Materials Science, VoL 34, No. I, 1998

CORROSION

RESISTANCE

IN AQUEOUS

SOLUTIONS

OF NITRIDED TITANIUM ALLOYS OF HYDROCHLORIC

ACID

V. M. Fedirko, I. M. Pohrelyuk, and O. I. Yas'kiv

UDC 620.193

It is shown that vacuum heating prior to nitridationcontributesto the formationof nitrided layers on a surface. The layers adequately protect titanium alloys from corrosion in aqueous solutions of hydrochloricacid. Moreover,vacuumheating makes it possible to performeffectivenitridation at low temperatures. For the protection of commercial titanium alloys from corrosion in solutions of hydrochloric acid, nitridation is recommended [1, 2]. Since the efficiency of nitride coatings is determined by an electrochemical mechanism rather than by a mechanism of coating [3], a search for ways of obtaining the optimum properties of surface films is of critical importance. The temperature of saturation significantly affects the characteristics of nitrided layers. The porosity and relief of nitrided films as well as their coefficient of saturation with nitrogen increase with temperature. Therefore, the aim of the work is to investigate the effect of nitridation temperature on the corrosion resistance of titanium alloys in aqueous solutions of hydrochloric acid. We investigated specimens of commercial titanium alloys of different structural classes: VTI-0 (commercially pure a-titanium), OT4-1 (pseudo-a-alloy), VT6s ( ( a + ]])-alloy), VT35 (I]-alloy)in 20% and 30% aqueous solutions of hydrochloric acid at room temperature and exposure time up to 60 days. Weight losses of the specimens were determined by a gravimetric method every 10 days. We used chemically pure acid and distilled water. The specimens were treated at temperatures of 950, 850, and 750~ for 10 h (modes I, II, and III, respectively) in nitrogen at atmospheric pressure. Prior to being forced into the reaction space of the furnace, nitrogen was dried and freed from oxygen by passing through silicagel and titanium chips. Since nitridation at a temperature of 750~ in nitrogen of a given purity does not proceed (the surface is covered with a thin oxide film), the specimens were heated in vacuum (3 MPa), and nitrogen was forced into the chamber at the temperature of saturation [4]. As a result of nitridation the rates of corrosion of titanium alloys in 20% and 30% aqueous solutions of hydrochloric acid decrease by factors of tens and hundreds, respectively (see Table 1). The efficiency of saturation of the alloys with nitrogen increases in the following order: VTI-0, VT6s, OT4-1, VT35. The rate of corrosion of the complex VT35 [3-alloy is half that of the other alloys. The corrosion resistance of titanium alloys depends on the specific conditions of nitridation (see Table 1). After saturation in mode I in a 20% aqueous solution of hydrochloric acid the VT6s ( a + [~)-alloy exhibits the lowest rate of corrosion. Mode II causes minimum corrosion losses of the VTI-0 a-alloy and VT35 [3-alloy. Mode III ensures high corrosion resistance of the VT35 p-alloy. The corrosion losses of nitrided alloys increase with increase in concentration of the acid. As the concentration of the acid is changed from 20% to 30% the rates of corrosion after nitridation in modes I - I I I increase in the following way: for the VT1-0 alloy by 1.3-1.4 times, for the OT4-1 alloy by 1.1 - 1.2 times, for the VT6s alloy by l . l - 1.5 times, and for the VT35 alloy by 1.3- 1.7 times (see Table 1). Hence, the OT4-1 pseudo-a-alloy is least sensitive to changes in acid concentration. After nitriding in mode III, its rate of corrosion in more concentrated solutions is lower than in less concentrated (4.7 and 6.2 mg/(m ~-• h)) as in the case of nitriding of VTI-0 (8.1 and 8.6 mg/(m 2 • h)) and VT-35 (7.6 and 9.2 mg/(m 2 • h)) alloys in mode I. Karpenko PhysicomechanicalInstitute, UkrainianAcademyof Sciences,L'viv. Translatedfrom Fizyko-KhimichnaMekhanikaMaterialiv, Vol. 34, No. 1, pp. 108-110, January-February, 1998. OriginalarticlesubmittedJuly 17, 1996. 1068~q20X/98/3401-0119 $20.00

9 1999 KluwerAcademic/PlenumPublishers

119

120

V.M. FEDIRKO,I. M. POHRELYUK,AND O. I. YAS'KIV

,4m/5~

4

g/m~O'. dissolution of TiNx "

I

"'1

I0

-

, am/5, g/mm 2 .

Z

J

.

5

Ol

I i O.08 O,O6

2

0O4 O.O2

~ 0

~

, I0

,t 30 (a)

, 50

9 ~, days

I0 ~

~'

i

-

t

I

I0 4

I

1

I

I

1

105 ~min (b)

Fig. I. Kinetics of corrosiondissolutionof nitridedtitaniumalloys in (1) 30% and (2-5) 20% aqueoussolutionsof hydrochloricacid: ( I) VT1-0, mode ll; (2) OT4-1, mode II; (3) VT35, mode ll; (4) VT6s, mode II; (5) VT35, modeIII (n = 1 lineardependence; n = 2 parabolic dependence). Thus, nitridation protects best from corrosion in aqueous solutions of hydrochloric acid of the investigated concentrations for the monophase alloys (the VT1-0 or-alloys and VT35 13-alloy). The high efficiency of nitridation in mode III of the OT4-1 pseudo-oc-alloy is due to the fact that heating in a vacuum of 3 MPa to the temperature of saturation (750~ favors the sublimation of alloying manganese and aluminum from near-surface layers, which is caused by the pressure of saturated vapors of these elements in the indicated conditions [5]. Therefore, the near-surface layers of the alloy practically do not contain alloying elements (similar to the VTI-0 r before saturation with nitrogen. A lowering of the temperature of nitridation causes a decrease in intemal stresses and the quantity of defects in the nitride film, which improves its quality. As a result, the corrosion resistance of these layers improves (modes II and III). A change in the temperature of nitridation exerts the greatest effect on the rate of corrosion of the VT1-0 and VT35 monophase alloys in a 20% solution of hydrochloric acid, and the OT4-1 pseudo-a-alloy in a 30% aqueous solution of hydrochloric acid (see Table 1). The kinetics of dissolution of nitrided alloys shows that the corrosion process consists of separate stages (see Fig. 1 and Table 1), which is connected with differences in the physicochemical properties of the components of the nitrided layer (the nitride film and the solid solution of nitrogen in o~-titanium) and, hence, with different rates of their dissolution in the process of corrosion. The nitride film dissolves in the main for 10-30 days. After 10 days exposure in solutions of hydrochloric acid the surface of the specimens coated with golden nitride film was 80%, after 20 days it was somewhat more than 50%, and on the 30 th day it was less than 20%; the specimens turn silvery-white, which is characteristic for a solid solution of nitrogen in o~-titanium. The kinetics of dissolution of the film and the solid solution of nitrogen in or-titanium can be described by a power law

where the exponent n depends on the properties of the object being dissolved. Dissolution of the nitride film TiN x proceeds according to both a linear law and a parabolic one, while corrosion processes in a solid solution of nitrogen in (z-titanium are mainly described by a parabolic dependence (see Fig. 1, b).

CORROSION RESISTANCEOF NITRIDEDTITANIUMALLOYSIN AQUEOUSSOLUTIONSOF HYDROCHLORICACID

121

T a b l e 1. Rate of C o r r o s i o n of Titanium Alloys in Aqueous Solutions

of Hydrochloric Acid a f t e r Nitridation K, mg/(m 2 x h)

VT i -0

Mode of nitridation

OT4-1

VT6s

VT35 20%

without treatment

79.5

715

132

1200

129

1242

271

2647

I

8.6

8.1

9.7

10.0

7.8

8.1

9.2

7.6

II

5.2

6.3

7.6

8.0

6.7

9.5

5.2

6.4

III

4.2

5.3

6.2

4.7

5.8

6.3

3.1

4.8

Thus, low-temperature nitridation is an effective way of protecting titanium alloys from corrosion in aqueous solutions of hydrochloric acid, and heating in vacuum to the temperature of saturation accomplishes this.

REFERENCES 1. N.G. Boriskina, E. M. Ksenina, T. A. Tumanova, et al., "Effect of nitridation on the corrosion resistance and wear resistance of AT3 and AT6 titanium alloys," Zashch. Met., 19, No. I, 61--64 (1983). 2. N.D. Tomatov, T. V. Chukalovskaya, and !. L. Medova, "Corrosion-electrochemical characteristics of oxide-carbide and oxidenitride coatings on titanium obtained in electrolytic plasma," Zashch. Met., 26, No. 2, 246 (1990). 3. K.B. Katsov, A. A. Trufanov, and V. N. Zhytomirskii, "Effect of coating with nitride on the low-cycle fatigue of VT1 and AT3 titanium alloys in corrosive media," Fiz~-Khim. Mekh. Mater., 21, No. 3, 102-103 (1985). 4. V.M. Fedirko, I. M. Pogrelyuk, and O. I. Yas'kiv, "On the advisability of application of vacuum technology in nitridation of titanium alloys," Metalloved. Tenn. Obrab. Met., No. I, 44-50 (1997). 5. G.G. Maksimovich, V. N. Fedirko, Ya. I. Spektor, and A. T. Pichugin, Thermal Treatment of Titanium and Aluminum Alloys in Vacuum and Inert Media [in Russian], Naukova Dumka, Kiev (1987).