Relative mass content of oxidized (valence state > 0) metals (wt%) in the outermost surface oxide, measured by means of XPS, of as-received austenitic stainless steel grades (AISI 201, 204, 316L, and 304), a lean duplex grade (EN 1.4162), and ferritic grades (EN 1.4003 and AISI 430 [only exposed to citric acid]) prior to (shown as 0) and after exposure (for 2, 26, and 240 h) in (a) citric acid (5 g/L, pH 2.4), and (b) in artificial tap water (pH 7.5). All exposures were conducted at 70°C for the first 2 h followed by exposures at 40°C for the remaining time periods. No oxidized nickel was observed in the outermost surface oxide.