The influence of steel pipe forming method on anti-corrosion treatment

SSAW Steel Pipe (spiral submerged arc welded steel pipe)

SSAW steel pipes are manufactured through a spiral forming process. The weld seam is spiral shaped, which makes the surface shape of the steel pipe relatively complex, especially the geometric shape of the weld seam is irregular. When carrying out anti-corrosion treatment, more attention needs to be paid to the coverage and protection of welds. For example, during electrostatic spraying of epoxy powder coatings, the electric field distribution at the spiral weld seam may be uneven, which can lead to inconsistent coating thickness. To ensure the quality of the coating at the weld seam, special spray gun angle adjustment or multiple spraying may be required.

Moreover, due to the presence of residual stresses during its molding process, these stresses may affect the adhesion between the anti-corrosion coating and the surface of the steel pipe. Before anti-corrosion treatment, it is necessary to better control the surface condition of the steel pipe, such as through appropriate stress relief processes (such as heat treatment), to avoid coating peeling due to stress release during subsequent use.

LSAW Steel Pipe (straight seam submerged arc welded steel pipe)

LSAW steel pipes are welded with straight seams, and the surface is relatively flat, which is advantageous for the construction of anti-corrosion coatings. When applying 3PE coating (three-layer polyolefin coating), it is easier to ensure the uniformity and good adhesion of each layer of coating. For example, when coating the bottom layer of epoxy powder, it is more convenient to pre treat and spray at the straight seam, so that the powder can better adhere to the surface of the steel pipe and the weld seam, reducing the probability of coating defects.

Meanwhile, the stress distribution of LSAW steel pipes during the manufacturing process is relatively uniform. Generally, under reasonable manufacturing processes, the adverse effect of residual stress on the adhesion of anti-corrosion coatings is relatively small. However, during the anti-corrosion treatment process, it is still necessary to perform appropriate treatment on the straight seam, such as grinding the weld seam, to ensure a good bond between the coating and the weld seam.

2. Differences in the construction process of anti-corrosion coatings

External coating construction

SSAW steel pipe: During the external coating construction, due to the presence of spiral welds, the construction difficulty is relatively high for some coatings that require continuous coating (such as coal tar enamel coating). During the coating process, it is necessary to evenly apply along the direction of the spiral weld seam, and attention should be paid to preventing the coating from accumulating or cracking at the weld seam. For the use of winding coatings (such as the outer polyolefin layer in 3PE coatings), it is necessary to ensure that the winding equipment can adapt to the shape of the spiral weld, so that the polyolefin layer is tightly adhered to the surface of the steel pipe, avoiding hollowing or wrinkling.

LSAW steel pipe: The external coating construction process is relatively simple. Taking epoxy powder coating as an example, straight seam steel pipes can be more conveniently sprayed through automated electrostatic spraying production lines. During the spraying process, special spray guns can be designed at the straight seam to better fill the weld seam and steel pipe surface with powder, ensuring the uniformity of coating thickness. For the construction of 3PE coating, straight seam steel pipes are also easier to achieve good bonding between layers, as the shape of the straight seam is more conducive to controlling the entanglement and adhesion of the adhesive and polyolefin layer.

Internal coating construction

SSAW steel pipe: Its internal spiral welds also pose certain challenges to the construction of the inner coating. If liquid epoxy coating is used, the surface roughness and shape of the weld seam during the spraying process may affect the flow and adhesion of the liquid epoxy. Pre treatment of the weld seam is required, such as grinding, cleaning, etc., to ensure that the liquid epoxy can evenly cover the weld seam and the inner wall of the steel pipe. And during the curing process, due to the different heat conduction at the weld seam compared to other parts, it is necessary to control the curing temperature and time reasonably to ensure consistent curing quality of the coating on the entire inner wall of the steel pipe.

LSAW steel pipe: For the internal coating construction of LSAW steel pipe, if cement mortar coating is used, the presence of straight joints makes it relatively easier to control the coating thickness and uniformity. When applying cement mortar, the straight seam can serve as a reference boundary, facilitating the operation of construction personnel and allowing the cement mortar to be more evenly distributed on the inner wall of the steel pipe. For liquid epoxy coatings, the inner wall of straight seam steel pipes is more conducive to achieving automation of spraying, improving coating quality and construction efficiency.

3. Differences in adaptability of cathodic protection systems

Sacrificial Anode Protection Law

SSAW steel pipe: When using sacrificial anode protection method, the spiral weld of SSAW steel pipe may affect the current distribution. The metal structure and residual stress at the spiral weld seam are different from other parts, which may lead to uneven distribution of local potential. Therefore, when installing sacrificial anodes, it is necessary to pay more attention to the distribution and connection method of the anodes to ensure that the current can be evenly distributed on the entire surface of the steel pipe, and to avoid local under protection or over protection at the weld seam.

LSAW steel pipe: The influence of the straight seam of LSAW steel pipe in sacrificial anode protection method is relatively small. Due to the regular shape of the straight seam, the distribution of current on the surface of the steel pipe is relatively easier to control. When installing sacrificial anodes, they can be arranged along the steel pipe at a certain distance, which can effectively provide uniform cathodic protection for the entire steel pipe.

External current cathodic protection method

SSAW steel pipe: For SSAW steel pipes, due to the presence of spiral welds on their surface, it is necessary to more accurately monitor the potential of each part of the steel pipe when applying external current cathodic protection. Because spiral welds may interfere with the electric field distribution, resulting in potential differences in different parts of the steel pipe. More reference electrodes need to be used to monitor the potential, and when adjusting the applied current, the influence of the spiral weld seam should be considered to ensure that all parts of the steel pipe can receive appropriate protective potential.

LSAW steel pipe: When using external current cathodic protection method, LSAW steel pipe has a relatively regular surface and relatively simple potential monitoring. By setting reference electrodes at key positions such as the ends and middle of the steel pipe to monitor the potential, and adjusting the applied current based on the monitoring results, the potential on the surface of the steel pipe can be maintained within an appropriate protection range, making it relatively easier to achieve effective cathodic protection.

The difference in anti-corrosion treatment cost between SSAW steel pipe and LSAW steel pipe

1. Cost difference of coating materials

Differences in dosage under the same coating type

Due to its spiral welds and relatively complex shape, SSAW steel pipes may require more coating materials to ensure uniform coverage during anti-corrosion coating construction. For example, when using epoxy powder coating, additional spraying may be required to ensure the required coating thickness at the spiral weld seam. In contrast, the straight seam structure of LSAW steel pipes allows for more effective distribution of coating materials. Typically, under the same anti-corrosion standards, SSAW steel pipes may require about 5% to 10% more epoxy powder than LSAW steel pipes.

For 3PE coating (three-layer polyolefin coating), SSAW steel pipes may generate more corner waste when wrapped with outer polyolefin due to the need to adapt to the shape of the spiral weld seam, resulting in relatively low material utilization. The straight seam structure of LSAW steel pipes results in less material waste during the 3PE coating construction process, which gives LSAW steel pipes a certain advantage in coating material cost.

Cost differences caused by the applicability of different coatings

Due to the characteristics of spiral welds, SSAW steel pipes may require more complex pre-treatment and construction equipment to ensure quality for coatings that require higher construction processes, such as liquid epoxy coatings. This may lead to an increase in the application cost of liquid epoxy coating on SSAW steel pipes. For example, in order to better adhere liquid epoxy to spiral welds, specialized spray guns and surface treatment tools may be required, and these additional equipment investments will increase costs.

The surface of LSAW steel pipe is relatively flat, and it has good applicability to various coatings, with higher flexibility in selecting coating materials. Some relatively low-cost but high-performance coatings (such as cement mortar coatings for inner wall corrosion prevention) are easier to apply on LSAW steel pipes, thereby reducing the cost of internal corrosion prevention.

2. Differences in construction process costs

Construction cost of external coating

SSAW steel pipe: During the external coating construction, the presence of spiral welds makes the construction difficult. For example, in the construction process of coal tar enamel coating, manual operation is required to ensure the quality of the coating at the spiral weld seam, which will increase labor costs. Moreover, in order to ensure a good fit between the coating and the spiral weld, more complex construction processes may be required, such as multiple applications, special heating and curing processes, which can prolong the construction time and indirectly increase the construction cost.

LSAW steel pipe: The external coating construction of LSAW steel pipe is relatively simple and can better adapt to automated production lines. For example, in the electrostatic spraying process of epoxy powder coating, straight seam steel pipes can be efficiently sprayed through automated equipment, reducing manual intervention and lowering labor costs. Meanwhile, the straight seam structure makes it easier to control the construction quality of the coating, reducing rework costs caused by quality issues.

Internal coating construction cost

SSAW steel pipe: Its internal spiral welds pose challenges to the construction of the inner coating. If liquid epoxy coating is used, special pretreatment is required for spiral welds, such as weld grinding, cleaning, etc., which increases labor and time costs. And during the spraying and curing process, due to the influence of spiral welds, stricter process control may be required, such as adjusting the spraying angle, controlling the curing temperature and time, which will also increase construction costs.

LSAW steel pipe: The straight seam of LSAW steel pipe has advantages in internal coating construction. For example, in the construction of cement mortar coatings, straight joints can serve as reference boundaries, facilitating the operation of construction personnel, improving construction efficiency, and reducing labor costs. For liquid epoxy coatings, the inner wall of straight seam steel pipes is more conducive to achieving automated spraying, reducing labor costs and rework costs caused by construction quality issues.

3. Cost difference of cathodic protection

Sacrificial cost of anode protection method

SSAW steel pipe: Due to the influence of spiral welds on current distribution, it is necessary to carefully consider the distribution and connection method of sacrificial anodes when installing them. More anodes may be needed to ensure uniform current distribution and avoid local under protection or over protection. This will increase the material cost of sacrificial anodes, and due to the more complex installation process, the manual installation cost will also increase accordingly.

LSAW steel pipe: The straight seam structure of LSAW steel pipe makes the installation of sacrificial anodes relatively simple, and anodes can be arranged at certain intervals. Under the same protective effect, the required number of sacrificial anodes may be relatively small, thereby reducing the material and installation costs of sacrificial anodes.

Cost of External Current Cathodic Protection Method

SSAW steel pipe: When using external current cathodic protection method, SSAW steel pipe requires more reference electrodes to monitor the potential of various parts of the steel pipe, as spiral welds may interfere with the electric field distribution. This increases the material and installation costs of the reference electrode. Meanwhile, due to the need for more precise adjustment of external current, higher requirements are placed on equipment and technical personnel, which may increase equipment maintenance and labor costs.

LSAW steel pipe: The surface of LSAW steel pipe is relatively regular, and potential monitoring is relatively simple. The required number of reference electrodes is relatively small, which reduces material and installation costs. In terms of equipment maintenance and labor costs, it is relatively low due to its easily controllable potential distribution.