Description
IS 4736 Hot-Dip Zinc Coatings on Mild Steel
IS 4736 is the Indian Standard specification that outlines the requirements for hot-dip zinc coatings on mild steel tubes. Introduced by the Bureau of Indian Standards (BIS), this standard ensures that mild steel tubes coated with zinc provide long-term corrosion protection, especially in industrial, agricultural, structural, and engineering applications.
Zinc coating (commonly referred to as galvanization) is an economical and effective method for safeguarding steel against rust and degradation. By immersing steel tubes in molten zinc, a metallurgical bond is formed, resulting in a robust coating that can withstand harsh environmental conditions.
Key Objectives
- To provide a uniform and protective hot-dip galvanized coating on mild steel tubes.
- To ensure that steel products meet minimum standards for corrosion resistance.
- To facilitate quality assurance and product reliability for both domestic and international markets.
- To standardize testing methods for evaluating coating thickness, uniformity, and adhesion.
Technical Specifications of IS 4736
1. Base Material: Mild Steel
The base tubes must conform to IS 1239 (Part 1) or any other relevant standards for mild steel tubes, depending on the application.
2. Galvanizing Method
Hot-dip galvanizing involves immersing the cleaned steel tube in molten zinc (usually at temperatures around 450°C). The coating provides superior corrosion protection by forming a metallurgical link with the steel surface.
3. Coating Thickness
IS 4736 specifies minimum coating mass or thickness depending on the diameter of the tube. Typically:
| Nominal Bore (NB) | Minimum Coating Mass (g/m²) | Approx. Thickness (µm) |
|---|---|---|
| Up to 40 mm | 300 | ~43 |
| Above 40 mm | 400 | ~57 |
These values may vary depending on the service environment and specific customer requirements.
4. Surface Finish
The zinc coating should be continuous, uniform, and free from defects such as flaking, bare spots, or blisters. A bright or matte grey finish is acceptable, depending on the cooling rate and steel composition.
Testing and Inspection
1. Visual Inspection
To find excessive zinc lumps, bare patches, and surface imperfections.
2. Coating Thickness Test
Measured using a magnetic thickness gauge or through weighing methods (mass of coating per unit area).
3. Adhesion Test
A manual or mechanical test where the zinc coating is scratched or bent to check for peeling or flaking.
4. Uniformity Test
Involves using copper sulfate solution (CuSO₄ test) to assess the uniformity and completeness of the coating. If the steel surface is exposed after a number of dips, the coating is considered non-uniform.
Applications
- Infrastructure & Construction
- Water Supply & Sanitation
- Agriculture
- Transport & Railways
- Rural Development
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