Researchers at Tohoku University have developed a new technique to identify the initiation sites of a destructive process called pitting corrosion, which occurs when aluminum (Al) alloys are exposed to sodium chloride solutions. This advancement is expected to accelerate the development of Al alloys with improved corrosion resistance. Since AI alloys are widely used in transportation equipment, improving corrosion resistance means we can develop more durable automotive engines, suspensions, and transmissions.

AI alloys are an excellent choice for transportation equipment due to their low weight and recyclability. However, their long-term durability is constantly being challenged by diverse and harsh operating environments. Because Al alloys contain complex microstructures, identifying the origin of corrosion has remained difficult.

"This innovative study combines real-time optical microscopy with a boric-borate buffer solution, in order to suppress discoloration around intermetallic compounds, prevent the deposition of corrosion products, and enable clear visualization of the exact locations where pits form," explains Masashi Nishimoto (Tohoku University).

The research team applied their technique to ADC12 (Al-12%Si-2%Cu), a die-cast Al alloy used for various automotive parts. Since alloys are a mixture of different metals and impurity elements (such as Cu, Fe, Mn, and Mg), numerous intermetallic compounds form during the solidification process, creating electrochemical inhomogeneities that are thought to cause pitting corrosion. However, conventional corrosion tests in sodium chloride solutions result in widespread discoloration of intermetallic compounds and deposition of corrosion products, obscuring pit initiation sites. This makes it difficult to pinpoint the precise cause.

By using a boric-borate buffer solution, the newly developed technique suppresses alkalization on intermetallic compounds. This prevents discoloration and reduces corrosion product deposition, allowing clear observation of pit initiation and early growth. Subsequent analysis by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed that pits originate in the final solidification region. However, not all final solidification regions become a pit, which suggests that pit initiation also depends on specific local chemical compositions and microstructural features.

"Being able to observe where and how these pits form is an exciting advancement, since it may help us find ways to prevent or slow down their formation for more long-lasting vehicle components," remarks Nishimoto.

The method can be applied beyond die-cast alloys to elucidate pitting corrosion mechanisms in other Al alloys. By clarifying causal relationships between microstructure and corrosion behavior, it contributes to guidelines for designing materials with higher corrosion resistance and supports the development of longer-lasting automotive components.

This research was published online in the Journal of The Electrochemical Society on January 20, 2025.