Effect of laser surface melting on the corrosion resistance of chromium-plated 9Cr-1Mo ferritic steel in an acidic medium

Abstract

9Cr-1 Mo ferritic steel forms an integral part of some of the nuclear power generating industries where it is used as the steam generating material. Its corrosion resistance could be further improved by employing a chromium coating over it. However, this chromium coating has been found to be unsatisfactory owing to the microcracks present in the coating. Laser surface melting (LSM) could be effectively used not only to remove these microcracks but also to form a better corrosion-resistant modified surface without affecting the bulk properties of the material. Studies were carried out on the laser surface melted chromium-plated 9Cr-1Mo steel. The specimens with chromium deposit thicknesses ranging from 30–70 μm were prepared and then laser irradiated. Optical microscopic studies on the cross-sections of these specimens revealed an average laser-melted thickness of around 0.1–0.5 mm, depending upon the irradiation parameter used. Aqueous corrosion behaviour of these specimens was studied by anodic polarization in 1N H₂SO₄ medium. Anodic polarization experiments were carried out for specimens after repolishing the same specimen until the 9Cr-1Mo base metal was reached. The passive and peak current density values, range of passivity, peak and transpassive potentials, were determined at each stage of polishing, and these were compared with those of pure chromium metal as well as 9Cr-1Mo alloy in the same medium. Observation of these data indicates that the laser surface melting could be beneficial in raising the aqueous corrosion resistance of such chromium-coated steels, to a level comparable with that of the pure chromium metal.