What is Salt Spray Test (SST) with reference to Strong Magnet coatings?

The salt spray test, also referred to as the salt fog test or salt corrosion test, is a standardized testing method utilized to evaluate the corrosion resistance of metal coatings, including magnetic coatings, that have been applied to metal substrates.

Additional coatings on magnets are usually applied to prolong the lifespan of the material especially super strong Neodymium magnetic material as these are naturally susceptible to corrosion.  

Why does corrosion take place in Neodymium magnets?

Neodymium iron boron (NdFeB) magnets such as those at Neodymium Magnets from Supreme Magnets - World's strongest magnets are susceptible to two types of corrosion that can damage them. Red corrosion occurs similarly to rust on iron, appearing on the surface.

White corrosion penetrates into the interior of the magnet along the grain boundaries of the magnetic material. White corrosion can eventually turn the magnet into a powder, destroying its magnetic properties.

The main problem is that these magnets contain rare earth metals like neodymium and samarium which have a highly negative electric potential. This means that exposure to elevated temperatures and humidity accelerates corrosion of the magnets. Samarium is often alloyed with cobalt, providing more protection from corrosion.

Neodymium magnets are more vulnerable because the magnetic grains are held together mechanically by neodymium-rich grain boundaries. These grain boundaries compose about 5% of the total volume of the magnet and can be considered nearly pure neodymium.

Higher temperature and humidity dramatically speeds up corrosion, which can then cause the magnet to disintegrate into neodymium hydroxide and loose NdFeB grains.

The primary purpose of conducting a salt spray test for magnetic coatings is to determine their ability to withstand corrosion when exposed to harsh environmental conditions, especially those involving salt-laden atmospheres.

How the salt spray test functions and why it is useful for magnetic coatings:

Appraisal of Corrosion Resistance: The test involves subjecting coated metal specimens, including those with magnetic coatings, to a steady or intermittent spray of a saltwater solution (typically sodium chloride) in a controlled chamber. This produces a highly corrosive setting that simulates the effects of saltwater exposure, such as that encountered in marine environments or coastal areas.

Image Reference : Research Gate, showing corroded and cleaned material surfaces

Speeding Up of Corrosion Process: The salt spray test speeds up the natural corrosion process, causing the coated specimens to undergo corrosion more quickly than they would under real-world conditions. This enables manufacturers to evaluate the long-term durability and performance of magnetic coatings over a relatively short time period.

Assessment of Coating Performance: By exposing coated specimens to the salt spray environment for a specified duration (e.g. hours or days), engineers and quality control professionals can assess various aspects of coating performance, including:

• Resistance to corrosion, such as rust formation, oxidation, or blistering.
• Adhesion strength between the coating and the substrate.
• Changes in appearance or color that may indicate coating degradation.

Quality Control and Product Certification: The salt spray test is commonly utilized as a quality control measure during the manufacturing process of magnetic coatings and coated products. It assists manufacturers in ensuring their coatings satisfy or surpass industry standards and customer requirements for corrosion resistance and durability. Additionally, certification bodies and regulatory agencies may mandate salt spray testing as part of product certification processes.

Research and Development: The test results obtained from salt spray testing can inform research and development efforts aimed at improving the formulation and performance of magnetic coatings. By identifying weaknesses or vulnerabilities in existing coatings, manufacturers can refine their formulations to enhance corrosion resistance and extend the service life of coated products.

Typical Salt Spray Test Machine

Therefore, the salt spray test is a valuable tool for evaluating the corrosion resistance and durability of magnetic coatings, providing critical data for product development, quality control, and compliance with industry standards.