S220GD+ZM Coating Weight: How to Choose ZM Coating for Outdoor Structures

ZM coating weight tells you how much zinc-aluminium-magnesium alloy is plated on the steel surface, measured in grams per square metre over both faces. For S220GD+ZM used outdoors — solar racking, purlins, fencing, agricultural buildings — picking the right ZM grade is the difference between a 15-year and a 30-year service life.
 

EN 10346 ZM Coating Weight Table

The ZM family is defined in EN 10346:2015. Common grades for S220GD substrate are ZM90 through ZM310.
 

Triple spot is the average over three test areas; single spot is the minimum anywhere on the coil. Thickness is calculated using the ZM alloy density of about 6.2 g/cm³.
 

Why ZM Outperforms Z at the Same Coating Mass

A common buyer question: is ZM120 lighter than Z275? Numerically yes, but performance is equal or better. The 1–3% magnesium in the alloy forms a stable corrosion-product film on cut edges and scratches, which is exactly where pure zinc fails first. In independent tests, ZM120 gives corrosion protection not less than Z275, and ZM310 matches Z600 — so you can specify a thinner, lighter ZM layer and still reach the same design life.
 

Match the Coating Weight to the Corrosion Class

For outdoor structures the practical minimum is ZM120. Anything lighter belongs to indoor or sheltered service only.
 

5-Point Selection Checklist

1. Service environment. Coastal salt, livestock ammonia and acid rain push you one class higher than the map alone suggests.
2. Design life target. 15 years vs 25 years vs 30+ years — each step up typically moves you one coating grade.
3. Cut edge exposure. Slit strips and punched parts benefit most from ZM180 and above because of the self-healing cut-edge effect.
4. Forming severity. Tight bends and deep draws are easier on thinner ZM (ZM90 to ZM150) than on thicker ones.
5. Budget vs life-cycle cost. ZM180 instead of ZM275 may raise material price by 10–20%, but cuts replacement and maintenance over the project life.

How to Write the Specification Clearly

Avoid ambiguous wording like “galvanized”. Use the EN 10346 designation: S220GD+ZM180 means 220 MPa minimum yield, hot-dip coated, ZM at 180 g/m² triple spot. Add the surface finish (zero spangle, skin passed, oiled or unoiled) and the standard test certificate (EN 10204 3.1).
 

Frequently Asked Questions

1. Can I order a ZM coating weight outside the EN 10346 table?
Yes, many mills accept non-standard weights between two listed grades, for example ZM160. Expect a small MOQ premium.

2. Is thicker ZM always better for outdoor use?
Not necessarily. Once the self-healing layer is established, additional mass gives diminishing returns. Pick the grade that matches the corrosion class and design life.

3. Does ZM coating need post-paint for outdoor colour?
Not for corrosion protection, but for colour you can powder coat or liquid paint directly on ZM after a standard pre-treatment — no special primer required.

4. How does ZM differ from AZ (Galvalume) on the same base steel?
AZ is roughly 55% Al / 43% Zn / 1.5% Si and excels at high temperature. ZM uses much less aluminium but adds magnesium, which gives far better cut-edge protection — the main reason ZM is preferred for outdoor structural parts.

Tell us your project country, target service life and profile drawing. We will recommend the most cost-effective ZM grade and supply mill-trial salt-spray data. Reach our sales team through the contact page.