| Tantalum coating by sputtering, one form of physical vapor deposition (PVD), has been investigated as a replacement for chromium coatings on gun bores to protect them from erosion and corrosion due to its high ductility and high corrosion resistance in aggressive environments. When deposited as a film on steel substrates by sputtering, either alpha-Ta, beta-Ta, or a mixture of both phases have been observed under varying deposition conditions. To evaluate corrosion behavior of Ta coatings, electrochemical impedance spectroscopy and potentiodynamic polarization were conducted as a function of coating thickness. The coating porosity was observed to decrease with increasing coating thickness and hence, coatings greater than 50 mum exhibited corrosion resistance consistent with the bulk phase. Substrate roughness appeared to have little to no effect on the coating duality with respect to corrosion performance for 50 mum alpha-Ta coatings.; Coatings produced in full scale processes revealed that for Ta coating (<50 mum), the corrosion process was dominated by dissolution of the steel substrate through open pores, however, at the end of 5 days, coating degradation was not observed. In contrast, while open pores were not observed with the Cr coatings, the corrosion resistance decreased as a function of time under acidic conditions, resulting in dissolution and oxidation of Cr. Initially, however, the sputtered Cr coating exhibited improved corrosion resistance over the electrodeposited one, potentially due to its oxide film.; The unique properties of tantalum oxide films produced from anodic oxidation and thermal oxidation demonstrates that the nanoscale oxide films formed exhibit an ordered local structure reflecting the very compact nature that enhances its corrosion resistance. |
