Study On The Deterioration Processes Of Organic Coatings Under The Wet-dry Cyclic Conditions

Organic coatings are widely used to control the corrosion of steel structure, both to maintain appearance and to prevent loss of structural integrity. Thus, organic coatings should isolate the metal from the corrosive environments. However, organic coatings can be degraded under various service conditions, because they comprise polymeric material, leading to reduction of its anti-corrosive effectiveness. The degradation process of organic coatings is complex since it is affected by several factors. Under weathering exposure, wet-dry cycles, H2O and O2 are three critical factors for coatings degradation.The evolution of the water absorption process in an organic coatings immersed in a solution is a very important phenomenon because it precedes phenomena such as paint/metal interface loss of adhesion, electrochemical reactions and the reduction of the resistance to the passage of ions through the coating itself. Kinds of important effects of water penetration have been reported and the parameter of water up-take is often used for predicting anti-corrosive performances of organic coatings. It has been proven that the loss of adhesion between the organic coatings and the iron sample is induced by cathodic delamination which is caused by reaction products, like radicals from the paint/metal interface redox. The synergistic effect of oxygen reduction and water penetration was strengthened by wet-dry cycles during the coatings degradation and interface metal corrosion processes.The corresponding experimental results are listed as follows:(1) Under immersed and wet-dry cyclic conditions, the deterioration processes of the organic coating on carbon steel surface have been comparatively studied using electrochemical techniques. The wet-dry cycles are carried out by exposure to 4 h immersion and 4h drying (4-4h cycles) and 12h immersion and 12h drying (12-12h cycles), respectively. The immersion condition is carry out in a 3.5% NaCl solution and drying at 298K and 50% RH. According to the EIS responses under above three states, the equivalent circuit models are established, and then the changes of coating performance and the underlying metal corrosion processes are evaluated by analyzing the fitting parameters.According to the EIS characteristics, the entire deterioration processes under above three mentioned conditions can be divided into three main stages, consisting of the medium penetration into coatings, corrosion initiation and corrosion extension underlying coatings. The deterioration processes are assessed depending on the EIS characteristics, the equivalent circuits, open circuit potential, the morphology pictures of the coatings surface and the corresponding metal substrate analysis. Comparing with single parameter analysis, The multi-parameter correlation analysis is able to provide rich correlated information from different analysis aspects on the same sub-process, giving reliable evaluation in details on the coatings performance and deterioration mechanism.Comparing with the immersed, the 4-4h wet-dry cycles greatly accelerate the entire deterioration process; especially during the corrosion initiation and the corrosion extension periods, leading the paint system lose its anti-corrosive performance in a short period. However, the 12-12h wet-dry cycles decelerate the entire deterioration process, prolonging the anticorrosive ability.During the medium penetration stage, the 4-4h wet-dry cycles and the immersed coatings experienced the same degrading period, shorter than the 12-12h wet-dry cycles. For the 4-4h wet-dry cycles, the cyclic ratio is so higher that the absorbed water during the wetting period do not lost completely when it comes into the drying process, and then the coatings enter the next immersion cycles again. So, the coatings wetting situation is the same like the immersed. But for the 12-12h wet-dry cycles, the penetrated water can be exudates completely for the long drying process.However, comparing with the immersed, both the 4-4h and the 12-12h wet-dry cycles accelerate the corrosion initiation period for the faster oxygen transportation and reduction at the polymer/metal interface during drying stages. The oxygen reduction at the coating/metal interface has to be regarded as the main detrimental reaction which gives rise to the destruction of the interface and, therefore, to the extension of the delaminated zone. The oxygen transportation is much faster in air pore of the coatings during drying process than the immersed state, and the high oxygen content in the wetting coating accelerate the cathodic reduction rate resulting in a rapid coating delamination from the metal substrate.When it comes into the corrosion extension stages, for the 4-4h wet-dry cycles, the quickly oxygen reduction at the coating/metal interface during drying stage accelerate coating delaminating process, making the coatings lost its anti-corrosive ability quickly. For the immersed state, without the fast oxygen transportation process, the delamination speed is lower, and the anodic reactions can only take place in the limited blisters, making heavy local substrate corrosion. However, although the 12-12h wet-dry cycles also experience the fast oxygen reduction processes during the drying stages, the longer dying period made the water desorbed completely and then the underlying metal corrosion stagnated without the connection of the electrolyte solution between the anodes and canodes. So, as the results show, comparing with the immersed, the deterioration process are decelerated.The current distribution results of the WBE show that the high anode current and cathodic current are found at the defect when the coating was immersed in the 3.5% sodium chloride solution, and then the new cathodic lacations are found around the defect, meaning that the coating delamination is accelerated by the oxygen reduction process in the interface.During the entire coating deterioration process, the EIS diagrams are dominated by the substrate corrosion process of the defect, the coatings and the underlying electrochemical process are "averaged" out. However, through detecting the current distribution information on the metal surface, the local electrochemical process occurring in the coatings and the underlying can be monitored.The responses of EIS and WBE are consistent with the progress of the degradation of organic coatings. Through the combination of the WBE and EIS, the local coating deterioration and substrate metal corrosion processes can be detected. The results show that the WBE and EIS techniques are available to study the progress of breakdown of organic coatings on a steel substrate.