Electrolytic Mill Scale Removal

[wayback machine]

As a corollary to this, I thought these photos might be interesting to some -
I sometimes restore vintage bicycles, and something I've found very useful is OXALIC acid (same stuff sold as wood bleach).
One of the interesting things about it, is that it ONLY attacks rust (ferric oxide), and won't affect steel, paint (except RED paint), or chrome plating.
Here are 2 photos of an old Raleigh bicycle, before and after an Oxalic acid soak :

 

[rwm]

Are you talking HCl or Muriatic acid (30% HCl)?

 

[pontiac428]

Last time I encountered oxalic acid was as a concrete etch.

 

[SLK001]

Are you talking HCl or Muriatic acid (30% HCl)?

Muriatic acid is Hydrochloric acid. I used pool acid (HCl) which I believe is 30%.

 

[RJSakowski]

Concentrated HCl as used in the c hem lab is 12 molar. A mole of HCl is 36.5 grams so 12 molar is 438 g/l HCl A liter of conc. HCl has a specific gravity of 1.18 g/l and weigh 1,180 grams so the w/w percentage of concentrated HCl is 37%. Muriatic acid that I purchase at the hardware store is 32% w/w.

 

[FOMOGO]

I use Oxalic acid for cleaning water filters, and it works great for rust removal, but needs to be immersed in it. I tried Muriatic acid on mill scale for my table build. Seemed to have no affect, I was surprised. Mike

 

[SLK001]

What does w/w mean?

 

[rwm]

Weight/weight. As opposed to weight/volume.
The Muriatic acid I have seen locally was only 20 something percent. That's why I asked about dillution.

 

[RJSakowski]

What does w/w mean?

weight /weight

Other formats are w/v (weight/volume) and v/v (volume/volume) Each has their own areas where they are the preferred standard. I mix my 2 cycle gas as v/v. The molar designation is actually a modified w/v standard. I believe that the hardware store label for muriatic acid is w/w. It all depends upon the tools used to make the solutions. In the lab, we have accurately known volume containers so dilution to a final concentration as w/v is fairly straightforward. Mixing gas, on the other hand, it is easiest to dump a pint of oil into a five gallon can and fill it so v/v. For a manufacturer diluting the acid to a final concentration it would be easiest to add a known weight of concentrate to a batch of water and then add water to the final weight.

Which one is used isn't too important as long as the user is aware of the standard used.

 

[SLK001]

Weight/weight. As opposed to weight/volume.
The Muriatic acid I have seen locally was only 20 something percent. That's why I asked about dillution.

When I create a pickling solution, I add water until I can no longer see fumes rising. Even a diluted solution like this is still strong enough to reduce the scale to sludge in less than 30 minutes. At most, you should see almost complete removal in less than an hour (the temperature may have an effect on the speed of removal - where I live 85º to 90º is the norm). YMMV