**1. Introduction**

For marine ships, the hull steel must withstand the impact of waves, the action of huge bending moments formed by surging waves, temperature changes in cold winters and hot summers, and the corrosion of seawater. Therefore, in addition to sufficient mechanical properties and good technological properties, hull steel should also have good corrosion resistance. Corrosion will reduce the strength of the ship structure and shorten the service life. It will also endanger the safety of navigation and reduce its operational performance. Therefore, corrosion prevention of the hull structure has always been an important issue in the design, construction and use of ships [1,2].

In the marine environment, the corrosion of marine equipment and marine engineering steel is electrochemical corrosion. Electrochemical corrosion refers to the corrosion of metals in the electrolyte due to the action of microcells on its surface [3,4]. The main reason for corrosion is the presence of dissolved oxygen. In the marine environment, the surface of steel and iron will form anode and cathode regions due to inhomogeneity. In the positive and negative regions, the formation of local batteries will cause a certain degree of corrosion [5,6]. A series of reports have shown that the surface of carbon steel does not

**Citation:** Zhang, H.; Yan, L.; Zhu, Y.; Ai, F.; Li, H.; Li, Y.; Jiang, Z. The Effect of Immersion Corrosion Time on Electrochemical Corrosion Behavior and the Corrosion Mechanism of EH47 Ship Steel in Seawater. *Metals* **2021**, *11*, 1317. https://doi.org/ 10.3390/met11081317

Academic Editor: Renato Altobelli Antunes

Received: 29 June 2021 Accepted: 16 August 2021 Published: 20 August 2021

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form a stable passivating corrosion film, but a porous rust layer, which makes it difficult to prevent further corrosion under marine conditions [7,8]. The passivation film of lowalloy steels has low stability and is easily broken and corroded in seawater. Chen et al. [9] showed that the passivating film could be formed on the surface of carbon steel. When immersed in 3% NaCl solution or seawater, the weak parts of the passivating film would crack quickly and induce corrosion. The corrosion of carbon steel and low-alloy steel is uneven in a seawater environment, and local pits are formed. Furthermore, a relatively closed microenvironment and occlusive corrosion cell can be formed under the rust layer, resulting in serious local corrosion. The strength of the occlusive corrosion cell directly affects the expansion of the pit and the local corrosion resistance of carbon steel and low-alloy steel. The pH of seawater is about 8.0 to 8.2, and seawater contains a large amount of chloride ions. With a decrease in Cl− content in the solution, the thickness of the passivation film decreases and its protection ability increases due to the failure of the passivation layer on the steel surface at the threshold of Cl− concentration [10,11]. In addition, with an increase in Cl− concentration, a porous and non-protective product is formed on the steel surface. Zhang studied the corrosion behavior of 304 stainless steel under different Cl− concentrations and soaking times. The results showed that with the increase in Cl− concentration, the stability of 304 stainless steel decreases with the passivation film [12]. The effect of Cl− on the passivation film and pitting corrosion is particularly serious with long-term use. Yang [13] used the potentiometric polarization method to measure pitting potential in the corrosion process. The results showed that with the increase in temperature and Cl− concentration, the size and number of corrosion pits increase.

Xia studied the erosion and corrosion of hull steel under different flow rates in the seawater environment of the East China Sea [14]. Jia et al. [15] analyzed the corrosion resistance of F690, F460 and Q235B steel in 3.5% NaCl solution. Melchers [16] proposed various models to explain the corrosion mechanism. Zayed et al. [17] and Panayotova et al. [18] investigated the main corrosion mechanisms of ship steels.

Pitting corrosion is a common form of corrosion of carbon steel and low-alloy steel in the marine environment [19,20]. It is uneven and comprehensive, and so is often called pitting corrosion, which is different from the typical blunt metal pitting corrosion. Pitting corrosion forms because of electrochemical inhomogeneity on the steel surface, which leads to the formation of local corrosion microcells. Typical forms of corrosion of carbon and low-alloy ship steels in the marine environment include uniform corrosion, pitting corrosion, oxygen concentration difference corrosion, etc. [21,22].

Electrochemical technologies and hanging plate tests, such as the corrosion potential measurement, the polarization test, linear polarization technology, electrochemical impedance spectroscopy, and the simulated block cell test, have been widely used in the study of the corrosion behavior of ship steels and the development of products [23–26]. The main advantage of the electrochemical test method is that it is fast and can obtain instantaneous corrosion information, but the main disadvantage is the lack of accuracy. In contrast to the electrochemical test, the results of the hanging test and field test are accurate, but the required time is longer and the process is more complicated [27].

In this study, due to the disadvantages of the long test period and significant influence of environmental factors, the seawater hanging sheet test was carried out in the laboratory, and the seawater was changed every seven days during the experiment, which overcomes the shortcoming that the medium changes due to the limitation of the amount of medium and the corrosion and dissolution of metal as the test progresses. In this paper, the electrochemical corrosion behavior of ship plate steel EH47 was studied by electrochemical techniques such as corrosion potential measurement, polarization curve and electrochemical impedance spectroscopy. At the same time, the corrosion mechanism of low-alloy and high-strength ship plate steel in seawater was investigated by actual seawater hanging plate test.
