[pmd34]

Hi, I am new to this whole area of anodising, but I was able to find a good old fashioned book on the subject.

With regards to Oxalic acid anodising the recomended strength is 1-5% with a voltage of 40-60V at 1-2A/dm^2 (strange units but hey!) The colour of the anodising depends on the voltage and temperature of ths solution, becoming progresively more golden in apearence incereasing the voltage to up to 100V.

With regards to temperature, apparetly there are various classes of Oxalic acid anodising. Using the above voltage and currents, at a temperature between 20-30 deg. C a very hard coating can be achieved. Requireing twice the force needed for that of sulphuric acid anodising to scratch the part (actual table of numbers not just me with a knife!) With a temperature above 35C however this reduces the scratch resistance to somewhat less than the sulphuric acid anodised part. (Some patents that can be found suggest even lower temperatures).

There was also a claim that addition of acetic acid to the oxalic gave a coating that was enamelled looking in appearence.

I have tried the regular oxalic acid anodising myself (as oxalic acid can be bought easily from a chemist here in Denmark and sulphuric acid is much hard to get hold of.) The result was a pale gold colour and was excedingly hard to scratch through.

However... I wonder if one of you might help... I did find that after sealing in boiling water for 1hr the surface was very chemicaly clean, so once done greasy finger prints etc left permanent marks on the coating, so I had to "dirty" the whole surface evenly to achieve a suitable coating, any thoughts? Is this normal? Would I be best coating the suface with say silicon oil as a final treatment? A slight chalky film was also observed on the surface, though I wonder if this may be a result of some calcium ions in solution?

Chears,
Dr Phil