Tin Plating Problem: deposits on the anode bags

A. I have seen precipitation start at the anode bags in nickel plating baths which contained very high sodium. Is this bath on an evaporator?  

A. It was mostly sodium, but if you are not concentrating and returning a lot of the tin bath back into the tank, the sodium is probably not building up. The speculation for the precipitating at the anode bags was that the total concentration of ions around the dissolving anode was higher that the solubility of the ions.

Another idea: The Canning Handbook [on eBay, Amazon, AbeBooks affil links], on their tin sulfuric/phenol sulfonic acid bath warns of passivity of anodes when the current density exceeds 20 amperes/square foot of anode area. Have you increased the current lately, or perhaps your anodes have been coated. Also the bags tend to tighten up over time.

You know, for barrel plating, you really don't need anode bags. Fines will settle to the bottom, it's true, but you can get a lot more current if you don't have to go through a bag. But; people become attached to their anode bags, and no one ever got fired for leaving them on. Personally, I think maintaining those bags is one of the most miserable jobs on this earth.

A. Mark,

Your problem of precipitation is one that afflicts all MSA (methanesulfonic Acid) based tin-bearing baths, both pure and alloy plate. The white or Bone colored precipitate is tetravalent Tin or "Stannate" ion which is sparingly soluble and therefore readily precipitates. You are probably doing little wrong, as precipitation of Stannate in this bath is spontaneous, and will occur if there is any dissolved oxygen present in the bath, which there will be if the bath surface is exposed to air. Divalent Tin, or Stannous Ion is the plateable species but is readily oxidized to Stannate ion by free oxygen in the bath. The vendors of MSA processes attempt to counter this by adding to the bath "antioxidants" (yes, precisely the same chemicals they are now selling as nutritional supplements). These antioxidants scavenge any free oxygen in the bath by outcompeting against Stannous ion for it, thereby depleting the bath of Oxygen potential and now the Stannous Ion conversion reaction is shut down due to lack of reactant (Oxygen). Sounds brilliant but it doesn't really work worth a DA__. But I do have some advise:

1. If you are air sparging, shut it off immediately and do something else for agitation.

2. This oxidation reaction is accelerated by stagnant flow conditions on the surface of the anode. As Tin dissolves at the anode, it first loses two electrons becoming Stannous, then can lose two more to become Stannate. As the "Plating Reactor Design Engineer", it is your job to provide a means of getting those newly hatched Stannous ions away from the metallic anode surface ASAP so that the anode does not have time to steal two more electrons from it forming Stannate. But you can NEVER solve this problem because the anode is immersed in Stannous bearing plating solution! But the "conversion ratio" can be minimized through correct reactor design. At first you may think to remove the anode bags to eliminate the stagnant anode condition, but this will cause nodules/rough-plate on your workpiece due to any unfiltered suspended Stannate formed by the above mentioned two mechanisms. So you still must do it through reactor design.

Regards,

A. Mark,

According to your description, this problem was mainly caused by the over-generation of stannate ion at anodes. As Dave said, stannous ion is very soluble in acid solutions while stannate ion tends to precipitate in the same solution. For a commercial tin plating bath, some tin-stabilizing agents are added to the bath to reduce the conversion of stannous to stannate ion, in addition to complexing agents. In your case, methylsulfonic acid is one of stabilizing agents. If this chemical is not in an adequate concentration range, it is unavoidable to generate stannate ions in the solution.

At tin anodes, three reactions could simultaneously take place: Sn to Sn(2+), Sn to Sn(4+), and Sn(2+) to Sn(4+) if anodic potential is quite high. The first reaction contributes to plating, while the second and third reactions contribute to the precipitation you observed. Under normal conditions, the first reaction dominates the anodic reaction. If any factors, such as high pH, bag clog, lack of complexing agents and overload, etc., lead to the significant increase in anodic potential, the second and third reactions will follow up.

Although you mentioned everything was consistent, something must be wrong. For example, I would like to ask if the pH meter was calibrated properly, standard solutions were correct, all meters functioned normally, and others. These things often happen in metal finishing shops in my experience. Regularly changing anode bags is very important. The dirt from tin anodes will initially clog the bags (you may not see it), and this leads to the increase in anodic polarization, which results in the second and third reactions. Then the precipitation of stannate oxide further clogs the bags, which causes a worse cycle mentioned above.

A. Let's say every thing you're doing is correct. Meaning temperature control, only pump filtration for solution movement, no drag-in, proper pH, proper levels of Sn/Pb and of additive.

I have found insoluble Sn can be caused by a number of unlikely ways. One of the ways I've run across is city water rinse. It introduces salts that can migrate to the anode film and help to form these insoluble forms of Sn. Another way is poor rinsing of anode bags before using. Trapped chemistry from manufacturing of bag if not properly soaked can trap and form insoluble Sn. Or although unlikely if the mesh of the bag if to fine it may not be allowing proper flow around anode. To see if it being caused by anode bag you might try removing one of your anode bags for a short time and seeing if you still have the problem. If the problem does not persist try changing anode bags. If you still continue to have a problem, you may have to carbon treat or plate at a low current density to remove possible metal contamination. I do not believe that you are having these problems but they are possibility.

A. Hi, Mark. Please check the contact between the contact bar and the basket.

A. The only time I have seen metal deposit on the anode was when I was plating copper parts with nickel and I left the unpolarised copper in the tank at some of it dissolved. It then formed an immersion deposit on the nickel. Perhaps you are achieving the same thing from silver being dragged in the tin and nickel tanks.

As far as getting tin trees growing on your acrylic guides, I suspect you have got some metal peeling off strip and it has got stuck in the guide. When you pass the charged strip through the line, the metal gets charged and lets metal deposit onto it - hence the tin tree.

I suggest you strip your line down and make sure all your rinses make sure your rinses are working properly and you are not getting any cross contamination. Then check your acrylic guides are not impregnated with bits of metal flake.

A. 1. Anode deposition through the bags, might be due to bi-polar effects?

A. This problem will occur anytime an anode looses current yet is in the field of anode/cathode current transfer. As stated before, a bipolar effect comes into play making the affected anode both an anode on one side and a cathode on the other side.

A. Hi Fir,

One possible scenario is conductive anode sludge or Sn(IV) colloid trap on anode bag surface whereby anode bag swollen during operation (strong solution flow, lose tie up to anode basket, etc) and accidentally touch lead frame (cathode) to cause tin plating extend to anode bag surface.

Regards,
David

A. This appears to be caused by bi-polar currents. Conductive Anode fines collect on the bottom of the bag but are not in electrical contact with the electrode. This causes them to bipole.

Possible corrective actions are to clean the tanks and change bags more frequently, and also to investigate if your tin anodes contain additives that make them corrode more evenly and generate less fines.

Lyle Kirman

A. Hi Jason,

It's unlikely tin plate out as metal on somewhere non conductive merely due to too high tin concentration in plating solution. Even it was, it should happen randomly rather than selectively per your describe "plating out near the electroplating site adhering to the polypropylene surface". Wonder if possible cause similar to my reply to Fir earlier.

To test out your theory, you may prepare an abnormal high tin concentration bath solution in lab with PP immerse inside to see if any tin metal plate out.

Regards,
David

A. Hi Ivana,

The insoluble tin colloid/sludge formed in MSA base tin plating solution was believed to be tin(IV) hydroxide, Sn(OH)4, arise from Sn(II) oxidation over time.

Sorry I don't have DTA/DSC ref of Sn(OH)4.

Regards,
David