Inorganic Corrosion Inhibitors in Acid Solution

Abstract

In dilute hydrochloric acid with excess potassium nitrate as a depolarizer, iron, zinc, and cadmium dissolve at, or nearly at, a maximum rate controlled by the rate of convection and the speed of diffusion of the hydrogen ion. The dissolution rates of these metals may be reduced to comparatively small values by addition of dichromate, molybdate, or tungstate. With very pure iron and zinc the dissolution rate in the presence of dichromate can be reduced still more by the addition of a soluble fluoride. While none of the solutions used are highly protective, the experiments described arc of aid in interpreting the mechanism of inhibition.

Corrosion Inhibition in Acid Solution

“…As shown previously (1), iron, zinc, and cadmium dissolve at approximately the same diffusion or transport-controlled rate, m dilute hydrochloric acid with excess of a statable depolarizer present. Low concentrations of dichromate ion reduce the dissolution rate considerably.…”

“…The pure metals used were described prevmusly (1). The cylinders varied from 1.75 to 2.0 cm 111 diameter, and were always rotated at 15000 -l-200 cm/min peripheral speed.…”

Inhibition of Iron Dissolution in Acid Solutions

“…Chromate and dichromate are perhaps the best; oxygen can inhibit in neutral and alkaline solution if its concentration is high enough. Molybdate and tungstate are somewhat effective (12) but offer difficulties in acid solution (2). The reduction products of permanganate, arsenite, and arsenate offer only porous mechanical barriers.…”

“…Maximum dissolution rates are attained with a suitable depolarizer; with excess nitrate present the dissolution rate can be that of convective-diffusive transport of acid to the metal surface (1). With 0.02M HC1, 0.06M KNO3 as a representative corroding solution, addition of 0.01M K2Cr~07 reduces the weight loss in a 5-rain run by 90% (2). Further addition of 0.01M sodium fluoride inhibits completely for 5 rain, and various complexing and chelating agents aid in dichromate protection for a longer time (3).…”

Electrochemical Corrosion in Nearly Neutral Liquids

“…Chromium compounds are sometimes present in the film, as shown chemically by Hoar (45); Mso by Brasher and Stove (46) and by Simnad (47), using radioactive tracer methods. It is not certain whether they contribute to the protection; King, Ooldsehmidt, and Mayer (48) consider that they detract from it.…”