Zinc-coated Steel Testing

Zinc steel inspection mainly conducts scientific testing and analysis on the coating property testing, base material composition and mechanical property testing, galvanized layer quality testing, dimension and geometric tolerance testing, environmental aging resistance testing and appearance quality 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

Coating property testing: It is the core of the quality of the products such as the zinc steel guardrails and shutters. The main testing items include the coating thickness, adhesion, glossiness, colour difference, impact resistance, pencil hardness, solvent rubbing resistance and salt spray resistance. These items jointly evaluate the protection ability of the coatings to the substrates and the durability of the decorative effects.

Base material composition and mechanical property testing: Ensuring that the quality of the steel itself reaches the standards. Usually including the chemical composition analysis of the steel (such as the carbon, silicon, manganese, phosphorus and sulphur contents), as well as the mechanical property testing such as the tensile strength, yield strength, elongation after fracture and bending testing. The strengths of the base material directly determine the load-bearing capacity and safety of the zinc steel structural parts.

Galvanized layer quality testing: The "zinc" of the zinc steel mainly comes from the hot-dip galvanizing process. The key testing items include the average thicknesses and local thicknesses of the galvanized layers, the galvanized layer uniformity (copper sulfate testing) and the bonding forces between the galvanized layers and the substrates (hammer testing). The galvanized layers are the first and most important barrier preventing the rust of the steel base material.

Dimension and geometric tolerance testing: Related to the installation adaptability and overall aesthetics of the products. The outer diameters, wall thicknesses, lengths, straightness, perpendicularity, flatness of the pipes or profiles, as well as the overall diagonal differences and installation hole position dimensions after the assembly need to be tested, ensuring that they meet the design drawings and assembly requirements.

Environmental aging resistance testing: Simulating the erosion that the products may encounter in the actual use environments. Mainly including the artificial accelerated aging testing (such as the xenon lamp aging and UV aging), damp heat resistance testing, acid mist resistance testing, alkali mist resistance testing and salt spray cyclic testing, comprehensively evaluating the weather resistance and service lives of the products.

Appearance quality testing: It belongs to the intuitive sensory inspection, but is indispensable. The inspection items include whether the coating surfaces are smooth and flat, and whether there are the defects such as the runs, orange peel, pinholes, bubbles and impurities, as well as whether the zinc layer surfaces have the defects such as the missed plating, roughness and zinc nodules, ensuring that the product appearances meet the commercial requirements.

Testing Scope

Zinc steel guardrail series: Including the balcony guardrails, stair handrails, road isolation guardrails, garden landscape guardrails and construction site edge protection guardrails, etc. The testing needs to cover their static load-bearing, impact resistance, weather resistance and installation firmness, especially paying attention to the strengths and anti-corrosion properties of the welding points or connection parts.

Zinc steel shutters and air conditioner enclosures: Such products have the higher requirements for the dimension precision, appearance flatness and corrosion resistance. The testing emphases are the uniformity of the blades (bars), assembly flexibility, and the anti-corrosion performance under the long-term exposure to the humid and salt-containing air (such as the air conditioner condensate water environments).

Zinc steel road and municipal facilities: Such as the street lamp poles, traffic sign poles and parking lot facilities, etc. Due to their long-term outdoor use and difficult maintenance, the testing scope needs to particularly emphasize the high salt spray resistance, wind load resistance, coating UV aging resistance and the continuity of the grounding anti-corrosion.

Zinc steel building decoration and structural parts: Including the gratings, anti-theft nets, awning frames and outdoor furniture frames, etc. The testing needs to take into account both the decoration and functionality, such as the coating colour durability, scratch resistance, as well as the mechanical property re-verification as the load-bearing or connection parts.

Zinc steel raw materials and semi-finished products: The testing scope extends to the upstream galvanized steel plates/strips, galvanized steel pipes, as well as the phosphating layer and passivation layer qualities before the spraying. The strict control of the raw materials is the key link of guaranteeing the final product quality from the sources.

Zinc steel assembly connection parts: Including the dedicated connection clips, anti-theft bolts and anti-tamper washers, etc. Although these accessories are small, they directly affect the stability and safety of the overall structures, and the special testing of their materials, coating thicknesses, torque coefficients and anti-rust properties needs to be carried out.

Testing Methods

Coating thickness measurement: Mainly adopting the magnetic thickness gauges (suitable for the non-magnetic coatings on the iron and steel substrates) and eddy current thickness gauges (suitable for the insulating coatings on the non-ferrous metal substrates). During the measurement, the principle of the multiple-point measurement taking the average values needs to be followed, and the operation is carried out according to the standards such as GB/T 4956 or ISO 2808.

Adhesion testing: The cross-cut method is commonly used (according to GB/T 9286 or ISO 2409). The dedicated cutting tools are used to scribe the grids on the coating surfaces, the tapes are pasted and quickly torn, and the grades are rated according to the coating falling off areas. For the thick coatings or high-hardness coatings, the cross-hatch method or pull-off method (using the adhesion testers) can also be adopted.

Salt spray testing: According to the standards such as GB/T 10125 or ASTM B117, the specimens are placed in the salt spray test chambers, sprayed with the 5% sodium chloride solution and continuously sprayed at 35±2°C. The surface corrosion, blistering and peeling conditions of the specimens are regularly observed, and the corrosion resistance is evaluated by the times (hours) of the red rust appearance.

Composition and mechanical property analysis: The chemical compositions of the steel can be quickly and non-destructively tested with the spectrometers (such as the direct-reading spectrometers). For the mechanical properties, the standard specimens need to be prepared and the tensile testing is carried out on the universal material testing machines according to GB/T 228.1, to obtain the accurate strength and plasticity data.

Galvanized layer thickness and uniformity testing: The thicknesses can be measured with the magnetic thickness measurement method. The uniformity traditionally adopts the copper sulfate testing method (GB/T 26989). The specimens are immersed in the copper sulfate solution, and the iron and steel substrates must not be exposed within the specified times. This method is simple to operate and is the common means of the fast on-site inspection.

Artificial accelerated aging testing: Mainly using the xenon lamp aging test chambers (according to GB/T 1865 or ISO 16474-2) or UV fluorescent aging test chambers (according to GB/T 23987 or ISO 16474-3), simulating the climatic factors such as the sunlight and rain and dew, evaluating the coating weather resistance in a short time, and quantitatively evaluating through the indicators such as the colour difference, chalking and loss of gloss.

Testing Standards

Domestic Standards:

1. GB/T 4956, Magnetic method for measuring the thickness of non-magnetic coatings on magnetic substrates

2. GB/T 9286, Paints and varnishes — Cross-cut test

3. GB/T 6739, Paints and varnishes — Determination of film hardness by pencil test

4. GB/T 1732, Determination of impact resistance of film

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

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

7. GB/T 1865, Paints and varnishes — Artificial weathering and exposure to artificial radiation — Filtered xenon-arc radiation

8. GB/T 23987, Paints and varnishes — Exposure of coatings to artificial weathering — Exposure to fluorescent UV lamps and water

9. GB/T 26989, Copper sulfate testing method for the uniformity of the hot-dip galvanized coatings on iron and steel articles

International Standards:

1. ISO 2808, Paints and varnishes — Determination of film thickness

2. ISO 2409, Paints and varnishes — Cross-cut test

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

5. ISO 16474-2, Paints and varnishes — Methods of exposure to laboratory light sources — Part 2: Xenon-arc lamps

6. ISO 16474-3, Paints and varnishes — Methods of exposure to laboratory light sources — Part 3: Fluorescent UV lamps

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: Divided into the magnetic induction type and eddy current type, and is the most commonly used basic equipment on site and in the laboratories. The high-precision digital coating thickness gauges can quickly and non-destructively measure the zinc layer, total coating thicknesses and the thicknesses of the various layers, with the data statistics, storage and transmission functions, convenient for the quality traceability.

Salt spray corrosion test chamber: The key equipment for assessing the corrosion resistance of the zinc steel products. By atomizing the sodium chloride solution, the constant-temperature and constant-humidity corrosion environments are created in the closed chambers, simulating the coastal atmosphere or salt-containing industrial atmosphere conditions. The various testing modes such as the continuous spraying or cyclic corrosion (CCT) can be set.

Universal material testing machine: Used to test the mechanical properties of the zinc steel materials, such as the tensile strength, yield strength and elongation. The equipment is equipped with the high-precision sensors and computer control systems, which can automatically draw the stress-strain curves and output the precise test reports. It is the core equipment of verifying whether the material reaches the standards.

UV/xenon lamp aging test chamber: Simulates the ultraviolet light, visible light and infrared radiation in the natural sunlight, and can control the temperature, humidity and spraying. The xenon lamp spectra are closer to the full sunlight spectra, while the UV lamps mainly strengthen the effects of the ultraviolet band. The combination of the two can comprehensively evaluate the weather resistance and aging properties of the coatings.

Precision dimension measurement tools: Including the digital callipers, micrometers, wall thickness micrometers, height gauges, R gauges, thread gauges, laser range finders and coordinate measuring machines (CMM). For the complex shapes and key installation dimensions, the CMM can provide the high-precision three-dimensional geometric dimension and geometric tolerance data.

Spectrometer and metallographic microscope: The direct-reading spectrometers are used for the fast and accurate quantitative analysis of the chemical compositions of the steel raw materials. The metallographic microscopes are used to observe the micro-structures, thicknesses and uniformity of the coatings and zinc layers, as well as whether there are the defects. They are the important tools for the failure analysis and process research.

Adhesion tester and hardness tester: The adhesion testers (pull-off method) can quantitatively measure the bonding strengths between the coatings and the substrates or between the coatings. The pencil hardness testers rate the hardness grades by scratching the coatings with the pencils of the known hardness. These two instruments are the common equipment of evaluating the coating mechanical properties.