AE Behavior for Corrosion on Carbon Steel in High Humidity Environments

Abstract

Corrosion on piping under thermal insulation is localized corrosion and a problem for plants which are located near the sea shore. Thermal insulation offers a corrosive environment where chloride ions allow concentration and humidity is high by penetrating rain water or splashes from the sea containing chloride ions to the insulation. A visual inspection can detect the corrosion after removing the insulator. However, removing and restoring the insulation needs enormous time and troubles. Therefore, a method for identifying the corrosion without removing the insulation is required. An acoustic emission (AE) method can detect elastic waves emitted by fractures of corrosion product (rust). In this study, AE monitoring was applied to evaluate the corrosion volume on a steel plate in high humidity with MgCl₂ solution. AE activity increased with time from several days later after providing MgCl₂ solution and then decreased. AE activity became high again after providing MgCl₂ solution. While AE activity was high, the thickness of the plate on the corrosion drastically changed. When the thickness did not change, externally low AE activity was detected. The relation between corrosion volume and cumulative AE count from the corrosion showed different linear relation in the early and following stages.