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Hard anodizing of unknown aluminum alloys

Doing hard anodise only for marine parts from some time, using sulfuric Acid 10%, conventional by constant voltage, some customers need to hard anodise aluminium pieces such as aluminum frames...etc of unknown aluminium, that when burning occurs.
Is it because of solution not suitable? poor aluminum?What do I need to change when doing such unknown aluminum jobs?

keeping in mind that before or after marine jobs are nicely done.
Thanks & Regards

Sam Buck
Industrialist - Australia
October 14, 2010

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Sam

Using a sodium hydroxide based etch, you should be able to identify the heavily alloyed series that are difficult to hard anodize- 2xxx & 7xxx. Etch some referee parts of a known alloy until there is enough smut present on the surface to yield a color- green for 2xxx and red/blue (w/ some green) for 7xxx. It takes some practice.

Consider that the burning may be the result of other operator controlled factors.

Willie Alexander
- Colorado Springs, Colorado
October 18, 2010

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10%/vol of 66 degree acid is OK. If you mean 10%/weight, then it is no wonder you are burning. Otherwise, is it sheet metal - then you are probably "not getting the current on the part" evenly and burning in places near the contacts. Agitation. Temperature, Need to know more. No shop should ever take in a job and not know the alloy.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

First of two simultaneous responses -- October 18, 2010

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Sam,

Personally, if the customer couldn't tell me the alloy they want anodised I wouldn't touch the job, because as you have found out there is the chance of burning the components and so scrapping the parts.

Brian Terry
Aerospace - Yeovil, Somerset, UK
Second of two simultaneous responses -- October 19, 2010