Pimples on hot dip galvanizing

A. Your operating conditions (makes it easier when people give a full picture like this), suggests that your zinc is close to saturation. There's quite low limits of solubility for the various elements we add to zinc, but adding excess means that one or more of these can precipitate out of solution. Your %Ni at 0.3% is very high. For the purpose of controlling sandelin steels alloy thickness, a level of 0.06% is adequate. Your Al at 0.02% is also high, and you'd find for most purposes, 0.001% is adequate. Your Pb at 1.28% is also high, and something under 1% is adequate. At your level, there's likely to be excess Pb on the kettle floor, as not much more can be dissolved in Zn than your 1.28%. So with all these alloying elements over the required levels, something will precipitate out, often Al does this as pimples. Suggest you add less of the Ni, Pb, Al until the levels are lower, and the pimples will likely disappear, and you'll save some money by not adding too much too!

A. I have noticed that this problem of pimples usually goes away with the addition of some lead.

I too have noticed that they do not occur on vertically oriented pieces, and have assumed this to be due to drainage of the surplus zinc, containing those pimples back into the melt. The pieces that have a horizontal component of their orientation however do not drain so well.

The reduction in the problem when lead is added seems to imply that they could be an impurity that requires some lead to increase the density and cause sinking.
Aluminium must have some part to play, as the problem is much less prevalent in low Al baths, but the shininess of low Al melt is not as attractive to those customers who think that bright shiny must be better.

Hi cousin Virendra. Dr. Cook has implied that dross pimples are one type of pimple, but that he currently had interest in another type which is always on the bottom, never on vertical surfaces, and which pimples test high for aluminum. Mr. Crowley offers some theories about lead and aluminum content for that specific case.

Please start by telling us whether the pimples you refer to are only on the bottom, never on vertical surfaces, and whether you have tested them for aluminum and lead. Or else, please tell us in good detail how they differ -- please get back to us with details. Thanks.

Regards,