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

•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.