Corrosion Prevention and Control by Design
At ambient temperatures significant corrosion occurs only if moisture is present. Thus, surfaces should be exposed as little as possible to moisture and arranged so that they dry out quickly after wetting. In practice all surfaces are at risk, vertical surfaces suffer ‘run off’, flat surfaces retain moisture on their top side and can attract dew and condensation on the under side.
Water retention by ‘V’, ‘H’ and other channel sections is obvious and drain holes should be provided, if mechanically acceptable. Overlaps and joints should be arranged to avoid the formation of water channels. Porous materials that can retain moisture should not be in contact with metals. The design should also make provision for inspection and maintenance during the service life of the structure.
Corrosion Prevention and Control by Coatings
Application of one or more coatings to a suitably prepared surface will isolate the metal from a corroding environment. Organic coatings, such as paints, pitch, tar, resins etc., form a protective barrier and are commonly used, often in conjunction with a metallic primer.
There is a wide range of products available, often for specialist purposes. Some metallic coatings will form a simple protective barrier, e.g. nickel or chromium on steel. However, with chromeplated steel, the chromium is more cathodic than iron so that if a small pit appears in the chromium, the steel underneath rusts away quite rapidly, as owners of old cars will testify.
All paint coatings, even of the highest quality and meticulous application, are only as good as the quality of the preceding surface preparation. Application, whether by brushing or spraying, should always be carried out on dry surfaces and in conditions of low humidity.
Steel that has been allowed to rust on site can be a problem, as the methods available for cleaning steel are often less than adequate, and some rust will inevitably remain at the bottom of the pits formed during rusting. These will contain sufficient active material for rusting to continue below any paint film.
The only real remedy is not to let rusting start by protecting the steel by priming coats as an integral part of the manufacturing process and, if these are damaged during erecting, to repair the damage as soon as possible. We will discuss the particular problem of the corrosion of steel in concrete, which is the cause of much deterioration and hence cost.
Corrosion Prevention by Cathodic Protection
If a metal in a corroding environment is connected to another metal that is more reactive, i.e. higher up the electromotive series, then the second metal will form the anode of the corrosion cell and hence will preferentially oxidise, thus protecting the first metal from corrosion. This can be used for example to protect buried steel pipelines by connecting them to zinc slabs buried nearby.
The slabs corrode, and are therefore called sacrificial anodes; they are periodically replaced. The same principle is applied in galvanising, in which a layer of zinc is deposited on the surface of steel by hot dipping. In moist air and in most other aqueous environments zinc is anodic to the steel and will thus protect it if there is any surface damage.
Furthermore the corrosion rate of the zinc is very slow because of the high ratio of anode to cathode surface area (Fig. 1). In addition to the use of sacrificial anodes, cathodic protection can be achieved by the use of an external power source to make the metal cathodic to its surroundings. Inert anodes are used, commonly carbon, titanium, lead or platinum. The procedure is not without its problems. For example in many cases the cost of replacement anodes is greater than the cost of the impressed power supply.
This method of cathodic protection has been quite widely used in marine environments, especially on offshore oilrigs. However in buried structures secondary reactions with other nearby buried structures may enhance, rather than control, corrosion and there is the possibility of hydrogen evolution at the cathode. This can diffuse into the metal and embrittle it.
The management and control of corrosion comprise one of the most difficult problems facing the design engineer. It is critical to recognise that the problems start at, and must be tackled at, the design stage. There are three requirements, all easy in theory but difficult in practice.
- Understand the environment in which the metal must work, whether polluted or not, whether facing or away from pervading sources of corrosion, whether wet and/or humid or dry, whether these conditions are stable or variable.
- Consider the ‘design life’. How long before the first major maintenance? Are you designing a ‘throwaway’ structure like a modern motor-car or are you designing a bridge for a century of service? If the component is not expected to outlive the structure as a whole, how easy is it to inspect and replace?
- Select the most appropriate method of control from those outlined above. You may be excused for imagining that the ‘most appropriate’ method is that one which involves the longest life, but you will, of course, be wrong. In the commercial world the most appropriate means of control is that one which produces the longest life at the least annual cost. So, on the whole, you would be best to master such matters as payback, rate of return, and discounted cash flow before deciding upon an appropriate technology .
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