Two processes share the name

Ask for galvanized steel and you may be offered either of two industries. The first is a coil finish. The Z275 coil page describes immersing a cleaned iron sheet into molten zinc at about five hundred degrees, the processing page notes that its line galvanizes continuously to produce a smooth product at Z30 to Z40 and up to twenty-five hundred millimetres wide, and the galvanized coil page sets the band at Z50 to Z275 over ten to thirteen hundred millimetres, with zero, minimized and regular spangle and chromated, oiled, skin passed, passivated or tension levelled treatments. That is bought by the tonne and made before anything is cut. The second industry appears on the hot-dip galvanized H-beam page: steel dipped into molten zinc reacts with it to form a solid zinc-iron alloy layer that keeps the environment off the steel and gives the member its gloss. That one is bought per assembly, after fabrication, priced by weight rather than by coating grade.

The coating grows out of the steel

Because that layer is a reaction product rather than a paint film, thickness is not something the galvanizer dials in. A rougher surface behaves like coarser ground: more steel meets more zinc and the alloy grows thicker. Steel chemistry matters too, since silicon and phosphorus influence how fast the layers form, which is why two heats of nominally identical steel can leave the same bath with visibly different coatings and both still comply. So the specification has to say which face matters and how it will be measured, because the inside corner of a welded box section never reads like its flat outer face. It also explains why the electroplated route, thinner and less corrosion resistant than hot dip sheet, cannot replace a structural dip, and why the galvanized H structural steel page offers the same members either way: its own line lists H section steel in Q355B as alkyd painting or hot dip galvanized, decided on service environment rather than on principle.

Coating numbers: mass, thickness and three habits

Conversion is easy once one line is kept in mind. The SECC page notes that fifty grams per square metre of pure zinc coating equals about seven point one microns, and GSM means grams per square metre. A continuous sheet designation therefore reads this way: Z275 is a total mass across both faces, so each face carries roughly a hundred and thirty-seven grams, landing at about nineteen and a half microns a side. The ladder runs from Z001 through Z060, Z080, Z120, Z180, Z250 and Z275 up to Z350, which SPGC Z350 coil shows on a real listing, while SGCC sheet carries zero point twelve to six millimetres over six hundred to fifteen hundred millimetres. Batch work is quoted in microns instead, usually the eighty-five micron class for structural sections, and a third habit shows up in names such as the Hot Dip 300 designation used on the galvanized beam page. Three notations, one question: how much, on which face, measured how.

Air has to get out and zinc has to get back in

Everything true of a fabricated assembly becomes relevant at five hundred degrees. A sealed volume is a hazard in the bath, because trapped air and residual moisture expand violently and can displace molten zinc rather than be displaced by it. The standard answer is a vent hole at the highest point and a drain hole at the lowest point of every sealed volume, sized generously and repeated along long runs, so air escapes on entry and zinc leaves on withdrawal. Overlapping surfaces need the same logic inverted. A lap joint that is continuously welded leaves a crevice holding pickling solution that will not fill with zinc, so it is either seal welded throughout so nothing enters, or interrupted so it drains and can be flushed. The rule underneath is simple: zinc enters through every opening it can reach and leaves through the lowest one available, and the design has to give it both.

Distortion is a bath problem, not a welding problem

An assembly entering a bath at about five hundred degrees receives a thermal cycle it never asked for, and asymmetric or slender fabrications are the ones that move. Long members bow, welded sections twist towards the heavy side, and straightening afterwards works against a coating harder than the steel beneath. Three decisions upstream avoid most of it: keep the fabrication as symmetrical as the design allows, brace long slender items through the dip, and agree who straightens and who pays before the order. Size brings the same constraint. Members longer than the bath are dipped in two passes, which leaves an overlap band to agree in advance, and long runs of rolled H sections or IPE beams are the usual offenders, since the same page notes a twenty foot container has five thousand eight hundred and ninety-eight millimetres of usable internal length and a twelve metre member will not travel in one.

The sequence matters more than the certificate

Galvanizing is the last step before assembly, and every operation after it costs more than the same operation performed earlier. Cut, drill, punch and weld first. The punching page notes that deburring, sanding or other treatment after punching has to be specified, and doing it before the bath means zinc covers the edge instead of exposing it. Preparation is stated plainly by the Z275 page, which puts a cleaned iron sheet into the molten zinc, and by the surface treatment page, which lists rust removal by sandblasting, shot blasting and pickling. Mill scale on hot rolled plate, cutting smear and paint residue all block the reaction, and the bare patch they leave looks identical to a coating fault on delivery. Forming afterwards carries the same risk: the bending page notes the edge takes a radius and parts may need deburring, polishing or painting later, which is what happens to coating on the outside of a tight bend.

Six lines for the enquiry

Put the coating specification first, with number, unit and whether it is per face or in total, since continuous sheet sells by Z designation while assemblies sell in microns. Say whether the item is batch dipped after fabrication or built from pre-galvanized material: DX51D+Z coil belongs to the first industry, while members listed under hot rolled H-beams and the wider steel beam programme belong to the second, and both sit side by side in the galvanized steel category. Name who owns vent and drain design. Set out who straightens and what is acceptable afterwards. State handling and packing rules, because members stacked while wet come out stained. Finally agree how thickness is verified and damage repaired, then add the processing needed first time around: cutting, shearing, bending and custom processing are priced separately, and asking before the dip rather than after is what keeps the coating intact at every edge.