Abstract
Alloy 22 is a nickel base alloy highly resistant to all forms of corrosion. In conditions where tight crevices exist in hot chloride containing solutions and at relatively high-applied potentials, Alloy 22 may suffer crevice corrosion, a form of localized attack. The occurrence (or not) of crevice corrosion in a given environment (e.g. salt concentration and temperature), is governed by the values of the critical potential (Ecrit) for crevice corrosion and the corrosion potential (Ecorr) that the alloy may establish in the studied environment. Also, it is generally accepted that as Alloy 22 becomes passive in a certain environment, its Ecorr increases and its corrosion rate (CR) decreases. This paper discusses the evolution of Ecorr and corrosion rate (CR) of creviced Alloy 22 specimens in six different mixtures of sodium chloride (NaCl) and potassium nitrate (KNO3) at 100°C. Two types of specimens were used, polished as-welded (ASW) and as-welded solution plus heat-treated (ASW+SHT). The latter contained the black annealing oxide film on the surface. Results show that, as the immersion time increases, Ecorr increased and the CR decreased. Even for concentrated brine solutions at 100°C the CR was < 50 nm/year after more than 100 days immersion. In most tested conditions, Ecorr in the aerated brines was lower than the Ecrit values from the literature.
