# Hot-Pressing of Ti-Al-N Multiphase Composite: Microstructure and Properties
## 3 Results and Discussion
### Corrosion Response Analysis
Sintered materials exhibit a different microstructure (Figure 13) that, in the presence of chloride ions (1 M NaCl), facilitates the initiation of pitting corrosion, as observed in electrochemical testing. Nevertheless, the applied sintering process (temperature: 1300 \({}^{\circ}\)C) promotes the formation of a passive layer through the self-passivating behavior of alloy components (Ti, Al), providing improved corrosion resistance compared to 3D-printed materials. The microstructure of the sintered AlN-TiN(BN) composite indicates that, due to the heterogeneity of the initial powder, complete homogeneity was not achieved (Figure 13a). This non-uniformity fosters galvanic corrosion in regions with high electrochemical heterogeneity. Furthermore, due to the high concentration of chloride ions (35.45 g Cl/dm\({}^{3}\)) in the corrosive solution, pitting corrosion predominantly occurs in areas enriched with oxides/nitrides of substrate components, which are particularly susceptible to halide-induced degradation (Figure 13b). The corrosion resistance of AlN-TiN(BN) composites was evaluated in a neutral solution (pH 6.7) at room temperature (20 \({}^{\circ}\)C). It can be inferred that the corrosion behavior of these materials will vary significantly under extreme pH conditions. This is particularly evident in metals that rely on the formation of a passive layer for corrosion resistance, where changes in pH can lead to alterations in corrosion rates. For example, in amphoteric metals such as aluminum, both decreasing and increasing the pH of the solution may result in the breakdown of the passive layer, thereby increasing the corrosion rate. In titanium alloys, an increase in pH enhances the corrosion resistance of the material by strengthening the passive oxide layer that forms on the surface. Conversely, a high concentration of hydrogen ions at \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Corrosion Potential,** & **Corrosion Current Density,** \\  & **Ecorr. (mV)** & **Icorr. (\(\mu\)A/cm\({}^{2}\))** \\ \hline AlN-TiN(BN) & \(-\)285 & 0.078 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrochemical properties of sintered AlN-TiN(BN) composite (LSV measurements). Figure 13: Surface topography of the sintered AlN-TiN(BN) alloy before (**a**) and after (**b**) corrosion tests in 1 M NaCl. low pH promotes the dissolution of the passive layer and causes the metal to transition to an active state, which accelerates the corrosion rate. For AlN-TiN(BN) composites, the effect of pH on the corrosion rate is influenced by the titanium and aluminum content. In acidic solutions, lowering the pH generally accelerates the corrosion rate. However, in alkaline environments, the corrosion behavior is determined by the relative amounts of titanium and aluminum in the composite. In composites with a higher aluminum content, the corrosion rate tends to increase with higher pH values, while for composites dominated by titanium, an increase in pH leads to improved corrosion resistance. Temperature is another significant factor influencing the corrosion rate. According to the Arrhenius equation, an increase in temperature accelerates electrochemical reactions, including the rate of corrosion. In aqueous environments, the corrosion rate increases with temperature up to approximately 80 \({}^{\circ}\)C. Above this temperature, the corrosion rate decreases gradually due to the reduced solubility of oxygen in water, limiting the availability of dissolved oxygen, which is essential for corrosion processes. Sintered AlN-TiN(BN) composites generally demonstrate high corrosion resistance, particularly in environments with moderate temperatures and mildly corrosive agents. Their primary protective mechanism involves the formation of a stable oxide layer, conferring resistance to oxidation and wear. Under extreme conditions, such as exposure to strong acids, high chloride concentrations, or elevated temperatures, localized corrosion or degradation of the oxide layer may occur.