The surface morphology and elemental composition of 316 stainless steel (316 SS) exposed in the solution consisting of acetic acid (HAc)

sulfuric acid

butyl acetate

and water with or without molybdophosphoric acid (HPMo) at 105℃ were investigated by using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).The nature of passive film forming and mechanism of corrosion inhibiting were discussed.The results showed that HPMo by means of physical absorption formed a protective layer on the surface of 316 SS

where redox reaction took place.The reaction products of hydrates

oxides

and insoluble salts deposited on the surface of 316 SS and constituted a compact protective passive film

which covered the surface active sites and retarded expanding of local corrosion and pitting.According to the bipolar model

a bipolar film consisted of a cation selective outer layer containing PMo12O403-

MoO42-

and CrO42- and an anion selectivity inner layer containing oxides.Aggressive anion SO42- from the solution was difficult to diffuse into the passive film

and cation from base metal was prevented from migrating into the solution.Therefore

the local corrosion was further inhibited.