Galvanized Bolt Testing

Galvanized bolt inspection mainly conducts scientific testing and analysis on the zinc layer thickness, zinc layer adhesion, zinc layer uniformity, appearance quality, hydrogen embrittlement susceptibility and corrosion resistance, 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

Zinc layer thickness: Referring to the average thickness or minimum local thickness of the zinc coatings on the bolt surfaces, it is the core indicator of measuring the anti-corrosion performance of the galvanized layers. Usually measured with the magnetic thickness gauges or metallographic microscope method, it is required to comply with the provisions of the relevant product standards (such as GB/T 5267.1 and ISO 10684), to ensure the sufficient sacrificial anode protection cycles.

Zinc layer adhesion: Evaluating the bonding firmness between the zinc layers and the bolt base metals, preventing the zinc layer peeling during the use. The commonly used testing methods include the hammering testing, friction testing and cross-cut testing. The good adhesion is the basis of ensuring the long-term effective protection of the coatings, avoiding the premature corrosion of the base metals caused by the mechanical damages.

Zinc layer uniformity: Checking the uniformity of the zinc layer distributions on the various parts of the bolts (especially the thread crests, thread roots, and the junctions between the heads and shanks). The copper sulfate solution immersion testing (such as GB/T 26991) is usually adopted, and the judgment is made by whether the red iron is exposed after the specified number of immersions. The non-uniform coatings will lead to the weak local protection and accelerate the corrosion.

Appearance quality: The surface conditions of the galvanized bolts are checked through the visual inspection or low-power magnifying glasses. The coatings are required to be continuous and smooth, without the defects such as the missed plating, blistering, peeling, cracks, roughness and dark spots. The thread parts should be inspected for the absence of zinc nodules or zinc dross, ensuring that the normal assembly and fastening functions are not affected.

Hydrogen embrittlement susceptibility: The important testing item for the high-strength bolts (usually referring to the property grades ≥ 8.8). The hydrogen atoms generated during the electroplating process may penetrate into the base metals and cause the delayed fracture. The testing methods include the sustained load testing (such as ISO 15330) or stress ring testing, evaluating the capability of the bolts to resist the hydrogen-induced delayed fracture under the sustained stresses.

Corrosion resistance: Simulating or accelerating the evaluation of the corrosion resistance of the galvanized layers in the actual environments. The commonly used methods include the neutral salt spray testing (NSS), copper-accelerated acetic acid salt spray testing (CASS) and cyclic corrosion testing. Through the recording of the times of the white rust and red rust appearance, their protection lives are evaluated, providing the basis for the material selection.

Dimension and tolerances: Testing the key dimensions of the bolts after the galvanizing, including the thread go/no-go gauge inspection, head width across flats, heights and shank diameters. The galvanized layers will increase the dimensions, so it is necessary to ensure that the post-plating threads still meet the standard tolerances (such as the 6g/6H grades in GB/T 197), guaranteeing their interchangeability and assemblability.

Testing Scope

Bolts of the different property grades: The testing covers the bolts of all the common property grades from the low-strength (such as the 4.8 grade) to the high-strength (such as the 10.9 and 12.9 grades). The bolts of the different strength grades have the significantly different requirements for the galvanizing processes (especially the dehydrogenation treatment), and the testing emphases are also different.

Products of the different galvanizing processes: Including the bolts treated by the different processes such as the electro-galvanizing, hot-dip galvanizing and mechanical galvanizing. The zinc layer structures, thickness ranges and adhesion characteristics formed by the different processes are completely different, and the testing methods and acceptance criteria need to be adjusted corresponding to their process characteristics.

Galvanized bolts of the different surface treatments: Covering the bolts with the pure zinc coatings, zinc-iron alloy coatings, as well as the passivation after the galvanizing (blue-white passivation, color passivation, black passivation, etc.) or the sealing agent coating. The post-treatments will affect the appearance, corrosion resistance and hydrogen embrittlement risks, and the testing needs to cover the comprehensive properties of the coatings and post-treatment layers.

Bolts of the different specifications and structures: The testing objects include the bolts of the various thread diameters (from M3 to M64 and above), lengths, head types (hexagon heads, socket heads, square heads, etc.), as well as the full-thread and partial-thread bolts. The special structure parts (such as the thread runouts and arc transition areas) are the difficulties and key areas of the testing.

Bolts applied in the specific environments: For the galvanized bolts used in the specific working conditions such as the outdoor atmospheric environments, marine climates, industrial pollution environments, high-temperature high-humidity environments or in contact with the concrete. The testing needs to simulate or intensify the corresponding environmental conditions, evaluating the reliability of their long-term service.

Production batches and incoming material acceptance: The testing is applicable to the process quality control and finished product factory delivery inspection of the bolt manufacturers, as well as the incoming material acceptance of the users (such as the construction, electric power, transportation and communication industries). Through the sampling testing, the quality of the whole batches of products is ensured to meet the technical agreements or standard requirements.

Failure analysis and quality arbitration: When the problems such as the early rust, coating peeling and fracture of the galvanized bolts occur during the storage, transportation or use, the systematic testing and analysis are needed to determine the failure causes, providing the technical basis for the responsibility determination, process improvements and quality claims.

Testing Methods

Magnetic thickness measurement method: The most commonly used non-destructive thickness measurement method, suitable for the measurement of the non-magnetic zinc coatings on the magnetic matrices (steel and iron). The magnetic thickness gauges are used to take multiple-point measurements on the main surfaces of the bolts (head top surfaces, shanks and thread parts), calculating the averages and minimum local thicknesses. The operation is simple and fast.

Metallographic microscope method: The destructive testing method, regarded as the arbitration method of the thickness measurement. After the sampling, mounting, polishing and appropriate etching of the bolts, the coating cross-section thicknesses are directly observed and measured under the metallographic microscopes. The results are precise and intuitive, and the coating structures and the bonding conditions with the base metals can be observed at the same time.

Copper sulfate immersion testing: The classic chemical method of the qualitative inspection of the zinc layer uniformity. The bolts are immersed in the standard copper sulfate solutions, and after each immersion for the specified time, they are taken out for the inspection. The uniformity is evaluated by the number of immersions that the bolts can withstand without exposing the red iron (base metals). The more the times, the more uniform and dense the coatings.

Friction testing method: The common method of evaluating the zinc layer adhesion. A smooth steel rod or blade is used to rub the galvanized surfaces for the specified number of times or lengths under a certain pressure, checking whether the coatings are peeling or stripping. This method has the standardized requirements for the tools and operation techniques, to ensure the comparability of the results.

Hammering testing method: A simple method of the qualitative evaluation of the adhesion. A hammer of a certain mass is used to strike the specific parts of the bolts (such as the head sides) in a specific way, and the adhesion is judged by observing whether the coatings are bursting or peeling. This method is relatively subjective, mostly used for the fast on-site screening.

Neutral salt spray testing: According to the standards such as GB/T 10125 and ISO 9227, the galvanized bolts are placed in the closed salt spray chambers, and the 5% sodium chloride solution is continuously sprayed to form the salt spray environments. The times of the first white rust (basic zinc salts) and red rust (base metal rust) appearance on the sample surfaces are regularly observed and recorded, evaluating their corrosion resistance.

Hydrogen embrittlement sustained load testing: The mandatory testing method for the high-strength bolts. The bolts are installed on the dedicated fixtures, and the sustained tensile loads reaching the specific percentages of their yield strengths or tensile strengths (such as 75%-90%) are applied. Within the specified time (such as 24 hours or 200 hours), whether the fracture occurs is observed, to determine their hydrogen embrittlement susceptibility.

Testing Standards

Domestic Standards:

1. GB/T 5267.1, Fasteners — Electroplated coatings

2. GB/T 26991, Fasteners — Hot dip galvanized coatings

3. GB/T 197, General purpose metric screw threads — Tolerances

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

International Standards:

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

2. ISO 10684, Fasteners — Hot dip galvanized coatings

3. ISO 15330, Fasteners — Test method for hydrogen embrittlement by parallel bearing surface loading

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 equipment, divided into the magnetic type (for the steel and iron matrices) and eddy current type (for the non-ferrous metal matrices). The instruments need to be calibrated with the standard sheets, and the probe sizes should adapt to the measurement of the complex shapes such as the bolt curved surfaces and threads. With the statistical calculation and data storage functions, the accurate and efficient measurement is ensured.

Metallographic microscope and sample preparation equipment: Used for the precise thickness measurement and micro-structure analysis of the coatings. The supporting equipment includes the cutting machines, mounting machines, grinding and polishing machines and etchants. Through the image analysis software, the quantitative analysis of the coating thicknesses and porosities can be carried out, providing the in-depth failure mechanism information.

Salt spray corrosion test chamber: The key equipment used to simulate the accelerated corrosion environments. The chambers need to have the precise temperature, humidity and salt spray sedimentation control functions, and be made of the corrosion-resistant materials. During the testing, the sample placement angles and spacings need to be strictly carried out according to the standards, to ensure the reproducibility of the testing conditions.

Hydrogen embrittlement testing machine (sustained load testing machine): The dedicated equipment, usually the hydraulic or mechanical loading type, which can apply the constant and precise tensile loads to multiple bolt specimens at the same time. The equipment needs to be equipped with the high-precision force transducers and timing devices, and the testing environments (temperatures) also need to be controlled as required, ensuring the reliable testing results.

Adhesion testing tools: Including the standardized friction testing machines (such as the scratch testers), dedicated hammering tools and the multi-blade cutting knives used in the cross-cut method. The designs and parameters of these tools (such as the friction head materials, pressures and cross-cut spacings) all need to comply with the relevant standard provisions, to ensure the consistency and comparability of the testing results.

Thread gauges and optical measuring instruments: Used to test the dimensions and precision of the threads after the galvanizing. Including the go gauges, no-go gauges and thread ring gauges. For the precision analysis, the thread profile projectors or coordinate measuring machines (CMM) can be used to precisely measure the effects of the coatings on the parameters such as the thread profiles, pitch diameters and pitches.

Environmental simulation and chemical analysis equipment: To comprehensively evaluate the properties, the constant temperature and humidity chambers, cyclic corrosion test chambers, pH meters, electrochemical workstations (used to measure the coating polarization curves, etc.) and spectrometers (used to analyse the coating compositions) may also be needed. These equipment provide the support for the in-depth research of the failure mechanisms and performance optimization of the galvanized bolts.