Localised Corrosion of Ni-base Superalloys in Seawater

摘要

Corrosion resistant alloys (CRAs) are used extensively within a number of different industries, e.g. chemical and petrochemical processing, marine engineering, pulp and paper manufacturing, oil and gas production and transport, and nuclear reactors. The main class of these are nickel-base superalloys, which exhibit good corrosion resistance[i] in a wide variety of demanding environments. In Oil & Gas, as wells are depleted, sea-water is often injected in order to maintain well pressure. The addition of a chloride rich electrolyte is known to facilitate damage, most commonly found as localised corrosion. The electrochemical behaviour of the alloy UNS N07718 (718) and UNS N07716 (625+) were investigated via potentiodynamic polarisation, in a temperature range of 25°C to 90°C, with specimens fitted with crevice formers to obtain information regarding the localised corrosion behaviour. The Tsujikawa-Hisamatsue Electrochemical (THE) technique was also employed to obtain the Crevice Repassivation Potential (E_(r.crev)). Here the crevice is initiated using potentiodynamic polarisation, and then held at a constant current during which crevice propagation takes place. The potential is then stepped downward allowing the crevice to repassivate[ii]. It was found that the alloy 718 shows resistance to pitting corrosion but is susceptible to crevic e corrosion at temperatures as low as 40°C in de-aerated 3.5 wt% NaCl solution. Alloy 625+ shows superior corrosion resistance compared to 718, with crevice corrosion initiating at 70°C. A further three different heat treatments were applied to the alloys to investigate the effects of the γ', γ" and 5-phases on the localised corrosion behaviour. In the solution anneal condition, both alloys become more susceptible to localised corrosion. [i] Rhodes PR. Environment Assisted Cracking of Corrosion Resistant Alloys in Oil and Gas Production Environments: A review. Corrosion. 2001. 57. 923-966. [ii]Mishra AK, Frankel GS. Crevice Corrosion Repassivation of Alloy 22 in Aggressive Environments. Corrosion. 2008; 64:836-844.