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I need to set a number of threaded bars into cement, to hold a new gatepost. When I cut the bar into shorter pieces, the metal near the cut became red hot, presumably hot enough to evaporate some or all of the zinc with which the entire rod was plated.

Now I want to plate zinc back onto the ends, and have had mixed success so far. I dissolved some zinc oxide with hydrochloric acid and hooked it up to a little PV panel I have, and after a while it seems there is a light-coloured deposit on the cathode. The pencil-lead (6B) anode is no longer as smooth as it first was, but it doesn't seem to be losing (much) of its bulk. I assume it to be zinc, because... well what else can it be? It plated thick enough that I had to (ab)use a nut to re-cut the thread, leaving a light-coloured shiny surface that gives me more confidence that it is, in fact, zinc.

Problem: the edges of the threads seem to be taking more zinc than the valleys. In some cases the thread's valley doesn't show any evidence of plating. The fractal-like deposit on the edges is quite pretty actually, pity I can't take a (good) closeup photo.

Is there anything I can do (perhaps using some other common(ish) kitchen chemicals?) that will result in a more uniform layer of zinc? Is it worth trying lower / higher current or pulsed current? Am I overthinking things - is it even necessary for the whole stud to have a layer of zinc?

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    $\begingroup$ Cool experiment! Not an answer to your question, but what you're experiencing in regards to differential rates of deposition on the edges versus the valleys is likely to be a type of diffusion limited aggregation. This is just a wild guess, but rapid agitation of your zinc solution (say, by a stirrer bar or sonication) might help prevent depletion of the solution in the thread valleys. $\endgroup$ – Richard Terrett Jul 26 '12 at 6:31
  • $\begingroup$ Yes, I discovered that stirring the rod now and again would disturb the bubbles (hydrogen?) that get trapped in the threads - which couldn't have been helping. That seemed to help, as the zinc then seemed to deposit over the whole surface. $\endgroup$ – Bernd Jendrissek Jul 27 '12 at 11:32
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I think the easiest way is to sputter coat the bars, if you can access one. One that can rotate your bars such that the rotation is vertical to the direction of sputtering would be the best. If not, switch sides manually.

Depends on the composition of your bars, you might want to coat something else before coating zinc. A wild guess: nickel. I suggest so because I don't recall zinc being sticky. So the coating might come off easily.

And I'd try sputter slow but for a long time. I think low sputtering rate tends to produce more densely packed and uniform coating. I am not sure. But dense and uniform coating provides good foundation of the zinc coat.

For electric coating, I think low voltage and uniform coating is still nice. I don't think you want formation of gas at the bar electrode, as that disturbs the structure of the coating. Not sure what's influencing the nucleation sites on the bars. I would guess that the current is not uniform on the bars, say there more on the hill tops. Still consider coating something else first. I would try whatever is available. I would try to get the solutions as pure as possible though. Organic compounds can act as surfactants which tend to aggregate the particles.

Whichever your method is, search for some journal articles characterizing similar processes. That might help.

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I see this is an old question ; however, in the real world steel rebar is protected with polymer coating or stainless steel rebar is used. Generally protection is unnecessary except for things like seacoast bridges.

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  • $\begingroup$ Threaded bar is not rebar. This question is about threaded bar, and about zinc plating, not about rebar and epoxy / polymer coatings. Also not about concrete structures where rebar is deeply embedded and not directly exposed to the elements. $\endgroup$ – Bernd Jendrissek Oct 4 '17 at 5:48
  • $\begingroup$ Oops ! Threaded rod and studs are routinely hot dipped and electrogalvanized , a thread die may be run over them as necessary. But not in sizes under 1/2 in. 12mm , they are more commonly polymer coated. $\endgroup$ – blacksmith37 Oct 5 '17 at 15:29
  • $\begingroup$ Lookup "throwing power" on the net to possibly get help plating into recessed areas. $\endgroup$ – blacksmith37 Oct 5 '17 at 15:42

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