Welding Phosphated Steel: Techniques and Precautions
Phosphating is a surface treatment applied to certain metals, including steel. This process involves washing the metal in a warm solution of phosphoric acid and trisodium phosphate, converting a thin surface layer into highly insoluble metal phosphates. Phosphating improves the adhesion of paints and removes grease, oil, and dirt, thanks to the strongly charged phosphate ions.
Despite its thin nature, phosphating does not significantly affect welding. However, in some cases, zinc coatings are applied over the phosphated layer. Underlying zinc-based coatings can interfere with welding, necessitating the removal of these coatings before welding.
Preparation for Welding Phosphated Steel
The preparation of the welding area is crucial for ensuring a successful weld. Regardless of the specific surface treatment, it is wise to sand or surface grind the weld area before welding. This step helps remove contaminants such as grease, oil, rust, or oxide scale.
Methods for Welding Phosphated Steel
Substrate Zinc Coating Removal: For ideal results, it is recommended to remove the zinc coating from the area where welding will take place. This method is the most straightforward and effective. However, if removing the zinc coating is not feasible, you can use a filler material on the zinc-coated portion of your welding area.
Arc Welding Galvanized Steel: When welding galvanized steel, the heat from the welding arc can vaporize the zinc coating in the weld area. This is because zinc has a lower boiling point (1600°F or 871°C) compared to the melting point of steel (2800°F or 1538°C). While the zinc is vaporized, there is minimal retention in the steel, making it less likely to cause contamination in the weld.
Protecting Welded Areas
After welding, a primer such as a zinc phosphate or zinc chromate can be applied to protect the welds from rust. These primers help create a barrier against moisture and other corrosive elements, ensuring the longevity and durability of the weld.
Conclusion
Welding phosphated steel requires careful preparation and, in some cases, the removal of underlying zinc coatings. By following the appropriate techniques and using the right materials, you can achieve successful and durable welds.