The anti-corrosion treatment of wrought iron garden fences is a core step in ensuring their long-term durability and aesthetics. Surface pretreatment, as the foundation of the anti-corrosion process, directly affects the adhesion between the coating and the metal substrate, as well as the anti-corrosion effect. The core objective of pretreatment is to thoroughly remove surface impurities, rust, and oxide layers, creating a clean substrate suitable for coating adhesion. Simultaneously, surface roughening enhances the mechanical adhesion of the coating, providing a reliable guarantee for subsequent anti-corrosion processes.
During the production and processing of wrought iron garden fences, the surface is prone to residual organic contaminants such as grease, rust-preventive oil, and cutting fluid. These substances hinder direct contact between the coating and the metal substrate, leading to decreased adhesion. The pretreatment stage requires degreasing using alkaline cleaning agents or solvents, removing surface oil through soaking, spraying, or manual wiping. For stubborn grease, heating or ultrasonic cleaning techniques can be used to improve cleaning efficiency. After cleaning, thorough rinsing with clean water is essential to avoid interference from alkaline residues in subsequent pickling processes.
Pickling is a crucial step in removing oxide scale and rust from the metal surface. Wrought iron garden fences are prone to rust (Fe₂O₃·nH₂O) and scale (Fe₃O₄) in humid environments. These corrosion products reduce coating adhesion and accelerate substrate corrosion. Pickling typically uses hydrochloric acid or sulfuric acid solutions to dissolve the oxide layer through a chemical reaction. Strict control of acid concentration and temperature is crucial during operation to avoid excessive corrosion of the substrate. After pickling, immediate rinsing with clean water and neutralization treatment (such as sodium carbonate solution) are necessary to remove residual acid and prevent rust re-emergence.
Surface roughening is a core process for improving coating adhesion. Methods such as sandblasting, shot blasting, or pickling passivation create a uniform roughness on the metal surface, increasing the contact area between the coating and the substrate. Sandblasting uses compressed air to propel quartz sand or steel shot onto the surface, creating a micro-textured structure; shot blasting utilizes a high-speed rotating impeller to propel shot onto the surface, achieving a more efficient roughening effect. The roughened surface roughness must meet the coating process requirements, typically needing to reach Ra 25-75μm to ensure the mechanical bonding strength between the coating and the substrate.
After pickling, a passivation film easily forms on the metal surface, requiring activation treatment to restore metal activity. Activation processes usually involve micro-etching the surface with a weak acid solution (such as phosphoric acid or oxalic acid) to remove the passivation layer and create a micro-roughened structure. For hot-dip galvanizing, the activation treatment also includes a flux coating step, using a mixed solution of zinc chloride and ammonium chloride to form a protective film on the metal surface, preventing secondary oxidation before galvanizing. The activated surface must be kept dry to avoid contamination.
After pretreatment, the wrought iron garden fence surface must be thoroughly cleaned to remove residual abrasive, dust, and impurities. Cleaning methods include high-pressure air blowing, vacuuming, or wiping with a clean cloth to ensure no visible contaminants are present. After cleaning, the surface roughness, cleanliness, and activation effect must be tested using visual inspection, tactile examination, or a portable roughness meter for verification. For critical areas (such as welds and corners), the frequency of inspections should be increased to ensure the quality of pretreatment.
The pretreated wrought iron garden fence should be applied with anti-corrosion coating or hot-dip galvanized as soon as possible to avoid secondary corrosion caused by prolonged exposure. If temporary storage is necessary due to process arrangements, moisture-proof packaging or desiccants should be used for protection, and the humidity of the storage environment should be controlled below 60%. During handling, surface scratches should be avoided to prevent damage to the pretreatment effect. Strict control of the pretreatment process can significantly improve the corrosion resistance of the wrought iron garden fence and extend its service life to over 20 years.
