Pre-galvanizing Steel Surface Considerations
Surface preparation is essential in order to ensure a top-quality galvanized coating. Steel must be entirely clean for the metallurgical reaction between it and molten zinc to occur and the galvanized coating to form.
Common residues found on steel, rough surfaces, heat-treating-induced surface coatings, rust, and other surface contaminants need to be removed prior to galvanizing, as they may prevent the formation of a consistent and uniform zinc coating.
It is important to understand that:
- Rough steel surfaces resulting from fabrication methods or abrasive cleaning processes may cause thicker zinc coatings,
- Casting surfaces must often be abrasively cleaned to remove sand that will prevent zinc adhesion,
- Heat-treating steel often introduces a surface coating that is not removed by conventional chemical cleaning processes and must be blast-cleaned,
- Steel surfaces that have varying degrees of rusting require varying amounts of time in the cleaning stages of the galvanizing process, resulting in a variety of surface finishes,
- Welding slag that adheres to the steel surface must be removed prior to galvanizing,
- Welding slag that adheres directly onto the weld is easily removed by grinding; welding slag that adheres around the weld areas is more difficult to remove and usually cannot be ground away; other means of mechanical removal, such as blasting, are used to remove this type of welding slag,
- The use of cutting oil should be minimized when planning to galvanize; if cutting oil use is unavoidable, the galvanizer should be consulted to determine if normal chemical cleaning processes will remove it from the surface or if another means of removal is necessary.
When welding steel, flux and molten metal particles (weld slag) may splatter away from the weld pool and adhere to the steel. Although these particles are commonly found on welded steel, they must be removed prior to galvanizing, because the standard chemical cleaning processes used by galvanizers do not remove welding slag. Additionally, if welding slag particles are carried over to the zinc bath, they will cause coating defects and areas that may be susceptible to corrosion problems. Welding slag particles that are not removed may lead to exposed areas unprotected from corrosion.