Effect Of The Seawater Flow Rate And Static Pressure On The Cathode Protection

With the development of the marine engineering, the metal structures, especially steelstructures have been more and more used and distributed widely. Cathode protectionwill still be one of the most effective methods for preventing steel corrosion in seawater. As a cathode protection project, the actual scene environment and the physicaland chemical properties of seawater t all affect cathode protection. The actualconditions of marine environment determine the design of the cathode protection andthe final protection effect. At present, some studies were conducted to research theinfluence of the Marine environment factors (including the velocity of the water, staticpressure, dissolved oxygen, etc.) on cathode protection, but this is not enough for thedemand and application in practical engineering. There is still a lack of adequate dataand theoretical results for the design of the cathode protection. Based on the DH36platform steel as the research object, this paper adopts the galvanostatic polarizationmethod to simulate cathode protection. The single factors experiments are conductedto study those effects on potential changes and the calcareous sediments, and thecurrent density demand is also been determined under corresponding conditions.This paper mainly studies the effect of the simulated seawater flow rate and staticpressure on the cathode protection. By a home-made pipe flow water circulationdevice, the velocities are control to be0.2,0.4,0.6,0.8,1.0,1.2,1.0and1.2m/s.Under each velocity, at least3different protection current densities are chose topolarize the cathode for7days. According to the different velocity, the experimentchoose current density within the scope of300~1600mA/m2. By a high pressurereaction kettle, the pressures are controlled to be0.1,2.0,5.0and10.0MPa. Undereach given pressure, four current density (50,100,150and200mA/m2) are used forgalvanostatic polarization for7days to study cathode protection with differentconditions. The results showed that:(1) The increase of flow velocity weakens the protective effect of cathode current andraises the current density demand. Under the experimental conditions, the criticalcurrent densities for0.2and0.4m/s were400and500mA/m2. At the velocity of0.6and0.8m/s, the critical current densities were both600mA/m2. The currentdensity of1000mA/m2was large enough to make the potential achieve theprotective potential. When the velocity was below1.2m/s, the results from thepotential measurements and the surface examination were consistent. When thevelocity was above1.2m/s, erosion corrosion maybe occurred even thepolarization potential has achieved an enough negative value. When the flowvelocity reached2.0m/s, to control the corrosion only with cathode protectionwas difficult and inappropriate. The polarization resistances after the polarizationas a whole are low and the electrode which have not reached protection potentialget a lower polarization resistance. Calcareous deposits have formed on thesurface of the specimens. Most of them were Mg-rich single layers. Only withlarge current density could the CaCO3deposit. And it has to attach on the Mg-richlayer. Because of the seawater flow, the thickness of the deposit was limited. Butit still had good protection ability to achieve protection potential. And the greatercurrent density could make the deposit grow much thicker and better.(2) The cathode protection condition of various pressures is almost the same, and theeffect of the increase of pressure on the cathode protection is not significant.Compared to50mA/m2, the polarization resistance under the current density of100mA/m2jumped significantly, and the protection potential achieve to an idealrange with good sediments formed on the electrode surface without hydrogenevolution. Under the polarization current of150and200mA/m2, the sedimentsare denser but have obvious hydrogen evolution with excess negative potential.With the same polarization current, the potential curves with time are almost thesame under different pressure. From the apparent morphology of the electrodesurface after polarization, with large current density (150and200mA/m2), the greater the pressure, the more serious the hydrogen evolution; with small Currentdensity, the greater the pressure, the harder to form sediment. When the currentdensity is100mA/m2, it is appropriate for each pressure to achieve goodprotection. The Sediments are mainly composed of aragonite of CaCO3. The jumpof the polarization resistance depends on the complete coverage of CaCO3. Themagnesium layer formed at the bottom of sediment, under the layer of CaCO3andvery thick. With the high pressure, when the current density is larger, the calcitewith magnesium can also been generated, symbiosis with aragonite.