A Study On Mechano-Chemical Behavior Of Marine Anti-corrosive Coating/Carbon Steel System

Corrosion is generally considered to be one of the most dominant degrading mechanisms of the structural strength of ships and marine structures,and the generation of corrosion can affect the structural lifetime significantly.From the viewpoint of structural safety,corrosion can cause serious damage and wall-thickness reduction,and might facilitate fatigue cracks,brittle fracture and unstable failure,resulting in tremendous loss of economic expenses and human lives.Protective coatings combined with cathodic protection are generally employed to protect marine structures against corrosion,and also have achieved some protective effect.However,such corrosion protection system is not always sufficient with the development of ships and marine structures towards the deep-sea applications.So far,there has been extensive work on studying the corrosion and corrosion protection of ships and marine structures.However,it still remains as a significant challenge due to the complexity and diversity of the combined effects of marine environment,cathodic protection potential and structural mechanical responses on the degradation behavior of marine anti-corrosive coatings and the corrosion behavior of carbon steel.In this paper,the separate and combined effects of cathodic protection potential and elastic stress on the protective properties of the composite coating system and the corrosion behavior of carbon steel in marine environment were studied theoretically and experimentally by employing the electrochemical measurement technique combined with some other physical testing methods.The main contents and conclusions of this study are as follows:The mathematical models of corrosion potential and corrosion rate of carbon steel under the effect of elastic stress in both uniform and localized corrosion systems were developed on the basis of the basic theories of thermodynamics,kinetics and mechano-chemistry,and thus they could lay the theoretical foundation for further mechano-chemical experiments,theoretical analyses and the establishment of corrosion models which could allow for the effect of mechanical factor on actual marine structures.The separate and combined effects of elastic stress and cathodic protection on protective properties of composite anti-corrosive coatings in artificial seawater were investigated by employing electrochemical impedance spectroscopy(EIS)combined with some other auxiliary technology instruments such as scanning electron microscopy(SEM),energy dispersive spectrum analyzer(EDS),Fourier transform infrared spectroscopy(FTIR),etc.Furthermore,a thermodynamic approach was adopted to evaluate the effect of stress on water diffusion coefficient,and meanwhile the effect of trapped water density in the coating system on the apparent water diffusion coefficient was also discussed.Experimental results showed that the elastic stress could have a significant influence on coating protective properties,and the extent of this influence depended on both the magnitude and direction of stress,and meanwhile it can be found that the applied elastic stress,independent of its direction,had no obvious effect on the chemical structures of the coating system.For the present coating systems,the enthalpy contribution played a major role on the modification of water diffusion coefficient than the entropic contribution,and meanwhile a high ratio of trapped water to free water in coating system could also decrease the apparent water diffusion coefficient.In addition,the obtained results indicated that the cathodic protection could promote coating degradation,and for the present coating systems,the more negative of the applied cathodic protection potential was,the more quickly and more seriously the coatings deteriorated.In simultaneous presence of elastic stress and cathodic protection,the two factors could influence each other and their interactive effect could reduce their respective impact on coating protective property.Their combined effect depends on the predominant one,and this may be associated with the intrinsic properties of coating system and the migration velocity and the degree of some aggressive particles,such as water molecules,sodium and hydroxyl ions.It is anticipated that some concluding remarks achieved in this study will provide some practical insights into the optimization design of anti-corrosive coating system of actual ships and marine structures.The separate and combined effects of elastic stress and cathodic protection on corrosion behavior of Q235B steel in 3.5%NaCl solution were investigated by employing the electrochemical measurement technique.Meanwhile,the impact of elastic stress on the interfacial tension between the metal and solution was also studied by using the contact angle measurement technique.Experimental results indicated that elastic stress could cause negative shift of corrosion potential and accelerate electrochemical corrosion of steel significantly.The non-unifonn distribution of elastic stress could lead to the formation of stress gradient cells,in which the higher stress locations act as anodes relative to the low stress locations,and thus could induce localized corrosion.The theoretical analysis and experimental results showed a good agreement.The corrosion product layer could also have a significant influence on metal corrosion behaviour,and meanwhile the extent of this influence depended on its own microstructure and also the types and characteristics of corrosion.Meanwhile,both interfacial tension and surface contact angle could also decrease significantly with the increase of stress level,and thus a new method to study the problem of mechano-chemical effect by using the interfacial tension between metal materials and corrosive mediums has been presented.In addition,the obtained results indicated that charge-transfer resistance firstly increased and then decreased with the negative shift of the applied cathodic polarization potential.As the charge-transfer resistance reached the peak value,the corresponding cathodic polarization potential was considered to be the optimum cathodic protection potential,and it can be found that there existed a good linear relationship between the optimum cathodic protection potential and elastic stress,and the optimum cathodic protection potential shifted negatively with the increasing of elastic stress level.Therefore,in the safety design of actual ships and marine structures and also the optimization design of impressed-current cathodic protection system,both the corrosion enhancement and the necessary adjustments of impressed current due to the elastic stress on structural surface should be considered.Through mechano-chemistry theory and experimental results,it can be found that the corrosion damage of Q235B steel under the combined effects of elastic stress and corrosive environment was greater than that under the single corrosive environment,and meanwhile this change could become more obvious with the prolongation of corrosion time,and thus the new time-variant physical corrosion model which could allow for the effect of mechanical stress on the corrosion process of actual marine structures has been proposed on the basis of the representative existing uniform corrosion models.The effect of corrosion models on lifetime assessment was studied by using a steel plate element subjected to the combined effects of corrosion and elastic stress.Analytical expressions were presented for the structure's lifetime,which is identified with the time when the stress reaches the ultimate allowable stress,and the corresponding analytical solutions for different corrosion models have been obtained.An example was presented to demonstrate the combined effects of corrosion and elastic stress on the structural lifetime assessment by using the measured experimental data,and meanwhile the accuracy of the new corrosion model was validated.The obtained results showed that different corrosion models could have a significant influence on the lifetime assessment of the plate element,even under the same stress level and corrosive environment.The proposed new stress corrosion model revealed that the mechano-chemical effect could have a significant influence on the accurate lifetime assessment of marine structures,and meanwhile this influence could become more significant with the increasing of steel strength grade.These indicated that the choice of an appropriate corrosion model would be crucial to accurately assess the lifetime of actual marine structures,and it is anticipated that the establishment of the new time-variant physical corrosion model will provide some reasonable references for the safety design and the accurmte assessment of corrosion wastage in actual ships and marine structures.