Enameled Steel Plate Testing

Enamel steel plate inspection mainly conducts scientific testing and analysis on the appearance and dimension inspection, adhesion strength, impact resistance testing, acid and alkali resistance, wear resistance testing and thermal shock resistance 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

Appearance and dimension inspection: Inspects the surface quality of the enamel steel plate, including the colour difference, gloss, flatness and whether there are the defects such as the bubbles, pinholes, cracks, orange peel and impurities. At the same time, the tools such as the callipers, tape measures and feeler gauges are used to precisely measure its length, width, diagonal difference, thickness and edge straightness, ensuring that it meets the design drawings and installation requirements.

Adhesion strength (adhesion) testing: This is the key item for evaluating the bonding firmness between the enamel and the steel plate substrate. The bending method, impact method or cross-cut method is usually adopted for the testing. For example, in the bending method, the specimen is bent to a certain angle on the mandrels of the specific radii, and it is observed whether the enamel layer peels off or produces the cracks, thereby judging the adhesion performance grade.

Impact resistance testing: Used to simulate the ability of the enamel steel plate to resist the external mechanical impacts during the transportation, installation or use. A falling weight impact testing machine is adopted, and the steel balls or falling weights of the specified masses and falling heights impact the specimen surface, checking whether the enamel layer produces the damages such as the cracks and peeling, and recording the impact energy values.

Acid and alkali resistance (chemical stability) testing: Evaluates the ability of the enamel layer to resist the chemical corrosion. The specimen is immersed in the acid (such as the citric acid) or alkali (such as the sodium carbonate) solutions of the specified concentrations for the specified time, or the reagents are dropped on its surface. After the testing, the surface gloss changes, weight losses or corrosion grades are observed and measured.

Wear resistance testing: For the enamel steel plates used in the places with the frequent contact or cleaning (such as the metro walls and laboratory bench surfaces), the surface wear resistance needs to be tested. A Taber abrasion tester is commonly used. Under the specified loads and revolutions, the standard abrasive wheels rub the surface, and the wear resistance grade is rated by measuring the mass loss or gloss reduction rate.

Thermal shock resistance testing: Detects the stability of the enamel steel plate under the drastic temperature changes. The specimen is heated to the specified high temperature (such as 150 ℃), and then quickly put into the room-temperature water for the rapid cooling, repeated for several cycles, checking whether the enamel layer produces the cracks or peeling, to evaluate its resistance to the cold and hot shock.

Weather resistance and UV resistance testing: For the enamel steel plates applied outdoors, this item is crucial. A xenon lamp aging test chamber or UV aging chamber is used to simulate the long-term exposure to the sun and rain, and after the accelerated aging for hundreds to thousands of hours, its colour changes (ΔE), gloss retention rates and surface deterioration conditions are detected, evaluating its long-term durability.

Testing Scope

Enamel steel plates for the building decoration: Mainly used for the walls and ceilings of the public buildings such as the metro stations, airports, tunnels and hospitals. The testing emphasis is on their decoration durability, including the weather resistance, anti-graffiti, easy cleaning, fire rating (Class A non-combustible materials), as well as the flatness and colour difference control of the large-size plates.

Enamel steel plates for the household appliances: Applied to the water heater inner tanks, ovens and stove panels. The testing focuses on their functionality, such as the extremely high acid and alkali resistance (resisting the scale and food acid and alkali corrosion), excellent adhesion and thermal shock resistance (bearing the repeated heating and cooling), as well as the good thermal conductivity.

Enamel steel plates for the industrial anti-corrosion: Used for the harsh environments such as the chemical vessels, reactors and storage tank linings. The testing core is the ultimate chemical stability. According to the characteristics of the contained media (strong acids, strong alkalis and organic solvents), the targeted corrosion resistance tests need to be carried out, and there are the strict requirements for the porosity of the coating (high-voltage spark detection).

Enamel steel plates for the signs and nameplates: Used for the durable signs on the roads and public places. The testing focuses on the long-term stability of the colours (anti-UV fading), the retention of the high contrast and the comprehensive durability under the harsh outdoor climates (rain, snow and freeze-thaw).

Special function enamel steel plates: Such as the antibacterial enamel, self-cleaning enamel and far-infrared radiation enamel. In addition to the routine performance testing, the declared special functions also need to be verified, for example, evaluating the antibacterial rate through the antibacterial testing, or testing the hydrophilic/hydrophobic properties of the surface under the light and the ability to catalytically decompose the organic matters.

Testing of the raw materials and semi-finished products: The testing scope also extends to the upstream of the production chain, including the inspection of the mechanical properties (yield strength and elongation) and surface cleanliness of the cold-rolled low-carbon steel plate substrates, as well as the testing of the composition analysis, fineness and melting temperature of the enamel frit powders, ensuring the controllable source quality.

Testing Methods

Standard reference method: The enamel steel plate testing strictly follows the domestic and international technical standards. China mainly follows GB/T 33284-2016 "Enamel inner tanks for household and similar electrical appliances" and JG/T 234-2008 "Enamel steel plates for building decoration". The commonly used international standards include ISO 2722 and ISO 2723 (the enamel testing method series), ASTM C282 (acid resistance) and ASTM C614 (alkali resistance). The testing is carried out according to the standards, and the results have the comparability and traceability.

Cross-cut method and bending method (adhesion): In the cross-cut method, a multi-blade cutting tool is used to cut the grids on the enamel layer surface, and after pasting the tape and quickly tearing it off, the grade is rated according to the percentage of the enamel layer peeling area in the grid areas. In the bending method, the specimen is bent on the conical or cylindrical mandrels until the fracture or the specified angle, and the enamel layer peeling conditions at and near the bend are directly observed. It is a more stringent qualitative evaluation method.

Falling weight impact testing method: Usually carried out in accordance with GB/T 1732 or ISO 6272. The specimen is fixed on the support, and the falling weight (punch) of the specified mass falls freely from the set height to impact the specimen centre. By gradually increasing the height or mass, the impact energy that just causes the cracks or peeling of the enamel layer is determined, or the damage states under the fixed impact energy are checked.

Chemical immersion and dropping testing method: In the acid and alkali resistance testing, the immersion method is to completely immerse the specimen in the constant-temperature corrosive liquid, take it out after the specified time (such as 24 h), clean and dry it, weigh the mass loss and observe the surface. In the dropping method, the quantitative test liquid is dropped on the horizontal surface of the specimen, maintained for the specified time under the specific temperature and humidity, and the gloss and colour changes of the enamel layer at the dropping points are observed.

Artificial accelerated aging testing method: The weather resistance testing mainly uses the xenon arc lamp aging chambers (simulating the full-spectrum sunlight) or fluorescent UV lamp chambers (mainly simulating the UV damage). By controlling the light intensity, chamber temperature, humidity and spraying cycles, the aging effects of several months or even years outdoors are reproduced in a short time, and the performance changes of the specimens are regularly monitored.

Non-destructive testing method: The high-voltage spark detection is an important non-destructive testing method, used to detect the through defects such as the pores and cracks in the enamel layer. During the detection, the high-voltage probe (usually several thousand to tens of thousands of volts) sweeps over the enamel surface. When encountering the defects, the high voltage will break down the air to produce the sparks and alarm, thereby locating the defect points.

Testing Standards

Domestic Standards:

1. GB/T 33284-2016, Enamel inner tanks for household and similar electrical appliances

2. JG/T 234, Enamel steel plates for building decoration

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

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

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

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

International Standards:

1. ISO 2722, Vitreous and porcelain enamels — Determination of resistance to citric acid at room temperature

2. ISO 2723, Vitreous and porcelain enamels — Determination of resistance to hot citric acid

3. ASTM C282, Standard test method for acid resistance of porcelain enamels (citric acid spot test)

4. ASTM C614, Standard test method for alkali resistance of porcelain enamels

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

6. 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

Coating thickness gauge: Used to precisely measure the dry film thickness of the enamel layer. The magnetic thickness gauge (applicable to the steel substrates) or eddy current thickness gauge is commonly used. Measuring the thickness is the basis for controlling the product quality and ensuring the coating performances (such as the corrosion resistance and adhesion), and it must be ensured that the thickness is within the specified tolerance ranges and uniform.

Colour difference meter and gloss meter: The colour difference meter measures the L* (lightness), a* (red-green value) and b* (yellow-blue value) between the sample and the standard plate, calculates the colour difference value ΔE, and objectively quantifies the colour consistency. The gloss meter irradiates the surface at a fixed angle (such as 60°), measures the reflected luminous flux, and evaluates the surface finish in the gloss units (GU). They are the key quantitative tools of the appearance quality.

Universal material testing machine and impact testing machine: The universal testing machine can carry out the bending testing, equipped with the bending mandrels of the different curvature radii, used for the adhesion evaluation. The dedicated falling weight impact testing machine or pendulum impact testing machine is used for the impact resistance testing, and its height scales, release devices and weight configurations need to be precisely calibrated, ensuring the accurate impact energy.

Constant temperature and humidity test chamber and chemical stability testing equipment: Including the constant-temperature water baths or precision ovens, used for the constant-temperature control of the chemical immersion testing. A precision analytical balance with the accuracy of 0.1 mg is also needed for measuring the corrosion weight losses. The dropping testing needs to be carried out under the standard temperature and humidity environments, and sometimes the specific dropping testing supports are used.

Aging test chamber: The xenon lamp aging test chamber is the core equipment of the weather resistance testing, and its equipped xenon arc lamp tubes, optical filters, black panel thermometers, humidity control systems and spraying systems need to comply with the relevant standards (such as GB/T 1865). The UV aging chamber uses the fluorescent UV lamps as the light sources, with the relatively simple structure, commonly used for the fast screening testing.

High-voltage spark detector: Composed of a high-voltage generator, detection electrode (brush type or ring type), alarm device and grounding device. Its output voltage can be adjusted according to the coating thickness. During the operation, it is necessary to ensure that the probe maintains a constant and suitable distance from the coating surface and moves at a uniform speed. It is the essential equipment for the online detection of the production lines and the quality acceptance.

Abrasion tester and hardness tester: The Taber abrasion tester evaluates the wear resistance by forming the abrasion tracks on the rotating specimen surface with two abrasive wheels. The pencil hardness tester is used to evaluate the scratch resistance of the enamel layer surface. A series of pencils from the soft to hard are used to scratch the surface, and the hardest pencil grade that does not cause the scratches is taken as the surface hardness value.