Deciphering the role and nature of phosphate species at the surface of stainless steel immersed in phosphoric acid solutions

摘要

Graphical abstractDisplay OmittedHighlights•In phosphoric acid solution, chlorides strongly influence the passive/active behavior of a stainless steel (SS).•From XPS data, a discrimination can be made between organic oxygen (adventitious contamination) and inorganic oxygen (oxides, phosphates).•An estimate of the oxygen due to phosphate species reveals the formation of a polyphosphate layer at the SS surface.•The polyphosphate layer plays a pivotal role on the stability of the passive film.AbstractWe investigate the evolution of the surface of a highly alloyed stainless steel (Sanicro 28) upon immersion in aqueous phosphoric acid solutions. For this purpose, both short- (fewhours) and long-term immersion (several days) were carried out. A detailed analysis of XPS spectra allowed a distinction to be made between oxygen originating from the organic adlayer (adventitious contamination), the passive oxide layer, and adsorbed phosphate species. By estimating the fraction of oxygen due to phosphate species (Oph), it was shown that theOph/Pmolar concentration ratio was ranging from about 2 to 3. This suggests the presence of a polyphosphate layer at the stainless steel surface, as also supported by Raman analysis, which influence the electrochemical behavior of SS in the acidic media.