The corrosion resistance and service life of prepainted steel sheets depend crucially on whether the zinc coating and paint film thickness meet the required standards. As a purchaser, quality inspector, or production staff, have you encountered these puzzles:

How to accurately calculate the weight of the zinc coating?

How to measure the paint film thickness in a standardized way?

How to convert indicators under different standards?

Today, we will use the most easy-to-understand method to help you thoroughly understand the calculation methods of these two key indicators for prepainted steel sheets!

I. Calculation of Zinc Coating

01 Calculation Method for Zinc Coating Weight (g/m²)

Core Formula: Zinc coating weight = (Weight before zinc removal - Weight after zinc removal) / Sample area

Operation Steps:

Sampling: Cut a standard sample of 10cm×10cm (with an area of 0.01m²).

Weighing: Use a one-ten-thousandth balance to weigh the initial weight W? (e.g., 50.235g).

Zinc Removal: Dissolve the zinc coating with 1:1 hydrochloric acid (Note: Do not over-corrode the substrate).

Re-weighing: Weigh the weight after zinc removal W? (e.g., 49.835g).

Calculation: (50.235 - 49.835) / 0.01 = 40g/m².

02 Conversion Method for Zinc Coating Thickness (μm)

Thickness (μm) = Weight (g/m²) / 7.14

(Calculated based on the density of zinc being 7.14g/cm³)

Example: If the measured zinc coating weight is 60g/m², then the thickness = 60 / 7.14 = 8.4μm.

II. Calculation of Paint Film Thickness

01 Direct Measurement Method

Magnetic thickness gauges for steel substrates

Eddy current thickness gauges for aluminum substrates

Key Calculation Points:

Measure at 5 different positions.

Remove the highest and lowest values.

Take the average of the remaining 3 points.

02 Weight Difference Method

Film thickness (μm) = (Weight after painting - Weight after paint removal) / (Area × Paint density)

Note: The density of ordinary polyester paint is approximately 1.2-1.5g/cm³.

III. Comparison Table of Key Indicators

Mastering these calculation methods will enable you to easily ensure the quality of prepainted steel sheets!