If "solvent (or flux) method" is adopted for pretreatment before galvanizing, various issues may arise on the surface of steel pipes, including oxide scale left behind due to inadequate acid pickling, latent hydrogen ions caused by over-pickling, incomplete removal of adherents such as oil contamination, adhesion of iron salt residues, incomplete removal of carbon black and corrosion inhibitor residues, inadequate activation due to too low solvent (or flux) concentration, excessively high iron content in the solvent (or flux), contamination of the solvent (or flux), deactivation of the solvent (or flux), burnout of the solvent (or flux), partial erasure of the solvent (or flux), and failure to dry the solvent properly. These issues can lead to missed galvanizing areas on the steel pipe surface or the formation of a false galvanized layer that consists only of a pure zinc layer without an underlying iron-zinc alloy layer. Therefore, the adhesion is poor.
If "protective gas reduction method" is used for pretreatment before galvanizing, the surface of the steel pipe should be fully reduced and activated 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 cannot be well controlled, and if the steel pipe surface becomes oxidized or insufficiently reduced, missed galvanizing areas or a false galvanized layer consisting only of a pure zinc layer without an underlying iron-zinc alloy layer may occur, leading to cracks or peeling during bending or cold working. Therefore, as with 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 layer brittle and prone to shedding.
