Galvanized Angle Steel Testing

Galvanized angle steel inspection mainly conducts scientific testing and analysis on the galvanized layer thickness, galvanized layer adhesion, galvanized layer uniformity, zinc layer chemical composition analysis, base material mechanical properties and appearance quality inspection, 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

Galvanized layer thickness: This is the key indicator of evaluating the anti-corrosion performance of the galvanized angle steel. The testing includes the local thicknesses and average thicknesses, usually measured with the magnetic thickness gauges or metallographic microscope methods, to ensure that the coating is sufficient to provide the long-term protection in the expected environments and prevent the rust of the base steel.

Galvanized layer adhesion: The item evaluating the bonding firmness between the galvanized layer and the angle steel substrate. The common methods include the bending testing, hammer testing or cross-cut testing. The good adhesion can prevent the coating peeling during the use, transportation or processing, ensuring the continuity of the anti-corrosion effects.

Galvanized layer uniformity: Checks the uniformity of the galvanized layer distribution on the angle steel surfaces, especially the parts prone to the weak coating areas such as the edges and sharp corners. The copper sulfate etching testing is usually adopted. Whether the iron is exposed after the specified times of etching is used to judge its uniformity and continuity.

Zinc layer chemical composition analysis: Analyses the contents of the zinc and other alloying elements (such as the aluminium, nickel, lead and iron) in the galvanized layer. This is crucial for distinguishing the pure zinc coatings and zinc alloy coatings (such as the zinc-aluminium coatings). The different compositions directly affect the corrosion resistance, workability and appearance of the coatings.

Base material mechanical properties: Referring to the properties of the angle steel substrate metal itself, including the tensile strength, yield strength, elongation after fracture and bending properties. These are the basis of the load-bearing capacity of the angle steel as the structural parts, and need to be tested before or after the galvanizing (considering the possible effects of the hot-dip galvanizing) according to the relevant material standards.

Appearance quality inspection: Through the visual inspection or low-power magnifying glasses, the surface conditions of the angle steel after the galvanizing are checked. The inspection items include the defects such as the missed plating, white rust, zinc nodules, burrs, uneven colours and mechanical damages, ensuring that the product meets the use and aesthetic requirements.

Dimension and shape deviations: Measures the geometric dimensions such as the side widths, side thicknesses, lengths, straightness and twist of the angle steel. The precise dimensions are the basis of ensuring its accurate installation and interchangeability in the steel structures, and need to meet the tolerance provisions in the product standards.

Testing Scope

Hot-dip galvanized angle steel: This is the most common type. The testing scope covers the whole process from the incoming raw materials to the ex-factory finished products. The emphases are on the coating thicknesses, uniformity, as well as the base material mechanical property changes (such as the hydrogen embrittlement tendencies) and angle steel deformations possibly caused by the high-temperature dipping.

Electroplated zinc angle steel: Mainly testing the coating thicknesses (usually thinner), adhesion, corrosion resistance (such as the neutral salt spray testing) and coating purity. Due to the low process temperatures, the porosity and bonding strengths of the coatings need to be additionally focused on.

Angle steels of the different specifications and materials: The testing is applicable to the various specifications from the small equilateral angle steels to the large unequal angle steels. At the same time, the base material materials (such as the low-alloy steels such as Q235B and Q355B) need to be considered, and the galvanizing process adaptability and final properties of the different materials need to be separately evaluated.

Galvanized post-treated workpieces: The galvanized angle steels after the post-treatments such as the passivation, oiling and painting are tested. The properties of the post-treatment films (such as the passivation film corrosion resistance), bonding forces with the zinc layers, and whether the effects on the subsequent processing (such as the welding and spraying) need to be evaluated.

In-service or failed parts: The galvanized angle steels that have been used for a period of time are tested, aiming to evaluate their remaining lives or analyse the corrosion failure causes. The testing emphases include the remaining coating thicknesses, corrosion product analysis, base material corrosion states and mechanical property degradation conditions.

Import and export trade commodities: The conformity testing is carried out according to the trade contracts, national mandatory standards or international common standards (such as ISO and ASTM), and the credible test reports are issued, to meet the customs supervision and customer acceptance requirements.

On-site sampling inspection of the steel structure projects: At the engineering sites such as the power transmission towers, communication towers and building steel structures, the sampling testing is carried out on the arrived galvanized angle steels. Usually focusing on the fast inspection of the coating thicknesses, adhesion and appearances, ensuring that the material qualities meet the engineering design requirements.

Testing Methods

Magnetic thickness measurement method: The most commonly used non-destructive thickness measurement method, based on the magnetic induction principle. The magnetic thickness gauges calibrated with the standard foils are used to measure at the sufficiently many points on the various angle steel surfaces (including the edges and corners), calculating the average thicknesses and minimum local thicknesses. The operation is simple and fast.

Metallographic microscope method: A destructive but extremely precise testing method. The specimens are cut from the angle steel, and the metallographic specimens are prepared after the mounting, grinding, polishing and etching. The thicknesses of the galvanized layer cross-sections are directly observed and measured under the microscope, and the results can be used as the arbitration basis.

Copper sulfate etching testing: Used for the qualitative inspection of the galvanized layer uniformity. The angle steel specimens are immersed in the copper sulfate solution of the specified concentration, etched for 1 minute each time, observing whether the iron substrate is exposed on the surfaces. The standards usually require that the iron is not exposed after the specified times of etching is qualified.

Bending testing: The main method of evaluating the galvanized layer adhesion. The angle steel specimens are bent to the specific angles (such as 90° or 180°) around the mandrels of the specified diameters, checking whether the galvanized layers on the outer sides of the bending areas have the peeling or spalling. The mandrel diameters are usually related to the specimen thicknesses.

Hammer testing: Another adhesion testing method. A dedicated hammer is used to strike the specific positions of the angle steel with the specific forces, judging the adhesion by checking whether the coating has the spalling or separation from the substrate. This method is more suitable for the fast on-site inspection.

Neutral salt spray testing: An accelerated corrosion testing, used to evaluate and compare the corrosion resistance of the galvanized layers. The specimens are placed in the closed salt spray chambers, continuously sprayed with the sodium chloride solution of a certain concentration, and the corrosion resistance is rated by observing the times of the red rust appearance.

Weighing method (gravimetric method): The galvanized layer weights per unit area (g/m²) are calculated by dissolving the galvanized layers and weighing the weight differences before and after the dissolution. This method has the high precision and is the reference method of the arbitration and calibration of the other thickness measurement methods, but is destructive.

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 1839, Test methods for mass of coatings on steel products — Gravimetric method

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

4. 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

Magnetic/eddy current thickness gauge: The core of the portable non-destructive testing equipment. The magnetic thickness gauges are used for the non-magnetic coatings (such as the zinc layers) on the iron and steel substrates; for the non-ferromagnetic substrates (such as the stainless steel angle steel galvanizing, relatively rare), the eddy current thickness gauges need to be used. The instruments need to be regularly calibrated with the standard thickness foils.

Metallographic microscope and image analysis system: Used to implement the metallographic method thickness measurement and observe the coating micro-structures. The system includes the microscope main bodies, image acquisition cameras and professional measurement and analysis software, which can precisely measure the coating thicknesses and analyse the bonding interface conditions between the coatings and substrates.

Salt spray corrosion test chamber: The accelerated corrosion testing equipment simulating the marine or industrial atmospheric environments. It can precisely control the chamber temperatures, humidities, salt mist sedimentation amounts and pH values, used for the long-term (such as hundreds to thousands of hours) corrosion resistance evaluation of the galvanized angle steel specimens.

Universal material testing machine: Used to test the mechanical properties of the angle steel substrates, such as the tensile testing (measuring the tensile strength, yield strength and elongation) and bending testing (measuring the coating adhesion and base material bending properties). The equipment needs to be equipped with the dedicated fixtures suitable for the angle steel shapes.

Spectrometer: Including the direct-reading spectrometers and X-ray fluorescence spectrometers, used to quickly and accurately analyse the base material compositions of the angle steel and the chemical compositions of the galvanized layers, ensuring that the materials meet the standard designation requirements and the coating alloy compositions reach the standards.

Coating adhesion tester: Such as the cross-cut testers and pull-off adhesion testers. The cross-cut testers combined with the tapes can carry out the qualitative adhesion testing; the pull-off testers can quantitatively determine the bonding strengths between the galvanized layers and the substrates, providing the more precise data.

Routine measuring and observation tools: Including the vernier callipers, micrometers, steel tape measures and right-angle rulers, used to precisely measure the dimension and shape deviations of the angle steel; as well as the magnifying glasses and observation lamp boxes for the naked eyes, used for the systematic inspection and recording of the product appearance quality.