LONG TERM ELECTROCHEMICAL BEHAVIOR OF CREVICED AND NON-CREVICED ALLOY 22 IN CaCl2 + Ca(NO3)2 BRINES AT 155°C

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. Chloride ion is known to be the most detrimental aggressive agent for Alloy 22 and is able to promote crevice corrosion when tight crevices exist in hot chloride containing solutions of different concentrations. Nitrate ion is an effective inhibitor of chloride induced crevice corrosion when present in a high enough [NO3 -]/[Cl-] ratio. The occurrence of localized corrosion in a given environment 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. If Ecorr is equal or higher than Ecrit, localized corrosion may be expected. This paper discusses the evolution of Ecorr and corrosion rate (CR) of Alloy 22 specimens in 18 m CaCl2 + 9 m Ca(NO3)2 and 18 m CaCl2 + 0.9 m Ca(NO3)2 brines at 155°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, in a few immersion days Ecorr reached a stable value higher than the open circuit potential of a platinum electrode in 18 m CaCl2 + 9 m Ca(NO3)2 for all specimens tested. Specimens tested in this solution did not suffer any type of localized attack. On the other hand, Ecorr showed oscillations of up to 600 mV in 18 m CaCl2 + 0.9 m Ca(NO3)2 during the entire immersion period. These oscillations were due to pitting corrosion development. Crevice corrosion was not observed in any testing case. Corrosion rates for specimens in the latter solution ([NO3 -]/[Cl-] = 0.05) were one order of magnitude higher than for specimens in the second one ([NO3 -]/[Cl-] = 0.5). Nitrate showed to be able to inhibit localized attack even in hot concentrated chloride brines when present in a ratio of [NO3 -]/[Cl-] = 0.5. Localized corrosion occurred only in condition where Ecorr > Ecrit.