Evaluation of passive films on stainless steel by photoelectron emission measurement

Abstract

Photoelectrons emitted from stainless steel and gold were measured under irradiation by monochromated ultra-violet light at room temperature. According to a modified Fowler equation, thresholds for the photoemission were determined as 4.19 and 3.93 eV for gold and filed stainless steel (AISI 304), respectively. The threshold for stainless steel was increased with oxidation time, and reached a constant (a saturation threshold). When stainless steel was oxidized in water at 50°C, the saturation threshold was 4.35 eV. On the other hand, the saturation value increased to 4.62 eV for the specimen oxidized in air at 100°C. The difference was explained by the change in composition of the oxide films which was confirmed by XPS measurement of the films. The thresholds for stainless steel were affected by the structure of the passive films which were formed by an electrochemical method. The thresholds for electrochemically treated stainless steel were determined as 3.92, 4.02, 4.25, 4.50 and 4.65 eV for the specimens prepared at a reductive region (potential was −0.7 V versus SHE), an active state (−0.3 V), a passive state (0.2 V), a second passive state (0.9 V) and a transpassive state (1.3 V), respectively. These results indicate that we can obtain information of surface films on stainless steel from a chemical shift of the threshold. The threshold for the transpassive film exceeded 4.35 eV, which was the threshold for an oxidized layer in water. This result can be explained by an alteration of the film composition. XPS measurements of the surface film revealed that the content of hydroxide ion (OH −) in the film treated at the transpassive region had decreased. It is concluded that the passive film on stainless steel can be evaluated from the chemical shift of the threshold of photoelectron emission.