Investigating the causes of corrosion in oil cooler, power transformer transformers and its prevention and control solution.

Failure analysis of oil cooler heat exchangers at Karkheh Dam Power Plant (KDPP) was investigated. In this shell & tube heat exchanger, oil is circulated inside copper tubes and got cooled by cooling water circulating inside a steel shell. Circulating water was taken from Karkheh river and contains corrosive elements and dissolved solids. 
Initially there were reports of water leakage in to the oil inside the heat exchangers. It was reported to be due to the deterioration of aluminum gasket, which had lost its sealing action. Such problems were also reported to be occurred in an another units of the same power plant. So an attempt was made to thoroughly investigate and study the corrosion of Al-gasket seals, and to find out the remedies for this problem. For this purpose, visiting and visual inspection from KDPP were done in three stages, in order to perform sampling from damaged Al-gasket as well as circulating water. Sample of circulating water were taken at inlet and outlet of heat exchanger. Optical microscope and scanning electron microscope (SEM) equipped with EDAX system, were used to identify the cause of corrosion and its mechanisms. X-Ray diffraction (XRD) analysis was also done on corrosion deposits, accumulated on surface of tube sheets and at the entrance of copper tubes. Corrosion rate of Al-gasket, copper tubes and tube sheet was determined by immersion method as well as by a potensiostat instrument. Cooling water was also analysed for its chemical composition at the inlet & outlet of heat exchanger.
The microscope examination of  Al- gasket at the damaged zones, revealed that, there are white brittle phases at these zones. Further study by EDAX showed that, these brittle phases are made of aluminum oxide (Al2O3). In fact they were corrosion products. Presence of chloride (Cl-) was also detected by EDAX in corroded areas of Al-gasket, which could be induced by cooling water. This fact was further proved by chemical analysis of cooling water. It can also be concluded that, pitting as well as crevice corrosion occurred on Al-gasket. These type of corrosions were clearly seen on gasket edges. The results of corrosion tests obtained by immersion testing, showed that corrosion of Al-gasket was increased when it was coupled with copper & steel. Thus it can be said that, galvanic corrosion is also effective when Al-gasket is in touch with Cu-tubes and steel made tube sheets. Therefore the main mechanisms drawn from this study, in order to control, prevent and reduce the corrosion of sealing gasket are : Proper material selection for gasket, that is changing to copper, titanium and PTFE ones. The other option is to control the condition and chemistry of circulating water, to reduce corrosion. It seems the former one is more simpler, cheaper and attainable.