Galvanized Flat Steel Testing

Galvanized flat steel inspection mainly conducts scientific testing and analysis on the zinc layer thickness, zinc layer adhesion, zinc layer uniformity, chemical composition analysis, appearance and dimension inspection 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: This is the core indicator of evaluating the corrosion resistance of the galvanized flat steels. The magnetic thickness measurement method (for the iron matrices) or eddy current thickness measurement method (for the non-iron matrices) is usually used for the non-destructive measurement, and the measurement results are expressed in micrometers (μm). According to the use environments (such as the atmosphere, soil and seawater) and product standard requirements, the zinc layer thicknesses have the different lower limit provisions.

Zinc layer adhesion: This testing item aims to evaluate the bonding firmness between the zinc layers and steel matrices. The commonly used methods include the bending testing, hammering testing and cross-cut testing. Through the observation of whether the zinc layers at the bends or impacted and scratched positions have the peeling, stripping or detachment phenomena, whether their adhesion properties meet the use requirements is judged.

Zinc layer uniformity: The uniformity of the zinc layers is qualitatively tested through the copper sulfate testing (also known as the copper immersion testing). The specimens are immersed in the copper sulfate solutions of the specified concentrations, and the number and degrees of the copper precipitation on their surfaces are observed within the specified time. If the iron-red copper plating adhesion appears, it indicates that the zinc layers at that position are too thin or discontinuous.

Chemical composition analysis: The chemical compositions of the flat steel matrix materials are analysed, to verify whether they meet the corresponding designation standards (such as Q235 and Q355). The main tested elements include the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S), whose contents directly affect the mechanical properties and weldability of the material.

Appearance and dimension inspection: Whether the flat steel surfaces have the defects such as the missed plating, bubbles, nodules, burrs and rust spots is checked. At the same time, the tools such as the callipers, micrometers and tapes need to be used for the precise measurement of their widths, thicknesses, lengths and flatness, ensuring that they meet the drawing or standard tolerance requirements.

Mechanical property testing: According to the relevant standards, the tensile testing is carried out on the flat steel specimens, determining the key mechanical property indicators such as the tensile strengths, yield strengths and elongations after fracture. For the occasions with the special requirements, the bending testing or hardness testing may also be carried out, to comprehensively evaluate their load-bearing and processing properties.

Testing Scope

Hot-dip galvanized flat steels: This is the most common type. The testing emphases are the zinc layer thicknesses, adhesion and uniformity. Their zinc layers are thicker with the long corrosion resistance lives, widely applied to the fields such as the power transmission towers, highway guardrails and building steel structures.

Electro-galvanized flat steels: The zinc layers are usually thinner and uniform, with the smooth and beautiful surfaces. During the testing, the continuity of their coatings and salt spray resistance need to be paid attention to, often used for the products with the higher requirements for the dimension precision and surface smoothness such as the electrical appliances, instruments and small hardware.

Cold-rolled flat steels and hot-rolled flat steels: The base material states before the galvanizing affect the final product properties. The hot-rolled flat steel surfaces have the oxide scales and need to be thoroughly cleaned; the cold-rolled flat steel surfaces are relatively smooth. During the testing, the original states of the base materials and their effects on the coating adhesion need to be distinguished.

Products of the different zinc layer quality grades: According to the standards such as GB/T 13912 and ASTM A123, the galvanized layers have the different quality grades (such as the Z80, Z100 and Z200, where the numbers represent the minimum zinc coating masses g/m²). The testing needs to implement the corresponding thickness or mass testing standards for the products of the different grades.

Special purpose galvanized flat steels: For example, the products used in the harsh corrosive environments such as the humid, marine or industrial pollution conditions, or the galvanized flat steels used for the subsequent painting (such as the "zinc + paint" composite coating systems). The testing items need to add the targeted contents such as the salt spray testing and adhesion testing.

Incoming material and finished product testing: The testing scope runs through the whole production process, including the raw material inspection of the incoming steel billets or flat steels, online or sampling inspection during the production process, as well as the final inspection before the finished product delivery, ensuring that the quality of each link is controllable.

Testing Methods

Magnetic thickness measurement method: According to the GB/T 4956 standard, using the magnetic attraction or magnetic flux changes between the probe magnets and steel matrices, the thicknesses of the non-magnetic zinc layers are measured. This method is fast, non-destructive and portable, and is the most commonly used method at the sites and laboratories. During the measurement, the stable matrix magnetism and clean and flat surfaces need to be ensured, and the calibration needs to be carried out in the areas with the small curvature radii.

Copper sulfate etching testing method: According to the GB/T 26974 or ISO 1460 standards, the specimens are immersed in the copper sulfate solutions, each time for 1 minute. The copper precipitation conditions on the surfaces are observed, and the zinc layer uniformity is evaluated by calculating the number of etchings. This method is simple to operate, but is a destructive testing, and has the strict requirements for the solution concentrations, temperatures and operation times.

Bending testing method: The classic method of evaluating the coating adhesion. According to the specimen thicknesses, the specimens are bent 180° or 90° around the mandrels of the specified diameters, and then the 4 times magnifying glasses are used to observe whether the zinc layers on the outer sides of the bends have the peeling or blistering. The mandrel diameters are usually related to the specimen thicknesses, with the clear provisions in the standards.

Weighing method (gravimetric method): The method of the arbitration or precise measurement of the zinc layer masses. According to GB/T 13825, the specific stripping solutions (such as the antimony trichloride or hydrochloric acid solutions containing the inhibitors) are used to completely dissolve the zinc layers on the specimens. According to the mass differences before and after the dissolution, the zinc layer masses per unit area (g/m²) are calculated.

Metallographic microscope method: The galvanized flat steels are made into the metallographic specimens, and the coating cross-section thicknesses are directly observed and measured under the microscopes. This method is intuitive and accurate, and can simultaneously observe the coating structures, phase layer thicknesses and the bonding interfaces with the matrices, but the sample preparation is complex with the higher costs, mostly used for the arbitration or in-depth analysis.

Salt spray testing method: The accelerated testing method of evaluating the corrosion resistance of the galvanized layers. According to GB/T 10125, the specimens are placed in the salt spray test chambers, simulating the marine or industrial atmospheric environments. Through the observation of the time of the white corrosion product appearance (white rust) or the red rust time, their corrosion resistance grades are evaluated.

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 4956, Coating thickness measurement of non-magnetic coatings on magnetic substrates — Magnetic method

3. GB/T 26974, Hot dip galvanized coatings on fabricated iron and steel articles — Test methods

4. GB/T 13825, Metallic coatings — Determination of coating mass per unit area of hot-dip galvanized coatings on ferrous materials — Gravimetric method

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

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

7. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

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

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

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

4. ASTM A123, Standard specification for zinc (hot-dip galvanized) coatings on iron and steel products

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 testing equipment, divided into the magnetic type (for the steel and iron matrices) and eddy current type (for the non-ferrous metal matrices). The instruments need to be regularly calibrated with the standard sheets. When selecting the probes, the workpiece shapes (flat/curved surfaces), substrate thicknesses and measurement ranges need to be considered. The high-precision models can be connected to the computers for the data storage and analysis.

Universal material testing machine: Used for the tensile testing and bending testing of the flat steel base materials, to determine their tensile strengths, yield strengths, elongations and coating bending adhesion. The equipment should be equipped with the force transducers of the different measuring ranges and the bending fixtures conforming to the standards. The testing process needs to be strictly carried out according to the relevant metal material mechanical property testing standards.

Metallographic sample preparation and observation system: Including the cutting machines, mounting machines, grinding and polishing machines, metallographic microscopes and image analysis software. Used to prepare the coating cross-section samples, and through the microscopic observation and software measurement, the coating thicknesses, structures and defects are precisely analysed. This system requires the higher professional skills of the operators.

Salt spray test chamber: Used to simulate the accelerated corrosion environments and evaluate the corrosion resistance of the galvanized layers. The equipment needs to stably generate and control the sedimentation rates of the sodium chloride salt spray, chamber temperatures, humidity and pH values. The test results are affected by the multiple factors such as the chamber structures, spraying methods and specimen placement positions, and the operation needs to be standardized.

Chemical analysis instruments: Such as the direct-reading spectrometers (OES) and carbon-sulfur analysers, used for the fast and accurate analysis of the chemical compositions of the flat steel matrices. These instruments can simultaneously determine the contents of the multiple elements, and are the key equipment of ensuring that the raw materials meet the designation requirements. They need to be regularly calibrated and maintained with the standard samples.

Precision dimension measuring tools: Including the outer diameter micrometers, vernier callipers, digital callipers, steel tapes, straight edges and feeler gauges. Used for the precise measurement of the shape dimensions such as the widths, thicknesses, lengths and cambers of the flat steels. All the measuring tools must pass the metrological verification and be used within the validity periods, to ensure the accuracy and traceability of the measurement results.