A New Method of Chromaticity Inspection for Evaluating Quality of Passive Film on Stainless Steel

A new method based on chromaticity reaction and color measurement was studied for passive film inspection during stainless steel manufacturing. The role of ferroxyl solution and phenanthroline solution during the inspection was compared and the relationship between the chromaticity measurement and electrochemical corrosion was explored. It is found that the lower the chromaticity measurement value is, the compacter the passive film is. The ferroxyl solution is sensitive to identify the passive films on different types of stainless steels, but it degrades the surface quality of stainless steels and is not stable during storage. The phenanthroline solution can effectively identify the passive film under various passive conditions, while it has no influence on the surface of stainless steel and also it is stable enough for long term storage. A good corrosion resistance of stainless steel is realized when the measured a* is below 3 by using phenanthroline solution, while relative low corrosion resistance is exposed at the range of a*>3.