## 1 Introduction  
The possible exploitation of any material is implied by previous research of material behaviour in the environment in which it will be applied. The usage of metallic materials, especially in the complex marine surroundings, or in the environment of the sea, atmosphere, semi-enclosed and closed spaces, certainly requires numerous studies on corrosion process causes, as well as predictions of the type, extent and rate of corrosion. In this regard, there are two types of methods for predicting the corrosion of metallic materials and structural elements. The first method is based on the application of statistical models and the analysis of empirical corrosion data of metallic materials in usage. These data further represent a basis on which are calculated the mean and Standard Deviations of the corrosion rate. The second method is based on the application of a model that predicts the probability of corrosion by identifying key variables and the mechanism of the corrosion process. Corrosion models based on a statistical analysis of large databases formed on the basis of the research of existing materials in usage, or smaller databases created on the basis of new materials, are considered the most adequate and practical in the formation of various new corrosion models. As the volume of empirical data grows (i.e., the number of samples of researched materials and collected data grows, as well as the amount of available information on exploitation conditions and influences of environmental factors), the models themselves describe the corrosion processes of the exploitation materials more realistically. The development of different corrosion models aims to understand the extent and speed of corrosion and predict the future state of materials, i.e., the technical and technological possibilities and economic justification of their further exploitation. A significant number of authors believe that the corrosion process is unstable and time-dependent, and that it has a constant velocity that can be expressed linearly [1]. However, experimental evidence of corrosion published by some authors has shown that a nonlinear model is, in specific cases, more realistic in terms of describing the corrosion process. In almost all models there are two key phases during the exploitation of the material, and these are the phase of surface coating stability when there is no corrosion, and the second in which a linear or nonlinear corrosion process takes place. Whether the corrosion process will be linear or nonlinear with the slowing down or acceleration of the corrosion process depends on the environmental conditions to which the metallic materials are exposed. The environment in which metallic materials are located (dry areas, wet areas, marine conditions, changing conditions of the sea and atmosphere, etc.) influences the oxidation process of metallic materials and the intensity of the corrosion process dominantly. One of the most important researches of corrosion processes, in which extensive databases are used, are those on ships in operation. In the past few decades, the most significant research on bulk carriers and tankers has been carried out, using databases with corrosion data detected on structures related to up to 100 ships [2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. The considered corrosion models describe the general and pitting corrosion of metal structural elements in a way that they can predict the degree of corrosion and the corrosion margin of individual structural elements, which was a prerequisite for the optimal design of the thickness of steel elements in operation [2; 6; 7; 8; 9]. Initially, the developed models were based on the linear dependence of the estimation of the thickness loss of steel structures due to corrosion (as suggested by [PERSON] et al. [12]). Lately, nonlinear models have been developed with widespread application. The most relevant nonlinear models were developed by [PERSON], [PERSON], [PERSON], [PERSON] and others.