Causes and solutions of missing plating caused by hot-dip galvanizing of seamless steel pipes:
1. The surface of the plated parts is not clean
⑴The surface of the plated parts contains paint, grease, and solder; it can be polished or scrubbed with related solvents.
⑵ iron oxide; under pickling; continue to pickling.
2. Pickling and operation:
⑴Under, over pickling. Under pickling, iron oxides (rust spots) are left on the surface of the plated parts; over pickling, the surface structure of the steel substrate is destroyed, and the Si contained in it and a small amount of active metal refractory oxides are attached to the surface of the plated parts. Prevents the binding of iron and zinc. Or due to pickling, the plated parts absorb hydrogen. When galvanizing, hydrogen gas expands and escapes due to thermal expansion, resulting in missing plating. Solution: Strictly control pickling and grasp the pickling time.
⑵When pickling, the plated parts are superimposed and backlogged, resulting in missed washing, or due to the structural reasons of the plated parts, they are neglected to turn during pickling and cause vortex air, resulting in missed washing. Solution: Spread the pickling as far as possible and turn it frequently.
3. Assisted plating
⑴ The concentration of additives is low, and the plating effect is not ideal; solution: adjust the appropriate concentration of additives.
(2) Improper proportion of additives and high zinc salt content make the additive salt film easy to absorb water and deliquescence, and the hydroxide decomposes into gas and explodes zinc, resulting in missed plating. Solution: adjust the ratio of additives.
4. Zinc liquid composition and pot operation:
(1) The aluminum content in the zinc solution is too high, resulting in missing plating; solution: add zinc or use ammonium chloride consumption to reduce the aluminum content;
⑵The speed of putting the plated parts into the pot is too slow, and the additives will be decomposed and invalidated by heat, resulting in missing plating. Adjust the cooking speed appropriately.
⑶ Zinc dipping time is too short so the iron-zinc reaction can not be carried out in the future.
(4) The zinc ash on the surface of the zinc liquid is not cleaned when the plated parts are put into the pot so the overheated zinc ash burns the additive and misses plating.
⑸ When the plated parts are put into the pot, due to the process hole or structure, when the high-temperature gas in the plated parts is heated and escapes, the auxiliary agent will be burned to cause missing plating. Solution: Sprinkle some ammonium chloride in an appropriate amount.
5. Plating material:
(1) The surface of the steel contains refractory oxides of silicon, aluminum, and other elements, which prevents the reaction of iron and zinc.
(2) The steel matrix contains too much carbon, or the steel produces too much iron carbide during the refining process, which makes it difficult to combine iron and zinc, resulting in missing plating.
⑶ Defects in the processing and molding of the plating material, resulting in cracking of the material, and the entry of acid, water, and additives, resulting in missing plating. At present, there is no good solution except by ammonium chloride.