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VARIOUS physical vapor deposition (PVD) techniques such as unbalanced magnetron sputtering,[1] high-power impulse magnetron sputtering,[2] pulsed laser deposition,[3] arc ion plating,[4] and the like have been applied to deposit pure titanium (Ti) and Ti-based ceramic coatings, e.g., TiN, TiAlN, and TiC. In particular, cathodic arc evaporation (CAE) is a commercialized method in which a high-rate and dense deposition is attainable without heating the substrate.[5,6] In response to the requirements of industries for highly corrosion-resistant materials, PVD methods have been developed for the deposition of protective coatings.[7,8] Such coatings must be either cathodically protecting or defect-free and inert to be applied as corrosion protective coatings. Formation of a nearly defect-free and inert coating is considered to be the case discussed herein. Moreover, the structure of coating may have a profound impact on its protective properties. A study of literature shows that

FARID REZA ATTARZADEH, Junior Engineer, is with the Research Institute of Petroleum Industry, Corrosion Division, Tehran, 14857-33111, Iran. Contact e-mail: [email protected], HASSAN ELMKHAH and ARASH FATTAH-ALHOSSEINI, Assistant Professors, are with Department of Materials Engineering, Bu-Ali Sina University, Hamedan 65178-38695, Iran. Manuscript submitted July 27, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS B

nano-grained or amorphous metals may offer more uniform and predictable sacrificial corrosion protection, essentially due to their small and uniformly distributed features.[9] According to Thornton’s structure zone model,[10,11] PVD coating in the low temperature and low inert gas pressure deposits a fine grain size structure. In the PVD technique, it is possible to achieve nanostructured (NS) coatings.[12,13] However, the main disadvantages of the coatings prepared by the CAE are the presence of porosity, pin holes, droplets, and other macro/micro surface defects which can adversely affect the passivation and corrosion resistance. In fact, some parts of the substrate may not be readily covered by the protective layer or surface defects may present serious pitting attacks.[14,15] On the bright side, applying double- or multilayer coatings not only improves the tribological properties of the coatings but also eliminates these defects to a large extent, thereby enhancing the corrosion resistance of the final product. Liu et al. have reported that a thin, yet well-bonded interlayer of Ti or TiN reduces the corrosion attacks effectively and modifies the pit propagation mechanism.[16] The purpose of the current study was twofold: first, a controlled two-step coating of Ti on the surface of 304 stainless steel (SS); second, comparing the passive behavior of pure Ti and NS Ti coating in strongly acidic solutions of H2SO4. It should be noted that the relatively thick coating that obtained herein renders the samples totally covered. Accordingly, the electrochemical behavior of Ti-coated samples was compared to the pure Ti rather than the underlyin