Effect of Tungsten on the Pitting and Crevice Corrosion Resistance of Type 25Cr Super Duplex Stainless Steels

Abstract

The oil and gas industry regularly uses Type 25Cr super duplex stainless steels (SDSS) for components exposed to seawater and hydrocarbon environments in topside facilities, downhole, and subsea equipment. Much debate still exists concerning the effect of tungsten on pitting and crevice corrosion resistance, particularly in standardization committees.Whereas some researchers claim that tungsten has a strong synergistic effect with molybdenum when added above a certain threshold value, others argue that tungsten additions at the expense of molybdenum could lower corrosion resistance. The objective of this investigation was to examine the effect of tungsten on localized corrosion of two super duplex stainless steels: a low-W (modified UNS S32750) and a high-W (UNS S39274) grade. Both crevice-free and creviced samples were studied. Tests were conducted in 3.5 wt% NaCl or natural seawater with temperatures ranging from 20°C to 95°C. Various independent methodologies including cyclic potentiodynamic polarization, electrochemical critical pitting temperature testing per ASTM G150, and long-term open-circuit potential exposure in natural seawater were used. Results showed that, in the solution annealed condition, tungsten additions to super duplex stainless steels had a marked positive effect on pitting and crevice corrosion resistance, increasing critical crevice temperatures by as much as 30°C. These findings suggested that tungsten-containing SDSS had a corrosion resistance on par with super austenitic stainless steel grades containing 6 wt% molybdenum. A new parametric definition of the pitting resistant equivalent is proposed to reflect the alloy's localized corrosion resistance and to support standardization efforts in the materials oil and gas community.

Crevice corrosion

“…A similar convention can be applied to tests using MCAs to determine E Crev and E R,Crev . [70] For creviced specimens, hysteresis, ΔE, is defined as:…”

“…Consequently, the duration of pitting tests is longer than a crevice test in the same environment. Lastly, CPT values are more reproducible than CCT (i.e., 2.5 vs. 10 °C) [47,70,94]. Precision and bias in critical temperature tests As detailed in ASTM G48, the precision of Methods D and F for measuring the pitting and crevice corrosion resistance was determined in an interlaboratory test program where seven laboratories carried out triplicate tests on four materials.…”

Use of the Critical Acidification Model to Estimate the Influence of W in the Localized Corrosion Resistance of 25Cr Super Duplex Stainless Steels

“…In this regard, Torres et al established a T R,Crev = 55 • C for UNS S39274 in 3.5 wt.% NaCl by the PD-GS-PD technique. In the same study, UNS S39274 polarized at +600 mV Ag/AgCl (simulating chlorinated seawater [43,44]) gave a CCT of 55 • C in natural seawater. For UNS S32750, that same work measured a CCT of 45 • C and a T R,Crev of 42.5 • C in natural seawater polarized at +600 mV Ag/AgCl .…”

Desenvolvimento de teste para avaliação comparativa de CPT para ligas de alta resistência à corrosão por pite

“…Cada metodologia proporciona resultados distintos de difícil comparação com outras técnicas. Pode-se tomar como exemplo resultados com aço superdúplex, que baseado nas técnicas descritas nessa revisão obteve valores de CPT entre 80°C, no ensaio de acordo com ASTM G48 método A modificado (SANDVIK SAF2507, 2018), e 63,5°C no ensaio de acordo com a ASTM G150, porém utilizando 3,5% NaCl, pH 8 e 600 mVAg/AgCl (HAUGAN et al, 2017). Além disso, os métodos de ensaios descritos em sua maioria não visam comparar materiais que apesar de indicados para aplicações semelhantes são diferentes, como aços inoxidáveis austeníticos, aços inoxidáveis dúplex e ligas de níquel.…”

“…Figura 25 e possui, assim como o material dúplex, microestrutura de matriz ferrítica com ilhas de austenita. A micrografia obtida apresenta as mesmas características da micrografia obtida porHaugan et al (2017) mostrada na Figura 4, mesmo tendo sido utilizado um reagente químico diferente para o ataque metalográfico. O gráfico obtido pelo Thermo-Calc ® mostrado na Figura 26 indica que para a formação de cerca de 50% de ferrita e austenita, que é o adequado para o material segundo a literatura, não há precipitação de demais fases em estado de equilíbrio.…”