Design And Development Of A Rapid Evaluation System For Protective Properties Of Organic Coating And Its Application In Mangnesium Rich Coatings

Organic coating is one of the important techniques to protect metal equipment and structure from corrosion.But the aggressive environments would cause serious damage to the coating,so that shorten the service life of the equipment.If the failure status of the coating is understood exactly,it is beneficial to the maintenance of the coating and greatly reduce the cost.In order to solve the problem,in this thesis a rapid detection of the coating protection performance and life prediction system was designed and developed.Meanwhile,the failure process and electrochemical behavior of Mg-rich coating and graphene modified Mg-rich coatings on AZ91D magnesium alloy in 3.5%NaCl solution and 0.05%NaCl+0.35%(NH4)2SO4 solution were studied by electrochemical impedance and potential monitoring methods,in order to furtherly improve the evaluation criteria for various coatings.The main conclusions are as follows:(1)The rapid detection and evaluation of the coating protection performance and life prediction system were designed,which comprises three main functions:? data maintenance function,which realizes the management,query and maintenance of on-site coating data and laboratory coating standard data;?fast detection and performance evaluation function for on-site coating,in which the users can intuitively understand the service status of coating on various parts of engineering equipment;?life prediction function,in which the residual life of the coating can be predicted by comparing the field test data and the standard data in database.The interface of the software is friendly,the operation is simple.Combined with portable impedance test instrument,the effective management of the service state of organic coatings on metal equipment is realized by the software system.(2)In the rapid detection of the coating protection performance and life prediction system,the phase angle value under the specific frequency proposed by our research group is used to rapidly detect the coating protection performance.According to the value of the phase angle,the coating performance can be classified into five grades:excellent,very good,good,poor and failure.According to more than 100 sets of coating impedance data in the standard database,which are the results of accelerated corrosion tests in our research group for many years,the curves of the same coating systems in similar corrosion environment are compared and the service life prediction of the field coating system is realized.(3)Epoxy Mg-rich coatings can provide good protection for magnesium alloys.The shielding performance of the coating can be improved and the cathodic protection duration of the coating can be extended by adding appropriate amount of graphene into the epoxy Mg-rich coating.It was found that by adding 0.5 wt.%graphene(0.5%Gr)the coating porosity is decreased and the cathodic protection duration of Mg-rich coating on AZ31 magnesium alloy in 3.5%NaCl solution could be extended twice.The calculation results of the diffusion coefficient of water in the coating show that the penetration rate of water in the graphene(0.5%Gr)modified Mg-rich coating is slower.(4)The protective effects of graphene modified Mg-rich coatings can be divided into three stages:in the first stage,the coating mainly plays a shielding role,at this time,the solution gradually penetrates into the coating;in the second stage,the magnesium particles are activated and provide cathodic protection for the magnesium alloy matrix;in the third stage,the corrosion products(Mg(OH)2)of the magnesium particles block the holes in the coating so as to improve the shielding performance.The protective properties of the four kinds of Mg-rich coatings in 3.5%NaCl solution follow the order:0.5%graphene modified Mg-rich coating>pure Mg-rich-coating>10%graphene content Mg-rich coating>double layer coating(upper layer of0.5%Gr modified Mg-rich coating+bottom layer of pure Mg-rich coating).(5)For pure Mg-rich coating and graphene modified Mg-rich coating,the variation of the phase angle at specific frequency with time is similar to that of the low frequency impedance(|Z|0.01Hz)value.It is still applicable to detect the degradation degree of the epoxy Mg-rich coatings by using the phase angle value at specific frequency.However,the failure evaluation criteria of two Mg-rich coatings should be justified.When the low-frequency impedance values of Mg-rich coatings in 3.5%NaCl solution decrease to around 107?·cm2,the coatings are considered as failure.