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Constant voltage anodizing issue: Current out is higher than current in

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June 29: Electrochemist [Niagara Falls NY]
June 2: Laboratory Technician [Albuquerque, NM]
Apr. 13: PRI (Nadcap) Chemical Processing Auditor (-----) May 26, 2022

Q. Hello,

I'm a newer engineer at an anodizing job shop and we have seen an unusual defect on a regularly processed job.

A lot of this job is processed in ~5K Pieces, on a Large Titanium Box Style Rack System.

We use magnesium bars for sacrificial corrosion in the process after the anodize tank - since the titanium racks are large enough that the stored energy within them will draw from the small pieces.

We recently have been seeing an issue where the magnesium bars as well as the parts are corroding.

Looking into the anodizing data for these jobs I've noticed that there is either a 2-5 amp drop, or in some cases an amp increase. (When processed correctly the drop for these loads is 15-20)

Additionally, the amps in is usually around 150. On one load this amps in were 30, and amps out where 35.

When digging in I can see this is something that has been occurring irregularly with other jobs as well.

When anodizing using constant Voltage how and why would the amps out be only slightly lower or increase?

Ive checked the resistance of the racks and load bars and they all seem to be consistent with there material properties.

Would really appreciate any support on where to look for answers! Thank you.

N Denesha
- Rhode Island
^


May 27, 2022

A. You cannot anodize "by voltage" because as the coating builds the resistance builds and unless you increase the voltage the amps fall off. If you are doing Type II, then you may use voltage only up to about 0.0008" thick, during the build time the amps will fall slightly. If you are doing Type III hard coat, then the resistance builds even higher and anything above 0.0008" will require increasing voltage to hold the current, like up to 60 volts for 0.002" in 180 g/L acid.

robert probert
Robert H Probert
Robert H Probert Technical Services
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Garner, North Carolina
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