(Invited) Rapid Photo-Charge Mapping of Stainless-Steel Under the Hydrogen Permeation

Abstract

Photocurrent techniques have been applied to analyze the properties of passive films on metals and their corrosion phenomena. Photocurrent mapping is also used to evaluate the inhomogeneity of, for example, non-uniform corrosion behavior by using a lock-in amplifier to detect a weak photocurrent signal combined with a laser light scanning. This method is, however, takes a long time to acquire a scanning image due to a rather slow time-constant of the lock-in amplifier measurement. In this work, a photo-charge mapping method is developed in which an intense UV laser pulse light (405 nm, 0.5 W in the present study) is focused on the specimen surface and a photo-charge evolved by a single laser pulse is evaluated by integrating the photocurrent using an electronic integrator, as shown in Fig. 1. An intense laser pulse provides a large photocurrent in some condition sufficiently for a single photocurrent pulse analysis. Fast scanning of the laser light is also achieved by using a galvano-scanner. Currently, a time required for one image acquisition is a few minutes which is limited due to the software execution time. In this work, the spatial distribution of hydrogen permeation in a stainless-steel sheet (SUS304, 0.1 mm in thickness) placed in a Devanathan cell was traced under the tensile stress test. Hydrogen was introduced from the backside of the sheet by using the galvanostatic cathodic polarization at –1mA cm–2, and penetrated hydrogen to the front side was oxidized under the anodic polarization at +0.8 V vs. Ag-AgCl R.E. The passive film was pre-formed before the hydrogen penetration. Absorption of hydrogen into stainless steel caused a drastic increase of photocurrent on the n-type semiconductor passive film due to the photo-assisted oxidation of permeated hydrogen atom via a stainless-steel sheet, i.e., hydrogen atoms can be effectively oxidized by the holes generated by UV irradiation in the passive film. In the photo-charge image shown in Fig. 1, left half of the observed area (4mm x 4 mm, 40 x 40 pixel) was penetrated by hydrogen from the backside where the increase of photo-charge was observed. The Devanathan cell was combined with a tensile tester, and a photo-charge map was traced during the hydrogen loading under the loading condition until the notched sample was cleaved. The photo-charge intensity increased around the notch before cleavage, probably due to an increase of hydrogen concentration penetrated or thickening of the passive film. Figure 1