Crevice Corrosion of Alloy 625 in Natural Seawater

Abstract

An experimental study was conducted to determine the influence of temperature on crevice corrosion initiation for Alloy 625 (UNS N06625) in natural seawater. These tests showed that there was a critical potential-temperature-time relationship needed to initiate crevice corrosion. The potential necessary to cause crevice corrosion on Alloy 625 decreased (became less noble) when the temperature was increased from ambient to 40°C. The crevice initiation potential decreased from 300 mV for ambient temperature seawater to 100 mV for 40°C seawater. Crevice initiation potentials were essentially unchanged between 40°C and 65°C, while the time required to initiate crevice corrosion decreased as temperature increased. In a second aspect of this work, natural seawater exposure studies were conducted to determine if there is a mechanistic connection between ennoblement (the gradual elevation of corrosion potential that occurs during long-term continuous immersion in natural seawater) and crevice corrosion initiation. It was found that ennoblement produced corrosion potentials that exceed the crevice corrosion initiation potential in ambient temperature natural seawater. At 65°C, the open-circuit potential did not exceed the crevice initiation potential. However, temperature transients from ambient to elevated temperature created temporary conditions where the corrosion potential was substantially higher than the crevice initiation potential if ennoblement had previously occurred at ambient temperatures.

Electroactive Bacteria Associated With Stainless Steel Ennoblement in Seawater

“…In this study, we were interested in gaining further insight into the bacterial community of the stainless-steel surface immersed in seawater and its electrochemical activity in relation to the potential ennoblement. Previous studies have shown an inhibition of the ennoblement activity above a critical temperature (Scotto et al, 1986; Dupont et al, 1997; Martin et al, 2003; Gümpel et al, 2006; Thierry et al, 2015). In our setting, this critical temperature was between 38°C and 40°C, above which the ennoblement was inhibited despite the continuing presence of bacteria.…”

“…While the mechanisms of ennoblement are still discussed, the seawater temperature has been identified as a critical parameter. Ennoblement is a temperature dependent process, undergoing a complete inhibition above a critical temperature around 40°C in temperate seawater and freshwater (Scotto et al, 1986; Dupont et al, 1997; Martin et al, 2003; Gümpel et al, 2006; Thierry et al, 2015). This critical temperature seems to vary with geography as it is 32°C in the Norwegian and Baltic sea (Bardal et al, 1993; Mattila et al, 2000).…”

Carbon Surface Modification for Enhanced Corrosion Resistance

“…Figure 2 also includes results published for Alloy 625 (left photo, an exotic Ni-22Cr-9Mo superalloy) for the same test conditions and crevice cell design. 3 These results indicate that the corrosion resistance of LTCSS-treated 316L was greatly increased compared to the untreated 316 and the more expensive, high-grade Ni alloy.…”

“…Electrochemical testing at NRL to determine pitting corrosion resistance 3 has shown that that there is dramatic increase in the pitting resistance of the LTCSS-treated 316 SS when compared to the untreated material. LTCSS treatment increased pitting potential from +320 millivolts (mV) for untreated material to +950 mV for the LTCSS-treated material.…”

Estudo da curva de polarização cí­clica da liga de níquel Inconel 625 em solução de NaCl.

“…).Em estudo para avaliação da suscetibilidade da liga Inconel 625 à corrosão em frestas, os autoresMartin et al (2003) realizaram um trabalho, em água do mar natural, para a avaliação da influência dos parâmetros potencial, temperatura e o tempo. Utilizando um dispositivo para simulação de uma fresta artificial e a aplicação de determinados potenciais eletroquímicos e períodos de tempo, entre a temperatura ambiente e 65 ºC, os autores realizaram avaliação da combinação de parâmetros críticos para inicio da corrosão em fresta.Como resultados, a análise mostrou que a correlação critica entre potencial, temperatura e tempo, para inicio da corrosão em frestas no Inconel 625, ocorreu no potencial de 300 mV (Ag/AgCl), na temperatura ambiente, após 58 h de exposição.…”