Stress corrosion cracking ammonia

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Virtuallv evety alloy system has its specific environment conditions which will prodiice stress-corrosion cracking, and the time of exposure required to produce failure will vary from minutes to years. Typical examples include cracking of cold-formed brass in ammonia environments, cracking of austenitic stainless steels in the presence of chlorides, cracking of Monel in hydrofluosihcic acid, and caustic embrittlement cracking of steel in caustic solutions. 

Brasses with up to 15 percent Zn are ductile but difficult to machine. Machinability improves with increasing zinc up to 36 percent Zn. Brasses with less than 20 percent Zn have corrosion resistance eqmvalent to that of copper but with better tensile strengths. Brasses with 20 to 40 percent Zn have lower corrosion resistance and are subject to dezincincation and stress-corrosion cracking, especially when ammonia is present. 

Finally, any living organism dies. Decomposition may generate ammonia at local concentrations high enough to produce stress-corrosion cracking of brass condenser tubes (Fig. 6.1). 

Figure 6.1 Stress-corrosion cracking of a brass condenser tube caused by ammonia from decomposing slime masses lodged on internal surfaces.

Figure 9.4 Both longitudinal and transverse stress-corrosion cracks on a brass heat exchanger tube that had been exposed to ammonia. Note the branching of the cracks.

Certain environments containing nitrate, cyanide, carbonate, amines, ammonia or strong caustic, due to the risk of stress corrosion cracking. Temperature is an important factor in assessment of each cracking environment ... 

Single-phase a-brasses are susceptible to stress-corrosion cracking in the presence of moist ammonia vapour or certain ammonium compounds Here the predominant metallurgical variable is alloy composition, and in... 

Nitrogen compounds These also arise from both natural and synthetic sources. Thus ammonia is formed in the atmosphere during electrical storms, but increases in the ammonium ion concentration in rainfall over Europe in recent years are attributed to increased use of artiflcial fertilisers. Ammonium compounds in solution may increase the wettability of a metaland the action of ammonia and its compounds in causing season cracking , a type of stress-corrosion cracking of cold-worked brass, is well documented. 

Stress-corrosion cracking (Section 8.10) New metal/environment combinations which produce stress-corrosion cracking are continually being found. Combinations discovered in service in recent years include titanium in red fuming nitric acid carbon steel in liquid anhydrous ammonia and in... 

For carbon steels, however, a full stress-relief heat treatment (580-620°C) has proved effective against stress-corrosion cracking by nitrates, caustic solutions, anhydrous ammonia, cyanides and carbonate solutions containing arsenite. For nitrates, even a low-temperature anneal at 350°C is effective, while for carbonate solution containing arsenite the stress-relief conditions have to be closely controlled for it to be effective . 

Cracknell, A., Stress corrosion cracking of steel in ammonia an update of operating experience. In Proceedings of AlChE Symposium on Safety in Ammonia Plants and Related Facilities, Los Angeles, 1982 (1982)... 

Lunde, L., and Nyborg, R., Stress corrosion cracking of different steels in liquid and vaporous ammonia. In Proceedings of Corrosion 87, San Francisco, 1987, paper 174, NACE, Houston (1987)... 

Little information is available on the performance of copper and of copper alloys in contact with concrete, but concrete sometimes contains ammonia, even traces of which will induce stress-corrosion cracking of copper pipe. The ammonia may be derived from nitrogenous foaming agents used for producing lightweight insulating concrete. 

Copper and silver tarnish readily in sulphide atmospheres, and copper in contact with sulphur-vulcanised rubber will sometimes react with the sulphur, devulcanising it in the process. The growth of conducting sulphide whiskers on silver is noteworthy as these whiskers may give rise to short circuits across silver-plated contacts. Ammonia has little effect on most metals, but traces will tarnish many copper alloys and cause stress-corrosion cracking of certain stressed brasses. 

Certain internal chemical treatments employed also need strict control to avoid risks of adverse chemical reaction and resultant corrosion. In particular, nitrogen-containing chemicals such as hydrazine and amines require effective monitoring to limit the concentration of ammonia release into steam because the presence of ammonia may, under certain conditions, cause stress corrosion cracking of copper and brasses. 

Yet another problem associated with ammonia is stress corrosion cracking (SCC or caustic embrittlement) of brasses (such as brass valves and other stressed components). Stress corrosion cracking of brass may develop in systems where ammonia steadily becomes available from a suitable source (such as the breakdown of sodium nitrate when it is added to inhibit SCC of steel) because it can concentrate in the steam. 

Materials of construction for ammonia are dependent on the operating temperature. Whilst mild steel may be used at ambient temperature, special costly steels are required at low temperatures to avoid embrittlement. Impurities in liquid ammonia such as air or carbon dioxide can cause stress corrosion cracking of mild steel. Ammonia is highly corrosive towards copper and zinc. Rubber lined steel construction is suitable for service at ambient temperature. 

One of the major problems encountered in the storage and transport of anhydrous liquid ammonia is the stress-corrosion-cracking (SCC) of carbon steel equipment. Cracks most often occur at the weld joints, where the leftover stress is at a maximum. The leftover stress is that which remains even after heat treatment. The hardness of the material and the presence of impurities and oxygenates in ammonia aggravate SCC88. 

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. 

This mode of failure is possible in all the copper alloys. The principal environment involved is ammonia. The evidence also exists for other media such as citrates, tartrates, nitrites, sulfur dioxide, carbonates, nitrogen oxides and phosphates to be conducive to a stress-corrosion cracking mode of failure of copper alloys. 

Mechanisms of SCC. Crack initiation of EAC is complex and not well understood till now. Most of the SCC systems exhibit short initiation times ranging from minutes to weeks and cracking often occurs due to the change in the environment rather than to a very long initiation time. Stress-corrosion crack growth rates are usually 10 11 and 10-6 m s In systems such as stainless steels in chloride solutions, localized corrosion may create the local conditions prone to crack development, but it is still difficult to explain the initiation of the crack in the absence of localized corrosion in environmental conditions different from that of the crack propagation.95 It should be mentioned that dealloyed surface layers such as certain copper alloys in ammonia-containing solutions are believed to cause SCC.54... 

Commercial grade is produced by ammonia synthesis while refrigeration grade is normally made from industrial raw ammonia by distillation. For ammonia shipped or pipelined in the United States, water content must be at least 0.2 weight% to inhibit stress corrosion cracking of the carbon steel.57... 

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