Assessing the Pitting Corrosion Resistance of Oilfield Nickel Alloys at Elevated Temperatures by Electrochemical Methods

Nickel alloys are broadly used in upstream oil and gas applications as a result of an advantageous set of properties including high mechanical strength accompanied by good ductility and toughness, excellent localized and environmentally assisted cracking resistance, and remarkable thermal stability. In general, their pitting corrosion resistance is ranked by calculating their pitting resistance equivalent (PRE) based on alloying element content, or by measuring the critical pitting temperature (CPT) in immersion tests in concentrated oxidizing acidic electrolytes. In spite of the fact that both concepts are crosslinked, the determination of CPTs is, from both approaches, clearly the best way to infer pitting corrosion resistance in service. Nevertheless, many of the electrolytes typically used for assessing CPTs suffer decomposition at temperatures higher than 85 degrees C. Therefore, highly alloyed nickel alloys having CPT higher than 85 degrees C cannot be ranked in these electrolytes. Electrochemical methods used to assess the pitting and repassivation potentials, which are well known for characterizing the pitting susceptibility of metallic alloys, emerge as one option to rank these at high temperatures. In the current study, the pitting corrosion resistance of four precipitation hardenable nickel alloys was electrochemically determined in 80,000 mg/L chloride solutions at ambient temperature, 80 degrees C, and 150 degrees C. Experimental results have shown that electrochemical methods such as open-circuit potential measurements and cyclic potentiodynamic polarization tests are suitable for ranking highly alloyed nickel alloys in chloride-containing environments at high temperatures. In addition, it was confirmed that the pitting corrosion resistance of these alloys was strongly influenced by its molybdenum content.