Localized Corrosion by Chromium Nitride Precipitation in Low-Temperature Plasma-Nitrided Inconel 718

Area of Science:

• Materials Science
• Corrosion Engineering
• Surface Engineering

Background:

• Inconel 718 is a critical alloy for applications in chloride-bearing environments.
• Localized corrosion resistance is paramount for Inconel 718's performance in such conditions.

Purpose of the Study:

• To evaluate the impact of continuous low-temperature plasma nitriding on Inconel 718's microstructure, hardness, and localized corrosion behavior.
• To determine the efficacy of plasma nitriding as a surface modification technique for enhancing Inconel 718's durability.

Main Methods:

• Continuous low-temperature plasma nitriding at 425 °C for 2 hours.
• Microstructural analysis using X-ray diffraction (XRD).
• Hardness testing and cyclic polarization tests to assess corrosion resistance.
• Determination of critical pitting temperature (CPT).

Main Results:

• Plasma nitriding increased surface hardness up to threefold, creating a 6.1-6.7 µm nitrided layer.
• XRD confirmed CrN precipitation without nitrogen-expanded phases.
• Corrosion rate doubled in nitrided samples, and CPT decreased by over 30 °C, falling below 10 °C.

Conclusions:

• Continuous plasma nitriding effectively enhances the surface hardness of Inconel 718.
• The nitriding process significantly degrades the localized corrosion resistance of Inconel 718 in chloride-rich environments.
• The compromised CPT indicates a reduced performance envelope for nitrided Inconel 718 in aggressive conditions.