Study On The Inhibition Mechanisms Of Precipitation Inhibitors

In this paper,the localized corrosion,galvanic corrosion,the synergistic effect of inhibitors and the ion selectivity of precipitation film were studied under the background of precipitation inhibitors by technologies of in situ observation,electrochemical testing and surface analysis.1.The localized corrosion behavior and mechanism of carbon steel under precipitation film,and the synergistic effect between Ce³⁺and other types of precipitation inhibitors were studied.The results show that:?I?The addition of Ce³⁺into NaCl solution raised the corrosion potential of carbon steel greatly,and area covered by the precipitation film formed on the surface had a more positive potential than the defect areas,so the covered area acted as cathode and the defect areas acted as anodes.The dissolution of the localized anodes were accelerated by the galvanic current,which promoted the occurrence of pitting corrosion;?II?The combination of Ce³⁺and Zn²⁺decreased the corrosion potential of carbon steel,and the potential of the area covered by the film was lower than that of the defect area,which indicated that the defect areas acted as cathodes.Then the defect areas were repaired rapidly under the action of the cathodic current,leading to the inhibition of pitting corrosion and remarkable reduction of the corrosion current density.A thin and invisible precipitation film was found on the metal surface.It can be attributed to the small amount of Zn²⁺doped in cerium oxide that changed the morphology of the film and greatly enhanced the corrosion protection ability of the film;?III?The combination of Ce³⁺and Ni²⁺increased the corrosion potential further and decreased the corrosion current density,however,failed to inhibit the pitting corrosion.The metal surface was covered with a thick precipitation film,mainly consisting of oxides of nickel and cerium;?IV?The combination of Ce³⁺and SiO₃^2-or salicylate decreased corrosion current density significantly,both of the uniform corrosion and local corrosion were strongly inhibited.The metal surface in the solution containing Ce³⁺and SiO₃^2-was bright and without obvious corrosion trace.The main components of the film may include cerium oxide,ferric silicate or silicate colloid.In the solution containing Ce³⁺and sodium salicylate,the metal surface was covered with a thick precipitation film,mainly composed of cerium oxide and complex of sodium salicylate and iron.2.The optimum ratio of Zn²⁺to 1-Hydroxyethylidene-1,1-diphosphonic acid?HEDP?in the binary corrosion inhibitor was studied theoretically and practically by means of MATLAB software and experiments.The results show that:?I?The synergistic effect was best when the molar ratio of Zn/P in solution was 1:1.Excessive content of any component between Zn²⁺and HEDP would lead to the decrease of corrosion inhibition efficiency;?II?In the process of the synergistic inhibition,HEDP reacted with the dissolved iron ions in the anode region to form insoluble complexes and inhibited the anodic process,Zn?OH?₂ was formed in the cathode region to inhibit the cathode process.3.The inhibition mechanism of ZnSO₄ and sodium sodium N-lauroyl sarcosinate?SLS?on the galvanic corrosion between carbon steel and stainless steel was studied.The results showed that:?I?ZnSO₄ mainly decreased the open-circuit potential of stainless steel,and SLS greatly increased the open-circuit potential of carbon steel,both of which reduced the potential difference between the couples and inhibited galvanic corrosion;?II?The current density of galvanic couple was affected by the potential difference between couples,the greater the potential difference,the stronger the driving force,the more serious the galvanic corrosion;?III?SLS mainly functioned on carbon steel surface by adsorption and chelating,The increase of potential difference will have a negative effect on the protective ability of the film on carbon steel surface.ZnSO₄ mainly functioned on stainless steel surface by forming precipitation film,the greater the potential difference,the better the film formation on the surface.4.A layer of anion-selective precipitation film of Al?OH?₃ and Zn?OH?₂ mixture was constructed on the surface of carbon steel by constant current polarization.The anion-selectivity of the film was enhanced or weakened by immersion in Ca?NO₃?₂ and Na₂SO₄ solutions,respectively.The effects of ion selectivity of precipitation film on corrosion resistance of carbon steel in solution containing Cu²⁺and Cl^-were studied.The results showed that:?I?The anion selectivity of precipitation film increased after immersion in Ca?NO₃?₂ solution,but decreased after immersion in Na₂SO₄ solution;?II?Film possessed stronger anion selectivity was more impermeable to Cu²⁺,and film possessed stronger cation selectivity was more impermeable to Cl^-.