Anticorrosion Steel Pipe Testing

Anti-corrosion steel pipe inspection mainly conducts scientific testing and analysis on the coating thickness, adhesion, holiday detection, cathodic disbondment performance, chemical medium corrosion resistance and impact resistance strength, 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 thickness: The coating thickness is the basic indicator measuring the protective ability of the anti-corrosion layers. The coating thickness gauges are used to measure at the uniformly distributed points on the steel pipe surfaces, and the average values and minimum values are both required to meet the provisions of the corresponding anti-corrosion standards (such as GB/T 23257 and SY/T 0413), ensuring that the coating has the sufficient physical barrier thicknesses to resist the permeation of the corrosive media.

Adhesion (peel strength): The key item evaluating the bonding firmness between the anti-corrosion coating and the steel pipe substrate. The pull-off method or cross-cut method is usually adopted for the testing. The pull-off method uses the adhesion testers to determine the forces required to pull the coating off, while the cross-cut method scores the lattice grids with the dedicated cutters and then observes the coating peeling conditions, ensuring that the coating does not peel off during the transportation, installation and service.

Holiday detection (electric spark leak detection): Used to find the discontinuity defects in the anti-corrosion coating, such as the pinholes, cracks and missed coating areas. During the detection, the probe of the high-voltage electric spark holiday detectors moves uniformly on the coating surface. When the holidays are encountered, the high voltage breaks down the air to generate the sparks and alarms, thereby accurately locating the defect positions for the repair.

Cathodic disbondment performance: The accelerated testing simulating the anti-disbondment ability of the coating under the cathodic protection environments. The artificial defects are prefabricated on the coating, immersed in the electrolyte solutions and applied with certain cathodic potentials. After the specified time (such as 28 days or 48 hours), the distances of the coating radial disbondment from the defects are measured, evaluating its long-term durability.

Chemical medium corrosion resistance: Inspects the tolerance ability of the anti-corrosion steel pipe in the specific chemical environments. The coating specimens are immersed in the specified acid, alkali, salt solutions or specific media (such as the crude oil and soil extracts), and after the specified time, the phenomena such as the appearance changes, blistering, softening and falling off of the coating are observed, evaluating its chemical stability.

Impact resistance strength: Simulates the ability of the steel pipe to bear the impacts of the hard objects such as the stones during the handling and backfilling. A falling weight impact testing machine is used to let the falling weight of the specific mass and height impact the coating surface, checking the damage conditions of the coating after the impact, ensuring that the coating has the sufficient mechanical strength.

Bending resistance: Inspects the anti-cracking ability of the coating during the cold bending or installation bending of the anti-corrosion steel pipe. The coated pipe sections or test plates are bent on the mandrels of the specific diameters, and after the bending, whether there are the cracks, peeling and other phenomena on the coating surfaces is checked, especially suitable for the flexible anti-corrosion layers such as the 3PE (three-layer polyethylene).

Testing Scope

Steel pipes for the oil and natural gas conveying pipelines: Mainly including the steel pipes for the long-distance onshore and submarine oil and gas transmission pipelines and gathering pipelines. Their testing strictly follows the pipe standards such as API 5L and GB/T 9711, as well as the anti-corrosion layer standards such as the relevant standards, with the emphasis on the comprehensive protective properties under the high-pressure and long-cycle service.

Steel pipes for the municipal water supply and drainage and heating pipe networks: Involving the drinking water conveying, sewage discharge and central heating pipelines in the municipal projects. The testing needs to meet the industry standards, and in addition to the routine items, the hygienic safety of the coating (such as no heavy metal precipitation) and long-term hot water resistance also need to be focused on.

Chemical and industrial process pipelines: The process pipelines used to convey the various chemical raw materials and corrosive media. The testing scope needs to be determined according to the specific medium characteristics, emphatically focusing on the resistance of the coating to the specific chemicals corrosion, temperature resistance and the adhesion retention under the internal pressure and temperature alternations.

Pipes for the marine engineering and port facilities: Including the wharf pile pipes, submarine pipelines and offshore platform structural pipes. Facing the harsh environments such as the high salt mist, wave scouring and microbial corrosion, the testing needs to additionally include the salt spray resistance, seawater immersion resistance, coating wear resistance and compatibility with the sacrificial anodes.

Pipes for the underground coal mines: Used for the ventilation, drainage and gas drainage of the coal mines. In addition to the routine items, the testing must meet the relevant coal mine safety regulations, especially emphasizing the antistatic properties, flame retardancy and long-term adhesion under the high-humidity environments of the coating.

Repair and in-service pipeline anti-corrosion layers: The anti-corrosion layer condition evaluation and post-repair testing are carried out on the laid or in-service pipelines. The testing scope includes the non-excavation ground potential testing (Pearson method and CIPS/DCVG), excavation direct evaluation (coating aging rating and damage point statistics), as well as the special testing of the field joint and repair areas.

Testing Methods

Magnetic method thickness measurement: Applicable to the thickness measurement of the non-magnetic coatings (such as the epoxy powder and polyethylene) on the iron and steel substrates. According to the GB/T 4956 standard, the magnetic flux or mutual inductance current changes between the probe and steel substrate are used to measure the coating thicknesses. The operation is simple and fast, and it is the most commonly used method on site and in the laboratories.

High-voltage/low-voltage electric spark holiday detection method: The high-voltage method (usually used for the coatings with the thicknesses >500 μm) is carried out according to the relevant standards, using the DC or AC high-voltage outputs; the low-voltage method (for the thin coatings or wet sponge method) is carried out according to the relevant standards. The coating defects are identified by whether the discharge circuits are formed between the electrodes and the metal substrates.

Pull-off adhesion testing: According to the ISO 4624 / GB/T 5210 standards, the test dollies of the specific sizes are bonded to the coating surfaces with the dedicated adhesives. After the curing, a hydraulic or mechanical tensile machine is used to pull them off perpendicularly, measuring the tensile force values at the failure and observing the failure interfaces (cohesive failure of the coating, adhesion failure or adhesive failure).

Laboratory accelerated aging testing: The accelerated testing simulating the environmental factors such as the sunlight, temperature, humidity and salt mist is carried out on the anti-corrosion coatings. The common methods include the salt spray testing (neutral salt spray, acetic acid salt spray and copper-accelerated acetic acid salt spray), ultraviolet aging testing and damp heat aging testing. After the specified test cycles, the coating blistering, rusting, peeling, chalking and gloss degradation are regularly observed, evaluating the weather resistance and long-term durability of the coating.

Cathodic disbondment testing method: The artificial defects of the specified sizes are prefabricated on the coating, and the specimen is placed in the electrolyte solution as the cathode. 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 radial disbondment distances are measured to evaluate the anti-disbondment ability of the coating under the cathodic protection.

Falling weight impact testing method: A falling weight impact testing machine is used. The falling weight of the specific mass freely falls from the specified height to impact the coating surface, and the damage conditions of the coating after the impact are checked, simulating the impact resistance ability of the steel pipe when subjected to the impacts of the hard objects such as the stones during the handling and backfilling.

Testing Standards

Domestic Standards:

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

2. SY/T 0413, Technical specification of polyethylene coating for buried steel pipeline

3. GB/T 9711, Petroleum and natural gas industries — Steel pipe for pipeline transportation systems

4. GB/T 4956, Magnetic method for measuring thickness of non-magnetic coatings on magnetic substrates

5. GB/T 5210, Paints and varnishes — Pull-off test for adhesion

6. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. API 5L, Specification for line pipe

2. ISO 4624, Paints and varnishes — Pull-off test for adhesion

3. NACE SP0188, Discontinuity (holiday) testing of protective coatings

4. ASTM B117, Standard practice for operating salt spray (fog) apparatus

5. ISO 4892-3, Plastics — Methods of exposure to laboratory light sources — Part 3: Fluorescent UV lamps

6. ASTM G154, Standard practice for operating fluorescent ultraviolet (UV) lamp apparatus for exposure of nonmetallic materials

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: Divided into the magnetic thickness gauges (for the non-magnetic coatings on the steel substrates) and eddy current thickness gauges (for the non-ferrous metal substrates). The coating thicknesses are measured at the uniformly distributed points on the steel pipe surfaces, and the average values and minimum values are required to meet the anti-corrosion standard provisions. It is the basic instrument of evaluating the protective ability of the anti-corrosion layers.

Electric spark holiday detector: Consisting of a high-voltage generator, detection electrode and grounding device. The output voltages are selected according to the tested coating thicknesses. When the holidays are encountered, the high voltage breaks down to generate the sparks and alarms, used to find the pinholes, cracks and missed coating areas in the anti-corrosion coating.

Adhesion tester: The common types include the hydraulic, mechanical and pneumatic types. The test dollies are bonded to the coating surfaces with the dedicated adhesives, and after the curing, a tensile machine is used to pull them off perpendicularly and measure the tensile force values at the failure, used to evaluate the bonding firmness between the anti-corrosion coating and the steel pipe substrate.

Falling weight impact testing machine: The falling weight of the specific mass and height impacts the coating surface, and the damage conditions of the coating after the impact are checked, simulating the impact resistance ability of the steel pipe when subjected to the impacts of the hard objects such as the stones during the handling and backfilling.

Cathodic disbondment testing device: Consisting of a DC power supply, reference electrode, auxiliary anode, electrolytic cell and constant-temperature device. The artificial defects are prefabricated on the coating and the cathodic potentials are applied, and the coating radial disbondment distances are measured, evaluating its long-term durability.

Salt spray test chamber: Simulates the corrosion accelerated testing under the marine atmospheric or salt-containing environments. The samples are exposed to the salt mist environments with the constant temperature and constant concentration, and the coating blistering, rusting and peeling conditions are regularly observed, evaluating its corrosion resistance.

Ultraviolet aging test chamber: Simulates the damage of the ultraviolet band in the sunlight to the coating, using the fluorescent ultraviolet lamps as the light sources, and the condensation or spray humidity can be controlled, accelerating the evaluation of the light aging, chalking and discoloration properties of the coating.

Digital microscope/coating profile instrument: Used for the micro observation and precision measurement of the coating defects and cross-section structures. The coating thicknesses can be re-verified with the metallographic method, the fusion states and porosity of the various layers can be observed, and the failure modes after the adhesion testing can be analysed, providing the intuitive image evidence.