Hot-dip Galvanized Cable Tray Testing

Hot-dip galvanized cable tray inspection mainly conducts scientific testing and analysis on the zinc coating thickness measurement, zinc coating uniformity inspection, zinc coating adhesion testing, appearance quality inspection, dimension and geometric tolerance testing and mechanical property verification, 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 coating thickness measurement: This is the core indicator of the quality assessment of the hot-dip galvanized cable tray. It is usually necessary to measure the total thickness of the zinc coating attached to the multiple representative areas of the tray surface (such as the planes, edges and weld positions), including the pure zinc layer and zinc-iron alloy layer. The measurement results need to meet the minimum local thickness and minimum average thickness requirements corresponding to the thicknesses of the steel components in the relevant standards (such as GB/T 13912).

Zinc coating uniformity inspection: This inspection aims to evaluate the uniformity of the zinc coating distribution at the various parts of the tray. The areas prone to the missed coating or excessively thin coatings, such as the edges, corners, welds and inner cavities, are emphatically focused on. The poor uniformity will affect the overall anti-corrosion performance, and is usually comprehensively judged through the appearance visual inspection combined with the thickness measurement data, requiring the coating to be continuous, complete and free of the missed coating.

Zinc coating adhesion testing: This testing is used to evaluate the bonding strength between the zinc coating and the steel substrate, preventing the coating from peeling off during the use. The common methods include the hammering testing and cross-cut testing. For example, a hammer of a certain weight is used to strike the specimen in a specific way, observing whether the zinc coating produces the peeling, stripping or lifting phenomena, thereby judging whether the adhesion meets the standard requirements.

Appearance quality inspection: The comprehensive visual inspection or low-power magnifying inspection of the tray surface is carried out. The main contents include: whether the surface is smooth, whether there are the missed coating, rough particles, zinc tumours, white rust (basic zinc oxide), dark spots and obvious flow marks. The excellent appearance usually means the good galvanizing process control.

Dimension and geometric tolerance testing: As a structural part, the geometric dimensions of the tray must meet the design drawings and installation requirements. The testing items include the length, width, height, diagonal difference, side perpendicularity, tray flatness and the positional tolerances of the connection holes, etc. These tolerances directly affect the assembly precision and overall structural stability of the tray.

Mechanical property verification: Although the hot-dip galvanizing is an anti-corrosion process, it is necessary to verify whether the process (especially the high-temperature zinc immersion) has the adverse effects on the mechanical properties of the tray base material. The load testing (such as the uniformly distributed load and concentrated load), deflection testing and the strength testing at the connections can be sampled, ensuring that its load-bearing capacity meets the use requirements.

Testing Scope

Trough-type trays: This is the most common type, and the testing needs to cover the bottoms, side edges and their connection flanges of the trays. The zinc coating thickness of the bottom planes, the coating uniformity at the transition arcs between the side edges and bottoms, and the zinc coating coverage at the edges of the punched holes are emphatically detected. These parts are prone to the coating defects due to the stress concentrations and fast heat dissipation.

Ladder-type trays: The testing objects include the side rails (side ribs) and rungs (ladder bars). For the side rails, the inner and outer surfaces and upper edges are emphatically focused on; for the rungs, the zinc coating thicknesses in the various directions of the cylindrical surfaces need to be detected, especially the junctions with the side rails by the welding or pressing. The zinc coating accumulation or excessive thinness is easy to occur at these places, and they are the weak links of the anti-corrosion.

Channel-type trays: As the fully enclosed structures, the testing scope includes the cover plates, channel bodies and internal spaces. In addition to the outer surfaces, the galvanizing quality of the inner cavities of the trays must be strictly inspected, because the exhaust and zinc flow of the enclosed spaces are relatively difficult during the galvanizing process, and the missed coating or uneven zinc coating problems are easy to occur.

Connectors and accessories: Including the various elbows, tees, crosses, reducer joints, supports, bolts and grounding wires. These parts are small in size and complex in shape, and are easy to stick together or have the uneven coatings during the galvanizing process. Their thread integrity, zinc coating coverage and installation matching dimensions need to be checked one by one.

Welded parts: The weld areas between the tray components are the top priority of the testing. It is necessary to check whether the weld surfaces are smooth, whether there are the blowholes and slag inclusions not filled by the zinc liquid, and whether the zinc coating structures and thicknesses in the weld heat-affected zones change due to the welding residual stresses, ensuring that the weld positions have the same corrosion resistance as the substrate.

Overall structural parts: For the large or combined trays, the overall testing needs to be carried out. In addition to the various components themselves, the matching conditions of the various connection surfaces after the assembly, and the conductive continuity of the grounding connections (the zinc coating has the good conductivity) also need to be checked, ensuring that the whole tray system meets the requirements in both the electrical and mechanical performances.

Testing Methods

Magnetic thickness measurement method: This is the most commonly used non-destructive testing method for measuring the zinc coating thickness. A magnetic thickness gauge is used, based on the magnetic induction or eddy current principle. Before the measurement, the instrument needs to be calibrated with the standard foils, and it is ensured that the measured substrate is the magnetic steel. During the measurement, the sufficient points need to be selected (usually not less than 10 points per square metre), avoiding the edges and welds, and the average and minimum values are taken for the assessment.

Metallographic microscope method: A destructive but extremely precise testing method. The specimens are cut from the samples, and after the mounting, grinding, polishing and etching, the metallographic specimens are made. Under a metallographic microscope, the total thickness of the zinc coating and the thicknesses of the various layers (such as the pure zinc layer and alloy layer) are directly observed and measured. This method is commonly used for the arbitration or the in-depth research of the zinc coating structures.

Hammering testing method: A traditional method for the qualitative evaluation of the zinc coating adhesion. A smooth hammer head with a weight of about 200 g is used to strike the grid on the zinc coating surface at the intervals of about 4 mm, observing whether the zinc coating peels off from the substrate. Or the pliers are used to repeatedly bend the specimen until the fracture, checking whether the zinc coating at the fracture peels or falls off. The method is simple and intuitive, but requires the empirical judgement.

Cross-cut testing method: Another quantitative/semi-quantitative adhesion testing method. A sharp cutting tool is used to cut the grids at the specified spacings on the zinc coating surface, and the cuts need to penetrate the zinc coating to the steel substrate. Then the adhesive tape is pasted on the cross-cut area and quickly torn off, and the adhesion grade is evaluated according to the peeling proportion of the zinc coating at the grid edges (such as the grades 0-5).

Copper sulfate testing method: A fast chemical method for inspecting the zinc coating uniformity. The copper sulfate solution of the specified concentration is dropped on the clean zinc coating surface, and the local zinc coating thickness is indirectly judged by calculating the number of times (immersion times) required for the solution to react with the zinc coating and change colour (from blue to dark). The more the times, the thicker the zinc coating at the position. This method has the high requirements for the operation standardization.

Appearance visual inspection method: Under the good and uniform lighting conditions (usually requiring the illuminance not lower than 300 lux), the inspector observes at a distance of about 0.5 m from the product with the normal vision (or corrected vision). The low-power magnifying glasses (such as 5-10 times) can be used for the auxiliary inspection of the fine defects. The inspection results are usually compared and judged with the standard samples or the limit samples agreed by both parties.

Testing Standards

Domestic Standards:

1. GB/T 13912, 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 229-2020, Metallic materials — Charpy pendulum impact test method

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

5. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

6. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

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

2. 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

Magnetic/eddy current thickness gauge: The core thickness measuring equipment. The range is required to cover above 0-300 μm, and the precision reaches ±(1-3)% of the reading or ±1.5 μm. The probe types (such as the flat-head probes and pointed-head probes) need to be selected according to the shapes of the measured trays (planes, curved surfaces and edges). The equipment needs to be regularly calibrated with the national reference materials or standard foils, ensuring the accurate and reliable data.

Metallographic microscope and sample preparation equipment: Used for the precision analysis. Including the precision cutting machines for the sampling, mounting machines, pre-grinding machines with the series of sandpapers from the coarse to fine, polishing machines, polishing agents and the chemical etchants for displaying the zinc coating structures (such as the nitric acid alcohol solutions). The microscope needs to be equipped with the eyepiece micrometers or image analysis software, to realize the precise thickness measurement.

Coating adhesion tester: The professional quantitative testing equipment, such as the cross-cut testers and pull-off adhesion testers. The cross-cut testers are equipped with the cemented carbide blades and spacing guides, ensuring the consistent cut spacings and depths. The pull-off testers determine the adhesion strength by pasting the test studs of the specified diameters on the coating surface and vertically pulling them off, with the results expressed in MPa, which is more precise.

Copper sulfate testing device: A set of the standardized chemical reagents and utensils. Including the analytical grade copper sulfate with the concentration required by the standards (such as the Appendix B of GB/T 13912), distilled water, dropping bottles, absorbent cotton and sandpapers. The test environment temperature needs to be controlled at 18±2 ℃, and the newly prepared solution needs to be used for each test, to ensure the comparability of the test results.

General measuring tools and testing bench: Used for the dimension and mechanical property testing. Including the high-precision vernier callipers, micrometers, right-angle rulers, feeler gauges and spirit levels; as well as the hydraulic or mechanical loading devices for the load testing, displacement sensors (for measuring the deflections) and data acquisition systems, to verify the load-bearing capacity and stiffness of the tray.

Environmental and auxiliary equipment: The infrastructure for guaranteeing the testing quality. Such as the standard light source boxes or illuminance meters, providing the stable and standard light source environments for the appearance inspection; ultrasonic cleaning machines, for the thorough cleaning of the oil stains and impurities on the specimen surfaces; and constant temperature and humidity chambers, for carrying out the testing under the specific environmental conditions or the sample state conditioning.