Spalling (Topic: 14099)

Spalling is a defect that is almost unique to electrolytic colouring, and it represents local disruption of the original anodic film leading to local film detachment. In very severe cases the film can become detached over large areas of the sample, but more usually small round pieces of film about 1 mm in diameter break away. The effect is caused by high colouring voltages or current densities and very long colouring times, and can be severely aggravated by contaminating ions such as sodium. It is probably most prevalent in nickel-based electrolytes, but can occur with most processes, like cobalt-based electrolytes. It is almost certainly caused by hydrogen generation within or beneath the barrier layer of the film, and the pressure produced can locally separate the film from the aluminium substrate.