Assessment of Cyclic Corrosion Test Protocols for Magnesium Substrates

Historically, accelerated cyclic corrosion test protocols utilized by original equipment manufacturers (OEMs) have been developed based on a great knowledge and abundant vehicle field data of primarily steel-containing vehicle components. Laboratory-accelerated cyclic corrosion tests with repeated cycles of wet, dry, humid, and/or corrosive media application have been developed both separately and in partnerships, such as in the case of SAE J2334, to simulate a severe corrosive field environment for evaluation of cosmetic corrosion performance of painted steel. With the interest in lightweight metals such as aluminum alloys and magnesium alloys in automotive applications, the validity and confidence of these accelerated test protocols with vehicle field data of lightweight metals is valuable to further increasing the usage of these metals. Over the last several years, Meridian has completed a long-term assessment of the corrosion performance of as-cast and powder-coated die-cast magnesium in an underbody vehicular environment. The corrosion performance of similarly prepared samples exposed to accelerated laboratory test protocols was also completed. The work presented here reviews the specifications of several of these OEM accelerated laboratory cyclic corrosion protocols and the results of as-cast and powder-coated magnesium substrates exposed to these protocols, as well as comparisons to die-cast magnesium in an underbody vehicular environment. To the author's knowledge, this work is the first published assessment of the corrosion performance of powder-coated magnesium exposed to an underbody vehicular environment and compared to accelerated laboratory test protocols. The results of this study discuss the range of exposure conditions between the accelerated test protocols and the range of observed conditions of the die-cast magnesium between the test protocols and are anticipated to encourage the development of corrosion testing protocols that include magnesium alloys as a consideration in multi-material testing and analysis.