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Environmental degradation of Ti alloys in artificial saliva and a role of fluorides


  Titanium and its alloys possess specific properties, among them high corrosion resistance and biocompatibility. Bioinertness of Ti assed in short time tests seems almost perfect: the corrosion rate in simulated body fluids varied between 0.01 and 0.1 μg/cm2d, and after 48 weeks from an implantation of the Ti6Al4V alloy into animal body, only traces of metallic elements were found in tissues [1]. The corrosion resistance significantly depends on acidity of solution: in lactic and formic acids the general corrosion of the Ti6Al4V alloy was observed after 4 weeks [2]. Even if titanium is bioinert in neutral solutions, corrosion may appear in physiological saliva [3]. The saliva can be assumed as relatively aggressive environment [4]. The weight corrosion rate in artificial saliva was assessed as 0.28 g/cm2. At pH = 3 or lower, the passive current density for commercially pure Ti increased markedly with decreasing pH. The corrosion was observed [5] in saliva as dependent on a surface state, with a few times higher corrosion after sandblasting than after polishing. Fluoride ions are aggressive ions for the oxide layers of Ti and its alloys. Their presence may initiate localized degradation by pitting and crevice corrosion. Such conditions may happen as the toothpastes and prophylactic gels contain fluoride ions. So far results are confusing. As shown in [6], the effect of fluoride ions was weak at pH from 6.15 to 3.0 but below the last value Ti and its alloys suffered from localized corrosion. On the other side, even if titanium revealed ion releases 0.01÷0.1 μg/cm2.d, similar as gold alloys, the ion release increased to 500 μg/cm2.d in presence of fluoride [7]. Low pH values accelerate this effect even more. Therefore it is recommended to avoid the presence of fluoride or to reduce contact time. In another paper [8] the strong effect of presence of fluorides on corrosion density was observed for pure Ti and Ti6Al4V[...]

Nowe metody badań korozji i pękania wodorowego materiałów eksploatowanych w umiarkowanych warunkach

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Przedstawiono założenia testu niskocyklowego zmęczenia nawodorowanego materiału oraz oceny odporności korozyjnej biomateriału bazującej na oznaczaniu ilości metalu w roztworze. Słowa kluczowe: biomateriały, degradacja wodorowa, korozja New research methods of corrosion and hydrogen degradation of materials used in moderate conditions The outlines of low cycle fatigues test of moderately hydrogen charged and of an assessment of corrosion resistance of biomaterial based on an assessment of quantity of dissolved metal are proposed. Keywords: biomaterials, corrosion, hydrogen degradation.Obecne metody badań niszczenia korozyjnego i mechano-korozyjnego są najczęściej testami przyspieszonymi, stosunkowo mało wiarygodnymi w przypadku oceny materiałów pracujących w łagodnych warunkach i dość odpornych na degradacje. Przykłady to korozja naprężeniowa w elektrowniach jądrowych [1], pękanie stali stopowych w olejach [2], czy korozja implantów tytanowych wcześniejsza od przewidywanej [3, 4]. Zachodzi więc potrzeba opracowania nowych testów opartych na analizie przewidywanych warunków eksploatacji, znajomości mechanizmów korozji i wiarygodności testów oraz utrzymania w miarę szybkiego czasu oceny, czemu poświęcony jest ten artykuł. 2. Metodyka badań podatności tworzyw metalowych na degradację wodorową Obecne procedury badawcze przewidują stosowanie metody rozciągania ze stałą małą prędkością (Slow Strain Rate Test SSRT) jako metody referencyjnej, a następnie metody rozciągania przy stałym obciążeniu (Constant Load CL) lub odkształceniu (Constant Elongation CE) jako zbliżonych do warunków rzeczywistych i pozwalających na określenie, czy materiał ulegnie w praktyce degradacji wodorowej. Warunki narzucane przez me[...]

Surface treatment of porous Ti13Nb13Zr alloy for biomedical applications

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The Ti and its alloys are considered as the best biomaterials for load-bearing applications. As the most popular Ti6Al4V alloy may be potentially harmful [1, 2], the number of another alloys have been proposed or implemented, e.g. Ti6Al7Nb, Ti29Nb13Ta4.6Zr, Ti13Nb13Zr, Ti15Mo5Zr3Al, Ti15Zr4Nb4Ta [3]. They are widely used, especially for load-carrying implants. However, in order to achieve the bioactivity, long term stability and bioactivity, new solutions are looking for. They may include: an use of scaffold/ porous structures, anticorrosion and nanooxidation, deposition of hydroxyapatite coatings with immersion techniques. The scaffold metallic materials possessing open porous structure are recently used to enhance bioactivity, i.e. the tissue in-growth rate and long term stability are considered. The pore size of 30÷400 μm seems the most plausible [4]. The greater pore size enhances the faster tissue growth, the smaller - better adhesion strength. Porous structures in a bulk or within the surface layer can be obtained by various methods: powder metallurgy (P/M) with or without space holders, rapid prototyping with an use of selective laser melting (SLM) or electron beam melting (EBM), plasma spraying etc. [5]. Corrosion rate of Ti measured may be substantial [6]; in electrochemical and gravimetric tests changes in simulated body fluids (SBF) between 0.01 and 0.1 μg/cm2·day, and after 48 weeks of an exposure of the Ti6Al4V alloy implant only traces of Ti, Al i V can be found in body tissues [7]. At low pH corrosion increases: in lactic and formic acids general corrosion occurs already after 3 weeks [8]. Ti alloys degrade in presence of chlorides saline and artificial saliva [9] and in phosphate buffered saline (PBS) [10]. Increase in roughness [11], increase in temperature, low pH and wear conditions increase corrosion rate of Ti alloys [6]. Improvement of corrosion resistance and bioactivity of titanium and it[...]

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