Physical Model for Electrochemical Oxidation of Composite Ceramics

The paper examines the corrosion behavior of dense ZrB2-based ceramic samples in simulated seawater (3% NaCl solution) using polarization curves of electrochemical oxidation (ECO). The dense ceramic samples of 3-5% porosity were produced by hot pressing and had the following composition (wt.%): ZrB2, 77 ZrB2-23 SiC, 70 ZrB2-20 SiC-10 AlN, and 60 ZrB2-20 SiC- 20 (Al2O3-ZrO2). The main ECO parameters were the conduction current i, corrosion current i(corr) (i value at which di/dE decreased through diversion of some oxygen ions to oxidize the material), and anode potential E-a (E value at which the protective oxide film failed (i > 0)). A two-stage model of the ECO process was proposed upon analysis of the experimental data. At the first stage (E < E-a, i = 0), an oxide film developed on the effective surface: the higher the protective function of the oxide film, the greater its thermodynamic stability. The second ECO stage (E > E-a, i > 0) had two steps of changing the conduction current i, carried by negative oxygen ions. The first step was characterized by an avalanche-like increase in i at E = E-a up to maximum i = i(corr), at which the rate of change in i decreased with increasing anode potential (di/dE). At higher i(corr) (second step), the increase in i(corr) with greater E slowed down through the interaction of oxygen with the test material, i.e., through oxidation. The higher the maximum i(corr) value, the greater the oxidation resistance of the material. According to the proposed model, the highest values of E-a and i(corr) in ECO conditions for ZrB2-SiC materials are reached when AlN is added as it promotes the formation of thermodynamically stable mullite in the protective film. An Al2O3-ZrO2 oxide addition increases the oxidation resistance of the material (high i(corr) values) but does not change the composition of the outer borosilicate glass film. This explains the close anode potentials of the 77 ZrB2-23 SiC (E-a= 0.1 V) and 60 ZrB2-20 SiC-20 (68 Al2O3-32 ZrO2) composites (E-a = 0 V).

Automated summary

The research investigates the corrosion behavior of dense ZrB2-based ceramic samples in simulated seawater using polarization curves of electrochemical oxidation (ECO). The addition of AlN was found to promote the formation of a stable protective film, leading to higher values of E-a and i(corr) in ECO conditions for ZrB2-SiC materials.