Metals are often thought of as strong, durable and long lasting, and generally speaking this correct. However, if the correct surface protection is not used for the intended purpose, they can very quickly corrode, losing their visual appeal and eventually their structural integrity. Although there are hundreds of different types of surface coatings, most are designed to either prevent corrosion, increase the aesthetic appeal, or both. Below is some basic information on the types of surface coatings that are commonly used by street furniture manufactures.
A very thin layer of zinc is imparted onto the surface of components using electrolysis. Generally street furniture items that are zinc plated are painted as well. Zinc plate and powder coat can offer good service lives of up to 15 years in low-corrosive environments, but is not suitable in coastal areas, other corrosive environments, or where vandalism and “hard” use is present. It is a relatively cheap process.
Hot Dip Galvanising
Components are dipped into a molten tank of Zinc, leaving a thick coating. Used extensively in outdoor products it offers a high level of protection against corrosion due to the volume of zinc present. Galvanised products initially have a bright silver look, but after several months will take on a matt grey finish. It will retain this finish until the zinc is consumed. Galvanised steel products can also have a secondary finish applied, such as powder coating. This gives the advantage of both a good corrosion protection and a visually appealing finish; however there are several steps that need to be taken to ensure good adhesion and finish, such as etching and de-gassing. Galvanising is more expensive than zinc plating, but is still reasonably economical. Galvanising creates a high level of protection against corrosion and is suitable for most environments.
Components are placed into a rumbling drum with zinc powder and plastic shot. The rotating action of the drum imparts the zinc onto the surface of the components. With similar protection levels to hot dip galvanising, the main advantage of this process is the uniform coating that is free from dags and slag that can be associated with hot dip. Due to the nature of the application, this process is only suitable for small components such as brackets and fasteners. It is an economical process when the labour costs of de-burring hot dip galvanised products are taken into account.
Zinc Rich Primers
Either through powder coating or a wet coat paint process, a primer containing zinc is applied to the material prior to a finish coat. Depending on the type of primers and paints used, protection levels vary. This generally offers a medium level of protection, but is usually a more expensive process and is not suitable where surface finishes are likely to be broken by physical damage.
Zinc Plated Sheet Steel
Either an alloy of aluminium and zinc (zincalloy, zincallume etc) or Zinc (gal sheet) is applied to sheet steel in a production line. Even if the material is cut into shape, the sacrificial properties of zinc will protect the raw edges to an extent, but if large areas of the finish are damaged during fabrication methods, such as welding, weak areas of protection will be created where corrosion can begin. This is a very cost effective method of achieving a good level of protection and excellent surface finish, but is acceptable only for low corrosive environments.
Components that are either coated with one of the above substrates, or a raw, are coated in a layer of powdered polymer, using electrostatic charge to hold the powder to the part. The coated part is then baked in an oven which melts the polymer and achieves a very smooth and consistent finish. Although there a different types of products available with different attributes, powder coat generally offers low levels of corrosion protection and for street furniture is really only an aesthetic finish (there are some powder primers and clear coats that bend this rule). Powder coating is suitable for steel and aluminium and is a very cost effective way of a achieving an attractive and uniform finish, with large range of colours available.
2 Pak Painting
Components are coated with primers and top coats of 2 part paint systems, consisting of a base and hardener. Many different brands of paint are available, but in general terms 2 pack paints offer a very hard wearing surface and a high level of finish. Depending on the primers used, 2 packs main defence against corrosion is that is seals the base material from the atmosphere. Due to the extreme durability of good quality 2 pack paints, this can offer a very high level of protection; however it relies heavily on the surface. If the surface is broken, then corrosion can instigate and proceed to spread underneath the paint, much like a car rusts from chips in the paint. The finish that can be achieved with this type of coating is very high, and good quality paints will retain finish quality far longer than powder coat. 2 pack paint is generally a more expensive finish than powder coating.
Components undergo an ectrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. In simpler words, aluminium naturally gains a skin that protects it from further corrosion. Anodising thickens this skin. Generally speaking all aluminium used for street furniture should be anodised; otherwise the surface will quickly become cloudy and unsightly, particularly in corrosive environments. Anodising is also a suitable preparation for powder coat. It is a reasonably cost effective process and offers a very high level of corrosion protection that is suitable for coastal areas.
Draffin Street Furniture has vast experience with surface coatings. If you have and questions or need advice when specifying a surface coating please contact one of our friendly engineers. We love to put our experience and resources to work helping solve our customer’s problems.