Sodium Phosphate Post-treatment on Al Coating: Morphological and Corrosion Study

Abstract

The coatings deposited by arc thermal spraying process possess defects and pores within the coating, which must be filled using a proper pore sealing agent. In the present study, we have deposited an Al coating on a steel substrate using arc thermal spraying. Post-treatment with different concentrations of sodium phosphate monobasic (NaH2PO4) salt solution was then carried out to fill the pores in the Al coating. The scanning electron microscopy results show that NaH2PO4 (SP) at 0.5 M concentration significantly reduces the porosity. When the concentration of NaH2PO4 is less than 0.5 M, the coating contains some pores and defects, whereas post-treatment with 1 M NaH2PO4 leads to the formation of a brittle film with wide cracks on the coating surface. The x-ray diffraction results of the treated coating confirm the formation of sodium aluminum hydrogen phosphate (Na3Al(OH)(HPO4)(PO4) film (SAHP). Electrochemical results obtained in artificial ocean water solution (ASTM D1141) show that treatment of the coating with 0.5 M NaH2PO4 enhanced the corrosion resistance properties for up to 7 days of exposure, but once the exposure periods are extended, the treated coatings exhibited deterioration because SAHP film started to dissolve in aggressive ions, i.e., artificial ocean water solution which makes the solution acidic owing to the formation of hydrogen phosphate and phosphoric acid. The AC sample shows protection after 60 days of exposure in artificial ocean water solution owing to the formation of protective, adherent, and uniform α-Al(OH)3 as corrosion products.