Occasionally, it arouses astonishment when, after anodising the products, profiles unexpectedly appear that show a differently pronounced iridescent colouring of greenish, reddish and violet tones. This phenomenon does not originate from a pioneering colouring technique, but is rather the result of challenges during anodising.
What is the cause?
The correct contact is undoubtedly an essential prerequisite for the successful and uniform formation of an oxide layer during the anodising process. If this contact is incorrectly dimensioned or arranged, burns can occur on the workpiece near the contact or an uneven layer formation can occur.
But what happens if the contact points to the product carrier slip during the anodising process? In such a case, a more or less intensive, iridescent discolouration of the displaced material can occur. Especially if a certain thickness of the oxide layer has already developed and the contact points then change their position, the current transfer from the contact with the material is suddenly interrupted. This happens because the current transfer is now on the insulating oxide layer instead of the conductive aluminium material.
Under these circumstances, there is no further increase in the thickness of the oxide layer. This highlights the importance of correct contacting and fixing goods on the racking system to ensure a high quality result.
The cause of the defect can usually be quickly identified by measuring the oxide layer thickness after anodising. While the correctly anodised product then has, for example, 20 µm nominal layer thickness, only 5 µm layer thickness is measured for the slipped component.
How can the error be avoided?
It is of crucial importance that the goods are securely fixed, especially if bulky components are involved, in order to prevent undesired shifting during the anodising process. When processing sheet materials, care should also be taken to ensure that the air supply in the anodising bath is not excessive and that the material is placed in such a way as to ensure a streamlined arrangement.
Depending on the fixing excessive pressure from air bubbles can otherwise cause continuous deformation of the sheet material surfaces, which can ultimately lead to loosening of the fasteners and slippage of the contacts on the sheets. This underlines the need for precise measures to ensure the quality of the anodising process and to avoid undesirable results.