Mirror Stainless Steel Plate Testing

Mirror-finish stainless steel plate inspection mainly conducts scientific testing and analysis on the surface reflectance, surface roughness, surface defect testing, surface cleanliness and contaminants, corrosion resistance testing, and film thickness and coating 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

Surface reflectance: This is the core indicator of measuring the mirror effects, usually measured with the gloss meters at the specific angles (such as 20°, 60° and 85°). The high reflectance is the basic requirement of the mirror-finish stainless steels, needing to meet the specified values of the customers or relevant standards (such as ASTM D523), ensuring that they have the excellent imaging definition and visual consistency.

Surface roughness: Directly affecting the fidelity and aesthetics of the reflected imaging. Through the contact (such as the profilometers) or non-contact (such as the white light interferometers) methods, the arithmetic average deviation (Ra) of the surface profiles is measured. The Ra values of the high-quality mirror plates are usually extremely low (for example, Ra ≤ 0.05μm), to ensure that the surfaces are smooth without the micro undulations, avoiding the light scattering.

Surface defect testing: Aiming to identify and evaluate the various imperfections affecting the appearances and properties. Through the visual inspection, optical microscopes or automatic optical inspection systems, whether the defects such as the scratches, roll marks, pits, orange peel, color differences, pits and uneven polishing lines exist is checked, and the qualitative or quantitative records are made.

Surface cleanliness and contaminants: The residual contaminants such as the oil, fingerprints, polishing wax and dust on the surfaces are tested. The commonly used methods include the white cloth wiping method, ultraviolet lamp irradiation to observe the oil fluorescence, or the Fourier transform infrared spectrometers to analyse the contaminant compositions, ensuring the adhesion of the subsequent processing (such as the coating and bonding).

Corrosion resistance testing: Evaluating the corrosion resistance of the mirror-finish stainless steels in the specific environments, ensuring that the appearances can still be maintained after the long-term use. The testing methods include the neutral salt spray testing, copper-accelerated acetic acid salt spray testing and acetic acid salt spray testing, etc., carried out according to the time periods specified by the standards (such as ASTM B117 and ISO 9227), observing whether the rust, discoloration or gloss loss appears on the surfaces.

Film thickness and coating testing: For the mirror-finish stainless steels with the physical or chemical coatings such as the PVD and CVD to enhance the colors or properties, the coating thicknesses need to be precisely measured. The X-ray fluorescence thickness gauges or metallographic sectioning methods are usually used, ensuring that the coating thicknesses are uniform and meet the design requirements, to guarantee the color consistency and protection effects.

Testing Scope

Material identification and designation verification: Determining the specific designations of the stainless steels, such as 304, 316 and 430. Through the spark spectrometers or handheld X-ray fluorescence spectrometers for the fast elemental composition analysis, whether the contents of the key elements such as the chromium, nickel and molybdenum meet the standards is verified, ensuring that they have the corresponding corrosion resistance and mechanical property bases.

Dimension and geometric precision: Measuring the thicknesses, widths, lengths, flatness (levelness) and diagonal differences of the plates. The tools such as the micrometers, callipers, laser distance meters, platforms and feeler gauges are used, ensuring that the product dimension tolerances meet the drawing or standard (such as GB/T 3280 and ASTM A480) provisions, satisfying the precision assembly requirements.

Mechanical property evaluation: Although it is not the direct indicator of the mirror effects, it is crucial for the material processability and service lives. Through the sampling for the tensile testing (measuring the yield strengths, tensile strengths and elongations), hardness testing (Brinell, Rockwell and Vickers) and cupping testing, whether they meet the material standards is verified.

Chemical stability testing: Evaluating the resistance of the mirror-finish stainless steel surfaces to the chemical reagent erosion. The commonly used methods include the drop testing (such as using the copper sulfate solutions to check the chromium-depleted layers), or the immersion of the specimens in the acid and alkali solutions of the specific concentrations, observing the gloss changes or corrosion conditions of the surfaces after a certain time.

Color and color difference evaluation: For the colored mirror-finish stainless steels (such as the titanium gold, black titanium and rose gold), the accuracy and consistency of the colors are the keys. The spectrophotometers are used to measure their L*a*b* values under the standard light sources, and compared with the standard color plates, the color differences ΔE are calculated, ensuring that the batch products have the uniform colors without the color differences.

Base material micro-structure analysis: Through the metallographic preparation technology, the specimens are prepared and observed under the optical or electron microscopes for the grain sizes, inclusion types and distributions, and phase compositions. The good micro-structures are the internal guarantee of obtaining the excellent surface qualities and comprehensive properties.

Testing Methods

Visual inspection method: The most basic and intuitive testing method. Under the standard light source observation boxes or the specific lighting angles (such as the 45° incident light), the inspectors observe the plate surfaces at the specific distances and angles, comparing with the defect standard drawings or limit samples, and qualitatively judge the surface quality grades. This method relies on the experience of the inspectors, and is often used for the fast initial inspection and online inspection.

Instrumental gloss measurement method: Using the digital gloss meters, a beam of light is projected onto the sample surfaces at the fixed incident angles, the reflected luminous fluxes in the specular reflection directions are measured, and compared with the reflected luminous fluxes of the black glass standard plates with the refractive index of 1.567, obtaining the gloss values (unit: GU). The different angles are suitable for the products of the different gloss ranges.

Non-contact three-dimensional topography measurement method: The high-precision instruments such as the white light interferometers or confocal microscopes are used for the micro-area scanning of the surfaces, obtaining the three-dimensional topography data of the surfaces. The surface roughness parameters (such as the Ra, Rz and Rq) can be precisely calculated, and the micro peak-valley structures can be visually presented. It is the effective means of evaluating the ultra-smooth surface qualities.

Salt spray testing method: An accelerated corrosion testing method. The specimens are placed in the closed salt spray test chambers, simulating the marine or industrial atmospheric environments. A certain concentration of sodium chloride solution is atomized and sprayed. After the continuous testing for a certain time (such as 24h, 72h and 240h), the specimens are taken out, and the corrosion initiation times, corrosion areas and degrees of their surfaces are evaluated.

Electrochemical testing method: Used for the in-depth study of the corrosion resistance mechanisms of the material. Through the electrochemical workstations measuring the open circuit potentials, polarization curves or electrochemical impedance spectroscopy of the material, the parameters such as the corrosion current densities and polarization resistances can be quantitatively obtained, scientifically evaluating the effects of the different process treatments on the corrosion resistance of the mirror-finish stainless steels.

Adhesion testing method: For the mirror plates with the coatings. The commonly used methods include the cross-cut method (using the cutters to scratch the grids, observing the coating peeling conditions after the adhesive tape sticking and tearing), scratch method (using the scratch testers to measure the critical loads when the coatings peel off) or bending testing, to evaluate the bonding strengths between the coatings and stainless steel matrices.

Testing Standards

Domestic Standards:

1. GB/T 3280, Cold rolled stainless steel plate, sheet and strip

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

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

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

International Standards:

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

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

3. ASTM A480, Standard specification for general requirements for flat-rolled stainless and heat-resisting steel plate, sheet, and strip

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

5. ASTM D523, Standard test method for specular gloss

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

Gloss meter: The dedicated instrument of measuring the surface specular gloss. Usually equipped with the multiple measurement angles such as 20°, 60° and 85°, to adapt to the different surfaces from the high gloss to matte. The modern digital gloss meters have the data storage, statistical analysis and computer connection functions, and are the necessary equipment of the factory delivery inspection of the mirror plates.

Surface roughness tester: The instrument used to measure the surface micro unevenness. The contact roughness testers directly measure through the diamond probes sweeping across the surfaces; the non-contact ones such as the white light interferometers use the optical interference principle, with the higher precision and no damage to the surfaces, especially suitable for testing the high-value and ultra-smooth mirror products.

Salt spray test chamber: The testing equipment used to simulate the accelerated corrosion environments. The chambers are made of the corrosion-resistant materials, equipped with the precision atomization systems, temperature control systems and saturated air barrels. The neutral salt spray, acetic acid salt spray and copper-accelerated acetic acid salt spray and other standard testing can be carried out. It is the key equipment of verifying the corrosion resistance service lives of the materials and coatings.

Spectroscopic analyser: Used for the fast quantitative analysis of the material compositions. The spark direct-reading spectrometers have the high precision, often used for the designation verification of the incoming materials in the laboratories; the handheld X-ray fluorescence spectrometers are convenient for the on-site, non-destructive and fast screening of the material compositions, preventing the material mixing.

Color difference meter/spectrophotometer: The instrument used for the precise color measurement. Through the measurement of the spectral distribution of the sample reflected light, the values of the various color spaces (such as the CIELAB) are calculated. The high-precision spectrophotometers can detect the tiny color differences, and are the core tools of guaranteeing the color batch consistency of the colored mirror-finish stainless steel products.

Metallographic microscope and image analysis system: The instrument used to observe the micro-structures of the material. After the polishing and etching of the specimens, they are placed under the microscopes, and the grain sizes, inclusions and phase compositions can be detected and observed. Combined with the image analysis software, the automatic quantitative analysis of the grain sizes and phase ratios can be carried out, providing the basis for the process optimization.