Influence of plasma electrolytic oxidation coating on reducing the corrosion rate of dissimilar FSW joint between AA5052 and AZ31 alloys by minimizing effect of different galvanic couple types

Abstract

This study investigated the different aspects of macro/micro-galvanic corrosion behavior of plasma electrolytic oxidation coating (PEO) applied on a dissimilar joint. In this regard, AA5052 and AZ31 were joined by friction stir welding. Then, the dissimilar joints were simultaneously coated with different electrolyte conductivities and current densities through PEO process. Despite of coated samples, the stir zone (SZ) of uncoated sample was severely corroded due to macro-galvanic couple (AA5052/AZ31) and micro-galvanic couple (intermetallic compound/surrounding matrix). In the optimal coated sample (CD-350), the corrosion current density of SZ compared with uncoated sample dropped from 8.18×10-4 to 1.69×10-5 A/cm2. The ratio between corrosion current density of SZ and that of uncoupled anode that is demonstrating the severity of galvanic effect; was less than 5 (icorr SZ/iuncoupled anode=4.17), therefore the various regions were totally compatible due to the effect of PEO coating on reducing different galvanic couple types. The corrosion rates for both heat-affected zone and base metal on each side of the coated samples were nearly identical, indicating the independence of the coating quality and properties from substrate microstructure. The findings revealed that rising electrolyte conductivity and current density led to the growth of coating thickness while the number of microspores decreased and their average diameter increased. Meanwhile, α-Al2O3, γ-Al2O3, and SiO2 amorphous formed on AA5052 side; and MgO, Mg2SiO4, and MgF2 on AZ31 side during coting process.