littleherb t1_iskq4v5 wrote on October 16, 2022 at 6:39 PM

You may be referring to "white rust", which is zinc hydroxide. The way galvanizing works is by depositing a sacrificial layer of zinc over the steel, usually by dipping the steel parts in molten zinc. As the zinc cools, it forms a protective layer of zinc oxide on its exterior surface. If it is allowed to get wet (directly or from high humidity/condensation), the zinc bonds with the hydrogen and oxygen from the water to form zinc hydroxide or "white rust". Stacked sheet metal is especially prone to this due to stacking, where moisture can be trapped between layers. You will also see this on larger structural members if they are not stored properly immediately after galvanizing.

Reference: I am a structural engineer with a lot of history in steel. Also: https://www.metalsupermarkets.com/what-is-white-rust/

TeaTimeSoon t1_isnoxx7 wrote on October 17, 2022 at 10:29 AM

I used to be the person responsible for the sale of over 30% of the entire UK demand for the artificial sweetener know here in the UK as saccharin. My largest customer in the UK was a large electroplating company called William Canning of Birmingham. They called saccharin a "brightener" but actually it is simply a zinc crystal disruptor and acts at the boundaries of the deposited zinc crystals as the zinc ions attach to the surface during electroplating. The optical effect of large zinc crystals is avoided using saccharin. The saccharin molecules act as a kind of traffic control officer at the boundary edges and new crystals are formed which are so small the surface will appear to be polished.

I believe a similar effect is seen with zinc/nickel and pure nickel electroplating as well. The effect is seen immediately after electroplating and is not a corrosion induced artifact.

It is always amazing to look at a zinc plated surface where a "brightener" has not been used. The crystal boundaries are so large - in fact I think they look attractive in their own right. No good if you want a shiny finish though!