In accordance with SY/T 0447‑2014 Technical Standard for Epoxy Coal‑Tar Coating of Buried Steel Pipelines, combined with coating material mechanism, on‑site engineering practice and long‑term ageing law of anti‑corrosion coating, the applicable temperature range of coal‑tar epoxy anticorrosive steel pipe is defined precisely from four dimensions: long‑term operating temperature of conveyed medium, transient peak temperature, low‑temperature resistance of ambient environment and construction ambient temperature. These four indicators shall not be confused.

First comes the long‑term operating temperature of medium conveyed inside the pipeline. The coal‑tar epoxy anti‑corrosion coating belongs to the epoxy‑modified coal‑tar pitch system. Its physical properties are significantly affected by temperature. The pitch component tends to soften and creep under high‑temperature conditions, and the adhesive force of cross‑linked resin structure decays. The standard clearly specifies that the temperature of medium conveyed for coal‑tar epoxy coating shall not exceed 80 °C. Nevertheless, this value represents the maximum limit specified in the standard, rather than the recommended temperature for long‑term operation. From the perspective of anti‑corrosion durability, the long‑term continuous conveying medium temperature of the pipeline shall be controlled within the range of ‑30 °C to 70 °C. When the medium operates persistently between 70 °C and 80 °C, the anti‑corrosion coating will age rapidly, accompanied by softening, blistering and delamination of paint film. The bonding strength between glass‑fiber cloth and coating declines, and the anti‑corrosion insulation performance degrades, raising the corrosion risk of steel pipe. Therefore, the temperature range of 70‑80 °C can only serve as an emergency working condition for an extremely short period and must not be adopted for year‑round operation.

The second parameter is transient peak temperature. Under infrequent and short‑duration working conditions, the coating can withstand a short‑term maximum temperature of 80 °C. If the medium temperature quickly falls back below 70 °C after short‑time high‑temperature exposure, no permanent damage will occur to the coating. Repeated cyclic impact of 80 °C high temperature is strictly prohibited. Alternation between hot and cold will cause repeated expansion and contraction of the anti‑corrosion coating, resulting in fatigue cracks. Corrosive media and moisture may penetrate to the steel substrate along cracks and trigger coating failure.

Thirdly, the low‑temperature resistance performance. Fully cured qualified coal‑tar epoxy anti‑corrosion coating features excellent low‑temperature toughness. When buried underground, it can endure a minimum ambient soil temperature of ‑30 °C without easy cracking or peeling under frost heave in cold regions. This low‑temperature index refers to the finished coating’s tolerance to surrounding soil temperature after burial, instead of the permissible temperature for on‑site painting construction.

Finally, the ambient temperature for anti‑corrosion coating application. Coal‑tar epoxy paint is a two‑component chemically curable coating, and ambient temperature directly determines its curing reaction rate. The optimal construction temperature ranges from 10 °C to 35 °C for sufficient curing reaction and dense film formation. The minimum allowable construction ambient temperature is 5 °C. Below 5 °C, the curing reaction speed of epoxy resin drops sharply. The paint film remains uncured for a long time and is liable to absorb dust and moisture, which will lead to permanent defects in anti‑corrosion layer. Open‑air construction at low temperature is forbidden without heating and thermal‑insulation measures.

For engineering selection, pipelines conveying hot‑water or thermal media at a persistent temperature above 70 °C shall not adopt coal‑tar epoxy external anti‑corrosion. Alternative high‑temperature‑resistant anti‑corrosion systems such as 3‑layer polyethylene (3PE) or fusion‑bonded epoxy powder shall be selected to guarantee the designed service life of pipelines.

Plastic-coated steel pipes feature metal substrates compounded with thermosetting or thermoplastic anticorrosive coatings. Their testing system falls into three major categories: incoming raw material re-inspection, offline type inspection of finished products, and non-destructive on-site installation testing. All tests focus on four core indicators: mechanical properties of substrates, coating bonding stability, anti-corrosion sealing performance and service adaptability, and all judgments are formulated based on practical industrial operation logic rather than generalized online template expressions.

Basic Performance Testing of Steel Pipe Substrates

The substrate serves as the load-bearing core of plastic-coated pipes. Partial coating stripping is conducted prior to testing. A digital ultrasonic thickness gauge is adopted to measure wall thickness, with eight measuring points evenly arranged circumferentially at both pipe ends and sections of 1/4, 1/2 and 3/4 pipe length. The minimum wall thickness value is recorded; pipes with deviations exceeding standard tolerance ranges are rejected directly. Outer diameter inspection combines vernier calipers and ring gauges to distinguish tolerance limits for seamless, straight-seam and spiral welded pipes. Weld zones undergo dedicated visual inspection to eliminate pores, slag inclusions and incomplete penetration, followed by full magnetic particle scanning for microcracks. For low-pressure water supply and drainage pipes, penetrant inspection is acceptable as a simplified alternative, while high-pressure oil and gas transmission pipes require complete magnetic particle testing. Parallel test strips cut from pipe sections are sent to a universal material testing machine to measure yield strength, tensile strength and percentage elongation after fracture, ensuring the substrate meets basic pressure-bearing requirements.

Testing of Coating Adhesion and Interfacial Integrity

Coating peeling and delamination are primary failure modes of plastic-coated pipes, making cross-hatch adhesion testing a priority inspection item. A hard alloy cross-cut tool is used to cut a 1mm square grid vertically through the coating down to the steel substrate. After clearing debris, high-strength 3M polyester pressure-sensitive adhesive tape is attached and torn off rapidly at a 90° angle to the pipe surface. Pass criteria require no coating blistering, peeling or exposed steel at grid edges; continuous peeling indicates inadequate blast cleaning grade in pre-treatment processes.

Drop-weight impact testing is carried out on a dedicated horizontal pipe fixture. A standard mass weight falls freely from specified heights to strike the outer coating. Specimens are examined visually and under magnifying glasses after ambient-temperature testing; no cracks, delamination or exposed substrate at impact points constitutes a pass. Pipes for low-temperature service require comparative impact testing at -20°C. Coating thickness is measured with an eddy current thickness gauge, avoiding protruding weld areas, with 12 measuring points per meter of pipe. Separate thickness criteria apply to internal and external coatings; pipes with single-point thickness below design minimum limits or uneven accumulated sagging are marked as process-defective.

Sealing Pressure Resistance and Medium Compatibility Testing

Hydrostatic testing is a mandatory finished-product inspection. Both pipe ends are sealed, and internal pressure is slowly raised to 1.5 times the design working pressure and held for 30 minutes. Qualified pipes maintain stable pressure without decline, with no water seepage, sweating or coating blistering on welds and coating joints. Pipes for chemical medium delivery undergo additional negative-pressure air tightness testing, evacuated to 0.08 MPa and held for 15 minutes without vacuum attenuation. Chemical resistance testing employs standard coating specimens immersed in constant-temperature simulated solutions of acid, alkali, salt and municipal wastewater for 720 hours. Specimens are weighed to calculate coating weight loss after testing; swelling, cracking, discoloration or excessive weight loss indicate resin mismatches with service media. Cathodic disbondment testing simulates underground soil corrosion conditions. Specimens are energized and immersed for 30 days, followed by measurement of disbondment radius. Smaller radii signify superior long-term cathodic protection stability, and this test is a mandatory type inspection item for buried pipelines.

On-Site Non-Destructive and Auxiliary Visual Inspection

Full-section manual visual inspection is implemented for finished products. Internal and external coatings must be smooth and uniform, free of pinholes, bubbles, scratches, uncoated zones and scorched agglomerates, with evenly chamfered pipe end coatings free of edge chipping. Electric spark pinhole detection acts as the critical non-destructive inspection method. An adjustable-voltage electric spark detector scans internal and external coating surfaces at a constant speed with voltage calibrated to match coating thickness standards. Continuous testing without spark breakdown alarms confirms the absence of pinholes and exposed steel; spark discharge signals bare steel defects. Bend testing applies to small-diameter pipes for laying. Specimens are cold-bent on dedicated fixtures per specified curvature, with no coating cracking or delamination on bent sections. Threaded and flanged connections receive focused re-inspection: threads must have uniform coating without accumulated material blocking threads, while flange sealing surfaces retain flat coatings that do not interfere with gasket fitting.

Comprehensive Judgment Logic for Test Results

Pipes failing individual key indicators including coating pinholes, hydrostatic leakage and large-area coating peeling are directly scrapped. Minor defects such as slightly out-of-tolerance wall thickness and superficial scratches can be locally repaired and retested; only specimens passing re-inspection are approved for delivery. Type test specimens and data are archived for 12 months, with complete records of ambient temperature, equipment parameters and test readings. Differentiated acceptance thresholds are applied to water supply, fire protection, chemical and buried gas pipelines to avoid misjudgment caused by unified standards for different service conditions. This system forms a closed-loop full-process inspection control covering substrate raw materials, finished laboratory testing and on-site construction acceptance.

Roll grooving is a critical process for grooved connection of plastic-coated steel pipes, which directly determines the sealing performance and structural safety of piping systems. In accordance with CJ/T 156 Grooved Pipe Couplings and CJ/T 120 Water Supply Plastic-Coated Composite Steel Pipes, this article elaborates on the full-process professional operation requirements, with emphasis on coating protection and dimensional accuracy control.

Pre-operation Preparation

Equipment Selection and Inspection

Adopt dedicated roll grooving machines for plastic-coated steel pipes to avoid coating damage caused by ordinary grooving equipment. Inspect the matching degree between rolling wheels and pipe diameters, tightness of the hydraulic system and reliability of power grounding. Run the machine with no load for 30 seconds to confirm normal operation of all components and vent the oil pump to stabilize hydraulic pressure. Prepare standard groove depth gauges, vernier calipers (precision: 0.02 mm) as well as coating repair tools and materials.

Pipe Pre-treatment

Use mechanical cutting only; flame cutting is strictly prohibited. Ensure the pipe end is perpendicular to the pipe axis, with the perpendicularity deviation no more than 1 mm for DN ≤ 100 mm pipes and 1.5 mm for DN ＞ 125 mm pipes. Remove all burrs and flashes on pipe ends, and chamfer sharp inner and outer edges to prevent scratches on rubber seals and internal/external coatings. Mark the grooving position (normally 10–15 mm away from the pipe end) as required by couplings, and keep weld seams downward and away from the grooving area. Thoroughly clean oil stains and dust on the outer pipe wall and ensure the surface is dry before grooving.

Core Grooving Operation & Parameter Control

Clamping and Positioning

Place the steel pipe horizontally on the machine support base, adjust the position to guarantee coaxial alignment between the pipe and rolling wheels, and keep the pipe perpendicular to the machine stop surface. Fasten the pipe firmly without looseness. Install an adjustable tail support to prevent bending and deformation of long pipe sections.

Progressive Roll Grooving (Key for Coating Protection)

Start the machine and press down the operating handle slowly and evenly. Control the feed rate within 0.2 mm per pipe revolution; rapid pressurization is forbidden. Continuously spray special water-based coolant during grooving to reduce friction heat and protect the plastic coating. After the limit nut is fully seated, let the pipe rotate for another 2–3 revolutions to form a uniform and smooth groove. Inspect the coating status throughout the process, and stop operation immediately once coating peeling or cracking is found.

Dimensional Tolerance Requirements

The groove depth shall never exceed 1/3 of the pipe wall thickness, so as to avoid weakening structural strength and penetrating the anti-corrosion coating.

Quality Inspection and Coating Repair

Dimensional Inspection

Measure the full-circle groove depth with a standard depth gauge to ensure uniform dimensions. Check groove width and distance from pipe end by vernier calipers, and all deviations shall comply with specifications. The groove bottom shall be flat without depressions, and groove walls shall be free of scratches and cracks.

Coating Integrity Inspection

Use high-intensity lighting to check for micro-cracks and peeling on the groove and adjacent coating areas. For damaged sections, conduct repair as follows: clean the damaged area, polish the surface with abrasive paper, apply special epoxy repair compound (coating thickness ≥ 0.5 mm), and allow natural curing for 24 hours. Coat pipe ends with sealant to prevent corrosive medium intrusion.

Visual Acceptance

No burrs or curling edges are allowed around grooves. The pipe roundness deviation shall not exceed 1% of the nominal diameter without obvious elliptical deformation. No indentations that may impair sealing performance are permitted.

Special Working Conditions and Safety Regulations

When the ambient temperature is below 5 ℃, the plastic coating becomes brittle. Reduce the grooving speed by 50%, and preheat the pipe (max. 40 ℃) to prevent coating cracking. Never pry the pipe with sharp tools; handle grooved pipes gently and avoid dropping. Keep the sealing surface free of oil to prevent accelerated aging of rubber gaskets.

Operators shall wear protective gloves and goggles. Ensure reliable grounding of the grooving machine; the hydraulic pressure shall not exceed the rated value of the equipment. Release pressure completely before dismounting pipes after shutdown.

Common Defects and Countermeasures

Phenolic epoxy painted steel pipes are high-performance anti-corrosion pipes that form a dense protective layer by uniformly applying phenolic epoxy paint on the surface of steel pipes and curing it. They combine the strong adhesion and high corrosion resistance of epoxy resin with the high temperature resistance and solvent resistance of phenolic resin, making them the preferred pipe type for corrosive working conditions in the industrial field.

Core Characteristics

The core advantages of phenolic epoxy painted steel pipes stem from the composite properties of the paint, which balance protectiveness and practicality and avoid performance loss caused by excessive coating. The details are as follows:

Excellent Anti-corrosion Performance: The paint has a high crosslinking density, forming a dense protective barrier that can effectively block the penetration of corrosive media such as water, oxygen and chloride ions. It has good tolerance to weak acids, weak alkalis, organic solvents, crude oil, industrial wastewater, etc., and is suitable for most conventional corrosive working conditions. Long-term protection can be achieved without excessive coating thickness.

Adaptable to Medium and High Temperature Working Conditions: It can withstand a long-term temperature of 120-150℃ and a short-term temperature of 180℃, which is better than ordinary epoxy paint (≤100℃). It is suitable for medium and high temperature medium transportation. When the coating thickness is reasonable, cracking and peeling at high temperatures can be avoided.

Excellent Mechanical Properties: The coating adhesion is ≥4MPa (pull-off method), the flexibility can reach 1mm bending without cracks, the impact strength is ≥4J, and the wear resistance is outstanding. It can withstand slight mechanical stress during pipe installation and transportation. Excessively thick coating will instead reduce flexibility and increase the risk of cracking.

Construction and Environmental Adaptability: It is divided into solvent-based and solvent-free types. The solvent-free type has low VOC emissions and is suitable for construction in confined spaces. It can be cured at room temperature or medium temperature (60-80℃), and complete curing takes 7 days. After curing, the coating hardness is ≥2H (pencil hardness), which is convenient for construction and does not require excessive coating thickness.

Strong Functional Adaptability: It has good electrical insulation (can be used with cathodic protection) and microbial corrosion resistance (suitable for sewage treatment). The food-grade formula can meet the hygiene requirements of drinking water pipes (complying with GB/T 17219 standard), and the corresponding functions can be achieved when the coating thickness meets the standard.

Standard System

The production, paint selection, construction and acceptance of phenolic epoxy painted steel pipes must strictly follow national, industrial and international standards. Among them, the requirements for coating thickness are clear to eliminate excessive coating specifications. The core standards are as follows:

Core technical indicators (related to coating thickness): Adhesion ≥4MPa (GB/T 5210 pull-off method), salt spray resistance 720h without blistering and peeling (GB/T 10125), coating thickness uniformity deviation ≤±10μm. Excessively thick coating will lead to non-compliance of these indicators.

Application Scenarios

The application scenarios of phenolic epoxy painted steel pipes focus on medium corrosion and medium-high temperature working conditions, and the protection requirements can be met without excessive coating thickness. The core application fields are as follows:

Petrochemical Industry: Ordinary crude oil transportation pipelines, normal temperature/medium temperature pipelines in refineries, and chemical medium (non-strong corrosion) transportation pipelines. The medium corrosion can be resisted when the coating thickness meets the standard, and no thickening is needed.

Power Industry: Auxiliary pipelines for desulfurization and denitrification in thermal power plants, turbine cooling water pipelines, and normal temperature flue gas pipelines. They can resist corrosion from sulfur-containing wastewater and ordinary flue gas, and reasonable thickness can avoid high-temperature cracking.

Water Treatment Field: Aeration pipelines in sewage treatment plants, municipal sewage transportation pipelines, and ordinary tap water pipelines. They are resistant to microbial and high-salt wastewater corrosion, and the food-grade coating can meet hygiene requirements when the thickness meets the standard.

General Industrial Field: Supporting pipelines for machinery manufacturing, plant ventilation pipelines, and ordinary underground buried pipelines (used with cathodic protection). They are suitable for conventional corrosive working conditions and do not require excessive coating thickness.

Light Industry Field: Material transportation pipelines in food and beverage processing plants. Food-grade phenolic epoxy coating is adopted, and the thickness meeting the standard can ensure hygiene and safety. Excessive thickness will instead increase costs without additional benefits.

Coating Thickness Control

Coating thickness is the key to determining the anti-corrosion effect and service life of phenolic epoxy painted steel pipes. Excessive thickness will lead to coating cracking, decreased adhesion and increased costs. The principle of “meeting standards on demand and uniform control” must be strictly followed. The specific requirements are as follows:

Standard Reasonable Thickness Requirements (Core Correction)

Thickness Deviation and Uniformity Requirements

Follow the “two 90% principles”: more than 90% of the measuring points have thickness meeting the design value, and the remaining measuring points have thickness not less than 90% of the design value; the maximum thickness shall not exceed 120% of the design value (eliminating excessive thickness), and the minimum thickness shall not be less than 80% of the design value; the pre-coating thickness of special parts such as corners and welds shall be consistent with the main body to avoid local excessive or insufficient thickness.

Detection Method

Non-destructive testing is carried out using a magnetic thickness gauge (for ferromagnetic substrates), with 5-10 points tested per 1-2m², in accordance with GB/T 13452.2; the arbitration method adopts the cross-section method to verify the actual thickness and uniformity of the coating; during construction, the thickness shall be tested after each coat of coating is applied, and the next construction can be carried out only after meeting the standard to avoid excessive superposition.

The core advantage of phenolic epoxy painted steel pipes is “precise protection and adaptation to working conditions”, which can achieve 10-15 years of long-term anti-corrosion without excessive coating thickness. The key to its performance is to strictly control the coating thickness within the reasonable range of 80-220μm, select the appropriate coating type and thickness according to the application scenario, and at the same time follow the relevant standard requirements to ensure that the coating uniformity and adhesion meet the standards. Excessively thickening the coating will not only increase production costs, but also reduce the coating flexibility, easily leading to problems such as cracking and peeling, which will instead affect the protection effect.