Anodised aluminium surfaces are characterised by their protective, ceramic-like oxide layer, which can, however, show fine cracks, so-called microcracks, in certain situations. These microcracks often only become visible when the light falls at an angle, whereas they are barely noticeable when viewed directly. They are an indicator of mechanical or thermal stresses on the surface and represent a typical form of damage to the oxide layer.
Development of microcracks
An anodised component consists of two different material layers: the soft base material aluminium and the brittle but firmly adhering anodised layer. Stresses between these two layers can cause the brittle top layer to develop fine cracks. These tensions are often caused by temperature differences or mechanical stresses.
An example of thermally induced cracking is an anodised, dark-coloured aluminium façade. During the day, it heats up strongly due to solar radiation, while the temperature drops rapidly at night, especially in winter. The resulting temperature changes stress the oxide layer, which can lead to microcracks, even if the layer appears intact on the outside and shows no flaking or splintering.
Mechanical and chemical influences
Microcracks can be caused not only by temperature changes, but also by mechanical stresses such as pressure or bending. The importance of chemical pre-treatment and coating properties is often emphasised in the literature. These factors have a considerable influence on the stability of the anodised layer.
Sealed and non-sealed anodised layers exhibit different resistance to cracking. While non-sealed anodising layers can withstand temperatures of up to around 150 °C, sealed anodising layers tend to crack at temperatures of around 80 °C and above. The type of sealing system used plays a decisive role here.
The following conditions can influence the formation of microcracks on anodic layers:
Influencing factor | Effect on microcrack formation |
Using thin anodised layers, < 5 µm | Risk decreases |
High current density (anodising) | Risk increases |
Low sulphuric acid concentration (anodising) | Risk increases |
Low bath temperature (anodising) | Risk increases |
Use of special hot sealing processes | Risk decreases |
Use of cold sealing (nickel fluoride) only | Risk increases |
Use of cold sealing (nickel fluoride) with hot water – ageing |
Risk is slightly reduced |
Sealed surfaces in very dry atmosphere | Risk increases |
Sealed surfaces in humid atmosphere | Risk decreases |