Effects Of Dry-Wet Alternation And Cathodic Protection On The Corrosion Resistance Of Epoxy Coatings

As the most effective protective technology,anti-corrosion coatings are widely used in various corrosive areas of offshore platforms.In all corrosive areas,the alternating dry-wet areas are the most severe corrosive areas where the coatings lose protection easily.For the better protection of marine platform metal materials,the corrosion protection coatings often combine with the cathodic protection technology.But macroscopic or microscopic defects will appear in the organic coatings during preparation process and service period.If the applied protective potential is improper when the coating is damaged,it may cause the coating peeling,even the corrosion occurring on the metal surface beneath the coating.In this paper,electrochemical impedance technique(EIS),open circuit potential(OCP),water absorption,porosity,coating adhesion test,fourier transform infrared spectrometry(FTIR),scanning electronic microscope(SEM)were used to study the failure processes and failure mechanism of epoxy coating(EP coating)and chemical bonding corrosion protection coating(CB coating)under full immersion and 6-6h dry-wet cycle conditions in artificial simulated sea water.The protective effects of different protective potentials on the metal substrate under damaged organic coatings were also studied.The main research results of this paper include:(1)The failure process of EP and CB coatings under the immersion condition was researched.EP coating failed faster than CB coating,And CB coating had higher coating resistance,charge transfer resistance and adhesion strength.CB coating had higher barrier property and good protection performance.The failure mechanism of two coatings under immersion was proposed that after the water absorption increased,porosity became bigger and the protection of coating deteriorated,leading to the aggravation of metal corrosion.(2)The failure process of EP and CB coatings under 6-6h alternating dry-wet cycle was studied.EP coating also failed faster than CB coating.The failure processes of both coatings were accelerated mainly in the later stage of service.The water absorption and desorption processes of the coatings were asymmetrical which the water absorption was slower than the water desorption.In the early and middle stages of alternating dry-wet,the failure of coating was slow.In the later stage,the coatings subjected to alternating swelling and shrinking,leading to more and bigger defects.The coatings cracked and formed the diffusion channel to let more corrosive medium penetrating,resulting in faster failure of coatings.(3)The protective effect of the cathodic protection on the damaged coating under the immersion condition was studied.With the potential shift to negative,the delamination area of the coatings increased.The cathode potential-750 mV(vs.SEC)was no longer applicable for the damaged CB coating,and-1050mV(vs.SCE)resulted in serious hydrogen evolution in the defect area,leading to the serious interface alkalization and the cathodic delamination area increasing.The cathode protection potential of-850 mV and-950mV(vs.SCE)can inhibit the corrosion of metal substrates.The deposited film under-950mV(vs.SCE)was composed by Mg(OH)2 and CaCO3 which was integrated and dense,leading to a good protective effect on the metal substrate.(4)Under the 6-6h alternating dry-wet condition,the cathode protection potential of-950mV(vs.SCE)had a certain protective effect on the metal substrate at the delamination areas of the coating.But the metal substrate beneath the damaged areas was still corroded.The cathodic delamination area was small and calcareous deposits film was loose.