On site, people ask for "MS bolts", "GI bolts" or "SS bolts". These words describe the material and the protective coating, and choosing the right one is mostly about where the fastener will be used. Here is a plain-language comparison to help you choose.

What the terms mean

• MS (mild steel): plain carbon steel with no corrosion protection other than a light oil, often called "black" finish. Strong and cheap, but it rusts quickly if left exposed.
• Zinc-plated or electro-galvanised (EG): steel with a thin layer of zinc applied by electroplating, often followed by a clear, blue, yellow or black passivation. Typical zinc thickness is only about 5 to 12 microns.
• GI or hot-dip galvanised (HDG): steel dipped in molten zinc, giving a much thicker coating, usually around 45 to 85 microns on fasteners. It looks dull grey and slightly rough. "GI" (galvanised iron) is the common trade name.
• SS (stainless steel): steel alloyed with chromium and nickel, so the whole fastener resists rust. SS304 (A2) is the common grade and SS316 (A4) is used for harsher, salty or chemical environments.

Side-by-side comparison

Which one should you use?

Inside a building, in dry areas

Zinc-plated fasteners are usually enough for false ceiling supports, cable tray hangers in dry shafts, furniture and panel work. They look neat and cost little.

Outdoors, terraces, basements and wet areas

Use hot-dip galvanised bolts, nuts, threaded rods and anchors. The thick zinc layer protects for years even when the surface is scratched, because zinc corrodes first and protects the steel underneath.

Near the sea, in pools, food plants and chemical areas

Use stainless steel, and choose SS316 (A4) where there is salt or chlorine.

Steel structures that will be painted

Black MS bolts are acceptable when the whole joint will be primed and painted, but galvanised bolts last much longer if paint is damaged.

Things that cause trouble on site

• Mixing metals. Stainless bolts in aluminium, or zinc-coated parts touching copper, can corrode faster because of galvanic action when wet. Use matching materials or insulating washers.
• Using plain nuts on HDG bolts. Hot-dip galvanised nuts are tapped slightly oversize to fit over the thick coating. Always buy HDG bolts and nuts as a set.
• Welding galvanised parts. Zinc fumes are harmful. Weld only with good ventilation and proper protection, and touch up the weld area afterwards with zinc-rich paint.
• Expecting plating to last outdoors. Bright zinc-plated fasteners often show white rust within months outside. That is a finish problem, not a material defect.
• Stainless galling. Stainless nuts and bolts can seize when tightened fast or dry. Tighten steadily and use anti-seize compound on larger sizes.

Also see our guide to bolt grades 4.6, 8.8 and 10.9.

This guide is general information for buyers. Always follow the anchor or fixing manufacturer's data sheet and your project engineer's design for load-bearing and safety-critical work.