Two GI coils can look identical on the truck and last completely different lengths of time on a roof. The difference is the zinc coating, and it is written in a short code on the mill certificate: Z120, Z275, G90. This page explains what those codes mean and which one to specify in the UAE.
What the Z number means
Under EN 10346, the European standard for continuously galvanized sheet and coil, the number after the Z is the minimum total zinc coating mass in grams per square metre, counting both sides together. Z275 carries at least 275 g/m² of zinc. Z120 carries at least 120 g/m².
Because the figure covers both sides, each face gets roughly half. Zinc has a density of 7.14 g/cm³, so every 7.14 g/m² on one side is about one micron of thickness. That lets you convert the code into an actual coating thickness.
| Coating class | Total zinc, both sides | Per side | Approx. thickness per side |
|---|---|---|---|
| Z100 | 100 g/m² | 50 g/m² | 7 µm |
| Z120 | 120 g/m² | 60 g/m² | 8.4 µm |
| Z140 | 140 g/m² | 70 g/m² | 9.8 µm |
| Z180 | 180 g/m² | 90 g/m² | 12.6 µm |
| Z200 | 200 g/m² | 100 g/m² | 14 µm |
| Z275 | 275 g/m² | 137.5 g/m² | 19 µm |
Zinc protects steel by corroding slowly in its place, so in a given environment the life of the coating is roughly proportional to its thickness. Z275 has more than twice the zinc of Z120, and in the same conditions it lasts roughly twice as long.
American and Japanese equivalents
ASTM A653 uses ounces per square foot, again counting both sides. JIS G3302 uses a Z number in tens of grams.
| ASTM A653 | Approx. g/m² | Closest EN 10346 | Closest JIS G3302 |
|---|---|---|---|
| G40 | about 120 | Z120 | Z12 |
| G60 | about 180 | Z180 | Z18 |
| G90 | about 275 | Z275 | Z27 |
These are practical equivalents for ordering, not exact matches. Each standard has its own test method and tolerances. If a specification names one standard, supply to that standard.
Which class to specify in the UAE
The UAE is harder on zinc than the dry climate suggests. Coastal humidity, salt in the air, and condensation cycles between air conditioned interiors and hot exteriors all speed up corrosion. As a general guide:
- Internal ducting in conditioned spaces: Z120 is commonly specified and is usually adequate where the sheet stays dry.
- Internal but humid or unconditioned: step up to Z180 or Z200. Plant rooms, car parks and warehouses without climate control fall here.
- Roofing, cladding and anything outdoors: Z275 is the sensible minimum. Thinner coatings save a little up front and cost a lot more when the roof needs replacing.
- Within a few kilometres of the sea: Z275 at least, and consider whether a painted or post galvanized product is the better answer for the life you need.
When in doubt, the consultant's specification decides. If there isn't one, tell us the application and the location and we will recommend a class.
Sheet galvanizing is not the same as hot dip galvanizing after fabrication
Everything above is about continuously galvanized sheet and coil, where the zinc goes on at the mill before the steel is cut. Structural sections, gratings and fabricated items are usually hot dip galvanized after fabrication to EN ISO 1461, and the coating is far heavier.
| Steel thickness | Minimum mean coating, EN ISO 1461 |
|---|---|
| Over 6 mm | 85 µm |
| Over 3 mm up to 6 mm | 70 µm |
| 1.5 mm up to 3 mm | 55 µm |
| Under 1.5 mm | 45 µm |
That is four to ten times the zinc on a Z275 sheet, and it covers every cut edge and weld because the dip happens after fabrication. It is why GI gratings and hot dip galvanized hollow sections are the normal specification for external structures.
Checking what you received
- Read the certificate. The coating class should be stated on the mill certificate alongside the steel grade.
- Check the coil label. Most mills print the coating class on the coil tag. It should match the certificate and your order.
- Measure it. A magnetic coating thickness gauge gives a reading in microns in seconds. Compare against the per side figures in the first table.
Frequently asked questions
Is Z275 the same as G90?
Close enough for ordering in most cases. Both are about 275 g/m² total, measured under different standards.
Why does my Z120 sheet show white rust already?
White rust is zinc corroding, usually because sheets were stored wet or stacked tight with moisture trapped between them. Store coil and sheet dry and ventilated.
Do you supply GI sheet and coil in different coating classes?
Yes. Tell us the thickness, width, coating class and quantity. See GI coil and sheet.
Getting a price
Send your requirement through the steel quote request form or on WhatsApp at +971 56 227 5722. Include thickness, coating class and quantity, and the delivery point in any of the seven emirates.