(Invited) On the Intergranular Corrosion in Non-stationary Conditions: Experimental Approach and Modeling by Cellular Automata
Authors: Guiso Simone , Brijou-Mokrani Nadia , De Lamare Jacques , Di Caprio Dung , Gwinner Benoit 
Published: December 2020

Intergranular corrosion can affect stainless steels even if they are in a non-sensitized state. This is the case when the corrosive medium is particularly oxidizing such as in hot and concentrated nitric acid [1]. Intergranular corrosion is characterized by the formation of triangular grooves at the level of the grain boundaries. Their progression leads to the detachment of grains with a certain periodicity. This corrosion type is quite well described and modeled by different techniques when the medium remains the same [2-3]. Nevertheless, the medium may vary in some industrial chemical processes. Therefore, this work has investigated the impact of non-stationary conditions on the evolution of the intergranular corrosion.

The intergranular corrosion kinetics of a 310L stainless steel was first studied in nitric acid with different levels of VO2
+ concentrations in stationary conditions. Then non-stationary experiments were performed by varying the concentration of VO2
+ as a function of time by periods. The corrosion kinetics was characterized by the evolution of the mass loss of the samples and of the geometry of the grooves (angle and depth) optically measured on cross sections.

Numerical simulations were also performed by cellular automata [3]. A hexagonal close-packed (HCP) grid was chosen for the simulations. A realistic 3D granular structure of the stainless steel was modeled with a Voronoi algorithm. Two corrosion probabilities (for the grains and the grain boundaries) were adequately chosen for driving the time evolution in each conditions. These probabilities were kept constant in stationary conditions and varied with time by periods in non-stationary conditions. The simulated mass loss and the surface morphology reflects the same pattern of the experiments.

In stationary conditions, the corrosion reaches a steady state after a transient period corresponding to the detachment of the first layer of grains: the corrosion rate and the specific surface in contact with the solution (increased by the formation of grooves) becomes constant. The characteristics of the steady state are directly dependent on the concentration of VO2
+. In non-stationary conditions, it appears that the characteristics of the intergranular corrosion (mass loss and geometry of the grooves) at a given period are influenced by the characteristics reached after the previous period (in other chemical conditions). This influence is only transient during a time corresponding approximately to the duration necessary to the consumption of a layer of grains.

[1] Fauvet, P., Corrosion issues in nuclear fuel reprocessing plants. Nuclear corrosion science and engineering. D. Feron, Woodhead Publishing. 22 (2012) 679-728.

[2] B.Gwinner et al., Towards a reliable determination of the intergranular corrosion rate of austenitic stainless steel in oxidizing media. Corrosion Science 107 (2016) 60-75.

[3] D.Di Caprio et al., 3D cellular automata simulations of intra and intergranular corrosion. Corrosion Science 112 (2016) 438-450.