finishing metals

REASONS FOR FINISHING

metals are finished to prevent corrosion in all ferrous and some non-ferrous metals and to improve or change the appearance of a metal

PAINTS

preparation - before painting the metal surface needs to be cleaned and keyed for a primer to bind to, often a red oxide is used as a primer with an undercoat, then multiple layers of paint are applied with a final topcoat to finish

cellulose paint - paints give colour and act as a corrosion barrier, the piece is cleaned and grease is removed, it is then keyed to allow paint to grip more easily. cellulose paint is used on cars as it dries quickly, cellulose must be applied in multiple coats to get a high-quality, smooth finish

acrylic paint - these paints are water-based, they require heat and professional equipment, they also need 24 hours to dry (cure), they give a durable and hard wearing finish good for outdoor applications, they can be applied with a brush, roller of spray and can require a primer for a better finish

METHODS

galvanising - galvanising is used on iron and steel using zinc as protection, this prevents corrosion. before galvanising the metal must be cleaned four times. galvanising can be done as electro plating or hot dipping the zinc at 460°C, then the metal is cooled and inspected for quality control, this is normally used on bigger pieces as the finish is thicker than electroplating, however, hot dipping is less aesthetically pleasing. hot dipping gives a longer lasting protection than electroplating

electroplating (a different type of galvanising) - when a metal is plated with a cheaper base metal, this gives protection and is also aesthetically pleasing. this process uses an electrolyte solution and a positive cathode with the coating metal and a negative anode with the coated metal

sealants - sealants are used to stop air and fluids flowing, often used on valves and moving parts, they are normally polymer based and can help decay, weathering and corrosion

preservatives - these often provide temporary instead of long-term protection , they are used in moulds and dies to prevent natural corrosion. they are used on machine beds as well to reduce friction and sometimes they can be used to control rust levels, they are oil based and are applied through electrostatic , spraying, immersion or with a cloth

anodising - anodising is used normally on aluminium to create corrosion resistant surface that is hard wearing, the aluminium is cleaned and etched using chemical and is then put in a large vat with a cathode with the aluminium as an anode to create an oxide layer, the chemical in the vat is an electrolyte and is often an acid. the metal is the coloured and sealed. anodised products come in a range of colours and they can also have increased colouration, resistance to wear, lubrication and electrical insulation.

polymer dip coating - dip coating is wear resistant and water resistant, it is also very thick creating an air tight finish. the metal is heated to 230°C and is then dipped in fluidised polymer powder with warm air, it is then cooled. metals can also be dip coated in tin

powder coating - powder coating gives an even coating using electrostatic charge and can be applied in awkward corners using an air gun the workpiece is then put in a oven where the coating bonds through melting. powder coating is often used on moving parts as it is more flexible than paint

cathodic protection - anodes are added to a large piece of metal which corrodes first as it is more reactive, this is done on large pieces such as oil rigs or boats, as they are too large to be fully finished