The use of inhibited acids, especially hydrochloric acid, in applications such as metal cleaning and oil well acidizing has increased considerably in recent years due to the development of effective corrosion inhibitors. Organic nitrogen compounds, particularly amines, have long been known to be effective as HC1 inhibitors for steel presumably because of their-ability to adsorb on the metal surface, thus forming a protective layer.
The development of an amine derivative of rosin led to an evaluation of this product as an HC1 inhibitor. Good results were obtained, but the relatively low solubility of rosin amine in hydrochloric acid imposed a concentration limitation. It was found that condensation of the amine with ethylene oxide formed oxyethylated amines which possessed good solubility in HC1 and good corrosion inhibition. Even more important were the discoveries that the oxyethylated amines would act as solubilizing agents for up to 15 percent of their weight of free amine, and that addition of free amine to the oxyethylated amines improved their inhibitive properties.
The optimum ethylene oxide content was determined by evaluating a series of condensates containing from one to 31 moles ethylene oxide per mole of rosin amine. The 5-mole condensate was found to be optimum from the standpoint of both inhibition efficiency and acid solubility. The addition of various amounts of free amine to the condensates improved their efficiencies and resulted in very good protection at exceedingly low concentrations. For example, the 5-mole condensate containing 15 percent free amine gave penetration values of 0.130 and 0.159 inch per year at concentrations of 0.20 and 0.05 percent as compared to about 17.0 inches per year for the controls. Test conditions involved mild steel in 15 percent HC1 at 165° F for four hours.
The 5-mole condensate containing 15 percent free amine was further evaluated at 0.20 percent concentration by varying both the acid concentration and contact time. Weight losses of mild steel in both inhibited and control acids were determined at 10 time intervals from eight to 240 minutes in 5, 10 and 15 percent HC1. Curves of corrosion rates plotted as a function of time show that in the uninhibited acids the rate increased rapidly for about 40 minutes, then increased slowly for the duration of the tests. In inhibited acids curves of corrosion as a function of time indicate a maximum rate at the shortest time intervals, and that the rate decreases logarithmically as a function of time. This is believed to be due to increased adsorption of the inhibitor during the. course of the test.
