Analysis of a UNS N08800 hydrogen unit preheat tube that failed in service. Based on chemical and metallographic analyses the failure was attributed to liquid metal embrittlement by zinc. The zinc source is thought to be the Cu/Zn catalyst for the low temperature shift converter located downstream of the preheat tubes.
Zinc and its alloys are used as sacrificial anodes because zinc is an active metal. Carbon steel can be coated with zinc to protect against corrosion. These metals are known as galvanized steel. In this work, microbiologically influenced corrosion (MIC) of pure zinc and galvanized steel caused by a sulfate reducing bacterium was investigated. After 7 days of incubation in 125 mL anaerobic vials with 100 mL culture medium and 1 mL inoculum, the sessile cell count on the galvanized steel was slightly higher than that on pure zinc. The abiotic weight loss for pure zinc was 1.4 ± 0.1 mg/cm2 vs. 4.6 ± 0.1 mg/cm2 for galvanized steel after 7 days of anaerobic incubation at 37oC. The weight losses for galvanized steel and pure zinc were 31.5 ± 2.5 mg/cm2 and 35.4 ± 4.5 mg/cm2, respectively, which were 10X larger than the previously reported carbon steel weight loss in the same SRB broth. Electrochemical corrosion tests confirmed the severe corrosion of these two metals. The corrosion current densities of galvanized and pure zinc were 25.5 µA/cm2 and 100 µA/cm2, respectvely at the end of the 7-day incubation with SRB, confirming that pure zinc was more prone to SRB MIC than galvanized steel. In both cases, the corrosion product was mainly ZnS. Three MIC mechanisms were possible for the severe corrosion. Extracellular electron transfer MIC is thermodynamically favorable for Zn. Furthermore, the detection of H2 evolution in the vials suggest that proton attack and H2S attack occurred against Zn in the SRB broth with neutral pH after passive film damage by the SRB biofilm.