## A Study of Zinc-Nickel as an Alternate Coating to C admium

for Electrical Connector Shells Used in Aerospace A pplications

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, 2011ii © Copyright 2011 by Odunayo Ogundiran All Rights Reservediii 

## 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 Col or ...................... 7
2.1.2 Class F & G finish: Electroless Nickel ............ ...................................... 7
2.1.3 Class W finish: Cadmium over nickel. Olive Drab Col or ..................... 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 ................................

................................................... .......... 12iv 2.5.1 Electrodeposition of zinc nickel alloy Coating Depo sits on Circular Connectors ........................................

................................................... 16
2.5.2 Properties of Cadmium and zinc nickel of Interest i n this Application 17

3. Experimental ......................................

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

................................................... ....... 19
3.1.1 Test Unit Set-up .................................. ................................................. 2 0
3.1.2 Mounting bracket .................................. ............................................... 31
3.2 Shield Braid Termination. .........................

................................................... .... 32
3.3 Salt Spray Test ...................................

................................................... ........... 33
3.3.1 Test Apparatus .................................... ................................................. 3 3
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 ................... ................................................. 4 3
4.2.1 Electrical Continuity Readings at 48 Hours ........ ................................. 43
4.2.2 Electrical Continuity Readings at 96 Hours ........ ................................. 45v 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 .................. ................................................ 77vi 

## LIST OF KEYWORDS

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

connection between harnesses or a harness and avion ics 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 prote ction 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 f rom being pulled out of the connectors Finish A protective layer of coating meant for corr osion 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 finis h to cadmium for circular

connectors Cadmium finish Widely used finish on electrical con nectors today. Plug Type of connector that is free moving and mate s with a receptacle. Harness An assembly of a group of wires that transm its 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 f inish CdC/Zn - Composite Cadmium finish on Zinc Nickel fi nish EnC/Zn - Composite Electroless Nickel finish on Zin c Nickel finish SS/Zn - Stainless steel passivate finish on Zinc N ickel 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 - Compositeviii 

## LIST OF TABLES
Table 1: Summary of Properties of Cadmium, Zinc Ni ckel 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 Conditi ons. ............................................. 37 Table 15: Measurement Points/Surfaces on test units

.................................................. ... 37 Table 16: Maximum and Minimum torques for Shell Siz e 19 Connectors. .................... 38 Table 17: Initial Mate Torque Values ..............

................................................... ............. 41 Table 18: Initial Electrical Continuity Reading (Mi llivolts) ......................................... .. 42 Table 19: 48 Hour Electrical Continuity Reading (Mi llivolts) ........................................ 44 Table 20: 96 Hour Electrical Continuity Reading (Mi llivolts) ........................................ 46 Table 21: 500 Hour Electrical Continuity Reading (M illivolts) ...................................... 4 9 Table 22: Unmating Torque Values after 500 Hours ..

................................................... . 50ix 

## LIST OF FIGURES
Figure 1: View of a Mated Plug and Receptacle Assem bled 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 Pl ug ........................................... 12 Figure 5: Average Corrosion Rates on Zinc Nickel Al loy 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 an d 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 Ap paratus [9] ................................. 34 Figure 21: Salt Solution Reservoir showing position ing of filter [11] ............................ 35 Figure 22: Measurement Points on test units .......

................................................... ......... 37 Figure 23: Test Sequence ..........................

................................................... .................... 40 Figure 24: 48 Hour Maximum Electrical Continuity Re adings Per Interface on Test
Units .............................................

................................................... ................................. 43x

Figure 25: 96 Hour Maximum Electrical Continuity Re adings Per Interface on Test
Units .............................................

................................................... ................................. 45 Figure 26: Minumum and Maximum Electrical Continuit y 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 Finis h on Receptacle after 48 hours .. 51 Figure 29: Test Unit 3 Backshell with Zinc Nickel F inish on Receptacle after 48 hours 51 Figure 30: Test Unit 1 Backshell and Plug with Cadm ium 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 wit h Cadmium Finish after 96 hours. 53 Figure 33: Test Unit 1 Backshell and Plug with Cadm ium Finish after 96 hours ........... 53 Figure 34: Test Unit 3 Backshell and Receptacle wit h 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 wit h 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 wit h Zinc Nickel Finish after 500 hours

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

## ACKNOWLEDGMENT
I would like to acknowledge the harness engineerin g 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 in vestigate the ability of zinc nickel coatings to provide adequate corrosion protection a nd 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 protec tion for circular connectors. Cadmium is the most widely used finish in this appl ication. 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 cir cular connectors; however there are no studies of how zinc nickel finishes couple with the materials that are used today on aerospace circular connectors, connector accessorie s, and mounting hardware. The need
to know more about the service behavior of zinc nic kel 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. Init ial inspections after exposure to salt fog showed rapid formation of oxide films on the surfac e of aluminum and stainless steel
connectors and backshells coated with the zinc nick el finish. Electrical continuity measurements taken at predetermined intervals confi rmed the presence of oxide films as voltage across the measurement points increased dra stically 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 rec ommended <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 to rques required to unmate the test
units after the 500 hours increased from the initia l values but were below the baseline value in most cases. No significant difference in c ontinuity readings collected wasxiii

determined between connectors with zinc nickel fini sh 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 p erform as well as the cadmium finish in this study, it met the industry requirement for fin ishes 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 h arnesses typically used in aerospace applications for terminating wires that t ransmit data or distribute current. They can also be found attached to a piece of equip ment. Circular connectors are usually
found at the ends of a harness where the wires term inate. Circular connectors can be one of two kinds; a plug which is attached to the harne ss alone and is free moving; or a receptacle which is not only attached to the harnes s, 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 Assem bled with Connector
Accessories.

Receptacles are also attached to avionic equipment as they are the only type of connectors used on this equipment. These two differ ent types of connectors are designed
to mate with one another. Circular connectors consi st of a shell which could be of different materials e.g. aluminum, composite, stain less steel, or cold rolled steel. The shell provides protection against abrasion and cont act with contaminants that could2

damage the mating contacts. They also provide shiel ding for the conductors in the harness against unwanted electromagnetic and radio frequency interference. Another component of the connector is an insulator placed i nside the shell. It contains inserts for
contacts and a seal for wires. The contacts serve a s terminations in the connector for the
wires in the harness. Due to the environments in wh ich aircraft operate, circular connectors are specifically designed to withstand h igh shock and vibration forces. Figure 2 shows cutaway views of the two types of connector s 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. On e can also see from the figure that the contacts in the plug are different from the contact s 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 aircr aft operates. Specifically, the effect of long term exposure to salt atmosphere environmen ts is a major concern. The mating
of connectors of different materials and finishes a lso poses a concern due to the3

electrochemical action of two dissimilar metals in contact with each other. This can cause galvanic corrosion given the presence of a co nductive path and an electrolyte. This is a primary problem for electrical connectors in t he environment in which the aircraft operates because moisture could serve as a medium f or current flow. In connectors, galvanic compatibility is required b etween 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 inter ference 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 elect rical conductivity over long term exposure to salt atmosphere environments.



### 1.3 Electromagnetic Interference Shielding
Electromagnetic Interference (EMI) can result in th e 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 t hat 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 station s that are not necessarily associated with the aircraft but are part of the environment i n which the aircraft operates. Because
the conductors in the cables can act as antennas an d pick up radiated signals, the reception of the interference can lead to inaccurat e or distorted data being transmitted. The examples stated are usually unintentional sourc es 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 ba ckshells helps prevent the interruption of communication in an aircraft system by electromagnetic interference (EMI). It allows for the harmless passage of the in terference from a harness shield braid to a grounding point through the backshell and conn ector. A backshell is a connector accessory that is used to shield the wire connectio n points when the harness is attached to the connector. The harness is shielded by ground ing a conductive cable over the4

harness to the backshell and connector. To shield a gainst electromagnetic interference, the harness is enclosed in a tightly woven metal br aid often referred to as the shield
braid. This braid is terminated at the backshell an d at the end of the cable. This serves as a path for the unwanted EMI from the harness to air craft electrical ground. Figure 3 shows a breakaway assembly of a connector and its a ccessories 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 expe rimental section of this study. The strain relief that prevents the wires from being pu lled out of the connector is mounted to
the rear of the backshell and also secures the back shell termination rings. The termination rings are used to secure the shield bra id to the backshell in order to provide a grounding path. The backshell is mounted on the rea r 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 cond uctive.

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



### 1.4 Project Background
The search for a suitable replacement for cadmium a s a protective finish for the circular connectors used in aircraft systems has be en going on for quite some time now. Cadmium is a generally preferred finish in this app lication 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 replacemen t for cadmium. Zinc nickel has been identified as an effective finish in protecting cir cular connectors from the environment [14]. It has been added to military specification M IL-DTL-38999 as one of the available finishes on connectors used in the aerospace indust ry [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 behavio r of zinc nickel finishes on circular
connectors if introduced into the field. This is du e to the lack of extensive testing of the effects of combining different materials and existi ng finish combinations of backshells, connectors, shield braids, and mounting hardware. T he requirement for prevention of connectors from long term corrosion degradation in this application when the finish is combined with different materials and existing fini shes demonstrates the need to perform a corrosion performance study. Maintaining the shell to shell continuity in order to keep the integrity of electromagnetic wiring shi elding is also required.



### 1.5 Overview of Project Goals
The purpose of this thesis is to provide insight in to the ability of the zinc nickel finish on the shell of circular connectors to provi de adequate protection from corrosion
when combined with the different materials and fini shes used today in aerospace applications. In addition to this, the ability of z inc nickel finish to maintain the required electrical conductivity and electromagnetic interfe rence shielding integrity when6

combined with different materials and finishes exis ting 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 resu lts from this study would reveal the electrical and physical properties, and corrosion r esistance level of a connector shell with zinc nickel finish after it has undergone a sa lt fog test for an extended period of
time when mated with connectors with the same and o ther finishes that exist today. The
results will be used to conclude whether or not zin c nickel finish is an acceptable replacement for cadmium finishes in this applicatio n.7 

## 2. Coatings for Circular Connectors

2.1 Existing finishes and their applications
According to MIL-DTL-38999, there are 22 finishes e xisting today for MILDTL-38999 circular connectors applied over the diff erent 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 conduct ivity for shells on circular connectors. Without the finish, the bare metal would come in co ntact with the environment and rapid degradation would occur. Once this happens, reducti on in conductive properties and strength of the shell begin which may result in ele ctromagnetic interference in the wiring interconnect system. Described below are some of t he finishes that exist today as listed in MIL-DTL-38999 [14].

2.1.1 Class A finish: Cadmium over nickel. Light Gold Col or This finish has a light gold color and is only avai lable 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 th e 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 h ours 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 solut ion to the connector shell using an autocatalytic process operating at elevate d temperatures. The autocatalytic process is a process where the product of the react ion 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 connect or shells. The final finish is electrically conductive and also provides corrosion resistance for the shell. The same8

finish is available for composite shell material, h owever it is identified as a class M finish. Class M finish provides electrical conducti vity only as corrosion is not a concern in composite materials. This finish is currently ra ted for up to 48 hours in a salt fog test for aluminum shells and 2000 hours for composite sh ells and is in use on some avionic equipment.

2.1.3 Class W finish: Cadmium over nickel. Olive Drab Col or
This finish has an olive drab color and is availabl e on connector shells made with aluminum. The same finish is available for composit e shell material, however it is identified as a class J finish. An electrodepositin g process is used to plate this finish on the connector shell. A nickel plate is applied on t he shell of the connector followed by a cadmium plate. The final finish is electrically con ductive 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 curr ently 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 an d backshells. This is because it provides ultimate corrosion protection. It continue s to provide corrosion protection when the finish is scratched. It also has good electrica l conductivity properties and low contact
resistance. A combination of connectors and backshe lls 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 w ith stainless steel. Connectors with this finish are used in harnesses exposed to h igh 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 w ith stainless steel. Similar to class K finishes, connectors with this finish are f ound in areas where high temperature is a concern. The electrodeposited nickel finish helps seal up contaminations that can become potential corrosion sites. An electrodeposit ing process is used to plate this finish on the connector shell. It is currently rated for u p 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 avail able 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-DT L-38999 as a replacement for
cadmium.

2.2.2 Class T finish: Nickel fluorocarbon polymer. This finish has a gun-metal gray non-reflective col or and is available on connector shells made with aluminum. An electrodepo siting process is used to plate this finish on the connector shell. A nickel plate is ap plied on the shell of the connector followed by a composite coating of electroless nick el phosphorus and polytetrafluoroethylene (PTFE). The final finish is electric ally 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 fin ishes 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 o ther colors depending on the conversion process used and is available only on co nnector shells made with aluminum. An electrodepositing process is used to plate this finish on the connector shell. The final10

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 repl acement for cadmium [14]. It is
also seen as a cost effective alternative to cadmiu m. This finish is further discussed later in this study.



### 2.3 How Circular Connectors Corrode
Corrosion is an electrochemical process that involv es the degradation of a metal due to electrochemical reactions it undergoes withi n its environment. The process of corrosion takes place at the most basic molecular l evel which is the atom and in most cases driven by electricity. Corrosion does not onl y occur in the presence of a liquid, it can also occur as a result of the presence of hot g asses. 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 proc essed from raw material or ores. [1]
Problems associated with corrosion on circular conn ectors 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 th em. 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 aeros pace applications, the source of ions is the atmosphere. The conductor of the electrical cur rent 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 ma terial finish for corrosion protection on electrical systems in aerospace appli cations. 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 prop erties. In corrosive environments, cadmium dissolves thereby leaving the substrate met al intact. In cases where the finish is broken, the surrounding finish still protects the s ubstrate. It also serves as a lubricious coating to reduce wear and thread galling when mate d with other connectors because of its softness and durability. It is also a very good electrical conductor thereby providing electromagnetic compatibility and resistance to ele ctromagnetic interference. Cadmium currently has an advantage over other finishes in t erms of cost because of its current high production use and plating method available fr om numerous sources. It’s ease of manufacturing and repair make this possible. Cadmiu m is solderable which is very important in this application. Cadmium also provide s 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 t o .00030 inches making it suitable for threaded components and components in which close t olerance 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 shel ls. However, Cadmium salts are toxic. Because of its sa lts, there is an increased risk of failure of steel components due to hydrogen embr ittlement, 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 hazard s when the fine dusts or fumes are inhaled or soluble compounds of the metal are inges ted. Hence there is a powerful motivation to look for alternative coatings for thi s application. Figure 4 shows a plug and receptacle with cadmium coating.12 Receptacle Plug Figure 4: Cadmium Plated Aluminum Receptacle and Pl ug



### 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 prot ected by the deposition of zinc and zinc alloys in order to test their perform ance in corrosive environments. The study revealed that electroplated zinc alloy deposi ts are superior and achieve a consistent and stable corrosion resistance performance over th e substrate alloy [7]. Zinc electrodeposits are widely used for corrosion protection in place of cadmium due to cadmium’s highly toxic nature and th e environmental hazard it represents. Similar to cadmium, zinc deposits are e ffective 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 req uirement 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 ca n be achieved by alloying it with some more noble metals such as tin, cobalt, and nic kel. Zinc coatings alloyed with nickel have generated th e highest interest in the aerospace industry because they meet most of the be nchmarks set by cadmium. Zinc coatings also have good corrosion resistance proper ties, superior formability and improved welding characteristics [6]. It has been s tated 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 s een 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 nick el coatings during corrosion [5]. Previous studies indicate that zinc nickel alloys c ontaining 10-15wt% nickel possess the highest corrosion resistance of the pos sible alloy composition however, they have more negative potential because of the high zi nc content thereby forming oxides instantaneously in corrosive environments [6]. Nick el 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 continuit y 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 o n 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 t o the substrate material. This is most likely due to the overall surface enrichment of nic kel by dezincification [15].14 Figure 5: Average Corrosion Rates on Zinc Nickel Al loy Deposits by Nickel Content [15].

The corrosion products of zinc nickel coatings grea tly affect the corrosion behavior of the substrate. The initial stage of cor rosion of zinc nickel coatings results in instant growth of an oxide film [5]. In the presenc e of chloride or sulphate ions, zinc hydrochlorides or hydrosulphates with water molecul es could be formed [5]. In a previous study under accelerated corrosion conditio ns, zinc hydroxychloride
Zn 5 (OH) 8 Cl 2 H 2 O was the main corrosion byproduct formed on the zi nc nickel surface [5]. Smaller quantities of sodium hydroxysulphate N aZn 4 (SO 4 ) Cl(OH) 6 H 2 O were also identified in the byproducts [5]. The composition of zinc nickel on circular connecto rs consists of 5-15wt% nickel and the balance is zinc in an aqueous soluti on that can be deposited from both acid and alkaline processes. The alkaline process i s 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 nick el 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 thick ness of the coating and selection of an appropriate conversion coating will improve resu lts [2]. Thicker zinc nickel coatings15

provide enhanced corrosion resistance. The form and fit of the component might be
affected but dimensions can be modified. Because th e corrosion performance of zinc nickel plating film is a function of the nickel con tent, 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 n ickel 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 c onnectors because of rough handling during harness assembly and installation. Nickel co ntents greater than 17% have a mixture of γ and α-phase [15]. Figure 6 is the phas e diagram for zinc nickel alloys. Evaluation in a previous study of zinc nickel on st eel sheets concluded that no deterioration in yield strength, tensile strength o r elongation properties of the substrate material was observed [17].

Figure 6: Zinc Nickel Phase Diagram [16]

Figure 7 shows the microstructure of zinc nickel co atings 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 presen ting 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 pre sence of salts which is a very stable byproduct in corrosive environments. The bas ic 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 nick el 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 Depo sits on Circular Connectors There is one class of zinc nickel coating identifie d by ASTM B841. It consists of 5-12wt% of nickel with the balance of zinc. There a re 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 circ ular connectors. The chromate17

conversion coating helps inhibit dezincification of the alloy. Manufacturers are allowed to select which conversion coatings to use. The coa ting is produced from an aqueous electroplating system. This aqueous system can eith er be alkaline or acidic as stated previously, but alkaline is preferred. As part of t he 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 electrop lating 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 disc ontinuities, and also free of cracks. The coating allows for normal handling storage conditio ns to occur without chipping, flaking or other coating damage [8]. Figure 8 shows zinc ni ckel finish with black conversion coating applied over an aluminum MIL-DTL-38999 plug and receptacle.

Receptacle Plug Figure 8: Zinc Nickel Plated Aluminum Receptacle an d Plug

2.5.2 Properties of Cadmium and zinc nickel of Interest i n this Application Table 1 shows a comparison between cadmium coatings , electrodeposited zinc nickel coatings, and pure zinc coatings based on pe rformance 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 Poo r 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 Ni ckel 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 provi de protection from corrosion and maintain the required electrical conductivity and e lectromagnetic interference shielding integrity, when combined with connectors and connec tor accessories with the same and other finishes that exist today. In addition to the se combinations, zinc nickel’s compatibility with the tin plated shield braid, whi ch is a key component of the ground path from the wires to the bulkhead, will be examin ed. 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 s pecifications. 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 periodi c intervals during the test, between 0 and 500 hours, will be used to determine the abilit y 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 f inish might come in contact with.
Test unit 10 has a nickel Teflon backshell on an el ectroless nickel connector mated with a nickel Teflon backshell on a cadmium connector, a ll with aluminum shells. The test unit combinations were arrived at by identifying th e materials and finishes currently used in today’s aircraft on connectors, connector a ccessories, shield braids, screws, washers and mounting flanges. Table 2 shows the mat erials 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 to day 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 uni ts simulate different potential conditions that arise in practice.

3.1.1 Test Unit Set-up Described are the test units simulating the differe nt combination of materials and
finishes if zinc nickel finish is introduced into t he 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.21Table 3: Test Unit Finish and Material Matrix (Al –Aluminum. C – Composite)Test Unit Backshell Plug Receptacle Backshell1 Cadmium (Al) Cadmium (Al) Cadmium (Al) Cadmium (Al)2 Cadmium (Al) Cadmium (Al) Zinc Nickel (Al) Zinc Nickel (Al)3 Zinc Nickel (Al) Zinc Nickel (Al) Zinc Nickel (Al) Zinc Nickel (Al)4 Zinc Nickel (Al) Zinc Nickel (Al) Electroless Nickel (Al) N/A5 Zinc Nickel (Al) Cadmium (Al) Zinc Nickel (Al) N/A6 Zinc Nickel (Al) Zinc Nickel (Al) Passivate (SS)N/A7 Cadmium (C) Cadmium (C) Zinc Nickel (Al) Zinc Nickel (Al)8 Electroless Nickel (C) Electroless Nickel (C) Zinc Nickel (Al) Zinc Nickel (Al)9 Zinc Nickel (Al) Electroless Nickel (C) Zinc Nickel) (Al) Cadmium (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 finis h

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 ex ists on many aircraft where cadmium plated connectors and backshells are mated with other cadmium plated connectors. The degradation of cadmium finish and t he interface between the shield braid and the rings will be reviewed. The results g athered 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 finis h

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 connect or 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 corrosi on resistance of zinc nickel finish connectors when mated with cadmium plated connector s. In addition the degradation of
the zinc nickel finish and the interface between th e shield braid and backshell rings will be reviewed. The possibility of this test unit in a ctual 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 finis h

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 connect ors and backshells used have a
zinc nickel finish. This unit will establish the co rrosion 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 investi gated 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 f inish
Backshell N/A
Shield Braid N/A Mounting Hardware Screw Carbon Steel, Cadmium finis h

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 nick el finish connector with a zinc
nickel finish backshell is mated with an electroles s nickel finish connector without a
backshell. Since electroless nickel finish connecto rs 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 fi nishes 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 finis h

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 fin ish

Backshell N/A
Shield Braid N/A Mounting Hardware Screw Carbon Steel, Cadmium finis h

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 nick el finish connector is mated with a stainless steel connector with a passivate f inish. These types of connectors can be found in areas of high temperature and when high st rength 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 connecto rs.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 finis h

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 subst rate material for the cadmium finish is composite. Results from this test unit sh ould be similar to Test Unit 2 because of the arrangement of the finishes on the component s 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 finis h

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 subst rate material for the electroless nickel is composite. The electroless nickel connect or 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 t est 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 finis h

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 thi s study. The substrate of the electroless nickel and cadmium finishes is composit e. This test unit will be used to
examine the corrosion resistance of a zinc nickel b ackshell on an electroless nickel finish31

composite connector and cadmium finish composite ba ckshell 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 unit s set up. This allows for more data on
this interface to be collected because this interfa ce is of major concern in this study. This also applies to the interface between the zinc nick el 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 wa s fabricated from Al 7076 and is 0.063 inches thick. The bracket surface was chemically he at treated and primed before the test
units were assembled. Chemical surface treatment is the application of alodine on aluminum and aluminum alloys where maximum corrosio n protection and electrical conductivity is required. The surface area where th e receptacles interfaced with the mounting bracket were masked off prior to the appli cation 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 p ath from the wires to the grounding point. The shield braid, made of copper material an d plated with tin, is applied over the
harness. The shield braid is terminated at the back shell. 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. Terminati on of the shield braid is important so as not to cause a break in this path. There are dif ferent 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. Howev er, two are used in this study because the third ring is meant for termination of the indi vidual wire braids that don’t apply to this study. The shield braid is woven between the r ings 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 in tegrity of the conductive path. The
effects of corrosion at this interface will be exam ined.

Figure 19: Shield Braid Termination [10].33



### 3.3 Salt Spray Test
This test involves exposing test units to a fine mi st 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 avenu e to assess the effects of salt-laden atmosphere on electrical connectors, connector acce ssories, finishes, and components through electrical continuity readings and visual i nspections. The apparatus, salt composition and procedure of the salt spray test wi ll 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 compres sed air, atomizing nozzles, supports for test units, and temperature control provisions. Figure 20 shows a salt spray test apparatus consist ing 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 affec ted by the salt fog. The chamber is constructed in a way that allows free circulation o f the salt fog to the same degree on all
the test units. It also allows for no direct encroa ching of the salt fog on the test units or condensation dripping on the units. The salt chambe r also contains the supports for the
test units made of a material that cannot be affect ed by the salt solution. The temperature of the exposure zone of the salt spray chamber is m aintained at 35 ± 2oC. The atomizers provide a finely divided dense fog to the test unit s. The nozzles delivers the fog and are made of materials that cannot be affected by the sa lt 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 suitabl e and finely divided dense fog in conjunction with the atomizers. Humidity and temper ature of the compressed air is controlled in order to meet the operating condition s. In order to protect the atomizers and nozzles from clogging by salt deposition, the relat ive humidity of the air at the point of release from the nozzle is 95-98%. An air saturati on tower is used to meet the requirements for the pressure and humidity. Air pre ssure varies based on the temperature of the water used to control the humidity and salt fog. The salt solution reservoir34

contains the salt solution that will be discussed i n the next section of this report. It is a separate piece from the salt chamber in order to pr ovide ease of maintainability. The part
of the reservoir’s construction that comes in conta ct with the salt solution is of a material that cannot be affected by the salt solution. The t emperature of the chamber is maintained at 35 ± 2oC in order to keep the salt so lution 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 Ap paratus [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 c reating 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 f ree of solids by a filter that is located in a35

reservoir (figure 21) as stated earlier in the repo rt. The chemical content of the salt does not include more than 0.3% by mass of total impurit ies. The pH value of the salt solution
is maintained at a value between 6.5 and 7.2 when m easured at a temperature range
between 34oC and 36oC.

Figure 21: Salt Solution Reservoir showing position ing 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 ch amber from the top using plastic hooks with the test fixture vertical thereby making the t est units horizontal in order to simulate the conditions of the aircraft. The plastic hooks u sed are fabricated from a material that
is non-reactive to the salt solution. The test unit s 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 in tervals when the exposure to salt fog will be interrupted and inspection and data collect ion 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 furthe r in the following sections.



### 3.4 Electrical Continuity Test
This test involves taking electrical continuity mea surements between shield braids and backshells, backshells and connectors, m ated plugs and receptacles, and between receptacles and the mounting bracket. The t est 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 delive ring up to 1.0 ± 0.1 Amps was also used in this test. The test probes have spherical e nds 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 surf aces that are being checked. The test probes are placed on the shells in a way t hat doesn’t cause any damage to the coating of the connector. Voltage drop across the t wo surfaces is measured from any point on the surfaces. Measurements are conducted a nd 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 Conditi ons. As stated earlier, electrical continuity measuremen ts are conducted on the
surfaces of the test units to measure the electrica l 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 co mplete 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 units38 Per MIL-DTL-38999, the voltage drop across the test units should be a maximum of 2.5 millivolts with 100% increase after 500 hour s of salt fog exposure.



### 3.5 Coupling Torque Test
This test involves taking measurements of force req uired to mate and unmate plugs and receptacles. The test equipment and proce dure 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 con nectors are then fully mated at a rate of 25.4 millimeters/minute until peak torque is ach ieved. The reading is recorded at this point. The same applies when unmating the connector s as the peak torque used to
unmate the connectors is recorded. Table 16 shows t he maximum and minimum torques for mating and unmating shell size 19 connectors us ed in this study per MIL-DTL-38999 [14].

Maximum Engagement and Disengagement Minimum Diseng agement

28 Inch Pounds 3 Inch Pounds Table 16: Maximum and Minimum torques for Shell Siz e 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 th e finished surfaces indicate that the normal protective coating is removed. White rust in dicates corrosion in the finish and39

successful protection of the substrate to the corro sive conditions. Red rust could indicate that the substrate has been exposed to the corrosiv e environment and degradation has begun to take place. This inspection is conducted a t 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 hou rs 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 fin ishes. The connectors are then torqued to values below the maximum engagement valu e specified in table 16. Once this is completed, electrical continuity measurements ar e 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 minut es under continuously running water colder than 38°C in order to remove the salt deposi ts 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 ri nsed, it is placed in a circulating air oven with dry air cooler than 38°C for a maximum of 16 h ours. 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 hour s and 500 hours with the addition of the unmating torque measurements after electrical c ontinuity measurements have been
recorded at 500 hours.40Figure 23: Test Sequence41 

## 4. Results and Discussion
Readings from the electrical continuity test are co llected at 0, 48, 96 and 500 hours of the test fixture’s exposure to salt fog an d 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 sa lt fog on the test units compared to no exposure at all. Coupling torque measurements and e lectrical continuity measurements
were recorded. The values recorded are within the s pecified 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 va lues 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 reco rded after 500 hours of exposure. This maximum unmating force is however allowed to i ncrease 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 readin gs 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 m aximum voltage is however allowed to increase 100% after 500 hours of salt sp ray test [14].42Table 18: Initial Electrical Continuity Reading (Millivolts)Test Hour: 0 HoursP-SideJ-SideShield Braid toBackshellBackshell toPlugPlug toReceptacleShield Braid toBackshellBackshell toReceptacleMounting Plate toReceptacleTest Unit 1 Reading 1 1.6 1.6 1.6 1.6 1.6 1.5Reading 2 1.6 1.6 1.6 1.6 1.6 1.6Test Unit 2 Reading 1 1.6 1.6 1.6 1.6 1.6 1.6Reading 2 1.6 1.6 1.6 1.6 1.6 1.6Test Unit 3 Reading 1 1.6 1.6 1.6 1.6 1.6 1.6Reading 2 1.6 1.6 1.6 1.6 1.6 1.6Test Unit 4 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 1.6 1.6 1.6 N/A N/A 1.6Test Unit 5 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 1.6 1.6 1.6 N/A N/A 1.6Test Unit 6 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 1.6 1.6 1.6 N/A N/A 1.6Test Unit 7 Reading 1 N/A 1.6 1.6 1.6 1.6 1.6Reading 2 N/A 1.6 1.6 1.6 1.6 1.6Test Unit 8 Reading 1 N/A 1.6 1.6 1.6 1.6 1.6Reading 2 N/A 1.6 1.6 1.6 1.6 1.6Test Unit 9 Reading 1 1.6 1.6 1.6 N/A 1.6 1.6Reading 2 1.6 1.6 1.6 N/A 1.6 1.643



### 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 rea dings 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 rea dings. The highest readings were
recorded between the shield braid and the zinc nick el 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 c onnectors used in this study are rated for 500 hours in the salt fog test. Values le ss 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 figur e 24 for maximum continuity readings recorded per interface after 48 hours of s alt exposure. Figure 26 shows the minimum reading collected across per interface for all the test intervals.

Figure 24: 48 Hour Maximum Electrical Continuity Re adings Per Interface on Test
Units44Test Hour: 48 HoursP-SideJ-SideShield Braid toBackshellBackshell toPlugPlug toReceptacleShield Braid toBackshellBackshell toReceptacleMounting Plate toReceptacleTest Unit 1 Reading 1 1.6 1.6 1.6 1.6 1.6 1.5Reading 2 1.6 1.6 1.6 1.6 1.6 1.6Test Unit 2 Reading 1 1.6 1.6 1.6 1.6 1.6 1.6Reading 2 1.6 1.6 1.6 3.8 1.6 1.6Test Unit 3 Reading 1 1.6 1.6 1.6 1.6 1.6 1.6Reading 2 36.1 4.1 1.8 22.5 38.2 1.6Test Unit 4 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 12.0 5.8 1.8 N/A N/A 1.6Test Unit 5 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 14.2 10.5 1.8 N/A N/A 1.6Test Unit 6 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 12.5 1.6 1.7 N/A N/A 1.6Test Unit 7 Reading 1 N/A 1.6 1.6 1.6 1.6 1.6Reading 2 N/A 1.6 1.6 13.5 1.7 1.6Test Unit 8 Reading 1 N/A 1.6 1.6 1.6 1.6 1.6Reading 2 N/A 1.6 1.6 1.6 1.6 1.6Test Unit 9 Reading 1 1.6 1.6 1.6 N/A N/A 1.6Reading 2 11.4 6.8 1.9 N/A N/A 1.6Table 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 cont inuity 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 wi th zinc nickel finish on the connector
or backshell remained below the recommended values. The measurements recorded
between the shield braid and the zinc nickel backsh ell in the test units showed values higher the maximum 2.5 millivolts potential drop wh en measured on some parts of the surfaces. Figure 25 shows the maximum values record ed per interface in this test. The
values below the recommended maximum value were ach ieved when the probes were moved to other parts of the surface of the backshel l. A slight increase in values was recorded for continuity measurements between the re ceptacles and the mounting bracket and a value above the recommended maximum voltage i n 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 Re adings Per Interface on Test
Units46Test Hour: 96 HoursP-SideJ-SideShield Braid toBackshellBackshell toPlugPlug toReceptacleShield Braid toBackshellBackshell toReceptacleMounting Plate toReceptacleTest Unit 1 Reading 1 1.6 1.6 1.6 1.6 1.6 1.6Reading 2 1.6 1.6 1.6 1.6 1.6 1.6Test Unit 2 Reading 1 1.6 1.6 1.6 1.6 3.9 1.6Reading 2 1.6 1.6 1.6 9.7 8.3 1.6Test Unit 3 Reading 1 4.6 2.3 14.6 2.0 1.6 1.7Reading 2 6.5 3.0 7.3 3.4 1.8 1.7Test Unit 4 Reading 1 1.6 2.3 1.6 N/A N/A 1.6Reading 2 19.4 1.9 9.0 N/A N/A 1.6Test Unit 5 Reading 1 2.2 5.2 1.6 N/A N/A 1.6Reading 2 12.2 4.6 1.6 N/A N/A 1.7Test Unit 6 Reading 1 3.2 2.0 1.6 N/A N/A 1.6Reading 2 4.1 3.7 1.6 N/A N/A 1.6Test Unit 7 Reading 1 N/A 1.6 1.6 6.7 1.6 3.2Reading 2 N/A 1.6 1.6 1.7 2.9 1.6Test Unit 8 Reading 1 N/A 1.6 1.7 1.6 1.6 1.6Reading 2 N/A 1.6 1.6 2.7 1.6 1.6Test Unit 9 Reading 1 N/A 11.8 1.6 N/A 1.6 1.6Reading 2 N/A 1.7 1.6 N/A 1.6 1.6Table 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 th e test fixture to salt fog was at 500 hours. The electrical continuity measurements i n table 21 show an additional increase in overall values when compared to the ini tial, 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 milliv olts recommended maximum voltage on some parts of the surface. The range of values r ecorded between the shield braid and
the zinc nickel backshell were from 1.6 to 29.7 mil livolts depending on where on the test unit’s surface the voltmeter probes were placed. Co mpared to cadmium which recorded a range from 1.6 to 2.8 millivolts, this range is m uch wider. The readings between the
zinc nickel backshell and the cadmium connectors sh owed a similar range of values but with a maximum voltage of 21.3 millivolts. The read ings between the zinc nickel
backshell and the zinc nickel connectors showed a m aximum 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 receptac les. 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 continui ty measurement on the same interface with cadmium, electroless nickel, and stainless ste el finishes. Overall, the maximum values recorded were a lot hig her 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 poten tial 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 pat h 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 Continuit y Readings Per Interface
on Test Units49Test Hour: 500 HoursP-SideJ-SideShield Braid toBackshellBackshell toPlugPlug toReceptacleShield Braid toBackshellBackshell toReceptacleMounting Plate toReceptacleTest Unit 1 Reading 1 1.7 1.6 1.6 1.7 1.6 1.6Reading 2 2.8 6.9 1.6 1.6 1.6 1.6Test Unit 2 Reading 1 1.6 1.2 1.6 3.5 1.6 1.6Reading 2 1.7 1.7 1.7 11.0 7.2 1.8Test Unit 3 Reading 1 3.9 1.6 1.6 5.6 2.3 1.6Reading 2 9.6 1.6 21.2 9.6 7.3 5.2Test Unit 4 Reading 1 6.8 1.4 2.7 N/A N/A 1.6Reading 2 29.7 8.9 16.9 N/A N/A 1.7Test Unit 5 Reading 1 16.0 1.9 1.6 N/A N/A 1.6Reading 2 25.0 21.3 1.8 N/A N/A 2.1Test Unit 6 Reading 1 9.1 1.5 1.6 4.8 2.5 1.7Reading 2 14.6 3.9 9.8 11.2 6.1 5.3Test Unit 7 Reading 1 N/A 2.9 1.6 4.8 2.5 1.7Reading 2 N/A 6.6 5.3 11.2 6.1 5.3Test Unit 8 Reading 1 N/A 1.6 1.7 8.1 1.6 1.6Reading 2 N/A 1.7 7.5 9.7 3.5 4.1Test Unit 9 Reading 1 N/A 2.2 1.6 N/A 2.5 1.6Reading 2 N/A 10.5 5.1 N/A 3.4 7.5Table 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 co nnectors after the 500 hours of salt spray. The values recorded are significantl y 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 i s 56 inch pounds after 500 hours of salt spray [14]. Initial mate and final un mate 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 un its except test units 6 and 7. Test unit 6 did not unmate as the plug was stuck to the recepta cle. This could be due to galvanic51

corrosion. The potential for significant corrosion buildup between the two materials of the plug and receptacle will cause this to occur. T est 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, i t was indicated that zinc nickel alloys produce oxide films as a byproduct and the oxide fi lms have high electrical resistance
but acts as a barrier film which reduces dissolutio n 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 te st probes and the backshell surface,52

high measurement values were achieved. The non-unif ormity of the oxide films allows for parts of the surface to be free of the film the reby 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 o xide film. This confirms that even though the higher continuity measurements were achi eved, the substrate was still protected from the salt environment and the groundi ng path remained intact. Figure 28
shows no residue on the cadmium backshell which con firms 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 backshe ll 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 f inish on the other hand still shows no sign of corrosion. See appendix for figures of a ll the test units after 48 hours.53 After 96 hours, there was an overall increase in th e continuity readings on
connectors and backshells with the zinc nickel fini sh from the 48 hour readings. Also
seen was an increase in the white residue on the te st 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
hours54 Figures 32 through 35 show test units 1 and 3 compa red earlier. Figure 34 and 35
show the zinc nickel finish backshell and the conne ctor 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 o r connectors. The cadmium finish on the other hand began to show a little corrosion near th e strain relief clamp, however continuity measurements were still below the maximu m 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 bet ween 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 nick el finish show similar values to
connectors with zinc nickel finish mated with conne ctors with the other finishes in test units 2, 4, 5, 6, 7, 8 & 9. After 500 hours of exposing the test fixture to sal t fog, significant changes in the test units were noticed from a visual inspection. T he 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 corro sion 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 backshe lls and connectors in the study. This can be attributed to the possibility of inadeq uate coating of the internal threads in the clamps of the strain relief. Another observatio n that brings us to this conclusion is the fact that cadmium finish, which is known for it s high performance in this environment, showed heavy corrosion on the clamps o f the strain relief. This heavy corrosion on the strain relief clamps was seen on t est 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
hours56

Overall, continuity measurements, coupling torque measurements and visual inspections indicate that the requirement of protec ting the substrate from corrosion while keeping the electrical grounding path intact has be en 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 differ ent materials and finishes that exist today are not significantly different from the meas urements taken between zinc nickel finishes and zinc nickel finishes except for the co upling measurements on the stainless steel connector and the unexplained higher torque v alue 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 u nits 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 d one 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 un it.57 

## 5. Conclusion
In this report, results of a study undertaken to in vestigate aerospace circular connectors and connector accessories, their means o f corrosion, their electromagnetic interference and shielding integrity behavior are p resented. As background and for completeness, a description of existing finishes an d proposed alternate finishes to
cadmium is included. Since zinc nickel finishes hav e long attracted interest in the industry, they were selected for this study. A summ ary review of the electrodeposition of zinc nickel is included. The rapid formation of oxi de films was anticipated on the zinc nickel finish and this is precisely what was observ ed during testing. Testing in this study included a salt spray test, a n 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 co nfirmed the presence of oxide films
on the zinc nickel backshells and connectors in thi s 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, w hich is the pass or fail point for this study, heavy corrosion was noticed on some of the t est 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 hi gh as 29.7 millivolts. Because of values as low as 1.2 millivolts recorded, a conclus ion can be made that the grounding path between the shield braid and the backshell, th e 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 a fter the 500 hours showed a delta from the initial values that were significantly hig her and in some test units above the recommended range, one of the test units did not un mate. The upside to this is that values outside of the recommended range were also r ecorded 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 inter face between zinc nickel finish and
other finishes were determined. Based on this, one can conclude that although the z inc nickel finish on aluminum connectors and backshells did not perform as well a s the cadmium finish in this study by58

not meeting all the benchmarks, it achieved the ind ustry 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” Aer ospace Electrical Interconnect System Symposium (AEISS) [2] Mason, Neidbalson & Melissa Klingenberg. “Upda te on Alternatives for Cadmium Coatings on Military Electrical Connectors” MetalFinishing. Mar ch 2010, p12-20 [3] Toshiaki Murai. “Performance Characteristics o f 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. Ramanauska s. “Corrosion resistance of phosphate Zn-Ni alloy electrodeposits” Surface and Coatings Technol ogy. 2009, p3072-3077 [6] Ganesan, Kumaraguru, N Popov “Development of c ompositionally modulated multilayer ZnNi deposties as replacement for cadmium” Surface and C oatings Technology. 2007, p7896-7904 [7] Gavrila, Millet, Mazille, Marchhandise, J.M. C untz “ Corrosion behavior of zinc-nickel coatings, electrodeposited on steel” Surface and Coatings Tec hnology. 2000, p164-172 [8] ASTM Standard B841, 2010, "Standard Specificati on for Electrodeposited Coatings of Zinc Nickel Alloy Deposits" ASTM International, West Con shohocken, PA, 2010 [9] Ascott Analytical Equipment Ltd "Salt Spray tes ting cabinets and chambers” http://www.ascott-analytical.com/howtheywork2/SSCsa ltsprayillo2.htm [10] Glenair, Inc " Series 38 - EMI-RFI Non-Environmental Backshells wi th Strain Relief - Glenair,
Inc.” http://www.glenair.com/backshells/circular/series_3 8.htm [11] EIA/ECA-364-26B, "Salt Spray Test Procedure fo r Electrical Connectors, Contacts and Sockets" Electronic industries Alliance, , 2006 [12] Official Journal of the European Union, Direc tive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restrictio n of the use of certain hazardous substances in electrical and electronic equipment. [13] Glenair Inc. “Wearable Solder electronics” Qwi kConnect. April 2005 Volume 10, number 1. [14] MIL-DTL-38999L, 2009, "General Specification f or 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 lite rature: 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, p59-6560 

## 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 hours61

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 hours62

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 hours63

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 hours64

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 hours65

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 hours66

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 hours67

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 hours68

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 hours69

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 hours70

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 hours71

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 hours72

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 hours73

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 hours74

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 hours75

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 hours76

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 hours777.10Mounting Bracket Detailed DesignCrossed hatched areas were free of primer in orderto allow electrical conductivity between the receptacle and the bracket.