Electroplated Steel Plate Testing

Electroplated steel plate inspection mainly conducts scientific testing and analysis on the coating thickness testing, coating adhesion testing, corrosion resistance testing, surface quality and appearance inspection, coating composition analysis and hardness and wear 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

Coating thickness testing: This is the most core testing item of the electroplated steel plate, directly related to the corrosion resistance, conductivity and appearance quality of the product. The average thicknesses and local minimum thicknesses of the coatings (such as the zinc, zinc-nickel alloy and tin) are usually measured. The non-conforming thicknesses will shorten the product life, while the excessive thicknesses waste the materials and may affect the subsequent processing. The non-destructive X-ray fluorescence spectrometry method and the micron-level metallographic microscope method are mainly adopted for the measurement.

Coating adhesion testing: Used to evaluate the bonding strength between the coating and the substrate steel plate, preventing the problems such as the coating peeling and blistering during the use. The common testing methods include the bending testing, cross-cut testing and cupping testing. By observing whether the coating cracks or peels after the specific deformations of the specimen, its adhesion is qualitatively or semi-quantitatively evaluated. It is the key to ensuring the processing formability and long-term reliability of the product.

Corrosion resistance testing: Directly simulates or accelerates the evaluation of the corrosion resistance of the electroplated steel plate under the specific environments. Including the neutral salt spray testing, copper-accelerated acetic acid salt spray testing and cyclic corrosion testing. By observing the times when the red rust and white rust appear on the specimen surfaces or the morphologies of the corrosion products, the protection grades are assessed. This testing is the core basis for verifying the effectiveness of the coating protection, and is crucial for the outdoor or harsh environment applications such as the automobiles and household appliances.

Surface quality and appearance inspection: The surface states of the electroplated steel plate are checked through the visual inspection or with the aid of the magnifying equipment. The main inspection items include the gloss and colour uniformity of the coating, and whether there are the defects such as the blistering, pitting, pinholes, missed coating, scratches, stains and inclusions. The appearance not only affects the aesthetics of the product, but may also become the initiation points of the corrosion or affect the painting quality, so it is the mandatory inspection item of the ex-factory inspection.

Coating composition analysis: For the alloy coatings (such as the zinc-nickel alloy and zinc-iron alloy), the precise determination of the content percentages of the various elements in the coating is crucial. The composition deviations will significantly affect the properties such as the corrosion resistance, solderability and paintability of the coating. The equipment such as the X-ray fluorescence spectrometers and inductively coupled plasma atomic emission spectrometers is usually adopted for the quantitative analysis, to ensure that the alloy coatings meet the preset chemical composition specifications.

Hardness and wear resistance testing: Mainly evaluates the mechanical properties of the coating itself or the coating-substrate composites. The coating hardness is usually measured with the micro-Vickers hardness testers. The wear resistance is evaluated through the specific friction and wear testing, assessing the ability of the coating to resist the mechanical wear. This is particularly important for the components (such as the fasteners and structural parts) that need to bear the friction, assembly or possible scratches during the transportation.

Paintability and weldability testing: Evaluates the suitability of the electroplated steel plate as the substrate material of the subsequent processing. The paintability testing includes the detection of the adhesion between the coating and the paints, solvent resistance and phosphating film quality. The weldability testing evaluates the stability of the welding process, weld point strength and whether the welding spatter or blowholes are produced through the process tests such as the spot welding and seam welding, directly affecting the feasibility and efficiency of the downstream manufacturing processes.

Testing Scope

Hot-dip galvanized steel plates: The testing emphasis is on the thickness uniformity of the zinc layer, surface structures of the coating (such as the sizes and morphologies of the zinc flowers), thicknesses and continuity of the alloy layers, and their excellent atmospheric corrosion resistance. The salt spray testing and coating bending adhesion testing are the routine testing items of this type of products, widely used in the buildings, household appliances and automobile structural parts.

Electrogalvanized steel plates: Compared with the hot-dip galvanizing, their coatings are more uniform and the surfaces are smoother. The testing focuses on the precise coating thickness control, lower surface roughness and excellent paint adhesion and welding performance. Commonly used for the automobile outer panels and high-end appliance housings with the high surface quality and formability requirements.

Zinc-nickel alloy electroplated steel plates: As a high-performance protective coating, the testing core is the precise control of the alloy compositions (usually the nickel contents of 10-15%), and the resulting corrosion resistance far exceeding the pure zinc coatings (especially the red rust resistance) and good machinability. The composition analysis and cyclic corrosion testing are the key.

Electrolytic tin-coated steel plates (tinplate): Mainly used for the food packaging cans. The testing items are highly special, including the tin layer thickness, continuity of the alloy-tin layer (FeSn2), passivation film quality, porosity, and the extremely strict food corrosion resistance and non-toxicity requirements. Ensuring that it does not contaminate the contents is the primary goal.

Electroplated chromium steel plates: Including the decorative chromium and hard chromium. The testing involves the micro-crack or micro-pore structures, hardness, wear resistance and corrosion resistance of the chromium coatings. For the decorative chromium, the brightness and colour uniformity of the appearance are the important indicators; for the functional hard chromium, the bonding force and internal stress testing of the thick coatings are particularly important.

Composite coating steel plates: Such as the pre-coated steel plates of the "zinc-based coating + organic coating". The testing scope covers the various properties of the bottom metal coatings, as well as the thickness, adhesion, hardness, flexibility, weather resistance and chemical medium resistance of the top organic coatings, and the systematic composite performance comprehensive evaluation needs to be carried out.

Special function electroplated steel plates: Such as the electromagnetic shielding coatings and conductive coatings. The testing scope goes beyond the routine protection and appearance, and needs to be set according to their functional characteristics, such as measuring the specific functional indicators including the surface resistivity, electromagnetic shielding effectiveness and thermal conductivity of the coatings.

Testing Methods

X-ray fluorescence spectrometry method: A fast, non-destructive and precise coating thickness and composition analysis method. Its principle is to use the X-rays to excite the coating elements to generate the characteristic fluorescence, and calculate the thicknesses or contents by analysing the fluorescence intensities. It is applicable to the on-line or laboratory measurement of the various single-layer or composite coatings such as the zinc, tin, nickel and chromium, and is the main means of the process control in the industrial production.

Metallographic microscope method: A destructive but very intuitive and accurate coating thickness and structure observation method. After the cutting, mounting, grinding, polishing and etching of the specimen, the coating cross-section thicknesses are directly measured under a metallographic microscope, and the micro-morphologies such as the bonding interfaces between the coating and substrate, alloy layer structures and pores can be detected and observed. It is the standard method of the arbitration and in-depth research.

Coulometric method (anodic dissolution method): A coating thickness measurement method based on the electrochemical principle. In a specific electrolyte, the coating is used as the anode for the electrolytic dissolution, and the average thickness of the coating is calculated according to the electric quantity (coulombs) consumed in the dissolution process. This method has the high precision, suitable for measuring the coating thicknesses of the local small areas, and especially has the advantages for the layer-by-layer dissolution analysis of the multi-layer coatings.

Salt spray testing method: The most commonly used artificial accelerated corrosion testing method, used to evaluate the corrosion resistance of the coating. The specimen is placed in an enclosed salt spray test chamber, and the sodium chloride solution of a certain concentration is sprayed, simulating the marine or salt-containing atmospheric corrosion environments. The protection grades are assessed by observing the times when the red rust or white rust first appears on the specimen surfaces, and the test cycles can range from several hours to over a thousand hours.

Cross-cut/scratch adhesion testing method: The common method of evaluating the bonding strength between the coating and substrate. A dedicated multi-blade cutting knife is used to cut the grids or parallel scratches at certain spacings on the coating surface, and then the dedicated tapes are pasted and quickly torn off. The adhesion grade is assessed according to the peeling proportion of the coating from the grids. This method has the simple operation and intuitive results, suitable for the fast laboratory and on-site inspection.

Bending adhesion testing method: The method of inspecting the coating adhesion by making the specimen produce the plastic deformations. The specimen is bent by 90° or 180° around an axis of a certain diameter, and whether the coating on the outer side of the bend peels or cracks is observed with the magnifying glasses. The cupping testing is another more stringent deformation testing, in which the steel balls or punches are used to press the specimen to form the bulges until the fracture, checking the coating states at the fracture positions. These two methods can effectively simulate the adhesion properties in the actual processing.

Electrochemical testing method: By measuring the parameters such as the open circuit potentials, polarization curves and electrochemical impedance spectra of the coating in the electrolyte solutions, its corrosion mechanisms, corrosion rates and protection properties are deeply studied. This method can provide the faster quantitative data than the salt spray testing, and is commonly used for the research and development of the new coating systems and the performance comparison studies.

Testing Standards

Domestic Standards:

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

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

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

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

5. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

International Standards:

1. ASTM B117, Standard practice for operating salt spray (fog) apparatus

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

3. 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

X-ray fluorescence spectrometer: The core equipment of the electroplated steel plate testing, used for the fast non-destructive determination of the coating thicknesses and chemical compositions. The modern instruments are equipped with the multi-dimensional CCD cameras, can realize the automatic focusing and positioning, and have the multiple built-in coating models and substrate correction programs. The measurement precision can reach ±0.01 μm, widely used for the on-line detection of the production lines and the laboratory analysis.

Metallographic microscope and image analysis system: Composed of an inverted or upright metallographic microscope, high-resolution digital camera and professional image analysis software. Used to observe the micro-structures of the coating cross-sections, and automatically or semi-automatically measure the parameters such as the thicknesses, porosities and grain sizes of the multi-layer coatings through the software. It is the necessary tool for the destructive precision analysis and failure analysis.

Salt spray test chamber: The constant temperature and humidity equipment for simulating the corrosive environments. It has the precise temperature and humidity control, automatic replenishment and atomization systems of the salt solutions, saturated air barrels and waste gas neutralization devices. Conforming to the requirements of the standards such as ASTM B117, ISO 9227 and GB/T 10125, it can carry out the various tests such as the neutral salt spray, acetic acid salt spray and copper-accelerated acetic acid salt spray.

Coating thickness gauge (non-XRF type): Including the magnetic thickness gauges and eddy current thickness gauges. The magnetic thickness gauges are based on the magnetic induction principle, used to measure the thicknesses of the non-magnetic coatings (such as the zinc, chromium and paints) on the iron and steel substrates; the eddy current thickness gauges are based on the eddy current principle, used to measure the thicknesses of the non-conductive coatings on the non-conductive substrates (such as the aluminium and plastics). Both are portable, suitable for the fast on-site testing.

Adhesion tester: Including the cross-cut testers, scratch testers and pull-off adhesion testers. The cross-cut testers are the manual or automatic multi-blade cutting tools; the scratch testers determine the critical values of the coating bonding forces by loading the diamond indenters and gradually increasing the loads; the pull-off testers paste the test studs of the specific sizes on the coating with the strong adhesives, and quantitatively measure the adhesion by the forces required for the vertical pulling-off.

Cupping testing machine and bending testing machine: Used to evaluate the adhesion and ductility of the coating during the forming processing. The cupping testing machine presses the specimen into the die cavity with the specified steel balls through the hydraulic or mechanical drives until the fracture, measuring the cupping depths and observing the coating states. The bending testing machines can carry out the precise bending at the different angles and with the different mandrel diameters, and are the key equipment for inspecting the processing properties of the material.

Electrochemical workstation: The precision instrument for the electrochemical testing, usually the integration of the potentiostat/galvanostat and computer control analysis systems. It can carry out the various tests such as the potentiodynamic polarization, linear polarization, electrochemical impedance spectroscopy and cyclic voltammetry. By analysing the electrochemical responses of the coating/substrate in the electrolytes, its corrosion behaviour and protection mechanisms are deeply studied.