Among hot-dip galvanized workpieces, in addition to machined parts, all forged, welded and cast parts must undergo surface treatment or surface cleaning. Machined parts are usually simply degreased and can be directly plating or zinc dipped. For forging, welding structures, rolling, and drawing workpieces, because there is generally a thick layer of oxide scale on the surface and other undesirable conditions, they must be pickled.
Casting workpieces must be shot blasted or shot blasted first, and then pre-plated. The residual molding sand dust, iron oxide scale and oil dirt on the surface of the workpiece are removed by sandblasting, so that the "iron-based material" of the workpiece is hot-dip galvanized in a clean state to ensure the quality of the galvanized layer.
Whether it is cast steel, malleable iron, ductile iron or gray cast iron, surface cleaning must be carried out using mechanical methods such as sandblasting to remove the molding sand bonding layer and oxide layer, so that the steel matrix of the workpiece is completely exposed, and then pre-plating treatment is performed.
High-strength bolts cannot be surface treated by pickling, but are mainly sandblasted.
Sandblasting is not recommended for general metal structure workpieces, mainly because structural dead ends, pipes, box-section internal structures, etc. cannot be completely sandblasted. In particular, it should be noted that before galvanizing, all chamfers (including bolt holes) must be polished, and the chamfer should not be less than 1.00 mm, because sharp corners can easily lead to missed plating.