In-Situ Detection of Early Corrosion of Ferritic Cr-Mo Steel in Aqueous Solutions of different Anions using Laser Raman Spectroscopy

We report the early-stage corrosion of modified 9Cr-1Mo ferritic steel in aqueous environments at natural corroding conditions. Uniform, pitting, and crevice corrosion was observed in acidic sulphate, neutral chloride, and fresh water environments, respectively. In-situ laser Raman spectroscopy (LRS) studies revealed the formation of Fe 3 O 4 , γ −Fe 2 O 3 , γ −FeOOH phases, and stable heterogeneous corrosion products of γ −FeOOH and α −FeOOH in all media, except in an alkaline solution. A stable passive film, composed of oxide and oxy-hydroxides of chromium and iron, is formed in an alkaline solution. X-ray photoelectron spectroscopy (XPS) results confirm the presence of Cr and Fe oxide and oxy-hydroxides in all corrosion products and enrichment of Mn and Nb oxides on the corroded surface in neutral chloride solution, but only Mn oxy-hydroxide in acidic solution. Chloride ion in the corroded surface in neutral chloride solution indicates a chloride-induced corrosion attack. In-situ LRS, together with ex-situ XPS enabled the identification of all corrosion products formed on modified 9Cr-1Mo steel. The presence of laminar γ −FeOOH and acicular α −FeOOH phases are confirmed from the FESEM images. Our results indicate that except in alkaline solution, the corrosive ions deteriorate the integrity of native film on modified 9Cr-1Mo steel.