Corrosion, stress corrosion cracking, and electrochemistry of the iron and nickel base alloys in caustic environments. Progress report, March 1, 1978--February 28, 1979

Abstract

The stress corrosion cracking (SCC) of 304 stainless steel in caustic solutions can be prevented by the use of proper inhibitors. Sodium chromate was found to be an effective inhibitor for preventing the caustic stress corrosion cracking. Sodium chromate causes the corrosion potential of 304 stainless steel to shift from its active region into the passive region where the alloy is less susceptible to SCC. The threshold concentration of sodium chromate in the boiling saturated caustic solution is 0.005 molar. Molybdenum is a very unstable metal in aqueous caustic solutions at room temperature and above. It dissolves at a very fast rate and does not passivate. Nickel, out of the three metals chromium, iron and nickel, is the most stable metal in caustic solutions in the temperature range 150 to 290$sup 0$C. The stability of iron base alloys decreases with an increase in either caustic concentration or temperature, increasing the nickel content in the alloys promotes corrosion resistance in caustic solutions. Molybdenum in fused caustic at 350$sup 0$C under an argon blanketing atmosphere dissolves at a catastrophic rate. The metal dissolves in an active state without any passivation. Nickel, unlike molybdenum, exhibits an active-passive transition in fused caustic at 350$supmore » 0$C under an argon blanket and also under a hydrogen blanket. However, the corrosion rate in the passive region may be of the order of 50 mils per year.« less