Some metal parts are exposed to heat within their area of application. This could be within a furnace, exhaust system or engine bay. The bare metal surface of such parts can rapidly deteriorate within such environments long before any mechanical wear out occurs.
Where Bare Metal Starts to Struggle
It’s well known that many metals can start to oxidise at relatively moderate temperatures. For steel this will often start to become visible at temperatures above 300°C, whilst aluminium can start to loose significant strength at temperatures above 150°C. Once a surface has started to oxidise then scaling can and does quickly follow, leading to rapid loss of dimensional accuracy and fatigue resistance.
Thermal Spray as a Barrier Layer
Thermal spray coatings are applied as molten or semi-molten droplets on metal surfaces and form a low-conductivity layer on top of the surface. Most ceramic-based thermal sprays have a continuous operating temperature of over 1,000°C. They are therefore ideal for exhaust duct work, furnace fixtures, and any other component that operates in proximity to high temperature combustion processes.
Hard Anodising on Aluminium
Aluminium components can be surface treated by hard anodising. This process converts the surface of an aluminium component to a hard aluminium oxide layer, which is much more able to withstand high temperatures than the aluminium below. The process does not significantly increase the dimensions of the part and is particularly suitable for parts that are required to remain accurately dimensioned at high temperatures. The maximum temperature to which an anodised component can be used is around 200°C for sustained periods.
Electroless Nickel and Phosphorus Content
The phosphorus content within an electroless nickel coating can have a major effect on its thermal properties. High phosphorus coatings, greater than 10% P, are generally amorphous within moderate temperature ranges. However, when exposed to prolonged high temperatures (above 300°C) the coating can begin to crystallize, during which time there can be an increase in the coatings hardness. In choosing the correct phosphorus grade of electroless nickel it is important to take account of the parts’ operating temperature.
Surface Treatments in Oxidation-Heavy Environments
For components such as furnace fixtures, exhaust manifolds, and engine components (such as brackets) that are exposed to high temperatures in service and are prone to scaling, treatments such as thermal spray or hard chrome can be beneficial in terms of extending component life through reduced maintenance. A useful reference for Surface Treatments is https://www.poeton.co.uk/surface-treatments/. UK employers must also note that there are special high temperature work equipment requirements to be fulfilled to ensure that equipment and components are fit for purpose when in use at high temperatures.
It is the coating’s temperature resistance that is critical, and needs to match the actual temperature to which the part will be exposed.
