CONTROLLING CORROSION IN NITRIC ACID ENVIRONMENTS: CURRENT UNDERSTANDING AND RECENT DEVELOPMENTS

The materials for use in nitric acid (HNO3) service have to be resistant to intergranular corrosion. The main reason for intergranular corrosion is sensitization, a process that has been thoroughly investigated and understood in the last few decades. Even non-sensitized stainless steels are known to undergo intergranular corrosion due to segregation of silicon and phosphorus at grain boundaries. End grain corrosion resulting from sulphide inclusions and due to segregation along dislocation bands needs to be minimized. This paper describes the strategies used in combating corrosion due to all these factors. The experience gained in optimizing the Nitric Acid Grade stainless steel and its quality and performance assessment has been described. It has been emphasized that proper fabrication practices have to be employed for the Nitric Acid Grade of stainless steels otherwise the advantages due to a clean and controlled chemistry and microstructure may not be realized. The problems associated with cold working during fabrication, contamination of the process fluid due to chloride and fluoride ions, fabrication of large diameter pipes with minimum delta ferrite content and oxide embedment and their remedial measures are described.