Coating thickness is an important steel specification that affects the steel’s durability and corrosion resistance. Steel coating thickness can be measured in two main ways:
- Directly as the physical thickness in microns (µm)
- Indirectly, as the coating weight per area, in g/m² or oz/ft² units.
Standard steel grade systems (such as ASTM and EN) also use the coating weight to indicate the galvanized coating thickness. Manufacturers use magnetic gauges, optical microscopy, and weighing techniques for measuring steel coating thickness.
Buyers often have confusion between measuring steel coating thickness and base metal thickness. This guide will clarify all such issues and explain how steel coating thickness is measured, the units, the standards, and what buyers should verify before ordering.
Why Steel Coating Thickness Matters
Coating thickness is the physical thickness of the protective metallic coating applied to the steel surface. In galvanized steel, it refers to the zinc coating thickness, while in Galvalume, it refers to the aluminum-zinc (AluZinc) coating thickness.
Steel Coating Thickness vs. Corrosion Resistance
Higher coating thickness grades provide longer sacrificial protection due to the amount of self-corroding zinc (or AluZinc) they contain.
However, the AluZinc coating of Galvalume performs better than galvanized coatings in harsher environments. Therefore, corrosion resistance cannot be compared only with the steel coating thickness. You need to consider the coating type, quality, and environmental conditions as well.
Coating Thickness and Base Metal Thickness
There is a relationship between the zinc coating thickness and the base steel thickness. Thicker steel sheets need higher zinc coating thicknesses to receive sufficient protection.
Corrosivity Level | Steel Sheet Thickness (mm) | Steel Coating Thickness (µm, per side) | Coating Weight (Grades) | Suitable Steel Types |
Interior and Dry Environments | 0.3 – 0.6 mm | 8 – 12 µm | Z80 – Z100 (G40 – G60) | Galvanized (GI) |
Standard Roofing & Cladding | 0.4 – 0.8 mm | 18 – 20 µm | Z275 (G90) | Prepainted Galvanized (PPGI) |
Urban and Outdoor Conditions | 0.5 – 1.0 mm | 20 – 25 µm | Z275 – Z350 | PPGI / Galvanized |
Industrial and Coastal Areas | 0.5 – 1.2 mm | 16 – 20 µm | AZ150 | Galvalume (Al-Zn) |
Long-Life Applications | 0.6 – 1.2 mm | 20 – 25 µm | AZ150 – AZ200 | Prepainted Galvalume (PPGL) |
Cost and Performance Balance
We can have a higher performance and durability with much larger coating thicknesses. But you should also be knowledgeable about the environment, level of corrosion protection needed, maximum thickness requirements, and cost constraints.
Proactively investing in surface preparation and coil coating process quality will bring more advantages than blindly increasing the coating thickness. A properly applied standard coating will mostly perform better than a poorly applied, thicker coating.
Steel Coating Thickness vs. Coating Weight: What’s the Difference?
Coating thickness is a direct thickness measurement, while coating weight is an indirect way of indicating the same.
Coating Weight
Coating weight is the total mass of metallic coating applied per unit surface area of steel. In most international standards (such as ASTM, EN, ISO, JIS), it refers to the combined coating on both sides of the steel sheet.
Coating Thickness
This is the length measurement of how thick the layer of coating is. This is typically measured in microns (µm) or mils.
Why Buyers Need to Understand Both Coating Thickness and Weight
In most commercial standards, weight (g/m² or oz/ft²) is used to represent the galvanized coating thickness measurement. However, inspections and testing often report steel coating thickness in µm. Therefore, buyers need to have the conversion knowledge of both designations.
Common Units Used for Measuring Steel Coating Thickness
Grams per Square Meter (g/m²) – EN, ISO, JIS Standards
This is the most widely used unit for galvanized steel coating thickness in global markets, and also in Europe and Asia. It represents the total coating weight on both sides of the steel sheet. As an example, Z275 means 275 g/m² of zinc coating measurement.
Ounces per Square Foot (oz/ft²) – ASTM Standards
This unit is used in ASTM-based systems. For example, G90 means 0.90 oz/ft² of zinc coating weight (on both sides). It is mainly used in the United States and is equivalent to metric units like g/m².
Microns
Microns measure the actual coating thickness, not weight. This unit is used for both metallic coatings and paint layers in PPGI and PPGL.
Steel Coating Thickness to Weight Unit Conversion Concepts
The best practice is to use standard tables or supplier documentation when converting units. Since the coating type and density also affect the steel coating thickness, using the values provided for the specific product is the safest method.
Common Coating Grades and Designations Explained
Coating grades usually indicate the total weight of the protective metallic coating applied on the steel surface.
G60 and G90
The Imperial “G” Series (G60, G90) is used widely in ASTM standards, indicating galvanized (pure zinc) by the letter “G.” The subsequent number represents the total coating weight across both sides of the sheet in hundredths of an ounce per square foot. So, a G90 designation means the steel carries 0.90 oz/ft² of zinc.
Z100 and Z275
The Metric “Z” Series is the international metric equivalent. “Z” denotes zinc, and the number dictates the total mass in grams per square meter (g/m²). For context, Z275 is the direct metric counterpart to G90.
AZ70 and AZ150
In the “AZ” series, “AZ” stands for an aluminum-zinc alloy (commonly known by the trade name Galvalume). The 150 represents the total mass in g/m².
Because aluminum is significantly less dense than zinc, an AZ150 coating is physically thicker than a Z150 pure zinc coating. However, even though AZ coatings appear thinner, they perform better due to aluminum barrier protection.
Steel Coating Weight to Thickness Conversion Chart
The table below shows coating weight and thickness relationships for common steel grades. These conversions are based on standard zinc density (7.14 g/cm³) used in the industry.
Standard Coating Designation | Grade | Coating Weight (Total Both Sides) | Equivalent (Approx.) | Thickness in µm (per side) |
G (ASTM) | G60 | 0.60 oz/ft² | 180 g/m² | 12 µm |
G90 | 0.90 oz/ft² | 275 g/m² | 19 – 20 µm | |
Z (EN / ISO) | Z100 | 100 g/m² | 0.33 oz/ft² | 7 µm |
Z180 | 180 g/m² | 0.60 oz/ft² | 12 – 13 µm | |
Z275 | 275 g/m² | 0.90 oz/ft² | 19 – 20 µm | |
AZ (Al-Zn) | AZ70 | 150 g/m² | 0.50 oz/ft² | 8 – 10 µm |
AZ150 | 150 g/m² | 0.50 oz/ft² | 16 – 20 µm |
How Steel Coating Thickness Is Measured
In industrial usage, there are a few different methods followed to obtain galvanized coating thickness measurements.
Weight-Based Laboratory Method
This is a highly accurate laboratory method used to verify standards like G90 or Z275. A coated sample is weighed, then the coating is chemically removed using a solution that dissolves only the coating layer. After weighing the sample again, the coating weight can be obtained from the difference.
Non-Destructive Magnetic Gauges
The magnetic thickness gauge is the most common instrument used for galvanized coating thickness measurement. Since steel is magnetic and the coating is not, the magnetic gauge measures changes in magnetic force to estimate coating thickness without damaging the material.
Cross-Section Analysis Using Optical Microscopy
Optical microscopy is used for detailed lab analysis, especially in failure investigations. Here, a selected sample is observed under a microscope. Optical microscopy allows direct steel coating thickness measurements with high precision.
X-Ray Fluorescence (XRF) Method
X-ray fluorescence is an advanced non-destructive method that uses X-rays to measure steel coating thickness. It detects the energy emitted from the coating material to determine thickness. This is highly effective for alloy coatings like Galvalume.
How Paint Coating Thickness Is Measured on PPGI and PPGL
Measuring the paint thickness on PPGI (pre-painted galvanized iron) and PPGL (pre-painted Galvalume) is quite challenging because pre-painted steels are multi-layered systems. You have bare steel, a metallic coating (zinc or Galvalume), a primer, and finally a topcoat.
Primer Coating Thickness Measurement
The primer is the base paint layer applied over the pretreated steel surface. In production, its thickness can be measured easily before the topcoat is applied. However, in finished sheets, it cannot be measured non-destructively. Inspectors use a Tooke Gauge, which creates a small V-shaped cut to expose the layers, allowing accurate measurement under a microscope.
Topcoat Thickness Measurement
The topcoat is the outermost layer exposed to the environment. The most common topcoats used are PE, SMP, and PVDF. Similar to primer, its thickness can be measured using a Tooke Gauge by analyzing the cross-section of the coating layers when factory data is unavailable.
Back Coat (Backer) Thickness
The back coat is a thin protective layer applied to the underside of the sheet, mainly to resist moisture and condensation. It is usually measured using a dual-technology coating thickness gauge (magnetic induction + eddy current) without damaging the coating.
Total Dry Film Thickness (DFT)
Total Dry Film Thickness of steel refers to the combined thickness of all paint layers on one side after curing (typically primer + topcoat). It represents the actual protective coating remaining after solvents have evaporated. DFT is a main quality parameter in prepainted steel.
How Buyers Can Read a Steel Specification Correctly
A typical steel specification contains multiple technical details that define the product’s performance, durability, and suitability in a specific application.
Example of a Full Prepainted Steel Specification
As an example, you can decode the prepainted steel specification represented by 0.5 mm × 1000 mm, Z275, PE 20+5 µm, as given below:
- 0.5 mm – base metal thickness
- 1000 mm – width
- Z275 – zinc coating thickness (indicated by grade/weight)
- PE – paint system (polyester)
- 20+5 µm – paint thickness
Common Mistakes Buyers Make When Reading Steel Specifications
Given below are a few of the common mistakes buyers make when reading steel specifications:
- Relying too much on paper specifications without considering the environmental conditions.
- Not distinguishing the total thickness from the coating thickness.
- Ignoring standard coating designation differences.
- Comparing zinc and aluminum-zinc coating measurements directly.
It’s important to understand and match the specifications with real-life industrial conditions before choosing a coated-steel product.
How to Choose the Right Coating Weight for Different Applications
Coating weight directly determines how well the material will resist corrosion, how long it will last in service, and whether it is suitable for the intended environment. But the ideal choice also depends on other factors like expected service life, change of environmental conditions, and the budget as well.
Interior Applications
Interior environments are usually dry and protected from rain, UV exposure, and corrosive elements. Therefore, heavy coatings like G90 or Z275 are often unnecessary. Lighter galvanized steel grades such as G30, G60, or Z80-Z100 are usually sufficient and more cost-effective.
Standard Roofing and Cladding
Roofing and cladding are exposed to rain, sunlight, temperature changes, and pollution. These applications require moderate to high coating weights and durable paint systems. You can go for steel grades such as G90, Z275, or AZ150 that offer a good balance between durability and cost.
Industrial or Coastal Environments
Industrial and coastal areas are highly corrosive due to salt spray, humidity, and chemicals. These conditions require heavy coatings such as Z350+, AZ150, and AZ200 to ensure long-term protection and performance. Choosing a prepainted steel grade is a smart decision for these environments since the paint system provides an additional barrier protection.
Long-Life Architectural Applications
For buildings like skyscrapers and stadiums, maintenance is difficult and costly. High-performance prepainted steels such as AZ150 or AZ200 combined with PVDF coatings are preferred for these applications. They provide excellent resistance to UV, corrosion, and long-term degradation.
What to Ask Your Supplier Before Purchasing a Steel Grade
Before ordering galvanized steel or prepainted steel grades, you need to confirm and clarify the following details:
- Quality standards and compliance were maintained during the hot-dip galvanizing and coil coating processes
- Testing methods used to verify steel coating thickness and quality (and whether they are done in-house or by a third-party)
- Available paint system and thickness options
- Their ability to provide an MTC (Mill Test Certificate)
- After-sales service and their support in repairs and replacements
FAQs
Is G90 thicker than G60?
Yes. G90 has 0.90 oz/ft², and G60 only has 0.60 oz/ft². That means G90 has more zinc than G60 and thereby greater zinc coating thickness and better corrosion resistance.
Is Z275 the same as G90?
Z275 and G90 are industrially equivalent in coating weight and corrosion protection.
How is AZ150 measured?
AZ150 is measured based on coating weight, which represents the total aluminum-zinc coating on both sides of the steel (150 g/m²). Thickness is typically estimated from this value or measured using magnetic gauges or XRF instruments.
Does a thicker coating always mean better steel?
Not always. In general, thicker coatings provide better corrosion resistance than thinner coatings of the same series. But you cannot compare different coatings like zinc and AluZinc only by the thickness. Because of the dual protection of the AluZinc coating, certain thinner Galvalume grades perform better than thicker galvanized grades.
How can I verify coating thickness before shipment?
You can verify steel coating thickness by requesting mill test certificates (MTC) or third-party inspection reports.
Conclusion
Steel coating thickness is a main durability factor of galvanized steel and galvalume. Therefore, buyers should understand units, standards, and testing methods clearly to make a knowledgeable decision when choosing the coating thickness. Picking the correct specification improves service life, provides better corrosion resistance, and reduces repair needs.
Need help selecting the right zinc, Al-Zn, or paint coating for your project? Contact us for technical support and steel coil specifications.
