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Troubleshooting Common Problems in Zinc Plating Baths


Metal Finishing

Jeff Grodecki, Columbia Chemical's resident expert on zinc, tackles your questions.

PROBLEMS WITH THICKNESS CONSISTENCY DESPITE SIMILAR PARAMETERS IN BATH SOLUTIONS

Q: I have two alkaline zinc plating bath of capacity 1200Lts with Zn metal 8g/L, NaOH 120g/L and Sodium Carbonate 100g/L with this i used to get 8-12microns thickness within 30 -35 min ( 5V 180 to 200Amps) but from last 10days it is taking 70-80 min to get 8-12 microns in both the baths with parameters being same as earlier. I have checked the rectifier output, which is normal. I analyzed the solution concentration found as mentioned above. What might be causing this problem?

 

A: Plating speed or cathode efficiency can be effected by a number of different variables within the alkaline non-cyanide plating solution. Please review the list of variables below.

1. Zinc metal - Low concentrations of zinc metal will cause slow plating speed or poor cathode efficiency. Zinc metal operating parameters can vary between chemical supplier. Maintaining your zinc metal concentration at 12 to 15 g/L will help to improve plating speed, but it is recommended checking with your supplier if any proprietary additions will be needed after the increase in zinc metal.

2. Temperature - Low bath temperatures can also cause slow plating speed or poor cathode efficiency. Maintaining a operating temperature of 26° to 35°C can help to improve plating speed.

3. Sodium carbonates - High concentration of sodium carbonates will cause slow plating speed or poor cathode efficiency. Concentrations of sodium carbonates above 75 g/L can very quickly reduce plating speed. It is recommended maintaining your sodium carbonate levels below 75 g/L. A very common treatment for high sodium carbonates would be to “batch freeze” when weather is permits. If the weather does not permit, a bath cuts would be the your second best option.

4. Proprietary additives – Over use of BRIGHTENERS and PURIFIERS can reduce overall plating speed or cathode efficiency. It is recommended having your supplier analyzing your plating solution for optimum additive concentrations.

Your bath analysis does indicate a low concentration of zinc metal and a high concentration of sodium carbonates. Making adjustments to the concentrations of zinc metal and sodium carbonates would be a great starting point for improving your overall plating speed.

 


PREVENTING BLISTERING IN ALKALINE ZINC BATHS

Q: Dear sir: I have two alkaline zinc baths--one is 1500 liters, zinc metal-8.9 g/l NaOH - 120 G/L; the other is of 2500 liters zinc metal-9.02 g/l NaOH- 130 G/L. I am doing the same plating over plain carbon steel and alloy carbon steel. I am facing blisters on the same when we heat the same component on 100 degree centigrade. Delay blistering is very common on these plated articles. The process sequence is as follows: 1-Hot soak 2-WR x 3 3-pickling 4-WRx2 5-Hot Anodic 6- WRx3 7-Acid activation 8-WRx2 9-NaOH neutralizer 10-Plating 11-drag out 12-WRx3 13- Nitric dip 14- Tri-passivation 15-WRx2 16-sealer Please tell me the solution of the same problem and send me the pretreatment sequence for the same if it differs from the existing. Thank you.


A: One of the most difficult substrates to process though an alkaline non-cyanide zinc plating process is high carbon or heat-treated steels. Although these substrates are challenging to process, improved adhesion is achievable with the proper pretreatment sequence.

There are two basic option for the pretreatment of high carbon or heat-treated steel substrates. These options will perform exactly what you would expect from the an excellent pretreatment system. They will remove oils, scale and excessive smut/carbon that are found on high carbon or heat-treated substrates.

Option #1 and Option #2 do have similarities. First, the use of an acid inhibitor within the hydrochloric acid pickle. An acid inhibitor will greatly reduce or even eliminate the formation of carbon on the surface of the high carbon or heat-treated substrate. Excessive carbon on the surface of the substrate will greatly decrease the overall adhesion of the zinc deposit. Second, both options will cycle though anodic cleaning prior to entry of the alkaline non-cyanide plating solution. Reverse current or anodic cleaning is critical for the removal of any carbon residue. As mentioned previously, any carbon is remaining on the substrate prior to plating will reduce adhesion of the zinc deposit.

OPTION #1

HOT ALKALINE SOAK CLEANER
Temperature - 65° to 85°C
Concentration - 60 to 90 g/L
Time - 4 to 6 minutes

HOT ALKALINE ELECTRO-CLEANER
Temperature - 65° to 85°C
Concentration - 60 to 90 g/L*
Time - 4 to 6 minutes
Current density - 2 to 15 amperes/dm2

TWO TO THREE COLD WATER CASCADING OR COUNTER FLOWING RINSES
Flow rate - 4 to 8 Liters per minute
Temperature - Ambient

HYDROCHLORIC ACID PICKLE
Temperature - Ambient
Concentration - 30 to 50% by volume
Time - 3 to 5 minutes
Acid Inhibitor - 0.2% to 2.0% by volume

TWO TO THREE COLD WATER CASCADING OR COUNTER FLOWING RINSES
Flow rate - 4 to 8 Liters per minute
Temperature - Ambient

ELECTRO-CAUSTIC
Temperature - Ambient
Concentration - 100 to 150 g/L Sodium Hydroxide
Time - 1 to 2 minutes
Current density - 2 to 15 amperes/dm2

ALKALINE NON-CYANIDE PLATE


OPTION #2

HOT ALKALINE SOAK CLEANER
Temperature - 65° to 85°C
Concentration - 60 to 90 g/L
Time - 4 to 6 minutes

TWO TO THREE COLD WATER CASCADING OR COUNTER FLOWING RINSES
Flow rate - 4 to 8 Liters per minute
Temperature - Ambient

HYDROCHLORIC ACID PICKLE
Temperature - Ambient
Concentration - 30 to 50% by volume
Time - 3 to 5 minutes
Acid Inhibitor - 0.2% to 2.0% by volume

TWO TO THREE COLD WATER CASCADING OR COUNTER FLOWING RINSES
Flow rate - 4 to 8 Liters per minute
Temperature - Ambient

HOT ALKALINE ELECTRO-CLEANER
Temperature - 65° to 85°C
Concentration - 60 to 90 g/L*
Time - 4 to 6 minutes
Current density - 2 to 15 amperes/dm2

TWO TO THREE COLD WATER CASCADING OR COUNTER FLOWING RINSES
Flow rate - 4 to 8 Liters per minute
Temperature - Ambient

ALKALINE NON-CYANIDE PLATE

As you can see, Option #1 will most likely be the best sequence for your application. A simple modification to the NaOH Neutralizer would be need to improve the overall plating quality.

 

ABOUT THE EXPERT

Jeff Grodecki is the Technical Support Manager for Columbia Chemical Corporation. His academic credentials include bachelor’s degrees in chemistry and biology from Tri-State University. Jeff works closely with the sales team and customers, both distributors and end-users, addressing issues involving zinc plating and its related processes. His eleven years in the industry have taken him to customers’ shops around the world including China, South Africa, Brazil, Indonesia, Vietnam, Spain, Thailand, and Italy. He has extensive experience in a range of finishing applications including the automotive, appliance, and electronic metal surface finishing markets. Jeff looks forward to answering your questions on zinc and zinc alloy plating processes. Feel free to submit your questions online.

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