## 2 Materials and Methods
### FIB Corrosion Depth Measurement Data Analysis  
The parameters of the environment, such as temperature, pressure, salinity, particle flow rate, conductivity, etc., and their influence on corrosion processes, are not considered in this paper. Corrosion layers of both different NiTi alloys were observed with the high-resolution Scanning Electron Microscope Sirion NC400 (FEI, Hillsboro, OR, USA) with field electron emission, which enables extremely high magnifications (up to a million times) and high resolution (1 nm). It was equipped with an EDS (Energy Dispersive Spectroscopy) analyser, Oxford Instruments (Oxford Instruments, Abingdon, UK) It enables qualitative and quantitative microchemical analysis in a single point and on the surface, as well as qualitative line analysis and surface distribution of elements (mapping analysis) from beryllium to uranium. The resulting corrosion layers were evaluated and their chemical composition identified by EDS (Tables 2 and 3). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & & & & & & & \\ \hline
**Spectrum** & **In stats.** & **C** & **O** & **Na** & **Cl** & **Ti** & **Fe** & **Ni** & **Total** \\ \hline Spectrum 1 & Yes & & & & & 30.87 & 24.69 & 44.44 & 100.00 \\ Spectrum 2 & Yes & & & & & 32.89 & & 67.11 & 100.00 \\ Spectrum 3 & Yes & & & & & 45.49 & & 54.51 & 100.00 \\ Spectrum 4 & Yes & 8.97 & 24.98 & 2.79 & 2.87 & 25.54 & 9.00 & 25.85 & 100.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of the NiTi-1 corrosion layers identified by EDS analysis. A Quanta 200 3D electron microscope with ion beam gun--FIB (Focused Ion Beam) (FEI, Hillsboro, OR, USA) was used for measuring the depth of the formatted corrosion layer. A focused beam of primary gallium ions was enabled for etching the surface of the samples and cutting a cross-section without contamination, and, thus, gaining a direct insight into the resulting corrosion layer. Figure 4a,b show NiTi-1 and NiTi-2 samples after 12 months of exposure to the sea and corresponding FIB measuring locations. A photo view of samples with 3000 magnifications and 20 \(\upmu\)m are shown in Figure 4c,d, while Figure 4e,f show the data on magnifications for samples, as well as corresponding corrosion depth in nm for the measured samples. All other samples of NiTi-1 and NiTi-2 exposed to the air, sea and tidal zone were scanned similarly. Figure 4g,h show the locations of the EDS analysis on the corrosion layer after the FIB cut was performed, directly on the surface and transversely in the corrosion layer. The results of the semi-qualitative EDS analysis in Tables 2 and 3 indicate the presence of elements that are typical of corrosion products, i.e., O, C, Na and others, and differ significantly from the initial chemical composition of the investigated alloys as shown in Table 1. The electrochemical process of corrosion and uniform corrosion on the surface of the alloy was observed on the samples shown in the figures. The number of measured corrosion depths for both observed NiTi alloys regarding three different seawater environments and after 6, 12 and 18 months of exposure, are shown in Table 4.  
Figures 5 and 6 show all raw data of the measured corrosion depth for the NiTi-1 and NiTi-2 alloys respectively. The \(x\)-axis shows the ordinal number of measurements, while the \(y\)-axis shows the value of the measured corrosion depth expressed in nm. Data representing the depth of corrosion related to the impact of different environments (air, tide, sea) are shown in different colours, in the way described by the corresponding legends. Figures 5 and 6 clearly depict the different behaviour of the alloys from the point of view of corrosion wear in the three considered environments. During the 18 months of the experiment, the NiTi-1 alloy was characterised by similar values of corrosion depth under the influence of air and tide, while the values of corrosion depth under the influence of the sea are significantly higher with a higher degree of scatter.