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Arc-like Pits around Contact Area in Sulphuric Acid Anodising

October 22, 2012

Q. We performed sulphuric acid anodising on Al6061-T6 & Al2219 forged rings. Recently, we found (arced like) pits around the contact surface, we notice that there were more pits on the part surface on contact area as compared to the contact jig surface (first two pics).

pits after sulphuric acid anodizing 1 pits after sulphuric acid anodizing 2

During our trouble shooting, we changed half of the anodising solution and to our shock, we experienced high shoot up of amperage and drop voltage (normal: 2 volt for 37 amp. abnormal: voltage dropped to 0.2V and amperage shot up ~1000 amps) during the test run with scrapped sample. Both sample and jig were severely pitted and the cathode plates were etched (see third picture).

pits after sulphuric acid anodizing 3

We changed the anodising bath completely. The anodising yield were good on the first two runs; there is no shoot up of current and volt at an acceptable range. However, on the 3rd run we found the (arced like) pits. We suspect this may be a tell-tale sign of returning of amperage shoot up and volt dropped.

Appreciate your sharing.

Jimmy Sim
- Singapore
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