Study For Calcareous Deposits Under Initial Cathodic Protection In Simulated Deep Ocean Environment

With the development of deep-sea engineering, deep-sea environment on the corrosion resistance of metal materials has drawn more attention. Cathodic protection is still one of the most effective ways to prevent the corrosion of steel structure in the deep sea. As a result of cathodic protection, steel surface will form calcareous deposits.It may limit the dissolved oxygen to the steel surface, increases the metal polarization resistance; simultaneously may prevent crack growth and improve corrosion resistance of the steel, enhance the cathodic protection.At present, the study mostly aims at how the various environmental factors in the shallow water to influence the formation of the calcareous deposits, but to the abyssal environment, the study of how calcareous deposits growth is less.In this paper, Q235 steel was studied by using galvanostatic polarization method, in different experiments environmental conditions take a comparative study of the formation and growth of the calcareous deposits.According, to the formation of calcareous deposits characteristics and real-sea environmental factors, this experiment mainly temperature, dissolved oxygen as well as liquidity as the main object of study. In each simulated environment, select the polarization 40-800mA/m2 current densities, has carried out 7 days galvanostatic polarization and studied the variation of polarization potential over time. The results showed that flow condition at normal temperature when the end of the experiment to ensure the polarization potential of the specimen to reach the minimum protection potential Q235 steel-0.85V (vs.Ag/AgCl) need more than 200mA/m2, the current density is bigger than normal temperature static condition; in low temperature flow environment, the polarization current density is greater than 400mA/m2 when it meet the minimum protection potential;in low temperature and low dissolve oxygen condition,150-800mA/m2 pole of current density, at the end of the experiment the polarization potential all achieved the minimum requirements to protect potential. Through electrochemical linear polarization experiments show that in different experimental environment, the polarization resistance of the specimen along with the increase of polarization current density increased. In the same polarization current density, the lower temperature and dissolved oxygen the smaller the formation of polarization resistance. At small current density, the polarization resistance values difference not to be big; with the current density increased, the polarization resistance at normal temperature increased rapidly, when the current density higher than 300mA/m2, the polarization resistance at normal temperature than low temperature as well as low temperature and low oxygen conditions, the polarization resistance values 2-3 times higher.Surface composition analysis showed that the calcareous deposits main components are CaCO3 and Mg(OH)2.Normal temperature conditions, the deposit and the rust layer co-exist at low current density, the formation of deposits has become increasingly obvious and thick with the current density increases, the magnesium content also increases with the current density. When the current density is greater than 400mA/m2 hydrogen evolution reaction will occurrence, the cracks will appear deposits off phenomenon, it can be observed that Mg(OH)2 inner and outer structural CaCO3.At low temperature, when the current density is less than 300mA/m2, no apparent calcareous deposits occur, only sporadic granular sediment, surface composition analysis showed that calcium and magnesium content in small, when the current density achieve 400mA/m2, the specimen surface is covered by sediment, but a very thin coating whose main component is CaCO3, Mg(OH)2 content is very little. At low temperature and low oxygen condition, when the current density is less than 400mA/m2, there is also only sporadic granular sediment, when the current density achieved 600mA/m2, the deposit morphology is similar to low temperature conditions.Comparative analysis can be seen that the calcareous deposit is closely related to the current density, in a certain environment, the initial large cathodic protection current density can achieve to protect potential more quickly; the formation of calcareous deposit will be more favorable. In this study, due to the impact of liquidity the negative polarization potential shift would be delayed, the cathode product ion of the metal surface such as OH- is difficult to enrichment in the cathode region, compared to static conditions;the deposit is difficult to format. Temperature and dissolve oxygen content are also important factor in the formation of calcareous deposit, it is difficult to achieve the protection potential in high oxygen content; at low temperature and low dissolved oxygen conditions,the deposit is difficult to form, the current density should be increased more than double the deposit layer occurs;so in deep sea water, because the low temperature and low oxygen content it need for greater current density and a longer time to form a good deposit layer.