Nickel base alloy UNS N07718 is a precipitation hardened material. Due to its properties, it is one of the most widely used materials in oil and gas industry. Mechanical and corrosion behavior of the material depends strongly on its microstructure. One of the microstructural elements, considered to have a detrimental effect on the material resistance to stress corrosion cracking, is the δ-phase precipitation. The mechanism of that phenomenon is not yet fully understood.

Therefore, the aim of the present work is to investigate the influence of δ-phase precipitates on the pitting performance of the material. Electrochemical measurements were performed in various chloride containing solutions at different temperatures. Two materials with different heat-treatment were used for this study. Those material variations were aged at 760°C and 870°C for 8 hours, respectively, to obtain microstructures with different δ-phase content. The corrosion behavior was evaluated by analyzing the current potential curve. Special attention was also paid to the passivation effect. The passive film formation was investigated via Electrochemical Impedance Spectroscopy measurements. Differences in corrosion performance of different microstructures were observed in all of the experiments. Specimens with an increased content of δ-phase precipitates were shown to be more active in the corrosion process. Among all of the studied parameters, temperature was found to have the most significant effect on the pitting behavior. The electrochemical measurements were performed on samples, which were charged with hydrogen for 24 hours before the actual experiment. It is assumed that dissolved hydrogen, as is the case with austenitic stainless steel UNS S30400 and UNS S31000, negatively effects the passivation of nickel base alloy UNS N07718.1 

Subject
Materials, Materials performance, Precipitates, Sodium chloride, Impedance, Impedance measurements, Sodium chloride solution, Corrosion behavior, Salt concentration, Pitting, Passivation, Nickel, Hydrogen
Keywords:
UNS N07718, delta-phase precipitation, pitting corrosion, passivation, hydrogen, potentiodynamic measurements
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2014
Association for Materials Protection and Performance (AMPP)
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