Phosphating instead of HCl Pickling before Galvanizing

A. Sir:
I believe I visited your plant about 1985, had dinner with one of your executives, and discussed a possible HCl reclaiming plant near Houston. Because the price of virgin HCl in South Africa was very high your plant was doing very well financially. In Houston HCl was cheap and a deep well disposal was nearby (near Houston), an HCl recovery plant was not financially reasonable.

HCl, H2SO4, and HNO3 are the three "strong" acids. HCl and H2SO4 are reducing acids, whereas HNO3 is an oxidizing acid.
Thus for pickling mild steel, HCl and H2SO4 are normally used. There are many organic weak acids (e.g. formic, acetic, etc.). H3P04 (phosphoric acid) could be classified as a medium strength acid which would have too slow of pickling (cleaning off mil scale off steel) to be useful. In addition, once the phosphoric acid became "spent" (saturated with iron and zinc phosphate), how would it be disposed of?

In the USA HCl and H2SO4 are both used for steel pickling. The "spent" HCl usually goes to deep well. The "spent" H2SO4 is cooled and crystals of Iron and Zinc heptahydrate are crystallized out, and the recovered acid is re-used again and again. These crystals are often used in plant fertilizers. I enjoyed the dinner, thank you.

A. Galvanizing isn't a coating(in the usual sense of the word), it's an alloying reaction. It will only take place between iron and zinc, and only when the iron (steel) is chemically clean.
Acid is used to clean the steel.
The type of acid depends on what is best cost. HCl is perhaps most common, as it's low cost, works at ambient temperature, and is relatively safe.
H2SO4 is next most popular, often due to cost locally, but usually needs heating, and "over pickling" starting to dissolve the steel - can occur.

Depositing something like phosphate (you didn't say which phosphate) onto steel won't leave a chemically clean surface for the zinc to alloy with.

If the steel isn't clean, then all you'll get is a hot piece of steel.