Dissimilar metal joints on macro- and micro scales: Impact on PEO processing-A review

Plasma electrolytic oxidation (PEO) processing of light metals has been established for decades and is in increasing industrial use, even as an alternative surface treatment to produce multifunctional coatings with environmental-friendly processing concept. One of the benefits of PEO processing claimed already a couple of years ago was the ability to treat dissimilar metal joints, which can obviously improve the surface homogeneity and stability at the interface of the dissimilar components, especially impeding the galvanic corrosion due to the different electrochemical properties of each component. However, the progress and breakthrough develop slowly especially for the macro scales due to the much larger gap between each component. This literature review firstly demonstrates the still low number of studies reporting successful PEO treatment of material combination such as Mg/Al, Mg/Ti, Al/Ti and scarcely light metal combinations with steel. The main issues and challenges to performing PEO processing on the macro-scale dissimilar weldments were stated. On the other hand, dissimilar metal joints also widely exist in micrometer scale in alloys and metal matrix composites (MMCs). Moreover, there is a huge knowledge base on PEO treatment of such multiphase substrates. PEO processing of such complicated mixed microstructures is reviewed as well to reveal the basic problems. To some certain degree, these PEO-related studies on alloys and MMCs can be good examples to have an insight into the coating formation mechanism on macro-scaled dissimilar metal joints. Conclusions are drawn from the micro- to macroscale. Finally, critical access to the problems is given and possible solutions and reaming limitations are discussed.