Corrosion resistance of steel for coiled tubing units

Abstract

The gravimetric method was used to determine the corrosion rate of a pipe for coiled tubing. Scanning electron and optical microscopy were used to study the microstructure and to determine the nature of corrosion damages. It has been found that corrosion processes of different nature occurred in the studied systems “metal – environment”, in particular, in acid solutions, corrosion was caused by the of hydrochloric acid and the ambient temperature of 70°С. In solution with a smaller acid content, along with the general corrosion, there is a significant localization of the corrosion process (deep corrosion damage is formed: macro pitting and corrosion ulcers). The general corrosion was observed in the HCl solution (13 mass %), which destroys the pipe walls after 576 h of exposure. The neutral solutions caused the general corrosion of smaller intensity in comparison with the acidic environments, even taking into account the temperature factor. The surface-active substances or petroleum products that are present in the solutions, form barrier films on the steel surface, which prevent the access of corrosive components from the environment to the surface of the material, especially during the short exposure time. With the increase of the exposure at the elevated temperatures, the barrier films break down and the steel surface undergoes the general corrosion.