Descriptions of metalic problems associated with brass casings that I have encountered.
While watching many of the gun forums, I have been interested in some of the conversations regarding the care of brass while cleaning as well as issues of casing failure due to care, structural damage, chemical exposure and improper inspection and loading. In this article I could show a few pics of some of the brass found while sorting. This is by no means a complete list as it is just the brass I have found. There are no doubt many more ways to distroy brass than I can show here. If I have missed something in this article that you are aware of or have witnessed and have pics or brass to show, please don't wait to e-mail me.
Season cracking: Is when the copper atoms within zinc alloy are selectively removed. In one gun forum, a man asked what was the harm  in cleaning brass with amonia. When brass is shaped and bent, little cracks are formed with each stress. When the stress cracks within the brass are exposed to ammonia, a cuprammonium ion, [Cu(NH3)4] is formed. Cuprammonium, being water soluble, is then spread and washed away by water. This problem is corrected by adding arsenic to the brass alloy or by annealing the brass cases to relieve the residual stresses caused durring formation and yet prior to ammonia exposure. The smallest amount of ammonia will start season cracking. a.k.a. Caustic embrittlement.
Dezincification: The dealloying of brass when zinc is selectively removed leaving the freshly polished brass  behind a red or pink color. The inferior sponge-like copper structure has little mechanical strength and is very suseptable to corrosion because of it's porous structure. Because zinc has a very weak atomic bond to other metals it is easily given up to liquids with aggressive characteristics, i.e., clorinated tap water, slightly acidic water, stagnant water or alkaline water with a high salt content. The process starts when the piece is exposed to moisture high in oxygen or carbon dioxide ions. The zinc  dissolves leaving the structurally weak, copper rich, sponge-like sructure with poor mechanical properties. The two pieces on the right after polishing show pink hue because of a uniform exposure. (Compaired to the single piece on the left.) Mineral staining and the presence of a white zinc oxide coating may be present prior to polishing.
Brass Separation: Let me set the scene. When the cartridige is fired, the  brass's mass 'migrates' forward thickening the shoulder and lengthening the case overall as well as thinning the center wall of the case. During the reloading process when the case is being resized, because of the case holder, the sizing die stops its sizing stroke at about the same location as were the cases' wall thickness thins from the thicker part near the head and rim. Each time that area is stressed with expansion and sizing, this flexing hardens the metal against the thicker metal of the case's head. On most rifle cases this is about 3/8" along the case wall. The thinning shows a thin line when the case is finished. This all being said, I have brass that has not been thined yet has seperated. The large piece in the foreground of this pic is an example. This may result from excessive headspace or a dangerously high chamber pressure. 
Neck Splitting: As the shoulder and neck of the case receive 'migrating' mass from each firing / sizing cycle (discribed in 'brass separation'). These stresses flex and reflex and further harden the metal at a point that has been getting thicker and less maliable. At some point these horizontal stresses exploit a vertical stress point and cause a casing failure. This also happens within the thinning case wall. Small chips or impacts on the edge of the mouth can also present a  weakness for the stresses to exploit.
Loading and Extraction wear. This is exactly what it sounds like. Brass that has been damaged while repeatedly loaded, fired, extracted and sized will present wear. This damage shown on the rim should not be reloaded.
Catastrophic case failure: Here are example cases for many reasons, both alone or in combination that spectacularly failed.
When I find more, I will add them. -ken
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