An effective and novel pore sealing agent to enhance the corrosion resistance performance of Al coating in artificial ocean water

Abstract

A new technique was accepted to fill the porosity of Al coating applied by arc thermal spray process to enhance corrosion resistance performance in artificial ocean water. The porosity is the inherent property of arc thermal spray coating process. In this study, applied coating was treated with different concentrations of ammonium phosphate mono basic (NH 4 H 2 PO 4: AP) solution thereafter dried at room temperature and kept in humidity chamber for 7d to deposit uniform film. The corrosion resistance of Al coating and treated samples have been evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic techniques with exposure periods in artificial ocean water. Electrochemical techniques, X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM) and field emission-scanning electron microscopy (FE-SEM) indicated that phosphate ion would have been retarding corrosion of Al coating effectively. The formation of AHP (Ammonium Aluminum Hydrogen Phosphate Hydrate: NH 4) 3 Al 5 H 6 (PO 4) 8.18H 2 O) on Al coating surface after treatment with AP is nano sized, crystalline and uniformly deposited but after exposure them in artificial ocean water, they form AHPH (Aluminum hydroxide phosphate hydrate Al 3 (PO 4) 2 (OH) 3 (H 2 O) 5) that is very protective, adherent, uniform and plate like morphology of corrosion products. The AHPH is sparingly soluble and adherent to surface and imparted improved corrosion resistance.