Corrosion pitting and environmentally assisted small crack growth

Area of Science:

• Materials Science
• Corrosion Science
• Fracture Mechanics

Background:

• Corrosion pits are known precursors to material cracking, but the transition process remains poorly understood, hindering accurate damage prediction.
• Existing models lack detailed insights into how cracks initiate and grow from corrosion pits, limiting quantitative analysis.

Approach:

• Utilized advanced 3D imaging techniques, including X-ray computed tomography (CT) and focused ion beam scanning electron microscopy (FIB-SEM), to visualize the pit-to-crack transition.
• Employed finite-element analysis (FEA) to support imaging data and investigate crack initiation sites.
• Applied the potential drop technique for quantifying the growth rates of newly formed small cracks.

Key Points:

• Novel insights into crack initiation location from pits were obtained using advanced imaging.
• A new concept proposes that growing pits induce local dynamic plastic strain, a critical factor for stress corrosion crack (SCC) development.
• The potential drop technique emerged as the most viable method for measuring small crack growth rates.

Conclusions:

• The study elucidates the pit-to-crack transition mechanism, highlighting the role of plastic strain induced by pit growth in SCC.
• Quantifying small crack growth rates presents challenges, but the potential drop technique shows promise.
• An electrochemical crack size effect is proposed to explain observed growth rate variations between small and long cracks.