| In the simulating oilfield downhole CO2 corrosive environment under high temperature high pressure condition,the structure characteristics of CO2 corrosion scale on general N80 steel and low Cr-containing N80 steel were studied,the relationship between the structure characteristics and anodic reactions was discussed. It is the first time that ion-passing-selectivity through corrosion scale was discovered and the influence of ion-passing-selectivity on the corrosion rate and local corrosion behavior was explained.It can be seen from the morphology characters that CO,corrosion scale on general N80 steel can be considered as three layers,which is composed of FeCO,crystal. The inner layer formed by primary corrosive product is compact and strong and well adheres on metal matrix,the outer layer formed by secondary corrosive product is loose and its strength is comparatively lower,and the middle layer probably is mixed by the primary corrosive product and secondary corrosive product.The corrosion scale formed by FeCO3 crystal has anion-passing-selectivity,it means that anions in corrosion electrolyte can penetrate through the corrosion scale and get to the interface between matrix and corrosion scale,as a result,result in metal matrix to be dissolved. During the process,Cl- could be enriched and congregated at the interface,that subsequently catalyzes the formation of pitting,and Cl" could be enriched again in pitting,thus leading to the further development of pitting.In the corrosion scale on 4%-5% Cr N80 steel. Cr could be enriched and form amorphous compound Cr(OH),,which has the character of cation-passing-selectivity through corrosion scale. This kind of scale would block anions such as Cl-,CO32- and HCO," penetrating through the corrosion scale and reduce the anionic concentration at the interface. As a result,the anodic reactions could be suppressed and corrosion rate be reduced. Because Cl" could not be enriched and congregated at the interface,pitting corrosion would be suppressed.The corrosion scale on l%Cr-containing N80 steel is composed of mixed product of amorphous compound Cr(OH),and FeCO3 crystal,that make the corrosion scale has the character of cation-passing-selectivity and as a result the anodic reactions could be suppressed and corrosion rate be reduced. But partial anions canpenetrate the FeCO3 crystal and get to the interface between matrix and corrosion scale,that can lead to formation of pitting.The electrochemical impedance spectroscopy (EIS) was used to analyze the electrode reaction of CO2 corrosion of N80 steel in various electrolytes,and corresponding EIS models were established to interpret the effect of electrode reactions on EIS curve. Moreover,these models were used to analyze the influence of temperature,Cl- concentration and Cr-containing on electrode reaction.It is found that the anodic EIS curve has three time-loops,in which at low frequency section the inductive arc is related with active dissolution of surface steel and the capacitive arc with formation of corrosion film. As increasing the coverage of corrosion scale,the inductive arc would reduce and capacitive arc expand. Moreover the relationship between formation of corrosion scale and electrode reactions is discussed. When anodic reaction rate of Fe + HCO,-FeCO3 + 2e + H+ and Fe + CO32- FeCO3 + 2e are comparatively higher,the formation of corrosion scale is easier,that causes bigger coverage of corrosion scale on steel,furthermore,the inductive arc in EIS curve would be reduced and capacitive arc be expanded. When anodic reaction rate of Fe -Fe2++ 2e are comparatively higher,the coverage of corrosion scale is lower,then,there generally is obvious inductive arc in EIS curve.During CO2 corrosion process of N80 steel in various electrolytes,the reduction of H+, H2O,H2CO3 and HCO3- all could be the cathodic reaction,but the reduction rates are different in various electrolytes. In acidic electrolyte saturated with CO2,the reduction rate of H+ is higher and controls cathodic reaction rate,the reduction ra...
|
PDF Full Text Request