The authoritative public forum
  for Metal Finishing since 1989
-----

Conductivity of 20% Sulfuric Acid

I would like to simulate an aluminum anodizing process using finite element analysis software to visualize the current distribution for various anode and cathode arrangements. Can anyone provide the typical value for the electrical conductivity (or its reciprocal, the resistivity) of a 20% sulfuric acid solution?

Mark Decker

---

ASMI's Electroplating Course by Faust lists the resistivity of one normal H2SO4 as 4.81 ohm-cm (conductivity 0.208 mho-cm) at 77 deg. F. (20 deg. C.). That's the closest reference I have, but you can determine it for yourself with a Haring Cell.

I hope you are not re-inventing the wheel in that companies such as L-Chem and ElSyCa commercially market this general type of software.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

---

According to a graph I got off the internet, 20% by weight H2SO4 exhibits 0.73 siemens/cm at 25C, 0.65 siemens/cm at 18C, and 0.49 siemens/cm at 0C.

Jake Miller

---

A few years back I designed a series of conductivity meters for a medical application. I looked back through my notes but I can not find any reference to the conductivity of sulfuric acid. If you decide to measure the conductivity you should be aware that it is very temperature sensitive.

As far as modeling your process is concerned you should also be aware that the electrode surface is a large contributor to the total impedance of the current path and can overwhelm the impedance of the sulfuric acid. Not only is it large but it is variable. All is not lost though. The impedance of the electrodes and wiring is small compared with the impedance of the sulfuric acid path and the surface impedance of the electrodes. From my experience in finite element modeling of such systems you will still get a useful indication of the current distribution on the surface of your part even if you are off in your guess by an order of magnitude.

To prove the point try modeling the FEA with a conductivity of 16 millimhos and then try with 160 millimhos. If you compare your results you will see very little difference in the ratio of current distributions and you will be able to refine your cathode design quite effectively. I have done this myself with excellent results.

If on the other hand you are trying to come up with precise current densities, with a certain voltage, at a certain temperature then you are embarking on a very significant project and the results are not likely to be much different to those obtained with less precise inputs.

Good luck and let us know how you get on.

John Holroyd