Influence of structural state of TiNi alloy on corrosion resistance in activating electrolytes
This work deals with the corrosion behavior of the TiNi alloy in the coarse-grained and ultrafine-grained states in inorganic media. Data on the microstructure of the TiNi alloy were obtained after corrosion tests by the gravimetric method. Research in active electrolytes has shown that the dissolution of titanium nickelide in the austenitic structure alloy is facilitated. The high activity of ultra-fine grained titanium nickelide is due to the smaller grain size of alloy and longer boundary lengths as well as a high density of dislocations along the boundaries.
In recent decades, ultrafine grained materials with a grain size of 100–300 nm have been developed, which have unique structures and properties that modify fundamental characteristics such as Debye and Curie temperatures, saturation of magnetisation, etc. Materials with a shape memory effect (SME) are already widely used in medicine as the implantable materials with long operating lifetime. The alloys based on titanium and nickel (NiTi alloys) make a special class of shape memory alloys. The range of their applications depends on the martensitic transformation temperature and the mechanical properties. They have high elastic properties, are able to change their shape with changes in temperature and will not fracture under alternating loads. Phase transitions in these alloys are characterised by broad hysteresis and a large temperature range in which the material exhibits shape memory and superelasticity effects [1–8].
Biochemical compatibility of physiological fluids and metal implants is largely determined by the electrochemical interaction between them, which usually results in the transfer of metal ions into the tissue fluids. In this case, an implant may also contain heavy elements that are toxic to an organism. However, it is not possible to assess biochemical compatibility by the concentration of toxic elements, especially if their incorporation into the implant leads to a significant increase in its corrosion resistance [9–23]. Since the corrosion properties of the implant are the most important indicators of biocompatibility, both direct corrosion processes involving transition of ions across the interface and reactions leading to formation of low conductive protective films must be considered. Typically, these protective films inhibit the release of toxic ions into a tissue, with the result that the implant containing the toxic elements poorly interacts with the surrounding tissue and becomes virtually inert in relation to biological environment. Corrosion resistance is largely determined by the degree of defectiveness of the material and the characteristics of its implantation as one of the most active carriers of aggressive media [15–20]. Tissues present a complex biological system that reacts to introduction of an implant by changing its own structure up to and including physical and mechanical destruction. Therefore, it is important to know the corrosion behaviour that affects the biochemical and biomechanical compatibility with the body tissues. In addition, it is important to know the corrosion behaviour of the material in various media where the material can be used.
A two-component alloy was chosen as the material of study: Ti49,1Ni50,9 alloy, which has a B2 austenite structure at room temperature, with a CsCl-type BCC lattice. The alloy was quenched from the homogeneity region (from 800 °C) into water to form a TiNi-based solid solution and to eliminate the precursor of the material. The average grain size of the quenched alloy was 200 µm. The TiNi alloy was subjected to 8 cycles of the Bc route at 400 °C to form an ultrafine grain size (UFG) structure [24–25]. An etchant of the following composition was used to reveal the microstructure of the original titanium nickelide: 60% H2O + 35% HNO3 + 5% HF. The microstructure was examined with the aid of an optical metallographic microscope OLYMPUS GX51, as well as by scanning electron microscopy (SEM) JEOL JSM-6490LV, inverted microscope AXIO OBSERVER Z1M in dark field mode (research was conducted in the laboratory of solid state physics of Institute of molecular and crystal physics of Ufa Federal Research Center of Russian Academy of Sciences). Before testing, the preliminary prepared samples were weighed on an analytical scale and then placed in a desiccator where the test sample came into contact with the corrosive medium at a temperature of 25 °C. The samples were placed in a solution for a specified time until visible signs of corrosion appeared. After testing the samples were washed with water, treated with alcohol, dried and weighed on an analytical scale. The rate of corrosion was determined by gravimetric method. After exposure the samples were brought to a constant weight and the weight loss was measured. The corrosion rate was estimated using the formula:
V= (m0-m)/S•t (1)
where m0 is the weight of the sample before corrosion testing, m is the mass of the sample after corrosion testing, S is the sample area, t is the corrosion test time.
The corrosion rate of Ti49,1Ni50,9 alloy in the coarse-grained and ultrafine-grained states was estimated by the gravimetric method. Table 1 shows the corrosion rates in various electrolyte solutions.
A study of corrosion of the alloy in 3% sodium chloride showed that the corrosion rate of UFG is 1.1 times higher than the corrosion rate of the alloy with CG structure. In a more aggressive corrosive environment (1 M HCl) corrosion rate of TiNi alloy in ultrafine-grained state is 1.2 times higher than in coarse-grained state. Increasing the molarity (concentration) of the solution to 5 M HCl showed that the corrosion rate in the ultrafine grained state increased manifold, since after soaking in this solution the sample in the UFG state was dissolved completely compared to the sample in the CG state. Summarizing the data obtained, it can be concluded that in aggressive media the alloy with UFG structure corrodes at a higher rate than the alloy with CG structure due to the different concentration of structural defects and the significant difference in the extent of grain boundaries. Similar behaviour is observed in sulphuric acid at different concentrations. At all values of sulphuric acid concentration, the corrosion rate of titanium nickelide with UFG structure is higher than that with CG structure. In 5 M sulphuric acid solution, the samples of ultra-fine grained alloy completely dissolved after 3 days of testing, while the samples in coarse grained state completely corroded by 15 days of testing. Thus the corrosion rate in the UFG state is several orders of magnitude higher than the corrosion rate in the CG state.
Figure 1 shows optical microscopy of the surface of Ti49,1Ni50,9 alloy samples in the coarse-grained and ultrafine grained states.
Figure 2 shows the surface structures of coarse- and ultrafine-grained alloy specimens after corrosion testing in 1 M H2SO4 solution. At this solution concentration no visible corrosion damage is observed, except for corrosion products in both coarse and ultrafine grained states on the surface of the samples.
Corrosion tests in 1 M HCl also do not result in damage to the specimens, accompanied by a slight loss of specimen mass. Corrosion products are observed on the surface; in the ultrafine grained state the volume fraction is higher than in the coarse grained state, while some corrosion is more evenly distributed over the surface (see Fig.3).
Studies in 3% NaCl showed that, compared with tests in acid solutions, fewer corrosion products were observed (see Fig.4). Thus, in the alloy structure the areas of different contrast are distinguished, and areas in ultrafine-grained condition have the size of about 200±20 micrometers (Fig.4c), in the coarse-grained condition – about 400±30 microns (Fig.4a).
Examination of the samples after corrosion tests in 5 M HCl using an inverted microscope (see Fig.5b, c) made it possible to determine the nature of the corrosion damage. In the coarse-grained state, deep pitting occupying more than 50% of the sample surface is observed, in the ultrafine-grained state the structure and surface cannot be evaluated (because dissolution of the samples occurred before the end of the corrosion tests). The scanning electron microscopy investigations also made it possible to estimate the average size and depth of the pitting pits in the coarse-grained state (Fig.5a). The average size of the pits is about 100 ± 10 µm.
Comparison of corrosion rates in various electrolytes has shown that for Ti49,1Ni50,9 alloy with CG structure the dissolution rate in sulphuric acid is 4.9 times higher than that in hydrochloric acid, and for titanium nickelide with UFG structure it is 5.6 times higher. High activity of titanium nickelide with UFG structure is explained by the significant reduction in grain size and the large extent of boundaries as well as by the high density of dislocations on the grain boundaries, which together lead to accelerated dissolution processes in interaction with the activation of the external carrier. Increasing the solution concentration leads to a significant acceleration of corrosion processes in Ti49,1Ni50,9 alloy with high Ni content up to complete dissolution of samples (5 M H2SO4 – coarse-grained, ultrafine grained state; 5 M HCl – ultrafine grained state). It was found that 1 M sulfuric acid and hydrochloric acid solutions after one month exposure did not change colour and did not show any precipitation, while 5 M hydrochloric acid and sulfuric acid solutions turned purple and then green, which is due to the predominant release of titanium ions (+4) and nickel ions (+2).
The process of dissolution of Ti49,1Ni50,9 alloy in acid solutions at high concentrations follows the mechanism of pitting, which is confirmed by the above mentioned photos of microstructures.
Studies have shown that corrosion dissolution is much more intense in the ultrafine grained state than in the coarse-grained state. Increasing the concentration of solutions leads to a significant growth of the corrosion rate, up to complete dissolution of samples. The dissolution process of Ti49,1Ni50,9 alloy in acid solutions at high concentrations follows the mechanism of pitting.
The research was supported by the Council for Grants of the President of the Russian Federation for state support of young Russian scientists – Candidates of Sciences (MK-6202.2021.1.2). ■
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.