The Marine Corrosive Environment In Zhanjiang Bay And Its Influence On Corrosion Behavior Of The Pipeline Steel

In this paper a corrosive environmental investigation is conducted in Zhanjiang Bay,to qualitatively analyze and evaluate the corrosivity of marine environment.The law of corrosion bahaviodr and the mechanism of corrosion on pipeline steel are also studied in the marine environment of Zhanjiang Bay,the corrosivity of the marine environment is quantitatively analyzed by studying the corrlation of corrosion factors and its influence on the corrosion of pipeline steel,the reasonable anti-corrosion measurements of pipeline steel are put forward based on the law of corrosion.In this paper 8 representative positions chose are investigated in Zhanjiang Bay,during the process,the water sample with different distances from sea level and submarine sediment are collected,the corrosion factors such as the value of pH and Eh,conductivity,temperature,salinity,dissolved oxygen,the characteristics of sediment type,ratio of Fe3+/Fe2+,and the content of organic carbon are analyzed,and oxidation-reduction environment of submarine sediment is divided,the evaluation of corrosivity of seawater and the submarine sediment are carried out by soft-sensor and statistical method.The results show that the seawater of Zhanjiang Bay is a weak corrosive environment.The position of Z25 has a strongest corrosive environment,the position of Z4 takes second place,and the position of Z25 is weaker than other positions among the positions.It also shows that the corrosivity of submarine sediment is stronger in Zhanjiang Bay,and it causes easily galvanic corrosion and microbiological corrosion.The submarine sediment is a strong corrosive environment on the position of Z4 and Z23,while the environment of position of Z25 that is divided into oxidative environment is weak corrosive.It can be concluded that they are in accordance with corrosion factors.The law of corrosion behavior of X70 pipeline steel in seawater-atmospheric environment and the benthic boundary layer of ocean has been studied by means of wire beam electrode technique,linear polarization,electrochemical impedance spectroscopy and other electrochemical analysis technique,coupled with the methods of corrosion morphology observation and corrosion products analysis.The results show that X70 pipeline steel has formed an corrosion cell in seawateratmospheric environment,which causes of galvanic corrosion.The areas of water-line are cathode region,near the areas of water-line and far from the areas of water-line are anode region.The electrodes formed into an oxygen concentration cell where the areas of water-line are full of oxygen and down from the areas of water-line are short of oxygen.It causes that the corrosion rate of the electrodes in the areas of water-line is lower and the electrodes suffer from slight corrosion.The corrosion rate of the electrodes down from the areas of water-line increase with the distance from the areas of water-line increasing,the electrodes suffer from serious corrosion.The corrosion rate of X70 pipeline steels in seawater-atmospheric environment is in decline during the corrosion process.The results show that X70 pipeline steel has formed an corrosion cell in the benthic boundary layer of ocean,which causes of galvanic corrosion.The areas of cathode and anode are uneven,in the first stage of corrosion,there are two oxygen concentration cells,one is the electrodes between water-line and the seawater,the other is in the vicinity of seawater and sea mud,and the former is stronger.With the development of corrosion,the corrosion rate of the electrodes in the seawater is greater than the electrodes in the sea mud.In the last stage of corrosion,the sulfate-reducting bacteria participate in corrosion reaction and the electrodes in the bottom of sea mud come into local cell corrosion,and the whole electrodes become a multiple electrode system of two oxygen concentration cells and SRB cell.As a result the electrodes in the vicinity of interface of seawater-sea mud suffered more severe corrosion with the multiple corrosion cells.The anti-corrosion measurements of electrochemical protection plus coatings protection are used in the places where X70 pipeline steels suffer from heavy corrosion in seawater-atmospheric environment and the benthic boundary layer of ocean,the anti-corrosion measurements of single electrochemical protection or coatings protection are used in the place where X70 pipeline steels suffer from slight corrosion.