How to Control the Quality of Coatings Before Galvanizing Seamless Steel Pipes

In the production system of seamless steel pipes, the pre-galvanizing pre-treatment process is the "first line of defense" to ensure coating quality. Its core task is to thoroughly remove oil, scale, rust, dust, and other impurities from the surface and inner wall of the steel pipe, creating a clean, uniform, and appropriately active surface condition. This lays the foundation for uniform zinc ion deposition and a strong bond between the coating and the substrate. Practice shows that most defects in the coating, such as peeling, flaking, uneven gloss, and insufficient corrosion resistance, are related to improper control of the pre-treatment process.

First, the Core Components of the Pre-galvanizing Pre-treatment Process for Seamless Steel Pipes.

The pre-galvanizing pre-treatment of seamless steel pipes is a continuous process system. The process steps must be rationally combined according to the steel pipe material, the degree of surface contamination, and the requirements of the subsequent galvanizing process. Its core components mainly include: surface degreasing, scale and rust removal, neutralization treatment, and water washing passivation. Each step is interconnected; negligence in any step will directly affect the pre-treatment effect.

1. Surface Degreasing and Oil Removal Process for Seamless Steel Pipes: During rolling, storage, and transportation, steel pipes easily accumulate mineral oil, cutting oil, animal and vegetable oils, and other oil contaminants on their surface. The goal of degreasing and oil removal is to completely remove these contaminants and prevent them from hindering the contact reaction between zinc ions and the base steel. Currently, the mainstream approach uses a composite process of "alkaline degreasing + surfactant assistance." To address the difficulty in removing oil contaminants from the inner wall of seamless steel pipes, circulating spraying or ultrasonic assistance technology is also used to ensure simultaneous removal of oil contaminants from both the inner and outer walls. After degreasing, multiple water rinses are required to prevent residual alkaline solution from interfering with subsequent pickling processes.

2. Oxide Scale and Rust Removal Process for Seamless Steel Pipes: After heat treatment, high-temperature processing, or long-term storage, a dense black oxide scale and rust products will form on the surface of seamless steel pipes. These substances have high hardness and strong adhesion, making them difficult to remove completely with simple pickling. A combined process of "mechanical pretreatment + chemical pickling" is required. Mechanical pretreatment can loosen oxide scale through polishing and sandblasting, while chemical pickling uses acids such as hydrochloric acid and sulfuric acid to dissolve residual oxide scale and rust, while simultaneously using corrosion inhibitors to prevent excessive corrosion of the steel.

3. Treatment of seamless steel pipes in neutralization and washing processes: After pickling, acid residue easily remains on the surface of the steel pipe. If not treated promptly, this can lead to localized corrosion, forming defects such as pitting and corrosion. Therefore, neutralization treatment with a weak alkaline solution, such as sodium carbonate, is necessary to neutralize the residual acid and adjust the surface pH value. After neutralization, a running rinse with clean water is required to ensure that there are no acid or alkali residues or impurities adhering to the surface, ultimately achieving a clean and uniform surface.

Second, the core influencing factors of the pretreatment process on the coating quality of seamless steel pipes.

The pretreatment process directly affects key quality indicators such as adhesion, uniformity, smoothness, corrosion resistance, and gloss of the subsequent zinc coating by altering the cleanliness, roughness, and activity state of the seamless steel pipe surface. Its influence runs through the entire coating formation process, specifically manifested in the following five aspects.

1. The Influence of Seamless Steel Pipes on the Adhesion of Galvanized Coatings.

Coating adhesion is a core indicator of coating quality and directly determines the service life of the casing. If pretreatment is incomplete, residual oil and oxide scale on the seamless steel pipe surface will form a physical isolation layer, hindering the adsorption and alloying reaction between zinc ions and the base iron, resulting in a weak galvanized coating. Such coatings are prone to defects such as peeling, flaking, and bulging during subsequent processing, transportation, or use, losing their protective function against the substrate. A clean surface after precise pretreatment allows zinc ions to fully adsorb and react with iron, forming a tightly bonded zinc-iron alloy transition layer, significantly improving the adhesion between the coating and the substrate, effectively resisting external impacts and environmental corrosion. Practical data shows that for every 10% increase in the pretreatment pass rate, the coating adhesion failure rate can be reduced by more than 15%.

2. Direct Impact on Coating Uniformity and Smoothness.

The uniformity and smoothness of the coating not only affect the appearance quality but also the stability of its protective performance. If there are localized impurities or uneven roughness on the surface after pretreatment, it will lead to significant differences in the deposition rate and amount of zinc ions during the galvanizing process: areas with residual impurities will have difficulty depositing zinc ions, easily forming thin coatings or uncoated areas; in areas with large differences in roughness, zinc ions will preferentially deposit on raised areas, resulting in uneven coating thickness and a rough surface. For example, in areas where oxide scale is not completely removed, the coating thickness may only be 60%-70% of that in normal areas, and the surface will be uneven; areas with residual oil are prone to pinhole defects, which can become entry points for subsequent corrosion. Conversely, a uniform and clean surface ensures uniform zinc ion deposition, forming a galvanized layer of consistent thickness and a smooth surface, laying the foundation for a uniformly glossy galvanized layer in the subsequent passivation process.

3. Fundamental Impact on Coating Corrosion Resistance.

The core function of the galvanized layer is to provide corrosion protection for the steel pipe, and the quality of the pretreatment directly determines the effectiveness of this protection. On the one hand, incomplete pretreatment leaving residual impurities such as oil and acid will react with moisture and oxygen in the subsequent use environment, causing under-coating corrosion or localized corrosion, accelerating coating failure. On the other hand, improper pretreatment leading to defects such as incomplete coating and pinholes will directly expose the base steel to a corrosive environment. Once the steel rusts, it will further damage the surrounding coating, creating a vicious cycle of "rust-coating damage." Related tests show that steel pipes with substandard pretreatment have a salt spray corrosion resistance time of only 30%-50% of qualified products. High-quality pretreatment can thoroughly remove various impurities, reduce coating defects, ensure the formation of a continuous and complete protective barrier in the galvanized layer, and fully utilize its corrosion resistance.

4. Indirect Impact on Coating Gloss.

For galvanized seamless steel pipes, surface gloss is an important appearance quality indicator. Its formation is closely related to the optical interference effect of the passivation film, but this requires a smooth and flat galvanized substrate. The pretreatment process indirectly determines the final gloss effect by affecting the uniformity and flatness of the coating. If the surface is rough after pretreatment, with zinc nodules or impurities, even with precise control of subsequent galvanizing and passivation processes, it will be difficult to obtain a uniform and vibrant iridescent luster. Problems such as dull luster, uneven color, and hazy whitening are likely to occur. By using pretreatment techniques such as mechanical polishing and precise pickling to control the surface roughness within a reasonable range of Ra≤0.8μm, a smooth and bright galvanized layer substrate can be formed, ensuring that the galvanized layer exhibits a uniform and vibrant iridescent luster after passivation, thus enhancing the product's decorative appeal and market competitiveness.

5. Impact on the Stability of Galvanized Layer Thickness.

The stability of the galvanized layer thickness is crucial for ensuring consistent protective performance, and the quality of pretreatment directly affects the stability of zinc ion deposition. Insufficient surface cleanliness and uneven activity will cause significant fluctuations in the zinc ion deposition rate, resulting in large differences in the galvanized layer thickness in different parts of the same sleeve, exceeding the standard range of 8-12μm. For example, in areas with severe oil residue on the inner wall, the coating thickness may be less than 5μm, failing to meet basic protection requirements; while in areas where oxide scale is thoroughly removed, the galvanized layer thickness may exceed 15μm, resulting in material waste. By precisely controlling the pretreatment process, ensuring uniform surface cleanliness and activity of seamless steel pipes can stabilize the zinc ion deposition rate, guarantee the uniformity of the galvanized layer thickness, and avoid inconsistent protective performance due to thickness variations.

Third, key control strategies for improving the quality of seamless steel pipe pretreatment.

Addressing the core impact of the pretreatment process on the quality of the galvanized layer, a comprehensive quality control system must be built, encompassing process parameter optimization, equipment upgrades, and quality inspection, to ensure stable and reliable pretreatment results.

1. Precisely Optimize Process Parameters.

In the degreasing stage, process parameters must be precisely adjusted according to the type of oil contamination: Use an alkaline solution system primarily composed of sodium hydroxide and sodium carbonate, with a concentration controlled at 50-80 g/L, a temperature of 60-80℃, and a treatment time of 5-10 minutes; add an appropriate amount of nonionic surfactant to improve degreasing efficiency; for heavily contaminated inner walls, use ultrasonic-assisted degreasing, with power controlled at 500-800W. In the oxide scale removal stage, mechanical polishing uses a 120-240 grit polishing wheel at a speed of 3000-5000 rpm; sandblasting uses 0.1-0.3 mm quartz sand at a compressed air pressure of 0.4-0.6 MPa; subsequent acid pickling uses a 15%-20% hydrochloric acid solution at a temperature of 20-30℃ for 3-5 minutes, with a corrosion inhibitor to prevent over-corrosion. The neutralization stage uses a 5-10 g/L sodium carbonate solution, and rinsing with running water ensures the surface pH reaches neutral.

2. Upgrade and Adapt Production Equipment.

Taking into account the structural characteristics of hollow sleeves, upgrade the specialized pretreatment equipment: adopt an internal wall spray-type degreasing tank to ensure thorough removal of oil stains from the inner wall through high-pressure spraying; configure automated polishing and sandblasting equipment to precisely control processing parameters and avoid uneven roughness caused by manual operation; select pickling and neutralization tanks with stirring functions to ensure uniform solution concentration and improve treatment effect; establish a multi-stage water washing system using a countercurrent rinsing process to ensure water washing effect while conserving water resources. Simultaneously, regularly maintain the equipment and calibrate key parameters such as polishing speed and spray pressure to ensure stable equipment operation.

3. Construct a Full-Process Quality Inspection System.

Establish quality inspection standards for each stage of pretreatment to ensure early detection and treatment of defects. After degreasing, use a water film test to check cleanliness, requiring the water film to continuously cover the steel pipe surface for 30 seconds without breaking; after pickling, test the surface roughness to ensure Ra≤0.8μm, without defects such as pitting or corrosion; after neutralization water washing, test the surface pH value to ensure it is within the neutral range of 6-8, without acid or alkali residue. A combination of sampling and full inspection is used to test each batch of products. Unqualified products must undergo re-pretreatment and are strictly prohibited from entering the subsequent galvanizing process. Simultaneously, process parameters and test results at each stage are recorded to establish a quality traceability system for subsequent process optimization.

4. Controlling the Production Environment and Material Quality.

The production workshop must be kept clean and dry, with the temperature controlled between 15-30℃ and relative humidity ≤60% to reduce secondary contamination of the steel pipe surface by dust and oil. The quality of chemical reagents used in pretreatment is strictly controlled to ensure that alkaline solutions, acid solutions, surfactants, etc., meet technical standards, avoiding the impact of excessive reagent impurities on the treatment effect. The concentration of the bath solutions is regularly tested and adjusted; acid and alkaline solutions are tested every 4 hours, and passivation solutions are tested weekly, with timely replenishment or replacement of the bath solutions to ensure stable solution performance.

The pretreatment process before galvanizing is the core link determining the quality of the seamless steel pipe coating. It directly affects the product's service life and market competitiveness by influencing key indicators such as coating adhesion, uniformity, corrosion resistance, and gloss. Practice has proven that only by precisely optimizing pretreatment process parameters, upgrading and adapting production equipment, and constructing a comprehensive quality inspection system can a clean, uniform, and appropriately reactive surface condition be obtained, providing a solid foundation for subsequent galvanizing processes. In the future, with the development of intelligent manufacturing technology, automated detection and parameter control systems can be further introduced to achieve precise and intelligent control of the pretreatment process, promoting the continuous improvement of the quality of galvanized seamless steel pipe products.