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A student question on tin electroplating

February 10, 2011

In this video:

SnCl2 dissolved in HCl is deposited on a stainless steel cathode. As you can see, it forms long fibrous crystals growing out from the cathode. My question is, how do they prevent this from occurring in electroplating? Because, tin plated obviously have a smooth tin coating, not a bunch of stringy crystals. Do they just shave off the excess?

My other question is, does anyone know WHY it forms in long fibres, rather than just uniformly coating the cathode?

Thanks for your time,

Jack M

Jack M
Student - Carcassonne, France

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Because tin baths platers use are formulated with additives that will reduce this.
The reason that they form is something on the ring is a tiny bit higher current density than the nearby surface. This forms the whisker that then becomes a very high current density and absorbs most of the amperage, so it grows faster and because it is rough, forms the other branches.

Too much voltage is a problem. No agitation is a problem.

They are normally delicate or you probably could find a market for them.

James Watts
- Navarre, Florida
First of two simultaneous responses -- February 10, 2011

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Second of two simultaneous responses -- February 11, 2011

Tin is usually electroplated from a sulphate, fluoborate, or MSA electrolyte, not chloride. There are two reasons to avoid the chloride electrolyte; the first is their generation of chlorine gas at the anode, and the second as you have found is that it results in acicular deposits or "trees" as in your example.

The very first layer of metal that is deposited is usually epitaxial, which means that it follows the structure of the metal being plated. After this initial layer, the tin starts to follow its own crystal structure. In this case, one face of the crystalline structure of tin is being favored over the others, resulting in long, needle-like deposits.

It is possible that a grain refiner would help. These and antioxidants and surfactants are all components of acid tin plating baths. Typical grain refiners are gelatin and protein-based materials. Gelatin produced from bones is usually better than gelatin produced from other sources. If you want to try this, first dissolve some gelatin in warm water and then add enough to your plating solution to provide 1-2 grams per liter of gelatin.

If this doesn't work, you will need to switch to a different electrolyte.

It is possible that a grain refiner would help

Lyle Kirman
consultant - Cleveland Heights, Ohio