Weathering Steel Plate Testing

Weathering steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, corrosion resistance assessment, surface rust layer stability and compactness analysis, welding property testing and dimension and shape inspection, 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

Chemical composition analysis: This is the basis of the weathering steel plate testing. Through the methods such as the spectrometry analysis, the contents of the elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulphur (S), copper (Cu), chromium (Cr) and nickel (Ni) in the steel are precisely determined. These elements, especially Cu, P, Cr and Ni, are the key to forming the dense protective rust layer, and their ratios directly affect the atmospheric corrosion resistance of the steel plate.

Mechanical property testing: Evaluates the load-bearing and deformation abilities of the steel plate under the stressed states. The key testing items include the yield strength, tensile strength, elongation after fracture and impact toughness (often tested at the low temperatures of -20 ℃ or -40 ℃). These properties ensure that the steel plate can still meet the structural safety and durability requirements under the harsh climates and complex loading conditions.

Corrosion resistance assessment: The core testing item, directly verifying its "weathering" characteristics. The cyclic immersion corrosion testing, salt spray testing or outdoor atmospheric exposure testing is usually adopted. By comparing the mass losses, corrosion depths or rust layer morphologies at the different time periods, its corrosion resistance is quantitatively or qualitatively evaluated and compared with the ordinary carbon steel.

Surface rust layer stability and compactness analysis: The excellent properties of the weathering steel originate from the stable, dense and firmly attached rust layer formed on its surface. The testing includes the rust layer micro-morphology observation (such as using the SEM), rust layer composition analysis (the XRD determination of the protective phases such as the α-FeOOH) and rust layer peeling testing, to evaluate the protection effects of the rust layer on the substrate.

Welding property testing: The weathering steel plates are mostly used for the welded structures. The testing items include the welding procedure qualification, mechanical property testing of the welded joints (tensile, bending and impact), hardness testing of the welds and heat-affected zones and the corrosion resistance comparison, ensuring that the overall structural properties after the welding do not significantly degrade.

Dimension and shape inspection: Ensures that the steel plate meets the ordering and use requirements. Including the precise measurement of the dimension deviations such as the thickness, width, length, flatness and camber. For the structural weathering steel plates, the strict dimension tolerances are the premise of ensuring the installation precision and connection quality.

Testing Scope

Raw material slabs/coils: The ex-factory inspection of the weathering steel raw materials produced by the steel enterprises is carried out, verifying whether their chemical compositions, basic mechanical properties and surface quality meet the requirements of the corresponding product standards (such as GB/T 4171 and ASTM A588). This is the first gate of the quality control.

Structural weathering steel plates/sections: Widely used for the steel structures exposed to the atmosphere for a long time such as the bridges, buildings and towers. The testing emphasis is on the mechanical properties (especially the low-temperature toughness), weldability and the performance uniformity of the large-section materials, ensuring the safety of the structures throughout the whole life cycle.

Weathering steel welded parts and structural components: The testing of the components after the completion of the welding or forming, such as the bridge box girders and building truss joints. The testing focuses on the comprehensive properties of the welded joints, local corrosion sensitivity and the overall dimensional precision of the components.

Steel plates after the rust layer stabilization treatment: The testing of the weathering steel plates after the artificial accelerated rusting or stabilization treatment (such as applying the rust-promoting agents), evaluating the colour uniformity, attachment firmness, anti-running resistance of the treated rust layers and whether the expected corrosion resistance effects are achieved.

Monitoring of the in-service weathering steel structures: The regular or irregular testing and monitoring of the weathering steel structures already in service, including the observation of the rust layer development states, thickness measurement of the key parts and stress corrosion cracking inspection, providing the basis for the structural maintenance and safety assessment.

Commodity inspection of the import and export trade: In the cross-border trade, according to the technical standards agreed in the contracts or the internationally common standards (such as the ISO, ASTM and JIS), the notarized inspection of the weathering steel plates is carried out, with the contents covering the quality, specifications, quantity, weight, packaging and safety and hygiene items.

Testing Methods

Spectrometry analysis method (OES/ICP): Used for the chemical composition analysis. The spark discharge atomic emission spectrometry (OES) is fast and accurate, and is the mainstream method of the furnace-front and finished product analysis; the inductively coupled plasma atomic emission spectrometry (ICP) is used for the more precise quantitative analysis and the determination of the trace elements.

Tensile testing and impact testing: Carried out in accordance with GB/T 228.1 and GB/T 229. The tensile testing is completed on a universal testing machine, obtaining the stress-strain curves and the strength and plasticity indicators. The impact testing uses a Charpy pendulum impact testing machine to test the energy absorbed by the specimen fracture at the specified low temperatures, evaluating the material toughness.

Cyclic immersion corrosion testing: An accelerated testing method simulating the corrosive environments such as the industrial and marine. The specimen is placed in the corrosion solution of the specific concentration (such as NaHSO₃), and the periodic immersion-drying cycles are carried out under the set temperature and humidity environments, and the corrosion resistance is evaluated through the corrosion weight loss per unit time.

Salt spray testing: The neutral salt spray testing is carried out in accordance with GB/T 10125 (equivalent to ISO 9227). In an enclosed chamber, the specimen is exposed to the continuously sprayed 5% sodium chloride salt spray, and after the testing of hundreds of hours, the appearance times, propagation conditions and grades of the rust are observed, commonly used for the comparative evaluation.

Metallographic examination and scanning electron microscope analysis: The metallographic examination is used to observe the microstructures, grain sizes and non-metallic inclusions of the steel. The scanning electron microscope combined with the energy dispersive spectrometer can carry out the high-power observation of the micro-morphologies, cracks and pores of the rust layers, and the qualitative or semi-quantitative analysis of the micro-area compositions.

X-ray diffraction analysis: Used to identify the phase compositions in the rust layers. The protective rust layers of the weathering steels are usually rich in the amorphous iron oxide hydroxides and stable α-FeOOH (goethite), while the rust layers of the ordinary steels contain more γ-FeOOH (lepidocrocite). The XRD analysis is an important means of distinguishing and evaluating the protection of the rust layers.

Testing Standards

Domestic Standards:

1. GB/T 4171, Atmospheric corrosion resisting structural steel

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 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

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

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

International Standards:

1. ASTM A588/A588M, Standard specification for high-strength low-alloy structural steel, up to 50 ksi [345 MPa] minimum yield point, with atmospheric corrosion resistance

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

Direct-reading spectrometer: The core equipment of the chemical composition analysis. By exciting the sample to generate the characteristic spectra, which are received by the grating dispersion and detectors, the contents of the multiple elements in the steel can be simultaneously determined in a short time (dozens of seconds), with the high precision and good repeatability, widely used on the production sites and in the laboratories.

Universal material testing machine: The key equipment for the mechanical property tests such as the tensile, bending and compression. Equipped with the high-precision load cells and extensometers, the computer control system controls the loading rates and collects the data in real time, automatically calculating and outputting the results such as the yield strength, tensile strength and elongation.

Pendulum impact testing machine: The dedicated equipment for measuring the impact toughness of the material. The notched specimen is placed on the supports, and the pendulum of the specified height is released to break it. Through the difference of the potential energies of the pendulum before and after breaking the specimen, the impact energy (KV2 or KU2) absorbed by the specimen is calculated, evaluating the brittle fracture resistance of the material under the dynamic loads.

Salt spray test chamber: The testing device artificially simulating the marine or salt-containing atmospheric corrosion environments. The sodium chloride solution is atomized by the compressed air and uniformly sprayed on the specimen surfaces in the enclosed chamber, and the constant temperature, salt spray settlement volume and pH value are maintained through the precise control systems, realizing the standardization of the test conditions.

Cyclic immersion corrosion testing machine: A programmable and controllable automated equipment. Through the mechanical devices, the specimen racks are periodically immersed into the corrosion solution tanks and lifted out, combined with the environmental chambers to control the temperature, humidity and drying conditions, precisely simulating the dry-wet alternating corrosion environments, with the controllable test cycles.

Scanning electron microscope: The high-end instrument for the micro-morphology observation and micro-area composition analysis of the material. The high-energy electron beam scans the sample surface, and the images are formed by receiving the signals such as the secondary electrons and backscattered electrons, with the resolution up to the nanometre level. It is the essential tool for studying the rust layer structures and corrosion product morphologies.