Tinplate Testing

Tinned plate (tinplate) inspection mainly conducts scientific testing and analysis on the tin coating weight testing, superficial Rockwell hardness, alloy-tin couple, chromate passivation film testing, porosity testing and surface oil film weight 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

Tin coating weight testing: Determines the weight of the tin layer per unit area on the tinned plate substrate, and is the core indicator for measuring its corrosion resistance and processing performance. It is usually expressed in grams per square metre (g/m²), and the common specifications include E1 (2.8/2.8), E2 (5.6/5.6), etc. The testing results are directly related to the corrosion resistance life and soldering performance of the product.

Superficial Rockwell hardness (HR30T) testing: The key item for evaluating the mechanical strength of the tinned plate substrate (steel base), directly affecting the subsequent forming and processing performances such as the stamping and drawing. The excessively high hardness value will make the processing difficult or even cause the cracking, and the excessively low value may result in the insufficient rigidity. A Rockwell hardness tester is usually adopted, and the indentation depth is measured under the specified load.

Alloy-tin couple (ATC) testing: An important means for evaluating the continuity and compactness of the alloy layer (FeSn₂) on the surface of the tinned plate. As the intermediate layer between the tin layer and the steel base, the quality of the alloy layer determines the adhesion of the coatings and the corrosion resistance. The grade is rated by observing the surface colour change or measuring the current after the treatment with the specific reagents.

Chromate passivation film testing: For the chromium-free (TFS) or low-chromium tinned plates, the weight and composition of the chromate conversion film on the surface need to be detected. The chemical titration method or X-ray fluorescence spectrometry (XRF) is usually adopted to determine the contents of the metallic chromium (Cr) and trivalent chromium (Cr³⁺) per unit area, to evaluate its sulphide resistance, discoloration resistance and paint adhesion.

Porosity testing: Checks whether there are the discontinuous micro-holes in the tin layer or passivation film. The common methods include the copper sulphate dropping test and iron rust test. These pores will expose the steel base and become the starting points of the corrosion, seriously affecting the corrosion resistance of the tinned plate, especially when the corrosive contents are contained.

Surface oil film weight testing: Measures the amount of the oil applied after the passivation remaining on the surface of the tinned plate, usually expressed in milligrams per square metre (mg/m²). The appropriate oil film can play the short-term anti-rust and lubrication roles, which is conducive to the subsequent processing and transportation, but the excessive amount will affect the adhesion of the coatings and printing.

Surface morphology and roughness testing: A profilometer or atomic force microscope (AFM) is used to analyse the micro-morphology and roughness (Ra value) of the tinned plate surface. This affects the levelling of the paints, adhesion and the fineness of the printed patterns, and is the basic testing for ensuring the high-quality appearance and functionality.

Testing Scope

Food and beverage packaging cans: Mainly used to test the tin coating weight, porosity, ATC value and the dissolved substances (such as the migration amounts of the lead and tin ions), ensuring the flavour safety of the contained food and preventing the leakage caused by the corrosion. It needs to comply with the food contact material safety standards such as GB 4806.9.

Chemical product packaging drums: Emphatically tests its chemical corrosion resistance, including the resistance to the specific acid and alkali solutions, passivation film integrity and adhesion. The tinned plate is required to have the stronger corrosion resistance and mechanical strength, to prevent the chemical leakage and corrosion perforation.

Aerosol cans: The requirements for the mechanical properties of the tinned plate are extremely high, and its hardness, tensile strength, elongation and weld strength need to be strictly tested. At the same time, the pressure resistance testing and bursting testing need to be carried out, ensuring that the can body is safe and reliable under the high pressure, without the deformation or rupture.

Crown caps and easy-open ends: As the sealing parts, the forming performance (hardness and cupping value), adhesion of the sealing compounds and the fatigue strength of the opening parts need to be emphatically tested. For the scoring parts of the easy-open ends, the micro-metallographic analysis also needs to be carried out, ensuring that the scoring depth is consistent and free of the micro-cracks.

Electronic and electrical components: Used to manufacture the capacitor casings and shielding covers. The testing focuses on the dimensional precision, surface insulation resistance, solderability and electromagnetic shielding effectiveness. There are the high requirements for the uniformity and purity of the tin layer, to ensure the good soldering and electrical performances.

Art and decoration materials: Such as the printed and tinned cans and nameplates. The testing emphasis is on the surface gloss, colour difference, printing adhesion (cross-cut test) and scratch resistance. The tinned plate surface is required to have the excellent coating bonding strength and anti-aging discoloration ability.

General industrial sheets: As the raw material coils or sheets, the testing scope covers all the basic items, including the thickness tolerance, width, camber, coating uniformity and surface defects (such as the scratches and roll marks), to meet the processing needs of the different downstream customers.

Testing Methods

Coulometric thickness measurement: The classic method for measuring the tin coating weight. The tin layer on the specimen surface is dissolved through the electrolytic cell, the electric quantity changes during the dissolution process are recorded, and the tin layer weight per unit area is precisely calculated according to the Faraday's law. This method has the high accuracy and is the main means of the arbitration and laboratory calibration.

X-ray fluorescence spectrometry (XRF): A fast and non-destructive testing method, widely used for the online or offline coating thickness (tin coating weight and chromium layer weight) testing on the production lines. By measuring the intensity of the characteristic X-rays of the elements to be tested, it is quickly converted into the coating thickness or weight, with the extremely high efficiency.

Electrochemical testing method: Including the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), used for the in-depth research of the corrosion behaviour, passivation film stability and porosity of the tinned plate in the specific media. The actual corrosion environments can be simulated, providing the kinetic parameters such as the corrosion rates and pitting sensitivity.

Chemical dissolution-titration method: The traditional method for determining the chromium content in the passivation film. The passivation film is dissolved with a specific solution (such as the sodium hydroxide), and then the redox titration (such as the potassium permanganate titration) is used to determine the total chromium and hexavalent chromium contents. The method is classic and reliable, but the operation is relatively cumbersome.

Salt spray testing: An accelerated testing method for evaluating the corrosion resistance of the tinned plate. The specimens are placed in an enclosed salt spray test chamber, simulating the marine or industrial atmospheric environments, with the continuous or intermittent spraying of the sodium chloride solution, and the corrosion resistance grade is rated by observing the time when the red rust appears on the surface.

Bending testing and cupping testing: The mechanical testing methods for evaluating the formability of the tinned plate substrate. The bending testing checks whether the outer side of the specimen cracks after bending to the specified angle; the cupping testing uses a spherical punch to press the specimen into the die, measuring the pressing depth until the cracks occur (cupping value).

Scanning electron microscope/energy dispersive spectroscopy (SEM/EDS): Used for the high-magnification micro-morphology observation and micro-area composition analysis of the tin coating, passivation film and alloy layer of the tinned plate. It can display the coating crystal morphology, defects and interface bonding conditions, and carry out the element mapping or line scanning analysis.

Testing Standards

Domestic Standards:

1. GB 4806.9, National food safety standard — Food contact materials and articles — Metal materials and articles

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. ISO 11953, Tinplate — Determination of coating weight — Coulometric method

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

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

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

5. 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: Including the coulometric thickness gauges and X-ray fluorescence thickness gauges. The coulometric method has the high precision and is used for the precise measurement in the laboratory; the XRF method is fast and suitable for the on-site and online testing, and is the standard quality monitoring equipment of the modern tinned plate production lines.

Hardness tester: The superficial Rockwell hardness tester (such as the HR30T scale) is mainly used. The instrument applies the specified initial load and main load, measures the indentation depth produced by the diamond cone indenter, and directly converts it into the hardness value reading. It is the key equipment for controlling the soft and hard state of the substrate.

Salt spray test chamber: Used to simulate the accelerated corrosion environment. The equipment has the precise temperature and humidity control, salt solution atomization and sedimentation amount collection systems, and can carry out the neutral salt spray (NSS), acetic acid salt spray (AASS) and other tests according to the standards such as GB/T 10125 and ASTM B117.

Electrochemical workstation: A precision instrument integrating the potentiostat, galvanostat and frequency response analyser. It is used to carry out the potentiodynamic scanning, electrochemical impedance spectroscopy and other tests, and is equipped with a three-electrode system (working electrode, reference electrode and counter electrode) and dedicated electrolytic cells.

Cupping testing machine and universal material testing machine: The cupping testing machine is specially used to determine the forming performance of the metal sheets. The universal material testing machine can carry out the various mechanical property tests such as the tensile, bending and shear, obtaining the key data such as the tensile strength, yield strength and elongation of the tinned plate substrate.

Surface morphology analyser: Including the contact profilometers and non-contact optical profilometers (white light interferometers). Used for the quantitative measurement of the two-dimensional/three-dimensional roughness parameters (such as the Ra and Rz) of the tinned plate surface, analysing the rolling textures and the micro-relief states of the coating surfaces.

Spectrum analyser: In addition to the XRF, the inductively coupled plasma optical emission spectrometer (ICP-OES) or atomic absorption spectrometer (AAS) is sometimes used, for the precise determination of the trace element compositions in the tinned plate substrate, coating or passivation film, such as the contents of the harmful substances like the lead and cadmium.