Troubleshooting Zinc Electroplating Baths In the Laboratory

Article From: Products Finishing, from Pavco, Inc.

Posted on: 2/1/2004

When things go wrong on a zinc electroplating line, quick solutions can save thousands of dollars. If the answer is not pre- or post-treatment, the laboratory is the place to troubleshoot the plating bath. A quick analysis and a few hull cells may be all it takes to put the operation back on its feet...

First Things First

Quick solutions to problems on an electroplating line can save thousands of dollars. Experience is by far the most valuable tool a troubleshooter can possess. So successful troubleshooting begins when the line is at its best!

Build your experience by walking the line when everything is running smoothly. Take note of solution color, smells, gage settings, etc. Intimate knowledge of your plating line and its idiosyncrasies will expedite the solution to future problems.

Now that a problem has developed, you must walk the line looking at temperature gages, current, anode baskets, pumps, etc. You must rule out the cleaning section of the line and the post plate section of the line.  These steps may take some time but they must be done. Ninety five percent of plating problems have nothing to do with the plating bath. Although occasionally a problem develops, which persists despite experience.

Into the Lab

Once the problem has been determined to be a result of the plating solution or the material being coated, the troubleshooting should be done in the laboratory.

Before starting a laboratory investigation, the first thing to do is ship samples of the plating bath and reject work to your supplier. Suppliers often have sophisticated labs with experienced people. Follow up with a phone call to your supplier. Speak directly with a technical service representative and discuss your problem and investigation.

Once in the laboratory, define the condition of the bath with a routine analysis and a routine hull cell. Correct any chemistry problems found by the routine analysis. The routine hull cell should be one you’re used to looking at.  Suggested conditions for a routine panel are:

  1. A two amp, five minute, unagitated panel for acid zinc.
  2. A one amp, 10 minute, agitated panel for alkaline zinc.

Compare the routine hull cell panel to ones from when the problem was not present. Measure thicknesses across the panel and again compare them to past hull cell panels. If the routine hull cell panel appears normal, chances are your problem lies outside of the plating bath. Re-walk the line and review your observations. Pay particular attention to the electrical portion of the plating bath, as poor electrical connections will make the plating bath appear to be at fault.  Investigate the material of the parts exhibiting the problem. Again, material problems will not manifest themselves in the lab. If you are still convinced that the plating bath is the source of the problem, continue with the lab investigation.

The next step is to use a hull cell to generate the problem. Make sure the conditions and time that all hull cells were run are clearly marked on the resulting panel. Vary the conditions in the hull cell to give yourself the best opportunity to produce the problem. Some variations, which may prove useful, are:

  1. Panels run at low amperage
  2. Panels run at high amperage
  3. Bent panels to create a shelf area
  4. Bent panels to create an extreme low current density area
  5. Panels run at a high temperature
  6. Panels run for 30 minutes then scribed with an exacto knife (to reproduce blistering.) Knowing how your bath appears when operating normally will make the interpretation of these hull cells easier.  Once the problem has been produced, we can proceed to the next step.

Target Identified

With the ability to produce the problem, one now needs to know how to solve the problem. The problem probably will fall into one of several broad categories:

  1. Organic contamination
  2. Metallic contamination
  3. Poor filtration
  4. Imbalance of proprietary chemicals
  5. Unknown

With an unlimited supply of solution, take the opportunity to begin multiple treatments. After each of the following treatments, rerun the hull cell test, which produced the problem. First, for organic contamination, treat 300 milliliters of solution with two grams of activated carbon. Mix the solution continuously for at least 30 minutes, then filter and run the hull cell.  Second, for metallic contamination, treat 300 milliliters of solution with one-half gram of zinc dust. Again, mix the solution continuously for at least 30 minutes, then filter and run the hull cell. Third, filter the solution thoroughly. The solution must be clear after this step. Use a filter aid if necessary. Fourth, if the solution is an acid bath, metallic or organic contamination may be affected by adding one tenth of a gram of potassium permanganate to 300 milliliters and mix the solution for five minutes. Filter thoroughly and run the hull cell. For an alkaline solution, freeze out carbonates by putting 300 milliliters in a lab refrigerator. Cool the solution to 30°F for 15 minutes.  Decant the solution, raise the temperature and run the hull cell test.  If one of these treatments affects the problem, you may be well on your way to solving the problem. Give yourself a pat on the back! Not too fast though. You now must translate the lab results to the production line. You must also locate and eliminate the source of the contamination. The quick results in the lab may take a couple of days to accomplish in production. But don’t give up. Plug away until the job is finished.

Unknown

If the above treatments did nothing to affect the problem, things just got a lot tougher. Get on the phone to your supplier and ask for their assistance on-sight. Review their analysis of your bath. Is your bath low on carrier, high in brightener, out of balance, etc? Many suppliers have “doctor” solutions. By comparing notes with your supplier, they will be able to send in a new arsenal of weapons along with technical assistance. Meanwhile, there are still a few hull cells to run:

For an alkaline bath try:

  1. Adding 1% sodium hypochlorite to the hull cell
  2. Adding one-half ounce per gallon of EDTA or Rochelle salts
  3. Diluting the bath by 25% with “virgin” solution 

For an acid bath try:

  1. Heating the bath above the cloud point, then carbon treat
  2. Reduce the pH of the solution with 50% hydrochloric acid to kick-out most organics. Usually a pH of 2.5 is suf- ficient. Filter the bath, raise the pH and add carrier.
  3. Diluting the bath by 25% with “virgin” solution.

Out of all the tests you have now run, hopefully something you can build on has emerged. If not, you are into the rare problem, which falls into the “unknown” classification. This type of problem will take time and effort to resolve. Calling in electricians, sending samples to outside laboratories, etc. are examples of the steps that may be necessary to solve the problem. In this case, the economics of dumping the bath and making a new one must also be considered.

Conclusion

Troubleshooting a zinc-electroplating bath will be much easier time is taken to observe the line when things are running well. When a problem develops, split the line into a precleaning section, the plating bath and a post-plate section. Run tests to isolate the problem to one of the three sections.

When the plating bath is identified, follow these steps:

  1. Send samples and reject parts to your supplier.
  2. Use the hull cell tests outlined above to treat the problem. Remember, even if you can treat the problem, you will also have to eliminate the source of the problem.
  3. Demand prompt service from your supplier.
  4. Label or identify all tests run during the troubleshooting process. Once the problem is solved, go back and review the troubleshooting process and learn from it. This will build your troubleshooting skills and shorten the duration of future problems.

 

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