Corrosion behaviour of TiN0.8O0.4 coating on 316L stainless steel alloy

Abstract

Among the different coating techniques used to protect metallic structures against corrosion in an aggressive environment, chemical vapour deposition and physical vapour deposition are probably the most popular. [1][2][3] Particularly, titanium compound coatings are widely used to protect the metallic structures due to their high hardness and corrosion resistance. However, several studies showed that the presence of porosity in the TiN coating on active substrate can generate an amplification of the corrosion phenomena. 4-7 Indeed, the presence of pores in the surface and the penetration of the aggressive agent through these pores lead to the creation of a "microbattery" leading to a local acidification process. For that reason, it is necessary in the corrosion studies of the coating on an active substrate, to take into account the contribution of the latter.In this paper we investigate the corrosion properties of TiN x O y thin film deposited at 700°C by low-pressure metalorganic chemical vapour deposition (LP-MOCVD) on 316L stainless steel alloy. At this temperature the phase formed is a conductrice. 2 ExperimentSample preparation: Before the development of the TiN x O y deposit 316L SS, the sample is polished mechanically and cleaned by ultrasounds at 70°C through the use of acetone, ethanol and trichlométhane. The coating is realised at 700°C by low-pressure (60 torr) LP-MOCVD, the sample is placed on a graphite support heated by induction with a radio frequency system. The isopropoxide of titanium (Ti(OCH(CH 3 ) 2 ) 4 ) is used as the precursor of titanium and the oxygen. This precursor is in a liquid state at ambient temperature and at 40°C its vapour pressure is 0.35 torr. It is maintained at 40°C and is paddled by nitrogen (gas carrier). Ammonia is used as a nitrogen source.The coating thickness is measured by scanning electron microscopy, observing the transverse section of the sample. For one hour of deposit at 700°C, the coating thickness is around 250 nm.Characterisation methods: The structural characterisation of the films was carried out by X-ray diffraction performed on a Siemens D5000 diffractometer using a Cu Kα radiation at 1.54 Å. The coating composition is determined with the help of a XPS apparatus equipped by a Riber spectrometer type Mac 2 and the source of X-ray used is Al Kα radiation (1486.6 eV) with power 300 W. The electrochemical test was carried out at 37°C, in NaCl solution and the polarisation behaviour was measured potentiodynamically with a potentiostatic apparatus (Amel 2049). All potential values are given versus a saturated calomel electrode (SCE). The scanning rate was 1mV/s. The polarisation was changed from -1.2 V. Before measurements, the corrosion potential E cor was monitored for 1h. Results and discussionThe coating composition analysis by photoelectron spectroscopy: The XPS analysis has been realised after ionic bombardment in order to eliminate the contamination layer. Fig. 1 presents an XPS (photoelectron spectroscopy) spectra obtained before (Fig. 1a) and after ioni...