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Reasons for missing plating in hot-dip galvanizing of seamless steel pipes

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1. The surface of the plated parts is not clean
⑴The surface of the plated parts contains paint, grease, and welding iron; it can be polished or scrubbed with relevant solvents.
⑵Iron oxide; lack of pickling; continue pickling.

2. Pickling and operation:
⑴ Under or over pickling. Under-pickling leaves iron oxides (rust spots) on the surface of the plated parts; over-pickling causes the surface structure of the steel matrix to be destroyed, and the Si and a small amount of active metal refractory oxides contained in it are attached to the surface of the plated parts. Prevents the combination of iron and zinc. Or due to over-pickling, the plated parts absorb hydrogen. During galvanizing, the hydrogen gas expands due to heat and escapes, resulting in missed plating. Solution: Strictly control pickling and grasp the pickling time.
⑵ During pickling, the plated parts are superimposed and accumulated, resulting in missed washing, or due to structural reasons of the plated parts, carelessness in turning during pickling causes eddies and missed washing. Solution: Open up the pickling as much as possible and turn frequently.

3. Assist plating link
⑴The additive concentration is low and the plating effect is not ideal; Solution: Adjust the appropriate additive concentration.
⑵ Improper proportion of additives and high zinc salt content make the additive salt film easy to absorb water and deliquesce, and the hydroxide decomposes into gas and explodes zinc, resulting in leakage plating. Solution: Adjust the additive ratio.

4. Zinc liquid composition and pot operation:
⑴The aluminum content in the zinc solution is too high, causing missed plating; solution: add zinc or use ammonium chloride consumption to reduce the aluminum content;
⑵ If the plating parts are put into the pot too slowly, the additives will be decomposed by heat and fail, causing leakage of plating. Adjust the pot loading speed appropriately.
⑶ The zinc immersion time is too short so the iron-zinc reaction does not have time to proceed.
⑷The zinc ash on the zinc liquid surface was not cleaned when the plated parts were put into the pot, causing the overheated zinc ash to burn the additives and cause leakage of plating.
⑸ When the plated parts are put into the pot, due to process holes or structural reasons, when the high-temperature gas in the plated parts is heated and escapes, the additives will be burned, causing missing plating. Solution: Sprinkle some ammonium chloride appropriately. ⑹

5. Material of plated parts
⑴The surface of steel contains refractory oxides of elements such as silicon and aluminum, which prevents the iron-zinc reaction.
⑵ The steel matrix contains too much carbon, or too much iron carbide is produced during the refining process of the steel, making it difficult to combine iron and zinc, resulting in missed plating.
⑶ Defects in the processing and molding of the plated parts, causing the material to crack and acid, moisture, and additives to enter, causing missing plating. At present, there is no good solution except using ammonium chloride.




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