Research paperThe effect of hydrostatic pressure on corrosion electric field considering the mechanochemical coupling effect
Graphical abstract
Introduction
The current field generated by the corrosion and anti-corrosion current of ship is called the corrosion electric field [1]. With the development of the detection technology of underwater electric field, the corrosion electric field of ship has become a new type of underwater signal source [1], [2]. At present, the research methods of ship corrosion electric field mainly include experimental methods such as real ship measurement and physical scale model (PSM), together with simulation methods such as equivalent electric dipole model and boundary element method (BEM). Experimental research takes a lot of cost and a long time, furthermore, it can only obtain the path distribution of corrosion electric field, and it is difficult to obtain the electric field distribution in a certain plane and three-dimensional space under water. However, the boundary element method is a high-precision numerical calculation method, which is very suitable for solving the open-field problem [3]. Therefore, this method has been widely used in the simulation of corrosion protection of marine structures such as oil rigs and ships [4], [5]. In recent years, some scholars have begun to apply the boundary element model (BEM) to the simulation study of ship corrosion electric field [5], [6], [7]. It has been studied that the effect of marine environmental factors such as seabed conductivity, seawater conductivity, temperature, velocity and depth on the ship corrosion electric field [8], [9]. Furthermore, the equivalent electric dipole model of corrosion electric field has been established, and the electromagnetic field generated by horizontal and vertical electric dipoles in multi-layer medium has been analyzed [10], [11], [12].
The complexity of ship and offshore engineering structure and the particularity of their service environment make the structure produce complex stress and strain, such as the welding between frame and ship hull, and corrosion defects on the hull surface, which will produce stress concentration [13], [14], [15], [16]. In addition, the structural stress of submarine will change significantly with the change of hydrostatic pressure in the process of submergence and floatation [17], [18]. The interaction between load and corrosion medium is defined as the mechanochemical effect of metal corrosion. It mainly includes the negative shift of corrosion potential and the increase of corrosion rate under the synergistic effect of load and corrosion medium [19], [20], [21]. It shows that the corrosion rate of Q235B steel in 3.5% NaCl solution increases by about 44% when the structural stress is close to 160 MPa [19]. Therefore, the mechanochemical effect of metal corrosion in seawater cannot be ignored. However, the effect of mechanical factor on the corrosion electric field is not considered in the existing research, but the research on the corrosion electric field under the synergistic action of load and corrosion medium has important engineering significance and practical value. Therefore, the purpose of this paper is to study the influence of hydrostatic pressure on the corrosion electric field of submarine in the process of submergence or floatation.
Relevant research shows that the structural stress of cylindrical shell in the state of pumping or inflation (internal pressure) can be used to replace the stress of submarine in the process of submergence (external pressure). Moreover, the effect of compression load and tensile load on mechanochemical effect is symmetrical [22]. Therefore, this paper took seamless steel cylinder as the research object, and the method of filling gas pressure with seamless steel cylinder was used to simulate the hydrostatic pressure on the submarine. The mechanochemical coupling model of corrosion electric field was established by COMSOL simulation software, and the parameterized scanning function in the software was used to realize the change of gas pressure with (0, 3, 9) MPa. Finally, the mechanochemical coupling model was verified by the experimental results of corrosion electric field.
Section snippets
Measurement of corrosion electric field
The experimental pool has a size of 12 × 5 × 3 m and a water depth of 0.8 m, as shown in Fig. 1(a). The inner walls of pool are made of resin-based composite material to reduce the influence of steel corrosion in concrete structure on experimental result. Seamless steel cylinder with the outer diameter of 140 mm and the height (including valve) of 810 mm meets the standard of GB/T 5099.3–2017. Its main material is 37Mn steel and the chemical composition is shown in Table 1.. The Ag/AgCl
Numerical simulation model
The mechanochemical coupling model of corrosion electric field of seamless steel cylinder was established by COMSOL multiphysics simulation software. The simulation model mainly analyzed the following three aspects: firstly, the elastoplastic solid stress of seamless steel cylinder; secondly, the electrode potential analysis of metal/solution interface under the synergistic action of solid stress and corrosion medium; thirdly, the electric field distribution in solution under the
Structural stress distribution of seamless steel cylinder
Fig. 2 shows the von Mises stress distribution on the surface of seamless steel cylinder when the gas pressure is 3 MPa, 6 MPa and 9 MPa, respectively. The stress level on the cylinder surface increases uniformly with the increase of gas pressure. When the gas pressure is 9 MPa, the maximum stress on the cylinder surface is 625 MPa, which is lower than the yield strength of 37Mn steel (735 MPa). Therefore, the linear elastic assumption in the structural stress simulation model is reasonable.
Conclusions
In this paper, the mechanochemical coupling model of corrosion electric field was established, and the effect of hydrostatic pressure on corrosion electric field was studied by a combined experimental and simulation approach. The conclusions were drawn as follows:
• Seamless steel cylinder is in the stage of linear elasticity within the pressure range studied. The von Mises stress of the cylinder body obtained by simulation is almost equal to the theoretical value, and there is a stress
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.
Acknowledgements
This work is supported by National Natural Science Foundation of China (Grant no. 41476153) and the author wishes to appreciate the technical support of COMSOL software company.
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