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Trouble Shooting Acid Zinc

Dull LCD Deposits
Poor Throwing Power
"Printing" in Barrel Processes
Poor Passivate Adhesion
HCD Burning
Blistering/Tinselling of the Deposits
Dissolution of Anode Baskets

Rough Deposits

Poor Deposit Brightness

This fault can be caused by a number of reasons:

  • Lack of brightener
  • High bath temperature
  • Low cloudpoint
  • pH Incorrect

When troubleshooting acid zinc systems, a good method of trouble shooting is to proceed as follows:

  • Check the operating pH of the bath and make sure it is within the parameters specified on the data sheet
  • Check the cloudpoint of the bath. If it is lower than 40°C, add Carrier or Wetter to increase the cloudpoint
  • Perform a Hull Cell test. If the deposit is still dull, try a brightener addition

Dull LCD Deposits

The causes of this defect are as follows:

  • Lack of brightener
  • High bath temperature
  • Bath contamination
  • pH incorrect
  • Incorrect ratio of chloride/zinc

Analyse the bath for zinc and chloride. Often, the zinc concentration of acid zinc processes tends to increase with time and this eventually produces dull LCD deposits. Check the bath pH and perform a Hull Cell. Provided that all of the analytical parameters are correct, if the deposits are still dull in the LCD, a brightener addition should correct the problem.

 

Poor Throwing Power

Poor thowing power (metal thickness distribution) may have several causes:

  • Incorrect ratio of chloride/zinc
  • Low bath temperature
  • High average current density
  • Poor cell geometry

The throwing power of acid zinc electrolytes is not as good as that of alkaline processes. The best throwing power is obtained by using a high chloride/zinc ratio, using as low a zinc content as practical and operating at a low average current density.

 

 "Printing" in Barrel Processes

This defect has the following causes:

  • Cell voltage too high
  • Iron contamination of the solution

Iron contamination is a common cause of "printing" in barrel electrolytes. Adding a small amount of hydrogen peroxide solution to the process tank will oxidise ferrous iron to ferric iron at which point, it will precipitate at the operating pH of the process and be removed by the filter.

 

Poor Passivate Adhesion

This is usually caused by the passivate film forming too quickly and is usually a fault in the passivation process. However, high brightener levels in the zinc plating process solution can also contribute to this problem. Make sure that brightener addition rates are carefully controlled in your process solution.

 

HCD Burning

This problem is most usually caused by the following:

  • Low zinc concentration
  • Low bath temperature
  • High current density
  • High bath pH
  • Low boric acid content of the bath

These parameters are easily checked and all should be within the limits specified on the relevant data sheet .

 

Blistering/Tinselling of the Deposits

This fault is caused either by low carrier concentration of the bath or poor cleaning/pretreatment. Check that the parts being processed are clean and completely free of water breaks before entering the process tank. If the cleaning is OK, the bath will require an addition of carrier. Refer to the relevant data sheet for the correct addition.

 

Dissolution of Anode Baskets

This is caused by the cell voltage being too high. A cell voltage of 10V should be considered as a maximum. Above this level, titanium begins to dissolve in the high chloride electrolyte. Make sure all of the anode cages are kept topped up with zinc domes and that there are sufficient anodes in the process tank.

 

Rough Deposits

Rough deposits are most usually caused by the presence of suspended solids in the process tank. Iron contamination that has been precipitated as ferric hydroxide may cause roughness. The cure for this fault is to improve the effectiveness of the filtration system. Particularly in rack solutions, the anodes must be bagged.