Epoxy Powder Anticorrosion Steel Pipe Testing

Epoxy powder anti-corrosion steel pipe inspection mainly conducts scientific testing and analysis on the coating appearance quality, coating thickness, coating adhesion, cathodic disbondment performance, impact resistance strength and bending resistance performance, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Coating appearance quality: Through the visual inspection or low-power magnifying glasses, it is checked whether there are the defects such as the bubbles, cracks, shrink holes, runs, orange peels and embedded impurity particles on the coating surface, ensuring that the coating is continuous, uniform and smooth, without the obvious flaws affecting the protective properties.

Coating thickness: The magnetic or eddy current thickness gauges are used to measure the multiple points uniformly distributed on the steel pipe surface, and their average thicknesses and minimum thicknesses are calculated, ensuring that they meet the design specifications (usually 300-500 μm). It is the key indicator guaranteeing the corrosion-resistant service life of the coating.

Coating adhesion: The cross-cut method or pull-off method is adopted for the testing. The cross-cut method uses the multi-blade cutters to score the lattice grids, observing whether the coating is peeled from the substrate; the pull-off method bonds the test dollies to the coating with the dedicated adhesives, measuring the force values required to pull them off, evaluating the bonding strength between the coating and the steel pipe substrate.

Cathodic disbondment performance: The artificial defects are made on the coating, placed in the specific electrolytes as the cathode, and the specified potentials (such as -1.5 V vs. CSE) are applied. After a certain time (such as 28 days or 48 hours), the disbondment radii are measured, evaluating the anti-disbondment ability of the coating under the cathodic protection.

Impact resistance strength: A falling weight impact testing machine is used to let the falling weight of the specified mass and height freely fall to impact the coating surface, checking whether there are the cracks at the impact points or the coating is peeled from the substrate, evaluating the anti-mechanical-damage ability of the coating during the transportation and installation.

Bending resistance performance: The coated steel pipe is bent around the mandrel of the specified diameter at a certain angle (such as 2.5° or 3°) at the normal or low temperature, checking whether the coating at the bending area has the cracks or peeling, evaluating the deformation adaptability of the coating during the pipe bending in the pipeline installation.

Porosity (holiday) testing: A high-voltage electric spark holiday detector is used, and the probe sweeps across the coating surface at a constant speed. When there are the defects such as the pinholes, the high voltage breaks down to generate the electric sparks and alarms. The detection voltage is calculated according to the coating thickness, and it is the important process ensuring the coating integrity.

Testing Scope

Raw material testing: Including the inspection of the chemical compositions, mechanical properties (yield strength and tensile strength) and geometric dimensions (outer diameter, wall thickness and ovality) of the steel pipe substrate, as well as the surface pre-treatment (such as the anchor pattern depth and cleanliness after the sandblasting derusting), ensuring that the basic requirements before the coating application are met.

Epoxy powder coating testing: Covering the particle size distributions, volatile content, gel times, cure times, densities and magnetic substance contents of the powder, ensuring the process applicability of the powder and the chemical and physical properties of the final coating.

On-line testing in the production process: On the coating application production lines, the key process parameters such as the steel pipe preheating temperatures, powder spraying amounts and curing oven temperature curves (peak temperatures and cure times) are monitored and controlled in real time, ensuring that the coating quality is stable and controlled.

Finished product coating property testing: For the finished steel pipes after the coating application and curing, the comprehensive property testing is carried out according to the relevant standards, including all the above testing items, to verify whether they meet the protection requirements within the design service lives.

Products for the different application environments: For the epoxy powder anti-corrosion steel pipes used in the different application environments such as the buried pipelines, submarine pipelines, overhead pipelines and high-temperature pipelines, the testing scope is adjusted accordingly, such as adding the specific performance verifications such as the cathodic disbondment resistance, high-temperature resistance and impact resistance required by the service environments.

Testing Methods

Coating thickness measurement method: The magnetic or eddy current thickness gauges are used to measure the coating thicknesses at the multiple points uniformly distributed on the steel pipe surface, and the average thicknesses and minimum thicknesses are calculated, ensuring that they meet the design specifications. The measurement point distributions need to be representative, covering the pipe body, weld areas and pipe end areas.

Adhesion testing method: The cross-cut method uses the multi-blade cutters to score the coating surface into the lattice grids, and the adhesion grade is evaluated according to the peeling conditions of the grids; the pull-off method bonds the test dollies to the coating with the dedicated adhesives, and after the complete curing of the adhesives, a tester is used to pull the dollies off perpendicularly at a constant rate, and the instrument directly displays the pull-off strength values (MPa).

Cathodic disbondment testing method: The artificial defects of the specified sizes are made on the coating, placed in the specified electrolytes as the cathode, and the specified potentials (such as -1.5 V vs. CSE) are applied through a DC power supply. After the specified time (such as 28 days or 48 hours), the coating around the artificial defects is cut open, and the disbondment radii are measured to evaluate the anti-disbondment ability of the coating under the cathodic protection.

Holiday (porosity) detection method: A high-voltage electric spark holiday detector is used, and the probe sweeps across the coating surface at a constant speed. When there are the defects such as the pinholes and cracks, the high voltage breaks down the air to generate the electric sparks and alarms. The detection voltage is calculated according to the coating thickness, and the detection is generally carried out after the coating is completely cured.

Impact resistance testing method: A falling weight impact testing machine is used. The falling weight of the specified mass falls freely from the specified height to impact the coating surface, checking whether there are the cracks at the impact points or the coating is peeled from the substrate, evaluating the anti-mechanical-damage ability of the coating during the transportation and installation.

Bending resistance testing method: The coated steel pipe is bent around the mandrel of the specified diameter at a certain angle (such as 2.5° or 3°) at the normal or low temperature, and the coating at the bending area is inspected for the cracks or peeling, evaluating the deformation adaptability of the coating.

Testing Standards

Domestic Standards:

1. GB/T 23257, Technical specification of epoxy powder anti-corrosion coating for buried steel pipeline

2. SY/T 0315, Technical specification of external fusion bonded epoxy coating for steel pipeline

3. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

International Standards:

1. ISO 21809-1, Petroleum and natural gas industries — External coatings for buried or submerged pipelines used in pipeline transportation systems — Part 1: Fusion bonded epoxy coatings

2. ASTM G8, Standard test methods for cathodic disbonding of pipeline coatings

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Coating thickness gauge: The magnetic or eddy current principles are used to measure the coating thicknesses at the multiple points uniformly distributed on the steel pipe surface, and the average thicknesses and minimum thicknesses are calculated, ensuring that they meet the design specifications (usually 300-500 μm). It is the key testing instrument guaranteeing the corrosion-resistant service life of the coating.

Electric spark holiday detector: Consisting of a high-voltage generator, detection electrode and grounding device. The probe sweeps across the coating surface at a constant speed. When there are the defects such as the pinholes, the high voltage breaks down the air to generate the electric sparks and alarms, used to detect the discontinuity defects of the coating and ensure the coating integrity.

Adhesion tester: The common types include the hydraulic, mechanical and pneumatic types. The test dollies are bonded to the coating with the dedicated adhesives, and after the curing, they are pulled off perpendicularly and the pull-off strength values (MPa) are displayed, used to evaluate the bonding strength between the coating and the steel pipe substrate.

Falling weight impact testing machine: The falling weight of the specified mass and height freely falls to impact the coating surface, checking whether there are the cracks at the impact points or the coating falls off, evaluating the anti-mechanical-damage ability of the coating during the transportation and installation.

Cathodic disbondment testing device: Mainly consisting of a DC power supply, reference electrode (such as the saturated calomel electrode), auxiliary anode (such as the graphite or platinum), electrolytic cell and constant-temperature device. Used to simulate the cathodic protection working conditions, precisely apply the potentials and measure the coating disbondment sizes, evaluating the anti-disbondment ability of the coating under the cathodic protection.

Differential scanning calorimeter: Used to analyse the glass transition temperatures, curing reaction heats and curing degrees of the epoxy powder. Through the programmed temperature control, the heat flow differences between the sample and reference are measured, providing the scientific basis for the powder quality control and curing process optimization.

Salt spray test chamber: Creates the artificially accelerated corrosion environments. The coated specimens with the artificial scratches are placed in the salt mist of a certain concentration, continuously sprayed for hundreds to thousands of hours, and the corrosion spreading at the scratches and the coating blistering conditions are regularly observed, evaluating their corrosion resistance.