Stress Corrosion Cracking:
A much more serious form of stress cracking could occur if the brass were exposed to an oxidizing agent or an acidic environment with the presence of ammonia or ammonium salts, because they form complex ions with zinc and copper, namely tetraaminezinc [Zn(NH3)4]++ and tetraaminecopper [Cu(NH3)4(H2O)2]++ ions (which gives the acqueous solution a blue colour).
The protective layers of zinc and copper oxides, which protect the metal underneath because they are insoluble in water, become soluble with the formation of the complex ions and are removed from the surface of the metal, exposing fresh metal to be oxidized. The oxidation continues unimpeded until the metal dissolves. If a stress is applied to the brass (such as hanging a weight from a brass hook), the brass will crack at the points of greatest stress when enough metal is oxidized to cause the lattice structure of the metal to fail. For example, if you hang a lamp from a brass hook in a barn (where there is plenty of moist air, carbon dioxide and ammonia produced by the animals), the moist air results in the formation of a layer of electrolyte (weak carbonic acid from carbon dioxide dissolved in water) and the right conditions are created for corrosion to take place, unimpeded by any protective layers of insoluble oxides or hydroxides as the ammonia makes the metal ions soluble. The warmth that rises from the lamp below accelerates the corrosion, the hook cracks and breaks, allowing the lamp to fall.
