Check Out 4 Crucial Coating and Qualification Procedures in the Specification of Three-Layer Polyethylene Coating for Pipes

Welcome to the Industrial Inspector site. Today, we will discuss the crucial coating and qualification procedures in the specification of three-layer polyethylene coating for pipes. First, it is important to highlight that inspectors in this field are corrosion and coating inspectors. This is a significant area as it aims to implement measures and procedures to increase the lifespan of metallic equipment, especially protecting them from oxidation.

Anticorrosive protection of pipelines is vital for the integrity and longevity of infrastructures, particularly in aggressive environments such as marine settings where equipment tends to oxidize more easily. A widely used method is the three-layer polyethylene coating (3LPE), which combines an epoxy layer, an adhesive layer, and a polyethylene layer, providing robust protection against corrosion and physical damage. However, before production begins, it is essential to perform a Procedure Qualification Test (PQT) to ensure that the plant, materials, and coating procedures result in a final product that meets the required material properties, relevant standards, specifications, and manufacturer's recommendations. Check out 4 crucial coating and qualification stages, where each stage includes certain procedures, totaling 21.

1. Initial Qualification Procedures

Tube Preheating: During the PQT, it is crucial to record temperature variations due to tube entry temperature, line speed variation, and interruptions. The proper functioning of the tube temperature monitoring and recording system must be demonstrated to ensure consistency during regular production.

Surface Preparation: Surface preparation is a very critical stage that includes visual inspection and verification of cleanliness. It is important to emphasize that the surface must be properly cleaned and prepared, as an inadequately prepared surface can compromise the coating's adhesion, thereby affecting the equipment's coating quality.

Coating Application: The tube surface temperature before applying the epoxy powder, line speed, and the temperatures of the copolymer adhesive and polyethylene must be established during the PQT. These values must be maintained during regular production to ensure the coating's quality and uniformity.

2. Qualification Tests of the Applied Coating

Epoxy Layer Thickness: The epoxy layer thickness must be checked every meter at 3, 6, 9, and 12 o'clock positions. The minimum acceptable thickness is 150 μm.

Adhesive Layer Thickness: The adhesive layer thickness must be checked at the same positions and spacing, with a minimum thickness of 150 μm.

Adhesion Test: The adhesion of the epoxy (FBE) must be determined separately at room temperature in two locations using the "St Andrews Cross" method. The epoxy layer must resist peeling when attempts are made to lift the coating with a sharp knife.

Thermal Analysis: A thermal analysis test must be conducted on a tube as specified in the CSA Z.245.20 standard or any other standard specified by the customer.

3. Tests on Tubes with All Three Layers

Peel Resistance: Five tubes must be selected for peel resistance tests. Two peel resistance tests must be performed on each tube at specified temperatures, without failures. The minimum acceptable force is 25 N/cm at 50ºC ± 2ºC and 20 N/cm at 80ºC ± 2ºC.

Impact Resistance: Five tubes must be tested for impact resistance, meeting the specified requirements. The acceptance criterion is the absence of failures at 11 J at 23ºC ± 2ºC.

Indentation Hardness: Indentation hardness must be tested on five samples at 23ºC ± 2ºC and 80ºC ± 2ºC. If a sample fails, the test must be repeated on four additional samples, and none of them should fail. The acceptance criteria are as specified by the customer.

Elongation at Break: Six samples from five tubes must be tested for elongation at break, with an acceptance criterion of 300% minimum.

Cathodic Disbondment Test: Two cathodic disbondment tests must be performed on two tubes from the total lot, one at 65°C for 48 hours and another at 80°C for 48 hours.

Coating Integrity Inspection: All tubes must be visually inspected to ensure the coating has no air bubbles, wrinkles, irregularities, discontinuities, or separation between the PE/adhesive layers. The internal surface of the tube must also be inspected for any foreign material or particles.

Coating Thickness Measurement: The coating thickness must be measured using a non-destructive gauge. At least 12 measurements (three locations per tube, equally spaced along the length of the tube) must be conducted on each tube. The minimum acceptable thickness of the complete coating is 2.9 mm.

4. Inspection and Testing in Regular Production

Visual Inspection: The resulting coating must have uniform gloss and appearance without air bubbles, wrinkles, irregularities, discontinuities, or separation between layers. The internal surface of the tube must also be inspected before coating to ensure the absence of foreign materials.

Holiday Detection: After verifying the thickness and cleaning the cut ends, each coated pipe length must be checked for holiday detection using a high-voltage holiday detector. If a defect is found, the defective area must be marked for repair.

Peel Resistance Test: The peel resistance test must be performed on coated pipe samples as per customer specifications. The minimum acceptable force is 25 N/cm at 50ºC ± 2ºC and 20 N/cm at 80ºC ± 2ºC.

Impact Resistance Test: Impact resistance must be tested on coated pipes to ensure the coating can withstand significant impacts without electrical failures. The test area must show no failures when checked for holiday detection after impact.

Indentation Hardness Test: Indentation hardness must be measured to ensure the coating can resist penetration under specified loads. The acceptance criteria are based on customer specifications.

Thermal Analysis: The thermal analysis test must be periodically conducted as specified by the customer to ensure the thermal stability of the coating.

Cathodic Disbondment Test: The cathodic disbondment test must be periodically conducted to evaluate the coating's resistance to disbondment under cathodic conditions. The acceptance criteria are defined by customer specifications.

The coating and qualification procedures described ensure that pipes coated with three-layer polyethylene meet the highest quality standards. Through thorough inspections, precise measurements, and comprehensive testing methods, it is possible to ensure that each coated pipe provides reliable protection against corrosion and physical damage, thus prolonging the lifespan of infrastructures and reducing the need for constant maintenance and repairs. Implementing these qualification procedures and regular tests is essential to achieving excellence in coated pipe production.

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