Baoyu Geng

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A joint protection device for seawater electrolysis antifouling and ICCP was constructed, and comparative experiments were conducted to study the composition of the EH36 surface deposition layer under joint protection in a marine environment. Surface morphology analysis, energy-dispersive spectroscopy (EDS) imaging analysis, and X-ray diffraction (XRD) composition analysis were performed on the surface deposition layers of the experimental samples. The experimental results showed that under joint protection, a sedimentary layer was rapidly formed on the surface of EH36 to isolate the seawater medium, and this layer was mainly composed of Mg(OH)₂ and a small amount of CaCO₃. There was no corrosion on the surface of the EH36 substrate. When only ICCP was used, a relatively thin layer of calcium magnesium was deposited on the surface of EH36. Marine fouling organisms adhere to the surface of calcium and magnesium sedimentary layers and the EH36 substrate, and their attachment affects the formation of calcium and magnesium sedimentary layers. Moreover, marine fouling organisms cause corrosion on the surface of the EH36 substrate. The joint protection of seawater electrolysis antifouling and ICCP can simultaneously prevent electrochemical corrosion and marine biological fouling corrosion on the surface of EH36.

Wire-beam, potentiodynamic polarization, electrochemical impedance spectroscopy and surface corrosion morphology analysis techniques were used to investigate the corrosion characteristics of 304 stainless steel (304SS) specimens subjected to different degrees of fouling; these analyses were conducted to clarify the corrosive effects of biofilms and barnacles. The experimental results indicated that the gradual thickening of the biofilm reduces the corrosion rate of 304SS (I_corr = 0.0677 μA/cm²). Mature biofilm acts as a barrier and significantly increases the protection of the substrate (Z = 905,000 Ω·cm²). The corrosion mechanisms of 304SS fouled with barnacles at various stages of growth are distinct: the corrosion rates of the microelectrodes fouled by living and recently dead barnacles are low because the overall structure is still intact. This structure creates a closed environment between the barnacles and the metal, limiting the transfer of corrosive factors from the outside to the inside. Despite the influences of corrosive bacterial decomposition, the E_corr values of recently dead barnacles dramatically decrease to −668.8 mV. The microelectrode covered by the empty-shell barnacles corrodes heavily. Bacteria decompose the barnacle body and calcite shell, and glue damages their originally closed structures. Direct contact between the metal and reactive ions occurs, resulting in the continuous ingress of Cl⁻ into the cracks, which intensifies crevice corrosion.

It is a controversial issue whether the electric current pulse (ECP) has an inoculation effect on the solidification process. Two different high chromium cast irons were modified with the ECP above the freezing point temperature to investigate this issue. According to the statistical results of the number and size of primary phase grains, it was found that the ECP did not have an inoculation effect on the hypoeutectic high chromium cast irons (HOHCCI) whose primary phase was austenite. However, it had a significant inoculation effect on the hypereutectic high chromium cast irons (HEHCCI) whose primary phase was M7C3 carbides. The result indicates that the response of different primary phases to the ECP inoculation effect was different. The cause of the phenomenon is further analyzed by simulating the diffusion of atoms in the melt and comparing the binding forces of atomic clusters. The analysis shows that although the ECP can increase the number of atomic clusters (composition close to the primary phase) in the melt, the number of clusters retained to the nucleation temperature after the ECP removal is different because the binding forces of clusters in two melt is different.

To investigate the influence of electric current pulse (ECP) on the size of primary carbides in the hypereutectic high chromium cast irons, an ECP (peak density: 19.66*105A/m2) was applied respectively to samples in different solidification stages (the pre-nucleation stage, the nucleation and initial growth stage, the late growth stage of primary carbides). Besides the latter two stages, the ECP can also refine primary carbides in the pre-nucleation stage. There are a few similar reports in the literature about other alloys. The samples were treated with ECP of different current density in the same stages to better study the refinement mechanism. The results show that the size of primary carbides decreased first and then increased as the ECP current density increased in the first two stages. The size continued to decrease in the last stage. The analysis shows that the main roles of ECP in different stages are different. The main roles of ECP are the free energy effect and the pinch effect both in the first two stages. That is the pinch effect in the last stage. The change of size of primary carbides is the result of competition between two effects of ECP.