Magnesium Rod Testing

Magnesium rod inspection mainly conducts scientific testing and analysis on the chemical composition analysis, electrochemical property testing, metallographic structure analysis, mechanical property testing, macro and micro corrosion morphology observation and dimension and appearance 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: Mainly testing the main content of the magnesium (Mg) in the magnesium rods, as well as the contents of the key alloying elements and impurity elements such as the iron (Fe), copper (Cu), nickel (Ni), silicon (Si) and aluminium (Al). The strict control of the impurity element contents is the core of ensuring the electrochemical properties and service lives of the magnesium rods. For example, the excessively high iron, copper and nickel will reduce their sacrificial anode efficiencies, and even cause the local failures.

Electrochemical property testing: Evaluating the core properties of the magnesium rods as the sacrificial anodes, including the open circuit potentials, working potentials, current efficiencies, capacitances and consumption rates, etc. The testing is carried out through the electrochemical workstations or galvanostats in the electrolytes simulating the use environments. These parameters directly determine the driving voltages, output currents and protection effects of the magnesium rods.

Metallographic structure analysis: The micro-structures of the magnesium rods are observed through the optical microscopes or scanning electron microscopes, including the grain sizes, morphologies, second phase distributions and whether the casting defects (such as the shrinkage cavities and inclusions) exist. The uniform and fine grain structures help to obtain the more stable and efficient corrosion electrochemical behaviors.

Mechanical property testing: The tensile, hardness and other testing are carried out on the magnesium rods, obtaining their tensile strengths, yield strengths, elongations and Brinell/Rockwell hardness values. The good mechanical properties ensure that the magnesium rods are not easy to fracture or deform during the production, transportation and installation processes, guaranteeing their structural integrity.

Macro and micro corrosion morphology observation: After the specific tests (such as the salt spray testing and immersion testing), the corrosion product morphologies, distribution uniformity, depths and densities of the corrosion pits on the magnesium rod surfaces are observed. The ideal sacrificial anodes should present the uniform and comprehensive corrosion consumption, avoiding the local perforation or pitting.

Dimension and appearance inspection: The tools such as the callipers and micrometers are used to precisely measure the key dimensions of the magnesium rods such as the diameters, lengths and thread specifications. At the same time, whether the surface has the defects such as the cracks, blowholes, shrinkage porosity, burrs and severe scales is visually checked, ensuring that they meet the installation and use requirements.

Testing Scope

Household appliance magnesium rods: Mainly applied to the cathodic protection of the liners of the electric water heaters and storage water heating boilers. The testing emphases are their dissolution uniformity, current efficiencies and service lives in the medium and low temperature (usually ≤75°C) fresh water environments, ensuring the long-term stable release of the protection currents.

Industrial water treatment system magnesium rods: Used for the anti-corrosion of the large central air conditioning cooling water systems, industrial circulating water systems and small water storage equipment. The testing needs to consider the more complex water quality conditions (such as the hardness, chloride ion contents and pH fluctuations) and the possible microbial effects.

Underground pipeline and storage tank magnesium anodes: As a form of the magnesium alloy sacrificial anodes, used for the outer wall anti-corrosion of the steel pipelines and storage tanks in the soil or fresh water environments. The testing needs to simulate the parameters such as the soil resistivity and moisture contents, evaluating their grounding resistances, output currents and actual protection distances.

Marine and ocean engineering magnesium rods: Used for the cathodic protection of the ship ballast tanks, small hulls or coastal facilities. The testing environment is the simulated seawater. The self-corrosion rates, potential stability and potential matching with the high-strength steels in the high chloride ion environments particularly need to be paid attention to.

New material and special magnesium rods: Including the high-potential magnesium rods, ribbon magnesium anodes and low-impurity magnesium rods. The testing scope extends to their special properties, such as the higher driving voltages, longer theoretical lives, and optimized properties in the specific media (such as the hot water and soil).

Production process quality control: Covering the incoming inspection of the magnesium alloy ingot raw materials, as well as the semi-finished and finished product sampling inspection of the various processes such as the melting, casting (extrusion), machining and surface treatment. Ensuring the stability of the whole production processes and the consistency of the products.

Testing Methods

Inductively coupled plasma optical emission spectrometry: The standard method of detecting the chemical compositions of the magnesium rods. After the samples are dissolved, the ICP-OES is used to determine the contents of the various elements, with the advantages of the fast analysis speeds, high precision and simultaneous determination of the multiple elements. It is the key means of controlling the raw material purity.

Galvanostatic method and potentiodynamic scanning method: The core methods of the electrochemical property testing. The galvanostatic method measures the electrode potential changes over time by applying the constant currents, calculating the current efficiencies; the potentiodynamic scanning method linearly changes the potentials to measure the current responses, obtaining the Tafel curves and calculating the parameters such as the corrosion current densities.

Weight loss method: The classic method of evaluating the actual consumption rates and current efficiencies of the magnesium rods. The magnesium rod specimens of the known surface areas and masses are energized and corroded in the simulated electrolytes for a certain time. After removing the corrosion products and precisely weighing, the actual capacitances and efficiencies are calculated according to the Faraday's law.

Salt spray testing: According to the standards such as GB/T 10125, the magnesium rods are placed in the salt spray test chambers to simulate the marine or high-humidity chlorine-containing atmospheric environments, examining their corrosion rates and corrosion morphologies. Often used for the fast screening and comparison of the corrosion resistance differences of the magnesium rods of the different formulations or processes.

Metallographic specimen preparation and observation method: After the sampling of the magnesium rods, the mounting, grinding, polishing and etching (commonly using the picric acid or nital solutions) are carried out to make the metallographic specimens. Then the observation and photographing are carried out under the microscopes, analysing their micro-structures and evaluating the casting or processing qualities.

X-ray diffraction analysis: Used to analyse the phase compositions of the corrosion products on the magnesium rod surfaces. Through the XRD patterns, whether the main compounds in the corrosion layers are Mg(OH)₂, MgO or other basic salts can be determined, thereby inferring their corrosion mechanisms and protection properties.

Testing Standards

Domestic Standards:

1. GB/T 17731, Magnesium alloy sacrificial anodes

2. GB/T 24488, Test method for electrochemical properties of magnesium alloy sacrificial anode

3. GB/T 13748, Methods for chemical analysis of magnesium and magnesium alloys

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

International Standards:

1. ASTM G97, Standard test method for laboratory evaluation of magnesium sacrificial anode test specimens

2. DNV-RP-B401, Cathodic protection design

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

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

Inductively coupled plasma optical emission spectrometer: The main equipment of the quantitative chemical composition analysis. Its high-temperature plasma light sources can effectively excite the magnesium matrix and alloying elements. Combined with the high-resolution spectrometers and detectors, the precise determination at the ppm level is realized.

Electrochemical workstation: The precision instrument integrating the potentiostats, galvanostats and frequency response analysers. Used to carry out the testing such as the potentiodynamic polarization, electrochemical impedance spectroscopy and galvanostatic discharge. It is the indispensable tool of studying the electrochemical behaviors and performance evaluation of the magnesium rods.

Precision electronic balance: Used for the precise weighing of the initial masses and post-corrosion masses of the samples in the weight loss method testing, requiring the precision up to 0.1 milligram. Its accuracy is directly related to the calculation results of the key parameters such as the current efficiencies and consumption rates.

Salt spray corrosion test chamber: The key equipment of simulating the accelerated corrosion environments. It can precisely control the chamber temperatures, sodium chloride solution concentrations, spraying amounts and pH values, providing the standardized accelerated corrosion test conditions for the magnesium rods and evaluating their long-term durability.

Optical microscope and scanning electron microscope: Used for the micro-structure and corrosion morphology observation. The optical microscopes are used for the routine metallographic analysis; the SEM can provide the higher-magnification surface morphologies and micro-area composition information (combined with the energy dispersive spectrometers EDS), deeply analysing the corrosion mechanisms.

Universal material testing machine and hardness tester: Used for the mechanical property testing. The universal testing machines can carry out the tests such as the tensile and compression; the Brinell or Rockwell hardness testers are used for the fast detection of the magnesium rod surface hardnesses, evaluating their work hardening degrees or material uniformity.

Reference electrode and saturated calomel electrode/silver/silver chloride electrode: The reference potential measurement devices in the electrochemical testing. When measuring the open circuit potentials or working potentials of the magnesium rods, the three-electrode systems must be constructed through the stable reference electrodes, to ensure the accurate and reliable potential data.