On the Inhibitor for the Corrosion by Hydrochloric Acid on High-Chromium Steels

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The action of potassium dichromate as the inhibitor for the corrosion by hydrochloric acid on high-chromium steels was investigated. High-chromium steels were made passive by addition of proper quantities of potassium dichromate in various concentration of hydrochloric acid. But the action of potassium dichromate is cathodic control and anodic inhibitor, therefore, it is a very dangerous inhibitor. The minimum quantity of potassium dichromate required for the complete passive state was proportional to square of specific concentration of chromium in alloys. Molybdenum added to high-chromium steels, so far as it existed in solid solution, was very effective to lower the minimum quantity of potassium dichromate and to prevent perfectly pit or cavity corrosion on high-chromium steels. For instance, the minimum numerical value of potassium dichromate was 100 g per litre for the steel containing 33 per cent chromium, but 0.01 g per litre for the steel containing 33 per cent chromium and 3 per cent molybdenum in 10 per cent of hydrochloric acid. When high-chromium-molybdenum steels were in heterogeneous structure, they became very difficult to get passive state. Tungsten was not so effective as molybdenum with respect to the corrosion resisting properties of high-chromium steels.
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