1.Is it necessary to re-galvanize if the coating is damaged or peeled off over a large area?
The zinc layer has lost more than 30% of its surface area due to mechanical damage (such as severe impact or scratching) or corrosion (such as prolonged exposure to strong acids or alkalis). The exposed substrate (mild steel) has developed noticeable red rust (iron oxide), and the rust is spreading (such as expanding, bulging, or flaking).
For example, a steel pipe was scraped by a sharp object during transportation, leaving a long strip of exposed iron. If not promptly addressed, extensive red rust will develop on and around the exposed area one to two months later.
2.How does corrosion depth affect structural strength?
Rust can significantly reduce the wall thickness of steel pipes (e.g., a 3mm-thick steel pipe may become less than 2mm after localized corrosion), or cause localized deformation in load-bearing or pressure-bearing applications (such as water pipelines and supports), compromising safety.
Note: Mild pitting (single-point rust depth less than 0.5mm) with no tendency to spread can be repaired with touch-up paint, without the need for re-galvanizing.
3.What is the impact of the usage environment?
If the steel pipe is used in a highly corrosive environment (such as a chemical workshop or coastal dock) or has mandatory standards for appearance/corrosion protection (such as food-grade pipes or precision equipment brackets), even if the coating is partially damaged (such as 5%-10% of the area exposed iron), touch-up painting cannot meet the long-term corrosion protection requirements and re-galvanizing is required to restore the complete zinc layer protection.
4.What are the scenarios where re-galvanizing may be considered?
If the coating is partially damaged but the repair cost is close to re-galvanizing, ◦ If the steel pipe surface has multiple, scattered, small areas of exposed iron or rust (such as multiple scratches or localized white rust accumulation), conventional repair methods (such as painting or cold galvanizing) are costly in terms of labor and materials, and the corrosion protection effect after repair is uneven. In this case, re-galvanizing is more economical (especially for batches of steel pipes). 2. Pretreatment before secondary use ◦ If the zinc coating on the surface of old steel pipes has aged after long-term use (e.g., the zinc coating thickness is less than 10μm), although it is not severely corroded, if it is planned to be used in a new outdoor or humid environment (such as fences or pipes in renovation projects), re-galvanizing is recommended to restore the corrosion protection to ensure a service life of more than 5 years.
5.What are the situations where re-galvanizing is not necessary?
If only a small amount of white rust (a corrosion product of zinc, appearing as a white powder) is present on the surface, with no exposed iron or red rust, it can be repaired by wire brushing and then applying a cold-spray zinc coating with a zinc content of ≥ 95%.
If there is a small area of exposed iron (<5 cm² per spot, <10% of the total area) without red rust, it can be sanded and then coated with an epoxy primer and zinc topcoat without re-galvanizing.