Baked Enamel Steel Plate Testing

Baked enamel steel plate inspection mainly conducts scientific testing and analysis on the paint film thickness, adhesion testing, hardness testing, impact resistance, corrosion resistance, weather resistance and light resistance, 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

Paint film thickness: Measures the average thickness and local thicknesses of the paint layer on the baked enamel steel plate surface. The magnetic thickness measurement method (applicable to the iron and steel substrates) or eddy current thickness measurement method (applicable to the non-ferromagnetic substrates such as the aluminium plates) is usually adopted, ensuring that the coating thickness conforms to the design specifications, which directly affects its corrosion resistance, mechanical strength and appearance effects.

Adhesion testing: Evaluates the bonding firmness between the paint film and the metal substrate. The common methods include the cross-cut method, cross-hatch method and pull-off method. The cross-cut method rates by cutting the coating grids and pasting the tapes for the peeling; the pull-off method uses the dedicated adhesives and pull-off testers to directly measure the failure strengths, and is the key testing for quantifying the adhesion performance.

Hardness testing: Determines the scratch resistance and indentation resistance of the paint film surface. The pencil hardness method is widely adopted. The standard graphite pencils from the soft to hard are used to scratch across the paint surface, and the grade of the hardest pencil that does not cause the scratches is taken as the paint film hardness value. In addition, the pendulum damping hardness method is also used to evaluate the mechanical strength of the dried paint film.

Impact resistance: Detects the cracking or peeling resistance of the paint film under the fast impact loads. A falling weight impact testing machine is mainly used. The punches of the specified weights and sizes fall freely from the specific heights to impact the back or front surfaces of the test panels, observing the deformation, cracking or peeling conditions of the paint film at the impact positions.

Corrosion resistance: Evaluates the protection performance of the baked enamel steel plate under the corrosive environments. The key tests include the neutral salt spray testing (NSS), copper-accelerated acetic acid salt spray testing (CASS), etc. By simulating the harsh corrosive environments, the coating blistering, rusting and adhesion degradation are investigated. It is the core indicator for evaluating its long-term durability.

Weather resistance and light resistance: Investigates the colour and gloss retention ability and anti-chalking performance of the paint film under the outdoor sunshine, temperature, humidity and other climatic factors. Mainly through the artificial accelerated aging means such as the xenon lamp aging testing and ultraviolet accelerated aging testing (QUV) to simulate the long-term outdoor exposure, the colour and gloss changes and the coating chalking grades are evaluated.

Appearance and colour difference: The visual and instrumental testing of the baked enamel steel plate surface is carried out, evaluating its gloss, distinctness of image (DOI), orange peel, levelling and the deviations from the standard colour plates. The equipment such as the gloss meters, colour difference meters and orange peel meters is used for the quantitative analysis, ensuring the consistency of the colour and appearance quality between the product batches.

Testing Scope

Baked enamel steel plates for the building decoration: Applied to the fields such as the building curtain walls, interior ceilings and partitions, including the baked enamel aluminium plates and colour-coated steel plates. The testing emphasis is on the weather resistance, colour difference control, corrosion resistance of the coatings and the fire resistance meeting the building fire protection specifications, ensuring their long-term aesthetics and safety under the complex climatic conditions.

Baked enamel steel plates for the household appliance housings: Used for the housings of the household appliances such as the refrigerators, washing machines, air conditioners and microwave ovens. The testing needs to emphatically focus on the hardness, chemical resistance (such as the cleaning agents), damp heat resistance, adhesion, as well as the anti-fingerprint and anti-stain properties of the paint films, to meet the requirements of the daily use and cleaning maintenance.

Baked enamel steel plates for the automobile parts: Covering the automobile bodies, interior parts and chassis parts. The testing requirements are extremely high, including the excellent salt spray corrosion resistance, stone chip impact resistance, weather resistance, temperature cycling resistance and the strict colour difference and appearance standards, and need to conform to the series standards such as the PV and SAE of the automobile industry.

Baked enamel steel plates for the office furniture and cabinets: Used for the products such as the filing cabinets, computer cases and server cabinets. The testing focuses on the wear resistance, scratch resistance, solvent resistance (such as the alcohol wiping), electromagnetic shielding effectiveness (if required) of the coatings, and the harmful substance limit requirements meeting the environmental regulations such as the RoHS.

Baked enamel steel plates for the industrial equipment and sheet metal parts: Applied to the industrial environments such as the machine tool housings, control cabinets and ventilation ducts. The testing core lies in the corrosion resistance (especially in the chemical environments), mechanical strength, oil stain resistance of the coatings and the high-temperature or low-temperature resistance under the specific environments.

Coil pre-coated baked enamel steel plates: I.e. the colour-coated steel/aluminium coils, with the coating completed on the continuous production lines. In addition to the routine items, the testing particularly focuses on the T-bend properties (flexibility), reverse impact properties, solvent wiping resistance (reflecting the curing degrees) and the anti-sticking properties between the coil layers.

Testing Methods

Magnetic/eddy current thickness measurement method: In accordance with the standards such as GB/T 4956 and ISO 2178. The magnetic method measures the thicknesses of the non-magnetic coatings on the iron and steel substrates through the probe magnetic flux changes; the eddy current method measures the thicknesses of the non-conductive coatings on the conductive non-ferrous metal substrates by using the high-frequency alternating magnetic fields to generate the eddy currents in the probe coils. Before the measurement, the instruments need to be calibrated with the standard foils.

Cross-cut adhesion testing: In accordance with GB/T 9286, ISO 2409 and ASTM D3359. A multi-blade cutting tool is used to cut the grids at the specified spacings on the paint film surface, and the pressure-sensitive tapes are pasted and quickly torn off. The grade is rated at 0-5 according to the peeling area proportions of the paint film (grade 0 is the best). It is applicable to the coatings with the dry film thicknesses not greater than 250 μm.

Salt spray testing: In accordance with GB/T 10125, ISO 9227 and ASTM B117. The specimen is placed in an enclosed salt spray chamber, and the neutral (pH 6.5-7.2) or acidic sodium chloride solutions are continuously or intermittently sprayed, creating a constant-temperature and constant-humidity corrosive environment. The specimens are regularly inspected, recording the times and areas of the first red rust and blistering. It is the benchmark method of the accelerated corrosion evaluation.

Xenon lamp aging testing: In accordance with GB/T 1865, ISO 16474-2 and ASTM G155. The xenon arc lamps are used to simulate the full-spectrum sunlight, and the outdoor sun and rain exposure environments are simulated through the optical filters, temperature and humidity control and water spraying cycles. By regularly detecting the gloss retention rates, colour changes (ΔE) and chalking grades of the test panels, the long-term weather resistance of the coating is evaluated.

Pencil hardness testing: In accordance with GB/T 6739, ISO 15184 and ASTM D3363. A set of drawing pencils from 6B (softest) to 9H (hardest) is used. The pencil leads are sharpened into the cylindrical surfaces and pushed across the paint film surfaces at a 45° angle with a 750 g load. The grade of the hardest pencil that does not scratch the paint film is taken as the pencil hardness of the paint film.

Impact testing: In accordance with GB/T 1732, ISO 6272 and ASTM D2794. The falling weight impact method is commonly used. The test panel is placed on the base of the impact tester, and the punches of the specified weights fall freely from the specific heights to impact the test panel. By checking whether the paint film at the impact pits cracks or peels off from the substrate, its fast deformation resistance is evaluated.

Testing Standards

Domestic Standards:

1. GB/T 4956, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness — Magnetic method

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 1865, Paints and varnishes — Artificial weathering and exposure to artificial radiation — Exposure to filtered xenon-arc radiation

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

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

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

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

International Standards:

1. ISO 2178, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness — Magnetic method

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

3. ISO 15184, Paints and varnishes — Determination of film hardness by pencil test

4. ISO 6272-1, Paints and varnishes — Rapid-deformation (impact resistance) tests — Part 1: Falling-weight test, large-area indenter

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

6. ASTM D3359, Standard test methods for rating adhesion by tape test

7. ASTM D3363, Standard test method for film hardness by pencil test

8. ASTM D2794, Standard test method for resistance of organic coatings to the effects of rapid deformation (impact)

9. ASTM G155, Standard practice for operating xenon arc lamp apparatus for exposure of materials

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

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

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: Including the magnetic thickness gauges and eddy current thickness gauges, and is the basic equipment for detecting the paint film thicknesses. The modern instruments usually have the dual functions (Fe/NFe automatic identification), data statistics, storage and transmission functions. During the use, the suitable probes need to be selected according to the substrates, and the zero calibration and range calibration need to be carried out on the standard plates.

Salt spray test chamber: The key equipment for the corrosion tests such as the neutral salt spray and acidic salt spray. Composed of the chamber body, spraying system, heating system and saturated air barrels. It is required to precisely control the chamber temperatures (such as 35 ℃±2 ℃), spray volumes and sedimentation rates, and be made of the corrosion-resistant materials such as the glass fibre reinforced plastics or PVC.

Xenon lamp aging test chamber: The precision equipment simulating the full-spectrum solar radiation and climatic aging. The core components are the xenon arc lamp tubes, optical filter systems, temperature and humidity control systems, spraying systems and black panel/black standard thermometers. The illumination, darkness and spraying cycles can be programmably controlled. It is the equipment for evaluating the weather resistance.

Colour difference meter/gloss meter: The colour difference meter quantifies the colour deviations by measuring the colour difference values (ΔE) between the sample and the standard plate in the CIEL*a*b* colour space. The gloss meter irradiates the beams at the fixed angles (such as 20°, 60° and 85°), measures the ratios of the specular reflection light intensities to the standard plate reflection light intensities, and gives the gloss units (GU).

Adhesion tester and impact tester: The adhesion testers include the cross-cut testers, tapes and the hydraulic or mechanical pull-off testers used in the pull-off method. The impact testers are mainly composed of the vertical guide pillars, falling weights of the fixed heights, hemispherical punches and bases, and the impact energies are changed by replacing the weights of the different masses and adjusting the heights.

Microscope and image analysis system: Used to assist in evaluating the test results, such as using the stereo microscopes to observe the cut edge morphologies after the cross-cut testing and the corrosion propagation conditions after the salt spray testing. Combined with the digital cameras and image analysis software, the more precise quantitative analysis of the blistering densities and rust areas can be carried out.