Corrosivity of smoke towards metals

Many people believe that smoke is corrosive only if it contains acid gases. This leads to associating corrosive potential of smoke from materials directly with the amount of acid gases emitted from them during combustion. In extreme cases, chemical composition (i.e. halogen content) is used as a substitute measure of corrosiveness and, therefore, of material suitability for certain applications. The reality is, however, that everything is usually heavily corroded in a fire, whatever the combustible materials involved. In the present project sets of 5 flat carbon steel coupons and 2 copper mirrors were exposed to smoke from several materials for 1 h, in a chamber at room temperature and at 100°C, and kept for 28 days at 75% relative humidity. Various surface treatments were also studied. The combustibles tested were: a PVC wire compound (PVC WR), a low acid PVC wire compound (PVC LH), a halogen free polyethylene wire compound (PE), wool, wood (Douglas fir, DFIR), polystyrene (PS), neoprene (NPR), nylon (NYL), and a blank (no sample). In order to determine corrosivity the resistance of the copper mirrors and the loss of metal from them was measured. All copper mirrors lost their electrical properties: an almost infinite resistance was measured post-exposure. The amount of copper lost was very similar for all smokes, but, in the 100°C experiments, the smoke corrosivity ranking was: All steel coupons were corroded too, at least to some extent. The amount of iron lost depended on treatment as well as on the smoke used. In particular, the highest degree of corrosion was found with coupons exposed at very high temperature, where, again, the most corrosive smoke came from nylon. Common wisdom suggests that acid gases result in corrosion of metal. The present work substantiates this fact. However, the acid gas concentration in the atmosphere does not normally correlate with the level of metal corrosion. The results of this work indicate that metal corrosion depends on factors in addition to acid gases: type of metal, temperature, humidity, length of exposure, surface treatment, and geometry. Furthermore all smokes tested are corrosive.