Tin-plated Steel Plate Testing

Tin-coated steel plate inspection mainly conducts scientific testing and analysis on the coating thickness determination, coating adhesion testing, tin layer surface structure analysis, porosity testing, alloy-tin couple testing and hardness and mechanical property 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 determination: By measuring the mass of the tin layer per unit area or directly measuring its geometric thickness, the uniformity and corrosion resistance of the tin plating are evaluated. The X-ray fluorescence spectrometry is usually adopted for the non-destructive and fast measurement, and the coulometric method can also be used for the high-precision analysis. This indicator directly affects the rust prevention ability of the steel plate during the transportation, storage and use.

Coating adhesion testing: The item evaluating the bonding firmness between the tin layer and the steel substrate. The common methods include the bending testing, impact testing and cross-cut method. The excellent adhesion can ensure that the coating does not peel off in the subsequent processing (such as the stamping and flanging), and is the core indicator guaranteeing the formability and durability of the product.

Tin layer surface structure analysis: The observation and analysis of the grain sizes, morphologies and alloy layers (FeSn2) on the tin coating surface. Usually completed with the aid of the optical microscopes or scanning electron microscopes. The surface structure affects the gloss, paintability and solderability of the coating, and is the important basis for evaluating the stability of the tin plating process.

Porosity testing: Detects the defects such as the pinholes and cracks in the coating. These defects will directly expose the steel substrate and become the initiation points of the corrosion. The common methods include the copper sulfate dropping testing, iron rust testing and electrochemical testing. The low porosity is the key to ensuring the long-term protection performance of the tin-coated steel plate.

Alloy-tin couple testing: Used to evaluate the corrosion resistance of the tin-coated steel plate under the specific environments (such as the food packaging), especially for the applications containing the acidic contents. This testing simulates the electrochemical corrosion process under the actual service states by measuring the potential difference between the tin layer and the alloy layer.

Hardness and mechanical property testing: The mechanical properties of the tin-coated steel plate are detected, including the Vickers hardness, Rockwell hardness, tensile strength, yield strength and elongation. These properties determine the performance of the material in the processing such as the can making and deep drawing, and need to meet the standard requirements of the specific applications.

Surface roughness and gloss measurement: The profilometers or gloss meters are used for the quantitative evaluation of the steel plate surface. The surface roughness affects the adhesion of the coatings and the retention of the lubricants, while the gloss is directly related to the appearance quality of the product, and is the important evaluation indicator of the high-end packaging materials.

Testing Scope

Electrolytic tin-coated steel plates: Usually referring to the products with the tin plated on the cold-rolled low-carbon thin steel plates through the electrochemical methods, divided into the equally coated and differentially coated tin plates. Widely used in the metal packaging fields such as the food, beverage cans and aerosol cans. The testing needs to focus on the coating uniformity under the high-speed production.

Hot-dip tin-coated steel plates: Produced by the hot-dip process, the tin layers are usually thicker, with the good corrosion resistance and solderability. Mainly used for the chemical drums, industrial cans and some traditional food cans. The testing focuses on the stability of the coating thicknesses and the continuity of the alloy layers.

Chromium-plated/tin-coated steel plates: Referring to the steel plates with a very thin chromate passivation film superimposed on the tin coating layer. The passivation film can significantly improve the corrosion resistance and paint adhesion. The testing scope needs to be extended to the passivation film thickness, coating amounts (calculated in chromium) and corrosion resistance testing.

Tin-free steel plates: An environmentally friendly steel plate using the chromium oxides and hydrated oxides as the coatings, partially replacing the traditional tin-coated plates. Its testing items partially overlap with the traditional tin-coated plates, but more focus on the quality, porosity and paintability of the chromium layer chemical conversion films.

Steel plates of the different tempers: The temper degrees (such as from T-1 to T-5) of the tin-coated substrates determine their hardness and formability. The testing scope needs to cover the corresponding mechanical property testing according to the temper degrees, ensuring that the steel plate can handle the different processing degrees from the shallow drawing to the deep drawing.

Special tin-coated steel plates: Including the high-corrosion-resistance tin-coated plates, low-tin-content tin-coated plates and the tin-coated plates for the special environments (such as the high-temperature sterilization). The testing needs to be customized according to their special uses, for example, evaluating the adhesion changes after the high-temperature and high-pressure sterilization and the corrosion rates in the specific media.

Processed products of the tin-coated steel plates: The testing of the tin-coated steel plate products after the printing, coating, welding and stamping forming. The testing emphasis shifts from the raw material properties to the comprehensive properties after the processing, such as the weld strength, coating adhesion and corrosion resistance of the formed parts.

Testing Methods

X-ray fluorescence spectrometry method: The preferred method of measuring the coating thickness, based on measuring the intensity of the characteristic X-rays of the tin element, and converting it into the coating mass or thickness through the standard curves. This method is fast, non-destructive and precise, suitable for the fast on-line or laboratory testing, and is the core means of the production quality control.

Coulometric thickness measurement method: An electrochemical dissolution method. The local coating is dissolved with a constant current, and the dissolution time is recorded to calculate the coating thickness. With the extremely high measurement precision, it is commonly used for the arbitration testing and the value determination of the reference materials. This method is destructive, but is insensitive to the coating compositions, with the accurate and reliable results.

Metallographic microscopic method: The cross-section metallographic specimens of the tin-coated steel plate are prepared, and the total coating thickness and the thicknesses of the various layers (tin layer and alloy layer) are directly observed and measured under an optical microscope or electron microscope. This method is intuitive, and is the method of studying the coating structures and analysing the causes of the defects.

Bending testing method: The classical method of evaluating the coating adhesion. The specimen is bent around an axis of the specified diameter to a specific angle, checking whether there is the peeling or blistering of the coating on the outer side of the bend. According to the different standards, the repeated bending and single bending modes can be adopted, simulating the deformation conditions in the actual processing.

Electrochemical corrosion testing method: Including the potentiodynamic polarization curve method and electrochemical impedance spectroscopy. By measuring the electrochemical parameters of the material in the electrolyte solutions, its corrosion tendency and corrosion rates are quantitatively evaluated. This method can quickly and sensitively reflect the protection performance and porosity conditions of the coating.

Chemical dropping testing method: A simple and fast qualitative or semi-quantitative method. For example, the copper sulfate solution is dropped on the surface, and the porosity of the coating is judged by observing the time when the red copper displacement spots appear. This method has the simple equipment, and is commonly used for the fast screening on the production sites.

Spectrometry analysis and energy spectrum analysis: The energy dispersive spectrometers equipped on the scanning electron microscopes or the dedicated glow discharge spectrometers are used for the composition analysis of the coating, detecting the distributions and contents of the elements such as the tin, iron and chromium. It is crucial for the alloy layer research and impurity element analysis.

Testing Standards

Domestic Standards:

1. GB/T 2520, Electrolytic tin coated cold rolled steel sheet and strip

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

6. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

1. ISO 11953, Tinplate — Determination of tin coating mass — Method of coulometric analysis

2. ASTM A630/A630M, Standard test methods for determination of tin coating weights for electrolytic tin plate

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

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 coating thickness gauge: The standard configuration of the modern tin plating production lines and laboratories. The instrument has the multiple built-in targets and optical filters, and can precisely measure the coating elements such as the tin and chromium. The advanced models have the functions of the automatic multi-point measurement, statistical analysis and SPC process control, and are the key equipment for realizing the automatic quality monitoring.

Coulometric thickness gauge: Composed of a potentiostat/galvanostat, electrolytic cell and data recording unit. The electrolytic cell is precisely designed to ensure the fixed electrolytic areas. The instrument needs to be calibrated with the standard samples, and its high-precision characteristics make it the essential instrument of the inspection agencies and standard laboratories.

Metallographic microscope and scanning electron microscope: The metallographic microscope is used to observe the macro-structures of the coating and measure the thicknesses; the scanning electron microscope can provide the higher-power micro-morphology observation, and its equipped energy dispersive spectrometers can carry out the micro-area composition analysis. It is the core tool of the in-depth research of the coating failure mechanisms.

Electrochemical workstation: Used to carry out the various electrochemical corrosion tests. Usually a three-electrode system (working electrode, reference electrode and auxiliary electrode), composed of a potentiostat and computer control software, which can carry out the various tests such as the polarization curves, impedance spectra and cyclic voltammetry, evaluating the corrosion resistance of the coating.

Mechanical property testing machine: Including the universal material testing machines, hardness testers (Vickers and Rockwell) and cupping testing machines. Used to determine the tensile, bending, hardness and forming properties (such as the Erichsen values) of the tin-coated steel plate, ensuring that it meets the requirements of the downstream processes such as the can making and stamping.

Surface morphology and roughness measuring instrument: The contact profilometers obtain the roughness parameters such as the Ra and Rz by scanning the surfaces with the probes; the non-contact white light interferometers can obtain the three-dimensional surface morphologies. The gloss meters quantify the surface gloss by measuring the reflected light intensities at the specific angles.

Constant temperature and humidity salt spray test chamber: Simulates and accelerates the atmospheric corrosion environments, used to evaluate the medium and long-term corrosion resistance of the tin-coated steel plates and their passivation films. By controlling the temperature, humidity, sodium chloride solution concentration and spraying modes in the chamber, the standard tests such as the neutral salt spray and acetic acid salt spray can be carried out.