The effect of hydrostatic pressure on corrosion electric field considering the mechanochemical coupling effect

Highlights:

• Developed the mechanochemical coupling model of corrosion electric field.

• Surface potential shifts linearly and negatively with the rising of pressure.

• Corrosion electric field increases with the rising of pressure.

• The effect of stress on corrosion electric field cannot be neglected.

Abstract

In order to study the effect of hydrostatic pressure on the corrosion electric field of ship, 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 using the solid mechanics and the current distribution (boundary element method, BEM) module in the COMSOL simulation software. The two physical fields were solved by using the sequential solver setup, and the structural stress simulated by the solid mechanics module was coupled to the expressions of equilibrium potential and exchange current density of electrode reaction, which were taken as the boundary conditions of the current distribution simulation module. Finally, the simulation results of the mechanochemical coupling model were compared with the measured corrosion electric field. The results show that the distribution characteristics of corrosion electric field and the variation trend of electric field with gas pressure obtained by simulation have a good agreement with the experimental results, moreover, the errors of peak-to-peak value of electric field obtained by experiment and simulation are within 20%, which demonstrates that the mechanochemical coupling model of corrosion electric field established in this paper can effectively predict the distribution characteristics of electric field under the synergistic action of stress and corrosion medium. When the gas pressure is 0 MPa, the maximum value of electric field modulus is 0.262 mV/m. When the gas pressure increases to 3 MPa, 6 MPa, and 9 MPa, respectively, the maximum value of electric field modulus correspondingly increases to 0.288 mV/m (9.92% increase), 0.317 mV/m (20.10% increase), and 0.348 mV/m (32.82% increase), therefore, the effect of stress on corrosion electric field cannot be neglected.

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.