Protecting Against the Corrosion of Iron

A common approach to preventing or limiting corrosion is to eliminate contact with the corrosive factors. The simple act of washing off road salt removes the ionic solution from auto bodies. Iron objects are frequently painted to keep out O2 and moisture, but if the paint layer chips, rusting proceeds. More permanent coatings include chromium plated on plumbing fixtures. 

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Davy made use of zinc to protect against the corrosion of copper or iron parts in ships. 

Thus, in the case of iron coated with zinc (galvanized sheet), zinc would protect iron by sacrificing itself, i.e., by anodically dissolving in the corroding media. However, in the case of iron coated with tin (tinned sheet), tin would protect iron against corrosion by virtue of its own corrosion-resistance properties however, any flaw in the coating would enhance the corrosion of iron since it is anodically disposed to tin according to their placements in the electrochemical series. 

However, in many cases, the oxide layer adheres, or sticks firmly, to the metal surface. This layer protects the metal from further corrosion. For example, aluminum, chromium, and magnesium are readily oxidized in air to form their oxides, AI2O3, Cr203, and MgO. Unless the oxide layer is broken by a cut or a scratch, the layer prevents further corrosion. In contrast, rust easily flakes off from the surface of an iron object and provides little protection against further corrosion. 

The oxidation of iron with steam is used technically as a means of protecting the metal against corrosion. This is the principle of the Bower-Barff process. 

Under certain corrosive conditions, many metals form covering layers. If these are sufficiently dense, they act as protective films against the corrosive removal of the material. An example of this is the protective layer of iron oxide formed in unalloyed or low-alloy boiler tubes. Corrosion with erosion is understood as the combined action of mechanical surface removal and corrosion. With some soft and loose layers, the shear forces obtained with pure flowing liqnids at medium flow velocities are sufficient to damage the protective layer without the involvement of abrasive solid particles. Where drop impingement or cavitation is involved, the mechanical removal of material is understandable. 

A metal pipeline was unlikely to be satisfactory in this application. Cast iron would be expected to last perhaps 20-30 years, whereas the design life was to be 50 years. It would be subject to internal attack from the effluent and external attack from the sea water. Protection against this type of corrosion would be difficult. Similar objections apply to use of mild steel, where again predictable protection would be difficult to achieve mild steel would also suffer rapid internal corrosion. A suitable grade of stainless steel could probably be selected to resist internal attack by the effluent but the problem of external corrosion would remain. Stainless steel is subject to corrosion pitting in sea water, especially when the oxygen content is low. In quiet water the rate of pitting can be 6.9 mm per year. 

The problem of corrosion is immense. One can say with certainty that all metals, except the noble ones (gold, platinum etc.), corrode to a certain extent. Some of them form oxide layers on the surface which protects them against further attack, but some, notably iron, do not form such a layer. Iron corrosion alone costs every year millions of pounds in protection and replacement. There are two main mechanisms by which corrosion occurs. One is by direct oxidation of the metal by air oxygen. Metal oxides are, as a rule, thermodynamically more stable than the metal and oxygen in their elementary states. This is the mechanism by which metals become covered with oxide even in very dry atmosphere. The second mechanism is an electrochemical one two electrode reactions take place on the surface of the corroding metal their products are the undesirable corrosion products. It is easier to understand electrochemical corrosion by considering the corrosion of copper plated iron as an illustration if the plating is broken, the iron, which is in contact with the copper, is exposed to the atmosphere, to rain and often to traffic fumes. The atmosphere contains carbon dioxide and some-... 

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