Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
February, 2002

Porous Pots

Q. We have three porous pots to reduce the stray metal content in our baths. 1. Do they work? 2. What level of contaimination do you start to get pitting? We run them a lot but don't see the results in plating. We seem to be sround 8000ppm most of the time.

A. You didn't mention which manufacturer's "pot" you are using. Some popular ones are made by Hard Chrome Plating Consultants Inc. in Cleveland, OH. They make two sizes, and I have seen a lot of their large size PPS1 at shops I have visited. They use a porous, high silica-alumina ceramic vessel as the "membrane" to separate the interior catholyte and exterior anolyte (plating bath). Two lead-alloy anodes are positioned outside the vessel, and a third lead-alloy cathode is positioned in the catholyte.

The porous ceramic electrochemical separation technique is voltage driven. A DC voltage, impressed across the cathode and anodes, provides the energy needed to drive dissolved cations toward the cathode. The impurities are thus collected as a sludge in the catholyte. Copper and iron can be removed, as well as nickel and zinc at lower rates. Additionally, unwanted trivalent chromium ions are reoxidized to the hexavalent state at the anode surfaces. This oxidation rate is not as fast as some dedicated "dummy" fixtures I have used.

Typically, a clean porous pot will draw about 200-300 amps @ 6-9 VDC. The current will steadily drop as the cell works, and the rate at which impurities are removed will drop as well. It is important, therefore, to clean the pot regularly. Under normal operation, it will need to be cleaned daily. If the unit is placed in a chromic acid environment with high degree of contamination, it will require cleaning twice per day instead. If cleaning is neglected, the removal rate suffers.

I have seen occasional articles or papers testifying to the effectiveness of the porous pot.

• "Practical & Theoretical Aspects of Regeneration of Chromic Acid Plating Solutions via Electrolytic Purification (Porous Pot Method)", N.V. Mandich, C-C. Li & J.R. Selman, Plating and Surface Finishing magazine, Dec. 1997, p.82. This is a technical article which describes chemical reactions and electrochemical mechanisms.
• "Removal of Metallic Impurities in Chromium Plating Solutions by Electrocoagulation (Porous Pot Method)", N.V. Mandich, paper was presented at the Chromium Colloquium, Kissimmee, FL, Jan 27-28, 1994, p. 51.
• "Existing Practices in the Purification of Chromium Plating Solutions", Rolando Santos, paper was presented at the Second Chromium Colloquium, Miami, FL, Feb. 8-9, 1990.

You didn't mention what kind of hard chrome plating bath you have. The single-catalyst bath tends to be especially affected by iron contamination, compared to mixed-catalyst baths, probably do the lower current efficiency of this type of bath. A portion of the fluoride catalyst in the mixed-catalyst bath can be rendered ineffective when iron and aluminum impurities form complexes.

There are varying opinions as to the maximum concentration of various metallic impurities that should be permitted in the chrome bath. My feeling is that total metallic impurities should be kept well below 1.5 oz/gal (11.2 g/L or 11,200 ppm). However, it is relative to chromic acid concentration. That is why platers will often increase the control levels of the bath as it ages, to make the bath more tolerant of the dissolved impurities. Also, thick deposits and rougher substrates often require a bath with lower contamination.

When levels of the metallic impurities get too high, the following defects and undesirable circumstances may present themselves:

• Roughness or nodulous deposits.
• Dull deposits.
• Reduced cathodic efficiency.
• Reduced plating rate.
• Increased electrical consumption.
• Higher bath resistivity.
• Higher voltage potential needed to achieve a given current.
• Decreased bright range.
• Reduced covering power.
• Reduced adhesion.
• Increased tendency for burning.

Pitting is typically associated more with high trivalent buildup in the bath than with iron, nickel, aluminum, etc. However, when nodules are ground or polished after plating, they will sometimes leave pits.

I hope this information is helpful.