GB/T 17456.2-2025Ductile iron pipes, fittings and accessories - External zinc-based coating - Part 2: Zinc rich paint with finishing layer (English PDF)

5.1 Substrate surface condition

5.1.1 The surface of the pipe component substrate shall be dry, free of dust, and free of any loosely adhering particles or foreign matter, such as oil or grease.

5.1.2 The zinc-rich paint coating may be applied directly to the outer surface of the annealed pipe component, or to the outer surface after sandblasting, shot blasting, or grinding, as determined by the manufacturer.

5.2 Coating method

5.2.1 The zinc-rich paint may be sprayed or brushed onto the outer surface of the pipe component.

5.2.2 The design and structure of the spraying equipment are not covered in this Document.

5.3 Coating requirements

5.3.1 The zinc-rich paint coating shall cover the outer surface of the pipe components and shall be free from defects such as exposed surfaces or poor adhesion.

5.3.2 When the mass of zinc-rich paint coating meets the requirements of 5.4, a spiral appearance is permissible.

5.3.3 The manufacturer shall specify in its product manual any abnormal conditions that do not affect the performance of the coating system.

5.3.4 For operational damage to the zinc-rich paint coating, the coating surface is considered qualified if the cumulative area of the damaged area per square meter does not exceed 5 cm² and the smaller side dimension of the damaged area is less than 5 mm. Larger area of damage shall be repaired according to the provisions of 5.5.

5.4 Coating mass

5.4.1 The mass of zinc-rich paint coatings determined according to 7.1 shall be graded as follows, with Grade I applied unless otherwise specified by the purchaser.

--- Grade I. Average mass no less than 150 g/m², local mass no less than 130 g/m²;

--- Grade II. Average mass no less than 235 g/m², local mass no less than 210 g/m²;

--- In highly corrosive soil environments, the supplier and purchaser may negotiate and determine a higher zinc-based coating mass.

NOTE. Zinc-rich paint coatings of 130 g/m², 150 g/m², 210 g/m², and 235 g/m² are equivalent to the masses of metallic zinc coating of 110 g/m², 130 g/m², 180 g/m², and 235 g/m², respectively.

5.4.2 Before fabricating the insulation layer according to GB/T 43492, a zinc-rich paint coating should be applied, with an average mass of no less than 70 g/m² and a minimum local mass of no less than 60 g/m². Depending on the manufacturer's production process or as agreed upon by both the supplier and the purchaser, a zinc-rich paint coating may not be necessary.

NOTE. 60 g/m² and 70 g/m² zinc-rich paint coatings are equivalent to the masses of metallic zinc coating of 50 g/m² and 60 g/m², respectively.

5.4.3 The manufacturer shall visually inspect the surface quality and uniformity of the coating on each pipe component and shall periodically test the quality of the zinc-rich paint coating according to the method in 7.1.

5.4.4 Factory mass control inspections shall ensure compliance with the requirements of this

Document. Factory mass control test shall be the sole determining factor for coating thickness acceptance. When on-site verification information is required, the supplier and the purchaser shall agree upon and agree to use one of the methods in Appendix A for inspection.

5.4.5 When selecting a coating based on soil characteristics, refer to Appendix B.

5.5 Coating repair

Uncoated areas, such as areas covered by the test piece, and areas with coating damage exceeding the limits specified in 5.3.4, may be repaired using a zinc-rich paint with a dry film zinc content (mass fraction) of no less than 85%. The repaired coating shall comply with the provisions of 5.3 and 5.4.

7.1 Zinc-rich paint coating mass

7.1.1 Before applying the zinc-rich paint coating, attach a rectangular test piece along the axial direction to the outer surface of the pipe component. The minimum size of the test piece shall be any of the following.

a) 250 mm × 100 mm;

b) 500 mm × 50 mm.

7.1.2 During applying the zinc-rich paint coating process, the density and thickness of the test piece shall not change, and its morphology shall be stable at the substrate temperature. It shall also serve as a substitute surface for coating thickness measurement.

7.1.3 Calculate the average mass (m) of the zinc-rich paint coating on the test piece according to Formula (1) based on the mass difference before and after coating.

Where.

m - Average mass of the zinc-rich paint coating on the test piece, in g, accurate to 0.1 g;

C - Correction factor for the difference between the surface roughness of the test piece and the surface roughness of the pipe component, depending on the test piece material;

m2 - Mass of the test piece after coating, in g, accurate to 0.1 g;

m1 - Mass of the test piece before coating, in g, accurate to 0.1 g;

A - Area of the test piece, in m2.

7.1.4 When a report is required to be issued, the C value shall be determined and specified by the manufacturer.

7.1.5 The mass (m2) of the test piece shall be measured after spraying and drying.

7.1.6 Visually inspect the surface quality and uniformity of the coating on the test piece. If the

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