| Applying organic coatings on the surface of carbon steel is an effective and economic way to protect carbon steel from corrosion in the atmospheric environment.The coatings can experience degradation in the environment,which will seriously affect the service life of coating/steel structure.Various factors such as temperature,humidity,soluble atmospheric pollutant salt and pretreatment process have effects on the failure of carbon steel/coating system.Therefore,the research on the degradation of the coating system in different atmospheric environments and service lifetime prediction are the important basis for reasonable reduction of corrosion cost and maintenance in time.In this thesis,the degradation process of the epoxy zinc-rich primer/epoxy micaceous iron oxide paint double-layer coating system and epoxy zinc-rich primer single layer coating in different atmospheric environment simulated solutions were investigated and compared.The atmospheric environments include four simulated typical atmospheric environments,which are marine atmosphere,industrial atmosphere,marine industrial atmosphere and coastal high salinity marine industrial atmosphere,and four special atmospheric corrosion pollutant salt solutions.The electrochemical impedance spectroscopy(EIS)technology was carried out to study the electrochemical behaviors of carbon steel/coating system.The adhesion test,Fourier transform infrared spectroscopy(FTIR),scanning electron microscopy(SEM)and X-ray diffraction(XRD)methods were used to study the physical and chemical changes,microscopic surface morphology and composition of the coatings before and after immersion.The Corrosion Master software was used to simulate the failure behavior of the coating system and predict the service lifetime.The main conclusions are as follows:(1)For the double-layer coating system in different atmospheric environment simulated solutions,the coating degradation effect follows in this order:coastal high salinity marine industrial atmosphere simulated solution>marine atmospheric simulated solution>industrial atmospheric simulated solution.The type of the atmospheric corrosion medium is the main factor affecting the protection performance of the coatings,which is manifested by the penetration rate of the corrosive ions in the coating.The faster the penetration rate,the easier the corrosion electrolyte penetrates through the coating and therefore react with the zinc powder in the primer.The lower the solubility of the corrosion product of the reaction,the slowly of the consumption rate of zinc powder,therefore extending the cathodic protection time of the zinc-rich primer and the service life of coating system.Generally,under the same condition,the permeation rate of Cl-ions in the coating is greater than that of SO42-ions,so the effect of the electrolyte on the coating degradation is:NaCl solution is much larger than(NH4)2SO4 solution.Hence,the coating system in the offshore marine industry simulated solution has the fastest degradation speed.For the NaCl+(NH4)2SO4 mixed solution,because of the dilution,the number of chloride ion particles in the mixed solution per unit volume is reduced,and the ion permeation rate decreases.Therefore,under the same condition,the effect of the solution on the coating is:NaCl solution>NaCl+(NH4)2SO4 mixed solution.(2)For double-layer coating samples with two thicknesses in four typical simulated solutions,the order of the coating degradation speed in marine industrial atmospheric simulated solutions is different.The solubility of the zinc powder corrosion product in the coating is also an important effect on the degradation of the coating.More(NH4)+ions in the solution will lead to more low solubility corrosion complex product formed by reaction with zinc powder.In the coatings with different thicknesses,the relative content of(NH4)+ ions around the interface of zinc powder is different,resulting in different corrosion products produced.The solubility of the products will affect the further corrosion of the zinc powder,thus affecting the degradation of the coating.(3)For the double-layer coatings in several special atmospheric corrosive medium simulated solutions,the effect of the solutions on the degradation of the coating is NaHSO3+NaHCO3 solution>NaHSO3 solution>NaHSO3+NaNO3 solution.The volume and conductivity of corrosive anions determine the penetration rate of ions in the coating,which furtherly affects the protective properties of the coating.Based on comparison of the results of the coatings in(NH4)2SO4 solution and NaHSO3 solution,it is confirmed that the influence of(NH4)+ions on the solubility of zinc corrosion products is an important factor to accelerate the degradation of the coating.In the simulated acid rain atmosphere solution,the conductivity of the electrolyte inside the coating increases and the ion permeation rate increases,the barrier property of the coating will decrease,leading to premature failure of the coating.(4)The influence of three typical atmospheric simulated solutions on degradation of epoxy zinc-rich single layer coating is:marine atmospheric simulated solution>marine industrial atmospheric simulated solution>industrial atmospheric simulated solution.The FTIR results show that the degree of degradation of the coating in NaCl solution is greater than those in the other two solutions.The failure of the zinc-rich coating in these three solutions is mainly due to the degradation of the ether bond in the epoxy resin.There is no obvious difference in the failure pattern in three kinds of solutions.The EIS results show that the coating still has a good protective effect on the carbon steel matrix when the low-frequency impedance(|Z|0.01HZ)of the zinc-rich coating is less than 1×106Ω·cm2.Therefore,the criterion of the coating complete failure using |Z|0.01HZ lower than 1×106Ω·cm2 is not suitable for the zinc-rich coating because of the conductive particles contained in the coating.(5)The Corrosion Master software was used to simulate the failure behavior of the coating systems in different simulated atmospheric environments and the service lifetime was predicted.The order of the simulated service life of the coatings is in good agreement with the experimental results.This indicates that the simulation technology can be used to study the failure process of carbon steel/organic coatings in the atmospheric environment,and the coating life prediction can be obtained to some extent. |
PDF Full Text Request