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Microbiological Induced Corrosion of Copper Heat Exchanger Tubes

An oil cooling heat exchanger was received for failure analysis. The shell and tube oil cooler is about three years old. The closed loop lube oil circuit operate at a maximum pressure of 90 psi (30- 35 psi operating pressure) and temperature of 130°F-170°F (flows at 90 gallons per hour). The lube oil flows inside the tubes. The shell side of the oil cooler was exposed to ash pond reclaim water (APR) at ambient temperature. 

It was reported that the reclaim water containing chlorides (12k to 16k ppm) was hooked to the oil cooler in 2020 for a short duration (a day or two). The copper tubes used in the oil cooler was rated for 300°F at 150 psi. The supplier of the heat exchanger recommended upgrading to a stainless steel from copper. The stainless steel model is rated to 450oF at 150 psi when compared to 300oF at 150 psi of copper based heat exchanger.

The deposit scrapings from the flange primarily exhibited iron, oxygen, copper, zinc, sulfur and silicon. These deposits primarily consisted of iron and copper oxides as corrosion products. The tube ID deposits primarily consisted of copper, sulfur and oxygen. Relatively high sulfur points to the possibility that the wastage was influenced by the microbiological activity. The sulfur reducing bacteria (SRB) likely caused the formation of hydrogen sulfide which created favorable conditions for ID corrosion. Upgrading the heat exchanger to 300-series stainless steel may have detrimental effects due to the presence of chlorine in the ID deposits. It is likely that the thick, black ID deposits consisted of copper sulfides. These deposits exhibited several contaminants in the form of zinc, sulfur, silicon, aluminum, molybdenum, calcium, nickel, magnesium, sodium, potassium, chromium, titanium, chlorine and vanadium. Ash pond reclaim water contains metals and other contaminants found in coal ash.

The lubrication oil cooling heat exchanger failed due to severe tube-side corrosion damage, resulting in overload failures in multiple tubes. The oil cooler tubes had thinned to a point where they could not withstand the internal pressure and ruptured with significant plastic deformation. Note that the copper alloys exhibit corrosion resistance by forming a protective oxide layer. Hydrogen sulfide, likely from microbial activity in the APR water, caused the formation of a sulfide layer rather than protective oxide scale, resulting in ID corrosion. Note that the shell side of the heat exchanger appeared to be in good condition. The oil cooler tube material is similar to ASME SB-75 UNS C12200 deoxidized high residual phosphorus copper (DHP). Relatively uniform hardness was observed; the hardness is consistent with cold-drawn copper tubes.

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