1. Introduction
Abundant marine resources have attracted the attention of countries all over the world. The European Union has been continuously updating marine development documents since 2007 to promote the development of marine industry [
1]. China put forward the major deployments of a “developing ocean economy” and “strengthening a strong marine country” at the 19th and 20th National Congresses of the Communist Party of China [
2,
3]. In 2017, the EU’s gross value added (GVA) reached EUR 184 billion, and the total turnover reached EUR 665 billion [
4]. In 2021, China’s total marine economy exceeded CNY 9 trillion, accounting for 8% of the growth of the national economy [
5]. With the rapid development of the marine industry, concrete has become the main material for the construction of marine infrastructures, such as deep-sea exploration and ship engineering, due to its advantages of low price, high strength, and high plasticity [
6,
7].
However, marine concrete often suffers from severe erosion, requiring large amounts of materials and funds to maintain the infrastructure and repair the damage. The annual cost of repair and replacement of chloride corrosion of marine concrete in the United States is reported to be approximately USD 1 billion [
8]. In 2014, the cost of marine corrosion in China was RMB 700 billion, accounting for one-third of China’s total corrosion costs [
9]. Marine corrosion can even cause human casualties. In 2021, a 12-story beachfront condominium in Miami, USA, collapsed due to seawater back-up corrosion, killing 97 people [
10]. It is clear that exploring the corrosion mechanisms of marine concrete structures and accurately assessing their service life are important long-term issues for the global engineering industry [
11,
12].
Marine concrete faces an extremely complex environment [
13]. Huang [
14] reported that each kilogram of seawater contains about 19.345 g chloride ions, which is the highest chemical composition of seawater. Damage to marine concrete structures mainly comes from chloride salt erosion [
15,
16]. It is the most lethal factor leading to the degradation of reinforced concrete structures [
17]. Tang [
18] investigated the resistance of calcium sulphoaluminate cements to chloride ions under dry and wet cycling conditions. Al Sodani [
19] developed five short-term empirical models for predicting the diffusion coefficients of chloride ions in the actual exposure environment. Most of the current studies on chloride ions are based on Fick’s second law, which explores the erosion process from the perspective of quantitative analysis. Among the most important parameters of Fick’s second theorem, the surface chloride concentration is the boundary condition for quantitatively analyzing the life of concrete and predicting the resistance of concrete to chloride ion erosion [
20]. And it has been shown that the differences in chloride ion concentration in concrete will cause the change of concrete surface shape; when the chloride concentration exceeds a certain limit, it will cause the corrosion of the passivation film of concrete reinforcement, leading to cracks and accelerating the corrosion of concrete [
21,
22]. Wang [
23] investigated coral aggregate concrete under alternating wet and dry conditions using the finite element software, and the chloride ion concentration was used as a key indicator. Bao [
24] studied the relationship between factors such as the water–cement ratio and drew conclusions by measuring the chloride concentration under different exposure durations. It shows that the study of surface chloride ion concentration in concrete is of great engineering value for the design, maintenance, and service life assessment of marine concrete [
25,
26,
27].
The influence of surface chloride ion concentration is intricate and affected by factors such as environment, material, concrete composition ratio, and application time [
28], and researchers often neglect the influence of coupling factors. The different mechanisms of surface chloride concentration in different exposure environments lead to the convection phenomenon of chloride concentration, which is often neglected in numerical simulations. In addition, despite the rise of machine learning, its application in chloride erosion studies lacks uniform generalization. Compared with the traditional empirical formula methods, the characteristics of machine learning are not yet clear. It remains a challenge to utilize and translate existing research results into practical outcomes.
Marine structures include many regions where stress contours may be severely perturbed, which in turn lead to severe stress concentrations [
29]. These regions are known as disturbed regions [
30]. Plie caps, beam column joints, loading points, Corbels, deep beams, dapped ends, openings, T-shaped deep beams, and joints between piers and cross beams are other examples of these severely disturbed regions [
31,
32]. These regions are considered to be the most likely places for failure and the first cracks to occur. Therefore, improving the performance of these regions can improve the overall performance of the structure. It is worth mentioning that FE methods [
33] (including ABAQUS 2022 R2/ANSYS 2022/SAP84) are considered to be powerful tools for dealing with such regions.
With the maturity of chloride erosion studies and the emergence of sulphate ions as another key factor in concrete durability problems [
34], researchers have also begun to focus on the coupled erosion of chloride and sulphate ions [
35]. Currently, the coupling of sulphate ions with chloride ions is mainly focused on elucidating their erosion mechanisms [
36,
37]. It is generally believed that chloride ions from Friedel’s salt together with hydration products refine the pore space and limit the diffusion of sulphate. Subsequently, sulphate triggers concrete damage and pore dilation, facilitating chloride ion diffusion [
38,
39]. However, calcium leaching by sulphate ions also hinders chloride ion migration [
40]. The mechanism by which chloride and sulfate ions couple to produce erosion is currently unknown. Numerical analysis is a common method of studying the interaction between these ions [
41,
42]. Zhuang [
43] developed a numerical model of the diffusion response of their coupled erosion and investigated the effects of porosity and diffusion coefficient on the ion distribution and service life of concrete under dry and wet cycling and calcium leaching conditions. The researchers also focused on protective measures against coupled ion erosion. Liu [
44] found that modified barium chloride–silica fume composite admixtures could improve the resistance of concrete to internal sulphate attack. Although some progress has been made, the coupled erosion mechanism is still unclear, and there are relatively few research tools. The coupling effect of chloride and sulfate ions still needs to be vigorously studied in the future. The present paper does not deal with this subject.
Here, this paper summarizes the erosion mechanism and diffusion behavior of surface chloride ion concentration. Considering various exposure environments and influencing factors, the current research status of the empirical formula method and the machine learning method are highlighted, and the shortcomings of different research methods are analyzed. Meanwhile, the advantages and future development of the machine learning method are revealed to provide reference for researchers’ studies.