Responses Of The Electrochemical And Nano-mechanic Parameters Of Coatings To Transmission Process Of Aggressive Medium

At present,the organic coating protecting technology is the most economical and effective measure in the field of corrosion protection of materials.The failure of organic coating usually leads to corrosion of metallic equipment,resulting in the decline of service life and environmental pollution.Sometimes,corrosion threats the people’s lives and property.Organic coatings were degraded by the permeation of aggressive medium into surface of the metal substrate through coatings.Therefore,it is of great practical significance to study the diffusion process of aggressive medium in organic coating and the influencing factors to evaluate the performance of organic coating,and to predict its service life.In this paper,the changes of permeation volume of aggressive medium,thickness and morphology of different organic coatings with immersion time in different aggressive environments were studied.Next,The electrochemical impedance spectroscopy of organic coating responding to penetration of aggressive medium was studied,and equivalent electrical circuits based on characteristics of impedance spectroscopy were established,Electrochemical parameters(equivalent capacitance and equivalent resistance)of organic coatings with immersion time were further investigated.Thirdly,the nano-mechanical parameters(nano-hardness,nano-elastic modulus)of coating responding to permeation of aggressive medium were studied using nanoindentation technique.The permeation depth of aggressive medium with different immersion time in organic coating was researched.The relationship between electrochemical parameters and nano-mechanical parameters of coating was studied based on ideal diffusion model of organic coating.Finally,the life prediction model of organic coating was established.The results show that the trends of the weight,thickness,equivalent capacitance and equivalent resistance of the organic coating during the diffusion and transmission of the aggressive medium in organic coatings were divided into three stages.At the first stage,the swelling and water absorption of coating is dominant,and dielectric constant of the aggressive solution is much larger than that of the coating component,leading to coating capacitance increases sharply.At the second stage,the swelling and water absorption of coating and dissolution of coating components reaches the dynamic equilibrium state,and weight,thickness,electrochemical parameters of coatings remain unchanged.At the third stage,permeation process of aggressive medium was back to be dominant,the coating completely loose the protection to metal substrates.The organic coatings with higher pigment/resin ratio have a larger volume of permeated medium due to higher volume of hydrophilic pigments and fillers.The volume of permeated medium in urban environment was higher than that in marine environment.The decreases of nano-hardness and nano-elastic modulus of organic coatings resulted from the permeation of aggressive medium into organic coatings,indicating that nano-mechanical parameters were sensitive to the penetration of aggressive media.It was found that H/E,which is a dimensional parameter,was suitable for describing the change of nano-mechanical parameters of organic coatings with immersion time.Finally,the relationship between the electrochemical parameters of the organic coating and the mechanical parameters of the organic coating was obtained by establishing the ideal corrosion model of the organic coating.