Does color steel sheet rust?

In many fields such as construction and manufacturing, color steel sheets are widely used due to their unique advantages. They are beautiful and practical, adding a unique charm to many buildings and facilities. However, a highly concerned question always lingers in everyone's mind: Will color steel sheets rust? This question not only concerns the service life of color steel sheets, but also affects the early planning and later maintenance of many engineering projects.

1. Substrate material: the foundation of corrosion resistance

The color steel sheet substrate mainly consists of hot-dip galvanized steel plate (GI), hot-dip aluminum zinc coated steel plate (GL), and electroplated galvanized steel plate (EG). Among them, hot-dip aluminum zinc steel plates (with aluminum content of 55% and zinc content of 43.4%) have 3-6 times higher corrosion resistance than hot-dip galvanized steel plates due to the formation of a dense aluminum oxide protective layer on the surface. If cold-rolled substrate (uncoated) is selected, red rust will appear after 3-6 months in a humid environment. A certain experiment showed that in the salt spray test (NSS), hot-dip galvanized steel sheets showed no corrosion after 720 hours, while cold-rolled steel sheets only showed corrosion points after 48 hours.

2. Coating quality: the key to protective performance

The coating of color steel sheet is divided into primer and topcoat. The primer is usually made of epoxy resin (with strong adhesion), and the topcoat is mainly made of polyester, silicon modified polyester or fluorocarbon resin. Fluorocarbon coating (PVDF) has a fluorine content of ≥ 25% and a weather resistance of over 20 years, while ordinary polyester coating only lasts 5-10 years. The coating thickness must meet the standard: primer ≥ 5 μ m, topcoat ≥ 15 μ m. If there are pinholes (diameter>0.1mm) or insufficient thickness in the coating, moisture will penetrate into the substrate and cause corrosion. A certain manufacturer improved the coating process, reducing the coating pinhole rate from 0.5% to 0.02% and increasing the product life by three times.

3. Usage environment: Variable of corrosion risk

Humidity: When the relative humidity is greater than 70%, condensation water is easily formed on the surface of the color steel sheet, accelerating electrochemical corrosion. Coastal areas have a corrosion rate 2-3 times higher than inland areas due to high levels of chloride ions in the air.
Temperature: High temperature environments (>60 ℃) can accelerate coating aging, while low temperatures (<-10 ℃) may lead to coating embrittlement.

Pollutant: When the concentration of sulfur dioxide (SO ₂) in the air of industrial areas exceeds 0.01mg/m ³, it reacts with moisture to generate sulfuric acid, which corrodes substrates.

4. Maintenance method: means of extending lifespan

Regular cleaning can remove surface dirt (such as dust, bird droppings) and prevent the formation of corrosive batteries. If there are scratches on the coating (depth>20% of coating thickness), it is necessary to repair the anti rust paint (such as epoxy zinc rich paint) in a timely manner. A certain construction project has achieved a coating retention rate of 90% after 10 years of annual cleaning of color steel sheet exterior walls, while only 60% remains for an unmaintained project.
Under normal conditions (humidity of 50% -70%, no pollution), high-quality color steel sheets (hot-dip aluminum zinc substrate+fluorocarbon coating) can have a lifespan of over 25 years; If hot-dip galvanized substrate+polyester coating is used, the lifespan is about 10-15 years; Cold rolled substrates with thin coatings may rust within 3-5 years. It is recommended to choose the appropriate type of color steel sheet according to the usage scenario and regularly inspect and maintain it.