Why do galvanized street light poles still rust?

Galvanizing is widely used in outdoor facilities such as street light poles. However, in real life, we can still see rust on the surface of galvanized street light poles. The reason behind this is not the failure of the galvanizing process, but complex factors involving process, environment, construction, design, and so on.

 

I. Quality defects of the galvanizing process itself

1. Insufficient coating thickness: This is one of the most common reasons. National standards (such as GB/T 13912) have clear requirements for the thickness of the galvanized layer in different use environments (usually above 70-85μm). If the manufacturer cuts corners to reduce costs, or the process control is not strict (such as insufficient zinc liquid temperature and zinc immersion time), resulting in a too-thin coating, its physical barrier effect and the protection life of the sacrificial anode will be greatly shortened. Thin zinc layers are more easily damaged by machinery or consumed quickly.

2. Uneven coating/leaky coating: During the galvanizing process, if the surface treatment of the workpiece is not clean (there are oil stains, rust, oxide scale), the pickling is not sufficient, the coating agent is unevenly covered, or the workpiece structure is complex (such as welds, sharp corners, inner cavities) resulting in poor flow of zinc liquid, it is possible that the zinc layer in some areas is not fully covered or even not coated with zinc at all (leaky coating). These "bare spots" are the starting point for steel to be directly exposed to the corrosive environment.

3. Poor bonding strength of the coating: If the pre-treatment is improper or the process parameters are not good (such as the temperature is too high, causing the zinc-iron alloy layer to be too thick and brittle), the bonding strength of the galvanized layer and the steel substrate will weaken. This coating is prone to peeling and flaking due to collision, vibration, and hot and cold cycles during transportation, installation, or use, and loses its protective effect.

4. Improper process selection: The coating of electro galvanizing (cold galvanizing) is usually much thinner than hot-dip galvanizing (generally 5-25μm), and the structure is different (mainly pure zinc layer, lacking a zinc-iron alloy layer), and its durability is far inferior to hot-dip galvanizing. If electro galvanizing or inferior hot-dip galvanizing process is mistakenly selected, the anti-rust ability will naturally be greatly reduced.

 

II. Accelerated erosion by harsh environmental factors

1. Highly corrosive environment:

Coastal areas: The air is rich in high concentrations of chloride ions (Cl⁻), which have strong penetration and can destroy the passivation film of zinc, significantly accelerate the corrosion of zinc (white rust), and easily penetrate the steel matrix to cause pitting and rust.

Industrial pollution area: The air contains high concentrations of acidic gases such as sulfur dioxide (SO₂) and nitrogen oxides (NOx), which dissolve in water to form acid rain or acid mist, which continuously corrodes the zinc layer.

Winter in the north: Deicing salt (mainly sodium chloride and calcium chloride) is used in large quantities, and the snow-melting salt water splashes or penetrates the soil at the bottom of the lamp pole, bringing extremely high concentrations of chloride ion corrosion.

2. Long-term moisture/water accumulation:

Bottom of the lamp pole/flange area: This is the most prone to water accumulation. Rainwater, snowmelt water, sprinkler water, soil capillary water, etc. accumulate here and remain moist for a long time. Continuous moisture provides the necessary conditions for electrochemical corrosion (electrolyte solution).

Internal cavity: Some lamp poles are designed with internal cavities or poorly sealed inspection ports, which can result in rainwater infiltration. Internal condensed water cannot be discharged, forming a continuous humid environment.

3. Physical and chemical damage:

Mechanical damage: collision and scratches during transportation, lifting, and installation; hard injuries caused by later maintenance (such as climbing work and vehicle scratches); even long-term erosion by strong winds carrying gravel will destroy the local zinc layer and expose the steel.

Welding damage: On-site welding during installation (such as fixing flanges and mounting brackets) will generate high temperatures, and the galvanized layer near the weld and in the heat-affected zone will be burned and oxidized. If effective secondary anti-corrosion treatment (such as spraying zinc-rich paint, cold galvanizing, etc.) is not carried out as required after welding, the area will rust quickly. The weld itself is often a stress concentration and composition segregation area, which is more susceptible to corrosion.

Galvanic corrosion (bimetallic corrosion): When galvanized steel is in direct contact with a metal with a more positive potential (more inert) (such as stainless steel bolts, and copper wire joints) in the electrolyte, a galvanic pair is formed. The zinc (or exposed steel) as the anode will accelerate corrosion. This is common at connectors.

 

III. Negligence in design, installation and maintenance

1. Design defects:

Water accumulation design: The flange design at the bottom of the lamp pole is unreasonable, lacking drainage holes or insufficient holes, resulting in long-term accumulation of rainwater that cannot be discharged smoothly.

Enclosed cavity: The internal cavity is not designed with effective drainage or ventilation measures, and moisture cannot dissipate.

Direct contact between different metals: Insulation measures at the joints of other metals (such as the use of insulating gaskets) are not considered.

2. Improper installation:

Untreated damage: The damage to the galvanized layer caused during the installation process was not promptly and professionally repaired.

Lack of post-welding treatment: No anti-corrosion repair was performed on the burned area after on-site welding.

Rough bottom treatment: The buried part of the lamp pole was improperly backfilled, or additional anti-corrosion wrapping (such as asphalt fiberglass cloth) was not performed according to the specifications, or the concrete base failed to effectively isolate soil moisture and corrosives.

3. Lack of maintenance:

Lack of regular inspections: A regular inspection system has not been established, and early rust spots cannot be discovered in time.

Failure to repair in time: After discovering small rust spots and zinc layer damage, surface treatment (rust removal) and repair (such as applying zinc-rich primer and topcoat) are not carried out in time, allowing them to develop and expand.

Improper cleaning: Use strong acid and alkali cleaners or hard objects such as wire brushes to clean the lamp pole, destroying the zinc layer or passivation film.

 

Summary and enlightenment

The rust of galvanized street light poles is not the failure of the galvanizing process itself, but the result of the interweaving of multiple factors. The poor quality galvanized layer (thin, uneven, poor bonding) is the inherent fragility; the harsh environment (chloride ions, acid, salt, continuous humidity) is the external driving force; and the negligence in the design, installation, and maintenance links artificially creates weaknesses or accelerates the corrosion process. In particular, water accumulation at the bottom of the lamp pole, damage to the heat-affected zone of welding, and coastal/industrial/de-melting salt environments are the "hardest hit areas" that cause rust.

 

To maximize the service life of galvanized street light poles, full-process and systematic prevention and control is required:

1. Source control: Select high-quality hot-dip galvanized products that meet national standards (or higher requirements) to ensure sufficient and uniform coating thickness and good bonding strength.

2. Optimize design: Avoid water accumulation design to ensure smooth drainage; consider ventilation and drainage for internal cavities; take insulation measures at different metal joints.

3. Standardized construction: Handle with care to reduce transportation damage; professional anti-corrosion repair must be carried out on the heat-affected zone after welding; additional anti-corrosion should be done for the buried part; protect the coating during installation and repair damage in time.

4. Scientific maintenance: Establish a regular inspection system; scientifically treat the rust and damage points found in time (after thorough rust removal, apply zinc-rich primer + matching topcoat); avoid using corrosive or hard tools when cleaning.

5. Environmental adaptation: In extremely corrosive environments (such as the seaside, heavy industrial areas, and roads that use a large amount of snow-melting agents), a higher-specification anti-corrosion system should be considered, such as the "double insurance" of hot-dip galvanizing + high-performance coating (such as polyester, PVDF coating), or cathodic protection measures should be considered in key areas (such as the bottom).