ФИЗИКОХИМИЯ ПОВЕРХНОСТИ И ЗАЩИТА МАТЕРИАЛОВ, 2008, том 44, № 6, с. 656-660
ФИЗИКО-ХИМИЧЕСКИЕ ПРОБЛЕМЫ ЗАЩИТЫ МАТЕРИАЛОВ
УДК 620.193.01:669
CORROSION PROTECTION STUDY OF NANOCRYSTALLINE PLASMA ELECTROLYTIC CARBONITRIDING PROCESS FOR CP-Ti
© 2008 r. M. Kh. Aliev*, A. Sabour*, P. Taheri**
*Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University,
Tehran, Iran
**Department of Materials Science, Faculty of Engineering, Tehran University,
Tehran, Iran Поступила в редакцию 19.06.07 г.
The effect of various nanocrystalline plasma electrolytic carbonitriding on commercially pure titanium in a Glycerol bath with different additives such as carbamide, natrium nitrate and triethanolamine was studied. Effects of electrolyte composition on chemical composition and corrosion resistance of the PEC/N films were examined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in Ringer solution. The results showed that the PEC/N films obtained in solutions with triethanolamine (T-film) had better corrosion resistance. Also a design of experiment (DOE) technique, the Taguchi method, has been used to optimize the process. The experimental design consisted of four factors (Triethanolamine concentration, electrical conductivity of electrolyte, applied voltage and duration of process), each containing three levels. An analysis of the mean of signal-to-noise (S/N) ratio indicated that the corrosion resistance of plasma electrolytic carbonitrided commercially pure Titanium was influenced significantly by the levels in the Taguchi orthogonal array. The optimized coating parameters for corrosion resistance are 1060 g/l for Triethanolamine concentration, 360 mS/cm for electrical conductivity of electrolyte, 260 Volts for applied voltage, 9 minutes for treatment time. The percentage of contribution for each factor was determined by the analysis of variance (ANOVA). The results showed that the applied voltage is the most significant factor affecting the corrosion resistance of the coatings.
PACS: 82.45.Bb
1. INTRODUCTION
Titanium and its alloys have a number of applications where weight reduction is of great concern [1]. Some surface modification techniques have been developed for protection of titanium [2-5]. Plasma electrolytic carbonitriding (PEC/N) [6-8], as a new and effective surface treatment technique, has been brought for surface modification of Ti alloys and some significant results have been reported [9-11].
In recent years, many investigators investigations have studied corrosion properties of Ti alloys coated with oxide films by plasma electrolytic oxidation process, but none of them have paid attention on the effects of composition of electrolyte on the properties of PEC/N films. In some of our previous studies, carbamide and natrium nitrate solutions were widely used for PEC/N process of different metals due to considerable corrosion and wear resistance of carbonitrides [8, 11]. Furthermore, some additives, such as formamide and triethanolamine are very commonly used for formation of carbonitrides [9]. These stable products can act as barrier layer preventing Ti substrate from corrosion in most corrosive intermediate. Undoubtedly, suitable additives are one of the effective ways to facilitate the formation of protective films on titanium and its alloys.
PEC/N process provides titanium thick, hard, high corrosion resistant ceramic-like nitride and carbide films composed of porous and barrier layers [9-11].
The additives effects on the properties of PEC/N films on Ti alloys have not been well studied; and the relationship between electronic characteristics and corrosion properties of PEC/N film is also seldom concerned. In this investigation, effects of some additives (natrium nitrate, carbamide and triethanolamine) in Glycerol base electrolyte on the structure, composition and corrosion resistance of commercially pure titanium (CP-Ti) with PEC/N film were studied. Based on the results, corrosion resistance of the PEC/N film was analyzed.
2. EXPERIMENTAL 2.1. Samples and solutions
Cylinder shape samples (with dimensions 5 mm x x 20 mm (dia)) made of extruded commercially pure titanium rod (CP-Ti) (Ti 99.7 ± 0.1%, Al 0.13%, Fe 0.05%, V < 0.05%, Cr < 0.01%, Cu < 0.02%, Mn < 0.01%, Mo <
< 0.03%, Nb < 0.01%, Sn < 0.05%, Zr < 0.01%, Si <
< 0.01%) were used as working electrode and 316L stainless steel electrode were used as anode during PEC/N treatment. Before PEC/N treatment, every surface of CP-Ti samples was successively ground down to 3000 grit SiC papers, then respectively ultrasonic degreased in eth-anol and distilled water. Three types of effective and representative electrolytes (see Table 1) were used for PEC/N treatment of CP-Ti and they were named respectively as solution C, solution N and solution T according to the additives species; correspondingly obtained PEC/N films
were called as C-film, N-film and T-film. All PEC/N processes were carried under direct voltage with 200-260 V for 3-9 minutes. Samples used in electrochemical experiments were placed in EG&G standard cell leaving an exposed area of 0.785 cm2.
Because CP-Ti is widely used for manufacturing human body implants, the conventional simulated human body electrolyte, Ringer solution (8.6 g/l NaCl, 0.33 g/l KCl, 0.3 g/l CaCl2 ■ 2H2O) was used for corrosion tests. All solutions were made from analytical grade reagents and distilled water.
2.2. Test methods
The surface and cross-section morphologies and chemical composition of PEC/N films were examined by XL-30 (PHILIPS) and Camscan MV-2300 scanning electron microscopy (SEM) equipped with energy dispersive analysis of X-rays (EDX). From the cross-sectional SEM morphologies, the approximate thickness of PEC/N films was estimated. Phase constituents of PEC/N film were detected by X-ray diffraction (XRD) with Cu^a radiation (k = 1.54178 Â).
Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization experiments were performed using a Princeton Applied Research (PAR) EG&G poten-tiostat/galvanostat model 273A with M352 corrosion software and EG&G 1025 frequency response detector. Poten-tiodynamic polarization was used to investigate pitting corrosion properties at a scan rate of 0.1 mV/s and EIS was used to analyze the corrosion nature of Ti electrode with and without PEC/N films during immersion in Ringer solution. The signal amplitude of EIS was 10 mV and the frequency ranged between 100 kHz to 10 mHz. All electrochemical measurements were conducted using a conventional three electrodes electrochemical cell with Ti samples as working electrode, a platinum plate as auxiliary electrode and a saturated calomel electrode (SCE) as reference. All experiments were carried out at room temperature.
3. RESULTS AND DISCUSSION
Because the substances of electrolyte participated in film formation reaction during PEC/N process, the structure, composition and other properties of PEC/N films would vary with the electrolyte compositions. In order to investigate the effectiveness of some useful additives (such as carbamide, natrium nitrate and triethanolamine) on properties of PEC/N film on CP-Ti, the morphologies, composition and corrosion resistance of PEC/N films were systemically studied.
3.1. Morphologies and composition of PEC/N films
Morphologies of PEC/N films obtained in solutions containing different additives were observed by SEM. As shown in Fig.1, there are many micro pores and some micro cracks on surfaces of these PEC/N films. Micro pores were formed by the molten compound layer and gas bubbles thrown out of micro-arc discharge channels; while micro-cracks were resulted from thermal stress due to rapid solidification of molten layer in the relatively cool elec-
Table 1. Electrolyte Composition for PEC/N processes of CP-Ti
Composition (g/l) Solution C Solution N Solution T
Carbamide 120 - -
Natrium nitrate - 80 -
Triethanolamine - - 180
trolyte. From these morphologies, it can be seen that these PEC/N films treated in solutions containing different additives represented different morphologies. Compared with the surface morphologies of C-film and T-film, the surface morphology of C-film (shown in Fig. 1a) was composed of bigger area of sintered compound layer containing both big and small size pores, and most of micro pores were blocked internally due to the essential of PEC/N process that the defected and unsubstantial surface in the compound film always gave birth to micro-discharge. The surface morphologies of N-film and T-film were similar, they had relatively uniform and "coral reef" like structure (Fig.lc and e) and the molten compound particles were randomly distributed in the film and integrated each other due to sintering effect during PEC/N process. The cross-section morphologies showed that PEC/N films were integrated firmly with Ti substrate by sintered interlocking and there were also some micro-pores and cracks to be seen in the cross-sections. These micro pores were neither connected with each other nor perforated through the whole compound films. The cross-section morpholo-
Fig. 1. Morphologies of PEC/N films on CP-Ti, respectively, treated in solutions containing carbamide(a), natrium ni-trate(c) and triethanolamine(e); (b), (d) and (f) - cross-sectional morphologies.
Table 2. EDX analysis of PEC/N films on CP-Ti treated in solutions containing different additives
Elements (at. %) Ti N
C-film 55.83 12.92
N-film 50.01 15.33
T-film 63.22 13.80
gies showed that C-film (Fig.lb) had relative bigger growth rate (higher thickness) than that of N-film (Fig.ld) and T-film (Fig.lf). And the T-film was composed of large bulk sintered carboxinitried but there were some large pores and cracks in the interior of PEC/N film. While C-film and N-film had relative uniform structure, and there were a few micro-pores and micro cracks in thei
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