Aminopolycarboxylic 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. Due to the commonplace usage of APCAs, it is important to understand the corrosion that can results from using 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.

The results obtained indicate that the corrosion rate of ethylenediaminetetraacetic acid (EDTA) was the highest at both temperatures with a rate of 1.07 lb/ft2 at 350°F and 0.858 lb/ft2 at 300°F. L-Glutamic diacetic acid (GLDA) had the lowest corrosion rates of 0.754 lb/ft2 at 350°F and 0.724 lb/ft2 at 300°F. Hydroxyethyl ethylenediamine triacetic acid (HEDTA) showed intermediate corrosion of 0.974 lb/ft2 at 350°F and 0.803 lb/ft2 at 300°F. Furthermore, the mechanism of 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 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. The work provided will provide better understanding of low-carbon steel corrosion by APCAs at high temperatures and opportunities to pursue an effective corrosion inhibitor.

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