Electroplishing Solution

[jeffg35]

We are heat treating parts made of 17-4PH to H900. We would like to setup a electropolishing station to remove the blue tint left on the parts after heat treating. Could someone advise us the formula for the solution you would use. These parts are .75 long, step down to .375 dia. with a .312 thru hole. Also,on a .700 bolt circle there are (2) 4-40 tapped thru holes. We would also appreciate some tips on the setup for electropolishing.

 

[TEV]

I suggest you remove the blue tint by acid pickling, per section 5.2.1 of ASTM A 380. This is simpler than electropolishing. Electropolishing will give you a brighter surface than acid pickling, however, if this is required for your product. If so, I would suggest use of a mixture of phosphoric and sulfuric acids. Do NOT use perchloric! Explosive! There are numerous proprietary solutions on the market which you can find using a Google search on stainless steel electropolishing. Also some companies and consultants that specialize in this process. I can give you more specific info if you need.

 

[kenvlach]

jeffg35,
I noticed that this was posted after your earlier thread " Heat Treating 17-4PH No Discoloration Allowed!" thread330-40071. Since you weren't satisfied then, I will give some info on electropolishing (EP) and on the racking of your parts and then a suggestion.

There is a rather basic EP article in the Metal Finishing Guidebook:

http://www.metalfinishing.com/WZ/MetalFinishing/guidebooks/MetalGB_SurfaceTreatments/000013/show/

Various solutions are given for EP of SS; one that I like (easy to prepare & maintain chemistry, wide T range & fairly low critical current density) is:

Sulfuric acid, 15%
Phosphoric acid, 63%
Current density minimum, 50 A/ft2
Temperature, 80-175°F

With this solution, a cathode (use copper) to anode spacing of 1-4 inches will work by adjusting the power supply. This is for the exterior surfaces. I would recommend EPing the holes first, then plugging them (for protection from 'frosting&quot, and EPing the exterior.
For throughholes, center an auxiliary Cu cathode wire that can carry adequate current (to avoid 'frosting' the part) and not make contact (to avoid burns from short circuits). Also, the part must be racked with the holes vertical to avoid the trapping of gas bubbles created at both the anode (part) and the cathode (or else, more 'frosting'). As the electropolish solution is quite viscous, small diameter threaded holes become very problematical. I suggest not even attempting to EP the 4-40 holes; leave them open or use platers' plugs (Shercon is one supplier).

Generally, EPing of such parts is best left to professionals. A common alternative is to use a nitric acid-fluoride pickle as in the ASTM spec. suggested by TEV. For best results, the parts should be examined frequently to avoid etching. As the amount of heat tint varies among parts treated en masse, this may entail an in-process sorting to remove descaled parts.

My preference: prevent the formation of scale via suggestions in the earlier thread: heat treat in vacuum or in high purity inert gas, or use SS foil or 'baggies' tightly crimped around the part.