Atomistic understanding of chlorine ingress into compact and wide-open grain boundaries of TiN

The ingress of corrossive Cl ions into the grain boundaries of TiN widely leads to the degradation of numerous realistic protective/functional coatings. The atomistic nature of the interaction between Cl ion and various coexistent grain boundaries in TiN coatings makes it challenging to precisely understand, predict, and control the Cl-ion behaviors. Here, the surface adsorption and inward diffusion of Cl ion on/in many prototypical grain boundaries, i.e., two compact $\Sigma 3\left\{112\right\}〈110〉$ twins and four wide-open intergranular boundaries (plus another four doped with oxygen), are studied using first-principles calculations. The active surface adsorption of Cl ion on the exposed grain boundaries is revealed by the low adsorption free energies, and the fast (blocked) Cl-ion diffusion trends on the intergranular-boundary walls (in the twin boundaries) are discovered and quantified by the calculated diffusion barriers and coefficients. The ionic-bonding and lattice-deformation mechanisms that jointly govern these kinetic trends are established by our electronic-structure analysis method. The fast Cl-ion ingress into wide-open intergranular boundaries is also portrayed by a multiscale simulation using Fick’s second law, and many previous saline-corrosion experiments on various TiN coatings are unifiedly explained. The strong blocking effect of compact twin boundaries against the Cl-ion ingress is further validated by our hydrochloric-corrosion experiment on a highly-twinned TiN nanofilm, which is precisely characterized by the X-ray reflectometry and electron microscopy techniques. The TiN nanofilm still encounters the corrosive surface adsorption of Cl ions, but the identified corrosion manner (pitting-initiated uniform thinning) presents a very low corrosion rate of $2.5\times 10^{-3}$ mm/year.