I. Main Causes of White Discoloration
1. Normal Protective Oxidation (Uniform and Slight White Discoloration)
When hot-dip galvanized pipes are stored in a dry and ventilated environment, the zinc layer on the surface will slowly react with oxygen and carbon dioxide in the air to form a very thin and dense alkaline carbonate zinc protective film, presenting a uniform matte white appearance.
This is a normal passivation process of the zinc layer and is not a quality issue; instead, it can further enhance the corrosion resistance of the zinc layer.
2. White Rust (Early Corrosion of the Zinc Layer, the Most Common Abnormal White Discoloration)
This is the main reason for the white discoloration of galvanized pipes. It is commonly known as "long white rust" in the industry and belongs to the corrosion loss of the zinc layer.
Formation Principle: When galvanized pipes are left in a humid, sealed, and poorly ventilated environment (such as being exposed to rain and water, sealed packaging in water, tightly stacked without ventilation, or during transportation with condensation and water accumulation), a layer of water film will form on the surface; zinc undergoes electrochemical corrosion in the water film, generating white powdery products such as zinc hydroxide and alkaline carbonate zinc, which is the white rust.
Common Inducing Factors:
Improper Storage and Transportation: In the construction site / warehouse, being exposed to rain and moisture, with pipes tightly stacked without ventilation gaps, are the most common causes on-site;
Galvanizing Process Defects: Insufficient or uneven surface passivation film treatment, or loss of the basic protective effect;
Surface Mechanical Damage: The zinc layer is scratched, and the damaged area of the passivation film is prone to corrosion, resulting in local white discoloration.
3. Other Rare Causes
Surface Mineral Residues: After contact with hard water or saline-alkali environment, water evaporates and leaves white water scale / salt frost. It can be completely removed by wiping.
Abnormal Zinc Layer Crystallization: Abnormal zinc liquid composition and cooling speed during galvanizing cause the zinc layer to become white due to crystallization, which is an appearance defect and does not affect the corrosion resistance.
II. Impact on Normal Use
The degree of impact on normal use mainly depends on the severity of the white discoloration and whether the underlying zinc layer and the steel pipe substrate are intact. There are three types of situations:
1. Slight Uniform White Discoloration / Mild White Rust: Does not affect normal use
Judgment Criteria: The white discoloration layer is thin and uniform. After wiping with a dry cloth, the underlying zinc layer is intact, has metallic luster, no scratches or pits, and no reddish-brown rust spots.
Explanation: Only affects the appearance. The sacrificial anode protection function of the zinc layer is basically intact, and the steel pipe substrate will not be corroded, and can be used normally for indoor wiring, structural supports, and ordinary water transmission scenarios, with negligible impact on the overall corrosion life.
Supplement: After cleaning the mild white rust, in a dry and ventilated environment, a protective film will be re-formed, and the corrosion rate will significantly slow down.
2. Moderate White Rust: Corrosion Life Decreases, Need to Evaluate According to the Scenario
Judgment Criteria: The white rust layer is thick and powdery, and after wiping, the zinc layer shows slight scratches, but the underlying steel (without reddish-brown rust spots) is not exposed.
Explanation: The zinc layer has already undergone substantial corrosion loss, and the overall corrosion resistance decreases, resulting in a certain reduction in service life.
Dry indoor, non-pressure scenarios (such as indoor wire protection pipes): Can still be used normally. It is recommended to wipe clean and apply a layer of cold galvanized paint for repair;
Outdoor, humid, and pressure water transmission scenarios: Not recommended for long-term use. The corrosion rate will accelerate, and there is a risk of premature perforation and leakage.
3. Severe White Discoloration Accompanied by Red Rust: Affects Use, Requires Treatment or Replacement
Explanation：At this point, the zinc layer has partially failed, losing its protection effect on the steel pipe substrate, and the underlying steel will quickly corrode, leading to a decrease in pipe wall strength and a risk of perforation and leakage in pressure pipes, and a potential bearing capacity deficiency in structural components.
Treatment Suggestions： Large-scale rusting suggests direct replacement; for local mild red rust, it can be remedied by sanding and spraying zinc-rich paint, but the corrosion resistance is far lower than the original factory galvanizing layer.
Supplement： Simple Prevention and Treatment When storing, place the material on a stand, keep it ventilated, avoid getting wet, and seal the package with a breathable hole. This can significantly reduce the occurrence of white rust;
For mild white rust, simply wipe it off with a dry cloth or fine steel wool. For moderate white rust, after wiping, apply cold galvanized paint to restore most of the anti-corrosion ability.
