Surface white spot and pitting corrosion of 316 L stainless steel

The purpose of this paper is to address the concern of some stainless steel users. To understand the effect of surface white spots on corrosion performance of stainless steel.

White spots appeared on some component surfaces made of 316 L stainless steel in some industrial applications. To address the concern about the pitting performance in the spot areas, the pitting corrosion potential and corrosion resistance were measured in the spot and non-spot areas by means of potentiodynamic polarization and electrochemical impedance spectroscopy and the two different surface characteristics were analytically compared by using optical microscopy, laser confocal microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy and auger energy spectroscopy. The results indicated that the pitting performance of the 316 L stainless steel was not negatively influenced by the spots and the white spots simply resulted from the slightly different surface morphology in the spot areas.

The white spots are actually the slightly rougher surface areas with some carbon-containing species. They do not reduce the pitting resistance. Interestingly, the white spot areas even have slightly improved general corrosion resistance.

Not all surface contamination or roughening can adversely affect the corrosion resistance of stainless steel.

Stainless steel components with such surface white spots are still qualified products in terms of corrosion performance.

The surface spot of stainless steel was systematically investigated for the first time for its effect on corrosion resistance and the conclusion was new to the common knowledge.