Corrosion Bahaviors Of Zn,Al,and ZnAl Coatings In Simulated Deep-ocean Environment

With the implementation of "Deep strategy" of China’s offshore oil industry,the exploration and exploitation of seabed resources need to make use of new instruments.However,the deep ocean has a very unique corrosive environment.Compared with the organic coatings,thermal sprayed Zn coating,Al coating and ZnAl coating further improve the anticorrosion performance of steel structure.Arc is one of the thermal sprayed coating technology,which has been successfully applied in the field of corrosion protection.There are few domestic reports so far for corrosion property and mechanism of arc sprayed coatings in deep-ocean environment and systematic research is still not enough.Meanwhile,The few research data of deep-ocean corrosion abroad remained confidential and rarely made public,as most of the studies are involved in the military field.In this paper,corrosion mechanism of Zn coating,Al coating and ZnAl alloy coating in simulated deep-ocean environment were studied,contrasting which in simulated shallow-sea environment.This paper also used metallographic microscope,stereoscopic microscope,X-ray diffraction,indoor corrosion bolts combined with electrochemical measurement to study the corrosion morphology、corrosion products and corrosion electrochemical performance of the coatings.Morphology observation and analysis of the porosity showed that porosity of Zn coating,Al coating and ZnAl coating were about 2.67%,7.2%,4.03% on average,repectively.In general,the coatings accorded with quality standards of thermal sprayed coatings.After immersion,the morphology and components of corrosion product covering on the coatings were analyzed by stereoscopic microscope and XRD.The results showed that,in simulated deep-ocean environment,there was relatively few corrosion products and the corrosion products cannot completely cover the coating surface.The coatings presented uniform corrosion morphology.The coatings were all of oxygen corrosion.Corrosion products of Zn coating was principally Zn5(OH)8Cl2·H2O,whicn is granular,loose and diffluent;Corrosion products of Al coating was flocculent Al(OH)3,which was not easy to dissolve in seawater;Zn5(OH)8Cl2·H2O and Zn0.7Al0.3(OH)2(CO3)0.15·xH2O were corrosion products of ZnAl coating comaparatively.While in the simulated shallow-sea environment,the coating surface was completely covered by a large number of white corrosion products.Results of corrosion potential of and potentiodynamic polarization curves showed that the coatings were able to provide effective sacrificial anode protection for the matrix in the deep ocean as corrosion potential of the coatings was always lower than that of Q235.Besides,Ecorr and potentiodynamic polarization curves of Zn coating and ZnAl coating were very close.Corrosion mechanism of coatings was understood through the analysis of electrochemical testing combined with corrosion morphology and corrosion products.In the simulated deepocean environment,generally speaking,as the temperature was much lower,electrochemical corrosion resistance was bigger and corrosion rate i obviously lower than that in simulated shallow-sea environment;As oxygen content was less,corrosion products was also less and have weak closed effect for coating porosity,corrosion rate was slightly lower in the immersion process.