Technological processes influence regarding titanium alloys biomaterials corrosion.

Catana, Dorin ; Scarneciu, Ioan ; Popescu, Rodica 等

1. INTRODUCTION

The increase of life span expectancy within the last century, especially in developed countries, has determined a gradual increasing of requests regarding the organs and tissues replacing as a result of exceeding progressive deterioration grade of tissue quality.

Titanium alloys research program has several stages and one of these stages was the study of heat treatment influence over material micro hardness. In order to gather all information regarding the behaviour of titanium alloy biomaterials used for prothesis was necessary to study the corrosion.

Regarding the materials used for prothesis was noticed passivity phenomenon (Rieu et al., 1991) which consists in formation of a compact small size and adherent to metallic layer protective film (3-100 [Angstrom]) by creating a barrier which determines kinetic inhibition of materials natural trend to react with contact medium (Rae, 1986).

In order to evaluate the corrosion of titanium alloy based biomaterials, following electro-chemical methods had been used:

--determination of passivity range, trans passivity and pitting corrosion by setting out cyclic polarizing curves using potential and kinetic methods (cyclic voltmeter);

--measuring the electrode potential in open circuit (E=f(t)). As electrolytes have been used highly acid HCl and [H.sub.2]S[O.sub.4] water based solutions and two Ringer solutions which simulate human body physiological environment.

The Ringer solution (1 and respectively 2) contain 8.6 g/l NaCl, 0.3 g/l KCl, 0.33 Ca[Cl.sub.2] and [H.sub.2]S[O.sub.4]. For this chemical composition the solution pH is of 7.3 (Ringer 2 solution). In order to obtain Ringer solution 1 in was added HCl in Ringer solution 2 until pH value attain 6.3 on the scale.

2. THEORETICAL CONSIDERATIONS

The biomaterials are synthetic materials used for parts, devices and artificial systems carrying out, with the object of replace and/or take over, totally or partially, the function of an alive tissue for a limited or unlimited time. In order to manufacture prosthesis appliance components, metallic biomaterials must have:

--an elasticity module as similar as possible, to the thighbone;

--good fatigue resistance and lastingness;

--wear and electrochemical corrosion resistance as advanced as possible.

The metallic materials that can be used for the hip articulations replacement are: stainless steels, Co- Cr alloys and titan alloys (Catana, 2002).

Out of presented metallic materials, the titanium and its alloys are considered to be the most biocompatible, with the best corrosion resistance in human body and with the most powerful relation between the implant and the surrounding osseous tissue. The alloys for the implants are described in table 1 (Catana et al., 2008).

Measuring the electrode potential in open circuit (POC) is a simple method of studying the formation of protective film layer and metallic material passivity at contact with the electrolyte. The increase of POC can indicate the formation of protective layer in time and its stability indicates film integrity and its protective purpose. A sudden dropping of POC value indicates either film dissolution or breakage.

Measuring the electrode potential in open circuit with acid HCl and [H.sub.2]S[O.sub.4] solutions, reveal that chemical composition and metallurgic processing can influence the formation and/or stability of protective film from metallic material surface.

Metallurgical processing applied to titanium alloys were: casting, plastic deformation, heat treatments (annealing). Therefore (Wusinczky et al., 2007), POC in Ti6Al4V system / HCl solution has growing and stabilization tendency for positive values which indicate the formation but mostly the stability of protective film (see figure 1).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Similar findings can be found for Ti6Ai7Nb alloy with the mention that for cast alloy the protective film has the tendency of breaking followed by a re-passivity process. A similar tendency has been noticed for the same plastic deformed alloy but it was used [H.sub.2]S[O.sub.4] solution instead.

Regardless acid solution and POC value for Ti6Al4V and Ti6Al7Nb alloys subdued at different metallurgic processing, the ratio between these values remain unaffected. By alloying titanium with Al and V electrode potential will decrease but replacing V with Nb (within the same percentage value) the same potential will increase as a confirmation for the titanium passivity after alloying it with Nb. By replacing a part of Nb with Ta and Mo this tendency is maintained or even augmented for HCl solution. Gathered data, based on measurement regarding the electrode potential in open circuit for HCl and [H.sub.2]S[O.sub.4] water based solutions, are presented in figures 2 and 3.

Ringer 1 and Ringer 2 solutions contain the same ion species, with the same concentration, except that of H and Cl ions, which have the different pH concentration. Different values of pH Ringer are not influencing (POC) values. But, for the system made of metallic material / Ringer 1 was noticed a slight value increase compared with metallic material / Ringer 2 solution system, excepting the values for Ti6Al4V, Ti6Al7Nb plastic deformed alloys, respectively Ti6Al2Nb1Ta1Mo cast alloy.

Is to be noticed again that Nb addition can determine POC electrode growth, in both solution, but by replacing Nb percentage with Mo no similar effect was noticed (see figure 4).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

For both the titanium alloys systems / Ringer solution it was noticed that for the Ti6Al4V and Ti6Al7Nb plastic deformed alloys, the electrode potential in open circuit has a lower values than the alloys in a cast or normalized state.

3. CONCLUSION

When alloying titanium with different chemical elements in order to improve mechanical characteristics it is recommended to take into consideration their effect into corrosion resistance. Based on previous affirmations we can conclude:

--metallurgical processing is favorable for POC value modifications by increasing them;

--by alloying titanium, POC values will be modified by increasing or decreasing them regardless the acid solution in use. The alloying with Nb, Ta, Mo has in effect the increasing of POC values and alloying with Fe or V will decrease POC values;

--chemical composition of acid solutions even if will modify POC values does not alter the influence of chemical composition or metallurgical processing of metallic materials.

In vivo corrosion, even if it can be evaluated using experimental methods, is a totally different process, main cause being complex composition of human body physiological environment. Therefore, present proteins can act as an initiating agent or inhibitory agent accordingly to the ionic environmental composition. For this step are required different in vivo corrosion investigation methods as well developing new materials for this purpose.

4. REFERENCES

Catana, D. (2002). Advanced Materials Processing, Lux Libris, ISBN 973-9428-73-8, Brasov

Catana, D.; Scarneciu, I. & Popescu, R. (2008). Thermal treatment influence on micro hardness titanium alloy biomaterials, Proceeding of the 19th International DAAAM Symposium "Intelligent Manufacturing & Automation: Focus Next Generation of Intelligent Systems and Solution", Katalinic, B. (Ed.), pp. 209-210, ISSN 17269679, Trnava-Slovakia, October 2008, DAAAM International, Vienna

Rae, T. (1986). The biological response to titanium and titanium-aluminium-vanadium alloys particles, I; Tissue culture studies, Biomaterials, Vol. 7, No. 1, 02-1986, 3036, ISSN 0142-9612

Rieu, J., Pichart, A., Rambert, L.M., Rambert, A., Chabrol, C. & Robelet, M. (1991). Ion implantation effects on friction and wear of joints prosthesis materials, Biomaterials, Vol. 12, No. 2, 04-1991, 139-143, ISSN 0142-9612

Wusinczky, D.; Wusinczky, E. & Popescu, R. (2007). Titan based biomaterials behaviour used in joint replacements, Bulletin of Polytechnic Institute of Jassy, Vol. LIII, No. 4, 05.2007, pp. 339-344, ISSN 1453-1690

Tab. 1. Main alloying elements for titanium implant alloys.

 Alloy Chemical composition [%]
 type Al V Fe Nb

 Ti6Al4V 5.5-6.5 3.5-4.5 0.25 --
 Ti6Al7Nb 5.5-6.5 -- 0.25 6.5-7.5
 Ti5Al2.5Fe 4.5-5.5 -- 2-3 --
 Ti6Al2Nb1Ta1Mo 5.5-6.5 -- -- 2