Letter 32077

Black spots in zinc plating  

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1.I WANT TO UNDERSTAND THE REASONS FOR THE GREY / BLACK SPOTS IN THE ZINC PLATING.

2. WHICH ARE THE GOOD COMPANIES IN SOUTH EAST ASIA THAT MAKE GOOD QUALITY OF BARRELS AND DANGLERS?

3. WHAT SHOULD BE THE LIFE OF A GOOD BARREL / DANGLER.

SANJAY NARULA
FASTENERS - ROHTAK, HARYANA, INDIA

Sanjay,

You don't include enough information, but sounds like corrosion. Are you chromating? If you let the part(s) sit long enough I'm guessing the dark spot will turn to "white rust".

Trent Kaufman
electroplater - Galva, IL

Dear Sanjay,

Black spots in barrel acid chloride zinc plating is a common problem there are many reasons for that some of the important reasons are Iron contamination, low metal content, higher content of brightener "R" if u r using Indian Chemistry.If there is no iron contamination in the bath check & adjust metal content above 30 gms/litre.

N K Praveen Kumar
- Secunderabad, ANDHRA PRADESH, INDIA

Dear Praveen,

I would like to know more about Iron Contamination in details.

Kindly respond to the same.

Sanjay Kumar
- Bangalore, Karnataka, India

Sanjay

These are possible causes of Black spots in Barrel Zinc plating from acid Zinc electrolyte:

1) Dissolved iron from dropped parts in solution.
2) lack of carrier or Wetter sold as "M" in India.
3) Lack of Chloride or low Chloride./ maintain 125 to 140 Gms/liter.
4) lack of Zinc as  metal . maintain 25 to 28 Gms/lit.
5) Excess brightener too sometimes causes Blackish deposits.
6) Recast Impure Zinc anodes containing lead or Copper cause black deposits.
7) using Local impure "A" salt ..
8) using Local impure "B" salt.
9) Proper pretreatment is essential.Clean parts plate good.
10) Continuous filtration is essential. Dirty Solutions don't plate well.
11) Passing higher volts than the solution can carry.
 

Like we said above this is a short list of POSSIBLE causes.

Barrels and danglers: You need to connect with a Professional maker in North India.

Good luck with your plating .

Asif Nurie Atlanta Global Limited New Delhi, India

Dear Sanjay ,

Sorry for delayed response. The reason being I didn't check ur question. Following are the some of the reasons for Iron contamination in Acid Zinc Plating (barrel):

1. Parts falling into the tank during loading, unloading or during the course of plating.

2. As the current density in barrel plating will be very low, parts to be plated are exposed to acidic solution and hence Iron gets dissolved into the solution.

Apart from the above poor quality of anodes and chemicals also result in iron contamination.

Removal : Depending on the extent of contamination you should treat the solution with Hydrogen peroxide 100 vol, @1-2 ml/litre. Then filter the solution after settling the solution overnight, and discard the sludge.

Note brightener "M" should be added after the above treatment @ 2-3 ml/litre.

N.K.Praveen Kumar
- Secunderadad, India

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