Stress corrosion cracking of Ni-resist ductile iron used in manufacturing brine circulating pumps of desalination plants

Recently, it has been reported that Ni-resist ductile iron (NDI) casings of major pumps in Arabian Gulf desalination plants suffered failures by stress corrosion cracking (SCC) in brine environments. Two brands of pumps have been reported to have different lives; one lasted 18 years while the other lasted 5 years only prior to failure. The former pumps were made out of NDI as per ASTM A439 D2 (D material), whereas, the latter as per its equivalent BS 3468 S2W (G material). In a previous study by the authors the microstructures, mechanical and electrochemical properties of both materials were compared. In this work, the SCC of each material has been investigated. Optical and SEM microscopic examinations of cracked samples removed from failed pump casings have been carried out. Using a specially designed testing apparatus, accelerated SCC tests have been conducted following the constant load proof ring method. Specimens have been stressed to stress levels between 70% and 102% of their respective 0.2% offset yield strengths. Results have indicated that the D material has a higher resistance to SCC when compared with the G material. This has been attributed to differences in mechanical and microstructural properties of the two materials.