Electrochemistry of Active Chromium: Part 1—Anomalous Corrosion and Products of Chromium Dissolution in Deaerated Sulfuric Acid

Chromium corroding in deaerated aqueous solution of sulfuric acid (H2SO4; pH 1 to 3) produces Cr(II) and Cr(III) ions simultaneously in the ratio 7:1, as well as H2. The corrosion potentials of electrochemically activated chromium are determined by the electrochemical processes as expected according to the Wagner-Traud model. However, the real rates of chromium corrosion determined by collecting evolved hydrogen, spectrophotometric determination of the accumulated Cr ions in the solution, or by weight-loss measurements are higher than the electrochemical dissolution rate by up to 12 times for pH 1.0. The effect is smaller for higher pH. This was due to the simultaneous “anomalous” (or chemical) dissolution process of the direct chemical reaction of Cr with H2O molecules, as proposed some time ago by Kolotyrkin and coworkers. Since “anomalous” dissolution cannot be detected by electrochemical means, it has been pointed out that in the presence of “anomalous” dissolution processes during metal corrosion, electrochemical corrosion rate measurements should be taken only as approximate, while the level of approximation should be determined by some other direct corrosion rate measurement method.

Electrochemistry of active chromium, part III: Effects of temperature

It was shown that the temperature in the range 20 ? 65 ?C has considerable effects on the electrochemical anodic dissolution of chromium in the active potential range as well as on the electrochemical hydrogen evolution reactions on bare and oxide covered chromium surfaces. Also, the chemical dissolution of chromium is strongly affected. The apparent energy of activation for anodic dissolution is 63.1 kJ mol-1, for hydrogen evolution on a bare Cr surface 19.5 kJ mol-1, for the same reaction on an oxide covered surface 44.0 kJ mol-1 and for the chemical ("anomalous") dissolution 66.9 kJ mol-1. The temperature dependences of the total corrosion rate, and the electrochemical corrosion rate alone, are presented in polynomial forms with the appropriate constants obtained by the best fit of the experimental data. For the hydrogen evolution reaction on both bare and oxide covered chromium, the Volmer-Heyrovsky reaction mechanism with the second step as rate determining was proposed.

Dissolution of chromium in sulfuric acid

By combining electrochemical corrosion rate measurements and spectrophotometric analysis of the electrolyte it was shown that at room temperature chromium dissolves in deaerated 0.1M Na2SO4 + H2SO4 (pH1) solution as Cr(II) and Cr(III) ions in he ratio Cr(II):Cr(III)?7:1. This process was stable over 4h without any detectable change. The total corrosion rate of chromium calculated from the analytical data is about 12 times higher, than that determined electrochemically by cathodic Tafel line extrapolation to the corrosion potential. This finding was confirmed by applying the weight-loss method for the determination of the corrosion rate. This enormous difference between these experimentally determined corrosion rates can be explained by the rather fast, "anomalous" dissolution process proposed by Kolotyrkin and coworkers (chemical reaction of Cr with H2O molecules) occurring simultaneously with the electrochemical corrosion process.