9.4.Degradation of Polymers and Glass 167 or polymers.Moreover,steel pipes,nails,or roofing material are often coated with zinc,leading to galvanized steel.One of these processes,involving molten Zn,called hot dip zinc gal- vanizing,consumes about 3 million tons of Zn per year in the United States.The protection is still maintained even when the zinc surface is scratched,since zinc is less noble than iron and therefore behaves as a sacrificial anode.The same cannot be said for tin-coated iron (used for cans),in which case the iron is attacked by the electrolyte (see Table 9.2). The protective film on metals may be mechanically disturbed in certain parts of machines such as pumps,valves,centrifuges, elbows in pipes,impellers,or agitated tanks due to fluid flow. This leads to accelerated corrosion in combination with ero- sion and is thus appropriately named erosion-corrosion. In specific cases,recirculated water (electrolyte)can be treated with chemicals,called inhibitors,which mitigate corrosion in various ways.For example,the trace elements preferentially migrate to the anode and increase the electrical resistance there,thus decreasing the corrosion current.As an example, chromate,phosphate,and borate salts are used among other constituents for this in automobile radiators. All taken,the corrosion of iron,steel,and other metals and al- loys is a serious problem.However,when properly understood and acted upon,it can be avoided or brought under reasonable control.Specifically,15%of corrosion costs are preventable by application of proper methods.Naturally,there is always an eco- nomic side for not exercising corrosion prevention as outlined at the beginning of this chapter. 9.4.Degradation of Polymers and Glass At the end of this chapter on environmental interactions of ma- terials,some brief remarks on the deterioration of polymers and glass are in place.Polymers are generally considered to be more resistant toward acids and alkaline solutions than most metals. Still,strong acids attack,for example,nylon and polyesters. Strong alkalis affect polymethyl methacrylate or polyesters.Ny- lon absorbs substantial amounts of water if exposed to it for suf- ficiently long periods of time(1.5%in 24 h).This can cause a re- duction in stiffness and hardness.Some polymers such as polyethylene [Figure 3.16(a)],polypropylene,and polyesters are attacked by oxygen and ozone.Other polymers are assaulted by organic solvents.As a matter of fact,the likelihood for dissolu- tion is understandably larger the greater the similarities betweenor polymers. Moreover, steel pipes, nails, or roofing material are often coated with zinc, leading to galvanized steel. One of these processes, involving molten Zn, called hot dip zinc gal￾vanizing, consumes about 3 million tons of Zn per year in the United States. The protection is still maintained even when the zinc surface is scratched, since zinc is less noble than iron and therefore behaves as a sacrificial anode. The same cannot be said for tin-coated iron (used for cans), in which case the iron is attacked by the electrolyte (see Table 9.2). • The protective film on metals may be mechanically disturbed in certain parts of machines such as pumps, valves, centrifuges, elbows in pipes, impellers, or agitated tanks due to fluid flow. This leads to accelerated corrosion in combination with ero￾sion and is thus appropriately named erosion-corrosion. • In specific cases, recirculated water (electrolyte) can be treated with chemicals, called inhibitors, which mitigate corrosion in various ways. For example, the trace elements preferentially migrate to the anode and increase the electrical resistance there, thus decreasing the corrosion current. As an example, chromate, phosphate, and borate salts are used among other constituents for this in automobile radiators. All taken, the corrosion of iron, steel, and other metals and al￾loys is a serious problem. However, when properly understood and acted upon, it can be avoided or brought under reasonable control. Specifically, 15% of corrosion costs are preventable by application of proper methods. Naturally, there is always an eco￾nomic side for not exercising corrosion prevention as outlined at the beginning of this chapter. At the end of this chapter on environmental interactions of ma￾terials, some brief remarks on the deterioration of polymers and glass are in place. Polymers are generally considered to be more resistant toward acids and alkaline solutions than most metals. Still, strong acids attack, for example, nylon and polyesters. Strong alkalis affect polymethyl methacrylate or polyesters. Ny￾lon absorbs substantial amounts of water if exposed to it for suf￾ficiently long periods of time (1.5% in 24 h). This can cause a re￾duction in stiffness and hardness. Some polymers such as polyethylene [Figure 3.16(a)], polypropylene, and polyesters are attacked by oxygen and ozone. Other polymers are assaulted by organic solvents. As a matter of fact, the likelihood for dissolu￾tion is understandably larger the greater the similarities between 9.4 • Degradation of Polymers and Glass 167 9.4 • Degradation of Polymers and Glass