There is a general misunderstanding among common people that every zinc coating is called galvanization but this is not true as there might be many processes through which a zing coating can be applied. Hot Dip Galvanizing is a coating which is formed after immersing a steelwork in molten zinc. There are however two different processes of hot-dip galvanizing which are pregalvanizing and batch galvanizing.
Batch galvanizing is a process that is used to coat fully fabricated items and includes chemical cleaning of work before immersing it in the galvanizing bath. The item gets completely coated including internal surfaces if hollow sections. The thick coating also provides the best level of available corrosion protection.
Pregalvanization is a process where you take a specific product and galvanize them through an automated process. Steel tubes can be galvanized through an automated process. The immersion time is less and after removal, steam can be blown on the bore of the tube which gives it a good surface finish. In the case of reselling, the tubes can be cut to different sizes which creates uncoated edges.
Strands of wire can be coated using an automated process. The immersion time is very short and the wiping procedure is used to remove excess zinc and give a smooth finish to the final product. It is better to use a mesh of wires so that thicker galvanized coating can be achieved. The steel sheet is uncoiled and passed through the reduced atmosphere before immersing in galvanizing bath for a brief period.
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After withdrawing the sheet from the galvanizing bath a mechanical wiper or air knife is used for removing excessive zinc so that the product has a smooth finish. The sheet after galvanizing is recoiled before the stockholder and then it is uncoiled and cut for sale which produces uncoated edges on the sheet.
Both batch and pregalvanized coating have different characteristics and some of them are being enlisted below.
- The coating continuity in batch galvanizing is continuous over whole item whereas in pregalvanizing the cut edges are left uncoated
- In batch galvanization, coating formation after galvanization is not recommended as it can damage the product whereas in pregalvanization coating can be normally formed without any damage
- Batch galvanization offers a high level of sacrificial protection whereas a pregalvanized product has less sacrificial protection because of the thin coating due to which rust formation can happen from cut edges of the sheet
- The coating appearance after batch galvanizing can be bright but variable whereas pregalvanized products the overall finish is bright without any variation
- There is high resistance to abrasion in batch galvanizing due to thick and hard layer of iron and zinc alloy whereas in pregalvanization the alloy layer is very thin which can cause abrasion easily
- The coating thickness in batch galvanizing is very thick as compared to pregalvanized coating
- The consideration of design for galvanization is required in both batch and pregalvanized products