Long-Term Corrosion Behavior of Alloy 22 in 5M CaCl2 at 120 C

Abstract

In conditions where tight crevices exist in hot chloride containing solutions Alloy 22 may suffer crevice corrosion. 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 (E,",) for crevice corrosion and the corrosion potential (E,,,). This paper discusses the evolution of E,,, and corrosion rate (CR) of creviced Alloy 22 specimens in 5 M calcium chloride (CaCI2) at 120°C. Tested specimens included non-creviced rods and multiple creviced assemblies (MCA) both non-welded (wrought) and welded. Results show that Alloy 22 suffers crevice corrosion under the open circuit conditions in the aerated hot CaC12 brine. However, after more than a year of immersion the propagation of crevice corrosion was not significant. The general corrosion rate decreased or remained unchanged as the immersion time increased. For rods and MCA specimens, the corrosion rate was lower than 100 nrnlyear after more than a year immersion time.

Stress-corrosion-crack initiation and growth-rate studies on titanium grade 7 and alloy 22 in concentrated groundwater

“…In any event, this would not affect type-316NG stainless steel specimens, which have also not failed, and would have affected Ti, which did fail. Finally, our results are consistent with the observation at LLNL [19] that significant polarization (ϩ300 to ϩ400 mV) is needed to produce SCC in slow-strain-rate tests.…”

“…Figure 19 shows curves for n between 0.8 and 1.4, which provide reasonable agreement with the observed rates in this and other studies that covered a range of environments. [19] Given the exceedingly low growth rates at moderate to high stress-intensity factors, the prospect of accurately measuring a threshold stress-intensity factor (K Iscc ) for Alloy 22 in this environment is challenging. However, there is broad concurrence that when the general corrosion rate exceeds the SCC growth rate (as has more commonly been observed in less passive materials like carbon steel), SCC will cease as the crack blunts.…”

“…While the temperature is much lower and the solutions are not similar to "pure water," the materials are highly resistant in this environment and are, therefore, expected to have rapid repassivation rates. In the slip-dissolution model of crack based on the existing data [19] and a K 4 dependency obtained for an assumed n value equal to 1 (in the Ford/Andresen slip dissolution/oxidation model, the K dependency for environmental cracking follows a K 4n dependency, so that in aggressive environments (e.g., sensitized stainless steel tested in an aggressive water chemistry) n Ϸ 0.5, and the observed K dependency is ϷK 2 ). Based on the current upper-bound values for general corrosion rates in Alloy 22, a K Iscc of about can be estimated.…”

SCC Initiation and Growth Rate Studies on Titanium Grade 7 and Base Metal, Welded, and Aged Alloy 22 in Concentrated Groundwater

“…In the slip dissolution model of crack advance, this corresponds to a high "n" value, e.g., n > 0.8. . Figure 23 shows curves for 'n' between 0.8 and 1.4, which provide reasonable agreement with the observed rates in this and other studies that covered a range of environments [24].…”

“…However, there is broad concurrence that when the general corrosion rate exceeds the SCC growth rate (as has more commonly been observed in k s s passive materials like carbon steel), SCC will cease as the crack blunts. Figure 24 presents a basis for conservatively estimating Klscc based on the existing data [24] and a K4 dependency obtained for an assumed n value equal to 1 (in the FordIAndresen slip dissolution I oxidation model, the K dependency for environmental cracking follows a K4" dependency, so that in aggressive environments (e.g., sensiked stainless steel tested in an aggressive water chemistry, n 0 0.5, and the observed K dependency is = Kz). Based on the current upper bound values for general corrosion rates in Alloy 22, a Klscc of about 13 MPadm can be estimated.…”