|Content Type||Conference Paper|
|Title||The Influence of Composition of Carbon Steel on the Anodic and Cathodic Reaction Rate in CO2 Corrosion|
|Authors||Ketil Videm, University of Oslo|
|Source||CORROSION 98, March 22 - 27, 1998 , San Diego Ca|
|Copyright||1998. NACE International|
|Keywords||C02 corrosion, carbon steel, electrochemistry, corrosion mechanism.|
Experiments were carried out with pure iron and carbon steels in C02 containing brine in the temperature range 25-700C. The anodic dissolution of iron takes place by two different mechanisms. Low pH and low corrosion rates favored dissolution in the active state. This process is charge transfer controlled and has a Tafel slope of 30-50 mV/decade. The reaction rate at a given potential increased about proportionally with the hydroxyl concentration. A change in the dissolution mechanism took place at a pH about 4 to so called prepassive dissolution. This process is little influenced by pH, but affected by agitation of the solution. The steels followed the same trend. However, the anodic dissolution rate at a given potential and pH differed for the various steels to a factor of three compared with pure iron. The response of the current density to an instantaneous pH change was sluggish. This and other observations support the view that the anodic dissolution of iron is controlled by species adsorbed at the metal surface.
Also the kinetics of the cathodic liberation of hydrogen is complex and varies significantly with the test conditions. The reaction rate is hardly 100% activation controlled under any conditions of practical interest and is seldom purely diffusion controlled. Tests with ten different carbon steels showed that both the anodic- and cathodic reaction rates at a given potential exhibited large variations with composition and structure of the steels. The behaviour of a steel at low pH gives little guidance for its performance at high pH. The parameters responsible for the individual behaviour of the steels were not identified.
The electrochemistry of the anodic dissolution of iron in acidic solutions is known to be complicated. Anodic dissolution and hydrogen reduction on iron have been treated in more than 400 relevant publications. Nevertheless, many phenomena are poorly understood even on a qualitative base. Carbonic acid and its dissociation products introduce additional complications. Purity of iron is known to be important. According to Drazic 1, even analytical grade iron and zone refined iron can behave rather differently. The effect of the composition of the carbon steel on C02 corrosion has attained much interest in the last years 2-5. As it has been difficult to draw general conclusions from the corrosion rate, the strategy of this work was to examine the anodic- and cathodic reactions separately. Experiments were carried out with pure iron and carbon steels in C02 containing brine, at varied pH in the temperature range 25-700C.
Ten different steels were studied. Only the results for some of them are presented in this report. Table 1 gives the composition of these materials. All steels mentioned in this report had a ferritic perlitic structure. The thermostatic glass cells with C02 bubbling used for the exposures had standard cone openings for specimen assembly, electrode, counter electrode and a pH electrode. The calibrated Ag/AgCl reference cell of the pH electrode was also used for potential measurements of the steels. A stirring magnet gave a flow rate of about 0.2 m/s. The test solution was deoxygenated at least for fifteen hours by C02 bubbling before the specimens were introduced. As the volume was 1.5 liters and the total area of the specimens was below 1.5 cm2 , the building up of the corrosion products in the water was slow.
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