LONG-TERM CORROSION POTENTIAL BEHAVIOR OF ALLOY 22 IN HOT 5 m CaCI_2 + 5 m Ca(NO_3)_2 BRINES

摘要

Alloy 22 is a nickel base alloy highly resistant to all forms of corrosion. In very aggressive conditions (e.g. hot concentrated chloride containing brines) Alloy 22 could suffer localized attack, namely pitting and crevice corrosion. The occurrence of localized corrosion in a given environment is governed by the values of the critical potential (E_(crit)) for crevice corrosion and the corrosion potential (E_(corr)) that the alloy may establish in the studied environment. If E_(corr) is equal or higher than E_(crit), localized corrosion may be expected. This paper discusses the evolution of E_(corr) of Alloy 22 specimens in 5 m CaCl_2 + 5 m Ca(NO_3)_2 brines at 100°C and 120°C. Two types of specimens were used, polished as-welded (ASW) creviced and non-creviced specimens and as-welded plus solution heat-treated (ASW+SHT) creviced specimens. The latter contained the black annealing oxide film on the surface. Results show that, for all types of Alloy 22 specimens the E_(corr) was higher at 120°C than at 100°C, probably because a more protective film formed at the higher temperature. Specimens with the black oxide film on the surface showed more oscillations in the potential. None of the tested specimens suffered crevice corrosion probably because of the relatively high concentration of nitrate in the electrolyte, R = [NO3]/[C1] = 1.