Corrosion degradation of pipeline carbon steels subject to geothermal plant conditions

Purpose: The electrochemical behaviour of two carbon steels exposed to the acidic solutions and their resistance to hydrogen induced cracking (HIC) is evaluated in order to determine the effect of hydrogen damage on the failure process of the steels used in line pipe and casing at a geothermal plant. Methodology/Approach: Samples of two different steels, ASTM A-53 grade B (line pipe) and API L-80 (casing) were immersed during 96 hours in the electrolyte proposed by NACE to evaluate the susceptibility to HIC. Samples of the two steels embedded in non-conducting bakelite were exposed to potentiodynamic polarisation at room temperature using brines obtained from different wells at Cerro Prieto geothermal plant as electrolyte. Hardness tests were performed before and after the HIC test on samples in order to determine hardness changes induced by hydrogen penetration, as field results indicated embrittlement of the steels after 4 months of service. Findings: The steels, ASTM A-53 grade B and API L-80 did not present cracking sensitivity, as no cracks were observed in the tests specimens, although they showed an increase in hardness. The steels presented high corrosion rate in the brine media at room temperature (3.3 mm/year), which is expected to increase at high temperatures. Originality: This work revealed that carbon steels used at line pipes and casing at geothermal plants present high resistance to hydrogen induced cracking; however they corrode at high rates and present embrittlement. It is suggested that due to the high operation temperature the damage induced by hydrogen results in hardness increase but was not sufficient to crack development.