Pit Initiation on Stainless Steels in 1 M NaCl With and Without Mechanical Stress

Abstract

The onset of pitting corrosion at MnS inclusions on 304 stainless steel in 1 M NaCl was studied with and without applied mechanical stress with use of microelectrochemical cells. Polarization curves of areas ͑100 m diam͒ without inclusion showed no pitting at potentials below that of oxidation evolution; stress had no effect on the corrosion behavior. Areas containing five round inclusions of about 4 m in size showed stable pitting at about 400 mV; the effect of stress shifted the pitting potential to values that were 150 mV more negative. Polarization curves measured on large deep MnS inclusions showed active pitting. Curves of areas with single large, shallow MnS inclusions showed multiple current transients during dissolution of the inclusion without stress, but the metastable events did not initiate stable pitting. The dissolution of shallow MnS inclusion did not form a deep microcrevice between the MnS and stainless steel matrix. However, under applied stress, cracks were formed within the shallow MnS inclusion and active pitting occurred. To explore whether such cracks might serve to generate locally high concentrations of aggressive species, the pH and chloride concentration inside a crack were simulated using a finite difference model. For experimental conditions where stable pitting was observed, the simulations predicted that the solution composition at the base of a typical 13 m deep crack correspond to a pH of around 2, and a chloride concentration of about 6 M led to stable pitting.

Local Electrochemistry and In Situ Microscopy of Pitting at Sensitized Grain Boundary of Type 304 Stainless Steel in NaCl Solution

“…For the solution-treated specimen, many current spikes were generated above 0.4 V, and the current density rapidly increased and exceeded 0.1 A m −2 at 0.67 V. The potential region for MnS dissolution has been reported to be around 0.5 V in NaCl solutions. 13,14,[17][18][19]24 The many current spikes around 0.5 V were due to the dissolution of the sulfide inclusions, suggesting that the inclusions were likely the initiation sites for pitting. 17 In the case of the sensitized specimen, a sharp increase in the current density appeared suddenly at 0.32 V. The pitting potential of the sensitized specimens were determined to be around 0.3 V.…”

“…Microelectrochemical measurements are powerful techniques for the analysis of the electrochemical properties of a small area of a material. [13][14][15] However, the occurrence of crevice corrosion between a sensitized grain boundary and the masking of the electrode area was expected. A micro-TIG (tungsten inert gas) welding technique was applied to prevent crevice corrosion.…”

A Summary of Corrosion Properties of Al-Rich Solid Solution and Secondary Phase Particles in Al Alloys

“…Furthermore, these basic corrosion data can be used to simulate pit initiation and propagation. One very successful method for determining the corrosion characteristics of secondary phase particles is the electrochemical microcell method, which uses a small capillary filled with electrolyte, the tip of which permits electrochemical analysis of specific particles [11,[35][36][37][38][39][40]. This method has been employed with both synthesized intermetallic compounds (IMCs) and/or actual alloy substrates [11,17,34,[41][42][43][44][45][46][47].…”

Interstitial Carbon Enhanced Corrosion Resistance of Fe-33Mn-xC Austenitic Steels: Inhibition of Anodic Dissolution

“…Anodic polarization measurements of the specimens with an exposed area of around 1 cm 2 indicated an increase in the pitting corrosion potential of the carburized stainless steels with interstitial carbon compared to untreated stainless steels. 1,[3][4][5] The microscopic anodic polarization measurements of the carburized stainless steel with a micro-scale electrode area (around 150 μm × 300 μm) containing an MnS inclusion, which is known to act as an initiation site of the pitting of stainless steels, [10][11][12][13][14][15][16][17][18][19][20][21] demonstrated that no pitting was initiated on the carburized stainless steel despite the presence of the MnS inclusion. 6 The micro-scale electrode areas were in situ observed by an optical microscope with a water immersion objective lens (a lateral resolution of ca.…”