The commonly used automated coating processes for seamless steel pipes, such as oil casing pipes, include the following four methods:

Process One

Utilizes a spray coating method with linear conveyor systems, applying the coating before, after, and between the rollers to form a coating film. The pipes are then transferred using forklifts and conveyed by a hook chain into a steam drying oven for heating and drying.

Process Two

Employs an electrostatic coating method, where inclined roller conveyors are used before, after, and in the middle to form the coating film. The pipes are then lifted by a spiral elevator to a rack for natural drying.

Process Three

Involves linear transportation of the pipes using a UV coating system with vacuum coating and airflow brushing to form the coating film. After coating, immediate UV radiation cures the film. The advantage is that both the formation and curing of the coating film occur between two rollers.

Process Four

Uses a heated airless spray coating method. The pipes are conveyed on a roller conveyor before coating, and after coating, they are transported using segmented synchronized “V-shaped tooth” chains to form the coating film. After coating, the pipes are transferred by a stepping machine to a transverse “V-shaped tooth” conveyor, which directs them into a steam drying box for heating and drying.

Comparison of Various Coating Processes

Process One

This method suffers from significant coating run-off due to the spray coating technique. Additionally, the design of the rollers and chains is inadequate, leading to two longitudinal and multiple circular scratches on the coating film. This process is being phased out, with the primary advantage being the heating and drying that occur after coating.

Process Two

Coating defects include run-off, spiral scratches, and whitening. Notably, the thickness of the coating at the spiral scratch areas is only one-fifth of the specified thickness, resulting in poor appearance. This process also poses fire hazards due to static electricity, with several fire incidents occurring in recent years, posing a threat to safety. The absence of a drying process is also a significant flaw. Due to many insurmountable contradictions, this method is increasingly viewed as outdated and will gradually exit the steel pipe coating field.

Process Three

This is a technologically advanced but not fully mature process. The instant spraying and curing between two rollers has clear advantages, but it also has insurmountable weaknesses, such as stringent pre-treatment requirements for the pipe surface. Any oversight can lead to significantly reduced adhesion. Additionally, UV coatings and equipment are expensive, and technical management requirements are high. The coating is brittle, making it prone to localized peeling during transport and difficult to recoat. These numerous issues restrict the promotion of this process.

Process Four

This is a relatively advanced and mature technology that has been developed in recent years. It overcomes the severe run-off, scratches, whitening, and brittleness issues present in other processes. The resulting coating has strong adhesion, flexibility, good rust resistance, minimal run-off, and an aesthetically pleasing finish. Ease of operation, comprehensive support, low technical management requirements, and high safety standards also characterize this method. Due to its technological completeness, it is referred to as the “Complete Technology for Airless Heating Spray Coating of Steel Pipes.

Advancements of the “Airless Heating Spray Coating Technology for Steel Pipes”

Processes One to Three reflect common coating defects in traditional methods, such as severe “run-off,” scratches, and “whitening.” The latest Process Four comprehensively addresses these issues and forms a complete “Airless Heating Spray Coating Technology for Steel Pipes.” This technology has the following technical advantages

Avoidance of Stripe or Spiral Scratches

In the late 1990s, experts from Beijing Boronus Coating Equipment Co., Ltd. and the Beijing Steel Design Institute analyzed the scratching issue in steel pipe coatings. At that time, scratches primarily appeared as two longitudinal scratches, each 40 mm wide, along the surface of the pipes.

In response, a technical solution was proposed to utilize synchronous chain transportation for the front roller before spray coating, rather than using roller conveyors. This solution was implemented at the Daqing General Machinery Factory’s oil pipe branch by Beijing Boronus Coating Equipment Co., Ltd. in the early 21st century.

Practical applications proved that this method effectively prevents longitudinal scratches on the pipes, with only two small point marks spaced 600 mm apart remaining after coating. Coupled with the self-healing properties of the coating, the resulting film is macroscopically intact, receiving positive feedback from users.

The further improved process (utilizing segmented synchronous belt chain transportation before and after spraying) ensures that the pipes maintain partial contact with the “V” shaped teeth of the chain while moving synchronously, minimizing contact between the spray surface and the support point, thus avoiding stripe scratches from roller transport.

The coating is aesthetically pleasing and intact. Currently, this technology is considered advanced both domestically and internationally. It is not only advanced but also mature, having been validated on 13 automatic oil coating lines in the country. Preventing spiral scratches is straightforward; simply replacing the spiral transport with this method suffices.

Overcoming the “Run-Off” Issue

Run-off can be categorized into five types: density type, excessive thickness type, low viscosity type, special shape type, and contact type. Anti-rust coatings for steel pipes generally have a relatively low density, making their impact on run-off negligible.

The primary types of run-off for steel pipes are excessive thickness and low viscosity, while special shape and contact types are secondary. Since the cross-section of steel pipes is circular and closely resembles an aerodynamic shape, the coating flows more easily, making runoff inevitable but manageable.

By using a coating with a higher original viscosity that decreases upon heating, and then returning to a higher viscosity as the temperature drops, the flow of the coating on the pipe surface can be reduced to overcome run-off. The contact type of run-off occurs when the support point contacts the pipe surface, creating a nearly vertical contact area that guides the coating away from the film, leading to run-off.

This is unavoidable, but can be minimized through thickness control. Fortunately, the quantity and area of such occurrences are small, with minimal impact on appearance.

Addressing Excessive Thickness Run-Off: Key Control Points

    • The pipes must maintain a certain operational speed, which can be adjusted within a specific range.
    • The transport of the pipes must be uniform.
    • The airless heating spray method allows for adjustable working pressure, which must remain stable and consistent. The heating temperature of the coating can also be adjusted.
    • The spray nozzle type must be selected correctly through experimentation, considering parameters such as flow rate and spray width.
    • The airless automatic spray gun must distribute evenly across the round cross-section of the pipe, with adjustable angles and distances to the pipe surface.
    • The center of the spray gun group must align with the center of the pipe.
    • The exhaust airflow and speed must match the spraying conditions.
    • The working condition of the nozzle must be optimal.

If these points are carefully adjusted for each specification of steel pipe, excessive thickness run-off can be overcome entirely. The challenge lies in establishing a comprehensive coating process management system to ensure orderly technical adjustments during product changes.

Preventing Coating “Whitening”

Mechanism of “Whitening” Defects

The mechanism of “whitening” defects in coating films is primarily due to moisture condensing and mixing within the coating, resulting in an emulsion that transforms into a translucent white film. Several factors contribute to this issue, including environmental humidity, excessive dispersion of the coating, temperature drops during dispersion, localized surface temperatures, water content in the coating, excessively low surface temperatures of the workpiece, inappropriate solvent boiling points or ratios, and moisture in the atomizing air.

Leading Causes in Steel Pipe Coatings

For steel pipe coatings, the first three factors—environmental humidity, excessive dispersion, and temperature drops—are the leading causes of “whitening.” These factors create specific conditions that facilitate the formation of defects. In electrostatic coating, high humidity and low temperatures lead to excessive dispersion of the coating after it exits the gap-type electrostatic atomizer, causing it to accumulate on the surface of the steel pipe.

Excessive Dispersion and Its Effects

When the coating disperses excessively into a vast number of extremely fine droplets, it significantly increases the specific surface area and the area in contact with humid air. This allows the coating to attract a large number of water molecules, which mix into the coating and ultimately lead to “whitening” defects. Thus, while environmental humidity is a contributing factor, the real cause lies in the excessive dispersion of the coating.

Preventing “Whitening” Defects

To prevent “whitening” defects in the coating film, controlling excessive dispersion while maintaining constant environmental humidity is essential. The use of heated airless spray coating is considered the best method to address these two causes. Heated airless spray involves spraying heated material at a specific temperature, typically between 40°C and 80°C, without the use of compressed air.

Advantages of Heated Airless Spray

In the airless spraying process, the liquid coating is pressurized with a pump and transported through pipes to the spray gun, where it exits through the nozzle to form a high-speed liquid film. This film becomes unstable and disperses into small droplets due to friction with the air, resulting in moderate atomization that prevents excessive dispersion, making it suitable for pipe coating.

Material Efficiency and Effectiveness

Heated airless spraying can reduce solvent usage and allows for the application of high-viscosity coatings, resulting in material savings compared to air spraying, though slightly more than in electrostatic spraying. This disadvantage can be offset by using recovery devices. Given its advantages in temperature control and atomization, heated airless spraying is the optimal choice for coating steel pipes.

Proven Results

The effectiveness of heated airless spraying is evident, as multiple manufacturers and coating lines have consistently reported no occurrences of “whitening.” Even coatings transported across oceans maintain excellent rust resistance. Additionally, the heating and drying processes following airless heated spraying are crucial in preventing “whitening” defects.