1.How to perform rust removal?
Mechanical rust removal: Rust and mill scale are removed through physical methods such as grinding (sandpaper, grinding wheels), sandblasting (quartz sand, steel shot), and shot blasting (high-speed rotating steel shot impacting the steel surface). This method is suitable for large steel surfaces (such as steel plates and steel sections). After rust removal, the surface roughness is moderate, which improves the adhesion of subsequent protective coatings. (Sandblasting/shot blasting achieves the best rust removal results, reaching Sa2.5, with virtually no residual rust.)
Chemical rust removal: Rust is removed through pickling (hydrochloric acid, sulfuric acid solution) or alkaline cleaning. This method is suitable for small, complex structural parts (such as bolts and pipe fittings). However, it is important to rinse thoroughly with clean water after pickling, and then perform a "passivation treatment" (such as applying a passivating agent). Otherwise, the residual acid on the steel surface may accelerate corrosion.
2.How to remove oil/dirt?
Steel may become contaminated with oil (such as cutting oil) and dust during processing and storage. Cleaning with an organic solvent (such as alcohol or gasoline) or an alkaline detergent (such as soapy water) is necessary to ensure that the surface is free of oil. This can be verified using a "water film test": pour water over the surface after cleaning. If the water film adheres evenly without breaks, the oil has been removed.
3.What are the common coating types and characteristics?
Anti-corrosion paint: low cost, simple application (brush, roller, spray), optional colors; short lifespan of a single coat (1-5 years), requiring regular recoating; average weather resistance (easy to age outdoors)
Powder coating: Solvent-free (environmentally friendly), uniform coating thickness (50-150μm), impact and scratch resistance; requires high-temperature curing (180-200°C), not suitable for large/complex structures; high initial equipment cost
Heavy-duty anti-corrosion coatings: Extremely weather-resistant (outdoor lifespan 5-20 years), resistant to salt spray, acid and alkali (such as polyurea, which can withstand seawater immersion); high cost (unit price is 3-10 times that of ordinary paint), and high construction requirements (requires professional spraying)
4.What are the key considerations in construction?
Coatings require multiple application coats: A primer (such as epoxy primer) is typically applied first, followed by a topcoat (for corrosion protection and decorative purposes, such as polyurethane topcoat). Heavy-duty applications require an intermediate coat (for increased thickness, such as fiberglass cloth).
Control coating thickness: Too thin can lead to missing coatings, while too thick can cause cracking. Application should be based on material requirements (for example, a total thickness of heavy-duty anti-corrosion coatings must be ≥ 200μm).
Avoid application conditions: Application should not be conducted on rainy days, when humidity exceeds 85%, or when temperature is below 5°C (the coating may blister and cure slowly).
5.What are the mainstream coating types and characteristics?
Galvanizing (most commonly used):
Methods: Hot-dip galvanizing (steel is immersed in molten zinc to form a 50-100μm thick zinc layer with strong adhesion), electrogalvanizing (a thin 10-30μm zinc layer is formed through electrolysis, resulting in a smoother surface).
Advantages: The zinc layer offers strong corrosion resistance (outdoor lifespan of 5-15 years, longer with hot-dip galvanizing), is reasonably priced, and is suitable for most steel materials (such as angles, pipes, and bolts).
Note: If painting is desired after galvanizing, phosphating must be performed first (to improve paint adhesion). Otherwise, the zinc layer may react with the paint and cause it to peel off. Aluminum/Zinc-Aluminum Alloy Coating:
Zinc-aluminum alloy coatings (such as Galvalume, which contains 55% aluminum and 43.5% zinc) offer greater corrosion resistance than pure zinc plating (salt spray resistance is 2-3 times that of zinc plating) and are suitable for high-humidity, high-salt environments (such as coastal buildings and marine equipment).
Disadvantages: Cost is 20%-30% higher than zinc plating, and processing is slightly more difficult (specialized equipment is required).
Tin/Chromium Plating:
Tin plating is primarily used for food contact steel (such as cans). The tin layer is non-toxic and resistant to weak acids.
Chromium plating offers a hard and wear-resistant coating, primarily used for decorative or wear-resistant parts (such as automotive parts and mechanical shafts). However, its corrosion resistance relies on being pinhole-free (damage to the plating can accelerate corrosion), resulting in a higher cost.