Tin-plated Sheet Testing

Tinned sheet inspection mainly conducts scientific testing and analysis on the tin coating weight testing, alloy-tin couple testing, superficial Rockwell hardness testing, surface chromium content testing and porosity 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: This testing is the core of the tinned sheet quality. The protective performance is evaluated by determining the mass of the tin layer per unit area. The testing results are usually expressed in grams per square metre (g/m²), and are divided into the single-side tin coating weight and the equal-thickness/differential-thickness coatings. The precise tin coating weight data are directly related to the corrosion resistance, soldering performance and cost control of the product.

Alloy-tin couple testing: This testing is used to evaluate the continuity and compactness of the alloy layer on the surface of the tinned sheet, and is the key indicator for measuring its corrosion resistance. The quality of the alloy layer is indirectly reflected by measuring the current value of the exposed part of the alloy layer in the specific electrolyte. The lower the value, the more complete the alloy layer and the better the protection of the steel substrate.

Superficial Rockwell hardness testing: The hardness is an important parameter for measuring the mechanical properties of the tinned sheet substrate, directly affecting its stamping formability, rigidity and dent resistance. A Rockwell hardness tester (such as the HR30Tm scale) is usually adopted for the multi-point measurement on the sheet surface, to ensure the uniformity of the substrate hardness and meet the processing requirements of the subsequent can making and lid stamping.

Surface chromium content testing: For the passivated tinned sheets (such as the ECCS), the chromium content in the surface passivation film is crucial. It determines the thickness and corrosion resistance of the passivation film, especially the influence on the paint adhesion and sulphide discoloration resistance. The chemical titration method or X-ray fluorescence spectrometry is commonly used for the determination.

Porosity testing: Detects whether there are the discontinuous micro-pores in the tin coating, and these micro-pores will expose the steel substrate and become the starting points of the corrosion. The common methods include the iron dissolution value testing and potassium ferrocyanide solution testing, and the continuity and compactness of the coating are evaluated through the chemical colour development reactions.

Testing Scope

Tinned sheets for the food packaging: Mainly used for the food contact materials such as the cans, beverage cans and milk powder cans. The testing emphasis is on the corrosion resistance (acid resistance and sulphur resistance) of the tin coating, coating film adhesion, safety of the dissolved substances (migration of the heavy metals such as the lead and cadmium) and the integrity of the soldered parts, and it needs to comply with the food contact material regulations of the various countries.

Tinned sheets for the chemical packaging: Used to package the non-food products such as the paints, oils and greases and pharmaceuticals. The testing focuses on the resistance of the tinned sheet to the specific chemical media, as well as the chemical corrosion resistance and sealing performance of the inner coatings, ensuring the safety and stability of the packaging under the long-term storage.

Tinned sheets for the electronics and electrical industry: Applied to the capacitor casings, shielding covers and connectors. The testing focuses on the solderability of the tin coating, surface gloss after the soldering, electrical conductivity and the oxidation resistance after the long-term storage, ensuring the good electrical connection performance and assembly reliability.

General industrial tinned sheets: Covering the fields such as the toys, daily necessities and hardware parts. The testing items are relatively comprehensive, including the coating appearance (gloss and colour uniformity), mechanical properties (hardness and toughness) and the basic weather resistance, meeting the diverse processing and use needs.

Differential-thickness tinned sheets: Referring to the products with the different tin coating weights on the two sides, usually with the higher tin coating weight on the inner wall to enhance the corrosion resistance and the lower one on the outer wall to save the cost. During the testing, the tin coating weights, alloy layer characteristics and passivation film quality on the two sides need to be precisely determined respectively, ensuring that its asymmetric performance meets the design expectations.

Testing Methods

Coulometric method for determining the tin coating weight: This is the internationally common standard method (such as ISO 11953 and ASTM A630). The tin coating is anodically dissolved from the specimen of a known area through the electrolysis, and the mass of the tin is precisely calculated according to the consumed electric quantity (coulombs). This method is accurate and fast, and is the first choice for the arbitration and precision measurement.

X-ray fluorescence spectrometry method: A non-destructive and fast testing method. The X-rays excite the tin atoms in the coating to produce the characteristic fluorescence, and the tin coating weight is calculated through the intensity. It is suitable for the online testing of the production lines and the rapid screening of the finished products, but needs to be calibrated with the standard samples.

Alloy-tin couple testing method: The specimen is used as the anode and placed in a specific deoxidized sodium gluconate electrolyte, and a constant potential is applied to measure its current-time curve. The quality of the alloy layer is quantitatively evaluated by calculating the average current value within the specific time (such as the ATA value).

Salt spray testing: Used to evaluate the corrosion resistance of the tinned sheet and its passivation film. The specimens are placed in an enclosed salt spray test chamber, simulating the marine or industrial atmospheric environments, and the corrosion resistance grade is rated by observing the area and degree of the rust or white rust appearing on the surface within the specified time.

Paint film adhesion testing: For the coated tinned sheets, the cross-cut method or pull-off method is commonly used. In the cross-cut method, a knife is used to cut the grid on the paint film, and the tape is used to tear it off, rating according to the area of the paint film falling off; in the pull-off method, a dedicated adhesive and tensile testing machine are used to quantitatively determine the adhesion strength between the paint film and the substrate.

Testing Standards

Domestic Standards:

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

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

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

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

5. GB/T 4340.1-2009, Metallic materials — Vickers 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. 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

Coulometric thickness gauge: A precision instrument specially used for the coulometric measurement of the metal coating thickness. Its core components are the electrolytic cell and potentiostat, which can precisely control the dissolution process, automatically integrate and calculate the electric quantity, and thereby convert it into the tin coating weight. The instrument is usually equipped with the various electrolytes to adapt to the different substrates and coatings.

X-ray fluorescence spectrometer: The key equipment for the fast and non-destructive analysis of the coating composition and thickness. It is composed of an X-ray tube, detector, spectroscopic system and data analysis software. In the tinned sheet testing, the dedicated tin coating measurement program can be configured to realize the single-point or multi-point automatic scanning.

Electrochemical workstation: Used for the electrochemical analyses such as the alloy-tin couple (ATC) testing. This equipment can precisely control the potential or current applied to the specimen and record the response signals in real time. It is an important tool for researching the micro-mechanisms such as the coating corrosion behaviour and passivation film performance.

Salt spray test chamber: The key equipment for simulating the corrosion environments. The sodium chloride solution is atomized and sprayed on the specimen surfaces at the constant temperature and humidity, accelerating the corrosion process. The chamber body needs to be made of the corrosion-resistant materials, and have the precise temperature, humidity, spray amount control and sedimentation amount collection functions.

Precision hardness tester: The superficial Rockwell hardness tester (such as the HR30Tm scale) is usually used to test the hardness of the tinned sheet substrate. The instrument needs to be equipped with the diamond cone indenter and small-diameter steel ball indenter, ensuring that when testing on the thin sheet surface, the back side does not produce the deformation interference, and the testing results are accurate and reliable.