Development of measurement system for ph distribution on metal surface under atmospheric corrosion

Abstract

Thin water film is formed on metal surfaces that are exposed to the atmosphere. The conditions of this film change according to environmental factors, such as temperature, wind, and rainfall. Hence, it is difficult to investigate atmospheric corrosion on metal surfaces.This paper aimed to develop a measurement system for the pH distribution on a metal surface in simulated atmospheric envi -ronment. The surface pH distribution was measured with agar film including pH indicators and MgCl 2 . The pH indicators used were a universal indicator and an MR ︲ BTB indicator. Agar film of 0 . 5 mm thickness was placed on an iron specimen. The specimen was set in a chamber with a relative humidity of 100 ％ to prevent the agar film from drying out. The surface pH distribution was deter - mined from the color of the agar film with an RGB color model. The surface pH distribution on the iron showed that anodic reac tions produced acidic regions and cathodic reactions produced basic regions. The observed phenomena were the same as that found under a water droplet which contained the pH indicator and MgCl 2 . Therefore, we conclude that the agar film could simulate thin water film in an atmospheric environment and allow measurement of the surface pH distribution on iron under corro sion.