On Fri, 6 Oct 2017 12:45:20 +0100, David Billington
wrote:

On 06/10/17 12:33, Christopher Tidy wrote:
Am Freitag, 6. Oktober 2017 13:17:30 UTC+2 schrieb David Billington:

Possibly the corrosion leads to a rougher surface which results in a
thicker zinc coating. Items are intentional roughened by sand blasting
for that reason.
That makes sense, especially among guys who are keen to save money.

I don't know if it would save money as my understanding is you pay by
the weight of zinc added. It's a very effective bulk process and my
neighbour priced up galvanising versus painting in his former job at
Hanson and the galvanising* was cheaper and more effective, the paint
system if used would have required sandblasting the steelwork anyway.
The galvanising didn't require sandblasting but it may have been done
for extra protection, the initial acid dip removes any rust and mill
scale anyway.

In high-volume commercial galzanizing, the steel is prepped with a
zinc ammonium chloride bath. Chemistry is not my thing but I think
it's actually a conversion coating that cleans the surface and leaves
a protective film of the conversion product on the steel.

Even slight oxidation of the steel weakens adhesion of the zinc.

--
Ed Huntress