Galling is an unexpected form of wear on engineers, who often expect it to only occur under high stress, but it can in fact occur under moderate loads in a very short amount of time. Understanding why galling occurs and how coating can prevent it and thereby save components from early failure is therefore very important.
What Galling Actually Is
Galling is defined as adhesive wear, which is a specific type of wear that can affect components operating under load. This type of wear can occur rapidly and often appears to occur under moderate load. At the point of contact between two surfaces there are numerous asperities, some larger than others. In cases where galling occurs, the asperities on opposing surfaces weld together momentarily and then pull apart taking some material with it. This process can repeat many times causing severe wear to the surface of components such as causing a fastener to seize in a threaded hole or to create deep scores on sliding surfaces.
Why Bare Steel Is Vulnerable
Steel on steel contact surfaces are prone to galling because of their similar hardness and surface energy. As a result, the surfaces can cold-weld together momentarily. However, when the surfaces are ‘un-stuck’ they can pull material with them, causing damage. A surface coated or made of a dissimilar material can help to reduce galling.
How the Alloy Layer Interrupts Adhesion
The alloy coating, in this case Zinc Nickel Plating, is deposited to a thickness of around 8-15 microns. This hard coating is significantly harder than steel and is a completely different material, with a different surface energy. As a result, the coating prevents the metal-to-metal adhesion, typical of steel on steel contact, that causes galling. The coating has low friction properties and means that contact asperities simply slide past each other as opposed to welding together.
Where It Makes a Practical Difference
Fasteners, flanges and sliding parts are typical areas where galling can occur. By applying a zinc nickel coating to the threads of bolts and other fasteners, or to the faces of flanges, threaded parts and sliding parts can be assembled and disassembled repeatedly without seizing or galling.
What to Specify
A minimum thickness of 8 microns and a confirmed nickel content of 12 per cent to 15 per cent within that thickness is acceptable for specification. If you are looking for Zinc Nickel Plating, see https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Getting the specification right the first time is a small cost in comparison to the cost of replacing failed parts.