30852 (Prot Mg Against Corrosion)

Protection of Mg Alloys Against Galvanic and Other Forms of Corrosion

Wenyue Zhenga, C. Derushieb and Jason Loc

CANMET Materials Technology Laboratory, Natural Resources Canada, 568 Booth Street, Ottawa, Canada K1A 0G1

[email protected], [email protected], [email protected]

Keywords: automotive, structural application, galvanic corrosion, coating, conversion coating, stress corrosion, compatibility, cyclic salt-spray testing, AM 50/60, AE alloy.

Abstract. Galvanic corrosion is a particularly important form of corrosion for Mg alloys used in

automobiles. Our research work focuses on corrosion protection using cost-effective Cr-free

coatings. The top-tanking coatings are found to be effective in preventing general corrosion; some

of these coatings are also good for reducing galvanic corrosion and stress corrosion.

A practical approach for mitigating galvanic corrosion is to increase the electrolytic resistance

between the coated steel and the Mg surfaces. This has been demonstrated in the case of a

conversion coating plus a powder coat applied on the surface of a magnesium alloy and in the case

of a thin Mylar isolation layer installed between the Mg and the steel surfaces.

Introduction

Mg castings of appropriate compositions used in the automotive interior condition can resist well

general corrosion. In terms of general corrosion rates modern Mg alloys can perform quite

satisfactorily in clean atmospheric conditions. For example, in the study by Hillis etal. [1]

, the

measured corrosion rate of AZ91D was less than 4 micrometers per year. In this type of alloys, the

harmful impurity elements are Ni, Fe and Cu. To increase the resistance to corrosion in natural

environment, rare-earth additions have shown to be beneficial [2]

and the good corrosion resistance

of AE alloys, along with their increased creep resistance, has made them the top candidate materials

for many applications.

However, the automotive exterior conditions in cold-climate countries can be very corrosive due

to the use of de-icing salts on the road. The most challenging form of corrosion is galvanic

corrosion. Under the sponsorship of the USAMP (US DOE) and the Canadian Lightweight

Materials Research Initiative (CLIMRI), a research project was initiated at the Materials

Technology Laboratories (MTL) of CANMET to identify cost-effective measures for protecting

magnesium alloys used as structural components such as an engine cradle [3,4]

. The susceptibility of

Mg to stress-corrosion cracking (SCC) is also investigated; this part is relevant to a typical

structural application where certain localized sites may be subject to high stresses.

Evaluation of advanced Chromate-free coating systems

Coating Selection. Although chromate conversion coatings were widely used for Mg alloys used in

aggressive conditions, their future use in automobiles is limited due to the environmental and health

concerns. In this work, a total of eighteen different types of Cr-free coatings were selected [4]

. In

selecting a coating, consideration was given to the technical performance, commercial availability

as well as the effects of the coating materials on the recycling of the component [5]

.

Screening Test Results. ASTM B117 standard was used as a screening tool for the coated and

uncoated Mg samples [4]

. Visual examination of the test plates was performed at preset time

intervals. In the case of the scribed samples, the creeping of corrosion from the scribe line and

degree of disbondment underneath the coating were the criteria for assessing a coating. Based on

the ranking of the test samples after 1000 hours testing, the top coatings were selected for the

validation phase. These coating are: (1) Alodine 5200 with an epoxy powder coat and Magpass

Materials Science Forum Vols. 488-489 (2005) pp. 787-791online at http://www.scientific.net© (2005) Trans Tech Publications, Switzerland

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with an epoxy topcoat, (2) Tagnite anodizing with an epoxy topcoat, (3) Anomag anodizing with

Ecoat and (4) Polyurea topcoat

Alodine 5200 [6]

is an organometallic titanium

based primer used as a chromate-replacement

conversion coating. An epoxy powder coat was

used as the topcoat. Figure 1 shows a micrograph of

the cross-section cut of the Mg-coating interface,

which was taken with a focused ion beam

microscope as the conversion layer is less than 0.5

micrometers in thickness. Magpass by AHC is a

new Cr-free conversion coating product with a very

comparable performance. The application processes

of the various conversion coatings are by and large

similar.

Tagnite [7]

is a Cr-free equivalent of the Dow 17 and HAE anodizing coating. The coating

consists mostly of a hard magnesium oxide with minor surface deposition of fused silicates.

Anomag [8]

is another anodizing coating for magnesium. In this work the typical thickness is

between 10 and 15 micrometers; the latest version of Anomag and Tagnite coatings can be made

thinner.

Polyurea [9]

is a fast-curing polymer resulting from the reaction of an isocyanate prepolymer and a

blend of primary and secondary amines. The selection of this product for this project is based on its

good elasticity and adhesion to magnesium and to an epoxy-type topcoat. Although it can be quite

useful for protecting Mg alloy to a certain extent by itself, the main advantage of this coating is that

it can resist chipping and impact damage of the road sand and gravels when used as a topcoat.

Compatibility testing by electrochemical technique

Potentiodynamic polarization testing is a fast way to compare the galvanic compatibility of different

materials. A Solatron 1287 system controlled with a Corrware software was used. The solution

consisted of a 1 M NaCl solution with the pH adjusted to 11 by adding appropriate amount of 0.1 M

NaOH solution.

Figure 2 shows the polarization curves for A356, Mg

alloy AM60 and a new experimental alloys in a salt

solution. AM60 showed a corrosion potential of –1.5 V

(SCE) and the A356 sample showed a potential of about –

1.2 V (SCE). The new experimental Al-Mg alloy showed a

corrosion potential in the vicinity of that of the AM60

alloy. As the galvanic corrosion is driven by the difference

in the corrosion potential of the two coupling metals, it is

conceivable that the galvanic current flow between Mg

alloys and the new Al-Mg alloy would be much smaller

than that between Mg and A356. Another interesting

feature of this comparison is that in the more anodic

potential range the behavior of the new Al-Mg alloys is

very similar to that of A356, indicating a similarity in long-term corrosion resistance.

The LPR resistance of AM60 in 1 M NaCl solution is about 170 Ohms*cm2 and AA6061 alloy

had a resistance of 23,000 Ohms*cm2. The new alloy had a LPR resistance slightly greater than

that of AA 6061 but they are generally in the same range.

Galvanic corrosion testing Using the GM9540 Test Method

GM9540P was chosen as the method for evaluating the galvanic corrosion susceptibility of Mg

(coated and uncoated) fastened with M8 and M10 fasteners. A Singleton CCT-10C cyclic corrosion

Fig. 1 A micrograph of the cross-section of the Mg-

coating interface taken with a FIB microscope.

Fig. 2 Polarization measurements for

AM60, A356 and a new Al-Mg alloy

Magnesium – Science, Technology and Applications788

chamber was used [4]

. Each fastening unit consists of one bolt (M8 or M10), two washers and a hex

nut. The selection of washer materials and their coatings were based on the current industry

recommendations and published performance data [10,11,12]

. A number of in-house made prototype

washers made of the Al-Mg chemistries were also evaluated.

Test results

Bare Mg plate. Figure 3 shows the appearance of the uncoated AM60 plate after 40 cycles of

GM9540 testing. The effects of washers on the galvanic corrosion of Mg are demonstrated very

clearly. The fastener unit which did not contain a washer showed perforation by corrosion in the

vicinity of the bolt, whereas the units with a washer showed much reduced corrosion damage. The

effect of a thin (about 50 micrometer thickness) layer of Mylar sheet on preventing galvanic attack

is shown in Figure 4. In this case, an uncoated steel washer was bolted against the uncoated Mg

plate with the insulating Mylar sandwiched between them. After 40 cycles of testing, the nut was

removed and no sign of corrosion can be seen underneath the Mayler sheet.

On the bare magnesium test plates, the best results were observed with the anodized and sealed

AA6061 washers. A ring of corrosion product can be seen to occur immediately adjacent to the

Ti washer. The experimental alloy washer, Exp2091 showed little effects of galvanic corrosion.

Alodine 5200 with a powder coat [Fig. 5]. In comparison with the bare AM60 plate, the

performance of a coated Mg AM60 plate fastened with various combination of fastening units

showed no visible corrosion on the Mg plate after 40 cycles of testing. There were a few spots of

red-rust emerging on the duplex GM3359 coating on the bolt head itself, likely due to damage

during the application of the torque loading.

Magpass with a powder coat [Fig.6]. Similar results were observed on the test plate coated with

a Magpass conversion layer and then a powder coat as the topcoat, Figure 6.

Magoxide without any topcoat. This series in which anodized plates were bolted with various

Fig.3 Appearance of uncoated AM60 plate after 40

cycles of GM9540 testing.

Fig. 4 Effectiveness of a Mylar sheet under the

washer (bottom center) in preventing galvanic

attack on the Mg plate

Fig. 5 AM60 plate coated with Alodine5200

and a topcoat after 40 cycles of testing. Fig.6 AM60 plate coated with Magpass and

a topcoat after 40 cycles of testing.

Materials Science Forum Vols. 488-489 789

combinations of bolts and nuts showed,

surprisingly, that without an organic topcoat

the semi-conducting oxide layer produced by

anodizing was not sufficient to prevent

galvanic attack by the coated steel bolts.

Figure 7. Note that a good anodizing film such

as Magoxide can very effectively prevent

general corrosion in the environments such as

that in the GM9540 and B117 conditions.

Stress corrosion tests

Stress corrosion tests were carried out using

LPDC AM50 samples. One series of the

samples were coated with an Alodine 5200

conversion coating with an epoxy topcoat; the

other series was tested as-cast. The maximum

stress used was 130% of the yield stress and

the R value was 0.90 to take into account of

any possible load fluctuation in the practice.

The load frequency used was 1.0 Hz, and the

solution used was the GM9540 solution

composition.

The coated sample survived 2.4 million

cycles without the formation of any visible

cracks on the sample surface. On the other

hand, two uncoated samples showed

significant cracking after about 0.4 million

cycles. Figure 8 shows a picture of the LPDC

AM50 alloys containing stress-corrosion

cracks.

Hydrogen embrittlement is thought to be the responsible SCC mechanism in Mg alloys. The

resistance to SCC of the sample coated with Alodine 5200 may be related to the reduction in

hydrogen generation rendered by the presence of the conversion product. Some of the conversion

treatments are known to significantly reduce the formation of hydrogen gas on the Mg surface [13].

Summary and conclusions

Results from completed tasks of this on-going project are very encouraging for future wide-scale

application of Mg alloys as structural components. Specifically, the following conclusions can be

drawn at this stage:

1. Some of the commercially available coating systems for Mg alloys can satisfactorily protect Mg

alloys against aggressive testing conditions established by the industry. For example, chromate-

free conversion coatings, such as Alodine 5200 and Magpass with an epoxy powder coat as well

a sealed anodizing coating, can last sufficiently long to survive the 1000 hours in the ASTM

B117 test chamber.

2. Galvanic corrosion between fasteners and Mg alloys can be effectively mitigated though the use

of a high-resistance insulating layer that separates the magnesium from the cathode material. In

this work, a thin Mylar sheet sandwiched between a steel washer and a magnesium plate as well

as a layer of powder coating on top of a Mg conversion film could effectively prevent the

galvanic attack.

3. In the preliminary SCC tests conducted so far, AM50 samples coated with a conversion product

and a topcoat did not exhibits visible cracking even after 2.4 million load cycles in a GM9540

Fig. 7 Galvanic attack in along the washer on the

anodized Mg plate.

Fig. 8 A uncoated SCC test sample showing cracks

developed along the main fracture surface.

Magnesium – Science, Technology and Applications790

solution whereas uncoated samples showed significant cracking after about 400,000 cycles

under the same test conditions. The beneficial effect of the coating on SCC resistance is likely a

result of its influence in reducing hydrogen generation on the Mg surface.

Acknowledgement

The author(s) acknowledge that this research was supported, in part, by Department of Energy

Cooperative Agreement No. DE-FC05-02OR22910, although such support does not constitute an

endorsement by the Department of Energy of the views expressed herein. Support by the Canadian

Lightweight Materials Research Initiative (CLiMRI) is also acknowledged. The authors wish to

thank all SCMD project members who have provided materials, helpful suggestions and ideas in the

past three years, especially Dick Osborne of GM, The project manager for the SCMD work.

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