Comparative Study On The Failure Process Of Organic Composite Coatings In Three Simulated Atmospheric Environmental Media

Carbon steel is widely used in the actual production and daily life due to its excellent mechanical properties and low cost.However atmospheric corrosion can cause great loss to carbon steel,and the most common method to prevent the corrosion of carbon steel is applying anticorrosive coating.When the coating/carbon steel system is exposed to different types of atmospheric environments,the various climatic factors will aggravate the deterioration of the coating and reduce the service life of the steel structure.Previous studies on the failure of coatings in various atmospheric environments are mainly designing simulation experiment to evaluate the performance of coatings according to different environmental factors.However,there are few studies about the comparison of the failure process of coatings in different atmospheric environments,the influence of the temperatures and the combined action of these two factors.In this thesis,the failure process of double layer coating(zinc-rich epoxy primer/acrylic polyurethane top coat)and three layer coating(zinc-rich epoxy primer/epoxy with micaceous iron oxide intermediate coat/acrylic polyurethane top coat)in marine atmospheric simulated solution(0.05%NaCl),industrial atmospheric simulated solution(0.35%(NH4)2S04)and coastal-idustrial atmospheric simulated solution(0.05%NaCl + 0.35%(NH4)2SO4)under 25℃condition,35℃ and 45℃ thermal cycling conditions,respectively,was studied.The physical and chemical properties of the coating systems under different conditions were compared and analyzed by using adhesion test,EIS,FTIR,SEM and DSC methods.The main conclusions are as follows:(1)The influence of atmospheric environmental medium on the anticorrosive properties of organic composite coatings is determined by the permeability of corrosive media in the coating.The more rapidly the corrosive medium penetrates in the coating,the easier the medium enter the coating and react with fillers and zinc powders,causing the increased pores of coating and thereby the failure.The penetration rate of NaCl solution in the coating is the fastest,then the NaCl +(NH4)2SO4 solution,and the penetration rate of(NH4)2SO4 solution in the coating is the slowest.The failure of the coating in three atmospheric environment simulated solutions is:marine atmospheric simulated solution>coastal-idustrial atmospheric simulated solution>industrial atmospheric simulated solution.(2)The influence of temperatures on the anticorrosive properties of organic composite coatings is determined by the degradation of coating and the penetration of corrosive media in the coating.The corrosive medium penetrates faster in the coating and easier to react with zinc powders at higher temperature.It caused pores of coating increased and reduces its shielding performance.Moreover,the chemical stability of the coating is decreased and the coating is easier to degrade.The degradation rate of the coating under 45 ℃ thermal cycling test condition is the fastest,then the 3 5 ℃ thermal cycling test condition,and under 25 ℃ condition the degradation rate is the slowest.(3)The coating absorbs water faster and the saturated water absorptivity is higher under high temperature condition.At 25 ℃,saturated water absorptivity of double layer coating is 8%,for 35 ℃ thermal cycling condition it is 11%and for 45℃ thermal cycling condition it is 13%.The corrosion media type only influences the water absorption rate of the coating,but doesn’t influence the saturated water absorptivity.The water absorption rate of the coating in 0.05%NaCl solution is the fastest,then in 0.05%NaCl + 0.35%(NH4)2S04 solution,and the water absorption rate in 0.35%(NH4)2S04 solution is the slowest.(4)In the combined action of atmospheric environmental medium and temperature on the anticorrosive properties of the coatings,the temperature plays a main role in the process of coating failure.The effect of atmospheric environment on the shielding performance of the coating is more obvious at higher temperature.It is because that the difference of the the penetration rate of the corrosive medium in the coating is bigger at higher temperature,therefore,the influence of the corrosive medium types on the anticorrosive property of the coating is more obvious.(5)The degradation of the coating systems under three simulated conditions shows no difference,but it is changed at different temperatures.Under the 25 ℃condition,the decreased intensity of C-O band of acrylic polyurethane top coat demonstrate the hydrolytic degradation of carbamate occurs.Under the 35℃and 45 ℃ thermal cycling conditions,the decreased intensity of the ester carbonyl and the C=O,N-H,C-N and C-O of carbamate,the degradation of top coat is the same.The zinc rich epoxy primer dosen’t degrade significantly under 25 ℃ condition and 35 ℃ thermal cycling condition,but the hydrolytic degradation of ether bond in epoxy group occurs under 45 ℃ thermal cycling condition.The main components of epoxy with micaceous iron oxide pigment and zinc rich epoxy primer are the epoxy resin,so they degrade in the same way.