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The agitation of anodising solutions by pumped flow eductors: serflco looks at the reasons for agitating anodising solution and the difference it makes to the process.

As the Surface treatment industry has developed and with ever increasing pressures exerted by customers, energy suppliers and legislators it is now more important than ever to assess the technology that is being used and the benefits that can be derived from new technologies. Whilst there are new developments in chemical additives, rectifiers, racking and pre and post treatments it is possible that the adoption of new methods of agitation can have as significant an impact on process efficiencies, film properties and environmental impact as any other development.

For many years electrolyte agitation has been performed by the simple use of an air compressor/blower connected to air sparge pipes within tanks. Whilst this is cheap and readily available air agitation also has its drawbacks. In this paper we will look at the factors contributing to the removal of air from anodising baths and address the best available replacement technology--Eductor agitation.

Why agitate at all?

Whilst the requirement for the inclusion of agitation in anodising solutions is well known, sometimes it is less clear why the process actually requires it--so simply put it is required primarily to transfer heat and to disperse aluminium and oxygen. Agitation is used to remove excess heat from the electrode-electrolyte interface and to disperse aluminium ions and any oxygen at the anode/electrolyte boundary layer.

If the heat is not dispersed effectively the anodic film will grow being influenced by the characteristics of the localised temperatures rather than those of the bulk solution temperature--so where we think the temperature has been carefully controlled we are actually anodising at elevated temperatures and these affect the temperature sensitive film properties:

* Film Porosity (density, dying capability and mechanical properties)

* Film Hardness

* Wear Resistance

A 5[degrees]C local change in temperature in hard anodising conditions can alter film properties by 20% or more, this is not necessarily the norm, but it is common to see +/-4[degrees]C. So in the choice of agitation if temperature can be controlled to +/-1[degrees]C then film properties will be enhanced.

Common methods of agitation

The most basic method of agitation would be mechanical anode movement, and whilst this is widely used in some plating applications it simply does not provide sufficient levels of agitation required in anodising.

Mechanical stirring has been adopted in hot seals and other cleaning applications, and whilst it is possible to use this method in anodising baths it is highly localised and can lead to differences within the tank.

The most commonly used method of agitation used in the anodising process is that of air agitation--the installation of sparge pipes on the floor of the tanks attached to either a blower of an air compressor. This method is a cheap and readily available technology and provides very visible patterns in the tank. But, air does create some process problems and certainly contributes to the environmental impact of the process.

Process wise we have said that agitation is used to disperse heat, and so we must consider how effective air agitation is when related to heat transfer It is generally understood that as agitation is increased heat is dispersed more quickly at the anode / electrolyte boundary. Using air agitation this means using more air, and this is where our problems start--when air use is increased the current density (c.d.) falls given a constant voltage and film thickness is reduced (see figure 1). In order to maintain the current density and the film thickness the voltage must be increased as the air agitation is increased. In short, when using air agitation film properties are either reduced or the energy requirement is increased.

A secondary issue with air is that of emissions--with changes to the classifications of both chromic and sulphuric solutions from hazardous to toxic it is now more important than ever that where possible mists above tanks are minimised. Unfortunately rather than minimise emissions above tanks, air agitation adds to them and increases them.

Finally the issue of dispersed air as an insulating second phase is one to be addressed--typically air agitation increases solution resistivity by 20-35% and a subsequent requirement for a costly increase in applied voltage.

So what is the alternative?

Eductor agitation has been increasingly adopted in the plating processes and in the last few years it has come to the attention of anodisers. Eductor technology has its roots in the 19th century and uses the Venturi principle to amplify the flow delivered by a pump.

The primary advantage of this method is as the flow is directed around the work piece it provides improved heat transfer--studies show that when we compare air agitated and eductor agitated baths the HRI (Heat Removal Index) is almost doubled in the eductor agitated tanks. This HRI figure and the nature of eductor systems provide for effective dispersal of heat and temperature control of typically +/-0.5[degrees]C--allowing for more control over, and enhancement of, the temperature sensitive properties of film porosity, film hardness and wear resistance.

Clearly the removal of air agitation from anodising baths will have the effect of reducing the resistivity of the solution and a reduction in the voltage and power requirements of the tank--typically it can be assumed that a saving of 10% is readily attainable.

Finally we can address the environmental benefits--simply by removing the air agitation we can remove the explosive air that is passing through the tank, absorbing chemicals and spewing them into the environment. If Eductor agitation is used to replace air then emissions can be reduced by in excess if 95%.

Conclusions

The use of agitation is vital and should be emphasized more--both good and bad agitation has more of an effect on the film characteristics than perhaps is believed to begin with--and opportunities exist to install a versatile system of optimised agitation. By working with eductor technology agitation can be improved, energy consumption reduced, emissions reduced and the possibility is there to enhance film properties.
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Title Annotation:ANODISING
Publication:Finishing
Date:Sep 1, 2013
Words:991
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