Coating Mass Explained — What Z Numbers Mean (GI & AZ Steel Guide)

One of the most misunderstood parts of buying galvanized or Galvalume coil is the coating mass number.

Coating Mass Tests & What Z Numbers Mean (GI / AZ)

One of the most misunderstood parts of buying galvanized or Galvalume coil is the coating mass number.

Buyers ask:

What does Z275 actually mean?
Is Z180 enough for roofing?
Is G90 the same as Z275?
How is coating mass tested?
Can coating vary across the strip?

Coating mass directly affects:

Corrosion resistance
Service life
Warranty validity
Export compliance
Insurance claims

If you do not understand coating mass, you are buying steel blind.

This guide explains:

What Z numbers mean
What AZ numbers mean
GI vs AZ differences
How coating mass is measured
Tolerance rules
How to verify compliance

1. What Is Coating Mass?

Coating mass refers to:

The total metallic coating weight applied to both sides of the steel.

It is usually expressed in:

grams per square meter (g/m²)

The coating provides corrosion protection.

More coating = generally longer corrosion resistance.

But application matters.

2. What Does “Z” Mean in GI?

For galvanized steel (GI):

Z stands for zinc.

The number represents:

Total zinc coating weight on both sides combined.

Example:

Z275 = 275 grams of zinc per square meter (both sides total)

That does NOT mean 275 per side.

It means approximately 137.5 g/m² per side (average).

3. Common Z Coating Levels

Z100
Z120
Z140
Z180
Z200
Z275

Higher number = thicker zinc layer.

Z275 is common in:

Structural and roofing applications requiring durability.

4. What About “G” Numbers (ASTM System)?

In North America:

Coating mass often expressed as:

G60
G90

These are ounces per square foot.

For example:

G90 ≈ Z275 equivalent (approximately)

But conversions must be handled carefully.

Always confirm units.

5. What Does “AZ” Mean?

For Galvalume (Aluminum-Zinc alloy):

AZ stands for Aluminum-Zinc.

AZ150 means:

150 g/m² total coating weight (both sides combined).

Galvalume coating is:

~55% aluminum
~43.5% zinc
~1.5% silicon

It offers different corrosion characteristics compared to GI.

6. GI vs AZ Coating Performance

GI (Z coating):

Strong sacrificial protection
Better for scratched surfaces

AZ coating:

Better long-term corrosion resistance
Better heat reflectivity
Less sacrificial at cut edges

Choosing correct coating type matters for climate conditions.

7. How Coating Mass Is Tested

There are two primary methods:

Non-destructive magnetic gauge
Destructive weight difference method

Magnetic Coating Thickness Gauge

Used in warehouse or field.

Measures coating thickness indirectly.

Quick and non-destructive.

But:

Less precise for official compliance.

Triple Spot Test (Destructive Method)

Used in laboratory.

Procedure:

Remove zinc coating chemically
Weigh before and after
Calculate coating mass

More accurate for compliance testing.

Often used for dispute resolution.

8. Single Spot vs Triple Spot

Standards often specify:

Single spot minimum
Triple spot average

Single spot = lowest reading allowed
Triple spot = average across multiple test areas

A coil may pass average but fail single spot.

Specification must clarify acceptance criteria.

9. Coating Tolerances

Coating mass is not exact.

There are allowable tolerances.

Example:

Z275 may allow variation within standard limits.

If coating falls below minimum defined tolerance:

Claim may be valid.

Without defined tolerance in PO, dispute becomes subjective.

10. Common Buyer Misunderstandings

Z275 does not mean 275 per side.
G90 is not exactly equal to Z275.
Thicker coating does not change BMT.
Magnetic gauge reading is not exact coating mass.
White rust does not always mean low coating.

Clarity prevents disputes.

11. How Coating Affects Thickness

Important:

Coating adds thickness.

But:

Coating mass measured in weight per area — not thickness in mm.

Two coils may have same BMT but different coating mass.

Never confuse BMT with total coated thickness.

12. Application-Based Coating Selection

Roofing in coastal areas:

Higher Z or AZ recommended.

Interior structural use:

Lower coating acceptable.

Agricultural buildings:

Often require higher corrosion resistance.

Correct coating selection affects lifespan and warranty.

13. Claim Strategy for Coating Disputes

If suspect low coating:

Verify specification in PO
Measure using magnetic gauge
Send sample for lab triple-spot test
Compare against tolerance
Notify supplier immediately

Coating disputes require technical proof.

Visual inspection alone is insufficient.

14. Coating Uniformity Issues

Coating may vary across width if:

Air knife imbalance
Bath contamination
Line speed fluctuation

Patterns may indicate mill issue.

Random variation may indicate measurement error.

15. Practical Inspection Checklist

Incoming inspection:

✔ Verify Z or AZ number on MTC
✔ Match heat number
✔ Perform spot gauge checks
✔ Record readings
✔ Compare to tolerance
✔ Inspect for bare spots

Documentation strengthens future claim.

FAQ Section

Does Z275 mean 275 per side?

No — total both sides.

Is G90 equal to Z275?

Approximately similar, but confirm.

Can coating vary slightly?

Yes, within tolerance.

Is AZ150 better than Z180?

Depends on environment.

Should coating tolerance be written in PO?

Yes.

Can white rust mean low coating?

Not necessarily.

Is magnetic gauge enough?

For quick check, yes. For dispute, lab test preferred.

Does thicker coating improve lifespan?

Generally yes.

Is coating weight same as thickness?

No.

Should coastal projects use higher Z?

Yes.

Conclusion

Coating mass is one of the most critical specifications in galvanized and Galvalume steel.

Z numbers represent:

Total coating weight per square meter.

AZ numbers represent:

Aluminum-zinc alloy coating weight.

Understanding:

Measurement methods
Tolerance rules
Application suitability

Protects you from:

Premature corrosion
Warranty disputes
Unnecessary rejection
Incorrect purchasing

Professional buyers define coating requirements clearly — and verify them upon arrival.

Specification clarity + proper testing = controlled procurement.