Galvanized Iron Wire Testing

Galvanized iron wire inspection mainly conducts scientific testing and analysis on the coating mass, coating uniformity, coating adhesion, mechanical properties, diameter and dimension deviations and surface quality, objectively assessing the product quality grade and issuing a detailed test report within 3-7 working days, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Coating mass: This is the core indicator of measuring the zinc layer thicknesses of the galvanized iron wires, usually expressed as the zinc layer masses per unit area (g/m²). Through the testing of the coating masses, the corrosion resistance of the material and the quality of the galvanizing processes can be evaluated. It is the important basis of the product grading and life prediction.

Coating uniformity: Testing the uniformity of the zinc layer distributions on the iron wire surfaces. The non-uniform coatings will lead to the excessively thin zinc layers in the local areas, becoming the weak points of the corrosion, seriously affecting the overall protection effects and service lives of the products.

Coating adhesion: Evaluating the bonding firmness between the zinc layers and iron wire matrices. Through the tests such as the wrapping, hammering or tensile, whether the zinc layers have the peeling, blistering or detachment is checked, ensuring that the coatings are intact during the use and processing.

Mechanical properties: Including the tensile strengths, yield strengths, elongations after fracture and reverse bending times. These indicators are directly related to the load-bearing capabilities, toughness and processing forming properties of the galvanized iron wires, and are the key of judging whether they meet the specific engineering application requirements.

Diameter and dimension deviations: Precisely measuring the nominal diameters, ovality and allowable tolerances of the iron wires. The dimension precision affects the standardized use of the products, gripping forces (such as the iron wires for the concrete) and the matching of the subsequent processing and assembly.

Surface quality: Checking whether the galvanized iron wire surfaces have the defects such as the cracks, scars, rust, missed plating, burrs and obvious zinc dross and zinc nodules. The good surface qualities are the basis of the beauty and corrosion resistance.

Coating composition and structure: Through the spectroscopic analysis or metallographic observation, the contents of the alloying elements (such as the aluminum and rare earths) in the zinc layers and the micro-structures of the coatings (such as the pure zinc layers and alloy layers) are analysed, to evaluate their corrosion resistance mechanisms and process characteristics.

Corrosion resistance: Through the accelerated corrosion methods such as the neutral salt spray testing (NSS), copper-accelerated acetic acid salt spray testing (CASS) or sulfur dioxide corrosion testing, the harsh environments are simulated, and the corrosion resistance lives of the galvanized layers are quantitatively evaluated.

Testing Scope

Electro-galvanized iron wires: The zinc layers are deposited on the iron wire surfaces through the electrolytic process. The coatings of this type of products are thinner but uniform and bright. The testing emphases are the coating thicknesses (masses), adhesion and appearances, often used for the occasions requiring the beauty and general rust prevention.

Hot-dip galvanized iron wires: The iron wires are immersed in the molten zinc to obtain the coatings. Their coatings are thicker with the strong corrosion resistance. The testing core is the coating masses, uniformity, adhesion and mechanical properties, widely used in the harsh environments such as the outdoor buildings, agriculture and bundling.

Alloyed galvanized iron wires: After the hot-dip galvanizing, the heat treatment is carried out to make the zinc and iron interdiffuse to form the zinc-iron alloy layers. The testing needs to pay attention to the alloy layer structures, compositions, adhesion and the resulting unique properties (such as the good paintability).

Iron wires of the different uses: The testing needs to be focused according to the final uses. For example, the bundling and packaging iron wires focus on the mechanical properties and coating adhesion; the construction iron wires (such as the rebar tying wires) focus on the strengths, toughness and diameter precision; the aquaculture fence iron wires focus on the corrosion resistance and coating masses.

Iron wires of the different specifications: The testing covers the full series of products from the fine diameters (such as 0.5mm) to the coarse diameters (such as 6.0mm). For the iron wires of the different diameters, the mechanical property testing methods (such as the gauge lengths of the tensile specimens) and coating mass requirements may be different.

Raw materials and semi-finished products: The testing is not only for the finished products, but also covers the wire rod raw materials (chemical compositions and surface qualities) used to produce the galvanized iron wires and the drawn iron wire semi-finished products (dimensions and mechanical properties), carrying out the process quality control.

Products of the new and old standards: The testing needs to comply with the current effective standards (such as GB/T 15393 and ASTM A641), and may also involve the evaluation of the stock products of the old standards, to ensure that they meet the current use requirements.

Testing Methods

Antimony trichloride method (gravimetric method): This is the arbitration method of determining the hot-dip galvanized layer masses. The specimens are immersed in the hydrochloric acid solutions containing the antimony trichloride, dissolving the coatings without eroding the matrices. Through the mass differences before and after the dissolution, the zinc layer masses per unit area are calculated. The results are accurate and reliable.

Magnetic thickness measurement method: Using the magnetic induction or eddy current principles, the thicknesses of the non-magnetic zinc layers on the magnetic steel matrices are non-destructively measured. This method is fast and convenient, suitable for the online testing at the production sites and the batch sampling inspection of the finished products, but needs to be regularly calibrated with the standard sheets.

Wrapping testing method: Used to evaluate the coating adhesion. The iron wires are tightly wrapped around the mandrels of several times their own diameters, observing whether the coatings are cracked or peeled. The wrapping speeds, numbers of turns and mandrel diameters need to be strictly carried out according to the standard provisions.

Tensile testing method: The axial tensile loads are applied to the specimens on the universal material testing machines until the fracture, thereby determining their tensile strengths, yield strengths and elongations after fracture. This is the most fundamental method of evaluating the mechanical properties of the iron wires.

Reverse bending testing method: One end of the specimen is clamped, and it is repeatedly bent 90° on the arcs of the specified radii until the fracture, recording the number of bends. This testing can effectively evaluate the toughness and processing properties of the iron wires under the dynamic loads.

Metallographic microscope method: The specimen cross-sections are cut, mounted, ground, polished and etched, and then the total coating thicknesses, layer thicknesses (alloy layers and pure zinc layers) and micro-structures are observed and measured under the metallographic microscopes. It is the important means of the in-depth research of the coating qualities.

Salt spray testing method: The specimens are placed in the closed salt spray test chambers, and the sodium chloride solutions of a certain concentration are sprayed to form the corrosive atmospheres. After the continuous spraying for a certain time (such as 72 hours and 240 hours), the corrosion conditions of the specimen surfaces are checked, evaluating their corrosion resistance.

Dimension measurement method: The micrometers, vernier callipers or laser diameter gauges are used for the multiple measurements at the different positions and directions of the iron wires, to determine their average diameters, ovality and dimension deviations, ensuring that they meet the tolerance requirements.

Testing Standards

Domestic Standards:

1. GB/T 15393, Zinc-coated steel wires

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

3. GB/T 238, Metallic materials — Wire rod — Reverse bend test

4. GB/T 2976, Metallic materials — Wire — Wrapping test

5. GB/T 1839, Test methods for determination of mass of zinc coating on galvanized iron and steel products

6. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

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

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

3. ASTM A641, Standard specification for zinc-coated (galvanized) carbon steel wire

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

Electronic balance: Used for the gravimetric determination of the coating masses, requiring the high precision (usually 0.1mg) and good stability. It is the core equipment of obtaining the accurate coating mass data.

Coating thickness gauge: Including the magnetic thickness gauges and eddy current thickness gauges, used for the fast and non-destructive measurement of the zinc layer thicknesses at the sites. The equipment needs to be equipped with the multiple probes to adapt to the different curvature surfaces, and equipped with the standard calibration sheets.

Universal material testing machine: Used for the mechanical property testing such as the tensile and bending. The equipment needs to have the sufficient load capacities, precise displacement and force value measurement systems, and can automatically draw the stress-strain curves.

Reverse bending testing machine: Specially used to carry out the standard-specified reverse bending testing. Its clamping devices, bending arms and bending arc radii must meet the provisions of the relevant standards, ensuring the consistency and comparability of the testing results.

Metallographic specimen preparation equipment: Including the cutting machines, mounting machines, pre-grinding machines, polishing machines and etching devices, used to prepare the coating cross-section samples that can be used for the microscopic observation. It is the basis of the micro-analysis.

Metallographic microscope/electron microscope: Used to observe the micro morphologies of the coatings, measure the layer thicknesses and analyse the defects. The scanning electron microscopes (SEM) combined with the energy dispersive spectrometers (EDS) can also carry out the micro-area composition analysis.

Salt spray test chamber: Used to simulate the marine or industrial atmospheric corrosion environments. The equipment needs to precisely control the chamber temperatures, salt spray sedimentation amounts and spraying cycles, and the test space needs to meet the standard specimen placement requirements.

Precision dimension measuring tools: Such as the digital micrometers, laser diameter gauges and optical projectors, used for the precise measurement of the geometric dimensions such as the diameters and ovality of the iron wires, ensuring that the product dimension precision meets the specifications.