Titanium alloys exhibit remarkable corrosion resistance and a high strength-to-density ratio making them attractive for various industrial applications. Today, additive manufacturing is rapidly becoming popular in manufacturing titanium alloys. Compared to conventional methods, additively manufactured titanium alloys exhibit better mechanical properties. However, the differences in the corrosion resistance of additive and subtractive manufactured titanium alloys have not been fully clarified in the literature, preferably due to the difference in the formation of martensite phases and the direction of stacking during their formation. This has led to extensive investigation of the corrosion behavior of additive manufactured titanium alloys, especially Ti–6Al–4V alloys.
Recent research revealed that the corrosion resistance of addictively manufacture Ti–6Al–4V alloys are mainly affected by the types of ions present in the aqueous solution, especially those containing chloride (Cl – ) and bromide (Br – ) ions. Consequently, reports have shown that titanium alloys are more susceptible to pitting corrosion in solutions containing Br– than Cl- due to its higher adsorption coefficient. Equipped with this knowledge Professor Jae-Bong Lee and his PhD candidate Dong-II Seo from Kookmin University in South Korea investigated the effects of the competitive anion adsorption of Br– and Cl–, and semiconducting properties of passive films on the carrion behavior of Ti–6Al–4V alloys manufactured via additive manufacturing method. They determined the relationship between the pitting potential and the equilibrium adsorption of the anions. Their work is currently published in the journal, Corrosion Science.
In their approach, the samples of the additive manufactured Ti–6Al–4V alloys used in this study were manufactured through the direct energy deposition technique. A combination of potentiodynamic polarization curves, Mott–Schottky plots, and the Langmuir isotherm simulation model was used to elucidate further the relationship between the equilibrium adsorption coefficients and pitting potentials. In particular, a microdroplet cell technique based on the passive films’ capacitance was used to perform micro-electrochemical experiments and measure the pitting potentials of the individual regions of the bright and dark grains.
Results showed that the pitting potential for Br – solution was lower than that for Cl – solution for both bright and dark grains. The higher equilibrium adsorption coefficient of Br – indicated that more Br – could be adsorbed on the oxide surface compared to Cl –. The high adsorption coefficient of Br – was ascribed to the aggressive attacks of the passive films of the alloy. X-ray diffraction images revealed that the phases of the Ti–6Al–4V sample comprised of only α and α’ without β phases. Moreover, the micro-Vickers hardness test results showed that dark grains exhibited greater hardness and lower corrosion resistance than bright grains because they contained relatively more acicular martensite α’ phase. Furthermore, the authors observed that the additive manufactured Ti–6Al–4V alloys with greater α’ phase distribution contained more defects owing to the higher donor density on the passive films.
In summary, the study reported an investigation of the corrosion behavior of additively manufactured Ti–6Al–4V alloys, explicitly focusing on the effects and relationship between the pitting potentials and the equilibrium adsorption of the Br – and Cl – anions. The pitting corrosion of the Ti–6Al–4V alloys was reported to significantly depend on the donor density, the α’ phase distribution, and equilibrium adsorption coefficients of Br – and Cl – anions. In a statement to Advances in Engineering, Professor Jae-Bong Lee said the current study would advance additive manufacturing of high-performance titanium alloys.
Seo, D.-I., & Lee, J.-B. (2020). Effects of competitive anion adsorption (Br− or Cl−) and semiconducting properties of the passive films on the corrosion behavior of the additively manufactured Ti–6Al–4V alloys. Corrosion Science, 173, 108789.