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Zinc plating is flaking off

2000

We have been trying to plate zinc on mixture of metals consists soldered copper attached on steel tubes.The article is refrigerating tube and shelf.The shape of the component is like a comb with three teeth. At the back of the shelf there is soldered copper thermostat groups brazed onto steel tubes. After zinc plating and blue chromating the parts are lacquered and cured at 120 °C.After curing the lacquer and bending the soldered copper zinc flakes over solder. Materials to be zinc plated are:

a.Steel tubes
b.Steel wire welded onto steel tubes
c.Copper tube soldered onto another copper tube
d.Brazed residues onto steel and copper tubes

Our plating sequence is as follows.
1.Hot Cathodic clean at 60 °C 3 A/dm2 for 3 min
2.Hot Anodic cleaning at 60 °C 3 A/dm2 for 25 sec.
3.CWR x2 counterflow
4.Phosphoric acid & sulfuric; Acid mixture 3 min
5.CWR x 2 counterflow
6.Acid Zinc KCl based 35°C,3.5 A/dm2 for 15 minBR
7.CWR x 2 counterflow
8.Nitric acid dip 0.2% for 5 sec
9.Blue 3 valent chromate 25 °C for 20 sec
10.CWR x 2 counterflow
11.Dry
12.Solvent based epoxy lacquer
13.Cure at 125 °C

Savas Altinok
- Istanbul-TURKEY

Sir your problem may be one of adhesion of the solder over the Copper tube.Poor adhesion of the dry solder may also reduce the adhesion of the zinc over the solder while bending.If there are cracks on the solder surface too, Zinc may come off.These need your attention if other process parameters and conditions are ok. ramajayam

S.RAMAJAYAM
- Bangalore,Karnataka, India
2000

The problem may be incomplete removal of flux residues from the soldering application. An initial cleaning stage with a high emulsifying soak cleaner should be tried. Initially this may be done offline to check the efficacy of the material and process. Also the addition of a proprietary acid salt for pickle or pickle additive may prove beneficial.

Gene Packman
process supplier - Great Neck, New York
2000

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2000

Dear Mr Altinok,

I am unable to understand why you are using phosphoric + sulfuric for pickling. If it is because you do not want to use HCl then try using H2So4 with a little HCL. In any case you will require to inspect if your pickle is removing all traces of the solder flux before plating. HCl will be the best pickling acid for your purpose. regards

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

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I am also unable to understand why you are using phosphoric + sulfuric for pickling. If it is because you do not want to use HCl then try using H2SO4 with a little HCL.

Thank you very much best regards,

Doruk Gurkan
- Istanbul, Turkey
2003