The base metal is usually the first criterion for choosing a coating. The pairing of a coating with a base metal is critical, and poor choice can lead to early failure of coatings that are otherwise applied correctly.
Aluminium Needs Its Oxide Removed First
The natural oxide layer that forms on the surface of Aluminium as soon as it is exposed to air has to be removed or etched prior to coating. The anodising process exploits this property and converts the surface layer to a hard, porous oxide which is an ideal surface for dyeing and sealing.
Steel Suits Hard Chrome or Zinc Nickel
When treating steel prior to coating it is a dense, relatively inactive material. The most common surface treatment for steel before coating is therefore Hard Chrome or Zinc Nickel. As the name suggests Hard Chrome is a very hard coating that is ideal for high-load sliding wear applications. Zinc Nickel is a more corrosion resistant coating and is generally more suitable for applications where the surface hardness of the coating is not as important as corrosion resistance. Steel fasteners and structural brackets that are exposed to salt are typical examples of items that are best coated with Zinc Nickel.
Copper Alloys Respond Well to Electroless Nickel If you want Surface Treatments, www.poeton.co.uk/surface-treatments/ is a good place to start.
Copper and other alloys of this metal are ideal for coating with electroless nickel. The coating process is very even, with no problem in recesses and blind holes. Valve bodies, and other components of this type such as hydraulic fittings, are typical examples of the use of electroless nickel on copper alloys.
Titanium Requires Specialist Processes
As one of the most reactive of all the metals, and a very hard material to apply a coating to due to its natural passivation, special activating treatments are necessary to enable coatings to adhere to this material. For information on the various treatments, and how to specify the correct ones for your application, refer to the materials and coatings standards from recognised engineering bodies.
Base Metal Hardness Sets the Ceiling
No coating can make up for a soft substrate under heavy load. A surface treatment which is suitable for hard anodised components will fail quickly on soft alloys as the substrate will deflect under load. It is common practice to specify the hardness of the substrate together with the coating thickness for wear resistant applications.