• Mantel, Marc
• Batalha, Weverson Capute
• Roche, Virginie
• Champion, Yannick
• Jorge, Alberto Moreira

Abstract

Corrosion is a substantial concern for marine applications. The application of amorphous alloys as thin-film coatings has been drawing attention in the last years since overcoming the typical limitations of these materials, such as the critical size and brittleness. Techniques based on Physical Vapor Deposition like DC magnetron sputtering are interesting once it allows parameter optimization to achieve the desired microstructure1,2,3. This study aims to understand the influence of formation, structure, thermal stability, and crystallization behavior on the electrochemical properties of novel FeCrMoNbB alloy containing 11%at of Cr. Ribbons were prepared by melt-spinning and annealed in a furnace under Ar protective atmosphere. Thin-film coatings were produced in a DC magnetron sputtering PVD system with 0.16Pa Ar working pressure, 92 W of sputtering power, and 400°C of deposition temperature. Corrosion tests were performed in a three-electrode cell set up in a Gamry 600+ potentiostat. The following sequence was defined: two hours of open circuit potential (OCP), pursued by electrochemical impedance spectroscopy (EIS) from 100kHz to 0,02 Hz, followed by twenty minutes of OCP, finishing with a potentiodynamic polarization test from -0,03 to 1,2V (vs. OCP). The specimens were the working electrode (WE), and a Pt grid was used as the counter electrode. The reference was a saturated calomel electrode. The solution was a 0,6M NaCl (Cl- content equivalent to seawater). X-ray photoelectron microscopy (XPS) was performed to obtain complementary information about the passive film. An XR3E2 apparatus from Vacuum Generator with an Mg-Kα source was used. The analysis was performed after stabilization in an open circuit for two hours. Corrosion behavior was analyzed by corrosion current (icorr) and corrosion potential (Ecorr). The alloy primarily presented outstanding corrosion behavior in the amorphous state, which exhibited a broad passivation plateau. However, Ecorr and icorr were sensitive to the crystallization process of the alloy. The value of icorr increased with increasing temperature and consequent reduction of the percentage of the amorphous phase, indicating less corrosion resistance for the crystallized alloys. Nevertheless, until the second state of thermal treatment, i.e., up to720°C, the alloys continued to show low values of corrosion current and passivation plateau that extended for more than 800mV concerning the OCP. Low-frequency impedance modulus (|Z|LF) can also indicate corrosion resistance and corroborate what was mentioned. A reduction of |Z|LF was observed with the increase in temperature. No occurrence of pitting was detected. Data from EIS was analyzed in SIMAD software based on the power-law model. Amorphous ribbons presented a capacitive-like behavior described by a simple constant-phase element (CPE) with capacitance values and α typical of a compact passive film layer. Through the model, it was possible to extract an approximate value for the resistivity of the metal/oxide interface (ρ0) of 1x10-13 Ω.cm. The thickness of the passive layer was estimated by Cole-Cole representation and compared with the results obtained by XPS. XPS analysis indicated the formation of a passive film composed of a mixture of the protective Cr, Mo, B, and Nb oxides. The XPS analysis also suggested that by increasing the temperature of heat treatment, the percentage of protective oxides such as Cr2O3 and Nb2O5 reduces, hence depleting the protectiveness of the passive film. Additionally, preliminary tests showed excellent performance of the coating applied on a 316L stainless steel substrate in slightly acid media (pH 5,5). The coating showed similar icorr and Ecorr values compared with 316L. However, the coating did not suffer pitting corrosion, typical for the stainless steel 316L near 500mV. Optimization of parameters and further analysis must be done; however, such results indicate that this novel alloy has the potential to be applied as a coating.

Topics

• amorphous
• stainless steel
• resistivity
• x-ray photoelectron spectroscopy
• melt
• laser emission spectroscopy
• physical vapor deposition
• pitting corrosion
• electrochemical-induced impedance spectroscopy
• drawing
• crystallization
• microscopy
• spinning
• cloud-point extraction