Oxidation kinetics of commercially pure titanium

Abstract

The aim of this work was to perform thermal characterization of commercially pure titanium in dry air to determine its oxidation kinetics and the structure of the oxide. The oxidation kinetics were determined thermogravimetrically under isothermal conditions in the temperature range 300 to 750 o C for 48 hours and the structure of the oxides was determined by differential thermal analyses and X-ray diffraction in the temperature range room temperature -1000 o C. The oxidation rate of titanium increased with increase in temperature. It was high in the initial stages of oxidation and then decreased rapidly with time, especially up to 600 o C. The kinetic laws varied between inverse logarithmic at the lower temperatures (300 and 400 o C) and parabolic at the higher temperatures (650, 700 and 750 o C). Evidences from X-ray diffraction and differential thermal analyses data revealed that the passive oxide film formed at room temperature crystallized into anatase at about 276 o C. The crystallized oxide formed in the range 276 -457 o C consisted of anatase, in the range 457 – 718 o C consisted of anatase and rutile sublayers, and at temperatures beyond 718 o C consisted of a layer of pure rutile. Scanning electron microscopy observations reveled that the oxidized surfaces were crack-free and the surface roughness increased steadily with oxidation temperature.

Full text

ISSN 1517-7076
Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10919
Corresponding Author: Gemelli, E.
Received on: 13/04/07
Accepted on: 10/07/07
Oxidation kinetics of commercially pure titanium
Gemelli, E.; Camargo, N.H.A.;
Santa Catarina State University (UDESC), Center of Technological Science (CCT), Department of
Mechanical Engineering (DEM), Campus Universitário, Bairro Bom retiro, P.O. Box 631, Zip code 89.223-
100 – Joinville/SC - Brazil
e-mail: gemelli@joinville.udesc.br; dem2nhac@joinville.udesc.br
ABSTRACT
The aim of this work was to perform thermal characterization of commercially pure titanium in dry
air to determine its oxidation kinetics and the structure of the oxide. The oxidation kinetics were determined
thermogravimetrically under isothermal conditions in the temperature range 300 to 750
o
C for 48 hours and
the structure of the oxides was determined by differential thermal analyses and X-ray diffraction in the
temperature range room temperature - 1000
o
C. The oxidation rate of titanium increased with increase in
temperature. It was high in the initial stages of oxidation and then decreased rapidly with time, especially up
to 600
o
C. The kinetic laws varied between inverse logarithmic at the lower temperatures (300 and 400
o
C)
and parabolic at the higher temperatures (650, 700 and 750
o
C). Evidences from X-ray diffraction and
differential thermal analyses data revealed that the passive oxide film formed at room temperature
crystallized into anatase at about 276
o
C. The crystallized oxide formed in the range 276 - 457
o
C consisted of
anatase, in the range 457 – 718
o
C consisted of anatase and rutile sublayers, and at temperatures beyond 718
o
C consisted of a layer of pure rutile. Scanning electron microscopy observations reveled that the oxidized
surfaces were crack-free and the surface roughness increased steadily with oxidation temperature.
Keywords: Titanium, oxidation kinetics, oxide film, structure.
Cinética de oxidação de um titânio puro comercial
RESUMO
O objetivo deste trabalho foi de realizar uma caracterização térmica de um titânio puro comercial
para estudar sua cinética de oxidação e seus produtos de corrosão formados em atmosfera de ar seco em
função da temperatura. A cinética de oxidação foi determinada por termogravimetria em condições
isotérmicas entre 300 e 750
o
C por 48 horas e os óxidos formados foram caracterizados por análise térmica
diferencial e difração de raios-X entre a temperatura ambiente até 1000
o
C. A cinética de oxidação aumenta
com a temperatura e é muito rápida nos primeiros instantes, diminuindo rapidamente com o tempo,
especialmente até 600
o
C. As leis cinéticas variaram entre a logarítmica inversa nas temperaturas mais baixas
(300 e 400
o
C) e a parabólica nas temperaturas mais altas (650, 700 e 750
o
C). As análises por difração de
raios-X e análise térmica diferencial mostraram que a cristalização do filme passivo de óxido, formado
naturalmente à temperatura ambiente, ocorre aproximadamente a 276
o
C. A estrutura do óxido é composta de
anatásia cristalina entre 276 e 457
o
C, anatásia e rutilo entre 457 e 718
o
C, e somente rutilo acima de 718
o
C.
Observações por microscopia eletrônica de varredura revelaram que as superfícies oxidadas não apresentam
fissuras e que suas rugosidades aumentam uniformemente com a temperatura de oxidação.
Palavras-chave: Titânio, cinética de oxidação, filme de óxido, estrutura.
1 INTRODUCTION
The biocompatibility of titanium or its alloys is attributed to the oxide film formed on the material
[1-3]. This oxide film promotes a good osseointegration and prevents the dissolution of metallic ions into the
surrounding tissue [4]. Cell proliferation and mechanical interlocking between the implant and the bone
depend on the morphology and composition of the oxide layer [5, 6]. Nevertheless, the characteristics of the
oxide film depend on the chemical composition, structure, morphology and mechanical conditions of the
implanted material. Many surface treatments have been proposed to improve the biological performance of

GEMELLI, E.; CAMARGO, N.H.A.; Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007.
526
the implants. Among these techniques, the oxidation of titanium has been investigated recently [7]. The
oxidation can be enhanced by thermal oxidation or by micro-arc oxidation, also known as plasma electrolysis
[5, 8]. This process is performed by applying a positive voltage to a titanium specimen immersed in an
electrolyte (anodic oxidation). When the applied voltage is increased to a certain point, a micro-arc occurs as
a result of the dielectric breakdown of the TiO
2
layer. At this moment, Ti ions in the implant and OH ions in
the electrolyte move in opposite directions very quickly to form TiO
2
again [7, 8]. Recent studies
demonstrated that this treatment increases the thickness and roughness of the oxide layer, which leads to a
beneficial effect on the biocompatibility of the titanium implants [7].
When the titanium is exposed to ambient air at room temperature, a passive oxide film is
spontaneously formed on its surface. This passive film is amorphous, very thin (5-10 nm thickness [9]), and
composed of three layers [10, 11]: the first layer adjacent to metallic titanium is TiO, the intermediary layer
is Ti
2
O
3
, and the third layer, which is in contact with the environment, is anatase TiO
2
. At room temperature,
anatase TiO
2
is the most important layer in thickness and responsible for the integration between the implant
and the human bone when the material is not submitted to a thermal treatment at high temperature. The
surface oxide film on titanium formed in the air is so protective that the further oxidation of titanium is
prevented in various circumstances and mediums [12]. Even during sterilization in autoclave under a
saturated water vapor pressure at 120
o
C for 1.8 ks, oxidation of titanium does not proceed [12].
Oxidation at high temperatures promotes the development of a crystalline oxide film. Increasing
temperature induces the formation of a thicker oxide layer, which is accompanied with dissolution of oxygen
beneath it [13]. Feng and al. [14] investigated the oxidation of a commercially pure titanium at 600
o
C for 30
min. in air, in the oxygen and in the water vapor with 1.13-1.15 Pa. Surface composition and crystal structure
analyses carried out by X-ray photoelectron spectroscopy (XPS) and by X-ray diffraction (XRD),
respectively, indicated that titanium was oxidized in every medium, and formed films of rutile TiO
2
instead
of anatase TiO
2
[14]. Nevertheless, thermal oxidation at 600
o
C and 650
o
C for 48 h at normal atmospheric
condition revealed that the oxidized surfaces of the Ti-6Al-4V alloy principally consist of rutile but the
anatase form of TiO
2
was also detected at limited number of diffraction angles, especially after oxidation at
600
o
C (13). However, at 650
o
C rutile totally dominated the oxide structure [13].
Although some works have reported the structure of the oxide film in many temperatures, none of
them have investigated the crystallization temperature of the passive oxide film and the stability domain of
anatase and rutile as a function of temperature. In this paper, a systematic study has been carried out to
evaluate the oxidation kinetics and the transition temperatures of each oxide phase formed on commercially
pure titanium.
2 MATERIALS AND METHODS
Samples of commercially cast titanium were gradually polished up to 1 m alumina, cleaned in
ethanol, and dried at room temperature. The average chemical composition, measured by X-Ray fluorescence
spectroscopy (Shimadzu RF-5301), was found to be Ti – 0.078 wt.% Fe – 0.065 wt.% Mn – 0.026 wt.% Zn.
Oxidation kinetics of this material were performed by thermogravimetry between 300 and 750
o
C for 48 h in
dry air with an accuracy of 10
-6
g. Characterization of the oxide films was made by differential thermal
analyses (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Thermogravimetry
(TG) and DTA tests were carried out on disks of 5 mm diameter and 2 mm thickness with a Netzsch
equipment (Jupiter STA 449C). DTA was performed in dry air between room temperature and 1000
o
C at a
heating speed of 5
o
C/min. Samples of 20 mm x 17 mm x 2 mm were oxidized in a furnace, at normal
atmospheric conditions, for 48 h between 200 and 800
o
C. After oxidation, a XRD-6000 Shimadzu X-ray
diffractometer was used to identifier the oxides formed on the titanium. CuK

radiation source was used and
the incidence beam scan was 2
o
/min. Diffraction angle range was between 10
o
and 80
o
, with a step increment
of 0.02
o
and a count time of 0.6 s.
3 RESULTS AND DISCUSSIONS
3.1 Oxidation Kinetics
Figure 1 shows that the oxidation kinetics are very fast in the beginning of the oxidation and
decrease gradually with time within the firsts 10 and 20 minutes. The steady state is reached after a short time
and depends on the oxidation temperature. At lower temperatures (300 and 400
o
C) the weight gain per
surface unity (mg/cm
2
) is very low and the oxide film oxidized in inverse ratio of the logarithmic law. At
650, 700 and 750
o
C the oxidation kinetics are parabolics and at 500 and 600
o
C the oxide film grows
somehow between a parabolic and an inverse logarithmic law. As a matter of fact, the oxidation kinetics are
very close to a paralinear law. Nevertheless, the weight gain remains relatively low up to 600
o
C.

GEMELLI, E.; CAMARGO, N.H.A.; Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007.
527
3.2 Differential Thermal Analyses and X-ray Diffraction
Figure 2 shows the transformations of the oxide film during the heating. The peak at 73,5
o
C is due
to volatilization of ethanol molecules adsorbed on the surface of the passive film. At 276.1
o
C the passive
film converted to a crystalline film. The next transformation is observed at about 444-470
o
C (peak at 457.2
o
C). Evidences from XRD have shown a crystalline film of anatase between these two peaks (Figure 3). The
major peak of anatase was found at 2 = 38.1
o
. Actually, between 276 and 500
o
C there are a double peak
composed of anatase (2 = 38.1
o
) and titanium (2 = 38.3
o
). A systematic investigation between 200 and 300
o
C confirmed that the transition between the passive film to a crystalline film of anatase might occur at
approximately 275
o
C. At this temperature the oxide film is very thin and the peaks observed by XRD are
from titanium substrate. At 300
o
C the anatase peak can be clearly detected by XRD (Figure 4). At
approximately 444
o
C (peak at 457
o
C) rutile begins to nucleate and then the oxide film is constituted of
anatase and rutile sublayers (Figure 2). The peak at 680-730
o
C (718
o
C ) indicates that anatase is no longer
stable, converting to rutile, which is the only stable oxide above 718
o
C. This results are also back up by
XRD, which indicates that the oxide film formed at 500 and 600
o
C is constituted of anatase and rutile, and at
700 and 800
o
C rutile totally dominated the oxide structure (Figure 3). At 700
o
C a slight peak of anatase was
identified after 48 of oxidation showing that the reaction needs more time or higher temperatures to complete
the transformation. The peak at 900
o
C, which starts at about 885
o
C, is due to the allotropic transformation of
Ti to Ti (Figure 2). Allotropic transformation of pure titanium is reported to be at 882,5
o
C as a result of a
new atomic structure arrangement, from hexagonal ( phase) at room temperature to cubic ( phase) between
882,5
o
C and its melting point at 1670
o
C [15].
Figure 1: Isothermal oxidation kinetics of titanium in dynamic air atmospheres.

GEMELLI, E.; CAMARGO, N.H.A.; Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007.
528
Figure 2: TG and DTA curves of titanium.
The formation of defective oxide structure after crystallization provides easier diffusion paths for
oxygen and/or titanium ions allowing oxidation progress with temperature. At 300 and 400
o
C the oxide film
is very thin and the matter transport is influenced by the electric field formed between the internal
(metal/film) and the external (film/air) interfaces. Therefore, migration of the ionic species is predominant
leading to an oxidation kinetic that corresponds to the well known Cabrera and Mott’s model, i.e. the oxide
film grows according to the inverse logarithmic law. At 650
o
C diffusion takeovers the migration and the
oxidation kinetic is controlled by diffusion of the species through the oxide layer. Therefore, an intermediary
mechanism is speculated at 500 and 600
o
C, which could explain the oxidation kinetics observed in Figure 1.
At these temperatures the oxide film is still somewhat thin and the migration due to the electric field might
play an important role along with the diffusion that is improved by the crystallization, which creates easier
diffusion paths through the grain boundaries. The large peak at 276.1
o
C (Figure 2) also represents the
oxidation energy, i.e. at low temperatures (200 - 400
o
C) the reaction is very fast leading to a thicker layer
which increases steady and very slowly between 400
o
C and 700
o
C. Over 700
o
C the reaction is accelerated
again as we can observe in the TG curve (Figure 2). This result shows that the oxide film is very effective
against corrosion up to 700
o
C. From this temperature the oxide film growth is stimulated by diffusion as a
result of temperature and oxide structure. Nevertheless, the effect of the oxide structure is overlapped by the
temperature effect and it is still unclear whether or not the rutile structure is less effective than anatase to
prevent matter transport in the scale.

GEMELLI, E.; CAMARGO, N.H.A.; Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007.
529
30 40 50 60 70 80
0
200
400
400
o
C
R
A
A
T
T
T
T
T
T
T
2 
0
200
400
500
o
C
R
R
R
R
A
T
T
T
T
T
T
T
0
200
400
600
600
o
C
T
T
R
R
R
R
R
R
R
R
R
R
A
A
0
1000
2000
A
R
700
o
C
R
R
RR
R
R
R
R
R
0
1000
2000
3000
800
o
C
R
R
R
R
R
R
R
R
R
R
R
R
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
Figure 3: XRD patterns of titanium surfaces oxidized 48 hours in air (T = titanium, A = anatase, R = Rutile).

GEMELLI, E.; CAMARGO, N.H.A.; Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007.
530
Figure 4: XRD patterns showing the crystallization of the passive film of titanium surfaces oxidized 48 hours
at 300
o
C in air (T = titanium, A = anatase).
SEM observations on top of the oxide films showed that the surfaces are crack-free and uniformly
roughness, which increases with the oxidation temperature (Figure 5). The surface roughness, measured on
the Ti-6Al-4V according to average roughness (Ra) values, which define the arithmetic mean of departure of
a surface profile from a mean line, indicated that the roughness of oxidized surfaces at 600 and 650
o
C for 48
h was 0,80 and 1,35 m, respectively [13]. Average roughness of untreated sample was 0.17 m before
oxidation [13].
Figure 5: SEM micrographs of surface appearances of oxidized samples for 48 h in dry air
at 300
o
C (a), 500
o
C (b) and 700
o
C (c).
4 CONCLUSION
Commercially pure titanium was thermally characterized up to 1000
o
C. Oxidation reaction is very
fast in the early stages of the oxidation process leading to oxide layers composed of anatase and rutile
structures of TiO
2
. After the initial period, the oxidation kinetics are very slow up to 650
o
C due to the
excellent thermal behavior of the scale. Crystallization of the passive film into anatase occurs at about 276
o
C. Rutile starts growing from about 444
o
C (peak at 457
o
C). Between 457 and 718
o
C the oxide film is
composed of anatase and rutile sublayers. After 718
o
C the oxide layer is constituted of rutile only. Oxidation
kinetics and SEM analysis have shown that the oxide film formed on the titanium is very effective against
corrosion, crack-free and uniformly roughness.

GEMELLI, E.; CAMARGO, N.H.A.; Revista Matéria, v. 12, n. 3, pp. 525 – 531, 2007.
531
5 REFERENCES
[1] SUL, Y.T., JOHANSSON, C.B., JEONG, Y. ROSER, K., WENENBERG, A., ALBREKTSSON, T.,
“Oxidized implants and their influence on the bone response”, Journal of Materials Science:
Materials in Medicine, v.12, pp.1025-31, 2001.
[2] SUL, Y.T., “The significance of the surface properties of oxidized titanium to the bone response: special
emphasis on potential biochemical bonding of oxidized titanium implant”, Biomaterials, v. 24, pp.
3893-907, 2003.
[3] THOMSEN, P., LARSSON, C., ERICSON, L.E., SENNERBY, L., LAUSMA, J., KASEMO, B.,
“Structure of the interface between rabbit cortical bone and implants of gold, zirconium and
titanium”, Journal of Materials Science: Materials in Medicine, v. 8, pp. 653-65, 1997.
[4] OKAZAKI, Y., GOTOH, E., “Comparison of metal release from various metallic biomaterials in vivo”,
Biomaterials, v. 26, pp. 11-21, 2005.
[5] LI, D., FERGUSON, S.J., BEUTLER, T., COCHRAN, D.L., SITTING, C., HIRT, H.P., BUSER, D.,
“Biomechanical comparison of the sandblasted and acid-etched and the machined and acid-etched
titanium surface for dental implants”, Journal of Biomedical and Material Research, v. 60, pp. 325-
32, 2002.
[6] WENNERBERG, A., “The importance of surface roughness for implants incorporation”, International
Journal of Machine and Tools Manufacturing, v. 38, pp. 657-62, 1998.
[7] LI, L.H., KONG, Y.M., KIM, H.W., KIM, Y.W., KIM, H.E., HEO, S.J., KOAK, J.Y., “Improved
biological performance of Ti implants due to surface modification by micro-arc oxidation”,
Biomaterials, v. 25, pp. 2867-2875, 2004.
[8] YEROKHIN, A.L., NIE, X., LEYLAND, A., MATTHEWS, A., “Influence of surface characteristics on
bone integration of titanium implants”, Journal of Biomedical and Material Research, v. 122, pp.
73-93, 1999.
[9] FENG, B., WENG, J., YANG, B.C., CHEN, J.Y., ZHAO, J.Z., HE, L., QI, S.K., ZHANG, X.D., “Surface
characterization of titanium and adsoption of bovine serum albumin”, Materials Characterization, v.
49, pp. 129-137, 2003.
[10] CHENG, X., ROSCAE, S.G., “Corrosion behavior of titanium in the presence of calcium phosphate and
serum proteins”, Biomaterials, v. 26, pp. 7350-7356, 2005.
[11] POUILLEAU, J., DEVILLIERS, D., GARRIDO, F., DURAND-VIDAL, S., MAHE, E., “Structure and
composition of passive titanium oxide films”, Materials Science and Engineering, v. B47, pp. 235-
243, 1997.
[12] HIROMOTO, S., HANAWA, T., ASAMI, K., “Composition of surface oxide film of titanium with
culturing murine fibroblasts L929”, Biomaterials, v. 25, pp. 979-986, 2004.
[13] GÜLERYÜZ, H., CIMENOGLU, H., “Effect of thermal oxidation on corrosion and corrosion-wear
behaviour of a Ti-6Al-4V alloy”, Biomaterials, v. 25, pp. 3325-3333, 2004.
[14] FENG, B., CHEN, J.Y., QI, S.K., ZHAO, J.Z., HE, L., ZHANG, X.D., “Characterization of surface
oxide films on titanium and bioactivity”, Jounal of Materials Science: Materials in Medicine, v. 13,
pp. 457-464, 2002.
[15] ROCHA MELLO, G.M., ALEIXO, G.T., CHAVES, R.R., CARAM, R., “Estabilidade e meta-
estabilidade em ligas de titânio e sua relação com teores de Nb, Ta e Zr”, Proceedings of the XVI
Brazilian conference in Materials Science and Engineering, Porto Alegre/RS, CDROM, Brazil,
2004.