Missed coating refers to localized areas on the steel surface that are not covered by zinc, exposing the bare metal underneath. This is usually caused by paint, marker ink, weld slag, or heavy oil residue remaining on the steel before it enters the plant, which prevents the pickling process from cleaning it thoroughly.
1. Small-area repair: The defective area can be ground smooth and then reinforced with a zinc-rich coating (cold galvanizing compound).
2. Large-area repair: The coating must be stripped and the steel re-pickled and re-galvanized.
When a steel component is withdrawn from the high-temperature zinc bath, uneven cooling or dripping of the molten zinc can form zinc drips, nodules, or burrs. For grating and other areas that people may touch, our plant performs surface smoothing and grinding before shipment to prevent injury during assembly or use.
No, it is not damaged. White rust is zinc oxide/zinc hydroxide formed when the galvanized coating reacts with moisture in a humid, waterlogged, and poorly ventilated environment.
1. Mild white rust: Simply brush it off with a stiff brush and keep the area ventilated and dry — it does not affect the structural rust protection.
2. Prevention: Store materials off the ground, away from rain, and leave ventilation gaps between components to avoid tight stacking.
The temperature of the hot-dip galvanizing bath is around 450°C. When a steel component is immersed, it releases internal stresses left over from prior welding or processing. If the structural design is asymmetrical, the plate is too thin, or welding heat is uneven, thermal stress deformation can occur. This can usually be corrected mechanically after galvanizing to restore flatness.
This is for safety and quality reasons:
1. Preventing explosions: If an enclosed tube has no holes, the internal air will expand under heat and be unable to escape, causing an explosion directly in the zinc bath.
2. Smooth zinc flow: This allows the high-temperature molten zinc to flow in and provide internal rust protection, and to drain out quickly when the component is lifted, preventing excess zinc from accumulating inside and adding unnecessary weight.
After hot-dip galvanizing, the component surface gains approximately 80-150 um (micrometers) of zinc coating thickness. Drain covers and grating: The outer frame gap is recommended to be 3~5 mm smaller than the intended drain edge, to avoid the piece becoming too tight to fit after galvanizing. Bolt holes: The hole diameter should be sized 1.5-2mm larger than the bolt, or re-tapped after galvanizing.
This is mainly due to the silicon (Si) content in the steel. When the silicon content falls within a certain range, it accelerates the chemical reaction between zinc and iron, producing a thicker zinc-iron alloy layer that shows through the surface. This is a normal physical phenomenon (known as the Sandelin effect) and does not affect rust prevention or durability at all — only the appearance shows color variation.