Plastic-coated Steel Pipe Testing

Plastic-coated steel pipe inspection mainly conducts scientific testing and analysis on the coating thickness testing, adhesion testing, pinhole testing, appearance quality inspection, impact resistance testing and chemical corrosion resistance testing, 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 testing: By measuring the overall thicknesses and uniformity of the inner and outer plastic layers of the steel pipe, whether they meet the design specification requirements is evaluated. The common standard requires that the coating thickness of the plastic-coated steel pipes for the fire protection is not lower than 200 μm, and for the water supply and drainage pipes, it is usually in the range of 150-300 μm depending on the diameters and application environments. This item is the basis for ensuring that the coating provides the sufficient physical barriers and corrosion resistance.

Adhesion testing: Evaluates the bonding strength between the plastic coating and the steel pipe substrate, and is the key indicator of preventing the coating peeling, blistering or cracking. The cross-cut method, pull-off method or bending method are mainly adopted. For example, a sharp blade is used to score the 1 mm × 1 mm lattice grids on the coating, observing whether the coating is peeled from the substrate, thereby rating the adhesion grades.

Pinhole (porosity) testing: Through the electric spark holiday detectors or high-voltage holiday detectors, the specific voltages are applied to the coating, checking whether there are the tiny through defects difficult to find with the naked eyes in the coating. The voltage values are set according to the coating thicknesses, usually about 5-10 kV detection voltages corresponding to each 100 μm thickness. The absence of the holidays is qualified.

Appearance quality inspection: The coating surfaces are checked with the naked eyes or with the aid of the magnifying glasses for whether they are smooth and flat with the uniform colours, and whether there are the defects such as the runs, orange peels, shrink holes and particles. At the same time, it is checked whether the coating is completely covered without the exposed iron and rust. This is the most basic and intuitive inspection link before the ex-factory of the products.

Impact resistance testing: Simulates the ability of the coating to resist the external force impacts during the transportation, installation or use. The falling weight impact testing is usually adopted. The impact head with the specified weight and falling height impacts the coating surface, observing whether there are the cracks or coating peeling. The impact energies are determined according to the pipe specifications and application standards.

Chemical corrosion resistance testing: The plastic-coated steel pipe specimens are immersed in the acid, alkali and salt solutions of the specific concentrations for a certain time, or the salt spray testing is carried out. After the testing, whether there are the phenomena such as the blistering, softening, swelling, discoloration, gloss loss or adhesion degradation on the coating surfaces is checked, to evaluate their stability in the specific corrosive environments.

Wear resistance testing: Evaluates the ability of the coating to resist the wear, especially important for conveying the fluids containing the solid particles. The common methods include the falling sand method or rubber wheel abrasion method. The wear resistance is quantified by measuring the mass losses or thickness reductions of the coating before and after the abrasion.

Bending property testing: Tests the integrity of the coating after the cold bending or hot bending of the plastic-coated steel pipe. Usually, the specimen is bent at a certain angle around the mandrel of the specified diameter, and then whether the coating at the bending area has the cracks, peeling or stripping is checked. This testing is crucial for the on-site installation and processing.

Testing Scope

Plastic-coated steel pipes for the fire protection: Mainly testing their fire resistance, high-pressure resistance, coating flame retardancy and long-term water pressure resistance. This type of pipes needs to meet the standards such as GB/T 5135.20, especially focusing on the adhesion retention ability and thermal shock resistance of the coating under the high-temperature and high-pressure water flow impacts.

Plastic-coated steel pipes for the water supply and drainage: Focusing on the hygienic safety of the coating, resistance to the drinking water corrosion (chloride ion resistance and microorganism resistance) and long-term water immersion resistance. Need to meet the hygienic standards such as GB/T 17219, ensuring that the coating does not precipitate the harmful substances and affect the water quality.

Plastic-coated steel pipes for the mines: The testing emphasis is on the extreme wear resistance, impact resistance and the ability to resist the corrosion of the complex mine chemicals (such as the acidic mine water). Their coatings are usually thicker, with the extremely high adhesion requirements, to adapt to the harsh underground environments.

Plastic-coated steel pipes for the cable protection: Mainly testing the insulation resistance, voltage withstand strength and flame retardancy of the coating. It needs to be ensured that while the pipes protect the cables, their own coatings have the good electrical insulation, preventing the leakage or becoming the fire spreading channels.

Plastic-coated steel pipes for the chemical industry: According to the chemical characteristics of the conveyed media, the abilities of the coatings to resist the specific acids, alkalis, solvents or organic compounds are targeted for testing. The testing scope may involve the immersion tests of the various chemicals and long-term aging tests.

Testing of the differentiated internal and external coating types: Distinguishing the internal plastic-coated, external plastic-coated and internal-external plastic-coated steel pipes. The internal plastic-coated pipes focus on the medium resistance, wear resistance and hygienic properties of the inner wall coatings; the external plastic-coated pipes focus on the weather resistance, ultraviolet resistance and soil corrosion resistance of the outer wall coatings; the internal-external plastic-coated pipes need the comprehensive testing.

Plastic-coated pipes of the different base pipe materials: The testing scope covers the plastic-coated products with the different substrates such as the welded steel pipes, seamless steel pipes and galvanized steel pipes. The effects of the surface treatments (such as the sandblasting derusting grades) of the different substrates on the final properties (especially the adhesion) of the coatings and the corresponding testing method adjustments need to be focused on.

Testing Methods

Magnetic thickness measurement method: The magnetic induction or eddy current principles are used, and the digital coating thickness gauges are used for the non-destructive measurement of the non-magnetic coating thicknesses on the iron and steel substrates. Before the measurement, the calibration on the standard panels is needed. During the measurement, the multi-point measurements (usually not less than 6 points) need to be carried out on the pipe body surfaces and the average values are calculated, to evaluate the coating thickness uniformity.

Electric spark holiday detection method: This method utilizes the characteristics that the coating is non-conductive while the metal substrate is conductive. During the detection, the instrument probe moves on the coating surface. When the defects such as the pinholes are encountered, the high-voltage electric field breaks down the air to generate the electric sparks and alarms. The detection voltages need to be precisely set according to the coating thicknesses, ensuring that the defects can be detected without damaging the sound coatings.

Pull-off adhesion testing: The dedicated adhesives are used to bond the test dollies of the specified sizes to the coating surfaces. After the curing, a hydraulic or mechanical tensile machine is used to pull them off perpendicularly, measuring the maximum tensile forces required to peel the coatings from the substrates, in MPa. This method can obtain the quantitative adhesion data, with the objective and accurate results. It is the important laboratory testing method.

Salt spray testing method: The specimens are placed in the salt spray test chambers, simulating the marine or industrial atmospheric environments, and the neutral salt mist is continuously or intermittently sprayed for hundreds to thousands of hours. After the testing, the corrosion, blistering and adhesion degradation conditions of the coating surfaces are checked. It is the accelerated testing method of evaluating the corrosion-resistant service lives of the coatings.

High-temperature furnace testing method: Mainly used for the fire resistance testing of the plastic-coated steel pipes for the fire protection. The specimens are placed in the high-temperature furnaces, heated according to the standard time-temperature curves (such as the ISO 834 curve), and after the testing, whether the pipes maintain the structural integrity without the fire penetration is checked, and their performance of the continuous water supply within a certain time is evaluated.

Immersion testing method: The plastic-coated specimens are completely immersed in the chemical reagents (such as the acid, alkali and salt solutions) of the specified concentrations and temperatures or the actual conveyed media for the specified time (such as 30 days and 90 days). The appearance changes, mass changes, hardness changes and adhesion losses of the coatings are regularly observed and finally evaluated, judging their chemical stability.

Falling weight impact testing method: The falling weight of the specified mass (such as 1 kg) is used to freely fall from the set height (such as 1 m) to impact the plastic-coated steel pipe specimens or plate specimens fixed on the bases. After the impact, whether the coating at the impact points has the cracks or is peeled from the substrate is checked, to evaluate its ability to resist the instantaneous mechanical impacts.

Testing Standards

Domestic Standards:

1. GB/T 28897, Steel-plastic composite pipes

2. CJ/T 120, Coated steel pipes for water supply

3. GB/T 5135.20, Automatic sprinkler system — Part 20: Coated steel pipes

4. GB/T 17219, Standard for safety evaluation of equipment and protective materials for drinking water distribution systems

5. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

1. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

2. ISO 834, Fire resistance tests — Elements of building construction

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

Digital coating thickness gauge: One of the core equipment, used for the non-destructive fast measurement of the coating thicknesses. Divided into the magnetic induction type (for the iron and steel substrates) and eddy current type (for the non-ferrous metal substrates). The high-precision instruments have the resolutions reaching 1 μm, with the statistics, storage and computer connection functions, convenient for the data management and analysis.

Electric spark holiday detector: Used to detect the coating continuity defects. The equipment consists of a high-voltage generator, detection electrode and alarm device. Divided into the DC type and AC type, with the wide adjustable output voltage ranges (such as 0-35 kV), and equipped with the detection brushes of the various shapes to adapt to the testing of the different parts such as the flat surfaces, welds and elbows.

Adhesion tester: Usually referring to the hydraulic or mechanical pull-off testers. The instruments provide the steadily increasing tensile forces until the dollies are pulled off, and automatically record the maximum tensile force values. The supporting dollies, dedicated adhesives and cutting tools are the indispensable components of completing the standard testing.

Salt spray test chamber: Used to simulate the accelerated corrosion environments. The chamber body is made of the corrosion-resistant materials, equipped with the precision spray systems, saturation barrels, heating and temperature control systems. The various standard tests such as the neutral salt spray testing (NSS), acetic acid salt spray testing (AASS) and copper-accelerated acetic acid salt spray testing (CASS) can be carried out.

Impact testing machine: Used for the falling weight impact testing. The equipment includes the vertical guide tubes, releasable falling weights, specimen fixing devices and height scales. The impact heads of the falling weights are usually hemispherical or with the specified curvature radii, and their masses and falling heights need to strictly meet the requirements of the relevant product standards.

Universal material testing machine: Can be used for the bending testing, flattening testing and tensile adhesion testing of the plastic-coated steel pipes. By changing the different fixtures and indenters, the bending, compression or tensile loads are applied to the specimens, and the load-displacement curves are precisely recorded, evaluating the mechanical properties of the coating and the bonding strengths with the substrate.

High-pressure airless spraying experimental equipment: Although mainly used for the production, in the research and development and process verification stages, it is used to prepare the laboratory coating panels conforming to the specific process parameters, providing the standardized specimens for the subsequent various performance testing. It is the key equipment connecting the production processes and testing results.