One dimensional wormhole corrosion in metals

Area of Science:

• Materials Science
• Corrosion Science
• Physical Chemistry

Background:

• Corrosion is a common material failure mechanism.
• Localized corrosion often leads to porosity.
• Previous classifications may have misidentified certain corrosion morphologies.

Purpose of the Study:

• To identify and characterize a previously miscategorized form of corrosion, termed 1D wormhole corrosion.
• To elucidate the underlying mechanism of 1D wormhole corrosion in a Ni-Cr alloy exposed to molten salt.
• To establish a foundation for designing materials with improved corrosion resistance.

Main Methods:

• Electron tomography was employed to visualize and confirm the 1D and percolating morphology of the corrosion.
• Energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) was combined with ab initio density functional theory (DFT) calculations.
• A novel vacancy mapping method with nanometer resolution was developed to quantify vacancy concentrations.

Main Results:

• Multiple instances of 1D wormhole corrosion morphology were identified and documented.
• A significantly high vacancy concentration, up to 100 times the equilibrium value, was detected in the diffusion-induced grain boundary migration zone.
• The study revealed a direct link between high vacancy concentrations and the initiation of 1D wormhole corrosion.

Conclusions:

• 1D wormhole corrosion represents a distinct and previously underappreciated corrosion morphology.
• High vacancy concentrations, particularly in grain boundary migration zones, are a key factor driving this corrosion mechanism.
• Understanding and mitigating 1D wormhole corrosion is essential for enhancing the durability of structural materials in corrosive environments.