Effect of shot-peening on the passive film formation and corrosion of carbon steel in LiBr aqueous solution

In absorption chillers using LiBr aqueous solution, corrosion problems of carbon steel heat exchangers become more serious as the temperature of the generator increases. To reduce the corrosion problems, various surface treatment methods are applied such as a shot-peening. In this study, corrosion characteristics were investigated for untreated and shot-peened surfaces. Pyrex tube samples containing the specimens and LiBr aqueous solution with Li2MoO4 were made and put into a high temperature oven at 200 °C After 2000 hours, the surface and cross section of carbon steel specimens were observed using field emission scanning electron microscope (FE-SEM) and energy dispersive spectroscopy (EDS). It was found that the passive films have the characteristic of a double layer consisting of a thick MoO2 layer and thin Fe3O4 layer. Thicker passive films were observed on the shot-peened surface than the untreated one. As a result of corrosion depth measurement, the shot-peened specimens showed a slightly smaller uniform corrosion depth, however, they showed a much smaller pitting corrosion depth than the untreated ones. It is supposed that the shot-peened surface is more effective in preventing the pitting corrosion than uniform corrosion. The results of this study are expected to be applied effectively to the absorption chillers using LiBr aqueous solution to estimate and reduce the corrosion.