Galvanising involves sinking the metal, usually steel or iron, in melted zinc. As it’s removed, the metal reacts with oxygen and carbon dioxide to form a layer of zinc carbonate. It’s particularly durable and doesn’t easily flake off. However, it’s expensive and not cost-effective for small components such as nuts and fasteners.
Anodising is the formation of a protective oxide layer on the metal’s surface. It’s most efficiently used on aluminium and a few other non-ferrous metals. It’s a strong coating that can be regularly cleaned using detergents, if mild. It rarely corrodes in standard conditions. However, it does not work on common ferrous metals like steel and iron. The process does not allow for many colours to be achieved, unlike thermal plasma spray.
Electroplating involves creating a layer of one metal on the surface of another. It is highly corrosion resistant and can boost the metal’s mechanical characteristics. It also looks good and is used in jewellery. It’s not really suitable for high-precision applications, due to its uneven surface and is also considered too expensive at industrial scale. The process also has potential for environmental contamination.
All of these methods are designed to save money. Reducing corrosion can offer savings of 15 and 35% in the long run, according to NACE: http://impact.nace.org/economic-impact.aspx.
Applying a liquid paint to the metal surface is usually considered the most easily achievable and affordable method of coating. There are specific paint formulations depending on the type of metal to be coated, where it is to be used and how it will be used. Some paints have toxic elements and other volatile compounds (VOCs), making them harmful to the environment unless dealt with properly and professionally. However, you can achieve a multitude of colours. Another way to achieve colouring and for a more durable finish is through thermal plasma spray.
Powder coating, is an electrostatic process where the object is coated with a powder-based substance. The coating particles have the opposite electrical polarity to the part to be coated, making them stick to the surface. It’s then heat-treated to make the coating hard. This makes the object durable and look good, with many colours achievable. Powder coating is considered good value for money in the long term but the initial costs are high because the process requires ovens, spraying equipment and special booths.