Low-Carbon-Steel Corrosion at High Temperatures by Aminopolycarboxylic Acids

Abstract

SummaryAminopolycarboxylic acids (APCAs) have been used in a variety of applications ranging from textiles to pharmaceuticals. They are also commonly used in the oil-and-gas industry for scale removal, standalone stimulation, and iron control. Because of the commonplace usage of APCAs, it is important to understand the corrosion that can result from the use of APCAs and the methods that can be applied to reduce corrosion damage resulting from their use. The objective of this work is to evaluate the corrosion rate of APCAs on low-carbon steel at high temperatures and to determine the mechanism of corrosion.At high temperatures, conventional acids such as hydrochloric acid (HCl) are extremely corrosive, lack penetration, and have sludging tendencies. Several organic acids such as formic acid and citric acid were proposed to overcome these shortcomings. However, these organic acids have displayed problems with solubility and compatibility. Chelating agents show good dissolving power, low corrosion, low sludging tendencies, and excellent iron control, and have been successfully used to replace HCl in certain applications. Furthermore, some of them are easily biodegradable and environmentally friendly.To study the mechanism of corrosion at high temperature, N-80 coupons were exposed to APCA solutions for 12 hours in the absence of corrosion inhibitors (CIs). At 350°F, the corrosion rate of ethylenediamine tetraacetic acid (EDTA), L-glutamic diacetic acid (GLDA), hydroxyethyl ethylene triacetic acid (HEDTA), and methylglycine diacetic acid (MGDA) had corrosion rates of 1.07, 0.754, 0.974, and 0.76 lbm/ft2, respectively. When the temperature was lowered to 300°F, the corrosion rates of each chelating agent decreased to 0.858, 0.724, 0.803, and 0.642 lbm/ft2 for EDTA, GLDA, HEDTA, and MGDA, respectively. The addition of a 1-vol% sulfur-containing CI to HEDTA and MGDA tests at 350°F caused a significant decrease in corrosion rates to 0.0102 and 0.00561 lbm/ft2, respectively. Furthermore, the mechanism of the APCA corrosion of low-carbon steel was found to be a combination of chelant-enhanced dissolution and cathodic reduction of the APCA. Chelant-enhanced dissolution involves the dissolution of the oxide layer on the surface of the metal, and is accelerated at high temperatures by reductive dissolution. Cathodic reduction of carboxylic-acid groups of APCAs was determined to be responsible for the corrosion of the bare metal layer.