The electrochemical mechanism in the erosion-corrosion damage of carbon steel by gas-liquid two-phase flow

The erosion-corrosion characteristics of a carbon steel commonly used as a piping material subjected to an impinging gas-liquid two-phase jet were investigated. The localized damage was observed, and the range of the erosion rate under the experimental conditions was between 45 mm/year and 87 mm/year. It became clear that the principal mechanism of this damage was the occurrence of pitting corrosion caused by the corrosion potential distribution on a specimen and the rapid growth of pits. The significance of flow-induced potential distribution on a specimen was demonstrated by use of an array of test pieces insulated electrically from each other. This potential distribution induced by the flow distribution on a specimen also gave crucial effects on the location where the erosion-corosion damage occurred. Namely, erosion-corrosion damage took place locally, just at the part of the specimen where the corrosion potential was shifted to more nobler than the pitting potential of the condition of interest. This corrosion potential distribution was caused by the distribution of the oxygen supply rate and deposition and/or removal of oxydizer corrosion product (γ-FeOOH). These were originally caused by the flow distribution. This erosion-corrosion damage was completely inhibited by keeping the pitting potential nobler than the corrosion potential; for example, by the addition of NaNO2 to the system. (author)