Pickling and Galvanizing Pretreatment Tanks

The galvanizing pretreatment sequence, tank by tank

Before steel meets molten zinc it passes through a fixed sequence of wet stages, and each stage puts a different demand on the vessel that holds it. Read left to right, a hot-dip line normally runs degrease, rinse, pickle, rinse, flux — with the rinses in place to stop drag-out carrying one chemistry into the next.

• Degrease — a hot alkaline bath, usually run warm to lift oils and drawing compounds; the tank must take sustained heat and a caustic environment
• Pickle — hydrochloric acid on most UK galvanizing lines, sulphuric on some steel-processing plants; aggressive, fuming, and the hardest duty of the four
• Rinse — mains or recovered water, often cascaded to cut consumption; the mildest tank, but still needs the same freeboard and drainage discipline
• Flux — zinc ammonium chloride, typically held warm; a salt solution that is unkind to bare and painted steel over time

Each of those is a different specification, not four copies of the same box. Designing them together as a set is what keeps transfer heights, drain runs and extraction flanges consistent across the line.

How fabricated thermoplastic and lined steel differ on acid pickling duty

A lined steel tank is a composite: a structural shell that corrodes, and a barrier layer that must remain perfect for the shell to survive. Once the lining is punctured by a dropped jig, a crane knock or years of thermal cycling, acid reaches carbon steel and the failure runs behind the lining where nobody can see it.

A fabricated polypropylene or PVC tank has no such split. The material that resists the acid is the material that carries the load, all the way through the wall. Polypropylene is widely used for warm alkaline and acid duty; PVC is the common choice for ambient acid service and is the lower-temperature option of the two. Operating temperature, concentration and any solvent content in the degrease stage all steer that decision, so those figures are the first thing worth discussing. Our materials guide sets out how the thermoplastics compare, and where a thermoplastic lining to an existing steel or concrete pit is the more sensible route.

Tank features that decide whether a line works

Most of the difference between a tank that runs for years and one that becomes a nuisance sits in details fixed at drawing stage.

• Freeboard — enough depth above working level to absorb displacement when a full jig of steel enters, plus splash and any gassing at the pickle
• Stiffening — external ribs or a frame sized for the static head of a full tank plus the working load, not just the empty vessel
• Lids and covers — heat retention on the degrease and flux stages, and fume containment when a bath is idle
• Lip extraction flanges — fabricated into the tank rim on both long sides so the take-off is part of the vessel rather than a bolted afterthought
• Weirs and overflows — cascade rinsing, oil skimming at the degrease, and a controlled route for overfill
• Heating provision — penetrations and support for the client's chosen heaters or coils, positioned clear of jig travel
• Drain, fill and transfer connections — flanged and placed for the bunding and pump layout that already exists on site

Acid fume capture and scrubbing

Hydrochloric pickling releases acid mist and vapour into the workshop, and a warm flux bath adds its own. Left uncaptured, that fume attacks roof steelwork, cranes, electrics and anything else in the building, and it is a straightforward occupational exposure problem as well.

Lip extraction along both rims is the usual answer, drawn into thermoplastic ducting chosen for its resistance to the acid it carries, then through a wet fume scrubber before discharge. Building the tanks and the extraction as one package avoids the common retrofit problem of flanges that do not line up with the hoods someone else has specified.

Phosphating and other pretreatment lines

The same tank-set logic applies well beyond galvanizing. Iron and zinc phosphating ahead of powder coating, chromate-free conversion coatings, passivation and general degrease-and-rinse lines all use a run of heated and ambient baths with drag-out between them. Details change — heater duty, sludge removal, tighter temperature control — but the vessel requirements rhyme, and they are built the same way in our Poole works. Our wider process tanks and vats page covers the single-vessel work.

Replacing one failed tank in a running line

Rarely does an entire line fail at once. More often a single pickle tank goes first, because it takes the worst of the chemistry, and the plant needs a replacement that drops into the same footprint without moving cranes, gantries or pipe runs.

For that work we measure from the tank in place — footprint, rim height, flange positions, extraction take-offs and every existing connection — and fabricate to match the existing footprint and connections, with the aim of keeping the changeover as simple as possible. Where the surrounding pipework needs adapting, that can be quoted as part of the same job; see pipework and controls.

Steel processing and stainless pickling

Beyond galvanizers, acid tanks turn up wherever mill scale, heat tint or weld discolouration has to come off steel. Stainless pickling and passivation baths, descaling tanks ahead of forming, and immersion tanks in fabrication shops all sit in the same technical territory: an aggressive bath, a fume problem above it, and a structure that has to survive both.

Talk to us about your pretreatment line

Beckox has fabricated in thermoplastics from the same Poole works since 1982, and designs, builds and installs in-house rather than subcontracting the site work. Bring the chemistry, temperatures, jig sizes and the space available, and we will work up a tank set around them — or price a single replacement measured from what is there now.

Call 01202 736 725 to arrange a free on-site consultation, or use the contact page and we will come and measure up.

PAGE REVIEWED — 30 July 2026