Aluminum has high resistance to attack in many environments but corrodes rapidly in most inorganic acids. The purpose of this investigation was to attempt to determine the rates and mechanism of the surface reactions for the corrosion of aluminum in hydrochloric acid vapors as a function of the vapor concentration, vapor composition, and temperature.

Corrosion tests were made by suspending aluminum foil samples, five centimeters by two centimeters, by a glass thread. The samples were located in the vapor over hydrochloric acid solutions. The glass thread with the sample was attached to the center of a glass rod. One end of the rod was fixed; the other was suspended from one end of the arm of an analytical balance, permitting periodic weighing of the corroding samples without removal from the flask.

The tests were conducted above hydrochloric acid solutions whose concentration varied from 2 to 32 weight percent. Ten temperature levels, from zero to 48°C, were employed so that the vapor pressures of the various acid concentrations overlapped. Weighings were made at 10 minute intervals for 200 minutes. The corrosion products were analyzed by standard x-ray diffraction techniques.

The rate of weight gain was found to be a linear function with a respect to time and exponential with respect to hydrochloric acid concentration. The rate passed through a maximum at a hydrogen chloride partial pressure of 1.77 mm Hg. The decrease in rate above the maximum is due to the formation of a protective aluminum trichloride six hydrate (Al Cl₃ • 6 H₂O) film. Temperature increased the rate of weight gain at all hydrogen chloride partial pressure. Based on the analysis of the corrosion products the following mechanism is proposed for the corrosion of aluminum.

For hydrogen chloride partial pressures below 1.77 mm Hg, the hydrogen chloride acts at the electrolyte for the following electrochemical reaction.

2 Al + (3 ÷ X)H₂O = Al₂O₃ • X H₂O ÷ 3 Hg

For hydrogen chloride partial pressures above 1.77 mm Hg., the hydrogen chloride enters into the ration as follows

2 Al + 6 H Cl + 12 H₂O = 12 Al Cl₃ • 6H₂O + 3 Hg

The energy of activation is approximately constant over the entire experimental range indicating that the same mechanism is controlling both proposed corrosion reactions. The controlling reactions would be the primary electrode reactions which are identical for both regions.