I am reading a Railway Gazette news item about Hunslet consolidations for Peru, 1948. The text says the flues are "galvanised Acquacidox". A Google search for Acquacidox returns nothing.

Anybody heard of this material before?

The article probably misspelled it.

Google finds an alloy "aquacidox" used for boiler tubes.

It is spelled wrong but even spelled correctly there are only 2 hits. One4 references a brochure by a British company making the tubes. The other to a book you have to pay to see. However the hit gives the recipe anf an AMS number. It is AMS 5030A.

If it proved useful, chances are we'd all know about it and use it today. Still, I'm interested in finding out more.

The British are masters at using long, complicated, Latinate-sounding names to describe what they want to keep proprietary. I believe AMS 5030A is similar to AISI grades 1006, 1008, and 1010, a low-alloy steel. Carbon and silicon are down in the range normally specified for sulfur and phosphorus, and small amounts of manganese (0.5% max) and copper (0.35 max) supply the structural refinement.

Of potential interest to the preservation community, cold-drawn tubing in this alloy was the subject of an anti-dumping hearing as recently as March 14, 2023 -- upon lobbying for domestic sources, the duty on this, which includes nominally 5030 tubing, was continued... so if price or availability of that stuff is restricted, you know where to go.