Chromium-plated Thin Steel Sheet Testing

Chromium-plated thin steel sheet inspection mainly conducts scientific testing and analysis on the coating thickness testing, coating adhesion testing, corrosion resistance testing, surface hardness and wear resistance testing, surface appearance and roughness inspection, and coating 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

Coating thickness testing: This is one of the core items for evaluating the quality of the chromium-plated thin steel sheet. By measuring the thickness of the chromium coating and the underlying nickel coating (if any), the corrosion resistance, wear resistance and the compliance with the standard specifications of the product can be judged. Usually, the average value and minimum value of the multiple measuring points are required to be measured, to ensure the coating uniformity.

Coating adhesion testing: Used to evaluate the adhesion between the chromium coating and the substrate steel sheet or the underlying coating. The poor adhesion will cause the peeling and shedding of the coating, seriously affecting the service life of the product. The commonly used methods include the bending testing, file testing and thermal shock testing, evaluating the bonding strength in the qualitative or semi-quantitative manner.

Corrosion resistance testing: The examination of the corrosion resistance of the chromium coating by simulating the environment is the key for evaluating its protective performance. The main methods include the neutral salt spray testing and copper-accelerated acetic acid salt spray testing, rating the grade by observing the time when the corrosion products (such as the red rust) appear on the specimen surface. The longer the time, the better the corrosion resistance.

Surface hardness and wear resistance testing: The chromium coating has the characteristics of the high hardness. This item measures the coating hardness with a micro Vickers hardness tester or Knoop hardness tester. The wear resistance is evaluated by simulating the actual wear conditions with a friction and wear testing machine, evaluating the anti-wear ability of the coating. It is crucial for both the decorative and functional chromium plating.

Surface appearance and roughness inspection: Particularly important for the decorative chromium plating, including the visual inspection of the gloss, colour uniformity, and whether there are the defects such as the pinholes, burrs and blistering. At the same time, a surface roughness tester is used to measure the arithmetic mean deviation of the profile, ensuring that the surface finish meets the requirements of the subsequent processing or direct use.

Coating porosity testing: Detects the number of the micro-pores in the chromium coating. The pores will reach the substrate directly and become the starting points of the corrosion. The commonly used methods include the filter paper method and electrographic method. The specific reagents react with the exposed substrate metal to generate the coloured spots, thereby calculating the number of the pores per unit area.

Coating microstructure analysis: The metallographic microscope or scanning electron microscope is used to observe the cross-section morphology of the coating. The layers, thicknesses and continuity of the various coatings (such as the copper, nickel and chromium) can be clearly distinguished, and whether there are the micro-defects such as the cracks and inclusions can be detected, providing the basis for the process improvement.

Testing Scope

Decorative chromium-plated thin steel sheets: Widely used in the products such as the household appliances, automotive interior and exterior trim parts, hardware parts and furniture, mainly providing the bright and beautiful appearance and a certain anti-rust ability. The testing emphasis is on the appearance, gloss, corrosion resistance and adhesion, and the requirement for the coating thickness is relatively thin but uniform.

Hard (wear-resistant) chromium-plated thin steel sheets: Used for the parts bearing the friction such as the mechanical dies, hydraulic piston rods and measuring tools. The testing core is the hardness, wear resistance, thickness and fatigue resistance of the coating. The coating is usually thicker, and there are the strict requirements for the crack control.

Chromium-plated thin steel sheets for the automotive use: For the specific standards of the automotive industry, such as the exterior trim strips and door handles. In addition to the routine items, the cyclic corrosion resistance, UV aging resistance and the compatibility testing with the plastic substrates (such as the adhesion after the thermal cycling) are particularly emphasized.

Chromium-plated thin steel sheets for the electronic and electrical use: Used for the electronic product shells and connectors. The testing needs to focus on the conductivity, solderability, electromagnetic shielding effectiveness of the coating, as well as the performance stability under the high and low temperature and damp-heat environments.

Chromium-plated steel sheet strips (coils): Supplied as the raw materials to the downstream stamping and forming manufacturers. The testing needs to sample at the head, middle and tail of the whole coil, comprehensively evaluating the coating thickness uniformity, surface cleanliness, sheet shape and mechanical properties, ensuring the quality of the subsequent processing.

Post-treatment products of the chromium plating: The chromium-plated steel sheets after the post-treatments such as the passivation, oiling and organic coating are tested. It is necessary to verify the properties of the post-treatment films (such as the passivation film thickness and coating adhesion) and their enhancement effects on the corrosion resistance and fingerprint resistance of the coating.

Testing Methods

Coulometric thickness measurement method: An electrochemical dissolution method. The coating is anodically dissolved at a constant current, and the thickness is calculated by recording the dissolution time corresponding to the potential jump point. This method has the high precision and can distinguish the multi-layer coatings (such as the Cu/Ni/Cr). It is a common method for measuring the absolute thickness, but is a destructive testing.

X-ray fluorescence spectrometry method: The X-rays are used to excite the coating elements to generate the characteristic fluorescence, and the coating thickness is calculated by measuring the fluorescence intensity. This method is fast, non-destructive and can measure the multiple layers, and is suitable for the online or laboratory rapid testing. It is currently the mainstream non-destructive thickness measurement method.

Metallographic microscope method: After the vertical cross-section of the specimen is mounted, polished and etched, the thicknesses of the various coatings are directly observed and measured under a metallographic microscope. This method is intuitive and accurate, and is the basis of the arbitration measurement, but the sample preparation is complex and it is a destructive testing.

Neutral salt spray testing: In accordance with the standards GB/T 10125 / ISO 9227, the specimen is placed in a salt spray chamber formed by the 5% sodium chloride solution for the continuous spraying. The time of the appearance of the red rust is regularly checked and recorded, thereby evaluating the corrosion resistance of the chromium coating and its underlying system. It is a classic method of the accelerated corrosion.

Adhesion bending testing: The specimen is bent 180 degrees around a mandrel of a certain diameter (or bent repeatedly), and it is observed whether the coating on the outside of the bending part peels or sheds. It is applicable to the thin steel sheets with the sufficient ductility of the substrate, and is a simple and effective qualitative evaluation method.

Thermal shock testing: The specimen is heated at a high temperature (such as 200-300 ℃) for a certain time, and then rapidly quenched in the room-temperature water. The stress is generated by utilizing the difference in the thermal expansion coefficients of the coating and the substrate, and it is checked whether the coating blisters or falls off. It is commonly used to test the adhesion.

Electrochemical testing: Through the electrochemical methods such as the potentiodynamic polarization and electrochemical impedance spectroscopy, the corrosion potential, corrosion current density and polarization resistance of the chromium coating in the corrosive media are studied, quantitatively evaluating its corrosion resistance and porosity from the kinetic perspective.

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. ISO 1456, Metallic and other inorganic coatings — Electrodeposited coatings of nickel, nickel plus chromium, copper plus nickel and copper plus nickel plus chromium

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

Coating thickness gauges: Including the X-ray fluorescence thickness gauges and coulometric thickness gauges. The X-ray fluorescence instruments are suitable for the fast non-destructive measurement, and the coulometric instruments are used for the high-precision arbitration analysis. The modern equipment is mostly equipped with the automatic stages and multi-point measurement software, which can generate the thickness distribution maps.

Salt spray test chamber: Used to simulate the corrosion environment of the marine or chloride-containing industrial atmospheres. The equipment needs to precisely control the sodium chloride solution concentration, deposition rate, chamber temperature and humidity, and have the various modes such as the continuous spraying and cyclic spraying, to meet the requirements of the different standards.

Metallographic microscope and image analysis system: Composed of an optical microscope, digital camera and dedicated analysis software. It is used to observe the microstructure of the coating, measure the thickness and analyse the pores and cracks. At the high magnification, the micro-crack networks can be clearly distinguished, evaluating the quality of the hard chromium coating.

Micro hardness tester: Used to measure the hardness of the chromium coating itself. Since the coating is thin, the micro Vickers or Knoop indenters need to be selected, and the testing is carried out under the micro loads (such as 10 gf-200 gf), to avoid the indentation penetrating the coating or being affected by the substrate, thereby obtaining the true hardness value of the coating.

Surface roughness tester: The diamond probe moves on the specimen surface, records the profile curves, and calculates the parameters such as the Ra and Rz. For the decorative chromium plating requiring the high finish or the wear-resistant chromium plating needing to store the lubricating oil, the surface roughness is the key control indicator.

Scanning electron microscope and energy dispersive spectrometer: The SEM provides the extremely high magnification and depth of field, used to observe the surface morphology, cross-section structure and the micro-morphology after the wear and corrosion of the coating. Combined with the EDS spectrometer, the micro-area qualitative and semi-quantitative analysis of the coating composition can be carried out.

Adhesion testing dedicated fixtures: Including the bending test mandrels of the different diameters, cross-cut testers and impact testers. These tools, combined with the corresponding testing standards, are used to implement the qualitative adhesion testing, with the simple operation and intuitive results.