Anode Plasma Electrolytic Saturation of Titanium Alloys with Nitrogen and Oxygen

doi:10.1016/j.jmst.2016.06.005

Abstract

Abstract:

In this work, we investigated the features of the anode plasma electrolytic saturation of titanium alloys with nitrogen and oxygen. In this case, the titanium samples may be heated to 1050 °C using aqueous solutions of ammonium chloride as working electrolyte. The weight of titanium samples is found to change due to their oxidation and anode dissolution. An X-ray diffractometer, a scanning electron microscope, nuclear proton backscattering and an optical microscope were used to characterize the phase and elemental composition of the modified layer. The electrolyte composition (10 wt% ammonium chloride, 5 wt% ammonia) and processing mode (850 °C, 5 min) of commercially pure titanium (CP-Ti) allowing to obtain the hardened surface layer up to 0.1 mm with microhardness of 220 HV were proposed. Surface roughness R_a of samples after their nitriding for 5 min at 800 °C decreases from 1.67 to 0.082 μm. The anode plasma electrolytic nitriding could decrease friction coefficient and increase wear resistance of the CP-Ti. It is found that the anodic nitriding of low alloy titanium alloys reduces their corrosion rate in an aqueous solution of sulphuric (4.5%) and salt (0.2%) acids by 2 orders of magnitude. Results of cyclic testing show that anodic nitriding of commercial titanium leads to a decrease in corrosion rate by 8 times in solution of hydrochloric acid (6%) with addition of protein and vitamin.

Key words: Plasma electrolysis, Titanium, Nitriding, Wear, Corrosion

Belkin P.N.,Kusmanov S.A.,Zhirov A.V.,Belkin V.S.,Parfenyuk V.I.. Anode Plasma Electrolytic Saturation of Titanium Alloys with Nitrogen and Oxygen[J]. J. Mater. Sci. Technol., 2016, 32(10): 1027-1032.

Figures/Tables 15

Fig. 1. Current?voltage characteristics of the heating of the CP-Ti samples (Ф8 mm × 15 mm) in NH4Cl solution. The NH4Cl concentration: 1-5 wt%; 2-10 wt%; 3-15 wt%.

Fig. 2. Temperature?voltage characteristics of the heating of the CP-Ti samples (Ф8 mm × 15 mm) in NH4Cl solution. The NH4Cl concentration: 1-5 wt%; 2-10 wt%; 3-15 wt%.

Fig. 3. Effect of the voltage on the mass loss of the titanium sample (1) and the titanium mass in the solution (2). The NH4Cl concentration is 10 wt%, the samples temperature changes from 700 to 950 °С, the processing time is 30 min.

Fig. 4. Effect of the processing time on the mass loss of the titanium sample (1) and the titanium mass in the solution (2). The NH4Cl concentration is 10 wt%, 1000 °С, 260 V.

Table 1 Variations in the sample mass of the treated CP-Ti

Voltage (V)	NH₄Cl concentration (g/L)	Time (min)	Mass change (g)
160	100	1	-0.1183
210	100	1	-0.0199
260	100	1	+0.0447
210	50	1	-0.0239
210	150	1	+0.0285
210	100	0.5	-0.0197
210	100	2	-0.0529

Fig. 5. Concentration profile of oxygen and nitrogen in the surface layer of the titanium sample (Ф8 mm × 10 mm) after nitriding in a solution of ammonium chloride (10%) and ammonia (5%) at 700 °С for 5 min.

Fig. 6. SEM image of CP-Ti surface after anode PES in a solution of ammonium chloride (10%) and ammonia (5%) at 850 °C for 5 min.

Fig. 7. Optical image of cross-section of Ti?3.5Al?1.2Mn after anode nitriding in a solution of ammonium chloride (10%) and ammonia (5%) at 220 V for 5 min.

Fig. 8. Dependence of the nitrided layer thickness on the treatment temperature (5 min).

Fig. 9. Dependence of the nitrided layer thickness on the treatment time (850 °C).

Fig. 10. Microhardness distribution in the surface layer after PES at 850 °C.

Fig. 11. Surface linear profile of layer before treatment (a) and fabricated layer after anode PEN in a solution of ammonium chloride (10%) and ammonia (5%) for 5 min at 700 °C (b), 800 °C (c), and 900 °C (d).

Table 2 Results of the wear testing of CP-Ti nitrided during 5 min

Treatment temperature (°C)	Untreated	750	800	850
Friction coefficient	0.70	0.25	0.22	0.15
Weight loss (mg)	37.00	1.77	0.52	0.88

Table 3 Results of testing CP-Ti samples in an aqueous solution of hydrochloric acid (6%) with addition of protein-vitamin concentrate (20%) at 20 cycles

Cooling media	Corrosion rate (mm/year)
In electrolyte	0.0098
In air	0.0099
Untreated sample	0.1380

Table 4 Corrosion rate (mm/year) of the titanium alloys (50 mm × 20 mm × 3 mm) at the testing temperature of 20 °C and 50 °C (in brackets). The testing media are a solution of sulfuric (4.5 wt%) and hydrochloric (0.2 wt%) acids

Samples	CP-Ti	Ti?3.5Al?1.2Mn	Ti?3.6Al?1.2V	Ti?3Al?2.2Zr
Untreated	0.449 (0.720)	0.493 (0.796)	0.456 (0.789)	0.385 (0.621)
Nitrided followed by cooling in air	0.001 (0.040)	0.001 (0.005)	0.002 (0.005)	0.002
Nitrided followed by cooling in solution	0.008 (0.002)	0.002 (0.002)	0.004 (0.004)	0.003

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