Everything about Season Cracking totally explained
The problem of
season cracking is known from
brass cartridge failures during the monsoon season in India in the 1920's, failures which were caused by attack of the metal by ammonia. The edges of rifle cases cracked and could cause misfiring of
rifles. The gas was produced from animal manures during the high temperatures and humidities of the monsoon, and by the high levels of
residual stress produced by forming the barrels of the cartridges. As a type of
stress corrosion cracking, it's characterised by deep brittle cracks which penetrate into affected components. If the cracks reach a critical size, the component can suddenly fracture, sometimes with disastrous results. However, if the concentration of ammonia is very high, then attack is much more severe, and attack over all exposed surfaces occurs. The problem was solved by annealing the brass cases after forming so as to relieve the residual stresses.
Ammonia
Attack takes the form of reaction between
ammonia and
copper to form the cuprammonium ion, a chemical complex which is water-soluble, and hence washed from the growing cracks. So the problem of cracking can also occur in
copper and any other copper alloy, such as
bronze. The tendency of copper to react with ammonia was exploited in making
rayon, and the deep blue colour of the aqueous solution of
copper oxide in ammonia is known as
Schweizer's reagent.
Materials
Although the problem was first found in brass, any alloy containing
copper will be susceptible to the problem. It includes copper itself (as used in pipe for example),
bronzes and other alloys with a significant copper content.
Also See
Further Information
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