Hobson technical article
A Different Shade of Grey: Why Hot Dip Galvanised Bolts Look Duller
A two-page Hobson article explaining why hot dip galvanised (HDG) products now look duller than they used to, and what affects the look of a galvanised coating. It quotes Australia's galvanising industry association.
What this covers
- Old versus new finish
- Coating mass and life
- Steel quality and chemistry
- Design and fabrication
- Cooling rate
- Thermal cutting
Applies to Threaded Rod (Allthread), Foundation Stud, Chemical Anchor Studs & Sleeves, Chemical Anchor Kit, Nylon Frame Plug, Nylon Wall Plugs & Screws, Through Bolt Anchor, Eye Bolts & Screw Eyes, Foundation Bolt, U-Bolt, Coach Screws (Lag Screws), Coach Bolts (Cup Head Bolts) and 46 more.
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- HDG products used to be dipped in a solution that gave a lustrous finish. That step is being phased out for environmental reasons, so the finish is now a flatter matt grey, with the same physical properties.
- All galvanised coatings turn matt grey over time as the zinc reacts with the environment.
- Brightness has no effect on performance. Corrosion resistance depends only on coating mass: the heavier the coating, the longer it lasts.
- Steel chemistry changes the look: silicon and phosphorus prolong the zinc reaction and can leave a duller grey zinc-iron surface. Steel kept hot for longer after dipping also comes out duller.
- Fabrication matters too: weld spatter shows through, unsuitable anti-spatter sprays and weld porosity leave bare spots, and thermally cut edges should be ground back before galvanising for an even coating.
Who it helps: Customers and staff asking why new galvanised bolts look different to old ones.
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