Steel's relatively high strength gives it many advantages as a construction material and steel products can be used in a wide variety of applications.
The most common problem with using steel in a marine environment, however, is that it corrodes easily. Hence, when steel is used, some knowledge of corrosion and of methods for protecting against it is necessary.
Steel corrodes (that is, rust forms) when it is exposed to oxygen or air. When other atmospheric agents, such as water (rain or moist air) and salt (sea spray), are also present steel corrodes at a much faster rate. Furthermore, when immersed in sea water, steel is also subjected to galvanic corrosion similar to that which occurs between steel and brass fittings on a boat. In Figure 104:
- Zone A is exposed to moist air and salt-laden spray, so is a corrosive zone for steel products.
- Zone B is constantly wet with salt-water which also contains a lot of dissolved oxygen. This is the most threatening zone for steel.
- Zone C is also very dangerous for steel because galvanic corrosion is taking place.
Figure 104
There are five possible ways of protecting steel products against the effects of corrosion:
Use stainless steel instead of normal steel. Stainless steel is normal steel mixed with other metals such as nickel and chrome. However, stainless steel is very expensive and so it is impractical for everyday use except for small fittings such as bolts and nuts.
Coat normal steel with zinc. This is generally referred to as "galvanizing". It is the most common way of protecting small manufactured objects, such as mooring rings, pipe bollards, bolts, clamps, chains, shackles and water pipes. The items to be galvanized are usually dipped in a bath of molten zinc. Once an object has been galvanized, it should not be welded, cut or drilled; these would damage the protective coating.
Coat normal steel with special wear-resistant plastics. The coating process, however, is done at special workshops and is expensive, making this method impractical for everyday use. Paint normal steel with special paints. This is by far the most common method of protecting large steel structures. Ideally, the surfaces to be painted should be sand-blasted to make them clean and smooth. If this is not possible, they should be thoroughly cleaned with a steel brush. The undercoat should be a zinc-based primer. The second and third coats should be a tar-based epoxy paint.
When painting steel remember:
- Common household paints are not suitable for the marine environment because they age very quickly when exposed to the sun's rays.
- Diesel, kerosene and petrol are not chemically compatible with marine paints and only the proper, recommended paint thinner should be used.
- When handling epoxy-based paints, rubber gloves should always be worn.
Protect normal steel with zinc anodes. This process is called cathodic protection. Zinc anodes are often used in conjunction with painting to further prolong the life of steel structures, such as steel piles, pontoons and metal floats, that are immersed in sea water. Aluminium fittings in contact with wet steel are also subject to galvanic corrosion.
