What Chemisorption on Alloy Surfaces can mean for Materials Performance

Earlier today I shared an article published in the Journal of the Electrochemical Society and written primarily by postdoctoral research associate Dr Huibin Ke. The article describes the application of DFT to study the competitive chemisorption of chloride versus oxide on to alloy surfaces, using the Ni-Cr family of superalloys as a basis for the study. Why is the chemisorption process significant? And what is competitive chemisorption anyway? Oh, and some of you might also ask, what in the heck is DFT?

Corrosion resistant alloys are usually protected from the environment by a very thin passive film that prevents further corrosion reactions from occurring. Chromium is often introduced into alloys because it forms very robust and protective passive films. Due to various reasons, however, over time these passive films might break, or be chemically attacked. When this happens the alloy surface becomes directly exposed to the environment. Depending on the circumstances, the passive oxide film could grow back- allowing the localized corrosion event to "heal", or corrosion could continue to occur at the bare metal surface, leading to pitting or crevice corrosion, or, if there is also a force acting on the material (like a cyclic or static stress) it might move into some form of stress-corrosion crack.

Figure: Some degradation processes that can occur at an alloy surface once the passive film is breached

Understanding the atomistic processes occurring on such an exposed alloy surface can help with learning how to design new alloys to resist such localized corrosion or cracking behavior. This is where DFT comes in. DFT (stands for density functional theory) is a computer modeling technique that allows very accurate simulation of the physics that governs the breaking and formation of chemical bonds in molecules, liquids and materials. It is one of the only techniques that can really be applied to directly learn what is happening in such scenarios. Think of it like creating a setup in Angry Birds and pulling back the bird in the slingshot, then hitting run and seeing how the atoms behave. But instead of houses with piggies in them, you have a simulated lattice for the metal alloy in contact with water molecules, and instead of a bird you have a big fat chloride ion ready to bust up the metal lattice.

In the simulations performed in Dr. Ke's paper (I am a co-author), we created simulated surfaces of alloys with Nickel, Chromium and other elements like Copper, Iron, Molybdenum, Tungsten, etc. and observed what the effect of changing the alloy composition was on the relative affinity of the surface to chemically bind to oxide, versus the effect it has on chemically binding chloride. A surface with a stronger tendency to bind oxide is more likely to repassivate- healing from the corrosion or stress-corrosion cracking state- whereas if it binds chloride, it is likely to continue to corrode. The DFT method directly provides a quantitative value for the binding energy of the ion to the surface, so the numbers can be put side by side. This process by which atoms and ions from the environment can bind to the alloy surface is called "chemisorption" and when two potential atoms can compete for those binding sites, it is called "competitive chemisorption".

Figure: Binding energies for Cl vs O for various alloy configurations.

In this first paper, we showed that typically alloying elements that bind oxide more strongly to the surface, also bind chlorine more strongly, so it wasn't clear immediately what the impact of different alloying elements would have in a corrosion scenario. That's why we moved ahead with a second paper that took the chemisorption energies and inserted them into an adsorption isotherm model, to determine what the overall impact would be at the chemical potentials (i.e. driving forces) at different conditions of pH, electrochemical potential and brine (chloride) concentration.

You can read this first paper here: https://iopscience.iop.org/article/10.1149/1945-7111/aba44e