Study of variables that affect the corrosion of sour water strippers

A laboratory study has been conducted under API sponsorship of the variables that affect the corrosion of sour water strippers. Variables included NH/sub 3/ concentrations, NH/sub 3/-H/sub 2/S ratios, KCN concentrations, temperature, and oxygen (air) in the system. Most of the experimental work was conducted in glass equipment at ambient pressures with steel and stainless steel (Type 304) being the alloys of interest. Corrosion rates were obtained from linear polarization measurements and coupon weight loss data. Hydrogen probes were included in the system to determine periods of active corrosion. Free sulfur, cyanide ion, thiocyanate ion, ferrocyanide ion, and pH were monitored by periodic sampling during test. The results of the study revealed that steel corroded at high rates in NH/sub 3/-H/sub 2/S solutions containing 1 to 10% NH/sub 3/ if cyanide ion was present. The function of the cyanide was to destroy the relatively protective sulfide film that forms on steel in these systems. The product of the cyanide-FeS reaction appeared to be soluble ferrocyanide ions although evidence for Fe(SCN)/sub 3/ was found in several tests. The corrosion rate of steel in NH/sub 3/-H/sub 2/S-KCN appeared to be greater at high NH/sub 3/ concentrations. The accelerated corrosion of steel appearedmore » to be associated with NH/sub 4/HS in the NH/sub 3/-H/sub 2/S-KCN system. Oxygen reduced the rate of attack on steel in the NH/sub 3/-H/sub 2/S-KCN system apparently by the formation of polysulfides; however, the oxygen was most effectivein retarding corrosion if introduced before a large reservoir of cyanide has built up in the system. Preliminary runs in autoclaves with 4% NH/sub 3/ + H/sub 2/S + KCN at 50 psig indicated that steel corroded more rapidly under the pressurized conditions. Type 304 stainless steel did not corrode under conditions that caused accelerated attack on steel, but could be activated to corrode at high rates by cathodic charging to destroy its normally protective film.« less