Agitation of anodising solutions by pumped flow eductors

The necessity for agitation of anodising solutions is well known but is par ticularly acute in the case of production of hard anodised surfaces. Throug hout metal finishing agitation is widely practised for a number of reasons, only two of which are appropriate to anodising: 1. To remove heat from the electrode-electrode interface. 2. To dissipate aluminum ions and any oxygen gas at the anode/solution inte rface. The use of eductors, which are available in many proprietorial forms, is in principle old technology but has recently been adapted for use in electrop lating applications. The paper will discuss results for eductor agitation and show that they are of importance in the context of surface heat dispersal, the effectiveness of which can be assessed in terms of anodic film hardness and porosity whic h are the two properties most sensitive to temperature rise beyond specifie d values. Limitations of the analogy will also be considered, notably that anodic fil m formation is not usually solution mass transfer controlled and so the opp ortunities for process rate-enhancement may be limited, unlike electrodepos ition.