How to Distinguish Omission Plating Defects in Galvanized Pipes

Omission plating defects in galvanized pipes can generally be identified from five perspectives:

1. Presence of Grease on the Base Pipe Surface
Grease on the outer wall of the base pipe increases surface tension and cannot be removed during the acid pickling stage before galvanizing. If the base pipe is rusted, the electroplating agent may fail to adhere, forming a protective saline residue. When the pipe enters the high-temperature molten zinc bath, it oxidizes rapidly, preventing any chemical or physical reaction between the zinc and the base material. Upon withdrawal from the bath, gravity causes uneven zinc flow and accumulation, leaving black oily marks in areas with thinner zinc coating.

2. Severe Corrosion on the Surface of the Alloy Base Pipe
If the outer surface of the base pipe is heavily corroded, iron oxides may remain even after acid pickling. Although the auxiliary plating agent can still adhere, the zinc liquid cannot directly contact the steel surface upon immersion, preventing any reaction. After the pipe is withdrawn from the zinc bath, the exposed areas appear relatively clean, while pitting corrosion may be observed on the surface of the galvanized pipe.

3. Thick Grease or Stubborn Stains on the Pipe Wall
Thick grease or stubborn stains on the base pipe wall inevitably lead to omission plating and complicate processing, as thorough cleaning along the entire length of the pipe is required. Stains resembling the size of mung beans may remain on the pipe wall. During baking, the zinc liquid at the pipe opening adheres tightly to the surface. Similar to rusted pipes, the resulting omission plating layers are consistent in nature, though such defects are generally smaller in scale and have smoother edges.

4. Surface Scratches and Roughness on the Base Pipe Wall
Obvious scratches on the base pipe wall allow contaminants to become embedded within them. These residues are difficult to remove during the acid pickling and alkaline washing stages prior to galvanizing. The resulting omission plating defects typically appear in long, narrow forms and are relatively rare.

5. Insufficient Temperature During the Galvanizing Process
If the temperature during galvanizing fails to meet the required standards, the galvanized pipe may be prone to leakage issues.