Relationship between silicon, phosphorus content and grain number in mild steels and its corrosion resistance in pickling hydrochloric acid

Abstract

The micro-structural effects on the corrosion resistance of three classes of mild steels (A, B and C) suitable for galvanizing industry, according to their silicon, phosphorus content, grain number and size of carbides (Fe3C) in concentrated hydrochloride acid solution was investigated by a series of known techniques, such as potentiodynamic polarization curves, electrochemical impedance spectroscopy measurements, optical microscope and scanning electron microscopy. Results showed that the corrosion rate of the three classes of mild steel depends on their silicon and silicon/phosphorus combinations contents. It is found also that the corrosion rate depends on grain number and size of carbide content in mild steels. These findings were confirmed by micro-structural characterization and scanning electron microscopy techniques. They indicated that the severe corrosion cavities formed on the carbon steel surfaces and their sizes depended to silicon and silicon/phosphorus combinations contents. These results explained by micro-galvanic corrosion process between cementites and ferrites which became more serious with silicon and silicon/phosphorus combinations contents.