Galvanized steel coils refer to steel that is covered with a layer of zinc by electroplating or hot-dip galvanizing. Its main purpose is to enhance the corrosion resistance of steel. In order to ensure the quality of galvanized coils, testing standards have become a key means to evaluate their performance and applicability.
Manufacturing Process Of Galvanized Steel Coils:
The manufacturing process of galvanized steel coils includes two main methods: hot-dip galvanizing and electro-galvanizing.
Hot-dip galvanizing is to immerse the steel coil in molten zinc liquid to form a layer of zinc-iron alloy, and then deposit a layer of pure zinc on the surface; electro-galvanizing is to reduce zinc ions by electrochemical methods and deposit them on the surface of the steel coil.
The Role Of The Galvanized Layer:
The galvanized layer mainly plays an anti-corrosion role. The zinc layer can form an oxide or zinc carbonate protective film in the air to prevent moisture and oxygen from entering the steel surface, thereby preventing corrosion. At the same time, the zinc layer can also provide certain mechanical protection and reduce surface damage.
Testing Standards Of Galvanized Steel Coil:
ASTM standards: The American Association for Testing and Materials has formulated and published technical standards for various materials. There are two standards for galvanized steel.
- ASTM A653/A653M: This standard applies to hot-dip galvanized steel sheets and strips, and specifies the thickness of the zinc coating, coating weight, and mechanical properties of the product.
- ASTM A924/A924M: This standard is a general requirement for galvanized steel sheets and strips, applicable to steel sheets with various coating types, including hot-dip galvanizing. The standard content includes dimensional tolerances, surface quality, marking, and packaging.
ISO standards: The International Organization for Standardization (ISO) has developed relevant standards for galvanized steel. For example:
- ISO 1461: This standard covers the requirements for hot-dip galvanized layers, especially the coating thickness, adhesion, and corrosion resistance of steel sheets and strips.
- ISO 2063: This standard specifies the technical requirements for thermal spray galvanized layers. It is applicable to zinc, aluminum, and their alloy coatings on the surface of steel, emphasizing the importance of coating thickness and uniformity.
Thickness measurement: The thickness of galvanized steel coils is a key indicator for evaluating their quality. Insufficient thickness may result in insufficient strength, while excessive thickness will increase material costs.
- The thickness of galvanized steel coils is usually measured by non-contact measurement methods, such as laser thickness gauges or eddy current thickness gauges. These instruments can achieve high-precision thickness measurement without damaging the surface of the material.
Galvanized layer weight: The weight of the galvanized layer is a key indicator for evaluating its quality. The weight of the galvanized layer directly affects the corrosion resistance of the steel coil, so special attention should be paid during the inspection. The weight of the galvanized layer is usually expressed in grams per square meter (g/m²). The measurement methods of the galvanized layer weight mainly include the stripping method and the weighing method.
- The stripping method is to peel the galvanized layer from the substrate by chemical or mechanical methods and then weigh it;
- The weighing method is to directly measure the total weight of the steel coil per unit area and subtract the weight of the substrate.
Galvanized layer adhesion: The adhesion of the galvanized layer is a key performance to ensure that the galvanized steel coil will not fall off or peel off during processing and use. Insufficient adhesion may cause the zinc layer to peel off, thereby losing its corrosion protection effect. Adhesion detection usually adopts methods such as the cross-cut method, bending test or impact test.
- The cross-hatch method is to draw a grid pattern on the surface of the galvanized layer and then test the adhesion of the galvanized layer by sticking tape;
- The bending test is to bend the galvanized coil at a certain angle to observe whether the galvanized layer falls off or cracks.
Surface quality: Common surface defects include uneven zinc flowers, coating shedding, surface scratches, blistering, pinholes, depressions and stains. These defects may affect the appearance and protective performance of the steel coil. Surface quality inspection is usually carried out by tools such as visual inspection, microscopic observation and surface flaw detector.
- Visual inspection is the most commonly used method and can quickly detect most surface defects;
- Microscopic observation is used to find tiny pinholes and cracks;
- Surface flaw detectors can detect internal defects such as bubbles and inclusions.
Chemical composition: The chemical composition of the galvanized layer mainly includes zinc and its alloy elements (such as aluminum, magnesium, etc.). The proportion of these elements directly affects the performance of the galvanized layer, such as corrosion resistance and adhesion. The substrate is usually low-carbon steel or alloy steel, and its main components include elements such as carbon, manganese, silicon, sulfur and phosphorus. The chemical composition is usually tested by spectral analysis, X-ray fluorescence analysis and other methods. The chemical composition of galvanized steel coils has a significant impact on their final properties. For example,
- An increase in aluminum content can improve the corrosion resistance of the zinc layer, but it may also reduce its adhesion;
- While excessive sulfur and phosphorus content in the substrate may increase the brittleness of the steel coil, affecting processability and service life.
Mechanical properties: There are three common mechanical property test methods, namely tensile test, bending test and impact test.
- Tensile test is a common method for testing the mechanical properties of galvanized steel coils, mainly used to determine its tensile strength, yield strength and elongation at break. According to standards such as ASTM A653 and GB/T 228, tensile tests are usually carried out at room temperature, and the specimens are cut from the steel coils.
- Bending test is one of the important methods for evaluating the plasticity and adhesion of galvanized steel coils. In the test, the specimen is bent at a certain angle to observe whether the surface coating cracks or falls off. Qualified galvanized steel coils should not have any coating peeling phenomenon under the specified bending conditions.
- Impact test is used to evaluate the toughness of galvanized steel coils under low temperature or impact load. The test is usually carried out using a Charpy impact tester, where an impact force is applied to the sample at a specific temperature to measure the energy absorbed. Qualified galvanized steel coils should show sufficient toughness to resist accidental impact during use.
Corrosion resistance: Common corrosion resistance test methods include salt spray test, damp heat test and atmospheric exposure test.
- The salt spray test is to apply salt spray to the galvanized steel coil under specific temperature and humidity conditions to observe the corrosion of the surface coating.
- The damp heat test simulates a high humidity and high temperature environment to test the corrosion rate of the galvanized steel coil under this condition.
- The atmospheric exposure test is to expose the sample for a long time in the natural environment to evaluate its corrosion resistance.
The testing standards for galvanized coils cover many aspects, from coating thickness, coating weight to adhesion, mechanical properties and corrosion resistance. Both ASTM standards and ISO standards are important bases for evaluating the quality of galvanized coils. Through strict testing and quality control, the service life and performance of galvanized coils can be greatly improved, providing a solid guarantee for applications in various industries. Our company is a steel manufacturer with many years of operation. If you are interested in our products, please contact us!
