Steel hot dip galvanization process

The nature of the hot dip galvanization requires special design consideration of the steel fabrications to effectively coat the materials.

1. Introduction

It is important to consider the corrosion protection of steel articles when they are being designed. The key factors to consider when designing for steel structure hot dip galvanizing are the design’s impact on safety during the process, coating quality and aesthetics.

When designing a structure which is to be hot dip galvanized, it should be considered that articles are immersed into and withdrawn from a bath with molten zinc heated to a temperature of up to 450ºC.

2. Design Detailing
  • Modular design techniques often produce economics in manufacture and assembly through simplified handling and transport.
  • Welding should be continuous and free from excessive pin-holing and porosity. Stick welding weld slag is not readily removed by acid cleaning and such slag must be removed prior to galvanizing.
  • Temperatures associated with flame cutting alter the surface properties of steel and if such surfaces are not thoroughly ground, a thinner galvanized coating will be formed.
3. Dimensions
  • The size of the structures should comply with the dimensions of the galvanization baths.
  • Articles which are larger than the bath dimensions can sometimes be galvanized by “double-dipping”. If made appropriately, the corrosion protection performed through double dipping is no different to that provided in a single dip
4. Distortion
  • Minimize distortion and warping by using symmetrical sections and sections with similar thickness for welded structures.
  • Use of stiffened sections, particularly when steel is unsupported and of less than 3 – 4mm thick
  • Use of balanced or sequence welding techniques to minimise stresses.
  • Large open fabrications, thin walled trough sections and rectangular tanks require temporary cross stays to prevent distortion during hot dip galvanizing.
5. Drainage and venting
  • Large open fabrications, thin walled trough sections and rectangular tanks may require temporary cross stays to prevent distortion during hot dip galvanizing
  • External stiffeners, welded gussets and webs on columns and beams and gussets in channel sections should have cropped corners. The gaps created should be as large as possible without compromising structural strength.
  • Closed sections must never be incorporated in a fabrication. Sections should be interconnected using open mitred joints.
  • Vent holes are required at each sealed end either in the end plate or the section.
  • For overlapping surfaces which sealed by continuous welding, holes should be drilled.
6. Conclusion

Early consultation between galvanizer, fabricator and designer is the key to obtaining the best results from the galvanizing process. Considerate steel detailing, which aid the access and drainage of molten zinc, will improve the quality and appearance of the coating.

For complete protection, molten zinc must be able to flow freely to all surfaces of a fabrication. With hollow sections or where there are internal compartments, the galvanizing of internal surfaces eliminates any danger of hidden corrosion during service.

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