Synergism in erosion-corrosion caused by interaction of electrochemical and mechanical factors

A serious synergistic effect arises from the coexistence of two basic material loss processes during erosion-corrosion, electrochemical dissolution due to corrosion and mechanical degradation due to erosion. This synergy contributes strongly to the total material loss. Consequently, it is critical to understand the interaction mechanisms between erosion and corrosion so as to develop means to address the loss of material. This thesis describes both the influence of erosion on corrosion and the influence of corrosion on erosion that contribute to the synergy between erosion and corrosion of carbon steel A1045.; The dependence of erosion-enhanced corrosion on the corrosion system was studied using both a non-passive and a passive corrosion system. In the non-passive system, erosion showed no significant acceleration effect on corrosion when corrosion was controlled by active dissolution. In the passive corrosion system, erosion caused an increase in the corrosion rate through impairment of the passive film. The effects of erosion were characterized using the parameters for an equivalent electrical circuit. The parameters for the passive film growth kinetics, including the diffusivities and densities of defects within the passive film, were determined using Mott-Schottky analysis. These parameters were correlated with the resistance of electrode to erosion-enhanced corrosion. In addition, electrochemical tests revealed that erosion promoted initiation of pitting corrosion by breaking down the passive film, but impeded propagation of metastable pits by impairing the occluded environment for pitting corrosion development.; A chemo-mechanical effect was employed to understand the influence of corrosion on erosion. The surface hardness of the electrode decreased when an anodic current was impressed, which led to a higher erosion rate. Nanoindentation tests confirmed that hardness was reduced, and demonstrated that the degradation in mechanical properties declined with increasing depth from surface. The chemo-mechanical effect was manifested as a linear relationship between the rate of corrosion-enhanced erosion and the logarithm of anodic current density.; Under high flow rate conditions in corrosive solution free of particles, the mass loss rates obtained from traditional weight loss measurements were found higher than those calculated using Faraday's law. Analysis of the data showed that the additional material loss arose from corrosion-induced erosion.