Abstract
The present paper describes a failure investigation of 70/30 cupronickel tubes of a heat-exchanger that showed pinhole leakages and localized thinning after about 5 years of operation. The shell and tube type heat-exchanger contained demineralized water on the shell side which was cooled by seawater flowing in the tube side. The examination of failed and partially-thinned tubes were done using stereo microscope, optical microscope and scanning electron microscope equipped with energy dispersive spectroscopy. Formation of thick biofilms together with dense colony of bacteria were observed on the inner diameter surface of the tubes. The biofilms were concentrated just ahead of the tube inlet insert due to low flow conditions of seawater at these regions. The bacteria within the biofilm was identified as Acidithio bacillus thiooxidans type which are sulphur-oxidizing bacteria. Pits were observed to initiate under the biofilm which finally led to pinhole leakage. Therefore, localized thinning and pinhole leakage of the tubes were concluded to be due to microbiologically influenced corrosion. Recommendations to avoid such failures are also suggested.
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