Copper-ammonium-salt solutions corrosion

The copper-ammonium-salt solutions are generally not corrosive to mild steel however, the gases evolved during regeneration can be quite corrosive due to the presence of carbon dioxide. To prevent corrosion in the vapor zones, it is good practice to use stainless steel (vessels or liners) in the exhaust-gas scrubber section and above the liquid in the evaporator section. 

The salt process employs scrubbing of the gas with cuprous ammonium salt solution. Carbon monoxide forms a complex with the solution at high pressure and low temperature in the absorption column. The absorbed pure carbon monoxide is released from the solution at low pressure and high temperature. In the Cosorb process, cuprous tetrachloroaluminate (CuAlCl ) in a toluene medium is used as the absorbing liquid instead of the copper salt solution. A low corrosion rate and low energy consumption are the advantages of the Cosorb process over the previous one. 

Copper does not form protective oxide fdms. Therefore, its corrosion resistance is poor against most acids and salts. Many gases-haloids, sulfurous anhydride, sulfur vapors, hydrogen sulfide, carbon dioxide, ammonium-destroy copper. However, copper is highly corrosion resistant to alkali solutions. 

Only certain specific environments appear to produce stress corrosion of copper alloys, notably ammonia or ammonium compounds or related compounds such as amines. Mercury or solutions of mercury salts (which cause deposition of mercury) or other molten metals will also cause cracking, but the mechanism is undoubtedly differentCracks produced by mercury are always intercrystalline, but ammonia may produce cracks that are transcrystalline or intercrystalline, or a mixture of both, according to circumstances. As an illustration of this, Edmundsfound that mercury would not produce cracking in a stressed single crystal of brass, but ammonia did. 

Copper is softer and more ductile than steel and is utilized frequently in the manufacture of pipes and tubing. Copper has good corrosion resistance but will corrode in the presence of nitric acid and other mineral acids. Organic acids do not corrode copper as readily. Dry ammonia does not corrode copper, but the presence of water in ammonia and ammonium hydroxide will corrode copper. Copper resists corrosion in the presence of caustic solutions, but the addition of zinc will increase corrosion rates. Also carbonate, phosphate, and silicate salts of sodium will corrode copper. See FIGURE 9-1. 

Nickel is used throughout industry because of its excellent corrosion resistance. In addition to itr+us37- oe/oe" nn< cladding material to provide corrosion resistance to tanks and production vessel surfaces, nickel is used as an alloying element in steel production. Nickel is resistant to attack by NaOH and other alkali solutions, but is not compatible with ammonium hydroxide. Nickel is resistant to corrosion by sodium chloride solutions, but is corroded severely by iron, copper- and mercury chloride salts. Also, nickel has excellent corrosion resistance to most organic acids. Some of the common nickel alloys are described below ... 

Copper and its alloys are resistant to alkalies with the exception of ammonium hydroxide and cyanides. Ammonium ions promote stress-corrosion cracking of copper and its alloys. Ferric and stannic salts are aggressive towards copper alloys. Ammonia and cyanide ions form tetramine copper and tetracyano copper complexes in ammonia and cyanide solutions, respectively. 

---