Evolution Of Local Corrosion Of Refinery Equipments And Pipeline Steels With Stress Under Coatings

The external corrosion of refinery equipments and piplines in coastal refinery threatens the long period safety of production seriously, the corrosion of refinery equipments with stress under coatings in marine atmosphere and corrosion of pipelines under coatings are very common, the two types of corrosion have similar corrosion law, namely the occurrence and the development process of corrosion under thin electrolyte environment layer.The evolution of local corrosion under thin electrolyte layer under coatings is the main scientific problems that the external corrosion of refinery pressure equipments and pipelines are faced with now, systematic research on this problem has important scientific value, and it can help clear the corrosion mechanism, and also can be used to detect corrosion control, monitoring and guiding device in engineering.For the evolution of local corrosion of refinery equipments under coatings in marine atmospheric environment,in this paper,we use the commonly used steel Q345 R of refinery equipments. We monitor Q345 R with stress for a long term in the dry-wet cycling of marine atmospheric environment by electrochemical impedance spectroscopy(EIS), local electrochemical impedance spectroscopy(LEIS) and beam wire electrode,also we work in with the corrosion morphology,to explore the evolution of local corrosion of the metal under the protection of coatings in marine atmospheric and the influence of stress.The results show that:(1) Equipments under coatings mainly have three stages:water percolation,accelerated corrosion and coating damage,when without stress,it mainly has some stages:water percolation, coating damage and the initiation stage of substrate corrosion,the stage of corrosion diffusion and coating losing effectiveness, coating damage and accumulation of corrosion product, the stage of coating losing effectiveness completely.In the stage of water percolation,the coating’s properties are intact,EIS of them shows captance arc in high frequency.Because the stress can speed up the process of the corrosion of metallic substrates,some stages without stress reflect not much,but in the whole corrosion period,EIS shows the characteristics of present semi arc and diffusion tail,and after coating losing effectiveness completely, the EIS shows a complete semi arc.(2) The beam wire electrode’s results show that:coatings and the characteristics of the electrode surface lead to the initial differences of the local environment,differences in penetrating speed of ion,solution,affect the local corrosion rate further,with the time, the difference gradually increases,shows apparently cathode and anticathode partition, accelerate the localized corrosion.(3)Protective effect on metal substrates increases with the thickness of the coating;If there is more stress,resistance,capacitance and other properties of the coating decreased more quickly,and the initial time of the substrate’s corrosion is put in advance,they all press the localized corrosion process.The process of stress corrosion of pipeline steel depends on the coating properties and the failure mode.For the evolution process of local corrosion of buried pipeline under coatings, in this paper,we use slow strain rate tensile test(SSRT) and beam wire electrode to study the stress corrosion sensitivity and the evolution of local corrosion of X80 pipeline steel under stripping coating in NS4 solution.The results show that,(1) The results of long-term monitoring of wire beam electrode show that,the distribution of electrochemical information is uneven because of the different distance from the leak point.With it’s increase,the diffusion of ion is more difficult, potential is lower, the corrosion rate is higher,the corrosion is more serious.As the corrosion products deposit on the metal surface,the corrosion rate decreases,but the local differences of different position of the pipeline steel under coating is more obvious.(2) SSRT results show that,with the increasing of the thickness of stripping coating,the extension rate decreased after a high rise,showing a "V" shaped law.At 100μm thickness,the stress corrosion sensitivity is the highest,and it fractured easily, it is because the difference of the internal and external corrosion environment at this thickness produces concentration cell and gives rise to local corrosion. When under a certain thickness, we find that when the cathodic protection potential is-700 mV, the stress corrosion sensitivity increases, this is because the inadequate cathodic protection leads to the significant difference of local corrosion electric field, and increases local corrosion evolution, easily leads to crack initiation. When apply different cathodic protection potentials to the specimens with 200μm thickness of liquid layer,stress corrosion sensitivity of-700 mV is higher,because the inadequate cathodic protection can lead to crack initiation easily;with the negative shift of potential to-900 mV it decreases,but when to-1100 mV, because of hydrogen evolution and interaction of stress caused by material increase the stress corrosion sensitivity.