Understanding Formic Acid Decomposition as a Corrosion Inhibitor Intensifier in Strong Acid Environments

Abstract For many years formic acid has been used to intensify, or aid in, the performance of acid corrosion inhibitors in hydrochloric acid-based fluids used for stimulation procedures in the oilfield industry. Even so, the picture of how formic acid accomplishes this and under what conditions it functions is incomplete. One theory for how formic acid aids in inhibition is that it undergoes a dehydration reaction to form water and carbon monoxide (CO), a known corrosion inhibitor. This paper confirms that CO is produced by decomposition of formic acid in strong acids under downhole conditions. In addition, the conditions for the release of CO were found to be dependent on several variables, including temperature, acid strength, and alloy. Delineation of these conditions is important for effective implementation of formic acid as a corrosion inhibitor intensifier. Introduction When acid is pumped downhole for damage removal, pickling, or matrix dissolution, corrosion inhibitors are used in the acid blends to reduce corrosion on any metallic materials wetted by the acid. At performance extremes for these inhibitors, additional compounds, often called intensifiers, are added to extend performance of the main corrosion inhibitor. Formic acid is one such commonly used intensifier. It may seem surprising that an acid is used to prevent corrosion of an acid. However, formic acid is more accurately described as a precursor to CO, the inhibiting molecule. Formic acid does not spontaneously decompose in any condition; rather, as will be described, it requires a strong acid---hydrochloric acid (HCl)---and heat. Carbon monoxide has been recognized as an inhibitor for steels for over 60 years. In 1940, Uhlig described how CO functions as an inhibitor for stainless steels under mild conditions.1 Carbon monoxide is believed to adsorb onto a ferrous surface, forming a strong nonpolar bond.2 Inhibition was found to be independent of the electrochemical potential if the current density was low. Thus, unless conditions are mild, CO performs most effectively when used as an intensifier in conjunction with a corrosion inhibitor that serves to depress the current density resulting from the corrosion process. Although formic acid was not used as a corrosion inhibitor intensifier until after Uhlig's discovery, the fact that CO could be produced from formic acid was known in the 1800s. Doebereiner reported in 1821 that mixing formic acid into sulfuric acid liberates CO.3 In fact, Uhlig used this reaction to generate CO for the corrosion testing in which he proved that CO was an inhibitor.1 Even though sulfuric acid readily produces CO at room temperature from formic acid, it was not until 1915, while studying the free energy of formation of formic acid, that Branch found that the decomposition of formic acid was catalyzed by HCl when heated.4 In his studies performed at 313°F (156°C) with 1.75 wt% HCl and 1.39 wt% formic acid, he found that the reaction came to equilibrium in about 13 days. Analyses of the decomposition products showed that the only gas formed was CO, thus supporting the decomposition reaction of formic acid to water and CO. Although formic acid has been utilized as a corrosion inhibitor intensifier for HCl for many years, the fact that it functions by decomposition into CO was either not disclosed or not recognized.5,6 Brezinski showed presumed CO gas production for 28% HCl/formic acid mixtures and cessation of the gas production by 3 hours when heated to 300°F (149°C); in 20% HCl, CO production slowed at about 7 hours. Corrosion loss results on N-80 specimens showed that inhibition failed in the same time frame as when gas production stopped.7 In another study by Al-Katheeri, et al., a decrease in the amount of formic acid as measured by capillary electrophoresis, was noted in 28% HCl at 250°F (121°C) over time. However in the same study, acid returns from pickling treatments in wells of temperatures up to 250°F (121°C) exhibited essentially no change in the formic acid intensifier.8