Mapping The Atmospheric Corrosion Level Of Southern Power Grid

With the rapid development of economy in China,the demand for electricity has increased greatly and led to a surge in the use of transmission tower,which is an important part of grid operation.The transmission towers are used in natural environment,which may cause damage and failure of the tower material due to atmospheric corrosion.Atmospheric corrosion will directly affect the safe and efficient operation of transmission networks.China Southern Power Grid has a vast service area(including Guangdong,Guangxi,Guizhou,Yunnan and Hainan provinces)with humid climate,and the environment types are complex(including industrial,marine,urban and rural types),which make a significant difference of corrosion risk among different regions.At present,there are lacks of research on corrosion risks and accumulation of material corrosion data in different environments in the region of China Southern Power Grid and especially the map of atmospheric corrosion level that can visually show the atmospheric corrosion risk in this region.In order to map the atmospheric corrosion level of southern network area,the corrosion rate data of 99 field exposure test points and the atmospheric environment data of 246 environmental monitoring stations were collected at first.Then a dose-response function suitable for the southern network region was established through the design and screening of the function models,which bridged a quantitative relationship between environmental data and atmospheric corrosion rate.Finally,the Kriging space interpolation algorithm with the ArcGIS software was used to realize the continuous surface formation from the known corrosion data points under the premise of taking into consideration the influence of industrial pollution sources and oceans on atmospheric corrosion and the atmospheric corrosion level map of Southern Power Grid region had been drawn to show visually the actual atmospheric corrosivity in the region.At the same time,two-year field exposure tests were carried out of three different materials(Q235,Q345 and galvanized steel)in atmospheric corrosion test stations(Guangzhou,Xishuangbanna,Wanning)of three different environments,including measurement and calculation of corrosion weight loss,corrosion product and morphology analysis.The main findings of the full text are as follows.According to the classification standard of atmospheric corrosion level of ISO 9223,the corrosion rate data of Q235 steel of 99 field exposure sites in 18 cities of Southern Power Grid region were classified.There are 28 sites of C2 level;67 sites of C3 level,3 sites of C4 level and only 1 sites of C5 level in Southern Power Grid region and the main atmospheric corrosivity is mild and moderate.Environmental data including temperature(T),relative humidity(RH),sulfur dioxide concentration(SO2),nitrogen dioxide concentration(NO2),ozone concentration(O3),and particulate matter(PM10))of 246 environmental monitoring stations in 78 cities in Southern Power Grid region were collected.Environmental factors have significant differences in the spatial distribution,which is the cause of different corrosion risks in different areas of the Southern Power Grid.Based on the corrosion rate results of the field exposure tests and the environmental data of the environmental monitoring stations,different functional models were tested by regression analysis,and the optimal dose-response function was selected.The environmental factors used in the DRF4 function include temperature,relative humidity,sulfur dioxide,nitrogen dioxide,and ozone.The goodness of fit of DRF4 reached 0.90,which was determined as the most suitable dose-response function for calculating the atmospheric corrosion rate in the Southern Power Grid region.The expression of DRF4 is:ln(rcorr)=0.4·ln(SO2)+0.24·ln(NO2)+0.847·ln(03)+0.394·ln(T)+3.14·ln(Rh)-14.939 This function establishes a quantitative relationship between atmospheric corrosion rate and environmental factors in the southern network region,and the environmental data could be transform to atmospheric corrosion rate by this function.The atmospheric corrosion level map of Southern Power Grid covering more than 1 million square kilometers was first drawn by means of ArcGIS software.The impact of industrial pollution sources and oceans on atmospheric corrosion were specially taken consideration by superimposing their corresponding corrosion rates on the basic atmospheric corrosion map for areas affected by marine and industrial pollution sources,since the corrosion data from exposure tests showed that industrial pollution sources and oceans accelerated the corrosion of materials.In order to better distinguish and display the difference in atmospheric corrosivity in the South Network,the C3 level was subdivided based on the ISO 9223 standard.The atmospheric corrosion level map of Southern Power Grid shows that the main atmospheric corrosivity was dominated by C2 and C3(C3(A),C3(B))grades,and there were very few areas at C4 level and above,which distribute in strips along the coastlines.Although the proportion of areas with high corrosion levels was small,special protective measures must be taken for materials serving in the area.The exposure test results at the three atmospheric corrosion test stations show that the corrosion risk order from high to low was Wanning station,Guangzhou station,Xishuangbanna station.Galvanized steel had the best corrosion resistance among the three materials,followed by Q235 and then Q345.The corrosion products of Q235 and Q345 carbon steels were y-FeOOH,a-FeOOH and Fe3O4.The corrosion products of galvanized steel were mainly ZnO and a small amount of Zn5(OH)6(CO3)2 and Zn(OH)2.The corrosion product of galvanized steel at Wanning had Zn5(OH)8Cl2H2O,which was caused by the reaction of chloride ions with Zn(OH)2.The structure of the rust layer of Q235 and Q345 was loose and porous,with cracks on its surface,which was not able to protect the substrate from corrosion.The corrosion products of galvanized steel is dense,with good protection.Under the presence of chloride ions,however,the galvanized layer will be corroded.The corrosion kinetic curves indicated that the carbon steel at Wanning station had accelerated corrosion rate in the second year.The materials at Guangzhou and Xishuangdian stations were in accordance with the power function law and their kinetic curves had a tendency to enter the stable zone.In Wanning station,the loose and porous rust layer on carbon steel became a place for enriching chloride ions,so that chloride ions continuously attacked the substrate.This made the atmospheric corrosion of carbon steel a three-dimensional form,which caused the accelerated corrosion.