1.What are some common causes of substrate defects?
Foreign contaminant intrusion: During transportation or storage, cold-rolled steel coils may accumulate iron filings, dust, oil, and other particulate matter on their surface. If these contaminants are not thoroughly removed before galvanizing, they will hinder the proper bonding between the molten zinc and the steel substrate during the galvanizing process, resulting in pitting after detachment.
Pre-existing defects in the substrate:
Rust: If the steel coil is stored for an extended period or in a humid environment, slight rust (surface rust) may occur on the surface. These rust spots are difficult to completely remove in the cleaning section, and will form pitting or black spots after galvanizing.
Rolling defects: Pits and indented iron oxide scale generated during cold rolling will be directly inherited by the galvanized surface.
2.What are the reasons for pretreatment before galvanizing?
The cleaning section's task is to provide an absolutely clean and activated steel surface for galvanizing.
Insufficient alkaline cleaning: Insufficient degreasing solution concentration or temperature, or inadequate brush roller pressure, results in the ineffective removal of grease and iron powder from the steel strip surface. These residues carbonize in the annealing furnace, forming stubborn stains that affect galvanizing.
Poor electrolytic cleaning: Scale buildup on the electrode plates, improper current density, etc., lead to a decrease in electrolytic degreasing power.
Incomplete rinsing: Alkaline or acidic solutions (if pickling is performed) remaining on the steel strip surface can cause boiling over or oxide formation upon entering the zinc bath, resulting in pitting.
3.What impact will the composition of zinc solution have?
Excessive Aluminum Content: In Galvalume (55% aluminum-zinc) or high-aluminum coatings, aluminum readily reacts with oxygen in the furnace gas to form suspended aluminum oxide (Al₂O₃) particles. These tiny, hard particles adhere to the steel strip surface, forming fine pitting.
Zinc Dross: When the zinc bath is saturated with iron, it reacts with zinc to form iron-zinc intermetallic compounds (such as FeZn₇), i.e., zinc dross. Zinc dross particles are suspended in the zinc bath or settle at the bottom, and are carried out by the moving steel strip, pressing into the coating and forming pitting.
Bottom Dross: Coarse particles settled at the bottom of the zinc pot.
Surface Dross: Oxides and slag particles floating on the surface of the zinc bath.
Suspended Dross: Tiny particles evenly distributed in the zinc bath due to the addition of aluminum or temperature fluctuations, posing the greatest risk.
4.What impact will process parameter issues have?
Excessive galvanizing temperature: Too high a temperature intensifies the reaction between iron and zinc, causing more iron to dissolve into the molten zinc and generating more zinc dross. It also accelerates aluminum oxidation.
Air knife issues: Improper air knife angle or distance, or nozzle blockage, leads to uneven zinc blowing, potentially blowing oxides from the zinc surface onto the coating.
Unstable production line speed: Speed fluctuations affect the stability of the steel strip in the zinc pot and the effectiveness of the air knife's blowing action.
5.What impact will equipment contamination have?
Furnace roll nodules: After prolonged operation in the annealing furnace, the oxide layer on the surface of the furnace rolls sinters with iron powder and other impurities on the steel strip, forming hard nodules. These nodules periodically press against the steel strip surface, causing indentations or damage, which manifests as periodic pitting or bumps after galvanizing.
Stabilizing roll/submerged roll contamination: Zinc dross accumulates on the surface of the stabilizing rolls and submerged rolls in the zinc pot. If these dross particles detach, they will directly adhere to the passing steel strip.