Box-Behnken design approach toward predicting the corrosion response of 13Cr stainless steel

Abstract

13Cr stainless steel, the most commonly used oil country tubular good material with good mechanical and corrosion behavior, has the drawback of sensitivity to localized corrosion, particularly in offshore downhole environments, limiting the life span of the parts. A careful assessment of the corrosion behavior of the material can be done by the perception of the most influential environmental factors combined with the material's intrinsic microstructure. This study aims to focus on the former, the effect of environmental factors, including pH, temperature, and chloride concentration, varying in the ranges of 4 to 7, 22°C to 80°C, and 1,000 mg/L to 22,000 mg/L, respectively, on the pitting corrosion behavior of 13Cr stainless steel. Adopting a response surface methodology, using a Box-Behnken design, a carefully designed set of corrosion tests at various combinations of the environmental factors were performed. Considering the pitting potential measured from the cyclic potentiodynamic polarization testing, as the response of each experiment, a quadratic model was developed correlating the studied environmental factors and the pitting potential values. Further analysis of the developed model was conducted through analysis of variance, followed by optimizing the model according to the highest, medium, and lowest pitting potentials. The optimized results confirmed that the best corrosion behavior occurs at approximately the lowest chloride concentration and temperature, and the highest pH value. However, contrary to the expectations, the worst corrosion response was detected at the medium temperature of 52°C, instead of the highest temperature of 80°C. It was concluded that at higher temperatures, the corrosion tends to be more uniform, resulting in the formation of a layer of corrosion products that covers the sample's surface. The corrosion product layer acts as a barrier against the diffusion of the aggressive ions, causing deceleration of the corrosion reactions.