Enhanced corrosion resistance properties of 15Al-85Zn coating by post-treatment with sodium hexa-meta phosphate in saline solution

Abstract

Different amounts of sodium hexametaphosphate (SHMP: Na6[(PO3)6]) has been used to fill the defects and pores of arc thermal sprayed 15Al-85Zn coating. 73.39% porosity is reduced by 0.1 M SHMP post-treated coating due to formation of hydrogen zinc phosphate hydrate (HZP: HZn4(PO4)3H2O). The corrosion resistance of post-treated coating is increased in 3.5 wt% NaCl solution with immersion periods attributed to the transformation of HZP into zinc phosphate (Zn3(PO4)2) as well as active Zn react with Cl- and form simonkolleite (Zn5(OH)8Cl2·H2O). Simonkolleite and zinc phosphate are sparingly soluble, therefore, post-treated coating provides two folds’ protection by HZP and corrosion products. © 2023 Elsevier Ltd