Physical Model for Electrochemical Oxidation of Composite Ceramics

The paper examines the corrosion behavior of dense ZrB 2 -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.%): ZrB 2 , 77 ZrB 2 –23 SiC, 70 ZrB 2 –20 SiC–10 AlN, and 60 ZrB 2 –20 SiC– 20 (Al 2 O 3 –ZrO 2 ). The main ECO parameters were the conduction current i , corrosion current i corr ( i value at which d i /d E 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 (d i /d E ). 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 ZrB 2 –SiC materials are reached when AlN is added as it promotes the formation of thermodynamically stable mullite in the protective film. An Al 2 O 3 –ZrO 2 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 ZrB 2 –23 SiC ( E a = 0.1 V) and 60 ZrB 2 –20 SiC–20 (68 Al 2 O 3 –32 ZrO 2 ) composites ( E a = 0 V).