Fighting corrosion 403

17.7 Fighting corrosion 403

O 2 O 2 Wet soil

Steel sheet

Figure 17.12 (a) Protection of steel pipes by a zinc sacrificial anode. (b) Galvanized steel

plate; the zinc protects the steel even when scratched.

Plating can work in the same way. Galvanized iron is steel sheet with a thin coating of zinc. The zinc acts as a sacrificial anode (Figure 17.12(b)): if scratched, the exposed steel does not rust because the zinc protects it even when it does not cover it. Other platings look good but do not protect so well. A plat- ing of copper sets up a cell the works in the opposite direction, eating the iron at a scratch rather than the copper. Chromium plating looks as if it should pro- tect, since chromium is below iron in the table, but the chromium protects itself so well with its oxide film that it becomes passive and no cell is set up. It is this passivity that makes the stainless alloys described in Section 17.3 good at resist- ing aqueous corrosion as well. Stainless steel is not just oxidation resistant, it is corrosion resistant too, and the same is true of the other alloys listed there.

When tricks like these cannot be used to prevent corrosion, the answer is to separate the reactive metal from the corrosive medium with a polymer film. Painting with solvent-based paints was, until recently, the most widely used method, but the evaporating solvent is generally toxic; it is a VOC (a volatile organic compound) and, for health reasons, their use is discouraged. Water- based paints overcome the problem but do not yet produce quite as good a paint film. Polymer powder coating works by plasma-spraying the polymer or dipping the component when hot into a fluidized bed of polymer powder, caus- ing a thick film of polymer to melt onto the surface. While intact, paint films and polymer powder coats work well, but as soon as the coating is damaged the protection ceases unless the underlying metal has a sub-coat of something— like zinc—that protects it.

Organic solvents and the solubility parameter

Organic solvents attack certain polymers. The nearest that this can be made sci- entific is via what are called solubility parameters. Polymer chains are bonded strongly (covalent bonding) along their length, but only weakly (van der Waals bonding) between chains. To enter a polymer the solvent, like a virus, must trick these bonds into allowing them in—that is where the ‘like dissolves like’,

404 Chapter 17 Durability: oxidation, corrosion and degradation

mentioned earlier, comes from. Once in—like viruses—they do things: mostly bad, a few, good.

The bad: like UV radiation, aggressive solvents cause discoloration, reduce strength, induce brittleness and trigger crazing (the whitening of the polymer because of many tiny crack-like expansion cavities). The good: certain organic solvents act as plasticizers, reducing the glass temperature but not the strength, converting a rigid polymer into a flexible, leather-like material. Artificial leather—plasticized PVC—is an example. Many plasticizers are phthalates but increasing concern for their potential toxicity generates pressure to use alterna- tives. Increasingly they are replaced by biochemical plasticizers—vegetable oils such as soybean oil or linseed oil—although these are more expensive.

Which organic solvents are the worst? Not an easy question. Amyl acetate (nail varnish) is particularly aggressive, as is chloroform (anesthetics), but this is no answer. To get a full answer you have to turn to the more specialized CES OPS database (which runs in the same system as that of the one used to make the charts and for the exercises in this book); it gives detailed rankings of a large number of individual polymers in a large number of different organic (and other) gases and liquids.