Morphology and stress evolution during the initial stages of intergranular corrosion of X70 steel

Abstract

Morphology and stress evolution during the initial stages of intergranular Morphology and stress evolution during the initial stages of intergranular corrosion of X70 steel corrosion of X70 steel

Electrochemical impedance spectroscopy analysis of corrosion product layer formation on pipeline steel

“…Prior to the corrosion exposures, the steel surface was cathodically polarized at -1.0 V for 5 min to remove native oxide. Cathodic polarization was followed immediately by either linear potential sweeps or constant potential corrosion exposures at -0.521 V. This potential is between the active dissolution peak of -0.60 V and the passivation potential of -0.40 V, and hence lies within the range of susceptibility to SCC and IGC [2,3,7,14]. Collection of impedance spectra was initiated immediately at the end of constant potential exposures of durations between 5 min and 20 hr.…”

“…The diffusion equation governs the anion concentration near the steel surface within a diffusion layer of thickness δ formed by anion consumption. The porous FeCO 3 product layer, once formed, significantly impedes anion diffusion to the metal interface [14]. Thus, the appropriate value of δ depends on the dominant diffusion resistance for a given experimental condition (Fig.…”

“…In recent work, we followed the progress of IGC at potentials in the critical range using morphological observations, mechanical stress measurements, and nanoindentation tests [14,15].…”

“…During extended corrosion exposures, the dissolution current density decays by 2 or 3 orders of magnitude, as the product layer accumulates on the steel surface [14]. The corrosion product 4 extends as "wedges" into grain boundary (GB) grooves formed by intergranular attack [14,16].…”

“…Compressive stress is generated during corrosion due to volume expansion accompanying formation of the product layer [14]. As the current density decays, the grooves sharpen significantly from their initially blunt shape, and eventually cracks initiate in the GBs [16].…”