Our outdoor steel and stainless steel postboxes are designed to be as weather-resistant as possible, and should last many years with little to no rusting or tarnishing. However, in certain circumstances tarnishing can occur, particularly in coastal areas, areas where the air has a high salt level, or areas with high levels of pollutants in the air. In such situations, you may wish to consider a PVC or plastic post box instead.
In the case of stainless steel postboxes, tarnishing can occur because of a process called galvanic corrosion. This occurs when moisture is present and stainless steel comes into contact with another form of metal such as plain steel or aluminium. If a stainless steel postbox has been manufactured with plain steel parts such as hinges or springs, or includes plain steel screws or other fittings, galvanic corrosion can result. It is also possible for pollutants in rainwater to cause very minor water marks or rusting on the surface of the stainless steel postbox due to pollutants.
In most cases, you can avoid such corrosion by occasionally cleaning the postbox with a soft cloth and an agent such as WD40. Applying some grease to screw heads and other fittings may also help prevent rust marks. Where tarnishing and rusting occurs, it can usually be wiped off quite easily, again by using a soft cloth and WD40. With a little care and maintenance, your stainless steel box should remain practically rust-free.