The use of very small amounts of air for the prevention of hydrogen attack leading to blistering and fissuring of equipment is described. This protection is achieved by producing a small amount of polysulfide in situ which converts cyanide to thiocyanate and inhibits the primary corrosion reaction. The method has been used successfully for about two years. Inspections have failed to show any hydrogen damage to equipment protected by adequate air injection; no degradation of product quality has been observed.
In the application of the method, a number of problems arose which have since been solved. These difficulties include: 1) accumulation of oxygen in stagnant vapor areas, 2) accumulation of acidic ammonium thiocyanate leading to high corrosion rates, 3) control of pH in the desired range to prevent deposition of ammonium salts and to prevent decomposition of ammonium polysulfide, and 4) limited reaction rate between oxygen and hydrogen sulfide to form polysulfide.
Several methods are considered for the control of air injection to meet the polysulfide requirement in the system. Hydrogen penetration probes and chemical spot tests for ferrocyanide and free cyanide have been found adequate for this purpose. Complete protection against hydrogen damage has been attained by only infrequent adjustment of the air injection rates.
