Galvanized Steel Plate Testing

Galvanized steel plate inspection mainly conducts scientific testing and analysis on the coating quality, coating adhesion, surface quality and appearance, chemical composition analysis, salt spray corrosion testing and painting performance 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 quality: Referring to the weight of the zinc layer per unit area of the steel plate surface, usually expressed in grams per square metre (g/m²) or micrometres (µm). This is the core indicator for evaluating the corrosion resistance of the galvanized plate, directly related to the service life of the product. During the testing, according to the product specifications and standard requirements, the weighing method or magnetic thickness measurement method needs to be adopted for the precise measurement.

Coating adhesion: The property evaluating the bonding firmness between the zinc layer and the steel substrate. The poor adhesion will cause the peeling and blistering of the zinc layer, seriously affecting the protection effects. The common testing methods include the bending testing, cupping testing and cross-cut testing, and the grades are judged by observing whether the coating in the deformed areas cracks or peels.

Surface quality and appearance: Checks whether there are the defects affecting the use and aesthetics on the galvanized steel plate surface. The main observation items include the zinc flower uniformity, and whether there are the missed coating, bubbles, nodules, scratches, colour differences, white rust (wet storage stain) and oil stains. The testing is usually carried out with the visual inspection under the standard light sources and compared with the standard samples.

Chemical composition analysis: The quantitative analysis of the chemical compositions of the steel substrate and zinc coating is carried out. The substrate composition affects its mechanical properties and weldability, while the elements such as the aluminium, magnesium and antimony added to the zinc liquid affect the coating structures and corrosion resistance. The spectrometry analysis method (such as the direct-reading spectrometer) or chemical titration method is usually adopted for the testing.

Salt spray corrosion testing: Simulates the marine or industrial atmospheric environments, and accelerates the evaluation of the corrosion resistance of the zinc coating. The specimen is placed in an enclosed salt spray test chamber, and the sodium chloride solution is sprayed to form the salt spray. The corrosion resistance grade is evaluated by recording the time when the red rust (substrate corrosion) appears on the specimen. The common standards include ASTM B117 and ISO 9227.

Painting performance testing: For the galvanized plates that need the subsequent painting, their compatibility with the coatings needs to be evaluated. The key testing items include the surface roughness, surface tension (wetting tension testing), alkali resistance (simulating the degreasing process), and the adhesion after the painting (such as the cross-cut method and pull-off method) and corrosion resistance (such as the cyclic corrosion testing).

Testing Scope

Hot-dip galvanized steel plates: Referring to the products with the coatings obtained by immersing the steel plates into the molten zinc liquid. The testing scope covers the structural steels for the buildings (such as the C/Z-section steels), colour-coated substrates, automobile plates and household appliance housings. The testing needs to emphatically focus on the uniformity of the thick coatings (such as for the buildings), zinc flower morphologies and the surface finish of the thin coatings (such as for the household appliances).

Electrogalvanized steel plates: The zinc layers are deposited on the steel plate surfaces through the electrochemical methods. Mainly applied to the fields with the high requirements for the surface quality, formability and weldability, such as the automobile body panels and high-end appliances. The testing emphasis is on the precise control of the coating thicknesses, coating purity, chromium-free passivation film quality and painting adhesion.

Alloyed galvanized steel plates: The heat treatment of the hot-dip galvanized plates is carried out, so that the zinc and iron diffuse with each other to form the zinc-iron alloy layers. The products have the excellent painting and welding properties, mainly used for the automobiles and household appliances. The testing core is the alloy layer structures (through the metallographic observation), powdering resistance and friction coefficients.

Processed parts of the galvanized steel plates: The finished or semi-finished products after the secondary processing such as the shearing, stamping, welding and bending of the galvanized steel plates. The testing needs to evaluate the damage of the processing processes to the coatings, such as the coating adhesion at the cuts and bends, corrosion resistance changes of the welding heat-affected zones, and whether there is the coating powdering or peeling.

Special coating steel plates: Including the alloy coating steel plates such as the aluminium-zinc (Zn-Al) and zinc-aluminium-magnesium (Zn-Al-Mg). In addition to the routine galvanizing items, the testing needs to add the precise analysis of the aluminium and magnesium contents in the coatings, as well as the special corrosion resistance evaluation methods based on their unique self-healing properties (such as the scratch salt spray testing).

Galvanized plates with the different post-treatments: Such as the galvanized plates after the passivation (chromate or chromium-free), oiling, phosphating and pre-priming treatments. The testing needs to cover the characteristics of the post-treatment films, such as the passivation film thicknesses and corrosion resistance, oiling amounts, phosphating film crystal morphologies and weights, as well as the effects of these treatments on the subsequent processes.

Testing Methods

Weighing method (dissolution method): The arbitration method of measuring the coating quality. The principle is to completely dissolve the zinc layer on the specimen with a specific stripping solution (such as the hydrochloric acid containing the inhibitors or antimony trichloride solution), and calculate the coating quality per unit area through the weight difference before and after the dissolution. This method has the high precision, but is a destructive testing, and the operation steps need to strictly follow the standards such as GB/T 1839 or ISO 1460.

Magnetic thickness measurement method: The non-destructive and fast method of measuring the thicknesses of the non-magnetic zinc layers on the magnetic steel substrates. The magnetic flux or magnetic attraction force changed by the non-magnetic coating between the probe and steel substrate is used to convert into the thickness. It is applicable to the fast on-site testing of the production sites, but needs to be regularly calibrated with the standard thickness foils, and is sensitive to the substrate magnetism, surface curvature and roughness.

Electron microscope and energy spectrum analysis: The scanning electron microscope is used to observe the micro-morphologies, cross-section structures and alloy phase distributions of the coating, combined with the X-ray energy dispersive spectrometers for the qualitative and semi-quantitative analysis of the micro-area chemical compositions. This method is crucial for studying the coating growth mechanisms, analysing the causes of the defects (such as the missed coating and cracks) and evaluating the phase compositions of the alloyed coatings.

Electrochemical testing: The corrosion resistance and mechanisms of the galvanized layer in the electrolyte solutions are evaluated by measuring its electrochemical parameters. The common techniques include the potentiodynamic polarization curve testing (obtaining the corrosion currents and potentials) and electrochemical impedance spectroscopy analysis. This method has the high sensitivity, and can quickly evaluate the corrosion resistance differences of the different coating systems or processes.

Bending testing: The classical method of evaluating the coating adhesion. The specimen is bent by 180° (or 90°) around the mandrels of the specified diameters, and then it is checked whether the coating on the outer side of the bend (tensile surface) peels or blisters. According to the standards (such as GB/T 232), the devices such as the roller type, vice type or V-die type can be adopted. The more stringent T-bend testing can also be adopted.

X-ray fluorescence spectrometry method: A fast and non-destructive element analysis method. The X-rays are used to excite the sample atoms to generate the characteristic X-ray fluorescence, and the qualitative and quantitative element analyses are carried out by analysing the fluorescence spectra. Commonly used for the on-line or fast detection of the coating thicknesses (based on the signal intensity ratios of the coating and substrate elements) and the alloy element contents in the coatings.

Testing Standards

Domestic Standards:

1. GB/T 13912, Hot dip galvanized coatings on fabricated iron and steel articles — Specifications and test methods

2. GB/T 1839, Test methods for gravimetric determination of the mass of hot-dip galvanized coatings on ferrous products

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

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

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

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

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

International Standards:

1. ISO 1460, Metallic coatings — Hot dip galvanized coatings on ferrous materials — Gravimetric determination of the mass per unit area

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

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

4. 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 gauge: The core on-site testing equipment, divided into the magnetic method and eddy current method. The magnetic method is used for the non-magnetic coatings on the steel substrates; the eddy current method is used for the non-conductive coatings on the non-conductive substrates or the non-conductive coatings on the conductive substrates. The modern equipment usually has both principles, with the data storage, statistics and computer connection functions, and the probes need to be selected according to the shapes (planes, convex surfaces and concave surfaces) and ranges.

Salt spray corrosion test chamber: Used to simulate the accelerated corrosion environments. The sodium chloride solution is atomized by the compressed air and sprayed on the specimens in the constant-temperature chambers. The equipment needs to precisely control the chamber temperatures, salt spray sedimentation rates, solution pH values and concentrations. According to the test standards, it can also be expanded into the cyclic corrosion test chambers, realizing the alternating cycles of the various environmental conditions such as the salt spray, drying and humidity.

Electronic balance and stripping device: Used for the weighing method of measuring the coating quality. The precision of the electronic balance needs to reach 0.1 mg. The stripping devices include the constant-temperature water baths (controlling the stripping solution temperatures), suitable stripping containers and ventilation facilities. The stripping solutions of the different standard formulations need to be equipped, to handle the different coatings such as the pure zinc and zinc-iron alloys.

Metallographic microscope and image analysis system: Used to observe the micro-structures of the coating cross-sections. Through the mounting, grinding, polishing and etching, the metallographic specimens are prepared, and the thicknesses, continuity and bonding conditions with the substrates of the various coating layers (such as the pure zinc layers and alloy layers) are observed under a metallographic microscope. The equipped image analysis software can automatically measure the layer thicknesses and count the phase proportions.

Mechanical property testing machine: Used to evaluate the mechanical and adhesion properties of the galvanized plate and its coating. For example, the universal material testing machines can carry out the bending testing and cupping testing to test the coating ductility and adhesion; the scratch testers can quantitatively test the bonding strengths between the coatings and substrates; the surface roughness testers are used to measure the profile arithmetic mean deviations of the coated plate surfaces, which affect the painting performance.

Spectrometer: Including the direct-reading spectrometers and X-ray fluorescence spectrometers. The direct-reading spectrometers (OES) are mainly used for the fast and precise quantitative analysis of the chemical compositions of the steel substrates, and are the key equipment of the incoming raw material inspection. The X-ray fluorescence spectrometers (XRF) are more commonly used for the fast non-destructive screening and analysis of the compositions and thicknesses of the finished product coatings.