Effect of Aging Treatment on Pitting Corrosion Behavior of Oil-grade Nickel Base Alloy 718 in 3.5 wt NaCl Solution

摘要

The increasing demand for strength and corrosion resistance of oil-grade nickel base alloy 718 with good toughness used in oilfield drilling application results from severe corrosive and aggressive conditions encountered in the deeper wells which involve high content of chloride salts and significant levels of hydrogen sulfide and carbon dioxide, high pressure and high temperature (HPHT). To increase the yield strength and optimize the impact toughness of oil-grade nickel base alloy 718, two-step aging, which consists of 4-5 hours for each step has been used in oil-grade alloy 718. However, the effect of aging treatment on the corrosion behavior is not well understood. In this paper the influence of aging treatment on pitting corrosion behavior of oil-grade nickel base alloy 718 in 3.5 wt% NaCI solution at room temperature was investigated by immersion test and electrochemical measurements including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The modified double-aging treatment can improve pitting corrosion resistance of oil-grade nickel base alloy 718 compared with the API 6A 718 standard aging treatment. The high amount of γ" precipitation in the modified double-aged microstructure increased the cathode-to-anode area ratio, leading to reduce the corrosion of γ matrix.