Metal spalling is a process of metallic surface failure in which the metal is broken down into small flakes (spalls) from a larger solid body. This process occurs for many reasons, such as when another material impacts it at a high speed resulting in chipping the material, or due to corrosion, weathering, cavitation or excessive rolling pressure.
In the metal spalling process, spontaneous fragmentation, chipping or separation of a surface or surface coating occurs. The most favorable location for metal spalling to occur in industrial equipment is either at the lowest point of the equipment or at the metal joints or crevices where the stresses are high and maximum shearing of the material occurs during operation of the equipment.
A simple form of mechanical spalling occurs when two metal plates collide with each other and produce metal shock waves (also known as compression waves) that are reflected back on the respective metal plates. These compression waves are further reflected to the regions where high tensile stresses are acting and cause further damage and failure.
Metal spalling is also caused by cavitation, when fluid at a low pressure causes a formation of vapor bubbles. A localized high-pressure area is created when these vapor bubbles collapse that causes spalling on adjacent or nearby surfaces.
Also, if a metallic surface is already corroded, it results in spalling as small flakes of the metal are chipped away, which further exposes the inner surface of the material to a corrosive environment.
Metal spalling due to a weathering process occurs due to exfoliation and salt spalling.