A study on the failure of pump casings made of ductile Ni-resist cast irons used in desalination plants

There have been cases of the corrosion failure of pressure parts of brine pumps in a desalination plant located on Arabian Gulf. The materials of the construction of failed parts are Ni-Resist Ductile Irons (NDI) and the environment is brine, which is concentrated seawater. Two brands of pumps had different lives to total failure; one lived 18 years while the other lasted only five years. The failed parts of former pumps were made out of NDI material as per ASTM A439 D2, whereas, the latter pumps were made out of NDI material as per BS 3468 S2W, which has better weldability. In this work, the material factor, as one of the possible factors that could have contributed to this different behavior has been evaluated. Metallurgical, mechanical and electrochemical tests were used to compare between the two alloys. Results have shown variations in the microstructure, tensile strength and hardness values of both alloys. However, electrochemical tests have shown similar corrosion rates between 0.20 and 0.25 mmpy in brine at room temperature, whereas, potentiodynamic sweeps have shown comparable Tafel plots.