Documentación del Sistema de Gestión de Calidad

2.1. Corrosion

Corrosion is a great concern when metallic biomaterials are implanted in the hostile aggressive environment of the human body. Implants face severe corrosion environment including blood and body fluids that contain different constituents like water, sodium, chlorine, proteins, plasma, amino acids [49]. Moreover, the pH of the human body is generally maintained at 7.0, but this value can undergo changes from 3.0 to 9.0 due to several causes such as accidents, infections, surgery…

Corrosion resistance of typically used materials; i.e., stainless steel, CoCr alloys and Ti alloys, is attributable to the protective oxide film formed on top of their surfaces.

Surface oxide film plays an important role as an inhibitor for the release of metallic ions. The composition, porosity and structure of this surface oxide film will determine the corrosion resistance of the biomaterial [50, 51]. The composition of the oxide layer depends on the biomaterial composition itself and on the reactions occurring between the material surface and surrounding living tissue and constituents [52].

However, it has been thoroughly demonstrated that even advanced materials used today, are prone to corrosion to a certain extent after long time exposures. This means that protective surface oxide layer cannot completely block the release of metallic ions with time.

In addition, it is necessary to remember that apart from corrosion, biomedical implants are subjected to different wear and fretting processes which will induce the rupture of the protective oxide layer [53-54]. This synergistic effect of wear and corrosion is known as tribocorrosion and will be deeply described in section 2.2.

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When the surface oxide film is disrupted, corrosion rapidly starts and metal ions are continuously released unless the protective film is build. The time taken for re-passivation is of sum importance since it conditions the quantity of released metal ions. Repassivation time for AISI 316L is higher than for Co-Cr alloys and much higher than for titanium alloys [55].

It is also of sum importance the type of metal ions that are exposed to living tissue, since adverse effects of several ions present on most common biomaterials have been already reported (Table 5.2).

AISI 316L alloy owns good corrosion resistance due to the formation of a protective oxide layer consisting mainly of chromium and iron oxides [56]. However, it doesn’t retain adequate corrosion resistance for long-term biomedical implants. Studies on retrieved implants show that more than 90% of AISI 316L implant failures result from pitting and crevice corrosion attack.

CoCr alloys have superior corrosion resistance than stainless steel. The high chromium content on its composition leads to the quick and spontaneous formation of passive Cr2O3 layer within the human body environment. Cobalt oxides are also present on the passive film [57]. Mo and Ni alloy elements further increase CoCr alloy’s corrosion resistance [58]. The main problem with CoCr alloys arises from the allergic reactions due to Ni and Co release.

Compared with stainless steel and CoCr alloys, Ti alloys have higher biocompatibility resulting from their superior corrosion resistance. Corrosion resistance of Ti-alloys come from the formation of highly stable, well-adherent and dense TiO2 protective layer [59]. Besides, titanium does not play any known biological role into the human body and is non-toxic even in large doses [60]. Surface oxide grown on Ti6Al4V consist of TiO2 with a small amount of aluminium oxide [61]. Protective layer formed

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on β-alloys surface is a mixture of TiO2 with highly stable niobium, tantalum and/or zirconium oxide, depending on the alloy type [62-63].

In terms of corrosion, Ti-alloys are considered superior materials of choice for biomedical implants.

2.2. Tribocorrosion

Apart from corrosion itself, biomedical implants are articulating systems that operate under sliding, rotation, vibration and loading conditions [64]. Hence, biomedical implants are exposed to degradation by different wear processes. When corrosion and wear occur simultaneously, these two processes lead to what is commonly known as “tribocorrosion” [65-67]. Tribocorrosion in biomedical implants is defined as a degradation phenomena of biomaterial surfaces (wear, cracking, corrosion…) subjected to the combined action of mechanical loading (friction, abrasion, erosion) and corrosion attack caused by human body environment (chemical and/or electrochemical interactions) [64, 68].

The coupling of the mechanical loading and chemical reactions taking place at the interface between the biomaterial and body fluids often results in accelerated degradation and implant damage than the one which would be expected by simply adding the degradation caused by individual processes [69-71].

Tribocorrosion is essentially a surface process, but it can negatively affect bulk mechanical properties of the whole material which have considerable impact on implants lifetime.

The effect of tribocorrosion on the wear rate of passivating biomaterials used for biomedical implants is directly related to some properties of the surface passive film.

Tribocorrosion phenomena is illustrated in Fig 5.1. Generally, when biomaterials are

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implanted in the human body, they tend to form a passive film (consisting mainly on oxides) that protect themselves from corrosion in the absence of mechanical loading.

However, once wear processes take place, the protective film is considered to be snatched in the contact area, exposing the bare biomaterial alloy to corrosion attack.

In this situation, the corrosion resistance of biomaterial is considerably reduced by wear. After mechanical loading removal, the biomaterial surface undergoes which is known as re-passivation phenomena, the re-building of passive protective film. The time taken for repassivation particularly in biomedical implants is crucial, since it limits the release of metal ions to the human body. Certain constituents of biomaterials (Ni, Co, Al, V…) have been found toxic and high ion release can cause adverse tissue reaction and loss of biocompatibility of implant material. Corrosion products generated during wear and repassivation processes are usually hard oxide particles that promote abrasive wear on the contact area. Thus, tribological resistance of the biomaterial can be diminished due to three-body wear mechanism induced by corrosion products.

It is hence demonstrated that friction and wear affect corrosion resistance of biomaterials and alternatively, corrosion enhance wear degradation of implants.

Corrosion and wear are widely studied modes of degradation of materials. However, in the majority of real life applications two degradation mechanisms take part at the same time and cannot be evaluated separately, since one directly affects the other.

Particularly in biomedical implants, the analysis and control of synergistic effect of corrosion and wear is of sum importance, since the implant damage induced by tribocorrosion involves the metallic ion release to the human body which has been proven to produce important health problems [5, 48, and 51]. Therefore, a dedicated testing protocol for tribocorrosion evaluation is crucial.

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Fig 5. 1: Schematic representation of tribocorrosion phenomena, illustrating corrosion accelerated by friction and wear and abrasion accelerated by corrosion products

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