IMPROVING PITTING CORROSION OF ۳۰۴ STAINLESS STEEL BY ELECTROPOLISHING TECHNIQUE

Several surface modification techniques such as ion implantation, surface laser melting, have been employed to improve pitting corrosion resistance of stainless steel. Electropolishing is a technique in which the surface roughness is eliminated through a selective electrochemical dissolution. The effect of electropolishing on pitting corrosion of ۳۰۴ stainless steel (SS) was investigated employing polarization technique in conjunction with the scanning electron microcopy examination. Electropolishing process was carried out on wire of ۲ mm diameter in ۷۰% phosphoric acid solution at room temperature for ۳۰ min. To elucidate the effect of roughness elimination on pitting corrosion, investigation was carried out on as-received specimen with surface finishing of ۶۰ SiC grit and electropolished specimen in ۰.۵M NaCl solution at room temperature. A significant decrease on passive current density and also shift of pitting potential towards noble value was recorded on electropolished specimen revealing a pronounce effect of this technique on surface modification. Further investigation was carried out by employing slow ramp anodic potentiodynamic polarization on as received and electropolished specimen. Plot of metastable pitting current transient revealed the reduction on the number and magnitude of metastable pitting transients prior to occurrence of stable pitting on electropolished specimen. EDX analysis of the surface area of as received and electropolished specimens showed modification in surface roughness during electropolishing was the main reason of pitting corrosion improvement. Scanning microscopy investigation of polarized specimens beyond the pitting potential revealed that in as-receives specimen pits were nucleated in at and in the vicinity of surface scratches that was created during surface abrading