1. Degreasing: Chemical degreasing or water-based metal degreasing detergent can be used to degrease the galvanized square pipe until it is completely soaked by water.
2、酸洗：镀锌方管可采用H2SO4 15%，硫脲0.1%，40～60℃或用HCl 20%，六次甲基四胺1～3g/L，20～40℃进行酸洗;加入缓蚀剂可防止基体过腐蚀及减少铁基体吸氢量。脱脂及酸洗处理不好会造成镀层附着力不好，镀不上锌或锌层脱落。
2. pickling: galvanized square tubes can be pickled by H2SO4 15%, thiourea 0.1%, 40-60 C or HCl 20%, hexamethylenetetramine 1-3 g/L, 20-40 C. Adding corrosion inhibitor can prevent matrix overcorrosion and reduce hydrogen absorption of iron matrix. Poor degreasing and pickling treatment will result in poor adhesion of the coating, and no zinc or zinc coating will fall off.
What steps should galvanized square tubes go through in galvanizing
3、浸助镀剂：也称结合剂，可保持在浸镀前镀锌方管具有一定活性，以增强镀层与基体结合。NH4Cl 15%～25%，ZnCl2 2.5%～3.5%，55～65℃，5～10min。为减少NH4Cl挥发可适当加入甘油。
3. Dipping aids: also known as binders, can keep galvanized square tubes active before immersion, in order to enhance the coating and matrix bonding. NH4Cl 15%-25%, ZnCl 2.5%-3.5%, 55-65 C, 5-10 min. Glycerol can be properly added to reduce volatilization of NH4Cl.
4. Heating intervention heat: In order to prevent the galvanized square pipe from deforming due to the sharp rise of temperature during dipping, and to remove residual moisture, to prevent zinc explosion, resulting in zinc liquid splash, the preheating temperature is generally 120-180 degrees Celsius.
5. Hot-dip galvanizing: It is necessary to control the temperature of galvanizing solution, dipping time and the speed at which galvanized square tubes are removed from the galvanizing solution.
6. Finishing: After galvanizing, the finishing of galvanized square tubes is mainly to remove residual zinc and zinc tumors on the surface. It can be done by vibration or manual method.
7. Passivation: The purpose is to improve the atmospheric corrosion resistance of galvanized square pipe surface, reduce or prolong the appearance time of white rust, and maintain a good appearance of the coating. Chromate passivation is used, such as Na2Cr2O7 80-100g/L and sulfuric acid 3-4ml/L.
8. Cooling: Cold water is generally used, but the temperature should not be too low to prevent galvanized square pipes, especially castings from cracking due to quenching and retraction.
9. Inspection: The coating appearance is fine, bright, no flow hanging, wrinkle phenomenon; Galvanized square pipe thickness inspection can be carried out by coating thickness gauge, the method is relatively simple. The thickness of the coating can also be obtained by calculating the zinc adhesion. The bonding strength can be bended by a bending press, and the specimen should be bent at 90-180 degrees. There should be no cracks and no coating peeling off. It can also be tested by hammering.