ABSTRACT: Cathodic protection is one of the most useful methods to stop corrosion when the deterioration process has started. When applying cathodic protection, the efficiency of this protection method under different experimental conditions is always a concern. High pH environments generally induces stress corrosion cracking (SCC) phenomena which occurs only in a relatively narrow cathodic potential range in the presence of a carbonate/bicarbonate compounds and for pH's greater than 9. In this work, the resistance to the stress corrosion cracking, of the API X-52 and API X-65 steels, was investigated using a modified wedge opening loading (WOL) fracture mechanics specimens. The specimens were loaded with 95% of the yield strength. The resistance of crack growth and the corrosion were evaluated under different protection potentials (-850 and -650 mV). The used electrolytes were simulated soils (carbonatebicarbonate solution). Evidence of crack propagation of the API X-52 and API X-65 steels were obtained by scanning electron microscopy. The results showed susceptibility to SCC on specimens even with cathodic protection. The applied protection potential (-850 mV vs Cu/CuSO4 electrode) decreases considerably the corrosion rate on the evaluated steels. Out the context, because Stress is one of the three factors influencing SCC.
INTRODUCTION Intergranular stress corrosion cracking (IGSCC) of carbon steel pipelines can occur in a highly concentrated bicarbonate-carbonate solution with high pH (~ 9 to 11). The mechanisms associated with SCC are film rupture and dissolution1. Many studies have focused mainly on the effect of electrochemical potential and solution composition. However, it was found that cathodic protection will lead to the formation of carbonate/bicarbonate solutions with high pH, promoting SCC on pipeline steels2. Cathodic protection has been employed for many years as the best method of corrosion prevention3. Cathodic protection is affected by forcing the potential to a negative region where the metal is completely stable. This can be done by using a sacrificial anode made from a more reactive metal, or using an external power supply to change the amount of charge on the metal surface4. Cathodic protections can complete the protection system by preventing corrosion of the steel surface at holidays which would develop on the coating in the areas surrounding the holidays. The potential difference between a buried pipeline and the soil is of considerable importance in evaluating the cathodic protection performance. It is almost universally accepted that a steel pipeline under cathodic protection is fully protected if the potential is at least -0.85 V vs copper/ copper sulfate electrode. A variety of electrochemical techniques is now widely being used for monitoring the cathodic protection condition of buried pipeline. However, the advantages and disadvantages of each respective method have resisted the acceptance of one unambiguous test method6. In this work the resistance of crack propagation and the corrosion rate were evaluated with different protection potentials -650 and -850 mV (with respect to a copper/saturated copper sulfate electrode) in simulated soil by means of polarization curves. 7The effect of the applied load was investigated using modified wedge opening loading specimens (MWOL).