Season cracking

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Season cracking is a kind of stress-corrosion cracking that affected brass cartridge cases. It was first noticed by British forces in India. During the monsoon, ammunition was stored in stables, exposing it to moisture and ammonia from horse urine. The combination of this ammonia with residual stress in the cold-worked brass caused deep, brittle cracks, especially where the case is crimped to the bullet. In 1921, Moor, Beckinsale and Mallinson explained the cause: ammonia and residual stress together lead to cracking. If the cracks reach a certain size, the part can suddenly break, which can be dangerous. Very high ammonia can cause more severe, widespread attack on exposed surfaces. The problem was solved by annealing the brass cases after forming to relieve the stresses. The chemistry involves ammonia reacting with copper to form a cuprammonium complex, [Cu(NH3)4]2+, which is water-soluble and washes out of the cracks, slowing the growth of the cracks. This issue can also occur in copper and copper-containing alloys, such as bronzes and copper piping. Detecting early cracks is hard, but the characteristic blue color of the copper-ammonia complex can be a clue. Microscopic inspection and X-ray analysis (EDX) can reveal ammonia traces by detecting nitrogen.