Corrosion Behaviour of Cold-Worked Steel in Slightly Acidified and Neutral Solutions

Abstract

A number of studies have been reported to clarify the corrosion mechanism and the structure of precipitated film (rust layer) in slightly acidified solution (pH>3.0). Usually, the structure of precipitated film are apparently affected by the physical conditions of environment such as fluid velocity and the chemical composition of materials and corrodents. Many of studies have been carried out on the effect of chemical composition of materials upon the precipitated film, but there seems to be a few study on the effect of internal structures induced by plastic deformation on the corrosion behaviour in slightly acidified solution. The present work has been carried out in order to study the relationships between the internal structure of cold-drawn mild steels and the corrosion behaviour in slightly acidified and neutral solutions. Results obtained are summarized as follows: 1) Effect of fluid velocity on the corrosion rate was studied in pH 4.0 hydrochloric and nitric acid solution. It was found that the dissolution rate of iron as ion increased in proportion to Re^0.6 in the turbulent flow, but in the laminar flow the rate was constant and equal to that observed in the stagnant solution. The rate of increase of iron in the precipitated film was proportional to Re^0.067 throughout the laminar and turbulent flow region. 2) The total corrosion rate of cold-drawn mild steel was higher than that of annealed steel in slightly acidified solution. This is mainly due to an increase of the amount of magnetite in the rust layer. Although the amount of total iron increased linearly with time of corrosion, the variation of structure of the precipitated film with time in slightly acidified solution differed in chloride and nitric solutions. Some considerations were made on the corrosion mechanism based on the above mentioned facts. 3) The precipitated film was observed preferentially on the pearlite grains and grain boundaries. Preferential attack of cementite and micro-pitting in the pearlite grains were observed under the rust layer. Relationships between the increased precipitation of magnetite and the preferential dissolution (or decomposition) of cementite was discussed. 4) Studies were also made on the corrosion in the neutral tap water. At the higher fluid velocity in the turbulent flow, specimens remained bright after long duration test and no effect of plastic deformation was observed. On the other hand, with decreasing fluid velocity the corrosion rate increased. In the stagnant solution, the amount of precipitated oxide of cold-drawn mild steel was larger than those of annealed one. The structure of rust layer and the attacked surface were investigated.