Comparative Corrosion Assessment of Coated Alloys for Multi-Material Lightweight Vehicle Architectures

Advanced characterization study of commercial conversion and electrocoating structures on magnesium alloys AZ31B and ZE10A

“…In single-and multi-layer coating concepts, the interface between the substrate alloy and the initial coating layer is critical to coating adherence and performance.Delamination or through attack in this region can allow the environment to access the underlying alloy and result in local corrosion and coating failure.Chemical conversion coatings are frequently used industrially as the first coating step after surface cleaning due to their relatively low cost and ease of implementation [17,19,29,30,33,38]. They are primarily used to provide a receptive surface for subsequent coating layer step adherence, but can also contribute to overall corrosion resistance of the multi-layer coating system [17,19,23,26]. Chromate conversion coatings provide the basis for good protection of…”

“…Such electrocoatings are one approach under consideration as the next protective layer above the conversion coating in multi-layered coating schemes used to protect Mg alloys [e.g. 22,23]. Conversion coatings and/or electrocoatings have been developed and successfully used to protect steels and aluminum, but are less well established for Mg alloys.…”

Magnesium Alloy Effects on Plasma Electrolytic Oxidation Electro-Ceramic and Electro-Coat Formation and Corrosion Resistance

“…Similar undercutting has been reported for multi-layer coatings on ZEK 100 using a similar initial PEO treatment as the present work. 15 In contrast, although lateral attack of the EC layer at the scribed material interface was observed for EC + Ecoated AZ91D and AZ31B, the alloy-EC interfaces remained intact with no undercutting observed, consistent with their greater degree of corrosion resistance after scribing. Further work will be needed to better understand the attack mechanisms and role of the substrate alloy in EC + E-coated Mg alloys, but the nature of the alloy-EC and EC-E-coat interfaces appears to be critical in determining the extent of corrosion susceptibility when scribing damage is introduced.…”

“…[13][14][15][16][17] These include an initial surface pre-treatment with chemical or electrochemical conversion coatings, surface alloying, or anodization primarily to improve adherence of subsequent coating layer(s), although they can also contribute to corrosion resistance; a second coating layer such as epoxy-base electro-deposition coatings, metal electro-platings, powder coatings, or organic coatings primarily to provide corrosion protection; frequently followed by a final layer(s) of sealant and/or paint to provide additional corrosion protection. [13][14][15][16][17][18][19][20][21][22][23][24][25] Plasma electrolytic oxidation (PEO), also referred to as micro-arc oxidation, is emerging as a key coating technique for the protection of Mg alloys, and is being investigated both as a single-coating in less-demanding environments and as the initial surface pre-treatment of a multi-layer coating system in more demanding environments. In the PEO process, the metal component to be coated is immersed in a liquid electrolyte and a high applied potential promotes chemical, thermal, and plasma reaction(s) to form ceramic surface layers(s) from the base metal.…”

“…As Mg alloys frequently require additional corrosion protection via a multi-layer coating scheme, EC-coated Mg alloys with and without a subsequent cathodic electro-coat epoxy-base overlayer (referred to as E-coat in the remainder of this paper for convenience) were studied. 14,15,34 Successful E-coat deposition for Mg alloys requires a surface pre-treatment such as chemical conversion or PEO EC coatings to promote adherence of the E-coat. The E-coat process is an electrophoretic deposition and can yield coatings tens of microns thick.…”