TECHNICAL BULLETIN

n. 25    

EXPERIENCE OF PARTIAL EXTERIOR CORROSION
OF AN INTERCOOLER IN A POWER PLANT

This experience was gained in a Cogeneration Plant with Diesel engines, where it was decided to perform the cleaning of the intercooler, by immersing it (once disassembled) in an aqueous bath of a detergent cleaner. The operation was carried out within the normal maintenance programme.

Once cleaning had been completed, the intercooler was reassembled and brought into operation satisfactorily (with highly effective cooling), although subsequently there were signs of the shedding of several flakes of copper originating from a distinct and delimitated area of the cooler.

The Cogeneration Plant commissioned our Department of Technical Consultancy to perform an investigation, which confirmed that this shedding of flakes was due to corrosion by sulphuric acid condensed from the combustion gases of the engine, where the exhaust circuit had produced a leak of gases that had affected the cooler, according to the Plant. This corrosion occurred prior to cleaning, and the flakes of copper became detached precisely when the soiling was removed along with the copper sulphate that helped in the precarious attachment of the flakes.

In order to verify that the cause of corrosion was the SO3 contained in the leaked exhaust gases, as we supposed from the outset, we decided to produce analyses to compare the used cleaning liquid with unused cleaning liquid. It was evident that the used liquid contained 17 times more copper than the unused cleaning liquid, and that the copper was in the form of copper sulphate in solution, due to the bluish coloration of the used cleaning liquid. This could only be caused by copper sulphate, which had adhered to the semi-corroded flakes prior to cleaning, given that the cleaning detergent contained a much lower level of SO4 ions:

		Used Liquid	Liquid prior to use
	Copper Content (Cu)	17 ppm	1 ppm
	Sulphate Content (SO4)	5500 ppm	476 ppm
	Colour of liquid	bluish (1)	colourless

(1) The coloration typical of CuSO4 . 5H2O (copper sulphate pentahydrate) and its aqueous solutions, formed by the reaction between SO3 , H2O from the gases and the Cu of the flakes.

The conclusion is that it is necessary to check for any signs of possible exhaust gases leaks and their effects on metallic components, given that the condensation of H2SO4 (derived from the SO3 from the gases) will cause corrosion.