Metal finishing Q&As since 1989
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Arc-like Pits around Contact Area in sulfuric Acid Anodising
October 22, 2012
Q. We performed sulfuric 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).
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).
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.
- Singapore