**5.1 Electrochemically induced self-healing**

*Advanced Functional Materials*

Al and Mo are shown as examples of slow diffusing elements from the group of slow diffusing elements, Others includes the other alloying elements, such as, V and Sn, which are close to Al, and Nb lies in between Al and Mo. Element Fe is shown as an example in the group of fast diffusing elements in the figure. However, Ni is slightly

*Temperature dependence of impurity diffusion coefficient in* α *titanium: Co, Fe, Ni, Mn, Cr and P in single* 

Subsequently systematic measurements were hitherto made for the diffusion of Fe [51] Ni, [52] Mn, [53]Cr [54] and P [55]. On the other hand, Raiszinen and Keinonen measured diffusivities of Al [56] and Si [56] in polycrystalline Ti by a nuclear-reaction method. A detailed analysis of the data is compiled in the form of Arrhenius plots in the review by [57] and presented in **Figure 7**. The findings has shown that transition metal elements and phosphorus exhibit fast diffusion, which are three to five orders of magnitude faster than the self-diffusion. While, measurements done on ultrahigh purity α titanium with respect to Fe, Ni, and Co impurities resulted in very low diffusivity rates for self-diffusion in titanium and about two

It is well known that research in the field metallic self-healing is still in its infancy stage. Self-healing metallic materials has received attention only in the past decade [13, 18]. While previous reviews on self-healing materials [59] have focused

faster, whereas Cr and Mn fall in between Fe and the βTi-self-diffusion line.

orders of magnitude slower than Fe, Co, and Ni [58].

*crystal and Si, Al and Ti in polycrystal, culled from [33].*

**5. Self-healing concepts in titanium based materials**

**132**

**Figure 7.**

Self-healing coatings inspired by biological systems possess the ability to repair physical damage or recover functional performance with minimal or no intervention. When the kinetics are extremely fast, the phenomenon is controlled by the diffusion (mass transport) of the species that enters or leaves the surface of the material under consideration. Consequently, the composition of the system will also be changing. Analogous effects have been found by other workers in systems of biological interest, e.g., with processes involving membranes and enzymes. It is well known that the basic diffusion controlled modes, such as surface diffusion, Ds; grain boundary diffusion, Dgb; vacancy diffusion, Dv and pipe diffusion, Dp, are fundamental to determine the rate of atomic diffusion in polycrystalline metals. In general, surface diffusion occurs much faster than grain boundary diffusion, and grain boundary diffusion occurs much faster than lattice diffusion. Atomic diffusion and indeed electrochemicalinduced self-healing in polycrystalline materials is therefore often modelled using a combination of diffusion kinetics (see previous section). More details of the transformation modes in titanium have been discussed elsewhere [31–33]. For this, electrochemically induced self-healing are said to be a good strategy to be exploited in metals. For instance, a damage to oxide films, which normally protect the surfaces of Ti materials from corrosion, can be repaired by reoxidation in air. Recently, Gang Lu et al. [60] studied the oxidation of a polycrystalline titanium surface by oxygen and water and found that at 150 K O2 can oxidize Ti to Ti5, Ti3 and Ti2, while exposure of Ti to H2O at this temperatures only produces Ti2 species. At temperatures above 300 K, H2O can by both O2 and H2O slightly increases a further oxidize Ti2 to higher oxidation states. They observed rising temperature promotes the diffusion of oxygen into the bulk of the sample, which increasing overall oxidation. This is because the thickness of the oxide coating on Ti surface depends on both the duration of O2 exposure and on the sample temperature. At a given temperature, Ti oxidation by both O2 and H2O slightly increases as exposure increases.

Additionally, a crack on the surface of a titanium component can also be healed, when the oxidation reaction products fill up the crack cap. Therefore, cracks developed due to operational related stress can be autonomously self-healed or repaired by re-oxidative reaction that occur in Ti-based materials. Although self-healing coatings are considered as an alternative route for efficient anti-corrosion protection, intense research and development effort are been done in the area of corrosion protection coatings of metals and alloys. However, in order to improve the equipment service prediction capabilities of infrastructure, the use of Ti-based materials in infrastructures are beneficial as it can act as a second line of safety assurance even after the coating has failed. In this context, autonomic healing materials respond without external intervention to environmental stimuli, and have great potential for advanced engineering systems [61]. However, the limitation of this self-healing approach is that the extent of oxidation depends on sample temperature. A recent study identified 550–600 K as maximum oxidation in Ti based alloy. Upon heating the oxidized Ti above 850 K the titanium oxide layer is completely reduced to Tio , which is effective.
