Date of Award

12-2007

Document Type

Thesis

Degree Name

Master of Science in Material Science Engineering (MSMatSE)

Department

Materials Science

First Advisor

Dr. Osama A. Abuzeid

Second Advisor

Dr. Abdel-Hamid 1. Mourad

Third Advisor

Dr. Mohamed Abou Zour

Abstract

Two cases of stress corrosion cracking failures of brine pumps casings have been reported. These pumps are installed in two desalination plants located on the coast of Arabian Gulf in UAE. The materials of 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 5 years. The failed parts of former pumps were made out of NDI material as per American standard ASTM A439 D2 (designated D material) whereas those of latter pumps were made out of NDI materials as per the British standard BS 3468 S2W (designated G material).

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, electrochemical tests and corrosion cracking tests were carried out 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, cyclic sweeps have shown comparable Tafel plots. To facilitate performing of SCC testing using the direct tensile test method a special testing apparatus has been developed. It comprised proof ring, stressing jig (frame), corrosion test cell and electric heater. The SCC test have been carried with durations till full fracture or a maximum of 8 to 10 days in case of no fracture. The results have clearly shown different behaviors with respect to SCC resistance. The time to failure of D material was much longer than that of G material. This difference in behavior was attributed to the higher yield strength of D material and its more favorable microstructure features compared to G material. This has confirmed the material factor as a main contributor to the different performance in resistance to SCC of the two Ni-resist ductile irons.

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