The Design Of Constant Current Method For Zinc-rich Paint And The Research Of The Correlation With The Open Circuit Potential Method

Open circuit potential (OCP) method has been widely used for evaluating zinc-richcoating’s cathodic protection performances. However, it usually took quite a long time.For this reason, galvanostatic dissolution (GD) method was designed in this thesis. Thepreparation method of the specimen, apparatus for testing, test parameters and theevaluation method were designed in order to quickly evaluate the cathodic protectionperformances of zinc rich paints.Ten kinds of coatings were prepared to study the correlation relationship between theGD method and OCP method. The coatings were different in zinc content, the morphologyof zinc powder or the addition of conductive pigment. The effects of the three influencefactors above on the performances of the coatings were also studied. Differential scanningcalorimetry (DSC) method was used to test the zinc content in the coatings, andconductive adhesive electrochemical impedance spectroscopy (EIS) method was used toevaluate the coatings’ percolation threshold and contact impedance. The area of active zincpowder in the coating was compared by the EIS spectra in early immersion stage. Thesurface and cross-section morphology of the coatings were observed by scanning electronmicroscope (SEM).Experimental results showed that there was a good correlation between GD and OCPmethod for evaluating the cathodic protection performances of zinc rich paints. It showedpositive correlation between the two methods above both when the current density in GDmethod was0.05mA/cm2for coatings containing flake zinc powder and when it was1mA/cm2for coatings containing spherical zinc powder. It means that the coating with longcathodic protection duration in OCP method is also have a long duration of activedissolution in GD method.Experimental results also showed that not only zinc content but also the addition ofgraphite powder and the shape of zinc powder were important for the cathodic protectionperformances of zinc-rich coatings. Zinc pigments were arranged orientally parallel to thesubstrate in flake zinc powder coatings, which resulted in face to face contact betweenzinc powders. It made the penetration of electrolyte more difficult. Moreover the area ofactive zinc was much smaller and only the outer part of the coating were dissolved in the test of GD method. Otherwise, there was point to point contact between zinc powders inspherical zinc powder coatings, which made the penetration of electrolyte in coatingsmore easily and the conductive skeleton can be saved more effectively, due to the largerarea of active zinc not only the outer zinc powder but also the inner zinc powder in thecoatings can be dissolved partly. Adding graphite powder to zinc-rich coating, thecoatings’ contact impedance was deduced, the active surface area of zinc powder wasincreased, and the cathodic protection performance was enhanced.