If the "solvent (or flux) method" is employed for pretreatment before galvanizing, the steel pipe surface may exhibit various issues such as oxide scale left from insufficient acid pickling, latent hydrogen ions caused by over-pickling, incomplete removal of oil contamination and other adhesives, adhesion of iron salt residues, incomplete removal of carbon black and corrosion inhibitor residues, insufficient activation due to too low solvent (or flux) concentration, too high iron content in the solvent (or flux), contamination of the solvent (or flux), solidification or deactivation of the solvent (or flux), burnout or partial erasure of the solvent (or flux), and failure to dry the solvent completely. These factors can lead to missed galvanizing areas on the steel pipe surface or the formation of false galvanized coatings that consist only of pure zinc without an underlying iron-zinc alloy layer, resulting in poor adhesion.
If the "protective gas reduction method" is used for pretreatment, the steel pipe surface should undergo sufficient reduction and activation to become pure iron before reacting with zinc to form an iron-zinc alloy layer. If factors such as the composition of the protective gas, dew point, furnace temperature, and sealing are not well controlled, and if the steel pipe surface becomes oxidized or insufficiently reduced, it can also lead to missed galvanizing areas or the formation of false galvanized coatings that consist only of pure zinc without an iron-zinc alloy layer, which can cause cracks or peeling during bending or cold working. Therefore, similar to the "solvent (or flux) method," poor adhesion issues can arise. During hot-dip galvanizing using the "flux method," the ammonium chloride flux on the surface of the molten zinc can easily make the galvanized coating brittle and prone to flaking.
