Using Atomic Force Microscopy to Measure Thickness of Passive Film on Stainless Steel Immersed in Aqueous Solution

Employing atomic force microscopy (AFM) to measure passive film thickness on stainless steel (SS) in aqueous solution is proposed. SUS304 austenite and SUS329J4L duplex SS samples partly covered by gold were set in a minicell. To remove the original film, the SS surface but gold was etched using dilute sulfuric acid. After cleaning, open circuit potential (OCP), and distance from the sample surface to the top of the gold were measured. They were then immersed in either 1.0% NaCl; 5.0% NaCl; or aqueous solution with pH ranging from 1.0 to 10.0 and measured again. Differences between the first and subsequent measures of OCP suggested a passive film had formed in solution with pH ranging from 2.8 to 10.0. Similarly, differences among AFM measures revealed the observed film thickness increased with increase in pH and with decrease in chloride ions. Also, film thickness in water was greater than that in a vacuum. Comparison of AFM measurements of passive film on the austenite and sigma phases in sensitized SUS329J4L duplex SS revealed the film was thinner on the sigma phase containing more chromium. Taken together, these findings suggest the proposed method is applicable for measuring the thickness of passive films in aqueous solution.