Assessment of Some Chromated and Non-Chromated 

Primers on Magnesium ZE41A, Aluminum 2024, and Steel 

4340 Via Electro-chemical Impedance Spectroscopy 

by John V. Kelley 

ARL-TR-3290 August 2004 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3290 August 2004 

Assessment of Some Chromated and Non-Chromated 

Primers on Magnesium ZE41A, Aluminum 2024, and Steel 

4340 Via Electro-chemical Impedance Spectroscopy 

John V. Kelley 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited.
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4. TITLE AND SUBTITLE 

 Assessment of Some Chromated and Non-Chromated Primers on Magnesium 

 ZE41A, Aluminum 2024, and Steel 4340 Via Electro-chemical Impedance 

 Spectroscopy 

5c. PROGRAM ELEMENT NUMBER 

5d. PROJECT NUMBER 

 622618-H80 

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6. AUTHOR(S) 

 John V. Kelley (ARL) 

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U.S. Army Research Laboratory 

Weapons and Materials Research Directorate 

Aberdeen Proving Ground, MD 21005-5069

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ARL-TR-3290

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Organic coating systems are one of the most efficient methods for protecting metals from corrosion, and the use of chromate 

primers vastly improves corrosion protection. However, environmental concerns about hexavalent chromium (Cr+6) toxicity 

have led to the development of many non-chromate coatings. In recent years, water-borne coatings have emerged as viable 

alternatives and provide the additional advantage of having low content of volatile organic compounds. Since the performance 

of chromated coatings tends to exceed that of their environmentally friendly counterparts, evaluations of some non-chromate 

alternatives need to be performed. This report compares the performance of three military primers: MIL-P-23377C 

(chromated), MIL-P-85582C (chromated), and MIL-P-85582N (non-chromated) on three common military alloys. 

15. SUBJECT TERMS

 chromated primers, coatings; magnesium corrosion; ZE41A 

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19a. NAME OF RESPONSIBLE PERSON

 John V. Kelley 

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iii

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Experimental Procedure 2

2.1 Test Coupon/Panel Preparation (3) .................................................................................2 

2.2 Coating Thickness Measurements...................................................................................2 

2.3 Electro-chemical Evaluation ...........................................................................................3 

3. Results and Discussion 4

3.1 Aluminum Alloy 2024-T3...............................................................................................5 

3.2 4340 Steel........................................................................................................................5 

3.3 Magnesium Alloy ZE41A ...............................................................................................6 

4. Conclusions 9

5. References 10

Distribution List 11
iv

List of Figures 

Figure 1. Schematic representation of a 3-inch × 5-inch test panel with an X indicating the 

approximate locations of each thickness measurement .............................................................3 

Figure 2. Electro-chemical corrosion cell configuration used for AC impedance 

measurements.............................................................................................................................4 

Figure 3. Low frequency impedance values versus exposure time in 0.5 N NaCl solution of 

primer- coated aluminum alloy 2024-T3 ...................................................................................5 

Figure 4. Low frequency impedance values versus exposure time in 0.5 N NaCl solution of 

primer-coated 4340 steel............................................................................................................6 

Figure 5. Area under corrosion cell of 4340 after 53 days of exposure: a) chromated primer 

23377C, b) chromate primer 85582C, c) non-chromate primer 85582N ..................................7 

Figure 6. Low frequency impedance values versus exposure time in 0.5 N NaCl solution of 

primer-coated ZE41A-T5 magnesium alloy ..............................................................................8 

Figure 7. Area under corrosion cell of ZE41A after 53 days of exposure: a) chromated 

primer 23377C, b) chromate primer 85582C, c) non-chromate primer 85582N.......................8 

List of Tables 

Table 1. Sample identification and thickness measurements..........................................................3 
1

1. Introduction 

Organic coating systems have been one of the most efficient ways of protecting metal substrates 

from corrosion. The use of chromated primers in these systems vastly improves the corrosion 

protection. However, because of environmental concerns about hexavalent chromium (Cr+6) 

and because of stricter environmental regulations, many non-chromated coatings have been 

developed and are now being investigated for use on military systems. Water-borne epoxy 

coatings are being considered as a viable alternative to chromated coatings. A water-borne 

epoxy system comes with additional advantages such as lower content of volatile organic 

compounds, faster curing times, and easier cleaning, among others (1). 

Although there are many environmental benefits for changing to the non-chromated primers, 

historically, the corrosion protection of these coatings tends to fall short of those containing 

chromate. Therefore, the goal of this study is to examine the performance of three military 

primers: MIL-P-23377C (chromated), MIL-P-85577C (chromated), and MIL-P-85577N (nonchromated) on three alloys that are commonly used in military systems: magnesium alloy 

ZE41A, aluminum alloy 2024-T3, and vacuum arc re-melted (VAR) 4340 steel. Specifically, 

electro-chemical impedance spectroscopy (EIS) was used to monitor and compare the corrosion 

behavior of the chromated versus non-chromated primers. 

The three alloys in this study have very different corrosion behavior in chloride environments. 

The most sensitive is the magnesium alloy ZE41A. It has been reported that this alloy can 

corrode at a rate greater than 400 mils per year (mpy) (2). Magnesium alloys are typically used 

in the automotive industry in an effort to lighten automobiles, thus increasing gas mileage. The 

primary uses for magnesium alloys are for static structures such as supporting brackets and 

machine and transmission housings. Similarly, these alloys have found their way into some 

aerospace applications. Because magnesium alloys are lightweight and exhibit good strength 

and stiffness at room and elevated temperatures, a great deal of weight can be saved when 

magnesium transmission housings are used on rotary winged aircraft, which illustrates the 

importance of having a protective coating for magnesium alloys. The steel and aluminum alloys 

were chosen as representatives of typical materials used for many Army materiel structural 

components. 
2

2. Experimental Procedure 

2.1 Test Coupon/Panel Preparation (3) 

Aluminum - Al2024-T3 samples were cleaned with an alkaline immersion tank cleaner (Turco 

4215 or equivalent), deoxidized, and then rinsed with tap water followed by a flowing deionized 

water rinse. Panels were then immersion tank cleaned and mechanically scrubbed with a ScotchBrite1 pad (commercial item description [CID] A-A-58054, Type I Grade B or equivalent) 

followed with a water rinse just before the coating was applied. 

Steel - 4340 panels were untreated (no cadmium plate) except that the surface was mechanically 

scrubbed with a Scotch-Brite pad (CID A-A-58054, Type I Grade B or equivalent) followed with 

a water rinse just before the coating was applied. 

Magnesium - ZE41A-T5 alloy panels were cleaned by vapor degreasing or solvent wipe with 

military performance specification (MIL-PRF)-680 Type II or IV followed by acetone and were 

then mechanically scrubbed with a Scotch-Brite pad (CID A-A-58054, Type I Grade B or 

equivalent) followed with a water rinse just before the coating was applied. 

All primers were applied in accordance with specifications MIL-P-23377 Type II Class C 

(control), MIL-PRF-85582 Type II Class C2, and MIL-PRF-85582 Type II Class N. Color-code 

383 olive drab green was used for panel top coating. The recommended primer coating thickness 

per the cited specifications was 0.6 to 0.9 mil. Primer was applied as one full wet coat to the 

specified coating thickness. 

2.2 Coating Thickness Measurements 

The thickness of the coating on each of the samples tested was measured with an Elcometer2

model 256FN eddy current thickness gauge. A total of five measurements was taken on each 

sample in the approximate locations shown in figure 1. The mean and standard deviations were 

recorded and are shown in table 1. 

 1 Scotch-Brite is a trademark of the 3M Company. 

2

 Elecometer® is a registered trademark of Elcometer, Inc. 
3

Figure 1. Schematic representation of a 3-inch 

by 5-inch test panel with an X indicating the approximate locations of 

each thickness measurement. 

Table 1. Sample identification and thickness measurements. 

Sample ID Substrate Material Primer (MIL-P- ) Mean Coating 

Thickness (mils) 

Standard Deviation 

(mils) 

33G4 Al 2024-T3 23377C 2 0.1 

36G4 Al 2024-T3 85582C 1.3 0 

39G1 Al 2024-T3 85582N 1.5 0 

34G4 4340 23377C 2.77 0.17 

37G4 4340 85582C 1.48 0.03 

40G2 4340 85582N 1.69 0.07 

35G2 ZE41A-T5 23377C 2.4 0.2 

38G2 ZE41A-T5 85582C 1.8 0.4 

41G2 ZE41A-T5 85582N 2.9 0.7 

2.3 Electro-chemical Evaluation 

EIS was employed to measure coating integrity over time. The apparatus used consisted of a 

Princeton Applied Research Model 283 potentiostat, a Schlumberger Model 1255 frequency 

response analyzer, and a Panasonic Model CF-71 laptop computer. The software package ZPlot3

was used for data acquisition and ZView4

 for data analysis. Periodic measurements were taken 

at 2, 26, 46, 168, 480, and 672 hours of continuous immersion exposure with the cell configuration illustrated in figure 2. The cell was filled with 25 mL of 0.5 normal (N) sodium chloride 

(NaCl) solution and allowed to equilibrate for at least 2 hours before measurements were taken 

at the corrosion potential of the sample over the frequency range of 100 KHz to 0.01 Hz. The 

single sine technique was used, with an applied amplitude of 5 mV. 

 3ZPlot® is a registered trademark of Scribner Association. 

4

ZViewis a trademark of Scribner Association. 
4

Figure 2. Electro-chemical corrosion cell configuration used for alternating 

current impedance measurements. 

The maximum impedance at low frequency can distinguish between good and poor coatings (4). 

Therefore, the total impedance of the specimen, defined as the log | Z | value at 10 mHz in the 

Bode magnitude plot5, was plotted as a function of exposure time for comparison. 

3. Results and Discussion 

The mean coating thickness of each sample represented in the EIS data is presented in table 1. 

All the primer thicknesses are greater than the recommended thickness cited in the specification. 

Some were as much as 3 times the recommended thickness. The low standard deviation 

indicates that the coatings on each of the aluminum 2024 samples were the most consistent. It is 

also worth noting that the 4340 sample with primer 23377C was at least 63% thicker than either 

of the 85582 formulations on steel. Similarly, the 2024 sample with primer 23377C was more 

than 30% thicker than the other two primers on aluminum. 

The surface condition of the ZE41A samples was much rougher than the other two alloys. 

Therefore, 10 measurements rather than 5 were taken on each sample in an attempt to gain more 

accurate measurements. However, there was no significant change in the standard deviation of 

these measurements. 

 5

A Bode plot is a standard way of graphically reporting electrochemical impedance values versus frequency. 
5

3.1 Aluminum Alloy 2024-T3 

The relative corrosion resistance of the aluminum alloy is evident in the EIS data presented in 

figure 3. As expected, the impedance value of the MIL-P-23377C primer remained in the 106 to 

107

 range. This supports the results of similar work previously reported by the U.S. Army 

Research Laboratory (5). The impedance values of the aluminum samples remain higher 

throughout the exposure period than those of either the steel or magnesium alloy. Although the 

performance of the 23377C chromated primer exceeded that of the other two primers on all 

alloys, the difference was not as significant in the case of the aluminum alloys as it was with the 

other substrates. After 46 hours of exposure, all three primers remained within an order of 

magnitude of each other, with the non-chromated 85582N exhibiting a slightly higher impedance 

value than the chromated 85582C. 

Aluminum 2024-T3 in 0.5 N NaCl

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

0 100 200 300 400 500 600 700 800

Exposure Time (hours)

Impedance |Z|

23377C

85582C

85582N

Figure 3. Low frequency impedance values versus exposure time in 0.5 N NaCl solution of primercoated aluminum alloy 2024-T3. 

3.2 4340 Steel 

The performance of the 23377C chromated primer sample significantly exceeded that of both 

85582C and 85582N on the steel samples, providing better corrosion protection as indicated by Z 

values that were several orders of magnitude higher. No red corrosion products were observed 

on this sample until the test was terminated and the cell removed. Conversely, corrosion was 
6

soon evident on the other two primer samples. In the case of 85582C, blistering was observed at 

approximately 46 hours of exposure. This coincides with the drop in impedance at that point. 

While no blistering was noticed at this interval on the 85582N sample, the poor performance of 

both the chromated and non-chromated 85582 primers on steel (see figure 4) indicates that both 

coatings were compromised at this point. Their performances were nearly identical and there 

appears to be no benefit to using 85582C (chromated) instead of 85582N (non-chromated) on 

4340 steel. It is also likely that the 23377C sample benefited from a coating 63% thicker than 

either the 85582C or 85582C primers. 

4340 Steel in 0.5 N NaCl

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

0 20 40 60 80 100 120 140 160 180

Exposure Time (hours)

Impedance |Z|

23377C

85582C

85582N

Figure 4. Low frequency impedance values versus exposure time in 0.5 N NaCl solution of primer-coated 

4340 steel. 

However, when the corroded surfaces in figure 5 are examined, a difference in the form of 

corrosion is clearly seen with the two 85582 primers. The chromated primer exhibited blisters 

that grew in number and size as exposure continued. The non-chromated primer appeared to 

erode somewhat uniformly before a blister was observed. 

3.3 Magnesium Alloy ZE41A 

It was anticipated that the primers on all the ZE41A magnesium alloy samples would perform 

inadequately for two reasons: first, historically, magnesium alloy ZE41A is very susceptible to 

corrosion. It has been reported that ZE41A corrodes at a rate greater than 400 mpy, making it 
7

difficult to protect with only a primer. Secondly, because the ZE41A panels were sand casted, 

the surface roughness created some additional challenges. The roughness of some of the samples 

was so great that it was difficult to obtain accurate coating thickness measurements. Also, an 

extra effort had to be made in order for the corrosion cell (figure 2) to be successfully attached to 

the panel without leakage. Most importantly, however, the peaks and valleys of a rough as-cast 

surface lead to a nonuniform coating thickness. Coatings tend to be thinner and thus more 

vulnerable to corrosion initiation at the peaks. Since impedance is proportional to the coating 

thickness, the thinner regions at the peaks are reflected in the lower impedance measurements 

(see figure 6). The ZE41A panels with either 85582C or 85582N began actively corroding 

immediately upon exposure to the 0.5 N NaCl solution. The test was terminated after 3 hours for 

these two primers. Chang et al. (2) reports considerably different results for ZE41A using the 

same test parameters. Chang’s samples were prepared with a 1-mil-thick coating system 

consisting of a Dow 17 pre-treatment and a Sermetel 1083 primer. These samples endured 

almost 100 days with an impedance value above 107

 ohms-cm2 while the best ZE41A sample in 

this experiment, primer 23377C, was at 105 ohms-cm2, well below 107. It maintained the highest 

impedance of the three primers tested on ZE41A, however. Although this is not meant to be a 

direct comparison, it may demonstrate the importance of using a good pretreatment on 

magnesium alloys. 

 a b c 

Figure 5. Area under corrosion cell of 4340 after 53 days of exposure: a) chromated primer 23377C, 

b) chromated primer 85582C, c) non-chromated primer 85582N. 

Exposure to the 0.5 N NaCl solution was allowed to continue to 53 days. Figure 7 shows the 

significant damage sustained by all the ZE41A samples. The yellow arrow in the middle 

photograph points to a spot where a hole was created when the ZE41A corroded through to the 

back side, which caused solution to leak out of the cell. It can be seen in figure 7c that there was 

considerable corrosion around the perimeter, which caused leakage underneath the o-ring. 
8

Magnesium Alloy ZE41A-T5 in 0.5 N NaCl

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

0 5 10 15 20 25 30

Exposure Time (hours)

Impedance |Z|

23377C

85582C

85582N

Figure 6. Low frequency impedance values versus exposure time in 0.5 N NaCl solution of primer-coated 

ZE41A-T5 magnesium alloy. 

 a b c 

Figure 7. Area under corrosion cell of ZE41A after 53 days of exposure: a) chromated primer 23377C, 

b) chromated primer 85582C, c) non-chromated primer 85582N. 
9

4. Conclusions 

 1. MIL-P-23377C provided the best initial corrosion protection across all three alloys 

tested. However, all primers on Al 2024 provided similar protection beyond 46 hours of 

immersion in 0.5 N NaCl. 

 2. Neither of the MIL-P-85582 primer formulations proved to be an effective alternative 

to MIL-P-23377C on either the VAR 4340 steel or the ZE41A magnesium. 

 3. Because the ZE41A magnesium was tested without a pre-treatment, the 0.5 N NaCl 

solution proved to be too aggressive, and all the coatings failed prematurely, illustrating the 

value of pre-treatments on magnesium alloys. 
10

5. References 

1. Chiang, R.; Athanasopoulos, C. Nonchromate, “Low Density, Waterborne Epoxy Primer,” 

Proceedings of the Triservice Conference on Corrosion, June 1994, 21-23, 513. 

2. Bartak, D. E. “A Protective Anodic Coating for Magnesium Alloys,” Proceedings of the 

Triservice Conference on Corrosion, May 1992, editor M Levy, 215. 

3. Helton, T. Procedure supplied by Senior Analyst, Titan Systems Corporation: Redstone 

Arsenal, AL 35898. 

4. Chang, F.; Huie, R.; Placzankis, B. “Assessment of Some Advanced Protective Schemes, 

Including Chromate and Non-Chromate Conversion Coatings for Mg Alloy ZE41A-T5 

Using Electrochemical Impedance Spectroscopy,” Proceedings of the Triservice Conference 

on Corrosion, June 1994, 549. 

5. Chang, F.; Levy, M.; Huie, R. “Assessment of Chromate and Non-Chromate Conversion 

Coatings for Al Alloys Using Electrochemical Impedance Spectroscopy,” Proceedings of the 

Triservice Conference on Corrosion, May 1992, editor M Levy, 181. 
11

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