What are the key steps in the surface treatment process before galvanizing gas appliance connectors?
Release Time : 2025-12-30
The surface treatment process before galvanized connectors is crucial for ensuring coating quality and improving corrosion resistance. Its key steps encompass five major processes: degreasing, pickling, washing, fluxing, and drying. Each step is closely linked and requires strict control of process parameters to create a clean and activated metal surface suitable for galvanized connectors.
Degreasing is the first step in surface treatment, aiming to remove oil, rust-preventive oil, cutting fluid, and other organic contaminants from the surface of the gas appliance connector substrate. If these contaminants are not thoroughly removed, they will hinder the contact between the pickling solution and the metal substrate, leading to incomplete rust removal and affecting the adhesion of the coating. Degreasing methods typically involve immersion in chemical degreasing agents, breaking down grease through saponification and emulsification. Some processes also supplement this with electrolytic degreasing or ultrasonic degreasing. Electrolytic degreasing uses bubbles generated by electrolysis to peel off oil contaminants and is suitable for connectors with complex shapes; ultrasonic degreasing accelerates oil removal through high-frequency vibration and is particularly suitable for cleaning tiny pores.
Pickling is a critical step in removing oxide scale and rust from the metal surface. The oxide scale on galvanized gas appliance connectors is primarily composed of iron oxides. Its dense structure and strong bond with the substrate necessitate a chemical reaction between hydrogen ions in the pickling solution and the oxides, dissolving them into soluble salts. Commonly used pickling solutions are hydrochloric acid or sulfuric acid. Hydrochloric acid is widely used due to its fast reaction speed and low cost, but its concentration and temperature must be controlled to avoid excessive corrosion of the substrate. During pickling, corrosion inhibitors must be added to prevent excessive corrosion of the substrate and reduce hydrogen absorption by the iron substrate. The workpiece must be periodically turned to ensure uniform contact with the acid solution, and floating debris should be promptly removed to prevent secondary contamination. After pickling, the metal surface should exhibit a uniform silver-gray color, free of residual rust or oxide scale.
The water rinsing step is integral to the entire pretreatment process. Its purpose is to remove residual acid, iron salts, and degreasing agents from the workpiece surface. The water rinse after pickling must be thorough to prevent residual acid from being carried into the fluxing process, causing flux failure or the formation of zinc ash or zinc slag. Water washing typically employs multi-stage counter-current rinsing, where fresh water flows in from the final stage and flows in the opposite direction at each stage. This improves water utilization and ensures effective cleaning. After washing, the workpiece surface should have a near-neutral pH and be free of visible impurities.
Fluid plating is the core step in pretreatment. Its purpose is to form a protective film on the metal surface, preventing re-oxidation during drying and hot-dip plating, while also activating the surface and improving the wettability of the zinc bath. Fluxes are generally composed of zinc chloride and ammonium chloride. By controlling their concentration and temperature, a thin and uniform salt film is formed on the workpiece surface. Strict control of the workpiece immersion time is crucial during flux plating to avoid an excessively thick salt film resulting in a rough coating, or an insufficiently thin film to effectively prevent oxidation. Some processes add a suitable amount of glycerin to the flux to prevent evaporation and ensure protective efficacy.
The drying process removes residual moisture from the workpiece surface through heating, preventing zinc bath splashing during hot-dip plating due to moisture evaporation. It also prevents moisture from reacting with the zinc bath to form zinc oxide, which would affect the coating quality. The drying temperature is typically controlled between 120 and 180 degrees Celsius, and the drying time is adjusted according to the size and shape of the workpiece to ensure complete evaporation of moisture. The dried workpiece surface should be dry and free of oil stains to create ideal conditions for subsequent hot-dip galvanizing.