Hot-dip galvanisation makes it possible to protect the underlying steel from corrosion. With the importance of material integrity to ensure the longevity of structures such as culverts, tunnels, and related products widely used in the mining and construction industries, it is imperative to select a supplier known for their focus on quality galvanisation.
What Is Galvanisation?
The hot-dip method entails a process whereby fabricated steel is immersed in a bath of molten zinc. The zinc bonds with the steel to create an exceptionally strong layer of protection to prevent corrosion. Although the techniques have been perfected over the years, it is a practice that dates back at least 140 years. With the process, the material requires very little maintenance over its full lifespan. As such, it can be seen as a sustainable practice to reduce the need for replacement of steel structures on a regular basis.
A Quick Explanation of the Process
The zinc is heated to an extremely high temperature to melt it. This melting makes it possible to have the zinc cover every part of the steel, leaving no weak points. The zinc-alloy forms in the process, creating the largest part of the thickness. Different grades of zinc are used for the hot-dip galvanisation process. The grades are determined by the percentage of zinc present. Always purchase galvanised steel from a local supplier, able to inform you of the percentage of zinc used in the galvanisation process.
The appearance that includes markings on the finished galvanised steel depends on the percentage of zinc used in the process. Uniform coating is essential to ensure aesthetic appeal and consistency in integrity. That said, even when purchasing from a trusted supplier, the galvanised steel may have some markings on it.
Markings and Their Meanings
We briefly look at these markings, many of which simply affect the appearance and not the quality and performance of the material. Some appearance defects are more serious than others. It is thus best to purchase from an experienced galvanisation firm that adheres to strict codes of conduct and compliance with quality standards.
- Rough Surfaces
Lighter coats are smoother than the heavier ones. Alloy layer irregularities become more apparent as the coat thickness increases. The roughness of the coat doesn’t have a significant effect on the durability of the coating, provided the zinc adhesion or bonding is correct. However, where two surfaces must be used together, the inconsistency in smoothness may affect the usefulness of the materials for a particular project. Make sure you work with a supplier, able to provide you with sheets or products with the same consistency.
- Impurity Protrusions
When the zinc and iron alloy settle at the bottom of the molten zinc bath, some surface bumps or protrusions can occur if the layer of impurity is agitated. Such bumps don’t really affect the durability of the material’s surface as the rate of corrosion stays pretty much the same. The only problem with such bumps is that it is easier to damage the surface coat when the galvanised product is subjected to mechanical type modification.
- The Coating Does Not Cover All the Areas
A small spot does not have a significant result on the durability of the coat since zinc sacrifices itself rather than the steel being compromised. As such, small spots should not be a major concern as the self-healing effect of zinc ensures the integrity of the coated steel.
However, some spots may need closer attention. These spots can be repaired, but if the galvanised product has uncoated spots that cannot be repaired, you have a reason for concern. This can be the result of inconsistencies in coating or improper preparation of the surface before galvanisation. Purchase from a supplier that has several quality control measures in place to avoid such issues.
- Flux and Ash Inclusion In the Coatings
If a zinc and ammonium chloride layer is present on top of the molten zinc, the flux moves to the side with the immersion of the steel into the hot bath. When the steel is withdrawn from the bath, the flux also moves to the side. If the flux is included, it can be as the result of stale bath flux that adheres to the coat. The supplier should reject the coating if the underlying surface coat is affected. With zinc ash, the oxide film or thin layer on the molten zinc surface can adhere to the steel if the material must be slowly withdrawn from the dip area. The zinc ash does not affect the durability of the material.
- Galvanised Material Shows Surface Lumps
Such lumps are caused by too low bath temperatures, excessively fast withdrawal or when the operator fails to drain the molten zinc back into the kettle or bath. If the material comes in direct contact with another material during the withdrawal process, it can also cause the lumpy appearance. The lumps don’t have a significant effect on the durability of the galvanisation, but to ensure quality finishes, the surfaces should be smoothed.
- Localised Matte Finishes
If there is not enough zinc on specific areas of the material, spots of matte finish are visible in the form of a localised dull appearance. This is mostly seen with steel galvanisation where the steel has heavier areas that don’t cool as fast as the others. It is not something that the galvanisation company can foresee unless the steel composition is known beforehand.
- Rust Spots on the Surface
It is not cause for concern if small rust stains are present if such are not caused through seepage at the seams after the material has been galvanised or because of storage along with corroded material. In the instance of rust as the result of poor quality galvanisation, the material is compromised. As such, it is imperative to purchase materials from a firm known for adhering to strict standards of quality in terms of galvanisation.
Where to Find Such Suppliers
Armco Superlite meets the requirements for superior quality galvanisation. The firm is listed in compliance with the BSI ISO 9001:2015 quality scheme, which is a testimony to its commitment to quality in galvanisation.
The company holds the SATAS mark for Hot Dipped Galvanised steel in accordance with SANS 121/ISO 1461 requirements. Clients are issued galvanisation certificates upon request. Furthermore, the firm carries the SATAS mark for production of W-beam guardrail systems in accordance with the specifications of SANS 1350.
Membership of the Hot Dip Galvanizers Association of South Africa, the Steel Engineering Industries Federation of South Africa, and the South African Institute of Steel Construction also proves the company’s commitment to quality and adherence to superior quality in terms of galvanisation.
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