Corrosion Screening of EV31A Magnesium and Other 

Magnesium Alloys Using Laboratory-Based 

Accelerated Corrosion and Electro-chemical

Methods

by Brian E. Placzankis, Joseph P. Labukas, Elizabeth Charleton, 

and Chris E. Miller

ARL-TR-6899 July 2014

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Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5066

ARL-TR-6899 July 2014

Corrosion Screening of EV31A Magnesium and Other 

Magnesium Alloys Using Laboratory-Based

Accelerated Corrosion and Electro-chemical

Methods

Brian E. Placzankis, Joseph P. Labukas, Elizabeth Charleton, 

and Chris E. Miller

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution is unlimited.
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Corrosion Screening of EV31A Magnesium and Other Magnesium Alloys Using 

Laboratory-Based Accelerated Corrosion and Electro-chemical Methods

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Brian E. Placzankis, Joseph P. Labukas, Elizabeth Charleton, and Chris E. Miller

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14. ABSTRACT

Test specimens of EV31A and other prospective magnesium alloys for U.S. Department of Defense systems were identically 

prepared and exposed under bare, uncoated conditions in chamber-based laboratory-accelerated corrosion test methods to assess 

their relative susceptibilities to general and pitting corrosion attack. The methods used were ASTM B117 neutral salt fog (NSF)

and General Motors (GM) Standard 9540P cyclic accelerated corrosion. The NSF specimens were compared at intervals of 18, 

72, and 168 h. The GM 9540P specimens were assessed at 1-, 5-, and 10-cycle intervals. The visual corrosion assessments were 

graphically obtained using flatbed scanning techniques. Following the exposures, mass loss measurements were obtained for all 

specimens to determine corrosion rates. Electro-chemical evaluations of corrosion susceptibility for each of the alloys were 

performed using potentiodynamic polarization. Based on the results of these studies, the overall corrosion resistance for EV31A 

was found to be good, and its relative susceptibility to general corrosion attack compared with the current military specification 

alloy AZ31B is discussed.

15. SUBJECT TERMS

corrosion, magnesium, alloys, EV31A, E21, ZE41A, AZ31B, AZ91C, WE43B, AMX602, ZAXE1711, salt fog, cyclic

corrosion, GM 9540P

16. SECURITY CLASSIFICATION OF: 

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Brian E. Placzankis

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410-306-2989

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Experimental Procedure 1

3. Results 5

3.1 Commercially Pure Magnesium (CP Mg).......................................................................5

3.2 AMX602..........................................................................................................................7

3.3 AZ31B-H24.....................................................................................................................8

3.4 AZ91C.............................................................................................................................9

3.5 EV31A...........................................................................................................................10

3.6 WE43B-T5 ....................................................................................................................12

3.7 ZAXE1711 ....................................................................................................................13

3.8 ZE41A ...........................................................................................................................14

3.9 Accelerated Corrosion Rate Determinations.................................................................16

3.10 Electro-chemical Behavior............................................................................................21

4. Discussion 22

5. Conclusions 27

6. References 28

Distribution List 31
iv

List of Figures

Figure 1. Corrosion rack configuration used for neutral salt fog (NSF) and General Motors 

(GM) Standard 9540P exposures...............................................................................................3

Figure 2. CP Mg after NSF..............................................................................................................6

Figure 3. CP Mg after GM 9540P cyclic corrosion.........................................................................6

Figure 4. Schematic illustration of SWAP equipment (a) to produce rapidly solidified 

AMX602 and ZAXE1711 Mg alloy powders and (b) morphology of coarse Mg alloy 

powder prepared by SWAP ......................................................................................................7

Figure 5. NSF corrosion of AMX602. .............................................................................................7

Figure 6. GM 9540P cyclic corrosion of AMX602. ........................................................................8

Figure 7. NSF corrosion of AZ31B-H24. ........................................................................................8

Figure 8. Filiform corrosion of AZ31B-H24 after 72-h NSF (6X)..................................................9

Figure 9. GM 9540P cyclic corrosion of AZ31B-H24 at 1, 5, and 10 cycles..................................9

Figure 10. AZ91C after NSF. ........................................................................................................10

Figure 11. AZ91C after GM 9540P cyclic corrosion. ...................................................................10

Figure 12. EV31A after NSF. ........................................................................................................11

Figure 13. Corrosion of EV31A localized pits after 168-h NSF (6X) as corroded (a) and after 

cleaning (b). .............................................................................................................................11

Figure 14. EV31A after GM 9540P cyclic corrosion. ...................................................................12

Figure 15. WE43B-T5 after NSF...................................................................................................12

Figure 16. WE43B-T5 after GM 9540P cyclic corrosion..............................................................13

Figure 17. NSF corrosion of ZAXE1711-D at 18, 72, and 168 h..................................................13

Figure 18. GM 9540P cyclic corrosion of ZAXE1711-C at 1, 5, and 10 cycles. ..........................14

Figure 19. ZE41A after NSF..........................................................................................................15

Figure 20. ZE41A after GM 9540P cyclic corrosion.....................................................................15

Figure 21. Corrosion rates in mils per year based upon mass loss measurements after NSF 

(red) and GM 9540P cyclic corrosion exposures (blue). .........................................................16

Figure 22. Front sides of 168-h NSF specimens after cleaning to reveal extent of substrate 

loss. ..........................................................................................................................................17

Figure 23. Rear sides of 168-h NSF specimens after cleaning to reveal extent of substrate 

loss. ..........................................................................................................................................18

Figure 24. Front sides of 10-cycle GM 9540P cyclic corrosion specimens after cleaning to 

reveal extent of substrate loss. .................................................................................................19

Figure 25. Rear sides of 10-cycle GM 9540P cyclic corrosion specimens after cleaning to 

reveal extent of substrate loss. .................................................................................................20
v

Figure 26. Potentiodynamic polarization analysis of Mg alloys....................................................21

Figure 27. 168-h NSF comparisons of (a) AZ91C-T6, (b) Al 5083-H131, and (c) Al 2024-T3...23

Figure 28. 10-cycle GM 9540P cyclic corrosion comparisons of (a) AZ91C, (b) Al 5083-

H131, and (c) Al 2024-T3.......................................................................................................24

Figure 29. Galvanic series showing approximately 1.8-V gap between Mg and graphite with 

other metallic materials between (37, 38) (Reprinted, with permission, from ASTM G82-

98 (2014) Standard Guide for Development and Use of a Galvanic Series for Predicting 

Galvanic Corrosion Performance, copyright ASTM International, 100 Barr Harbor Drive, 

West Conshohocken, PA 19428. A copy of the complete standard may be obtained from 

ASTM International, http://www.astm.org..............................................................................26

List of Tables

Table 1. Chemical compositions for Mg alloys (%). .......................................................................2

Table 2. Mechanical properties for Mg alloys. ................................................................................2

Table 3. Temper temperatures used for AMX602 and ZAXE1711.................................................2

Table 4. GM 9540P (15) cyclic corrosion test details. ....................................................................4

Table 5. Chemical certification/analysis for AZ91C. ....................................................................24
vi

Acknowledgments

The authors thank Tyrone Jones of the U.S. Army Research Laboratory Armor Mechanisms

Branch, Steve Brown of the U.S. Naval Air Systems Command, and Katsuyoshi Kondoh of the 

Joining and Welding Research Institute of Osaka University, Japan, for supplying magnesium 

samples used to produce the specimens in this study. The authors also thank Magnesium 

Elektron (Madison, IL) for supplying additional magnesium alloy samples and detailed 

properties for some of the alloys studied.
1

1. Introduction

The U.S. Department of Defense (DOD) has historically used magnesium (Mg) in various

applications, the majority being military aircraft, but despite its weight advantage over other 

materials, Mg has been limited in wider use (1). The prevailing factors limiting greater usage 

remain centered on poor corrosion resistance and persistent concerns about flame resistance. 

Recent regulatory pressures such as updated Corporate Average Fuel Economy standards have 

renewed interest in lighter-weight designs to comply with the mandates for more fuel-efficient 

automotive systems (2). The push for lighter-weight designs to increase fuel economy for 

civilian vehicles coincides with the renewed interest in Mg for ground-based defense 

applications such as structures and armor (3–5).

In 2009, the first U.S. military specification (MILSPEC) for Mg armor plate was established. 

This specification, known as MIL-DTL-32333 (6), Armor Plate, Magnesium Alloy, AZ31B, 

Applique, sets minimum acceptance values for mechanical properties, ballistics, and corrosion 

resistance in the bare, uncoated state. These minimum values are critical for manufacturers to 

deliver consistent and predictable materials performance for vehicles and weapons platforms. 

The addition of a minimum acceptance value for corrosion resistance was a critical parameter for 

the specification creation and is unique to Mg armor plates. Although significant research and 

development efforts (7) to expand the available Mg alloy plate have occurred since establishing 

MIL-DTL-32333, as of 2013 AZ31B-H24 remains the sole qualified alloy for this specification. 

This report provides additional basic corrosion screening data using the two most common 

laboratory-accelerated corrosion methods and electro-chemical polarization. Although these 

methods are useful for understanding alloy corrosion, predicting the long-term corrosion 

performance is complicated and requires more rigorous testing in a variety of environments. As 

with aluminum (Al) and steel materials, additional corrosion vulnerability data from other 

sources such as the Aberdeen Test Center Automotive Test Track, longer-term outdoor 

exposures, and prior corrosion data from actual fielded platforms should play greater roles in the 

final design decision process. This study consolidates and extends previous work, provides new 

data on the Magnesium Elektron alloy EV31A (8) (also known by the trade name Elektron 21), 

and adds baseline electro-chemical data for previously screened alloys (9).

2. Experimental Procedure

In order to assess the inherent corrosion resistance capabilities of bare, unprotected Mg alloys 

currently in use or proposed for use by the U.S. DOD, a wide selection of cast and wrought Mg

alloys were chosen from aviation, industry, and specialty alloys proposed for ground 
2

applications. The detailed list of Mg including pure melt-grade Mg followed by Mg alloys of 

various tempers, in alphanumerical order, included commercially pure (CP) melt-grade Mg 

(CPMg 9980B) (10), AMX602 (11), AZ31B-H24 (12), AZ91C-T6 (13), EV31A-T6, WE43B-T5

(13), ZAXE1711 (14), and ZE41A (13). The chemical composition and mechanical properties of 

these alloys are listed in tables 1 and 2, respectively.

Table 1. Chemical compositions for Mg alloys (%).

Table 2. Mechanical properties for Mg alloys.

For simplicity’s sake, mechanical properties listed for the powder and extruded alloys AMX602 

and ZAXE1711 were average values obtained using the longitudinal extruded direction from the 

three bars prepared from each alloy using temper temperatures listed in table 3.

Table 3. Temper temperatures used for AMX602 and

ZAXE1711.

Designation and Tempering Temperature (o C)

AMX602 ZAXE1711

1: 350 B: 350

2: 300 C: 250

3: 250 D: 200
3

The actual Mg specimens were cut to 1.75- × 1.5- × 0.25-in nominal dimensions using a watercooled nonmetallic abrasive blade saw (Buehler, Lake Bluff, IL). When cutting the specimens,

outer or parent material adjacent to exposed surfaces were used. The preference for using outer 

surfaces of specimens was two-fold. For the cast alloys or alloys originating from ingots, the use 

of outer material was to minimize the grain size. For the wrought Mg, the outer surfaces were 

selected for the most accurate representation of the alloy as it would actually be used, and thus 

specimens were down-sectioned to 0.25 in via the short transverse plane with only the outward 

facing exterior surfaces used. All specimens were then surface finished to 600 grit using 

metallographic grinding techniques. Following grinding, all specimens were cleaned and rinsed 

using acetone. Due to the varieties of plates and ingots, there were slight variations in thickness 

precision among the specimens. To normalize weight loss data collected among the specimens, 

the sample dimensions and initial mass were measured in addition to the initial masses. Finally, 

following the measurements for dimensions and mass, the specimens were organized in racks as 

shown in figure 1, and then placed into their respective chambers.

Figure 1. Corrosion rack configuration used for neutral salt fog (NSF) and General Motors (GM) Standard 

9540P (15) exposures.

A standard wet-bottom style test Harshaw model 410 chamber was used for NSF testing and an 

Attotech model CCT-NC cyclic corrosion chamber was used for cyclic testing. The NSF 

operating parameters were in accordance with ASTM B117 (16) at 95 °F with saturated humidity 

and an atomized fog of 5% NaCl solution. In deference to previous studies where Mg alloys had 

corroded rapidly, the observation intervals for the specimens in NSF were modified to 4, 8, 18, 

72, and 168 h rather than the 18, 72, and 168 h previously used for Al alloys. The image 

acquisition scans were made at the usual 18, 72, and 168 h with 4 and 8 h optional. If severe 

corrosion was observed early at 4 or 8 h, additional scans were then acquired. The 

GM 9540P (15) cyclic accelerated corrosion test consisted of 18 separate stages that included the 

following: saltwater spray using 0.9% NaCl, 0.1% CaCl2, 0.25% NaHCO3 test solution, high

humidity, drying, ambient, and heated drying. The environmental conditions and duration of 

each stage for one complete cycle are provided in table 4.
4

Table 4. GM 9540P (15) cyclic corrosion test details.

In addition, the cyclic chamber was calibrated with standard cold-rolled steel mass loss 

calibration coupons as described in the GM 9540P test specification. Although the GM 9540P 

procedure was developed for steel substrates, previous studies (17) have shown that the cyclic 

nature of the exposure and the electrolyte used can also have a significant impact on the 

corrosion behavior of magnesium alloys. The observation and scanning intervals for the 

GM 9540P specimens were 1, 5, and 10 cycles. 

To visually assess and characterize the corrosion, all specimens were scanned at 1200 dots per 

inch (dpi) optical resolution at their respective intervals using color flatbed scanning techniques.

Specimens were vigorously rinsed under a stream of deionized water to remove loose corrosion 

products and thus better reveal the corrosion damage to the substrate surface. Following the 

rinse, the specimens were then allowed to dry prior to scanning. After the final optical scan at the 

conclusion of exposures in NSF and GM 9540P, and prior to final weighing to determine the 

mass loss, the specimens were all cleaned in accordance with ASTM G97 (18) to remove all 

remaining corrosion products. After the final weights were recorded, the corrosion rate in mils 

(thousands of an inch) per year (mpy) (19) was calculated using the following formula:

mpy

K x (m m i f

(A x T x d )

 - 

 



(1)

where K = constant (534,000 for mpy), mi = initial mass (g), mf = final mass (g), A = area (in2), 

T = time (hours), and d = density (g/cm2).
5

For GM 9540P cyclic corrosion where duration is measured in cycles, each cycle was 

normalized to 24 h. Therefore, the final exposure time in hours for the GM 9540P specimens was 

240 h.

Potentiodynamic polarization of each Mg alloy was carried out in 3.5% (w/v) NaCl (aqueous) 

without stirring at ambient laboratory conditions. A Flat Cell (Princeton Applied Research, Oak 

Ridge, TN) was used and comprised approximately 250 mL of electrolyte, a Mg alloy working 

electrode, a Pt mesh counter electrode, and a saturated calomel electrode. The working electrode 

was polarized with a potentiostat (Ametek/Princeton Applied Research, VersaSTAT 3F) from 

500 mV less than open circuit potential (OCP) to 500 mV greater than OCP at 1 mV/sec. The 

open circuit potential (OCP or Ecorr) and corrosion current (Icorr) were calculated by fitting the 

linear portion of a plot of E versus I in the range of +/–100 mV around OCP using the RP fit 

function of the VersaStudio software (Ametek/Princeton Applied Research, version 2.40). For 

fitting the linear portion of the curve at such small overpotential, the constant β (Tafel slope) was 

fixed at 100 mV for both the anodic and cathodic branches. Reported values of Icorr and Ecorr are 

the average of at least four potentiodynamic scans. 

3. Results

Upon the start of chamber-accelerated corrosion testing, the decision was made to include 

additional scans of more actively corroding alloys. This decision was found to be fortuitous, as 

there was significant corrosion amongst some of the NSF specimens. These optional early image 

scan intervals at 4 and 8 h were needed for visually assessing the rapid corrosion progression for 

the more active alloys before the damage became too severe to observe the type and its 

origination. 

3.1 Commercially Pure Magnesium (CP Mg)

The CP Mg examined was 9980B melt-grade Mg that is typically used for alloying additions to 

other base metals such as Al. As was obvious from scans in both NSF and from GM 9540P, the 

CP Mg sample was corroded more severely that any of the Mg alloys based on the high density 

of pits and visual mass loss, and the earlier 4- and 8-h observation intervals were indeed useful. 

The corrosion rate was so high that the NSF exposure was concluded at 72 h to avoid the risk of 

having no specimen remaining from which to measure mass loss at 168 h. The corrosion rates 

calculated from mass loss were two orders of magnitude higher than the worst of the remaining 

alloys for NSF and one order of magnitude greater for cyclic corrosion. Optical scans of the 

corrosion progression for CP Mg in NSF and GM 9540P are shown in figures 2 and 3,

respectively. 
6

Figure 2. CP Mg after NSF.

Figure 3. CP Mg after GM 9540P cyclic corrosion.
7

3.2 AMX602

This advanced powder-based alloy and ZAXE1711 (both from Japan) were produced using a 

Spinning Water Atomization Process (SWAP) to yield powder particles with very fine 

microstructures. These powders were compacted into a billet that was then hot-extruded into bars 

for further mechanical, ballistic, and corrosion studies (16). A schematic of the SWAP process 

and example of the powder produced is included in figure 4. This alloy contains amounts of Al 

similar to those found in AZ61 but lacks zinc and contains calcium. Images shown in figures 5 

and 6 indicate very good response to accelerated corrosion environments with excellent 

performance under NSF and cyclic corrosion testing. AMX602 had the second lowest corrosion 

rate after AZ91C based on mass loss measurements. 

Figure 4. Schematic illustration of SWAP equipment (a) to produce rapidly solidified AMX602 

and ZAXE1711 Mg alloy powders and (b) morphology of coarse Mg alloy powder 

prepared by SWAP (16).

Figure 5. NSF corrosion of AMX602.

 

(a) (b)

Molten metal

Collected Mg powder
8

Figure 6. GM 9540P cyclic corrosion of AMX602.

3.3 AZ31B-H24

While CP Mg exists in this study as a scientific reference point, the AZ31B-H24 alloy can 

correspondingly be considered as the baseline engineering counterpart mainly due to its 

existence as the sole MILSPEC alloy for armor plates. The main alloying element in 

AZ31B-H24 is Al. This alloy is used for a variety of applications including aircraft fuselage 

materials and tooling plate. For armor, the AZ31B-H24 alloy was selected for its balanced 

mechanical properties and corrosion resistance. For both NSF and cyclic corrosion, the 

AZ31B-H24 performed as expected. The 7.1-mpy measurement for AZ31B-H24 in NSF was 

within the allowable acceptance range stated in the MILSPEC. The NSF corrosion initiated as 

filiform and was most visible at 72 h, and then propagated outward to encompass larger areas by 

the exposure conclusion at 168 h. The GM 9540P progression followed a similar mode but was 

less severe and stained darker. Figures 7–9 chronicle the corrosion of AZ31B-H24 including a 

close-up of the NSF filiform corrosion in figure 8.

Figure 7. NSF corrosion of AZ31B-H24.
9

Figure 8. Filiform corrosion of AZ31B-H24

after 72-h NSF (6X).

Figure 9. GM 9540P cyclic corrosion of AZ31B-H24 at 1, 5, and 10 cycles.

3.4 AZ91C

This particular alloy exhibited the lowest mass loss and cosmetic corrosion attributes among all 

of the alloys in this study. Similar to AZ31B-H24, the main alloying element in AZ91C is Al,

though in significantly higher amounts. This alloy is popularly used for aerospace castings but is 

not used for structural applications due to its low inherent as-cast strength. The cosmetic damage

from corrosion observed was less than typically observed for some Al alloys. For instance, it 

matched AA5083-H131 in appearance for similar exposure durations under NSF and cyclic and 

easily outperformed AA2024-T3 under those same conditions. Figures 10 and 11 illustrate the 

near complete lack of corrosion for this alloy under NSF and cyclic exposures. 
10

Figure 10. AZ91C after NSF.

Figure 11. AZ91C after GM 9540P cyclic corrosion.

3.5 EV31A

The EV31A alloy, also known commercially as Elektron 21 or E21 is a cast alloy produced by 

Magnesium Elektron (20). It is well known for its excellent mechanical properties and corrosion 

resistance and appears in SAE Aerospace Material Specification AMS4429 (21). Similar to 

WE43, it contains rare earth alloying additions though overall in lower concentration. Its 

composition is based upon zirconium and zinc additions with neodymium, gadolinium, and small 

amounts of other rare earths. The mass loss values determined under accelerated methods were 

quite low and generally fell between values observed for AZ31B and WE43. Most discolorations

observed under NSF and cyclic were from staining; however, the actual corrosion produced 

under NSF on EV31A consisted of very deep (greater than 100 μm) but localized elongated pits. 

Figures 12–14 depict the corrosion of EV31A and include a close-up of the localized pitting

morphology produced under NSF in figure 13.
11

Figure 12. EV31A after NSF.

Figure 13. Corrosion of EV31A localized pits after 168-h NSF (6X) as corroded (a) and after cleaning (b).

(a) (b)
12

Figure 14. EV31A after GM 9540P cyclic corrosion.

3.6 WE43B-T5

Mg alloy WE43B is best known for relatively good corrosion resistance and good mechanical 

properties, making it a prospective candidate for plates. It is used in commercial and military 

aviation applications and has demonstrated good flame resistance (22). For this study. the 

wrought form of WE43B (T5) was exposed. It shared similar corrosion resistance in the 

exposures as AZ31B-H24 and EV31A. The mode of corrosion for this alloy was similar that of 

AZ31B, with dark staining and diffuse pitting. There was little or no filiform corrosion observed. 

The NSF and cyclic scans for WE43B-T5 are displayed in figures 15 and 16.

Figure 15. WE43B-T5 after NSF.
13

Figure 16. WE43B-T5 after GM 9540P cyclic corrosion.

3.7 ZAXE1711

Extruded ZAXE1711 is the second alloy in this study produced in Japan using the SWAP 

powder extrusion method. From an alloy composition standpoint, it resembles an AZ61 type 

alloy with additions of calcium and lanthanum. It is marginally stronger than AMX602 and has 

slightly better ballistic response (23). Results for corrosion resistance were better than AZ31B 

under cyclic corrosion but had about the same or slightly lower performance than AZ31B under 

NSF. The corrosion observed under NSF and cyclic exposures in figures 17 and 18 was mainly 

filiform attack and dark staining with little to no pitting. The cyclic was less severe than the NSF.

Figure 17. NSF corrosion of ZAXE1711-D at 18, 72, and 168 h.
14

Figure 18. GM 9540P cyclic corrosion of ZAXE1711-C at 1, 5, and 10 cycles.

3.8 ZE41A

Mg alloy ZE41A has been used for decades in military and commercial aviation systems. It is 

best known in the U.S. Army for its use by Sikorsky in UH-60 Blackhawk transmission housings

and has long been known for issues with corrosion and under this application in particular

(24–29). In both NSF and cyclic corrosion exposure, the ZE41A indeed corroded more severely 

that the other alloys though not nearly as much as the CP Mg. As with CP Mg under NSF, the 

corrosion was very active and necessitated additional imaging from the earlier 4- and 8-h 

observations. The NSF damage to this alloy was characterized by extremely deep pits extending 

well into within the bulk of the specimen. This extensive pitting was the main mechanism for the 

appreciable mass loss measurement. In addition to somewhat milder pitting compared with NSF, 

the cyclic specimen also showed traces of filiform attack. Good images of the pitting and the 

mixed mode corrosion are displayed in figures 19 and 20.
15

Figure 19. ZE41A after NSF.

Figure 20. ZE41A after GM 9540P cyclic corrosion.
16

3.9 Accelerated Corrosion Rate Determinations 

Mass loss values for the alloys in NSF and GM 9540P cyclic corrosion were collected and the 

resultant corrosion rates in mils per year were calculated and plotted in figure 21. The overall 

corrosion rates were determined using mass loss and because the mass loss occurred from 

corrosion attack on all surfaces, post cleaning scans for the front and also of the backsides of the 

NSF and cyclic corrosion specimens were acquired and are displayed in figures 22–25.

Figure 21. Corrosion rates in mils per year based upon mass loss measurements after NSF (red) and 

GM 9540P cyclic corrosion exposures (blue).
17

Figure 22. Front sides of 168-h NSF specimens after cleaning to reveal extent of substrate loss.
18

Figure 23. Rear sides of 168-h NSF specimens after cleaning to reveal extent of substrate loss.
19

Figure 24. Front sides of 10-cycle GM 9540P cyclic corrosion specimens after cleaning to reveal extent of 

substrate loss.
20

Figure 25. Rear sides of 10-cycle GM 9540P cyclic corrosion specimens after cleaning to reveal extent of 

substrate loss.
21

3.10 Electro-chemical Behavior

To better understand Mg alloy corrosion at a more fundamental level, the electro-chemical

behavior of a metal or alloy is often studied. Specifically, potentiodynamic polarization provides 

a methodology to relate the OCP of an alloy (Ecorr) to the corrosion current density (Icorr). 

Figure 26 shows this relationship between Ecorr and Icorr for all of the alloys reported in this study. 

Figure 26. Potentiodynamic polarization analysis of Mg alloys. 

-1.95

-1.85

-1.75

-1.65

-1.55

-1.45

0 20 40 60 80

Open Circuit Potential (V)

Icorr (uA)

CP Mg

AMX 602

AZ31B-H24

AZ91C

EV31A

WE43B-T5

ZAXE1711

ZE41A
22

Results from this type of analysis are not always in agreement with what is observed using more 

qualitative approaches such as NSF, cyclic corrosion exposure, or outdoor exposure, and the 

reasons for this disagreement are not trivial. Frankel (30) provides some insight into the 

discrepancy between corrosion rates calculated from electro-chemical data and rates determined 

from mass loss measurement. One example of such discrepancies in our data can be seen by 

comparing the mass loss data for EV31A and ZE41A with the electro-chemical data for the two 

alloys. Clearly, the mass loss data show EV31A meeting the 7-mpy mass loss limit for Mg alloys 

to be used by the DOD for ballistic purposes, whereas ZE41A corrodes at a rate of 

approximately one order of magnitude faster and does not meet the corrosion requirements to be 

incorporated into the ballistic performance specification (6). Potentiodynamic polarization 

suggests that these alloys would corrode at near equivalent rates because of similar corrosion 

current densities (figure 26). The electro-chemical data will be discussed in the following 

section. 

4. Discussion

Corrosion will always remain a consideration for Mg alloys just as it is for many other materials.

However, designers considering Mg-based materials are now better informed of the galvanic and 

general corrosion issues. Higher-purity alloys limiting iron (Fe), cobalt (Co), nickel (Ni), and 

copper (Cu) contaminants have significantly improved corrosion resistance. Modern methods

limiting direct Mg substrate contact with corrosive environments include electro-chemical

plating, conversion coatings, anodizing, vapor-phase deposition techniques, laser surface 

alloying/cladding, and organic coatings (31). Coating solutions applied in clean environments 

with robust process controls such as Tagnite* anodizing, plasma gel anodizing, and Gardobond†

X4729 composite coating have provided superior corrosion, abrasion, and wear resistance (32). 

Since 1996, the Tagnite coating process has protected Mg components on the U.S. Navy’s 

Sikorsky CH-53 Sea Stallion helicopter, the U.S. Air Force F-22 Raptor fighter, U.S. Marine 

Corps Expeditionary Force Vehicle prototypes, and the U.S. Army’s Boeing Apache helicopter. 

Novel repair techniques have also been recently developed to address corrosion in fielded 

systems. For instance, cold-spray technology developed at the U.S. Army Research Laboratory 

has been a cost-effective and technically feasible solution for field repair and surface protection 

of corrosion damaged Mg components (33). In addition to alloy design and coatings, engineers 

now mitigate corrosion through proper joint design, insulation of dissimilar metals, 

manufacturing process controls, and appropriate maintenance schedules. 

 

* Tagnite is a registered trademark of Technology Applications Group Inc., Atlanta, GA.

† Gardobond is a registered trademark of Trakehner Straße 3, Frankfurt am Main, Germany.
23

While significant corrosion damage resulted in the accelerated exposure experiments on 

substrates such as CP Mg and ZE41A, other alloys such as AZ91C and the SWAP-prepared 

alloys experienced little to almost no corrosion damage. This result provides evidence that 

supports ongoing research and development aimed at corrosion-resistant Mg alloys with a higher 

degree of corrosion resistance. For the accelerated corrosion procedures, there was reasonable 

agreement of overall alloy trends (figure 21) between the NSF and the GM 9540P cyclic 

corrosion procedures. The largest variations occurred among the SWAP-prepared alloys where 

greater relative gaps in mils per year between NSF and cyclic corrosion experiments were 

evident. The sampling size was small due to a lack of material; therefore, standard error bars 

were not generated for the mass loss. For NSF exposure, the order of corrosion resistance 

calculated from mass loss from highest to lowest was AZ91C, AMX602, ZAXE1711, WE43B, 

EV31A, AZ31B, ZE41A, and CP Mg. For the GM 9540P cyclic corrosion, the order of corrosion 

resistance based on mass loss in mils per year was AZ91C, ZAXE1711, AMX602, AZ31B, 

WE43B, EV31A, ZE41A, and CP Mg. The corrosion damage on the EV31A was very 

interesting. The overall mass loss for this alloy was low but the deep localized damage that did 

occur from pitting under NSF suggests the possible presence of tramp elements such as Fe, Ni, or 

Cu, or intermetallic phases that act either as local cathodes or anodes. Were it not for this 

localized attack, the EV31A have visually resembled AZ91C. If this localized attack can be 

eliminated or reduced, the cast alloy EV31A would be worthy of further development as a 

wrought plate product.

The corrosion resistance of AZ91C was surprising. The visual evidence of corrosion or lack of it 

resembled highly corrosion-resistant 5000-series Al alloys, such as AA5083. The high level of 

corrosion resistance is visible in figure 27, comparing alloy AZ91C with Al-series alloys 

AA5083-H131 (34, 35) and AA2024-T3 (36) at 168 h of NSF. 

Figure 27. 168-h NSF comparisons of (a) AZ91C-T6, (b) Al 5083-H131, and (c) Al 2024-T3.
24

Similarly, figure 28 compares the same three alloys at 10 cycles of GM 9540P.

Figure 28. 10-cycle GM 9540P cyclic corrosion comparisons of (a) AZ91C, (b) Al 5083-H131, and (c) 

Al 2024-T3.

Unexpectedly, the performance of this batch of alloy was beyond typical AZ91C. Personal 

communications with Magnesium Elektron regarding this high resistance to corrosion suggested 

that the material sold as AZ91C, from which the samples originated, may have actually been 

AZ91E (11). The AZ91E alloy is similar to AZ91C but has much tighter controls on the 

permitted concentrations of harmful impurity elements such as Fe, Ni, and Cu. A formal 

certification of composition (37) for this particular sample stock was obtained through the 

U.S. Navy Air Warfare Center and is listed in table 5. A close examination of the AZ91C 

constituents confirms low concentrations of silicon, Cu, and Ni, all factors that greatly improve 

corrosion resistance. 

Table 5. Chemical certification/analysis for AZ91C.

The electro-chemical measurements on the Mg alloys evaluated in this research provide a 

guideline for estimating the susceptibility of each alloy to corrosion. Based on these 

measurements, alloys with the lowest corrosion current density are expected to be the most 

resistant to corrosion, and those with the highest Ecorr are expected to resist galvanic corrosion 

when coupled to a slightly more noble metal. The data in figure 26 suggest that all of the alloys 

in this research will corrode at about the same rate based on the Icorr. Some alloys such as 

AZ31B, AZ91, and AMX602 may exhibit somewhat stable galvanic behavior when coupled to 

lightweight Al alloys. Certainly, further study is necessary to evaluate the corrosion behavior of 

these Mg alloys when coupled to more noble other metals. Although there was no exact 

correlation between electro-chemical experiments and mass loss data, alloys with the highest 
25

open circuit potential (AMX602, ZAXE1711, AZ91C, and AZ31B) did have good corrosion 

resistance in accelerated corrosion experiments. The SWAP-produced alloys also had a smaller 

range of corrosion current density compared with the AZ-series alloys. Electro-chemically, 

CP Mg, ZE41A, and EV31A had similar Ecorr and Icorr; however, accelerated corrosion testing 

showed EV31A to have minimal mass loss in these tests. Perhaps the mass loss of EV31A 

happens rapidly and early in accelerated corrosion testing followed by a passivation. Polarization 

curves of EV31A show a slight passivation occurring in some curves just above the OCP. 

Research into this passive region and the surprisingly low Ecorr and Icorr of WE43 is ongoing. 

While the improved corrosion resistance of Mg alloys is indeed encouraging, the designers must 

not become complacent regarding their selection of what other materials are selected for contact 

with the Mg alloy. Designers must not forget that Mg is anodic to every other metallic material 

available for engineering design. A specific and extreme example of poor design decision would 

be the use of graphite in contact with Mg; similar to Mg, it is very lightweight and is very strong 

as a composite. The temptation can therefore be very great to use the two materials together in 

the design of new systems. The galvanic series (38) for metals, in figure 29, shows a voltage gap 

of nearly 2 V between Mg and graphite and clearly highlights the need for vigilance on the part 

of designers with regards to materials selection in conjunction with Mg alloys. 
26

Figure 29. Galvanic series showing approximately 1.8-V gap between Mg

and graphite with other metallic materials between (37, 38)

(Reprinted, with permission, from ASTM G82-98 (2014) Standard 

Guide for Development and Use of a Galvanic Series for 

Predicting Galvanic Corrosion Performance, copyright ASTM 

International, 100 Barr Harbor Drive, West Conshohocken, PA 

19428. A copy of the complete standard may be obtained from 

ASTM International, http://www.astm.org.

Increased corrosion resistance of alloys such as AZ31B-H24, AZ91C-T6, WE43B-T5, EV31A, 

and the SWAP-produced alloys, are encouraging signs of increasing quality, research, and 

innovation directed at a wide variety of lightweight and durable products and applications. It is 
27

the responsibility of design engineers to ensure safety and environmental robustness of Mg-based 

products through proper materials selection and electrical and moisture isolation of the Mg from 

all dissimilar metals.

5. Conclusions

• Overall, there was good agreement between the NSF and GM 9540P in the order of the 

mpy performance of the very best and the very worst of the alloys; however, results were 

mixed amongst the intermediate performing alloys.

• The excellent performance of the AZ91C-T6 was most likely due to a no-cost “upgrade” to 

AZ91E-T6.

• Avoidance of Fe, Co, Ni, and Cu, as seen in the AZ91C-T6, results in significantly 

improved corrosion resistance and is a good practice to follow for other Mg alloys.

• In addition to AZ31B, the rare-earth-based alloys EV31A and WE43 exhibit a good 

balance of corrosion resistance and mechanical properties.

• The SWAP-produced alloys AMX602 and ZAXE1711 provided encouraging results both 

mechanically and for corrosion resistance. Further studies and development of up-scaled 

plates are encouraged.
28

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International: Houston, TX, 2010.
29

12. ASTM B92/B92M-07. Standard Specification for Unalloyed Magnesium Ingot and Stick for 

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for Armor Development; ARL-TR-4828; U.S. Army Research Laboratory: Aberdeen 

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Precipitation Heat Treated; SAE Aerospace Material Specification AMS4429; Society of 

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30

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31

1 DEFENSE TECHNICAL

(PDF) INFORMATION CTR

DTIC OCA

2 DIRECTOR

(PDF) US ARMY RESEARCH LAB

RDRL CIO LL

IMAL HRA MAIL & RECORDS MGMT

1 GOVT PRINTG OFC

(PDF) A MALHOTRA

1 RDRL ROE M

 (PDF) S MATHAUDHU

1 CORROSION POLICY AND

(PDF) OVERSIGHT OFC OF THE

UNDER SECRETARY OF DEFNS FOR

ACQUISITION TECHLGY AND 

LOGISTICS

R HAYS

2 UNIV OF VIRGINIA

(PDF) R KELLY

J SCULLY

1 MONASH UNIV

(PDF) N BIRBILIS

2 OHIO STATE UNIV

(PDF) R BUCHHEIT

G FRANKEL

1 US NAVAL AIR WARFARE CMND

(PDF V SANTIAGO

C MATZDORF

1 BAE SYSTEMS

(PDF) J DORSCH

1 GENERAL DYNAMICS

(PDF) L KNISLEY

5 DIR USARL

(PDF) RDRL WMM C

J LABUKAS

C MILLER

B PLACZANKIS

RDRL WMM D

K CHO

RDRL WMP E

T JONES
32

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