Why Galvanised Coating Is Never Exactly Right? Analysis of Z Tolerance In Practice

One of the most common misunderstandings when working with galvanised steel sheet is: “Z275 must be exactly 275 g/m²”. This is a theoretical understanding, but not true in industrial production reality.

In reality, the galvanised coating always has tolerance due to the nature of the continuous hot-dip galvanising process. Without understanding this, buyers can easily dispute with suppliers or misjudge material quality.

1. Physical – metallurgical nature of the galvanised coating: why can't it be absolutely uniform?

To understand why the galvanised coating is never “absolutely exact”, it needs to be viewed from the perspective of fluid mechanics + heat transfer + metallurgical reactions, not just a simple coating.

1.1. Not a “coating”, but an Fe–Zn alloy layer system

Many people think galvanised is just “coating a layer of galvanised onto steel”. In reality, during the hot-dip process, at the contact surface, diffusion reaction between Fe (iron) and Zn (galvanised), creating multiple intermediate alloy layers:

• Gamma (Γ)
• Delta (δ)
• Zeta (ζ)
• Eta layer (η) – pure galvanised on the outside 

To make it easier to visualize, you can refer to the cross-section of galvanised steel sheet below: The Eta layer is coated on the outside with 100% pure galvanised (100% Zn), decreasing gradually from top to bottom.

This is a multi-layer structure, not a homogeneous layer. The overall thickness Z275 or Z180 is actually a composite of these layers, where each layer has a different formation rate depending on temperature and contact time.

1.2. Influence of flow dynamics and “air knife”

After exiting the molten galvanised bath (~450°C), the steel does not naturally have the desired galvanised coating thickness. The actual thickness is adjusted by the air knife (dao khí).

Essentially, this is a problem of fluid mechanics (fluid dynamics):

• molten galvanised flow adhering to the steel surface (viscous film)
• compressed air blowing to “cut” excess galvanised layer
• creating a balance between adhesion force and cutting force

But in reality:

• air pressure is never absolutely stable
• nozzle – surface distance has microscopic errors
• line speed changes with the coil

→ leading to galvanised layer thickness variations along both the length and width of the steel coil.

1.3. “Edge effect” phenomenon – edges are always different from the center

An important phenomenon in continuous galvanising is edge overcoating:

• coil edges are usually thicker than the center
• air flow difficult to control evenly at the edge
• galvanised flow pushed to the edge

This causes the coating distribution horizontally to always have a gradient, it cannot be perfectly flat.

1.4. Influence of base steel (substrate effect)

The chemical composition of the base steel directly affects the Fe–Zn reaction rate. Specifically:

• Si (Silicon) content
• P (Phosphorus) content

can increase or decrease the alloy layer formation rate.

This phenomenon is called Sandelin effect:

• some steel grades will react more strongly with galvanised → thicker coating
• others react more slowly → thinner coating

→ even with the same production line, same parameters, but different steel grades → coating still differs.

1.5. Thermal shrinkage and uneven solidification

After air knife, the galvanised layer changes from liquid to solid state in a very short time.

This process is influenced by:

• temperature gradient
• cooling rate
• heat exchange with the environment

Therefore:

• coating layer may shrink unevenly
• micro-variations in thickness appear

1.6. Technical nature conclusion

Galvanised coating layer is the result of a complex system comprising:

• metallurgical reaction (Fe–Zn diffusion)
• fluid mechanics (zinc flow + air knife)
• substrate material variation
• thermal solidification process

Therefore, the absolute unevenness of the coating layer is not a manufacturing defect, but an unavoidable physical nature.

Technical standards therefore do not require “absolute accuracy”, but specify according to average value and permissible tolerance.

2. Z275 is actually the minimum value

According to standards, symbols like Z275 are usually understood as average minimum value, not an absolute value at every point.

For more details, you can refer to the article about the "Z" symbol in galvanised steel sheet here: What is the coating tolerance standard Z120, Z180, Z275? Permissible error in galvanised steel sheet

This means:

• has points higher than 275
• has points lower than 275
• the average of all samples meets standard requirements

Therefore, measuring a single point as 260 or 290 does not mean the material is incorrect, if the overall remains within the permissible standard.

3. Concept of coating tolerance (coating tolerance)

Coating tolerance is the permissible range of variation of the galvanised coating compared to the nominal value. This is a mandatory factor in production because absolute precision cannot be achieved.

International standards (EN, ASTM, JIS) all stipulate:

• average minimum value
• local minimum value

These two concepts are very important but are often overlooked when purchasing.

4. Common mistake when inspecting the coating

Many people use a measuring device at a single point then conclude “insufficient galvanised coating”. This is an incorrect inspection method because:

• the galvanised coating is not absolutely uniform
• measuring equipment has its own error
• the standard requires average assessment, not a single point

The correct inspection method is to:

• take multiple measurement points
• calculate the average
• compare with the applicable standard

5. Why is tolerance important in practice?

Understanding tolerance helps you:

• avoid meaningless disputes with suppliers
• correctly read CO, CQ and mill test
• compare quotations more accurately
• correctly assess material quality

Conversely, if you do not understand tolerance, you are very likely to:

• reject compliant material but measured “off-point”
• or accept substandard material due to incorrect inspection

6. Practical problem in B2B purchasing

In mechanical, factory, MEP, FDI projects, misunderstanding tolerance can lead to:

• disputes between contractors and suppliers
• project delays due to unnecessary material rejection
• increased re-inspection costs

Meanwhile, if you understand the standard correctly, you will handle issues faster and more professionally in the eyes of partners.

7. Conclusion

The galvanised coating is never absolutely exact – and that is completely normal in industrial production.

What is important is not a number at a single measurement point, but Do all materials meet tolerance standards?.

Understanding this correctly helps you make more accurate decisions, save costs, and avoid many unnecessary risks.

Phu Cuong Steel Sheet – Leading Reputable Large Width Steel Sheet Distributor

If you need materials with clear control over coating thickness, tolerance, specifications, and CO, CQ documents, Phu Cuong Steel Sheet provides a complete solution from materials to fabrication.

• galvanised steel sheet, EG, GA with diverse specifications
• large width 1500mm and above – rare in the market
• fabrication, decoiling, bending, stamping according to drawings
• modern machinery system, precise cutting, minimal scratches
• full CO, CQ available for construction project
• fast delivery by private truck

Contact information:
Address: 2177C–2177D National Highway 1A, District 12, HCMC
Hotline: 0933 919 899
Website: tonthepphucuong.com

See more articles with related content:

• How Does Galvanised Steel Sheet Lifespan Depend on Z120 – Z180 – Z275 Coating Layer?
• Distinguish Between Sumikin, Dong A, Hoa Sen, Nam Kim Galvanised Steel Sheet – Which Type to Choose For Construction Project?
• What is EG Steel Sheet? SECC, EG, Electro-Galvanised Steel EG – A–Z Overview & Applications in District 12
• Compare Large Width and Standard Width Galvanised Steel Sheet: Which Option is More Effective?