Cathodic protection is an important method to prevent the corrosion of tank bottom plate. In this article,the research status in recent decades about the current density- and potential-distribution of tank bottom plate under cathodic protection is summarized. The research status and the relevant issues related with the analytical method and numerical analysis method applied for the status of cathodic protection of tank bottom plates are described and discussed. The authors prospect that the analytical method will be gradually replaced by numerical method, while the mathematical model and boundary conditions need to be further studied and improved. In addition,the combined use of a variety of numerical methods will be a trend in the development of numerical simulation in the future.

In this paper, the basic principle of scanning vibrating electrode technique is simply interpreted. Especially, the research progress of scanning vibrating electrode technique in the field related with localized corrosion of materials, inhibitor and evaluation of coating performance is illustrated. In the end, the relevant limitations are summarized for the application of the technique.

Organic coatings on metal surface usually present inherent local defects, thus are easy to be deteriorated. The characteristic of corrosion beneath coatings with damages (i.e. the inherent or artificial ones) is that the inherent defect and the debonding defect may be coupled together to induce non-uniform corrosion of the metal substrate. Hence, the debonding process of organic coating with artificial defects nearby the waterline in 3.5% (mass fraction) NaCl solution was studied by means of wire beam electrode (WBE) method coupled with electrochemical impedance spectroscopy (EIS) technique. Through analyzing the variations of current distribution, coating impedance, impedance spectroscopy in the disbonding process, the debonding mechanism for the damaged coating in areas nearby the waterline could be revealed. The result showed that, the cathodic disbonding was expedited in the area around the inherent defect or the artificial defect. The characteristic of disbonding process for damaged coating near waterline is that, the cathodic disbonding first happened nearby the inherent defect as well as the artificial defect, and then expanded around. Besides, the position of the artificial defect can affect the cathodic disbonding process. When the artificial defect just located on the waterline, the disbonding rate above the waterline was higher than that below the waterline. It was because that the higher the oxygen content is, the faster the disbonding rate well be.

The degradation behavior of three solvent-free epoxy coatings on carbon steel Q345E in simulated flowing sea water with 1% (mass fraction) sand by different flow velocities (2, 4 and 6 m/s respectively) was investigated at 60 ℃ by means of open circuit potential (OCP) measurement and electrochemical impedance spectroscopy (EIS) as well as confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS). Results showed that the failure process of the three coatings was greatly shortened in the flowing slurry, and the reason for the accelerated degradation of the coatings may be ascribed to the facts that: on one hand, the wear effect of sends, which led to the generation and development of pits or holes on/in coatings which shorten the diffusion path of the corrosive medium from the outside environment to the coating/metal interface, on the other hand, the high flow rate of the slurry could accelerate the transmission of Cl^- to the coating.

AC interference corrosion of X80 pipeline steel beneath a delaminated coating with an artificial damage is investigated via a simulated crevice cell by means of microelectrode technique in artificial neutral soil solution NS4. The results show that the applied AC interference may induce the negative shift of the free corrosion potential of X80 steel. With the increase of AC current density, the anodic dissolution rate of X80 steel increases, while the corrosion pattern changes from uniform corrosion to local corrosion. The corrosion of pipeline steel just on the damaged spot is the most serious, however, the corrosion attack of the steel beneath the delaminated coating around the damaged spot is slightly alleviated. When the AC interference current density increases up to 100 A/m², serious pitting corrosion appears on X80 steel surface, where locates even far from the damaged spot beneath the delaminated coating. The influence of AC interference on the corrosion behavior of pipeline steel beneath the damaged coating was discussed from aspects of the rectification effect of the AC interference, the irreversibility of anode reaction and the effect of AC on the double layer structure at the interface steel/solution.

Microstructure evolution and corrosion behavior of a degradable Zn-7Mg alloy after heat treatment was investigated by means of inductively coupled plasma emission spectrometer (ICP) and energy dispersive spectrometer (EDS), optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD) as well as electrochemical test in phosphate buffered saline (PBS). Results indicated that, the microstructure of the as-cast Zn-7Mg alloy was mainly composed of α-Zn and MgZn₂, while the peritectic reaction occurred rapidly during heat treatment. After heat treatment the Zn-7Mg alloy was composed of stable phase Mg₂Zn₁₁ and a little of residual MgZn₂. The phase Mg₂Zn₁₁ showed lower corrosion resistance in PBS solution than the pure zinc, therefore, the phase Mg₂Zn₁₁ should be avoided in actual production.

Three hot-dip galvanizing coatings i.e. pure Zn, Zn-5%Al and Zn-2%Al-2%Mg were prepared respectively on sheets of a commercial interstitial free (IF) steel by a hot-dip process simulator (HDPS). Corrosion behavior of the coated steel sheets was studied by means of immersion test, electrochemical measurement and scanning electron microscope (SEM). Results show that the Zn-Al-Mg coating has the largest corrosion current density at the early stage of immersion. While with the increasing immersion time, the corrosion current density of the Zn-Al-Mg decreases but those of the pure Zn and Zn-5%Al coatings increase. For the Zn-5%Al coating, diffusion characteristics could be found for its Nyquist curve, which means higher corrosion rate. However, for the Zn-Al-Mg coating, limited diffusion characteristics could be found for its Nyquist curve, which means a perfect coverage of corrosion products on the coating surface, in other word, the corrosion resistance was enhanced.

Black oxide films on Mg-alloy AZ40M were prepared by means of a two-step micro-arc oxidation process in electrolytes of 15 g/L Na₃PO₄+ 3 g/L NaF+5.6 g/L KOH and 20 g/L Na₃PO₄+5 g/L NaF with different additions of ferric citrate respectively. The microstructure and composition of the films were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-Ray diffractometer (XRD). The electrochemical corrosion property of the films was assessed by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves in 0.1 mol/L NaCl solution. Results showed that the concentration of ferric citrate strongly affects the surface morphology, composition and thickness of the formed oxide films. The oxide films have good corrosion resistance. The thicker oxide film with less iron content presents better corrosion resistance.

The marine atmospheric corrosion of Al-alloys could be originated from the localized corrosion beneath thin electrolyte layers. The pitting initiation of Al-alloy AA6061 beneath thin electrolyte layer of 3.5%NaCl and bulk solution, as well as the corrosion inhibition of cerium ions were comparatively investigated by means of polarization curves, electrochemical impedance, electrochemical noise and micro-morphological observation. Results demonstrate that, as a cathodic corrosion inhibitor, the density and coverage of the deposited CeO₂ film is mainly related to the thickness of thin electrolyte layer, the thinner the electrolyte layer is, the more compact the deposited CeO₂ film becomes. As a result, the oxygen reduction on the secondary intermetallic phase (Mg₂Si as cathode) of AA6061 alloy would be suppressed and therefore further refrain the initiation and development of pitting corrosion. The thinner the electrolyte layer, the higher the inhibition efficiency of cerium ions. Whereas, the inhibition efficiency of Ce³⁺ on the metastable pitting corrosion of AA6061 alloy in bulk NaCl solution decreases much more in the contrast to that beneath thin layer of NaCl solution.

The effect of 40% (mass fraction) ethylene glycol on the performance of CO₂ saturated 3.5% (mass fraction) NaCl solution and relevant condensates was assessed by measurements of contact angle, pH and viscosity. Effect of 40% ethylene glycol on both top of line corrosion and bottom of line corrosion for X65 mild steel pipeline was investigated by means of weight loss, electrochemical method and scanning electron microscopy (SEM). Results indicated that contact angle, pH and viscosity of 3.5%NaCl solution saturated by CO₂ and relevant condensates were increased due to the incorporation of 40% ethylene glycol at 35 ℃ and 1 atm. In the presence of solution of 3.5%NaCl or 3.5%NaCl+40% ethylene glycol, the pH value of the condensates formed on the top of line was lower than that on the bottom of pipeline. Due to the addition of 40% ethylene glycol to 3.5%NaCl solution, the deposition of corrosion products and pitting corrosion were accelerated, while the general corrosion rate of the bottom of pipeline was decreased. In fact, the general corrosion rate of the top of line for X65 mild steel pipeline was not obviously affected by 40% ethylene glycol, but the pitting corrosion was speeded up.

Cathodic polarization curves of steel bar were measured via potentiodynamic polarization method. Then it gets the first-order differential analysis of polarization curves and acquires the critical hydrogen evolution potential and the hydrogen evolution current density of the steel bar. Next, the steady-state critical current density of hydrogen evolution was obtained by means of steady-state measurement. Results show that potentiodynamic polarization curves can monitor the electrochemical reaction process of the cathode. Before the abrupt point of the differential curve, the cathode reaction is dominated by the oxygen consumption reaction, and the hydrogen evolution reaction is the main control reaction after the abrupt point. With polarization curves, the steady-state critical current density of hydrogen evolution of the steel bar in reinforced concrete could be determined and thereby the probability of hydrogen embrittlement could be predicted. By this method, the steady-state hydrogen evolution critical current density of the test steel bar is acquired to be about 0.355 A/m².

Composite coatings of Ni-SiC were prepared on Ti-alloy TA15 by composite electroplating technology, while the effect of electroplating parameters on the coating structure was predicted by means of artificial neural network approach. The results showed that the increase of SiC particles in the plating bath and the stirring speed could lead to higher SiC content of the composite coating, which in turn resulted in higher coating hardness. Increase in cathodic current density caused higher coating growth rates, but too higher cathodic current density would also cause cracks in the coatings. Predictions of the coating growth rates and coating hardness were carried out via artificial neural network. After training, the neural network model was available for the prediction of the thickness and the hardness of the coating.