Hot-dip Galvanized Steel Pipe Testing

Hot-dip galvanized steel pipe inspection mainly conducts scientific testing and analysis on the zinc layer thickness testing, zinc layer adhesion, appearance quality testing, uniformity, chemical composition analysis and mechanical property 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

Zinc layer thickness testing: This is the core indicator of evaluating the anti-corrosion performance of the hot-dip galvanized steel pipe. The magnetic thickness gauges are usually adopted to carry out the multi-point measurements at the specified positions of the steel pipe, to obtain the average thicknesses and minimum local thicknesses of the zinc layers. The testing needs to be carried out in accordance with the standards such as GB/T 13912 or ISO 1461, ensuring that the zinc layers can reach the expected anti-corrosion years and service lives.

Zinc layer adhesion (bonding force) testing: Used to evaluate the bonding firmness between the zinc layer and the steel pipe substrate. The common methods include the hammer testing and bending testing. The hammer testing strikes the galvanized layer with the specific tools, observing whether it has the cracks or spalling; the bending testing bends the specimen to the specified angle, checking whether the zinc layer at the bending area has the peeling or stripping.

Appearance quality testing: Through the visual inspection or low-power magnifying glasses, the surface conditions of the steel pipe are checked. The qualified galvanized layers should be continuous and complete, and the minor defects not affecting the anti-corrosion performance are allowed, such as the slight draining runs, colour differences or a small number of zinc nodules. However, there should be no missed plating, blistering, black spots, rough burrs and the impurity attachments that may damage the coating.

Uniformity (copper sulfate testing) testing: This test is used to qualitatively evaluate the uniformity of the zinc layers. The specimen is immersed in the copper sulfate solution of the specified concentration. After the specified times of immersions, whether the substrate metal (iron) is exposed due to the too thin or uneven zinc layers and produces the red displacement copper is observed, thereby judging the continuity of the coating.

Chemical composition analysis: The chemical composition analysis is carried out on the steel pipe substrate material, ensuring that it meets the corresponding steel grade standards (such as Q235 and Q345). At the same time, the compositions of the zinc ingot raw materials can also be tested, controlling the contents of the elements such as the lead, aluminium and tin, which affect the zinc liquid fluidity and coating quality.

Mechanical property testing: Although mainly targeting the steel pipe substrate, the galvanizing process (hot dipping and cooling) may have the minor effects on the material properties. Therefore, when necessary, the samples need to be taken from the galvanized steel pipes for the tensile testing and bending testing, verifying whether their tensile strengths, yield strengths and elongation rates still meet the standard requirements.

Testing Scope

Steel pipe substrate types and specifications: The testing is applicable to the seamless steel pipes and welded steel pipes (such as the longitudinally welded pipes and spiral welded pipes) made of the various carbon structural steels and low-alloy structural steels. The testing needs to cover the steel pipes of the different nominal diameters (DN), wall thicknesses and shapes, ensuring that the standards are applicable to all the specification products.

Galvanizing process control: The testing scope extends to the key processes of the hot-dip galvanizing production such as the pre-treatment (pickling and fluxing), hot-dip galvanizing (zinc liquid temperatures and dipping times) and post-treatment (cooling and passivation). The monitoring of the process parameters is the premise of guaranteeing the final coating quality.

Zinc ingots and fluxes: As the main raw materials, the purity (usually not lower than 99.995% zinc) and chemical compositions of the zinc ingots directly affect the coating properties. The concentrations, pH values and iron ion contents of the fluxes (usually the mixed solutions of the zinc chloride and ammonium chloride) also need to be regularly tested, to prevent the missed plating defects.

Finished pipe materials: This is the core object of the testing, including the straight pipes, fixed-length pipes and galvanized pipe fittings after the processing (such as the threading and grooving). The testing needs to ensure that after the ex-factory, transportation and on-site installation of the finished products, the quality of their galvanized layers still meets the standards.

Service environment adaptability: For the different use environments (such as the atmospheric, soil, marine or industrial corrosion environments), the testing needs to evaluate whether the corrosion resistance of the zinc layers matches. The accelerated corrosion tests such as the salt spray testing and cyclic corrosion testing can be used for the simulation verification.

Key parts of the engineering applications: In the engineering fields such as the construction, water delivery, electric power and communications, the hot-dip galvanized steel pipes used for the structural load-bearing, long-term exposure or the key parts at the high corrosion risks should be subjected to the stricter sampling testing and on-site re-inspection.

Testing Methods

Magnetic method thickness measurement: The most commonly used non-destructive testing method. The magnetic flux or magnetic attraction changes between the probe cores and the steel pipe substrate (ferromagnetic) are used to measure the thicknesses of the non-magnetic zinc layers on the surfaces. The operation is simple and fast, but needs to be regularly calibrated, and has certain requirements for the curvature and roughness of the measured surfaces.

Metallographic microscope method: A destructive precise measurement method. The specimens are cut from the steel pipe, and the metallographic specimens are prepared after the mounting, grinding, polishing and etching. The thicknesses of the various phases of the zinc layers (such as the pure zinc layers and alloy layers) are directly observed and measured under the microscope. The results are accurate and reliable, often used as the arbitration method.

Hammer testing method: A qualitative method of evaluating the adhesion. A dedicated hammer weighing about 0.25 kg is used to strike the specimen surface with the specific forces and frequencies, or a chisel is used to chisel a notch on the zinc layer, observing whether the zinc layer is peeled from the substrate. This method is simple and intuitive, suitable for the fast on-site judgement.

Bending testing method: Another quantitative or semi-quantitative method of evaluating the adhesion. The specimen is bent 90 degrees or 180 degrees around the mandrel of the specified diameter, and then whether the zinc layer on the outer surface of the bending area has the peeling or spalling is checked with the naked eyes or 10 times magnifying glasses. The mandrel diameters are usually related to the specimen thicknesses.

Copper sulfate testing method: The classic chemical method of evaluating the uniformity of the zinc layers. The cleaned specimen is immersed in the temperature-controlled copper sulfate solution for 60 seconds, taken out, washed with water and observed. This process is repeated until the substrate metal is exposed and the red displacement copper appears. The immersion times are recorded, and the standards usually require not less than 4 or 5 times.

Salt spray testing method: An accelerated corrosion testing, used to evaluate and compare the corrosion resistance of the galvanized layers. The specimen is placed in a closed salt spray test chamber, and the neutral or acidic salt mist is continuously or intermittently sprayed, simulating the marine or industrial atmospheric environments. The corrosion resistance is evaluated by observing the times of the red rust appearance.

Testing Standards

Domestic Standards:

1. GB/T 13912, Metallic coatings — Hot dip galvanized coatings on fabricated iron and steel articles — Specifications and test methods

2. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

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

International Standards:

1. ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles — Specifications and test methods

2. ISO 1460, Metallic coatings — Hot dip galvanized coatings on ferrous materials — Gravimetric determination of the mass per unit area

3. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

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

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: The core equipment, divided into the magnetic thickness gauges and eddy current thickness gauges. For the hot-dip galvanized steel pipes, the instruments with the magnetic principle should be selected. The equipment should have the data statistics, storage and transmission functions, and the probes need to be selected with the suitable shapes according to the pipe diameters, and regularly calibrated with the standard foils or standard substrates.

Metallographic sample preparation and microscope system: Including the cutting machines, mounting machines, grinding and polishing machines, metallographic microscopes and image analysis software. Used to prepare the high-precision coating cross-section samples, and through the microscopic observation and software measurement, precisely analyse the total thicknesses of the zinc layers and the thicknesses and structures of the various alloy sub-layers.

Mechanical property testing machine: Mainly the universal material testing machines, used for the mechanical property tests such as the tensile, bending and flattening of the galvanized steel pipe substrates. The equipment should have the high-precision load and displacement transducers, and meet the requirements of the testing method standards such as GB/T 228.1.

Chemical analysis instruments: Including the direct-reading spectrometers (OES), X-ray fluorescence spectrometers (XRF) or carbon-sulphur analysers. Used to quickly and accurately analyse the chemical compositions of the steel pipe substrates and zinc ingots, ensuring that the raw materials meet the standard provisions and controlling the quality from the source.

Environmental corrosion test chambers: Such as the salt spray test chambers, cyclic corrosion test chambers and damp heat test chambers. These equipment can simulate and strengthen the corrosion factors of the natural environments, used to evaluate the long-term corrosion resistance and life prediction of the hot-dip galvanized steel pipes under the different climatic conditions.

Dedicated testing tools and measuring tools: Including the dedicated hammers for the hammer testing, bending testing machines or pipe benders, constant-temperature water baths and glass containers required for the copper sulfate testing, lighting lamps and magnifying glasses for the appearance inspection, as well as the callipers, micrometers, thickness gauges and thread gauges for measuring the geometric dimensions of the steel pipes.