S350GD+ZM310 Vs S350GD+Z: Which Coating Is Better?

Oct 08, 2026

Both S350GD+ZM310 and S350GD+Z start from the same substrate: an EN 10346 structural steel with a guaranteed minimum yield strength of 350 MPa, tensile strength of 420 MPa and elongation A80 of 16 % at 0.70 mm and above. The difference is the bath. +Z is pure zinc; +ZM310 is a zinc-magnesium alloy with aluminium additions, applied at 310 g/m² over both faces. Which is better depends on where the part spends its life, and the answer below gives you the decision table.

For indoor parts, painted products and dry inland roofing, S350GD+Z does the job at the lowest coating cost. For anything unpainted that faces rain, salt or ammonia - solar structures, coastal profiles, agricultural buildings, parts with many cut edges - S350GD+ZM310 outlasts zinc by a wide margin at equal coating mass. The substrate decides how far the material can be formed and loaded; the coating decides how long it survives in service.

1. Coating Composition Compared

Property	S350GD+Z	S350GD+ZM310
Coating alloy	Pure zinc (Zn ≥ 99 %)	Zn balance with Al and Mg additions
Designation basis	Z-class, EN 10346	ZM-class, EN 10346
Typical reference mass	Z275: 275 g/m² both faces	ZM310: 310 g/m² both faces
Coating hardness	60–80 HV	around 100–140 HV
Corrosion products	Loose, non-protective white rust	Dense, adherent, self-healing layer

EN 10346 does not fix a single ZM recipe; common commercial baths are Zn-2Al-2Mg and Zn-3.5Al-3Mg types. Magnesium is the active element: it forces the coating to form compact simonkolleite-type corrosion products instead of the loose oxide that lets zinc keep consuming itself.

Ask for Sample & Quote

2. Corrosion Performance

Criterion	S350GD+Z (Z275)	S350GD+ZM310
Inland atmosphere (C2–C3)	Reference life	Roughly 2–4× at equal coating mass
Coastal / industrial (C4–C5)	Limited unpainted life	Up to about 3–10× at equal coating mass
Cut-edge and scratch protection	Sacrificial, edge rusts first	Corrosion products migrate over exposed steel
Salt-spray results (ASTM B117)	Around 300–500 h published	ZM coatings of this class commonly 2,000 h plus

Two caveats keep these numbers honest. First, salt-spray hours are indicative published ranges, not a design life; real service life depends on ISO 9223 corrosivity category, orientation and drainage. Second, the multiples compare coatings at equal mass - ZM310 against Z275 carries more coating, so its advantage over Z275 specifically is larger still. Always verify delivered coating weight on the EN 10204 3.1 certificate.

3. The Weight-and-Cost

Because a ZM coating lasts longer per gram, you can reach the same service life with less coating mass than a zinc solution would need - less bath weight per square metre, and a thinner, lighter product at equal protection. The harder ZM surface also means less die wear in roll forming and fewer surface scratches in transit, which matters when the panel or profile ships unpainted.

4.How to Choose

Your situation	Choose
Parts painted after forming (coating only needs to survive until paint)	S350GD+Z
Indoor ducting, appliance backs, dry interiors	S350GD+Z
Budget-limited inland roofing with planned maintenance	S350GD+Z
Unpainted solar structures, coastal or farm environments	S350GD+ZM310
Heavy cut edges, punched holes, frequent handling	S350GD+ZM310
Sandwich panel faces with visible matt surface	S350GD+ZM310

If you are unsure which side your project falls on, send us the service environment - distance to coast, indoor or outdoor, painted or bare - and we will recommend the coating class with the reasoning, not just a price.