515 Enhancement in Corrosion Resistance of Laser-Melted Stainless Steel by Passivation Treatment

Martensitic stainless steel AISI 440C was surface melted by a 2.5-kW CW Nd : YAG laser, followed by 50 vol-% nitric acid passivation treatment at 50 ℃ for further improvement in corrosion resistance. The corrosion behavior in 3.5% NaCl solution of various specimens before and after passivation was studied by potentiodynamic polarization technique. It was found that the pitting corrosion resistance of the laser-melted specimens was significantly improved by HNO_3-passivation as evidenced by a shift of the pitting potential in the noble direction, a wider passive range, and a lower passive current density. After passivation treatment, the laser-melted specimens possess higher pitting corrosion resistance than that of as-received and conventionally hardened 440C which were passivated under the same conditions. The pitting corrosion characteristics of the passivated 440C specimens were strongly dependent on the microstructures. The enhanced pitting corrosion resistance is due to the combined effect of dissolution or refinement of carbide particles by laser surface melting and removal of carbides inclusions and free iron by passivation treatment.