مشخصه يابي و ارزيابي رفتار خوردگي پوشش هاي روي و آلومينيوم به روش پاشش حرارتي بر زير لايه چدني

چكيده انگليسي :

148Evaluation of corrosion behavior of zinc and aluminum coatings produced by thermal spraying on cast iron substrate Salar Fatoureh Bonabi s fatoureh@ma iut ac ir Date of Submission 7 29 2015 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisor F Ashrafizadeh Professor ashrafif@ma iut ac irAbstractDespite many advantages of ductile iron pipelines they are susceptible to corrosion and must beprotected against it Customary protection solution is to apply sacrificial metal coatings such aszinc and aluminum and arc spraying as one of thermal spray methods has been vastly used forsuch coatings in recent years In this research coatings made up of pure zinc Aluminum individual layers of them and co spray of zinc aluminum were applied on ductile cast iron substratethrough arc spray process In order to investigate the effect of coating thickness on corrosionprotection and mechanical performance samples were prepared in two approximate thicknesses 50and 100 m Scanning electron micrographs of coatings cross sections revealed crack free coatingswith intimate contact with the substrate and low oxide content For evaluation of coating adhesionto cast iron substrate bending tests were carried out on typical samples the co sprayed sample of50 m and the double layer coating showed the best and lowest performance respectively The co sprayed samples had the ability of bending up to 28 degrees Results of micro hardness testingindicated that the maximum and minimum values of hardness belong to aluminum coating and zinccoating of 100 m thickness respectively With thickness increase in aluminum the hardness roseand in zinc coatings decreased In the case of co sprayed samples the increase in thickness did nothave significant change in hardness values In order to evaluate the corrosion behavior of sampleswith and without coating potentiodynamic cyclic polarization test in 3 5 wt NaCl and salt misttests were employed The results indicated higher rate of corrosion of aluminium and zinc coatingscompared with their bulk metal due to penetration of corrosive solution through surface porositiesto the interface and formation of galvanic cell Zinc coated samples and its uniform corrosion trendresulted in proper sacrificial corrosion performance On the other hand aluminium by aggregationof corrosion products in porosities and confining entrance of corrosion agents to coating protectedthe substrate But by progression of test pitting corrosion had occurred on its surface Thecorrosion behavior of double layer samples was similar to aluminium but in its pitting corrosionsites zinc acted its sacrificial performance and protected the substrate against corrosion Thus double layer sample had lowest corrosion current density The results of salt spray test during 750hours showed that after 70 hours the surface of ductile iron sample was completely corroded Butno signs of red rust was noticed in all test long on the surface of coated samples Corrosion signswere observed at the corners of aluminium coated samples after 500 hours The corrosion trend ofdouble layer coated sample was similar to aluminum without signs of corrosion at corners Co sprayed samples corrosion trend was just darkening the surface of sample and had betterperformance Keywords Ductile cast iron Thermal spraying Corrosion behavior Zinc coating Aluminiumcoating

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مشخصه يابي و ارزيابي رفتار خوردگي پوشش هاي روي و آلومينيوم به روش پاشش حرارتي بر زير لايه چدني