What Is Galvanized Steel?
Galvanizing puts a layer of zinc over steel. The zinc protects the steel in two ways: it seals the surface off from water and air, and because zinc corrodes before steel does, it keeps protecting small scratches and bare spots nearby.
Hot galvanized, hot dip galvanised and hot-dipped all describe the same process, where cleaned steel is dipped in a kettle of molten zinc at around 840°F (450°C) and pulled out with the zinc bonded to its surface.
Hot-Dip Galvanizing
Hot-dip galvanizing happens after the steel is fabricated, at a galvanizing plant. Because the whole piece goes into the kettle, the zinc covers the inside of tubes, the backs of plates and every weld.
Hollow sections such as rectangular tube and pipe need vent and drain holes, so molten zinc can flow in and out and trapped air can escape. Those holes belong on the shop drawings and in the fabrication labor. Each piece also has to fit the galvanizer's kettle, so long or wide assemblies may be split or dipped twice.
Pre-Galvanized Steel
Pre-galvanized steel, also called mill galvanized, is coated on a continuous line while it is still coil, sheet or tube. It costs less and has a smoother finish, and it is what most ductwork, strut and light-gauge framing is made from.
The catch is that cutting, punching and welding happen after the coating. Every cut edge and weld is bare steel, and the specification usually calls for those areas to be repaired with zinc-rich paint, which is labor someone has to carry.
How Galvanizing Shows Up in an Estimate
Galvanizers usually price hot-dip work by the weight of the steel, so the steel takeoff in pounds or tons drives the galvanizing cost directly. The galvanizing cost calculator roughs out that line from your tonnage, and the guide to hot-dip galvanizing cost per pound covers the rates.
- The galvanizer's charge, by weight
- Shipping to and from the galvanizing plant
- Vent and drain holes on hollow sections
- Extra handling for long pieces that need a double dip
- Touch-up of field welds and cut edges, including on pre-galvanized material
- Time in the schedule, since the steel leaves the shop before it is delivered
Common Mistakes on a Galvanized Scope
The one that costs the most is pricing hot-dip galvanizing like paint, by area, when the galvanizer bills by weight. Heavy members with little surface area cost far more to galvanize than a square-foot price would suggest.
The other regulars are leaving out the trip to the galvanizer, forgetting vent holes on tube steel, assuming pre-galvanized material needs no touch-up, and missing a duplex system, where paint goes over the galvanizing and needs its own surface preparation.
