9: Test Unit 6 Parts List ......................................................................................... 27

Table 10: Test Unit 7 Parts List ....................................................................................... 28

Table 11: Test Unit 8 Parts List ....................................................................................... 29

Table 12: Test Unit 9 Parts List ....................................................................................... 30

Table 13: Salt Spray Test intervals. ................................................................................. 36

Table 14: Electrical Continuity Measurement Conditions. ............................................. 37

Table 15: Measurement Points/Surfaces on test units ..................................................... 37

Table 16: Maximum and Minimum torques for Shell Size 19 Connectors. .................... 38

Table 17: Initial Mate Torque Values .............................................................................. 41

Table 18: Initial Electrical Continuity Reading (Millivolts) ........................................... 42

Table 19: 48 Hour Electrical Continuity Reading (Millivolts) ........................................ 44

Table 20: 96 Hour Electrical Continuity Reading (Millivolts) ........................................ 46

Table 21: 500 Hour Electrical Continuity Reading (Millivolts) ...................................... 49

Table 22: Unmating Torque Values after 500 Hours ...................................................... 50
ix 

LIST OF FIGURES 

Figure 1: View of a Mated Plug and Receptacle Assembled with Connector Accessories.

 ........................................................................................................................................... 1

Figure 2: Cutaway view of a Circular Connector Plug and Receptacle [13] ..................... 2

Figure 3: Connector and Accessory Assembly for EMI shielding [10] ............................ 4

Figure 4: Cadmium Plated Aluminum Receptacle and Plug ........................................... 12

Figure 5: Average Corrosion Rates on Zinc Nickel Alloy Deposits by Nickel Content 

[15]. .................................................................................................................................. 14

Figure 6: Zinc Nickel Phase Diagram [16] ...................................................................... 15

Figure 7: Micrograph of Zinc Nickel Coatings before and after 48 hours Immersion in 

NaCl Environment [16] ................................................................................................... 16

Figure 8: Zinc Nickel Plated Aluminum Receptacle and Plug ........................................ 17

Figure 9: Test Unit 1 ........................................................................................................ 22

Figure 10: Test Unit 2 ...................................................................................................... 23

Figure 11: Test Unit 3 ...................................................................................................... 24

Figure 12: Test Unit 4 ...................................................................................................... 25

Figure 13: Test Unit 5 ...................................................................................................... 26

Figure 14: Test Unit 6 ...................................................................................................... 27

Figure 15: Test Unit 7 ...................................................................................................... 28

Figure 16: Test Unit 8 ...................................................................................................... 29

Figure 17: Test Unit 9 ...................................................................................................... 30

Figure 18: Test bracket set up. ......................................................................................... 31

Figure 19: Shield Braid Termination [10]. ...................................................................... 32

Figure 20: Sample Composition of Salt Spray Test Apparatus [9] ................................. 34

Figure 21: Salt Solution Reservoir showing positioning of filter [11] ............................ 35

Figure 22: Measurement Points on test units ................................................................... 37

Figure 23: Test Sequence ................................................................................................. 40

Figure 24: 48 Hour Maximum Electrical Continuity Readings Per Interface on Test 

Units ................................................................................................................................. 43
x 

Figure 25: 96 Hour Maximum Electrical Continuity Readings Per Interface on Test 

Units ................................................................................................................................. 45

Figure 26: Minumum and Maximum Electrical Continuity Readings Per Interface on 

Test Units ......................................................................................................................... 48

Figure 27: Comparison of Torque values at 0 and 500 Hours ......................................... 50

Figure 28: Test Unit 1 Backshell with Cadmium Finish on Receptacle after 48 hours .. 51

Figure 29: Test Unit 3 Backshell with Zinc Nickel Finish on Receptacle after 48 hours 51

Figure 30: Test Unit 1 Backshell and Plug with Cadmium Finish after 48 hours ........... 52

Figure 31: Test Unit 3 Backshell and Plug with Zinc Nickel Finish after 48 hours ........ 52

Figure 32: Test Unit 1 Backshell and Receptacle with Cadmium Finish after 96 hours. 53

Figure 33: Test Unit 1 Backshell and Plug with Cadmium Finish after 96 hours ........... 53

Figure 34: Test Unit 3 Backshell and Receptacle with Zinc Nickel Finish after 96 hours

 ......................................................................................................................................... 53

Figure 35: Test Unit 3 Backshell and Plug with Zinc Nickel Finish after 96 hours ........ 53

Figure 36: Test Unit 1 Backshell and Receptacle with Zinc Nickel Finish after 500 hours

 ......................................................................................................................................... 55

Figure 37: Test Unit 1 Backshell and Plug with Zinc Nickel Finish after 500 hours ...... 55

Figure 38: Test Unit 3 Backshell and Receptacle with Zinc Nickel Finish after 500 hours

 ......................................................................................................................................... 55

Figure 39: Test Unit 3 Backshell and Plug with Zinc Nickel Finish after 500 hours ...... 55
xi 

ACKNOWLEDGMENT 

 I would like to acknowledge the harness engineering department at Sikorsky 

Aircraft Corporation for providing the funding for this study. I would also like to 

acknowledge the materials laboratory personnel for their efforts in this study. 
xii 

ABSTRACT 

This report presents results of a study aimed to investigate the ability of zinc nickel 

coatings to provide adequate corrosion protection and electromagnetic interference 

shielding for circular connectors used in aerospace applications and in comparison to 

cadmium coatings. The effects of aqueous corrosion in circular connectors and 

connector accessories were investigated. Substrate material degradation and 

electromagnetic interference concerns are the main reasons protective finishes are used 

on circular connectors. 

Several finishes used today provide adequate protection for circular connectors. 

Cadmium is the most widely used finish in this application. However, the hazardous 

nature of these finishes has long driven the search for an alternative. Zinc nickel has 

been identified as an alternative to cadmium on circular connectors; however there are 

no studies of how zinc nickel finishes couple with the materials that are used today on 

aerospace circular connectors, connector accessories, and mounting hardware. The need 

to know more about the service behavior of zinc nickel coatings provided the motivation 

for this study. 

 Tests in this study include a salt spray test, an electrical continuity test, a coupling 

test, and visual inspection of the test units. Initial inspections after exposure to salt fog 

showed rapid formation of oxide films on the surface of aluminum and stainless steel 

connectors and backshells coated with the zinc nickel finish. Electrical continuity 

measurements taken at predetermined intervals confirmed the presence of oxide films as 

voltage across the measurement points increased drastically from the first set of 

readings. 

At 500 hours, which was the pass or fail point for this study, heavy corrosion was 

noticed on most test units regardless of the finish. Continuity measurements at the 500 

hour interval ranged from values lower than the recommended <5.0 millivolts to much 

higher values. However, the electrical path between the shield braid and the ground was 

shown not to have been compromised. The measured torques required to unmate the test 

units after the 500 hours increased from the initial values but were below the baseline 

value in most cases. No significant difference in continuity readings collected was 
xiii 

determined between connectors with zinc nickel finish mated with connectors with the 

same finish and connectors with zinc nickel finish mated with connectors with other 

finishes. Although the zinc nickel finish did not perform as well as the cadmium finish in 

this study, it met the industry requirement for finishes on circular connectors and should 

be seriously considered as cadmium replacement. 
1 

1. Introduction 

1.1 Overview of Circular Connectors. 

Circular connectors are common components of wire harnesses typically used in 

aerospace applications for terminating wires that transmit data or distribute current. 

They can also be found attached to a piece of equipment. Circular connectors are usually 

found at the ends of a harness where the wires terminate. Circular connectors can be one 

of two kinds; a plug which is attached to the harness alone and is free moving; or a 

receptacle which is not only attached to the harness, but also to a frame or structure to 

keep it stationary. Figure 1 shows a mated plug and receptacle on two harnesses as they 

are mounted on an aircraft. 

Figure 1: View of a Mated Plug and Receptacle Assembled with Connector 

Accessories. 

Receptacles are also attached to avionic equipment as they are the only type of 

connectors used on this equipment. These two different types of connectors are designed 

to mate with one another. Circular connectors consist of a shell which could be of 

different materials e.g. aluminum, composite, stainless steel, or cold rolled steel. The 

shell provides protection against abrasion and contact with contaminants that could 
2 

damage the mating contacts. They also provide shielding for the conductors in the 

harness against unwanted electromagnetic and radio frequency interference. Another 

component of the connector is an insulator placed inside the shell. It contains inserts for 

contacts and a seal for wires. The contacts serve as terminations in the connector for the 

wires in the harness. Due to the environments in which aircraft operate, circular 

connectors are specifically designed to withstand high shock and vibration forces. Figure 

2 shows cutaway views of the two types of connectors and some of their components. It 

shows how the wire from the harness is connected to the contacts and how this 

connection is encapsulated within the insulator. One can also see from the figure that the 

contacts in the plug are different from the contacts in the receptacle and that they are 

designed to mate with one another. 

Plug Receptacle 

Figure 2: Cutaway view of a Circular Connector Plug and Receptacle [13] 

1.2 Corrosion on Circular Connectors 

Circular connectors used in the aerospace industry need adequate protection from 

corrosion due to the environment in which the aircraft operates. Specifically, the effect 

of long term exposure to salt atmosphere environments is a major concern. The mating 

of connectors of different materials and finishes also poses a concern due to the 
3 

electrochemical action of two dissimilar metals in contact with each other. This can 

cause galvanic corrosion given the presence of a conductive path and an electrolyte. This 

is a primary problem for electrical connectors in the environment in which the aircraft 

operates because moisture could serve as a medium for current flow. 

In connectors, galvanic compatibility is required between the base material or the 

shell and the coating for conduction. Conduction of the coating is required on connectors 

because this is what prevents electromagnetic interference from disruption of the flow of 

data in the harness, by providing a path to ground. Cadmium is a widely used finish in 

this application for corrosion resistance and electrical conductivity over long term 

exposure to salt atmosphere environments. 

1.3 Electromagnetic Interference Shielding 

Electromagnetic Interference (EMI) can result in the disruption of transmitted 

data or performance of an electrical circuit within an aircraft system. This could be due 

to electromagnetic conduction or radiation produced from an external source. A number 

of sources can produce electromagnetic interference (EMI) in an aircraft system. Coils, 

electromagnets, electric motors, and transformers that can be found in a number of 

locations in an aircraft are well known generators of electromagnetic interference. 

Others are high power radars, and broadcast stations that are not necessarily associated 

with the aircraft but are part of the environment in which the aircraft operates. Because 

the conductors in the cables can act as antennas and pick up radiated signals, the 

reception of the interference can lead to inaccurate or distorted data being transmitted. 

The examples stated are usually unintentional sources of EMI. However, due to 

electronic warfare, other sources that are used to intentionally jam radio signals can also 

affect aircraft systems. 

The conductive property of plated connectors and backshells helps prevent the 

interruption of communication in an aircraft system by electromagnetic interference 

(EMI). It allows for the harmless passage of the interference from a harness shield braid 

to a grounding point through the backshell and connector. A backshell is a connector 

accessory that is used to shield the wire connection points when the harness is attached 

to the connector. The harness is shielded by grounding a conductive cable over the 
4 

harness to the backshell and connector. To shield against electromagnetic interference, 

the harness is enclosed in a tightly woven metal braid often referred to as the shield 

braid. This braid is terminated at the backshell and at the end of the cable. This serves as 

a path for the unwanted EMI from the harness to aircraft electrical ground. Figure 3 

shows a breakaway assembly of a connector and its accessories as they are assembled on 

a harness. It shows the connection from the shield braid to the connector. Termination of 

shield braids will be discussed further in the experimental section of this study. The 

strain relief that prevents the wires from being pulled out of the connector is mounted to 

the rear of the backshell and also secures the backshell termination rings. The 

termination rings are used to secure the shield braid to the backshell in order to provide a 

grounding path. The backshell is mounted on the rear of the connector thus providing a 

ground path from the shield braid to the connector. Since the connector is mounted on 

the aircraft bulkhead, a full path to the aircraft electrical ground is established. All these 

components and their finishes are electrically conductive. 

Figure 3: Connector and Accessory Assembly for EMI shielding [10] 
5 

1.4 Project Background 

The search for a suitable replacement for cadmium as a protective finish for the 

circular connectors used in aircraft systems has been going on for quite some time now. 

Cadmium is a generally preferred finish in this application because of its corrosion 

resistance, conductivity, lubricity, and low cost. However, cadmium is a heavy metal 

and it has been identified as a hazardous material now listed by OSHA and on the 

Restriction of Hazardous Substances (RoHS) document as a workplace hazard [12]. This 

has caused a ramp up in the search for a replacement for cadmium. Zinc nickel has been 

identified as an effective finish in protecting circular connectors from the environment 

[14]. It has been added to military specification MIL-DTL-38999 as one of the available 

finishes on connectors used in the aerospace industry [14]. In addition to nickel 

fluorocarbon polymer, and electrodeposited aluminum, zinc nickel finish has also been 

identified as a potential alternative to cadmium in this and other types of applications. 

The existing test data regarding zinc nickel finish on circular connectors in MILDTL-38999 states it’s successful passage of the 500 hour dynamic salt spray test [14]. 

However, concerns still exist regarding the behavior of zinc nickel finishes on circular 

connectors if introduced into the field. This is due to the lack of extensive testing of the 

effects of combining different materials and existing finish combinations of backshells, 

connectors, shield braids, and mounting hardware. The requirement for prevention of 

connectors from long term corrosion degradation in this application when the finish is 

combined with different materials and existing finishes demonstrates the need to 

perform a corrosion performance study. Maintaining the shell to shell continuity in order 

to keep the integrity of electromagnetic wiring shielding is also required. 

1.5 Overview of Project Goals 

The purpose of this thesis is to provide insight into the ability of the zinc nickel 

finish on the shell of circular connectors to provide adequate protection from corrosion 

when combined with the different materials and finishes used today in aerospace 

applications. In addition to this, the ability of zinc nickel finish to maintain the required 

electrical conductivity and electromagnetic interference shielding integrity when 
6 

combined with different materials and finishes existing today will be examined. The 

observed behavior of the zinc nickel finish will be compared to cadmium finish that is 

widely used today in the same application. The results from this study would reveal the 

electrical and physical properties, and corrosion resistance level of a connector shell 

with zinc nickel finish after it has undergone a salt fog test for an extended period of 

time when mated with connectors with the same and other finishes that exist today. The 

results will be used to conclude whether or not zinc nickel finish is an acceptable 

replacement for cadmium finishes in this application. 
7 

2. Coatings for Circular Connectors 

2.1 Existing finishes and their applications 

According to MIL-DTL-38999, there are 22 finishes existing today for MILDTL-38999 circular connectors applied over the different types of shell material and 

connector series [14]. All these finishes are rated per period of exposure to salt fog and 

have been qualified based on material, finish color. and connector series. Finishes 

provide corrosion resistance and electrical conductivity for shells on circular connectors. 

Without the finish, the bare metal would come in contact with the environment and rapid 

degradation would occur. Once this happens, reduction in conductive properties and 

strength of the shell begin which may result in electromagnetic interference in the wiring 

interconnect system. Described below are some of the finishes that exist today as listed 

in MIL-DTL-38999 [14]. 

2.1.1 Class A finish: Cadmium over nickel. Light Gold Color 

This finish has a light gold color and is only available on connector shells made 

with aluminum. An electrodepositing process is used to plate this finish on the connector 

shell. A nickel plate is applied on the shell of the connector followed by a cadmium 

plate. The final finish is electrically conductive and also provides corrosion resistance 

for the shell. It is currently rated for up to 48 hours in a salt fog test and is inactive for 

new designs. 

2.1.2 Class F & G finish: Electroless Nickel 

This finish is applied from an aqueous acidic solution to the connector shell 

using an autocatalytic process operating at elevated temperatures. The autocatalytic 

process is a process where the product of the reaction is also the catalyst for that 

reaction. In this case, the deposited nickel alloy is the catalyst and the product in the 

process. This finish is applied on aluminum connector shells. The final finish is 

electrically conductive and also provides corrosion resistance for the shell. The same 
8 

finish is available for composite shell material, however it is identified as a class M 

finish. Class M finish provides electrical conductivity only as corrosion is not a concern 

in composite materials. This finish is currently rated for up to 48 hours in a salt fog test 

for aluminum shells and 2000 hours for composite shells and is in use on some avionic 

equipment. 

2.1.3 Class W finish: Cadmium over nickel. Olive Drab Color 

This finish has an olive drab color and is available on connector shells made with 

aluminum. The same finish is available for composite shell material, however it is 

identified as a class J finish. An electrodepositing process is used to plate this finish on 

the connector shell. A nickel plate is applied on the shell of the connector followed by a 

cadmium plate. The final finish is electrically conductive and also provides corrosion 

resistance for the shell. Class J finishes provide electrical conductivity only as corrosion 

is not a concern in composite materials. It is currently rated for up to 500 hours in a salt 

fog test and 2000 hours for composite shells (class J finish). The Class W finish is the 

most commonly used finish on circular connectors and backshells. This is because it 

provides ultimate corrosion protection. It continues to provide corrosion protection when 

the finish is scratched. It also has good electrical conductivity properties and low contact 

resistance. A combination of connectors and backshells with Class W finish is used as 

the baseline for the tests performed in this study. 

2.1.4 Class K finish: Passivate. 

This finish is available on connector shells made with stainless steel. Connectors 

with this finish are used in harnesses exposed to high temperature such as on the engine 

firewall. The passivate finish uses an oxidant such as nitric acid solution to enhance the 

spontaneous formation of protective passive films. These film helps to remove 

contamination that can be potential corrosion sites. It is currently rated for up to 500 

hours in a salt fog test. 
9 

2.1.5 Class L finish: Electrodeposited Nickel 

This finish is available on connector shells made with stainless steel. Similar to 

class K finishes, connectors with this finish are found in areas where high temperature is 

a concern. The electrodeposited nickel finish helps seal up contaminations that can 

become potential corrosion sites. An electrodepositing process is used to plate this finish 

on the connector shell. It is currently rated for up to 500 hours in a salt fog test. 

2.2 Currently identified alternate finishes to Cadmium 

2.2.1 Class P finish: Electrodeposited Aluminum. 

This finish has a non-reflective color and is available on connector shells made 

with aluminum. An electrodepositing process is used to plate this finish on the connector 

shell. The final finish is electrically conductive and also provides corrosion resistance 

for the shell. It is currently rated for up to 500 hours in a salt spray chamber. The Class P 

finish is one of the newer finishes approved MIL-DTL-38999 as a replacement for 

cadmium. 

2.2.2 Class T finish: Nickel fluorocarbon polymer. 

This finish has a gun-metal gray non-reflective color and is available on 

connector shells made with aluminum. An electrodepositing process is used to plate this 

finish on the connector shell. A nickel plate is applied on the shell of the connector 

followed by a composite coating of electroless nickel phosphorus and polytetrafluoroethylene (PTFE). The final finish is electrically conductive and also provides 

corrosion resistance for the shell. It is currently rated for up to 500 hours in a salt spray 

chamber. The Class T finish is one of the newer finishes approved for MIL-DTL-38999 

as a replacement for cadmium. 

2.2.3 Class Z finish: Zinc Nickel 

This finish has a black color but can be found in other colors depending on the 

conversion process used and is available only on connector shells made with aluminum. 

An electrodepositing process is used to plate this finish on the connector shell. The final 
10 

finish is electrically conductive and also provides corrosion resistance for the shell. It is 

currently rated for up to 500 hours in a salt spray test. The Class Z finish is one of the 

newer finishes approved for MIL-DTL-38999 as a replacement for cadmium [14]. It is 

also seen as a cost effective alternative to cadmium. This finish is further discussed later 

in this study. 

2.3 How Circular Connectors Corrode 

Corrosion is an electrochemical process that involves the degradation of a metal 

due to electrochemical reactions it undergoes within its environment. The process of 

corrosion takes place at the most basic molecular level which is the atom and in most 

cases driven by electricity. Corrosion does not only occur in the presence of a liquid, it 

can also occur as a result of the presence of hot gasses. In this application, hot gases are 

derived from fumes caused by fuel combustion in an aircraft. 

Most metals are unstable in their metallic states. Metals used for circular 

connector shells are refined from raw materials and ores in order to preserve their 

chemical stability when in corrosive environments. Aluminum is the most common 

material used on circular connectors and it is processed from raw material or ores. [1] 

Problems associated with corrosion on circular connectors are made worse 

because of the need for electrical conductivity and the ability to provide adequate 

environmental protection. [1] 

Migration of electrons occur between the anode and the cathode along a metallic 

path because of the electrical potential between them. The combination of positively 

charged atoms produced at the anode and negatively charged ions from the environment 

form rust which is a product of corrosion. In aerospace applications, the source of ions is 

the atmosphere. The conductor of the electrical current is the mating point for most of 

the components such as the threads of the plug and receptacle when mated. The metallic 

path for the current flow is between the shell and its plating when the finish is broken. 

An unbroken plating prevents the connection of the electrolyte to the cathode and anode 

elements thereby preventing ionic current flow. In aerospace applications, a sacrificial 

protection method known as cathodic protection for corrosion control is deployed. [1] 
11 

2.4 Cadmium Protection 

 Cadmium plating has historically been the ideal material finish for corrosion 

protection on electrical systems in aerospace applications. It acts as a sacrificial coating 

when used as a plating over more noble metals such as nickel [1]. It has superior 

environmental resistance; corrosion resistance properties. In corrosive environments, 

cadmium dissolves thereby leaving the substrate metal intact. In cases where the finish is 

broken, the surrounding finish still protects the substrate. It also serves as a lubricious 

coating to reduce wear and thread galling when mated with other connectors because of 

its softness and durability. It is also a very good electrical conductor thereby providing 

electromagnetic compatibility and resistance to electromagnetic interference. Cadmium 

currently has an advantage over other finishes in terms of cost because of its current 

high production use and plating method available from numerous sources. It’s ease of 

manufacturing and repair make this possible. Cadmium is solderable which is very 

important in this application. Cadmium also provides a low resistant electrically 

conductive surface which allows for part marking to be accomplished. The amount of 

deposit thickness is easy to control within .0010 to .00030 inches making it suitable for 

threaded components and components in which close tolerance is a critical requirement 

[1]. It can operate in a temperature range of -65C to 175C and has been tested to provide 

over 500 hours salt fog protection on aluminum shells. 

However, Cadmium salts are toxic. Because of its salts, there is an increased risk 

of failure of steel components due to hydrogen embrittlement, and there are health 

hazards from exposure to cyanide baths when plating cadmium on a substrate material. 

It is one of the most dangerous occupational hazards when the fine dusts or fumes are 

inhaled or soluble compounds of the metal are ingested. Hence there is a powerful 

motivation to look for alternative coatings for this application. Figure 4 shows a plug 

and receptacle with cadmium coating. 
12 

 

Receptacle Plug 

Figure 4: Cadmium Plated Aluminum Receptacle and Plug 

2.5 Zinc Nickel finish 

The performance features expected of the thin film alloy deposits include 

corrosion sacrificially to substrate, stability of corrosion byproducts, coating adherence 

to non-conversion films, and low dissolution rates in corrosive atmosphere [3]. Because 

the coating must be sacrificial in relation to its substrate, the positioning of the 

substitution metals in relation to the substrate in the corrosion potential scale in a highly 

corrosive environment is important [7]. 

In a previous study performed, substrates were protected by the deposition of 

zinc and zinc alloys in order to test their performance in corrosive environments. The 

study revealed that electroplated zinc alloy deposits are superior and achieve a consistent 

and stable corrosion resistance performance over the substrate alloy [7]. 

Zinc electrodeposits are widely used for corrosion protection in place of 

cadmium due to cadmium’s highly toxic nature and the environmental hazard it 

represents. Similar to cadmium, zinc deposits are effective in isolating a substrate from 

the environment and provide sacrificial protection to the substrate in a case where the 

coating is damaged [4]. However, in long term cases, pure zinc coatings offer 

insufficient corrosion resistance. The industry requirement to reduce coating thickness 

makes it difficult to plate the required thickness for optimal corrosion resistance [5]. 
13 

Improvements to the corrosion resistance of zinc can be achieved by alloying it with 

some more noble metals such as tin, cobalt, and nickel. 

Zinc coatings alloyed with nickel have generated the highest interest in the 

aerospace industry because they meet most of the benchmarks set by cadmium. Zinc 

coatings also have good corrosion resistance properties, superior formability and 

improved welding characteristics [6]. It has been stated in several studies that the 

corrosion resistance of zinc nickel alloy coatings can be 5 to 6 times higher than that of 

pure zinc and that higher corrosion resistance is seen in zinc nickel coatings with smaller 

grain size, higher uniformity or grain distribution, and higher number of lattice 

imperfections in their microstructure [5]. This is due to the protective properties of the 

oxide layers formed on the surface of the zinc nickel coatings during corrosion [5]. 

Previous studies indicate that zinc nickel alloys containing 10-15wt% nickel 

possess the highest corrosion resistance of the possible alloy composition however, they 

have more negative potential because of the high zinc content thereby forming oxides 

instantaneously in corrosive environments [6]. Nickel slows down the dehydration of 

zinc hydroxide into zinc oxide. Zinc hydroxide is a byproduct of corrosion. Since the 

hydroxide has a lower level of electrical continuity than the oxide, the gain in electrons 

is weaker thus reducing the rate of corrosion [7]. Figure 5 is a chart showing the average 

corrosion rates from a study previously conducted on zinc nickel deposits in a 3.5% 

NaCl solution. As the nickel content of the alloy composition increases, the alloy 

becomes more noble thus becoming less sacrificial to the substrate material. This is most 

likely due to the overall surface enrichment of nickel by dezincification [15]. 
14 

Figure 5: Average Corrosion Rates on Zinc Nickel Alloy Deposits by Nickel 

Content [15]. 

The corrosion products of zinc nickel coatings greatly affect the corrosion 

behavior of the substrate. The initial stage of corrosion of zinc nickel coatings results in 

instant growth of an oxide film [5]. In the presence of chloride or sulphate ions, zinc 

hydrochlorides or hydrosulphates with water molecules could be formed [5]. In a 

previous study under accelerated corrosion conditions, zinc hydroxychloride 

Zn5(OH)8Cl2 H2O was the main corrosion byproduct formed on the zinc nickel surface 

[5]. Smaller quantities of sodium hydroxysulphate NaZn4(SO4) Cl(OH)6 H2O were also 

identified in the byproducts [5]. 

The composition of zinc nickel on circular connectors consists of 5-15wt% 

nickel and the balance is zinc in an aqueous solution that can be deposited from both 

acid and alkaline processes. The alkaline process is easier to control and provides a more 

consistent coating composition [2]. Current studies have shown that paint adhesion and 

hydrogen embrittlement tests conducted on zinc nickel coatings were successful [1]. 

Paint adhesion is preferred on circular connectors because of part marking. Studies have 

also shown that corrosion resistance of zinc nickel on circular connectors is less than 

cadmium baselines, however an increase in the thickness of the coating and selection of 

an appropriate conversion coating will improve results [2]. Thicker zinc nickel coatings 
15 

provide enhanced corrosion resistance. The form and fit of the component might be 

affected but dimensions can be modified. Because the corrosion performance of zinc 

nickel plating film is a function of the nickel content, the corrosion potential for zinc 

nickel deposits with a high nickel content -0.9 to -1.0V is closer to being noble than 

basic. This slows down the dissolution rate of zinc nickel coating [3]. 

Zinc nickel alloys have different phases depending on the nickel content of the 

composition. Zinc nickel alloys with 10-17% nickel usually have a single γ-phase in 

electrodeposited coatings. Yield strength of zinc nickel alloys in this phase is reported to 

be 260MN m-2 [15]. Hardness values range from approximately 260 to 400HV [15]. 

Anti-scratch performance is preferred in circular connectors because of rough handling 

during harness assembly and installation. Nickel contents greater than 17% have a 

mixture of γ and α-phase [15]. Figure 6 is the phase diagram for zinc nickel alloys. 

Evaluation in a previous study of zinc nickel on steel sheets concluded that no 

deterioration in yield strength, tensile strength or elongation properties of the substrate 

material was observed [17]. 

Figure 6: Zinc Nickel Phase Diagram [16] 

Figure 7 shows the microstructure of zinc nickel coatings deposited from an 

alkaline bath before and after a 48 hour corrosion test in a sodium chloride environment. 
16 

The alloy had a nickel content of 6-10% thus presenting a two phase structure of γ and δ 

with γ-phase being predominant [16]. 

Before Test After 48 hrs 

Figure 7: Micrograph of Zinc Nickel Coatings before and after 48 hours Immersion 

in NaCl Environment [16] 

Zinc nickel alloys produce zinc chloride in the presence of salts which is a very 

stable byproduct in corrosive environments. The basic zinc chloride that is produced at 

the surface corrosion site acts as a barrier film, however it has high electrical resistance 

and a reduced dissolution rate [3]. Due to the nickel content, zinc nickel is harder and 

thus provides superior anti-scratch performance. As stated earlier, this is a favorable 

characteristic because of the rough handling of the connectors when the harness is 

assembled and installed. 

2.5.1 Electrodeposition of zinc nickel alloy Coating Deposits on Circular 

Connectors 

There is one class of zinc nickel coating identified by ASTM B841. It consists of 

5-12wt% of nickel with the balance of zinc. There are 5 types of conversion coatings 

that can be applied, however type D which is black chromate conversion coating is what 

is identified in MIL-DTL-38999 for coatings on circular connectors. The chromate 
17 

conversion coating helps inhibit dezincification of the alloy. Manufacturers are allowed 

to select which conversion coatings to use. The coating is produced from an aqueous 

electroplating system. This aqueous system can either be alkaline or acidic as stated 

previously, but alkaline is preferred. As part of the coating requirements, the substrate 

must be free of defects that are detrimental to the zinc nickel coating. Application of 

procedures such as cleaning, pickling, and electroplating occurs after all base metal heat 

treatments have been performed. This is required in order to ensure satisfactory adhesion 

and corrosion resistance performance of the coating. After electrodeposition, the coating 

must be adherent and free of blisters, pits or discontinuities, and also free of cracks. The 

coating allows for normal handling storage conditions to occur without chipping, flaking 

or other coating damage [8]. Figure 8 shows zinc nickel finish with black conversion 

coating applied over an aluminum MIL-DTL-38999 plug and receptacle. 

Receptacle Plug 

Figure 8: Zinc Nickel Plated Aluminum Receptacle and Plug 

2.5.2 Properties of Cadmium and zinc nickel of Interest in this Application 

Table 1 shows a comparison between cadmium coatings, electrodeposited zinc 

nickel coatings, and pure zinc coatings based on performance in properties of interest in 

the application of this study. These properties are preferred in coatings for circular 

connectors. 
18 

Properties of Interest Cadmium Zinc Nickel Zinc 

Corrosion Resistance Good Good Good 

Sacrificial Protection Good Good Good 

Galvanic Compatibility with Aluminum Good Good Poor 

Friction Coefficient Low Low Intermediate 

Electrical Conductivity Good Good Fair 

Solderability Good Good Poor 

White Rust Formation Low Low High 

Uniform Deposition Good Good Good 

Adhesion Good Good Good 

Table 1: Summary of Properties of Cadmium, Zinc Nickel and Pure Zinc Coatings 

[4]. 
19 

3. Experimental 

The goal of this study was to provide insight into the ability of the zinc nickel 

finishes on circular connectors to adequately provide protection from corrosion and 

maintain the required electrical conductivity and electromagnetic interference shielding 

integrity, when combined with connectors and connector accessories with the same and 

other finishes that exist today. In addition to these combinations, zinc nickel’s 

compatibility with the tin plated shield braid, which is a key component of the ground 

path from the wires to the bulkhead, will be examined. The different terminations of the 

shield braid used in this study will be discussed. The study was conducted using the 

following tests. 

• Salt spray test 

• Electrical Continuity Test 

• Coupling torque Test 

• Visual Inspection. 

All testing was carried out per industry accepted specifications. ASTM B117 for 

the salt spray test, EIA-364-83 for the electrical continuity test, and EIA-364-13D for 

the coupling torque test. Data collected at periodic intervals during the test, between 0 

and 500 hours, will be used to determine the ability of zinc nickel to provide adequate 

protection needed. 

3.1 Test Unit Identification 

There are 10 test units used in this study, however only 9 of them are applicable 

to this study. This is because the tenth test unit does not simulate any condition that a 

connector or connector accessory with zinc nickel finish might come in contact with. 

Test unit 10 has a nickel Teflon backshell on an electroless nickel connector mated with 

a nickel Teflon backshell on a cadmium connector, all with aluminum shells. The test 

unit combinations were arrived at by identifying the materials and finishes currently 

used in today’s aircraft on connectors, connector accessories, shield braids, screws, 

washers and mounting flanges. Table 2 shows the materials and finishes that were 

identified. 
20 

 Material Finish 

Connectors Aluminum Cadmium over Nickel 

Electroless Nickel 

Composite Cadmium over Nickel 

Electroless Nickel 

Stainless Steel Passivate 

Backshells Aluminum Cadmium over Nickel 

Composite Cadmium over Nickel 

Composite Electroless Nickel 

Shield Braids Copper Tin 

Screws Carbon Steel Cadmium 

Washers Aluminum Anodized 

Mounting Flanges Aluminum Epoxy Polyamide Primer 

Nuts Steel Cadmium 

Table 2: Matrix of existing materials and finishes 

Introducing zinc nickel finish into the industry today will involve the possible 

combination of the materials and finishes listed in table 2 depending on what part of the 

interconnect system is being replaced. The test units simulate different potential 

conditions that arise in practice. 

3.1.1 Test Unit Set-up 

Described are the test units simulating the different combination of materials and 

finishes if zinc nickel finish is introduced into the field. The parts list and figure of the 

test units applicable to this study is depicted in the description of the test units. Table 3 

shows a matrix of the test units based on connector and backshell materials and finishes. 
21 

Table 3: Test Unit Finish and Material Matrix (Al – Aluminum. C – Composite) 

Test Unit Backshell Plug Receptacle 

1 Cadmium (Al) Cadmium (Al) Cadmium (Al) C2 Cadmium (Al) Cadmium (Al) Zinc Nickel (Al) Z3 Zinc Nickel (Al) Zinc Nickel (Al) Zinc Nickel (Al) Z4 Zinc Nickel (Al) Zinc Nickel (Al) Electroless Nickel (Al) 5 Zinc Nickel (Al) Cadmium (Al) Zinc Nickel (Al) 6 Zinc Nickel (Al) Zinc Nickel (Al) Passivate (SS) 7 Cadmium (C) Cadmium (C) Zinc Nickel (Al) Z8 Electroless Nickel (C) Electroless Nickel (C) Zinc Nickel (Al) Z9 Zinc Nickel (Al) Electroless Nickel (C) Zinc Nickel) (Al) C
22 

3.1.1.1 Test Unit 1 (Test Baseline) 

 Component Material and Finish 

Plug Side Connector Aluminum, Cadmium finish 

Backshell Aluminum, Cadmium finish 

Shield Braid Copper, Tin plated 

Receptacle Connector Aluminum, Cadmium finish 

Backshell Aluminum, Cadmium finish 

Shield Braid Copper, Tin plated 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 4: Test Unit 1 Parts List 

Figure 9: Test Unit 1 

Test Unit 1 simulates a condition that currently exists on many aircraft where 

cadmium plated connectors and backshells are mated with other cadmium plated 

connectors. The degradation of cadmium finish and the interface between the shield 

braid and the rings will be reviewed. The results gathered from this test unit will serve as 

a baseline for this test. 
23 

3.1.1.2 Test Unit 2 

 Part Number Material and Finish 

Plug Connector Aluminum, Cadmium finish 

Backshell Aluminum, Cadmium finish 

Shield Braid Copper, Tin plated 

Receptacle Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 5: Test Unit 2 Parts List 

Figure 10: Test Unit 2 

Test Unit 2 simulates the introduction of a connector and backshell with zinc 

nickel finish into an existing aircraft today where one side of the interconnect system is 

replaced. This test unit will establish the corrosion resistance of zinc nickel finish 

connectors when mated with cadmium plated connectors. In addition the degradation of 

the zinc nickel finish and the interface between the shield braid and backshell rings will 

be reviewed. The possibility of this test unit in actual use in the industry is high if zinc 

nickel finish is introduced. 
24 

3.1.1.3 Test Unit 3 

 Part Number Material and Finish 

Plug Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Receptacle Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 6: Test Unit 3 Parts List 

Figure 11: Test Unit 3 

Test Unit 3 simulates a situation where all connectors and backshells used have a 

zinc nickel finish. This unit will establish the corrosion protection strength of zinc nickel 

finishes on connectors when mated with one another and also the degradation of the zinc 

nickel finish and the interface between the shield braid and backshell rings will be 

reviewed. A part of this test unit has been investigated by previous studies and results 

have been deemed favorable. 
25 

3.1.1.4 Test Unit 4 

 Part Number Material and Finish 

Plug Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Receptacle Connector Aluminum, Electroless Nickel finish 

Backshell N/A 

Shield Braid N/A 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 7: Test Unit 4 Parts List 

Figure 12: Test Unit 4 

Test Unit 4 simulates a situation where a zinc nickel finish connector with a zinc 

nickel finish backshell is mated with an electroless nickel finish connector without a 

backshell. Since electroless nickel finish connectors are mostly found on equipment, a 

backshell is not required in this simulation. This test unit will be reviewed to establish 

the corrosion protection strength of zinc nickel finishes when mated with electroless 

nickel plated connectors. 
26 

3.1.1.5 Test Unit 5 

 Part Number Material and Finish 

Plug Connector Aluminum, Cadmium finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Receptacle Connector Aluminum, Zinc Nickel finish 

Backshell N/A 

Shield Braid N/A 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 8: Test Unit 5 Parts List 

Figure 13: Test Unit 5 

Test unit 5 simulates a situation where a backshell with a cadmium finish is 

replaced with one with a zinc nickel finish without a change to the connector on one side 

of the interconnect system. This case is likely to exist if only a connector swap is 

needed. This test unit will be used to examine the interface between the connector and 

backshell of the different finishes. The other side of this test unit is a zinc nickel finish 

connector. 
27 

3.1.1.6 Test Unit 6 

 Part Number Material and Finish 

Plug Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Receptacle Connector Stainless Steel, Passivate finish 

Backshell N/A 

Shield Braid N/A 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Nuts Steel, Cadmium finish 

Table 9: Test Unit 6 Parts List 

Figure 14: Test Unit 6 

Test Unit 6 simulates a situation where a zinc nickel finish connector is mated 

with a stainless steel connector with a passivate finish. These types of connectors can be 

found in areas of high temperature and when high strength is required. This unit will be 

used to establish the corrosion protection strength of zinc nickel finishes when mated 

with passivate finishes on stainless steel connectors. 
28 

3.1.1.7 Test Unit 7 

 Part Number Material and Finish 

Plug Connector Composite, Cadmium finish 

Backshell Composite, Cadmium finish 

Shield Braid N/A 

Receptacle Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 10: Test Unit 7 Parts List 

Figure 15: Test Unit 7 

Test Unit 7 is similar to Test Unit 2 but the substrate material for the cadmium 

finish is composite. Results from this test unit should be similar to Test Unit 2 because 

of the arrangement of the finishes on the components of the test unit. The possibility of 

this configuration in actual use in the industry is high if zinc nickel finish is introduced. 
29 

3.1.1.8 Test Unit 8 

 Part Number Material and Finish 

Plug Connector Composite, Electroless Nickel 

Backshell Composite, Electroless Nickel 

Shield Braid N/A 

Receptacle Connector Aluminum, Zinc Nickel finish 

Backshell Aluminum, Zinc Nickel finish 

Shield Braid Copper, Tin plated 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 11: Test Unit 8 Parts List 

Figure 16: Test Unit 8 

Test Unit 7 is similar to Test Unit 4 but the substrate material for the electroless 

nickel is composite. The electroless nickel connector has an electroless nickel backshell 

that is also composite. Results from this test unit should be similar to Test Unit 4 

because of the arrangement of the finishes on the test unit components. 
30 

3.1.1.9 Test Unit 9 

 Part Number Material and Finish 

Plug Connector Composite, Electroless Nickel 

Backshell Aluminum, Zinc Nickel alloy 

Shield Braid Copper, Tin plated 

Receptacle Connector Aluminum, Zinc Nickel finish 

Backshell Composite, Cadmium finish 

Shield Braid N/A 

Mounting Hardware Screw Carbon Steel, Cadmium finish 

Washer Aluminum, Anodized 

Mounting Flange Aluminum, epoxy polyamide primer 

Table 12: Test Unit 9 Parts List 

Figure 17: Test Unit 9 

Test unit 9 comprised of 4 finishes examined in this study. The substrate of the 

electroless nickel and cadmium finishes is composite. This test unit will be used to 

examine the corrosion resistance of a zinc nickel backshell on an electroless nickel finish 
31 

composite connector and cadmium finish composite backshell on a zinc nickel finish 

connector. 

It is noted that the combination of the tin plated copper shield braid and the zinc 

nickel backshell occurs in several of the test units set up. This allows for more data on 

this interface to be collected because this interface is of major concern in this study. This 

also applies to the interface between the zinc nickel backshell and connector. 

3.1.2 Mounting bracket 

The mounting bracket in this study was used to hold the test units in place and 

also to simulate the bulkhead in an aircraft. It was fabricated from Al 7076 and is 0.063 

inches thick. The bracket surface was chemically heat treated and primed before the test 

units were assembled. Chemical surface treatment is the application of alodine on 

aluminum and aluminum alloys where maximum corrosion protection and electrical 

conductivity is required. The surface area where the receptacles interfaced with the 

mounting bracket were masked off prior to the application of primer in order to allow 

electrical continuity through the bracket. This is the same practice used on aircraft. (See 

appendix 7.10 for detailed design of the bracket) 

Figure 18: Test bracket set up. 
32 

3.2 Shield Braid Termination. 

As stated earlier in the report, a shield braid is a tightly woven braid in which the 

harness is enclosed and is part of the conductive path from the wires to the grounding 

point. The shield braid, made of copper material and plated with tin, is applied over the 

harness. The shield braid is terminated at the backshell. Any unwanted EMI travels from 

the harness to the shield braid, to the backshell, to the connector, and to the bulkhead 

which is attached to the aircraft ground. Termination of the shield braid is important so 

as not to cause a break in this path. There are different termination styles depending on 

the backshell, but the one used in this study has a ring termination style. Two or three 

rings are utilized for grounding the shields. However, two are used in this study because 

the third ring is meant for termination of the individual wire braids that don’t apply to 

this study. The shield braid is woven between the rings and is secured at the end of the 

backshell by the strain relief. This allows contact between the shield braid and the 

backshell thereby providing a conductive path. The interface between the zinc nickel 

finish and the shield braid is important for the integrity of the conductive path. The 

effects of corrosion at this interface will be examined. 

Figure 19: Shield Braid Termination [10]. 
33 

3.3 Salt Spray Test 

This test involves exposing test units to a fine mist of salt solution in order to 

establish an evaluation of the corrosion resisting properties of various materials and 

protective coatings. Electrolytic corrosion is the failure mechanism induced by the salt 

spray on the test units. The test provides an avenue to assess the effects of salt-laden 

atmosphere on electrical connectors, connector accessories, finishes, and components 

through electrical continuity readings and visual inspections. The apparatus, salt 

composition and procedure of the salt spray test will be discussed. 

3.3.1 Test Apparatus 

The apparatus required for this test consists of a salt fog chamber, a salt solution 

reservoir, a supply of suitably conditioned compressed air, atomizing nozzles, supports 

for test units, and temperature control provisions. 

Figure 20 shows a salt spray test apparatus consisting of the salt fog chamber 

which is the container in which the test fixture is placed. It is made of materials such as 

glass, hard rubber and plastic that cannot be affected by the salt fog. The chamber is 

constructed in a way that allows free circulation of the salt fog to the same degree on all 

the test units. It also allows for no direct encroaching of the salt fog on the test units or 

condensation dripping on the units. The salt chamber also contains the supports for the 

test units made of a material that cannot be affected by the salt solution. The temperature 

of the exposure zone of the salt spray chamber is maintained at 35 ± 2oC. The atomizers 

provide a finely divided dense fog to the test units. The nozzles delivers the fog and are 

made of materials that cannot be affected by the salt solution. The compressed air 

entering the atomizers is free from impurities such as oil and dirt and is passed through a 

filter. Air pressure is used to produce the suitable and finely divided dense fog in 

conjunction with the atomizers. Humidity and temperature of the compressed air is 

controlled in order to meet the operating conditions. In order to protect the atomizers and 

nozzles from clogging by salt deposition, the relative humidity of the air at the point of 

release from the nozzle is 95-98%. An air saturation tower is used to meet the 

requirements for the pressure and humidity. Air pressure varies based on the temperature 

of the water used to control the humidity and salt fog. The salt solution reservoir 
34 

contains the salt solution that will be discussed in the next section of this report. It is a 

separate piece from the salt chamber in order to provide ease of maintainability. The part 

of the reservoir’s construction that comes in contact with the salt solution is of a material 

that cannot be affected by the salt solution. The temperature of the chamber is 

maintained at 35 ± 2oC in order to keep the salt solution at the required temperature. 

During the testing, the salt solution is drawn from the reservoir by the chamber’s 

peristaltic pump through a filter unit. This filter unit removes any salt crystals that are 

not dissolved and debris that are contained in the salt solution. Figure 21 shows the 

placement of the filter in the reservoir. 

Figure 20: Sample Composition of Salt Spray Test Apparatus [9] 

3.3.2 Salt Composition 

The salt solution used in this study is prepared by dissolving 5 ± 1 parts mass of 

sodium chloride in 95 parts mass of water thereby creating a 5% sodium chloride salt 

solution. Distilled water is used for this solution and does not contain more than 50 parts 

per million of total solids. The solution is kept free of solids by a filter that is located in a 
35 

reservoir (figure 21) as stated earlier in the report. The chemical content of the salt does 

not include more than 0.3% by mass of total impurities. The pH value of the salt solution 

is maintained at a value between 6.5 and 7.2 when measured at a temperature range 

between 34oC and 36oC. 

Figure 21: Salt Solution Reservoir showing positioning of filter [11] 

3.3.3 Procedure 

The test fixture in this study consists of the test units mounted on a bracket (see 

figure 18). The test fixture is suspended in the chamber from the top using plastic hooks 

with the test fixture vertical thereby making the test units horizontal in order to simulate 

the conditions of the aircraft. The plastic hooks used are fabricated from a material that 

is non-reactive to the salt solution. The test units are free of oil, dirt, and grease and 

cleaned as necessary before they were put into the chamber. As stated earlier, the test is 

conducted at a temperature maintained at 35oC ± 2oC in the exposure zone. The salt 

solution deposited on the test unit in the exposure zone is about 0.5 milliliter to 3.0 

milliliters of solution per hour. The length of the test is 500 hours but data is collected at 

the intervals indicated. Table 13 shows the test intervals when the exposure to salt fog 

will be interrupted and inspection and data collection will occur. 
36 

Test Intervals Length of exposure in the Salt Spray Chamber 

A 48 Hours 

B 96 Hours 

C 500 Hours 

Table 13: Salt Spray Test intervals. 

At each test interval, the test fixture was dipped in running tap water at a 

temperature of 39oC for 5 minutes and dried for 16 hours in a circulating air oven at a 

temperature of 38oC ± 3oC. Once this was completed, examination and measurement 

taking was conducted which will be discussed further in the following sections. 

3.4 Electrical Continuity Test 

This test involves taking electrical continuity measurements between shield 

braids and backshells, backshells and connectors, mated plugs and receptacles, and 

between receptacles and the mounting bracket. The test equipment and procedure of the 

test units will be discussed. 

3.4.1 Equipment 

The shell to shell conductivity test equipment uses a voltmeter capable of 

measuring voltage within ±2% and an ammeter capable of measuring applied current 

within ±1%. A regulated current power supply delivering up to 1.0 ± 0.1 Amps was also 

used in this test. The test probes have spherical ends of 1.27 mm min radius and are used 

to take voltage measurements on the bonding points.

3.4.2 Procedure 

A 1.0 amp DC current is passed through the two surfaces that are being checked. 

The test probes are placed on the shells in a way that doesn’t cause any damage to the 

coating of the connector. Voltage drop across the two surfaces is measured from any 

point on the surfaces. Measurements are conducted and recorded at the test intervals 

listed in table 14. 
37 

Test Intervals Length of exposure to salt spray in chamber 

A 0 Hours 

B 48 Hours 

C 96 Hours 

D 500 Hours 

Table 14: Electrical Continuity Measurement Conditions. 

As stated earlier, electrical continuity measurements are conducted on the 

surfaces of the test units to measure the electrical continuity per interface. Table 15 

shows the different interfaces that are studied in this report as referenced in figure 22 

except the mounting bracket. Figure 18 shows the complete assembly of the test units 

mounted on the bracket. 

Measuring Point/Surface 1 Measuring Point/Surface 2

Shield Braid Backshell 

Backshell Receptacle/Plug 

Receptacle Plug 

Receptacle Mounting bracket 

Table 15: Measurement Points/Surfaces on test units

Figure 22: Measurement Points on test units 
38 

Per MIL-DTL-38999, the voltage drop across the test units should be a maximum 

of 2.5 millivolts with 100% increase after 500 hours of salt fog exposure. 

3.5 Coupling Torque Test 

This test involves taking measurements of force required to mate and unmate 

plugs and receptacles. The test equipment and procedure of the test units will be 

discussed. 

3.5.1 Equipment 

The equipment for this test is a torque gauge. When measurement are taken, 

readings are between 25% and 75% of the scale with nominal full scale accuracy of 

+2%. 

3.5.2 Procedures 

 The mating connectors are brought to the position where mechanical mating 

begins and the torque gauge is set to zero. The connectors are then fully mated at a rate 

of 25.4 millimeters/minute until peak torque is achieved. The reading is recorded at this 

point. The same applies when unmating the connectors as the peak torque used to 

unmate the connectors is recorded. Table 16 shows the maximum and minimum torques 

for mating and unmating shell size 19 connectors used in this study per MIL-DTL-38999 

[14]. 

Maximum Engagement and Disengagement Minimum Disengagement 

28 Inch Pounds 3 Inch Pounds 

Table 16: Maximum and Minimum torques for Shell Size 19 Connectors. 

3.6 Visual Inspection 

This involves examining the connectors for exposure of base metals, pitting and 

porosity of finishes. Cracking and delamination of the connector and coatings is also 

examined. Abnormal nicks, cracks or scratches on the finished surfaces indicate that the 

normal protective coating is removed. White rust indicates corrosion in the finish and 
39 

successful protection of the substrate to the corrosive conditions. Red rust could indicate 

that the substrate has been exposed to the corrosive environment and degradation has 

begun to take place. This inspection is conducted at the test intervals indicated in table 

14. 

3.7 Test Sequence 

The study is performed following the test sequence described in figure 23 until it 

reaches the total salt fog exposure time of 500 hours at the completion of the test. After 

assembling the test units onto the test fixture, a visual inspection of the test units is 

performed in order to check for damage to their finishes. The connectors are then 

torqued to values below the maximum engagement value specified in table 16. Once this 

is completed, electrical continuity measurements are collected. Since this is done prior to 

exposing the test fixture to salt fog, the readings are consistent across all test units. At 

this point, the test fixture is exposed to 48 hours of salt fog in the salt spray chamber. 

The test fixture is rinsed for a maximum of 5 minutes under continuously running water 

colder than 38°C in order to remove the salt deposits on the test units thus reducing the 

chances of confusing salt deposits with oxide film formation. In order to remove 

moisture from the test fixture after it has been rinsed, it is placed in a circulating air oven 

with dry air cooler than 38°C for a maximum of 16 hours. Visual inspection is conducted 

at this time on the test units at room temperature and electrical continuity measurements 

are collected. This sequence is repeated at 96 hours and 500 hours with the addition of 

the unmating torque measurements after electrical continuity measurements have been 

recorded at 500 hours. 
40 

Figure 23: Test Sequence
41 

4. Results and Discussion 

Readings from the electrical continuity test are collected at 0, 48, 96 and 500 

hours of the test fixture’s exposure to salt fog and recorded. Coupling torque 

measurements are taken at 0 and 500 hours only. 

4.1 Pre-Salt Fog Exposure Readings 

Before the test fixture was exposed to salt fog, a set of readings were collected in 

order to determine the effect of the exposure to salt fog on the test units compared to no 

exposure at all. Coupling torque measurements and electrical continuity measurements 

were recorded. The values recorded are within the specified range that MIL-DTL-38999 

recommends for these test units. 

4.1.1 Coupling Torque at 0 Hours 

As stated earlier, the connectors are torqued to values within the range 

recommended by MIL-DTL-38999 of 3 – 28 inch pounds (See table 17) [14] before 

exposure to salt fog. Unmate torque values are recorded after 500 hours of exposure. 

This maximum unmating force is however allowed to increase 100% after 500 hours of 

salt spray test [14]. 

Test Unit Mate Torque Test Unit Mate Torque 

1 8” inch lbs. 6 3”inch lbs. 

2 8” inch lbs. 7 8”inch lbs. 

3 8”inch lbs. 8 5”inch lbs. 

4 7” inch lbs. 9 5”inch lbs. 

5 10”inch lbs. 

Table 17: Initial Mate Torque Values 

4.1.2 Electrical Continuity Readings at 0 Hours 

As stated earlier, the electrical continuity readings recorded prior to the 

beginning of the test are consistent and below the maximum voltage of 2.5 millivolts 

recommended by MIL-DTL-38999 (See table 18). This maximum voltage is however 

allowed to increase 100% after 500 hours of salt spray test [14]. 
42 

Table 18: Initial Electrical Continuity Reading (Millivolts) 

Test Hour: 0 Hours P-Side J-Side 

Shield Braid to 

Backshell 

Backshell to 

Plug 

Plug to 

Receptacle 

Shield Braid to 

Backshell 

Backshell to 

Receptacle 

Test Unit 1 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 1.6 1.6 1.6 1.6 1.6 

Test Unit 2 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 1.6 1.6 1.6 1.6 1.6 

Test Unit 3 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 1.6 1.6 1.6 1.6 1.6 

Test Unit 4 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 1.6 1.6 1.6 N/A N/A 

Test Unit 5 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 1.6 1.6 1.6 N/A N/A 

Test Unit 6 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 1.6 1.6 1.6 N/A N/A 

Test Unit 7 Reading 1 N/A 1.6 1.6 1.6 1.6 

Reading 2 N/A 1.6 1.6 1.6 1.6 

Test Unit 8 Reading 1 N/A 1.6 1.6 1.6 1.6 

Reading 2 N/A 1.6 1.6 1.6 1.6 

Test Unit 9 Reading 1 1.6 1.6 1.6 N/A 1.6 

Reading 2 1.6 1.6 1.6 N/A 1.6 
43 

4.2 Post-Salt Fog Exposure Readings 

4.2.1 Electrical Continuity Readings at 48 Hours 

 The first set of readings recorded after exposing the test fixture to salt fog was at 

48 hours. The electrical continuity measurement readings in table 19 show that all the 

test units except the ones with zinc nickel finish on the connector or backshell remained 

the at same values when compared to the initial readings. The highest readings were 

recorded between the shield braid and the zinc nickel backshell and between the 

connectors and backshells with zinc nickel finish. These values were unexpectedly high 

for this test interval. It was expected that values would be closer to the 2.5 millivolts 

maximum potential drop because all but one of the connectors used in this study are 

rated for 500 hours in the salt fog test. Values less than the maximum voltage drop of 2.5 

millivolts were achieved when the probes were moved to other parts of the backshell 

surface. The readings recorded stayed constant for the continuity measurements between 

the receptacles and the mounting bracket. See figure 24 for maximum continuity 

readings recorded per interface after 48 hours of salt exposure. Figure 26 shows the 

minimum reading collected across per interface for all the test intervals. 

Figure 24: 48 Hour Maximum Electrical Continuity Readings Per Interface on Test 

Units
44 

Test Hour: 48 Hours P-Side J-Side 

Shield Braid to 

Backshell 

Backshell to 

Plug 

Plug to 

Receptacle 

Shield Braid to 

Backshell 

Backshell to 

Receptacle 

Test Unit 1 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 1.6 1.6 1.6 1.6 1.6 

Test Unit 2 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 1.6 1.6 1.6 3.8 1.6 

Test Unit 3 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 36.1 4.1 1.8 22.5 38.2 

Test Unit 4 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 12.0 5.8 1.8 N/A N/A 

Test Unit 5 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 14.2 10.5 1.8 N/A N/A 

Test Unit 6 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 12.5 1.6 1.7 N/A N/A 

Test Unit 7 Reading 1 N/A 1.6 1.6 1.6 1.6 

Reading 2 N/A 1.6 1.6 13.5 1.7 

Test Unit 8 Reading 1 N/A 1.6 1.6 1.6 1.6 

Reading 2 N/A 1.6 1.6 1.6 1.6 

Test Unit 9 Reading 1 1.6 1.6 1.6 N/A N/A 

Reading 2 11.4 6.8 1.9 N/A N/A 

Table 19: 48 Hour Electrical Continuity Reading (Millivolts)
45 

4.2.2 Electrical Continuity Readings at 96 Hours 

As stated earlier, the second set of readings after exposing the test fixture to salt 

fog were collected at 96 hours. The electrical continuity measurement readings in table 

20 show an increase in overall values when compared to the initial and 48 hour 

measurements. All the test units except the ones with zinc nickel finish on the connector 

or backshell remained below the recommended values. The measurements recorded 

between the shield braid and the zinc nickel backshell in the test units showed values 

higher the maximum 2.5 millivolts potential drop when measured on some parts of the 

surfaces. Figure 25 shows the maximum values recorded per interface in this test. The 

values below the recommended maximum value were achieved when the probes were 

moved to other parts of the surface of the backshell. A slight increase in values was 

recorded for continuity measurements between the receptacles and the mounting bracket 

and a value above the recommended maximum voltage in test unit 7. See figure 26 for 

minimum continuity readings recorded per interface after 96 hours of salt exposure. 

Figure 25: 96 Hour Maximum Electrical Continuity Readings Per Interface on Test 

Units
46 

Test Hour: 96 Hours P-Side J-Side 

Shield Braid to 

Backshell 

Backshell to 

Plug 

Plug to 

Receptacle 

Shield Braid to 

Backshell 

Backshell to 

Receptacle 

Test Unit 1 Reading 1 1.6 1.6 1.6 1.6 1.6 

Reading 2 1.6 1.6 1.6 1.6 1.6 

Test Unit 2 Reading 1 1.6 1.6 1.6 1.6 3.9 

Reading 2 1.6 1.6 1.6 9.7 8.3 

Test Unit 3 Reading 1 4.6 2.3 14.6 2.0 1.6 

Reading 2 6.5 3.0 7.3 3.4 1.8 

Test Unit 4 Reading 1 1.6 2.3 1.6 N/A N/A 

Reading 2 19.4 1.9 9.0 N/A N/A 

Test Unit 5 Reading 1 2.2 5.2 1.6 N/A N/A 

Reading 2 12.2 4.6 1.6 N/A N/A 

Test Unit 6 Reading 1 3.2 2.0 1.6 N/A N/A 

Reading 2 4.1 3.7 1.6 N/A N/A 

Test Unit 7 Reading 1 N/A 1.6 1.6 6.7 1.6 

Reading 2 N/A 1.6 1.6 1.7 2.9 

Test Unit 8 Reading 1 N/A 1.6 1.7 1.6 1.6 

Reading 2 N/A 1.6 1.6 2.7 1.6 

Test Unit 9 Reading 1 N/A 11.8 1.6 N/A 1.6 

Reading 2 N/A 1.7 1.6 N/A 1.6 

Table 20: 96 Hour Electrical Continuity Reading (Millivolts)
47 

4.2.3 Electrical Continuity Readings at 500 Hours 

The last set of readings recorded after exposing the test fixture to salt fog was at 

500 hours. The electrical continuity measurements in table 21 show an additional 

increase in overall values when compared to the initial, 48, and 96 hour measurements. 

The measurements recorded between the shield braid and the zinc nickel backshell in the 

test units showed values higher than the 5.0 millivolts recommended maximum voltage 

on some parts of the surface. The range of values recorded between the shield braid and 

the zinc nickel backshell were from 1.6 to 29.7 millivolts depending on where on the test 

unit’s surface the voltmeter probes were placed. Compared to cadmium which recorded 

a range from 1.6 to 2.8 millivolts, this range is much wider. The readings between the 

zinc nickel backshell and the cadmium connectors showed a similar range of values but 

with a maximum voltage of 21.3 millivolts. The readings between the zinc nickel 

backshell and the zinc nickel connectors showed a maximum value of 8.9 millivolts. The 

readings for continuity between the zinc nickel and cadmium connectors showed a 

maximum value at 10 millivolts which was less than that recorded for the zinc nickel 

finish plugs mated with zinc nickel finish receptacles. The readings for the continuity 

between the zinc nickel connector and the mounting bracket exhibited a maximum value 

of 7.5 millivolts which is higher than the continuity measurement on the same interface 

with cadmium, electroless nickel, and stainless steel finishes. 

Overall, the maximum values recorded were a lot higher than the recommended 

maximum voltage of 5 millivolts after 500 hours of salt fog exposure. The minimum 

values on the other hand remained at the same potential drop values recorded during the 

initial readings. In some cases, values as low as 1.2 millivolts were recorded on the 

voltmeter. This signifies that a good grounding path existed at the interfaces examined in 

this study. Figure 26 shows charts with the minimum and maximum values recorded 

after 500 hours of exposing the test units to salt fog per interface. 
48 

Figure 26: Minumum and Maximum Electrical Continuity Readings Per Interface 

on Test Units 
49 

Test Hour: 500 Hours P-Side J-Side 

Shield Braid to 

Backshell 

Backshell to 

Plug 

Plug to 

Receptacle 

Shield Braid to 

Backshell 

Backshell to 

Receptacle 

Test Unit 1 Reading 1 1.7 1.6 1.6 1.7 1.6 

Reading 2 2.8 6.9 1.6 1.6 1.6 

Test Unit 2 Reading 1 1.6 1.2 1.6 3.5 1.6 

Reading 2 1.7 1.7 1.7 11.0 7.2 

Test Unit 3 Reading 1 3.9 1.6 1.6 5.6 2.3 

Reading 2 9.6 1.6 21.2 9.6 7.3 

Test Unit 4 Reading 1 6.8 1.4 2.7 N/A N/A 

Reading 2 29.7 8.9 16.9 N/A N/A 

Test Unit 5 Reading 1 16.0 1.9 1.6 N/A N/A 

Reading 2 25.0 21.3 1.8 N/A N/A 

Test Unit 6 Reading 1 9.1 1.5 1.6 4.8 2.5 

Reading 2 14.6 3.9 9.8 11.2 6.1 

Test Unit 7 Reading 1 N/A 2.9 1.6 4.8 2.5 

Reading 2 N/A 6.6 5.3 11.2 6.1 

Test Unit 8 Reading 1 N/A 1.6 1.7 8.1 1.6 

Reading 2 N/A 1.7 7.5 9.7 3.5 

Test Unit 9 Reading 1 N/A 2.2 1.6 N/A 2.5 

Reading 2 N/A 10.5 5.1 N/A 3.4 

Table 21: 500 Hour Electrical Continuity Reading (Millivolts)
50 

4.2.4 Coupling Torque at 500 hours 

Test Unit Unmate Torque Test Unit Unmate Torque 

1 65” inch lbs. 6 > 300” inch lbs. 

2 45” inch lbs. 7 80” inch lbs. 

3 60” inch lbs. 8 50” inch lbs. 

4 51” inch lbs. 9 60” inch lbs. 

5 45” inch lbs. 

Table 22: Unmating Torque Values after 500 Hours 

Figure 27: Comparison of Torque values at 0 and 500 Hours 

Table 22 shows the values recorded to unmate the connectors after the 500 hours 

of salt spray. The values recorded are significantly higher than the values the connectors 

were torqued to initially. Test units 1, 3, 6, 7 & 9 recorded unmating torques higher than 

the maximum disengagement force required after 500 hours of salt spray. The maximum 

disengagement torque recommended by MIL-DTL-38999 is 56 inch pounds after 500 

hours of salt spray [14]. Initial mate and final unmate torque values are compared in 

figure 27. Surprisingly, test unit 1, which is the baseline of this test and is used widely 

today, recorded a higher value than all the test units except test units 6 and 7. Test unit 6 

did not unmate as the plug was stuck to the receptacle. This could be due to galvanic 
51 

corrosion. The potential for significant corrosion buildup between the two materials of 

the plug and receptacle will cause this to occur. Test unit 6 had an aluminum plug mated 

to a stainless steel receptacle. 

4.3 Discussion 

The test units all performed well at the first set of measurements after exposure 

to salt solution. Increase in electrical continuity was seen in the test units with zinc 

nickel plating on the connectors or backshell. 

Figure 28: Test Unit 1 Backshell with 

Cadmium Finish on Receptacle after 48 

hours 

Figure 29: Test Unit 3 Backshell with 

Zinc Nickel Finish on Receptacle after 

48 hours

Figure 29 shows a white residue on the zinc nickel backshell that was seen after 

48 hours of salt exposure. Earlier in the report, it was indicated that zinc nickel alloys 

produce oxide films as a byproduct and the oxide films have high electrical resistance 

but acts as a barrier film which reduces dissolution rate. This can be an explanation for 

the high continuity measurements that were recorded after 48 hours of salt fog exposure. 

Since the oxide films act as a layer between the test probes and the backshell surface, 
52 

high measurement values were achieved. The non-uniformity of the oxide films allows 

for parts of the surface to be free of the film thereby allowing for the lower readings to 

be achieved. The lower readings were also achieved when the probes were pressed 

harder into the shell thereby cutting through the oxide film. This confirms that even 

though the higher continuity measurements were achieved, the substrate was still 

protected from the salt environment and the grounding path remained intact. Figure 28 

shows no residue on the cadmium backshell which confirms the continuity measurement 

that was recorded for test unit 1. 

Figure 30: Test Unit 1 Backshell and 

Plug with Cadmium Finish after 48 

hours 

Figure 31: Test Unit 3 Backshell and 

Plug with Zinc Nickel Finish after 48 

hours 

Figures 30 and 31 show the plug side of test units 1 and 3 compared earlier. 

Figure 31 shows that the zinc nickel finish backshell and the plug both produced the 

white residue earlier discussed. This was the same for the rest of the test units that had 

zinc nickel backshells or connectors. The cadmium finish on the other hand still shows 

no sign of corrosion. See appendix for figures of all the test units after 48 hours. 
53 

After 96 hours, there was an overall increase in the continuity readings on 

connectors and backshells with the zinc nickel finish from the 48 hour readings. Also 

seen was an increase in the white residue on the test units with zinc nickel finish. 

Figure 32: Test Unit 1 Backshell and 

Receptacle with Cadmium Finish after 

96 hours 

Figure 33: Test Unit 1 Backshell and 

Plug with Cadmium Finish after 96 

hours 

Figure 34: Test Unit 3 Backshell and 

Receptacle with Zinc Nickel Finish after 

96 hours 

Figure 35: Test Unit 3 Backshell and 

Plug with Zinc Nickel Finish after 96 

hours 
54 

Figures 32 through 35 show test units 1 and 3 compared earlier. Figure 34 and 35 

show the zinc nickel finish backshell and the connector both produced additional white 

residue when compared to 48 hours in figures 29 and 31. This was the same for the rest 

of the test units that had zinc nickel backshells or connectors. The cadmium finish on the 

other hand began to show a little corrosion near the strain relief clamp, however 

continuity measurements were still below the maximum recommended voltage. See 

appendix for figures of all the test units after 96 hours. No apparent effects had been 

seen at this point of the test at the interface between the zinc nickel finish on connectors 

and the mounting hardware. Readings from test unit 3 mating a connector with zinc 

nickel finish with another connector with zinc nickel finish show similar values to 

connectors with zinc nickel finish mated with connectors with the other finishes in test 

units 2, 4, 5, 6, 7, 8 & 9. 

After 500 hours of exposing the test fixture to salt fog, significant changes in the 

test units were noticed from a visual inspection. The buildup of oxide layers still existed 

on most of the connectors and backshells with zinc nickel finish. Through visual 

inspections, corrosion was seen on test units with aluminum shell connectors and 

backshells with cadmium, electroless nickel finish, and the stainless steel connector with 

passivate finish. There was presence of heavy corrosion on the finish and the substrate of 

the strain relief clamp on backshells with cadmium and zinc nickel finish. This heavy 

corrosion was however not seen on any other backshells and connectors in the study. 

This can be attributed to the possibility of inadequate coating of the internal threads in 

the clamps of the strain relief. Another observation that brings us to this conclusion is 

the fact that cadmium finish, which is known for its high performance in this 

environment, showed heavy corrosion on the clamps of the strain relief. This heavy 

corrosion on the strain relief clamps was seen on test units 1 and 3 shown in figures 36 

through 39. The backshell on the receptacle on test unit 8 did show much more corrosion 

of the finish when compared to the other test units (See appendix 7.8). This happened 

only on this test unit. Based on visual inspection, one cannot determine how much 

degradation had occurred on the substrate, if any, in test unit 8. 
55 

 

Figure 36: Test Unit 1 Backshell and 

Receptacle with Zinc Nickel Finish after 

500 hours 

Figure 37: Test Unit 1 Backshell and 

Plug with Zinc Nickel Finish after 500 

hours 

Figure 38: Test Unit 3 Backshell and 

Receptacle with Zinc Nickel Finish after 

500 hours 

Figure 39: Test Unit 3 Backshell and 

Plug with Zinc Nickel Finish after 500 

hours 
56 

 Overall, continuity measurements, coupling torque measurements and visual 

inspections indicate that the requirement of protecting the substrate from corrosion while 

keeping the electrical grounding path intact has been demonstrated by zinc nickel 

finishes after 500 hours exposure to salt fog. The measurements taken to examine the 

interaction of zinc nickel finishes with the different materials and finishes that exist 

today are not significantly different from the measurements taken between zinc nickel 

finishes and zinc nickel finishes except for the coupling measurements on the stainless 

steel connector and the unexplained higher torque value achieved on test unit 7. 

4.3.1 Recommendations for Additional Testing 

Below are some observed points and recommendations for continued testing that 

were not performed in this study. 

• Additional testing can be performed on these test units to collect data at failure 

point. This will help determine the limits to which zinc nickel can provide 

corrosion protection before the substrate component begins to corrode 

• Test unit 6 was simulated for connectors on engine firewalls where high 

temperature might be of a concern. This study was done at a temperature of 35 ± 

2oC. The effect of high temperature is not seen on this test unit. Further testing at 

higher temperatures is recommended for this test unit. 
57 

5. Conclusion 

In this report, results of a study undertaken to investigate aerospace circular 

connectors and connector accessories, their means of corrosion, their electromagnetic 

interference and shielding integrity behavior are presented. As background and for 

completeness, a description of existing finishes and proposed alternate finishes to 

cadmium is included. Since zinc nickel finishes have long attracted interest in the 

industry, they were selected for this study. A summary review of the electrodeposition of 

zinc nickel is included. The rapid formation of oxide films was anticipated on the zinc 

nickel finish and this is precisely what was observed during testing. 

Testing in this study included a salt spray test, an electrical continuity test, a 

coupling test and the careful visual inspection of the test units. The electrical continuity 

measurements taken at the various time intervals confirmed the presence of oxide films 

on the zinc nickel backshells and connectors in this study as voltage across the 

measurement points increased from the first set of readings with respect to the value 

observed in the untested specimens. At 500 hours, which is the pass or fail point for this 

study, heavy corrosion was noticed on some of the test units regardless of the finishes on 

the connectors. Continuity measurements at the 500 hour interval ranged from values 

lower than the recommended <5.0 millivolts to as high as 29.7 millivolts. Because of 

values as low as 1.2 millivolts recorded, a conclusion can be made that the grounding 

path between the shield braid and the backshell, the backshell and the plug, the plug and 

the receptacle, and the receptacle and the mounting bracket had not been compromised 

in all cases. The coupling torque values measured after the 500 hours showed a delta 

from the initial values that were significantly higher and in some test units above the 

recommended range, one of the test units did not unmate. The upside to this is that 

values outside of the recommended range were also recorded for the baseline of the test. 

From this study, no significant differences between readings collected at the interface 

between zinc nickel finish and itself and the interface between zinc nickel finish and 

other finishes were determined. 

Based on this, one can conclude that although the zinc nickel finish on aluminum 

connectors and backshells did not perform as well as the cadmium finish in this study by 
58 

not meeting all the benchmarks, it achieved the industry requirement for finishes on 

circular connectors and can be used in combination with the different materials and 

finishes that exist today. 
59 

6. References 

[1] Bert Bergsrud. Glenair Inc. “Replacing Cadmium surface plating in electrical systems: the 

research and development of Alternate Finishes” Aerospace Electrical Interconnect System 

Symposium (AEISS) 

[2] Mason, Neidbalson & Melissa Klingenberg. “Update on Alternatives for Cadmium Coatings on 

Military Electrical Connectors” MetalFinishing. March 2010, p12-20 

[3] Toshiaki Murai. “Performance Characteristics of Zinc-Nickel Alloys and Dip-Spin Coatings” 

MetalFinishing. April 2009, p41-46 

[4] Bowden, A. Matthews. “A study of the corrosion properties of PVD Zn-Ni Coatings” Surface 

and Coatings Technology. 1995, 508-515 

[5] Girciene, Gudaviciute, Juskenas, R. Ramanauskas. “Corrosion resistance of phosphate Zn-Ni 

alloy electrodeposits” Surface and Coatings Technology. 2009, p3072-3077 

[6] Ganesan, Kumaraguru, N Popov “Development of compositionally modulated multilayer ZnNi 

deposties as replacement for cadmium” Surface and Coatings Technology. 2007, p7896-7904 

[7] Gavrila, Millet, Mazille, Marchhandise, J.M. Cuntz “ Corrosion behavior of zinc-nickel coatings, 

electrodeposited on steel” Surface and Coatings Technology. 2000, p164-172 

[8] ASTM Standard B841, 2010, "Standard Specification for Electrodeposited Coatings of Zinc 

Nickel Alloy Deposits" ASTM International, West Conshohocken, PA, 2010 

[9] Ascott Analytical Equipment Ltd "Salt Spray testing cabinets and chambers” 

http://www.ascott-analytical.com/howtheywork2/SSCsaltsprayillo2.htm

[10] Glenair, Inc " Series 38 - EMI-RFI Non-Environmental Backshells with Strain Relief - Glenair, 

Inc.” http://www.glenair.com/backshells/circular/series_38.htm

[11] EIA/ECA-364-26B, "Salt Spray Test Procedure for Electrical Connectors, Contacts and Sockets" 

Electronic industries Alliance, , 2006 

 [12] Official Journal of the European Union, Directive 2002/95/EC of the European Parliament and 

of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in 

electrical and electronic equipment. 

[13] Glenair Inc. “Wearable Solder electronics” QwikConnect. April 2005 Volume 10, number 1. 

[14] MIL-DTL-38999L, 2009, "General Specification for Electrical Circular” Defense Supply Center 

Columbus OH, 2009. 

[15] G. D. Wilcox and D. R. Gabe. “Electrodeposited Zinc Alloy Coatings” Corrosion Science 1993, 

Vol.35, Nos 5-8, p1251-1258 

[16] Journal of Phase Equilibria. Supplemental literature: Ni-Zn (Nickel-Zinc). Vol 24, Number 3, 

p280. 

[17] D. E. Hall “Electrodeposited Zinc Nickel Alloy Coatings – A Review” Plating and Surface 

Finishing. 1983, p59-65 
60 

7. Appendix 

7.1 Test Unit 1 

Test Unit 1 Backshell and Receptacle 

with Cadmium Finish before exposure 

Test Unit 1 Backshell and Receptacle 

with Cadmium Finish after 48 hours 

Test Unit 1 Backshell and Plug with 

Cadmium Finish before exposure 

Test Unit 1 Backshell and Plug with 

Cadmium Finish after 48 hours 
61 

Test Unit 1 Backshell and Receptacle 

with Cadmium Finish after 96 hours 

Test Unit 1 Backshell and Receptacle 

with Cadmium Finish after 500 hours 

Test Unit 1 Backshell and Plug with 

Cadmium Finish after 96 hours 

Test Unit 1 Backshell and Plug with 

Cadmium Finish after 500 hours 
62 

7.2 Test Unit 2 

Test Unit 2 Backshell and Receptacle 

with Zinc Nickel Finish before exposure 

Test Unit 2 Backshell and Receptacle 

with Zinc Nickel Finish after 48 hours 

Test Unit 2 Backshell and Plug with 

Cadmium Finish before exposure 

Test Unit 2 Backshell and Plug with 

Cadmium Finish after 48 hours 
63 

Test Unit 2 Backshell and Receptacle 

with Zinc Nickel Finish after 96 hours 

Test Unit 2 Backshell and Receptacle 

with Zinc Nickel Finish after 500 hours 

Test Unit 2 Backshell and Plug with 

Cadmium Finish after 96 hours 

Test Unit 2 Backshell and Plug with 

Cadmium Finish after 500 hours 
64 

7.3 Test Unit 3 

Test Unit 3 Backshell and Receptacle 

with Zinc Nickel Finish before exposure 

 

Test Unit 3 Backshell and Receptacle 

with Zinc Nickel Finish after 48 hours 

Test Unit 3 Backshell and Plug with 

Zinc Nickel Finish before exposure 

Test Unit 3 Backshell and Plug with 

Zinc Nickel Finish after 48 hours 
65 

Test Unit 3 Backshell and Receptacle 

with Zinc Nickel Finish after 96 hours 

Test Unit 3 Backshell and Receptacle 

with Zinc Nickel Finish after 500 hours 

Test Unit 3 Backshell and Plug with 

Zinc Nickel Finish after 96 hours 

Test Unit 3 Backshell and Plug with 

Zinc Nickel Finish after 500 hours 
66 

7.4 Test Unit 4 

Test Unit 4 Receptacle with Electroless 

Nickel Finish before exposure 

 

Test Unit 3 Receptacle with Electroless 

Nickel Finish after 48 hours 

Test Unit 4 Backshell and Plug with 

Zinc Nickel Finish before exposure 

Test Unit 4 Backshell and Plug with 

Zinc Nickel Finish after 48 hours 
67 

Test Unit 4 Receptacle with Electroless 

Nickel Finish after hours 

Test Unit 4 Backshell and Plug with 

Zinc Nickel Finish after 500 hours

Test Unit 4 Backshell and Plug with 

Zinc Nickel Finish after 96 hours 

Test Unit 4 Receptacle with Electroless 

Nickel Finish after 500 hours 
68 

7.5 Test Unit 5 

Test Unit 5 Receptacle with Zinc Nickel 

Finish before exposure 

 

Test Unit 5 Receptacle with Zinc Nickel 

Finish after 48 hours 

Test Unit 5 Zinc Nickel Finish 

Backshell with Cadmium finish Plug 

before exposure 

Test Unit 5 Zinc Nickel Finish 

Backshell with Cadmium finish Plug 

after 48 hours 
69 

Test Unit 5 Receptacle with Zinc Nickel 

Finish after 96 hours 

Test Unit 5 Receptacle with Zinc Nickel 

Finish after 500 hours 

Test Unit 5 Zinc Nickel Finish 

Backshell with Cadmium finish Plug 

after 96 hours 

Test Unit 5 Zinc Nickel Finish 

Backshell with Cadmium finish Plug 

after 500 hours
70 

7.6 Test Unit 6 

\ 

Test Unit 6 Zinc Nickel Backshell and 

plug mated with stainless steel 

receptacle before exposure 

 

Test Unit 6 Zinc Nickel Backshell and 

plug mated with stainless steel 

receptacle after 48 hours 

Test Unit 6 Zinc Nickel Backshell and 

plug mated with stainless steel 

receptacle after 96 hours 

Test Unit 6 Zinc Nickel Backshell and 

plug mated with stainless steel 

receptacle after 500 hours 
71 

7.7 Test Unit 7 

Test Unit 7 Zinc Nickel Backshell and 

Receptacle before exposure 

 

Test Unit 7 Zinc Nickel Backshell and 

Receptacle after 48 hours 

Test Unit 7 Composite Cadmium finish 

Backshell and Plug before exposure 

Test Unit 7 Composite Cadmium finish 

Backshell and Plug after 48 hours 
72 

Test Unit 7 Zinc Nickel Backshell and 

Receptacle after 96 hours 

Test Unit 7 Zinc Nickel Backshell and 

Receptacle after 500 hours 

Test Unit 7 Composite Cadmium finish 

Backshell and Plug after 96 hours 

Test Unit 7 Composite Cadmium finish 

Backshell and Plug after 500 hours
73 

7.8 Test Unit 8 

Test Unit 8 Zinc Nickel Backshell and 

Receptacle before exposure 

 

Test Unit 8 Zinc Nickel Backshell and 

Receptacle after 48 hours 

Test Unit 8 Composite Electroless 

Nickel finish Backshell and Plug before 

exposure 

Test Unit 8 Composite Electroless 

Nickel finish Backshell and Plug after 

48 hours 
74 

Test Unit 8 Zinc Nickel Backshell and 

Receptacle after 96 hours 

Test Unit 8 Zinc Nickel Backshell and 

Receptacle after 500 hours 

Test Unit 8 Composite Electroless 

Nickel finish Backshell and Plug after 

96 hours 

Test Unit 8 Composite Electroless 

Nickel finish Backshell and Plug after 

500 hours
75 

7.9 Test Unit 9 

Test Unit 9 Composite Electroless 

Nickel finish backshell on Zinc Nickel 

finish Receptacle before exposure 

 

Test Unit 9 Composite Electroless 

Nickel finish backshell on Zinc Nickel 

finish Receptacle after 48 hours 

Test Unit 9 Zinc Nickel finish Backshell 

on Cadmium finish Plug before 

exposure 

Test Unit 9 Zinc Nickel finish Backshell 

on Cadmium finish Plug after 48 hours 
76 

Test Unit 9 Composite Electroless 

Nickel finish backshell on Zinc Nickel 

finish Receptacle after 96 hours 

Test Unit 9 Composite Electroless 

Nickel finish backshell on Zinc Nickel 

finish Receptacle after 500 hours 

Test Unit 9 Zinc Nickel finish Backshell 

on Cadmium finish Plug after 96 hours 

Test Unit 9 Zinc Nickel finish Backshell 

on Cadmium finish Plug after 500 hours
77 

7.10 Mounting Bracket Detailed Design 

Crossed hatched areas were free of primer in order to allow electrical conductivity between the receptacle