An ultrathin corrosion-resistant film on a steel surface formed by dipping in a tungstate solution

Abstract

The surfaces of steel specimens after dipping in a solution with or without tungstate addition, followed by a corrosion test, were investigated by various techniques, e.g., cross-sectional Cs-(S)TEM, SEM and XPS, to gain an improved understanding of the role of tungstate in corrosion resistance. The corrosion reaction during the test was drastically suppressed when tungstate was added. A crystalline film approximately 4 nm thick consisting of Fe and O was formed on the surface after dipping in the solution without tungstate. After dipping in the solution with tungstate, a film consisting of Fe, O and W was formed with the thickness of about 4 nm. The latter film consisted of two layers; the outer layer showed lower crystallinity and a higher W concentration. This film improved corrosion resistance. It is supposed that (1) barrier properties are improved by the low crystalline layer or (2) W6+ in the film inhibits the corrosion reaction. The WO2-4 in the solution also protected the surface during the test. When the ultrathin film is destroyed, WO2-4 reacts with iron ions eluted from corrosion pits to form thicker protective films and pit propagation is also suppressed by segregation of W6+ in the corrosion products above pits.