Peichang Deng

Marine engineering components of 304 stainless steel inevitably form crevice structures in service, which readily trigger severe crevice corrosion in seawater. Conventional protection strategies, including cathodic protection, organic coatings and corrosion inhibitors, show evident limitations within occluded crevices because of IR drop and mass transfer barriers. In this work, a 10 × 10 array electrode was employed to construct a simulated crevice device with a 0.3 mm gap width. Static immersion tests were carried out in natural seawater collected from Zhanjiang Bay for 75 days, using a blank group and an intermittent anodic polarization group (0.1 V vs. SCE applied for 10 min every 30 min; three independent specimens per group). In situ galvanic potential/current distribution mapping, electrochemical impedance spectroscopy (EIS) and ultra-depth-of-field microscopic observations were performed. Stable oxygen concentration cells were gradually built inside the crevices of blank specimens, producing band-shaped heterogeneous galvanic signals and massive pitting initiation. Intermittent anodic polarization homogenized the crevice internal potential distribution, reduced the galvanic current magnitude and effectively repaired the passive films: the inner crevice film resistance increased from 2.62 × 10⁶ Ω·cm² to 2.59 × 10⁷ Ω·cm², and the pitting density decreased markedly. Intermittent anodic polarization ameliorates the occluded electrochemical environment and mitigates crevice corrosion of 304 stainless steel in seawater, offering a potential supplementary strategy for marine stainless-steel components with crevice structures.

The aim of this investigation was to study nonuniform corrosion in the vertical direction of Q345 steel in the South China Sea. First, a metal corrosion monitoring system was designed and applied based on the coulometer principle and array electrode technology. Then, through coulometer testing, weightlessness analysis, electrochemical testing, and surface morphology observation, the corrosion behavior of Q345 steel used in an offshore wind turbine in Zhanjiang Bay in the South China Sea was studied. The results revealed that within the first 30 days of the experiment, the array electrodes connected by electricity exhibited sacrificial anode/cathode protection with self‐coupling. Therefore, the corrosion rates of the electrodes were slowed. Between 60 and 120 days, the array electrodes underwent a transition in the anode/cathode region. In actual marine environments, the most severely corroded areas of long metal structures were near the sea surface and seabed and had to be prioritized for protection.

Based on the principle of a micropore-filling electrolyte, a graphene composite conductive coating combined with impressed current cathodic protection (ICCP) technology was constructed and applied in a marine atmospheric environment. To further explore the optimal protection parameters of the graphene composite conductive coating combined with ICCP technology in a marine atmospheric environment, the effects of the coating damage area (A), impressed voltage (B), and distance from the contact point (C) on the protective performance of the coating were investigated via orthogonal experiments. The optimal protection voltage and effective protection distance were verified by super-depth-of-field morphology observations and electrochemical tests. The orthogonal experimental results show that the primary and secondary orders affecting the protective performance of the conductive graphene composite coating are as follows: applied voltage (B) > coating damage area (A) > distance from the point of contact (C). The optimal protective parameters of the coating in the marine atmospheric environment are an applied voltage of 0.7 V, a damage rate of ≤1%, and a distance from the point of contact of 190 mm. The experimental results show that the corrosion potential of the sample is the highest under an applied voltage of 0.7 V, and the corrosion products do not diffuse to the surface of the coating. When the polarization resistance (R_p) values at 110 mm and 190 mm from the negative electrode at the point of contact are greater, the corrosion rate is lower, and the coating protection performance is better.

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.

In this study, a new anticorrosion method combining impressed current cathodic protection (ICCP) with coatings that can be applied to marine atmospheric environments is proposed. As the corrosion medium fills the cracks and pores of the coating, an electrolyte film layer is inevitably formed on the metal surface. Therefore, a graphene conductive coating with excellent chemical inertness and shielding performance is selected as the intermediate coating to form an electrolytic cell system with a metal substrate serving as the cathode and a graphene coating serving as the auxiliary anode. By studying the surface corrosion morphologies and electrochemical signals of the coating samples at different protection potentials and coating thicknesses, the optimal potential is determined to be 0.6 V, and the optimal coating thickness is determined to be 20 μm. The samples protected by the joint method have lower corrosion rates and better anticorrosion performance than those protected by the coatings alone.

Marked changes in temperature, pH, dissolved oxygen (DO) content, and nutrient content typically occur in marine thermoclines, which are key factors that affect the corrosion of metals. Offshore platforms require marine metals to be exposed to deep-sea environments and thus increase their penetration into the marine thermocline. This study investigates the galvanic corrosion of E690 steel in a marine thermocline using a simulated marine thermocline (SMT). Specifically, the corrosion of E690 steel was analyzed using the wire beam electrode (WBE) technique, linear polarization (LP), corrosion morphology, and weight loss measurement. Results indicated that the SMT had a stable multilayer structure, and the variations in temperature, DO, pH, and nutrient concentration in the SMT were similar to those in the natural marine thermocline. There were two forms of E690 steel corrosion in the SMT: galvanic corrosion and seawater corrosion. The corrosion rate of seawater corrosion was influenced by the DO concentration. Galvanic corrosion occurred after the intrusion of E690 steel into the marine thermocline. The driver of galvanic corrosion was the difference values for E_corrs of E690 steel at various depths of the marine thermocline. The E_corr of E690 steel was influenced by the temperature, pH, and DO of the seawater, in the following order: DO >> T > pH. The continuous reduction in E_corr with depth contributed to large-scale galvanic corrosion, and the oscillation variation in E_corr with depth was the reason for small-scale galvanic corrosion. The primary anodic regions of galvanic corrosion were located in the area with the fastest temperature variation in the thermocline, and the position of the anodic regions rose with time. The anodic regions gradually expanded with time. The proportion of galvanic corrosion in the average corrosion rate could increase up to approximately 80% in the stable anodic region. There were many hemispherical corrosion pits on the surface of the single electrodes that were at the depths of 75 cm, 105 cm, and 135 cm. These single electrodes comprised a long-term, sustainable anodic region of galvanic corrosion.

Marked changes in temperature, pH, dissolved oxygen (DO) content, and nutrient content typically occur in marine thermoclines, which are key factors that affect the corrosion of metals. Offshore platforms have required marine metals to be exposed to deep-sea environments and have thus increased their penetration into the marine thermocline. This study investigates the galvanic corrosion of E690 steel in a marine thermocline using a simulated marine thermocline (SMT). Specifically, the corrosion of E690 steel was analyzed using the wire beam electrode (WBE) technique, linear polarization (LP), corrosion morphology, and weight loss measurement. Results indicated that the SMT had a stable multilayer structure, and the variations of temperature, DO, pH and nutrient concentration in the SMT were similar to those in the natural marine thermocline. Galvanic corrosion occurred after the intrusion of E690 steel into the marine thermocline. The driver of galvanic corrosion was the of E690 steel at various depths of the marine thermocline. The Ecorr of E690 steel was influenced by the temperature, pH and DO of seawater, and the order was DO >> T > pH. The continuous reduction of Ecorr with depth contributed to the large-scale galvanic corrosion, and the oscillation variation of Ecorr with depth was the reason for small-scale galvanic corrosion. The primary anodic regions of galvanic corrosion were located in the area with the fastest temperature variation in the thermocline. The proportion of galvanic corrosion in the average corrosion rate could increase up to approximately 80% in the stable anodic region. There were many deep corrosion pits in the long-term and stable anodic region of galvanic corrosion.

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.

In this study, the galvanic corrosion behavior of the side surface of titanium‐clad steel plate (TCSP) and below the defects, which penetrate through the titanium layer of TCSP, have been focused on. Electrochemical corrosion tests and infinite focus three‐dimensional measurement results revealed that galvanic corrosion on the side of TCSP will lead to widespread pitting of the steel, and the distribution positions of pits in splash, tidal, and submerged zones are different. Severe corrosion occurs below the defects in the titanium layer of TCSP, and the larger the defect, the deeper the corrosion depth, but TCSP with smaller defects have larger corrosion width on the Ti‐Stl line (the joint between the titanium and steel). This shows that the size of the defect will influence the density of corrosion products and the effect of TCSP defect internal corrosion on titanium layer peeling is extremely limited.