Analysis for Corrosion Behavior of Mild Steels in Various Hydroxy Acid Solutions by New Methods of Surface Analyses and Electrochemical Measurements

Abstract

Corrosion behavior of mild steel in various hydroxy acid solutions of glycolic, lactic, malic, tartaric, and citric acids was investigated by corrosion tests, electrochemical measurements, solution analysis, and new methods of surface analyses. The corrosion rates of mild steel decreased in the order of malic (ca. 3000 mdd1 at 30 weight percent (w/o) of acid concentration) > tartaric > lactic > citric > glycolic acids. The corrosion rate of mild steel in aqueous malic, tartaric, lactic, and citric acid solutions increased with increasing acid concentration (0–30 w/o), but in glycolic acid solution the corrosion rate was kept constant in such a concentration range. The degree of complex formation between iron ion and malic acid was found to be superior to that between iron ion and glycolic acid as detected using ultraviolet and visible (UV‐vis) absorption spectroscopy. In the scanning tunneling microscopy (STM) images and scanning electron microscopy (SEM) photographs, the surface of the mild steel corroded in malic acid was microscopically uneven under this condition. The degree of unevenness was accelerated with increasing immersion time, but the surface in glycolic acid was even. Applying bias voltages in the range of 0.5–2.0V, the STM image in malic acid held constant, but in glycolic acid it was shifted with bias voltage and a step structure was also observed in the STM image. A carboxylic anion (COO−) peak was not observed for the steel sample in malic acid using the attenuated total reflection spectroscopy (ATR) technique, in glycolic acid a peak for carboxylic anion was observed, which was also confirmed by x‐ray photoelectron spectroscopy (XPS). In malic, tartaric, lactic, and citric acids, the corrosion behavior is not ascribed to the passivation but to the active dissolution of metal. In glycolic acid, the molecules of glycolic acid are adsorbed on the steel surface. The corrosion rate in glycolic acid was found to be constant at a low rate, independent of acid concentration.