Studies on the role of heavy metal ions on the corrosion behaviour of iron base alloys

Abstract

Corrosion is the major cause of components or materials failure in desalination and power plants. A seawater desalination plant offers numerous corrosion problems due to its process conditions, including factors such as temperature and pH and operation in relatively aggressive environments consisting of seawater, seawater-air and salt-air aerosols, corrosive gases, slow moving or stagnant liquids or deposits forming liquids. Though one or more forms of corrosion are involved during corrosion failure in desalination plants general and pitting corrosion are more common modes of failures. In seawater processing systems, if dissolved oxygen and pH are under control, general corrosion is the predominant mode of attack on conventional construction materials such as carbon steel. This form of corrosion is easily controllable and is desirable in the sense that it permits predictive estimates of service life. Pitting, the most detrimental form of attack, is often responsible for the corrosion failures of components in desalination and power plants. Though there are several causes of pitting, the attack of certain aggressive anions on the protective oxide film on the metal appears to be most widely referred to in the corrosion literature. Stainless steels particularly austenitic lose their protectivity in presence of chloride ions and undergo pitting. Steels invariably undergo pitting under stagnant or low fluid velocity conditions. Contact with heavy metal ions such as copper is another cause which has been attributed to pitting of steels. This aspect has been given little attention and is least understood yet has great relevance to seawater desalination and power plants. A number of cases have been reported regarding the copper induced pitting corrosion of iron and galvanized pipes and tanks in recirculating hot water system. A copper concentration of 0.1 mg/l was found to be sufficient to cause accelerated attack.