## 4 Discussion  
In this research the corrosion behaviour of two nickel-titanium alloys in three different seawater environments was analysed by statistical methods. Empirical data on the depth of corrosion were obtained by the FIB method of measurement after 6, 12 and 18 months of continuous exposure to the effects of air, tide and sea. A database for the corrosion rate of the two observed alloys was formed based on the empirical data. In the preliminary modelling the corrosion rate was observed as a constant value during the exposure of each alloy to the effects of the seawater environment, but assuming that corrosion begins to develop immediately after the sample is exposed to the environment. Based on this, adequate linear corrosion models were formed for each NiTi alloy and each environment. Three important assumptions were made in this preliminary modelling:  
* NiTi-1 alloy behaved similarly in conditions in which the dominant influence on the corrosion rate is air or tide, while corrosion processes were significantly more pronounced in conditions of exposure to the sea;
* NiTi-2 alloy did not show similarities from the point of view of corrosion processes in any of the three observed environments;
* NiTi-2 alloy behaved completely differently from the NiTi-1 alloy in all three environments. These assumptions were verified further by appropriate statistical analyses, because their confirmation would prove that, in addition to the composition of the alloy and the properties of the corrosive environment, the production method had a great influence on the corrosion behaviour of each observed NiTi alloy. It is generally known that corrosion processes are influenced by various factors that have uncertain characteristics. To capture this property of the corrosion rate, in the following statistical analysis, the corrosion rate was observed as a continuous real-valued probability variable. Normality tests have shown that empirical data do not follow a normal distribution, and that more complex distributions need to be considered. By detailed analysis of the data and preliminary testing, it was found that the two-parameter continuous distributions were not adequate for modelling such complex corrosion processes of the observed NiTi alloys. Therefore, 27 three-parameter continuous distributions were selected for further consideration. In the process of fitting three-parameter theoretical distributions into a probabilistic empirical corrosion model, statistical analysis revealed the continuous distributions that best reflect the shape, scale and location of the empirical data. The following theoretical distributions were detected as the most favourable for describing the corrosion processes of the two observed alloys:  
* For air influence, three-parameter Log-Logistic and Burr distribution;
* For tide influence, Burr and three-parameter Log-Logistic distribution;
* For sea influence, Generalised Extreme Value and General Pareto distribution. The positive tail of all the selected distributions extended out quite a long way, whereas the negative tail stopped near zero. As distributions with positive tails are detected to fit data in the best way, it can be concluded that the highest corrosion rate values are usually much higher than typical corrosion rate values, whereas the lowest corrosion rates are seldom much less than typical corrosion rate values. In the case of the dominant influence of the sea or tides, especially with the NiTi-1 alloy, high values of material wear due to corrosive processes were observed frequently.