Pit Initiation at Single Sulfide Inclusions in Stainless Steel

Abstract

Electrochemical measurements were performed on single sulfide inclusions within a microcell to study their dissolution behavior and role in the initiation of pitting corrosion of 304 stainless steel in NaCl solutions. It was found that chloride ions catalyzed inclusion dissolution and caused metastable pits, and that no metastable pits formed in thiosulfate solutions without chloride. The thiosulfate ion accelerated inclusion dissolution, and when the thiosulfate ion was present above a critical concentration it caused stable pitting, at low concentrations the chloride ion inhibited the aggressive nature of the thiosulfate ion and stifled stable pitting, while at high concentrations the thiosulfate ion inhibited the ability of the chloride ion to cause metastable pitting events. The addition of xanthate ion in combination with electrode activation in copper sulfate was found to inhibit the rate of inclusion dissolution and increase the pitting potential. The initiation of stable pitting was found to depend on the inclusion geometry.The addition of sulfur to stainless steel ͑SS͒ improves machinability but decreases the corrosion resistance due to the presence of sulfur-rich inclusions on the surface. 1-3 Such inclusions, mainly sulfide inclusions and mixed sulfide/oxide inclusions, have been the emphasis of extensive studies to understand their important role in the initiation mechanism of localized corrosion of SS. 4-9 It is generally agreed that anodic dissolution of sulfide inclusions leads to locally aggressive solution compositions near the inclusion, under which the passive film on SS cannot be sustained. However, direct experimental measurement of the local environment during initiation is usually difficult owing to the small distance-and time-scales over which critical events occur. In the present series of papers, a suite of techniques was applied in order to obtain electrochemical data at single inclusions by a microcell technique ͑Part I͒, local chemical compositions in the vicinity of a single inclusion ͑Part II͒, and a mathematical model ͑Part III͒.Mechanisms for the dissolution of sulfide inclusions and initiation of localized corrosion have been proposed by many researchers. Eklund 4,5 performed thermodynamic calculations and concluded that MnS was thermodynamically unstable above a potential of approximately Ϫ100 mV ͑SHE͒ and can only exist in the pH range 4.8-13.8, and suggested the following dissolution mechanismEklund proposed that the sulfide inclusions were unstable at the passive potential and dissolved, with dissolution occurring primarily at the edge of the inclusions. Thus a small microcrevice formed with one wall consisting of the MnS inclusion surface and the other wall consisting of the metal matrix. He proposed initiation due to the local acidification within the microcrevice region, and that the local acidification was attributed to the presence of a significant proportion of Cr within the sulfide inclusion. Similarly, Wranglen 6 suggested the following dissolution mechanism of sulfid...

Phase-field model of pitting corrosion kinetics in metallic materials

“…Finite element method, Galerkin method, 62 is used for space discretization while Backwards differentiation formula (BDF) method 63 is used for the time integration of the governing partial differential Eqs (17,18,21,(24)(25)(26)(27)(28). Triangular Lagrangian mesh elements were chosen to discretize the space.…”

“…(21) coupled with the governing Eqs. (18) and (24)(25)(26)(27)(28). The diffusivity D i is a function of the order parameter η.…”

“…These numerical models can be divided according to the method in which a moving interface is incorporated in their models. Several steady state 9,10,[13][14][15][16][17][18] and transient state [19][20][21][22][23][24][25][26][27][28] models have been developed over the years that did not allow for changes in the shape and dimensions of the pits/crevices as corrosion proceeds.…”

Nonequilibrium collective phenomena in the onset of pitting corrosion

“…Damage of sulfide inclusions in stainless steel have been considered in many publications (23)(24)(25)(26). Surface damage in commercial stainless steels (Fe-Cr-Ni-Mo-Mn alloys) is brought about by partial dissolution of mixed metal manganese sulfide Mn(Cr,Fe)S inclusions particularly in acids (15).…”

“…Surface damage in commercial stainless steels (Fe-Cr-Ni-Mo-Mn alloys) is brought about by partial dissolution of mixed metal manganese sulfide Mn(Cr,Fe)S inclusions particularly in acids (15). These often form occluded sites and enable sulfide ion release that enhance pitting susceptibility and can lower E pit determined from current rise (25,27). This phenomenon can occur on other passive metals containing discrete defect sites such as precipitation age-hardened Al-alloys containing constituent particles.…”