Electrochemical Investigation On Water Transport Behavior Of Organic Coatings And Degradation Mechanism Of Coated-metals

The corrosion behavior of metals in nature environment is a spontaneous process from the viewpoint of the chemical thermodynamics, which can result in destructure and function-loss of metallic materials. Among the corrosion protection methods of metals, such as electrochemical protection, treatment of corrosive media and coatings protection, organic coatings is one of the most frequent applications at the present time. The corrosion of coated-metals is closely related to organic coatings performance. However, the corrosion protection performances of coatings are influenced by many factors such as adhesion between the coating and the metal substrate and barrier property of coatings to penetration of water and ions. In this dissertation, water transport behavior in coatings, corrosion and degradation mechanism of coated-metals were investigated by using electrochemical methods and various physical testing technologies. The continuous evolutions of impedance of coated-metal systems in the long-term immersion were measured by electrochemical impedance spectroscopy (EIS), and the electrical equivalent circuits (EECs) corresponding to different immersion stages were established. The EIS data of composite electrodes were fitted to suitable EECs and the change of electrical parameters corresponding to coating performance and metal corrosion as a function of immersion time were obtained from the fitting results. The quantitative evaluations of the coating performance were indicated from the parameter changes. The main contents are listed as follows:(1) Water transport behavior in epoxy and polypropylene varnish coatings covered on mild steels and LY12 aluminium alloys and corrosion of coated-metals were investigated.Coating capacitances of polypropylene coatings were calculated from the imaginary part of impedance at a fixed high frequency. The coating capacitance (Cc)-time (?) curves (lnCc~tl/2) of coated-metals showed that water transport behavior followed the Fickian diffusion law during the initial period of immersion. According to the evolution of the coating capacitance with time, the water diffusion coefficient (D) was obtained.The evolution of impedance models for the polypropylene coated-mild steels was studied. The results suggested that the impedance of coated-metals acted as a capacitive characteristic in the initial time of immersion. With the increasing of immersion time, the number of time-constant in impedance spectra increased. The diffusion components emerged in impedance spectra after an intensive immersion. The diffusion behavior did not act as typical Warburg impedance.The analyses of impedance models of epoxy coated-metal electrodes were investigated during the whole immersion time. Four distinguished impedance models were used to fit the EIS data. In the beginning immersion period, the EEC only consisted of coating capacitance (Cc) in parallel with coating resistance (Rc). Along with the immersion time, water penetrated from the coating to the interface of metal/coatings and EEC contained double-layer capacitance (Cdi) and charge transport resistance (Rct) corresponding to electrochemical reactions at metal interface. After that, the diffusion impedance related to diffusive characteristics of corrsion products emerged in EEC. After longer immersion for aluminium alloy/epoxy coating systems, Cl" diffusion through the paint film to reach the interface of metal/epoxy coatings and the formation of chloride ion-incorporation salt films on the surface of metal substrate resulted in salt film capacitance (Csi) and salt film resistance (i?Sf) in the EEC. Water diffusion coefficients in epoxy coatings with different thickness and painted on different metal substrates were calculated from time dependence of coating capacitance. The results suggested that water diffusion coefficients were slightly influenced by the coating thickness. Different glass transition temperature (Tg) values were characterized for coatings prepared on various substrates by differential scanning calorimetry (DSC) measurement. After a certain time of immersion, Tg of coatings decreased dramatically. Nevertheless, the decrease of Tg of aluminium based organic films was smaller than that of steel based ones. Accordingly, thedifferences in water diffusion coefficient in the coatings painted on various substrates were explained. In addition, changes of Cdi and i?ct with immersion time were employed to characterize the corrosion behavior of coated-metal systems.The influence of curing agent (polyamide) content on the performance of epoxy coatings on mild steels was studied. The analysis of impedance data of the coated-metals showed that only one time-constant appeared in the impedance spectra at the beginning times of immersion. The electrochemical corrosion reaction occured at the metal/coating interface after a certain time of immersion, and three types of diffusion behavior (i.e. the semi-infinite length Warburg diffison, the open boundary finite length diffusion (FLD) and the blocked finite length diffusion (BFLD)) were presented in the impedance spectra after intensive immersion. The coating capacitance-time (lnCc-t1 ) curves showed that water transport behavior followed the Fickian diffusion law during initial period of immersion. Water diffusion coefficients (D) decreased and then increased with the increase of curing agent content, with the minimum value at 2.2:1 molar ratio of epoxy resin to polyamide. Fourier transform infrared spectroscopy (FTIR) measurements indicated that curing agent content had a marked influence on the structure of epoxy coatings. The curing reactions reached the maximum completeness at the ratio of 2.2:1. DSC measurements showed that T$ increased and then decreased with increasing the curing agent content, with the maximum also at molar ratio of 2.2:1. Both the structure Tg variations were used to interpret the curing agent content depedent water transport coefficients.(2) The influence of surface pretreatment of metal substrates on performance of organic coated metals was investigated. The selected pretreatment processes included conventional chromate conversion and a novel environmentally friendly protective silanization.The results indicated that the total impedance of chromate treated followed by epoxy coated-metals increased with immersion time in testing solution. In the initial period of immersion, the diffusion impedance was shown in low-frequency range, which was attributed to the self-healing effect of the chromate layer to the metal interface. At the same time, the EIS data during the long-term immersion were fitted to EECs and thechanges of electrical parameters corresponding to metal corrosion states and coating performance with immersion time were obtained from the fitting results, which could be employed to evaluate the protective performance of coatings. FTIR measurements showed that hydrolyzed silane initially adsorbed on metal surface and then a compact conversion layer was formed after cured at high temperature. Polarization curve measurturements indicated that silane films effectively barriered the penetration of corrosive media to metal substrates and increased the corrosion resistance of metals. In addition, scanning electron microscope (SEM) was used to characterize the morphologies of various silane layers.(3) Silane agents were attempted to modify epoxy resin followed by painting on metals. The results showed that water transport coefficients of silane modified-coatings obviously decreased and protective performance of coatings distinctly increased. This might result from the continued action of silane in coatings with water during the immersion, resulting in condensation of hydrolyzed silane and thereby in enhancing the cross-linking and denseness of epoxy coatings, which could reinforce the barrier of penetration of aggressive particles to metal interface. DSC measurements showed that, T% slightly increased for silane-modified epoxy coatings after immersion, which may indicate that silane reagents could enhance the resistance to destruction and degradation of organic coatings. The change trends of Cai and i?ct with immersion time revealed that silane-participation in coatings reinforced adhesion between coatings and metal substrates and improved corrosion protection of organic coatings.