Erosion–corrosion of mild steel in hot caustic. Part II: The effect of acid cleaning

Corrosion rates of 1020 steel in 2.75 M NaOH solution at a temperature of 160 °C and velocities of 0.32 and 2.5 m/s were studied. The focus was on the effect of the acid cleaning which was performed by using strong, inhibited sulphuric acid in between the exposures to caustic. In situ electrochemical methods were used to measure the corrosion rate such as the potentiodynamic sweep and the polarization resistance method. Also used were the weight-loss method and scanning electron microscopy (SEM).Eight electrodes/coupons were used to monitor the metal loss rate, four were placed at the low velocity section, while the other four were placed in the high velocity section of a high temperature flow. The first three coupons in each section were placed within the disturbed flow region, while the fourth was placed in a fully developed flow region.During the exposure of mild steel to the inhibited acid, following the first caustic period, the corrosion rate increased significantly to between 3 and 10 mm/y with a few electrodes experiencing as high as 50 mm/y. The second caustic period following the acidic period typically started with very high corrosion rates (20–80 mm/y). The length of this corrosion period was typically 2–3 h with a few exceptions when the high corrosion period lasted 7–10 h. Following the very high corrosion rates experienced at the beginning of the second caustic period, the corrosion rates were reduced sharply (as the corrosion potential increased) to nearly the same levels as those observed during the passive part of the first caustic period.