Flow-assisted erosion-corrosion of steel in pulping liquors containing particulates

Abstract

Erosion-Corrosion is a type of corrosion where mechanical (erosive) and chemical (corrosive) effects combine to accelerate material loss due to corrosion. Erosion-corrosion can cause a higher rate of wastage compared to the sum of its components through its negative impact on the reformation of a protective oxide layer on the surface of metals. The most important industrial occurrences of erosion-corrosion are in the pulp and paper, nuclear, mining, petrochemical and pharmaceutical industries. Most of these industries have alkaline process streams such as pulping liquors in the pulp mill, where erosion-corrosion has been observed. While erosion-corrosion maps have been produced and modeling has been done in certain environments, the effect of fluid properties such as viscosity and second phase particles had not been explored systematically. This study systematically characterizes the mechanism of erosion-corrosion of steels in pulping liquors by exposing C1018 carbon steel, 304L and 316L austenitic stainless steels and 2205 duplex stainless steel to simulated pulping liquors and monitoring the corrosion rate as a function of fluid properties and additives. Within this work, flow regimes and their effects within pulping liquors were characterized and a method to control viscosity in simulated pulping liquors through addition of agar powder was developed. It was found that acceleration of material loss due to erosion-corrosion happened at a critical Reynolds number and not a critical speed, pointing out that viscosity was an important factor in the erosion-corrosion mechanism. Further results demonstrated that a fluid with higher viscosity caused higher erosion-corrosion rates at the same speed as a fluid with lower viscosity. Above a critical value, the wall shear stress acting on the passive film and the erosion-corrosion rate were found to be directly proportional. Hard particles contained in white liquor increased corrosion rate at and above 1 g/L concentration, but higher concentrations did not affect the corrosion rate significantly. It was found, however, that a combination of high viscosity and second phase particles increased the corrosion rate much more severely than the individual factors. It was thus determined that a synergistic effect between high viscosity and hard particles within the fluid caused the passive film to become more vulnerable to chemical and mechanical attack by the corrosive environment and the particles contained in the fluid respectively. The mechanism of erosion-corrosion was determined to be weakening of the passive film for active-passive alloys and more efficient removal of corrosion products from the surface for active alloys. The entire dataset generated was used to create a machine learning model using a support vector machine method.