In situ atomic force microscope observation of initial stages of corrosion at 18Cr–8Ni stainless steel

Atomic force microscope (AFM) was applied to the investigation of the initial stages of corrosion at 18Cr–8Ni stainless steel. Surfaces of electrochemically or mechanically polished specimens were in situ observed by AFM in a 0.89 M NaCl solution or a 1 M buffered $Na2SO4$ solution while measuring potentiostatic polarization curves. The mechanical polishing resulted in a substantial increase in corrosion rates, and a dramatic change in surface morphology to a facet structure has been observed in a 0.89 M NaCl solution during an anodic potential sweep up to 0.5 V. The electrochemical polish or the anodic polarization in 1 M buffered $Na2SO4$ leads to a lower corrosion rate, and the transition from passive to transpassive corrosion states has been clearly detected by AFM as well as in potentiostatic polarization curves. The detailed in situ observation has revealed that the transpassive film is formed on the surface with an oriented rectangle morphology.