2.6.2 Cracking of copper alloys
Stainless steel is not the only metal to fall victim to SCC. One of the first discoveries of SCC occurred in India in the early part of the nineteenth century, when that country was still part of the British Empire. There was a large standing army that was always in need of live ammunition. The brass cartridge cases would occasionally split, and often at the worst possible time (when being fired), frequently causing injury to the marksman.
So what caused such failures? The two factors needed for stress corrosion cracks are, first, a tensile stress in the outer layers of the brass and, second, an active chemical that will attack brass or copper. The stress could be caused by the manufacturing forces used to shape the cartridges, since the cases were made from cold-deformed brass (70% copper, 30% zinc). The process involved successive stages of deformation of flat discs punched out from 3.25 mm thick sheet (Figure 22). After each stage, the product was annealed in order to recrystallise the metal, and pickled with sulphuric acid to remove oxide at the surface. The annealing process was intended to relieve residual stresses set up in the cases, but the process was not always successful in completely removing these stresses.
After some detective work, an association was seen between the rate of cracking and the season of the year. Cracking tended to occur during the monsoon season when humidity and temperatures were high, rather than during the cooler months. Yet although the rate of most chemical reactions increases with temperature, controlled experiments showed that this could not have been the only cause of the problem. Then the Woolwich Arsenal undertook a series of trials with many different chemicals. They exposed bent strips of brass to the chemicals and observed the metal surfaces at the most highly stressed zones. They found that ammonia gas and water vapour were, in combination, the two most potent agents needed to initiate brittle cracks. Bearing in mind the experience of stainless steel in chlorine-doped water, it is interesting that failure times for many samples dipped into ammonia solutions were longer than for exposure to ammonia gas and water vapour.
The mystery was therefore solved, because it was realised that ammonia is produced by manure and dung, so would have been present in the stables of the army horses, for example. If ammunition had been stored near the stables, it is most likely that trace amounts of ammonia in humid air could have cracked the brass cases extremely quickly. Hairline or microscopic cracks would have been formed, and then grown to a critical size by the time the ammunition was needed.
So why does cracking or highly localised attack occur in such a case, rather than general corrosion? The active agents attack at stress raisers, at the upper edge where the case makes contact with the bullet (Figure 23). The formation of a galvanic cell is unlikely, because a thin film of water on the surface is insufficient to provide the electrolyte. However, the final stage of manufacture, when the bullet is put in the explosive-filled case, will put the lip under a radial or hoop stress. The edge is unlikely to be totally level, and small degrees of roughness there will be attacked by the ammonia. Once a crack has formed, it will grow under the influence of the hoop stress, with the corrosive solution seeping away to leave a fresh crack tip ready for further attack.
The problem of chemical attack on brass and other copper alloys is not uncommon, as the example described in Box 3: Pump failures demonstrates.
Box 3: Pump failures
Brittle ceramic products are frequently impregnated with softer and tougher materials to strengthen them. Ceramics usually have an open pore structure, and filling the pores with a crack-resistant material toughens the final product. Such a process is used to improve the toughness of anodes used for the electrolytic production of aluminium, and involves applying a vacuum to a chamber in which the anodes are placed. Liquid pitch is then pumped into the chamber to fill the pores, before the anodes are removed for baking so as to solidify the pitch.
After six months’ operation, the vacuum in one such chamber deteriorated and investigation pointed to failure of an impellor used to apply the vacuum. The impellor was made from brass and had suffered severe corrosion, with the formation of a green deposit over all the surfaces (Figure 24). The impellor was replaced, but the vacuum again began to deteriorate. The time had come to perform a serious investigation, especially as the impellors were rather expensive.
Suspicion fell on the liquid pitch, as it had a high sulphur content, but an alternative explanation quickly became apparent. Several operators had smelled ammonia in the pitch, but it took an alert manager to recognise the cause of the corrosion. As an OU student studying forensic engineering, he correctly identified that the ammonia had attacked the copper component of the brass to form cuprammonium salts, attack being most severe at the tips of the flight vanes and corners of the design where the local stresses were highest.
At these points, stress corrosion cracking had further weakened the impellor. The net result was loss of material leading to loss of evacuation power, with a lower vacuum for impregnation.
So what was the solution to the problem? One possibility was to replace the rotor with stainless steel, but this was an expensive option. A lower-cost solution would be to apply a resistant coating to the surface of existing brass impellors, which would prevent the ammonia contacting the brass surfaces. Several different polymer coatings could be used, such as sintering powdered PVC or polypropylene onto the part, but the final solution involved using epoxy resin. The pumps have, since this innovation, proved entirely trouble-free.