Keeping Corrosion at Bay

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According to the U.S. Energy Information Administration, Americans pumped about 143 billion gallons of gasoline into their vehicles in 2017. And the prices are rising - as we close out 2018, each gallon of gas costs on average $2.86, up from $2.41 in 2017. Incorporating lighter-weight materials, such as magnesium alloys, in newly manufactured automobile designs would help improve fuel economy. Indeed, the automotive industry is looking at multi- material solutions in its manufacturing processes to drive down vehicle weight. But first, the automotive industry must overcome challenges with incorporating magnesium alloys-specifically with respect to material joining and galvanic corrosion, especially when the alloys are coupled with dissimilar materials.External appearance of the bare AZ31 coupons after the ASTM B117 salt spray (fog) test at 0 hours and at 1200 hours.

Researchers at Pacific Northwest National Laboratory have joined forces with automotive supplier Magna to develop strategies for joining magnesium with magnesium/dissimilar metals that will meet structural requirements-while also mitigating corrosion and achieving a Class-A (e.g., glossy and defect free) surface finish. Environmental corrosion chamber tests and microstructural characterization are used to evaluate corrosion behavior of joints made of magnesium, magnesium/magnesium, magnesium/aluminum, and magnesium/steel. The team assessed a base magnesium material (AZ31) as well as two proprietary coating-based protection schemes-Henkel pretreatment (HP) and Henkel pretreatment + E coated (HPEC). A comprehensive test matrix was developed for testing 144 coupons with similar joints and 96 coupons with dissimilar joints. Corrosion behavior testing was performed using the ASTM B117 salt spray (fog) test method-where the team retrieves coupons at regular intervals, gently washes them with tap water, and immediately dries the coupons. Then the coupons are measured to determine change in weight due to corrosion buildup. For microstructural characterization, the team prepared a cross sections of corrosion tested coupons, using a variety of imaging and analytical techniques to distinguish corrosion products, protective coatings, and AZ31.

In the first year of this two-year project, the team evaluated the corrosion behavior of the base AZ31 in three conditions - uncoated, HP, and HPEC - using the ASTM B117 salt spray (fog) method for up to 1500 hours. The team noted that the uncoated AZ31 was severely corroded, while the HP and HPEC coupons both showed minimal to no corrosion. The HP coupons showed some porosity and loss of original shine, but the coating protected the underlying AZ31 substrate. The topmost layer (E-coat) of the HPEC coupons was shown to be pore-free with almost no degradation, although after 1500 hours of testing some cracks were observed at the AZ31 base metal/HP-layer interface. Thus, the team determined that both protection schemes (HP and HPEC) protected the AZ31 from corrosion. In the upcoming second year, the team will perform similar corrosion testing on magnesium-similar and –dissimilar joints and will evaluate the interactions among the joint-type and coating schemes. The team used an extensive suite of advanced electron microscopy and microstructural characterization tools to quantify the rate of corrosion and evolution of corrosion product in order to compare the effects of various protective coatings on magnesium sheet metals. The PNNL team has decades of experience in analyzing corrosion in metals and developing strategies to mitigate corrosion that degrades service life of metals.

By investigating strategies for mitigating corrosion on magnesium alloys with protective coatings, PNNL and its automotive supply partner will make available to industry lighter-weight, yet durable, components for future vehicle manufacturing.