Required Current Density for Electropolishing

A. Hi Kinh. Sorry that I don't really quite understand the graphics, but is it possible that the cone shape is welded to the tube shape, and that this defect is occurring as electropolishing reveals the weld line? If so, the problem must be resolved with better welding or better welding material.

Luck & Regards,

A. Hi Jim,

Starting with the first question: yes, you seem to have a too low CD for your electropolishing activity. If typically you use 100 - 150 A for a surface area of 50 dm2, you will only have 2 - 3 A/dm2 where an average of 10 A/dm2 would be a much more "normal" value.

Concerning the water coming into your electrolyte: I always learned that the concentrated sulfuric acid has hygroscopic properties and so slowly attracts water from the ambient atmosphere. Maybe your air supply contains water or, by applying air agitation, you increase the contact area between your solution and the outside air, thereby enhancing the influx of moisture.

From theory, the solution works best with as little water in it as possible. If the water concentration becomes too high, instead of polishing, you will get an etching of the surface and in the worst situation, you will even get a grain boundary attack. By increasing the CD, you will come back in an electropolishing current density range, however this has a limit as well and your operation window becomes smaller and smaller.

Why people add water? Maybe to stabilise the concentration and so the working conditions, as inevitably it will contain water after some period of time.

The 3400 mm2 part:
If your 2 - 3 A/dm2 worked always OK in the past, that will give you a nice starting point. In your situation, not knowing if the geometry is complex or not, I would calculate with an average CD of 5A/dm for a complex geometry and if it's only a plain sheet I would go for 10 A/dm2, which will mean a current of 1,7A only for this part of 3400 mm2 in the complex form and 3,4 as plain sheet.
Please make sure that you have a suited rectifier for a job like this, you cannot use your usual one! The reading will be too inaccurate. Use a 5 or 10 A max. rectifier.

The question for the time is a nice one, but unfortunately this is, together with your remark that you need "a high quality result" impossible to answer scientifically. You should know the actual roughness now and have to define the required roughness after electropolishing. If we assume that the electropolishing process will dissolve the surface with the same parameters as for Ni-deposition, you will dissolve 0,205 µ/Aminute/dm2. This will mean that after 10 minutes at this 1,7 A/dm2 you will have a situation that on average 3,5 you is dissolved and that should be more than enough to obtain the optimum result possible for the complex part. Double the CD and use 5' for the simple part. Longer electropolishing will make no sense.

Make sure that you enter the solution with either a dry part or remove excessive water first before entering and then dip the part in the solution turn it around a few time and switch on the rectifier.

So this is what I should do in your place and before the start, I would cross my fingers for a moment too, in order to exclude Murphy and O'Toole (Murphy: What can go wrong, will go wrong, O'Toole: Murphy's law always occurs at the most unwanted moments).
In order to minimise your risk, you could spend some time in additional experimenting with similar base material and investigate for example your current density range. Higher CD's are the standard (Even up to 25 A/dm2).

By the way, I assume that the temperature of your electrolyte is somewhere in the range of 60 - 80 °C? If not, go for the lower current densities you used to apply!

Best regards and Success, always eager to hear if you succeeded with the job.

Harry.

A. Hi Srinivas. Harry has offered good advise on current density, saying about 10 A/dm2 (93 A/ft2) is "normal", and up to 25 A/dm2 (230 A/gt2) may be doable. This concurs well with sources such as the Electroplating Handbook, which says 50-500 A/ft2. I would figure 14 A/dm2 (150 A/ft2) is a good number. Voltage is whatever is required to sustain that amperage considering the conductivity of the solution, and will depend on cathode placement and the conductivity of the chosen electropolishing solution, but 6-12 V would be pretty normal.

The electricity will follow the path of least resistance, so there will be little polishing on the ID if you use tank anodes.

Regards,

A. Hi Santosh. Amperage and voltage are discussed reasonably deeply on this page already. Temperature is probably not lower than 45 °C, but 60-80 °C is more typical. Time is completely dependent on how much material you want to remove -- 3 to 4 minutes might be right for metallographic work, 20 minutes is probably more typical for production, and even more time than that is not unusual.

"Specific" answers require specific questions. What are you electropolishing and why please? What problem motivates you to ask? Thanks!

Luck & Regards,

A. Hi Aimran.

'Etching' is a common defect in electropolishing. Although you say that the problem is random, is it possible that it effects low current density areas rather than high current density ones?

Luck & Regards,

A. Hi again, Aimran. Per your earlier postings you know that electricity "follows the path of least resistance", so the anode arrangement can affect current distribution. As a general rule, increasing the current density in electropolishing usually doesn't do much harm, whereas reducing it can let you drop into the etching zone. So if experimentation is practical, you might see what happens at higher current density.

A library may have a copy of the Electroplating Engineering Handbook, which has a good chapter on electropolishing.

Luck & Regards,