45053

Sudden increase of current  

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We have a sulphuric acid anodizer . According to our standard we have to keep voltage constant i.e 18 V +- 3,For a particular load we used to get current 130 - 160 amp. But suddenly we are getting 490-560 amp. I do not know why? Can you help me in that?

Abhishek Ummat
Chemical engg. - Toronto, ON, Canada

Maybe the anodic surface is very higher.

Alex Delgado
- Spain

First, just to make sure, calibrate the meters.
Possibilities are, a much larger load.
Some of the parts are much closer to the cathode is a possible but not very probable cause.
Most likely is a short in the system or one of the leads is broken and it is trying to force all of the current thru an inadequate lead.
Check your temperature, if it is high, the parts will be very slow to anodize and you will pull a much higher current for the voltage, possibly never getting a decent anodize coating.

James Watts
- Navarre, Florida

I'd like to know the sudden incresing of current happened at first few second(according to my experience it lasts less than 30 sec if any) or full time of the ramp?

John Hu
- Singapore

At the fist moment of aluminum anodizing process the oxide film is very thin (2-3 nm), and high electrical current is used to grow this film to 15-20nm, which correspond to voltage set up. After this barrier layer is created, electrical current is used only to grow porous film, thus decreasing significantly.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania

This may also be an effect from anodizing a different alloy. See the chapter 'Anodizing Characteristics of Commercial Aluminum Alloys in Sulfuric Acid,' pages 13-31 in The Finishing of Aluminum, G. H. Kissin et al.(ed.) (1963). As mentioned in Letter #42837, lower voltages (13-14 V) may be sufficient for 5052, 6063, 7072 & 7075, while a higher voltage (18-19 V) is required for 2014. One voltage does not fit all!

Best to operate under current control. If using voltage control, to avoid an excessive initial current (may overload equipment or burn parts), start at lower V and after about 5 minutes anodizing (when oxide thickening has reduced the current), raise V to the desired value.

Ken Vlach - Goleta, California