Direct Observation of Pit Initiation Process on Type 304 Stainless Steel

Self Cite

The initiation site and morphology during the early stage of pitting on AISI 1045 carbon steel that has a microstructure of primary ferrite and pearlite were investigated in boric-borate buffer solutions with and without NaCl at pH 8.0. The pits initiated by micro-scale polarization were in the pearlite only and not in primary ferrite. In situ real-time observations during the micro-scale polarization of pearlite in a boric-borate buffer solution with 100 mM NaCl indicated that the pits were polygonal or rod-like in shape. In addition, it was found that the pit growth direction was the same as that of the pearlite lamellae that consisted of ferrite and cementite. Field-emission electron probe micro analysis detected segregated points of sulfur in the ferrite lamellae. On the basis of their etching behavior in 3% nital, the corrosion resistance of the cementite was estimated to be higher than that of the ferrite lamellar structure. Thus, pits readily initiated in the ferrite lamellae and proceeded along the ferrite lamellae. Ferrite-pearlite steel is widely used in the production of bars, plates, steel wires, etc.1-3 because of its high strength, ductility, toughness, wear resistance, and low cost.3-5 Pearlite has a lamellar structure that consists of ferrite and cementite (Fe 3 C) phases. A fine pearlite structure improves the mechanical properties of steels in terms of strength and ductility.6-8 While ferrite-pearlite steels exhibit excellent mechanical properties, their pitting corrosion resistance is relatively low. It is widely believed that the boundaries between the different phases act as initiation sites for localized corrosion in chloride environments. 9,10When ferrite-pearlite steel that does not undergo any surface treatment is exposed to chloride environments, the steel readily suffers from pitting corrosion. Although the steel is successfully protected from corrosion by coating and/or painting, to prolong their service life and improve their reliability, it is necessary to elucidate the initiation mechanism of pitting on ferrite-pearlite steel.The pitting corrosion process involves two stages: initiation and propagation.11 In chloride environments, pitting is initiated by a breakdown of the passive film on steel during a local active dissolution process that exposes the bare steel surface to the environment. This film-free dissolution involves the hydrolysis reaction of metal cations and results in acidification. Chloride ions migrate into the pit to maintain electrical neutrality, and the simultaneous accumulation of chloride ions combined with the high level of acidification accelerates the active dissolution inside the pit. There have been many reports about the propagation stage of pitting corrosion in ferrite-pearlite steels. A comparison of the active dissolution rates and the electrochemical properties of ferrite and cementite have been widely discussed. It is commonly believed that cementite has a lower dissolution rate than ferrite. Yumoto et al. synthesized stoichiometric Fe 3 C films with...

Section: Resultsmentioning

confidence: 99%

Real-Time Microelectrochemical Observations of Very Early Stage Pitting on Ferrite-Pearlite Steel in Chloride Solutions

Kadowaki

Muto

Sugawara

et al. 2017

Self Cite

“…However, the pit generation preceded the intergranular corrosion; therefore, the predominant factor is thought to be the existence of the inclusion at the grain boundaries. Chiba et al 18,19,29 proposed the pit initiation mechanisms at MnS inclusions in stainless steels. When MnS inclusions dissolve, the pH on and around the MnS is thought to decrease, 30,31 and is followed by the precipitation of elemental S. 19 In chloride solutions, the depassivation of the boundary between the inclusion and steel matrix occurs due to the synergistic effect of Cl − ions and elemental S. The trench is generated as selective dissolution at the boundary of the inclusions and steel matrix, and a pit is initiated inside the trench as a local transition from passive to active state.…”

Section: Effect Of Electrode Size On Pit Initiation-comparison Ofmentioning

confidence: 99%

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

Ida

Muto

Sugawara

et al. 2017

Self Cite

To elucidate the pit initiation behavior of sensitized stainless steels, the anodic polarization of a single grain boundary was examined in 0.1 M NaCl (pH 5.4) at 298 K using a micro-electrochemical system. For Type 304 heat-treated at 923 K for 2 h, no pitting was initiated on a small area (ca. 100 μm × 100 μm) with a sensitized grain boundary without MnS inclusions. However, stable pitting was observed on the electrode area that was larger than ca. 200 μm × 200 μm. In situ microscopy revealed that the first step in corrosion was a spherical pit generated at a MnS inclusion at a sensitized grain boundary, and that intergranular corrosion started at the pit. The local depassivation of the Cr-depleted zone along the sensitized grain boundary was thought to be introduced by the dissolution of the inclusion. The co-existence of the MnS inclusion and the Cr-depleted zone was considered to be the critical factor in the pit initiation of sensitized stainless steels in NaCl solutions. Stainless steels are widely used in chloride environments because of their high corrosion resistance, which is attributed to their high Cr content, at above 12 mass%.1 When stainless steels are heated to around 900 K (e.g., welding), sensitization occurs due to the formation of Cr-depleted zones at the grain boundaries. The susceptibility to intergranular corrosion and pitting at the grain boundaries increases as a result of sensitization-treatments. 2To assess the degree of sensitization and/or the mechanisms of intergranular corrosion, acidic solutions are usually used. ASTM A262 testing specification contains the following intergranular corrosion tests: 1) Huey test (boiling HNO 3 ), 2) Strauss test (boiling H 2 SO 4 + CuSO 4 + metallic Cu), 3) and Streicher test (boiling H 2 SO 4 + Fe 2 (SO 4 ) 3 ). After immersion, the specimens are visually examined and/or measured for weight loss. These methods have been widely and successfully used in corrosion engineering. ISO 12732 specifies a method that uses the double loop electrochemical potentiodynamic reactivation test (based onĈihal's method 3 ). In this method, the standard solution is 0.5 M H 2 SO 4 -0.01 M KSCN, and the degree of sensitization can be evaluated quantitatively from the comparison of the peak current densities between the anodic scan and the subsequent cathodic scan.Although acidic solutions are generally used to evaluate the degree of sensitization, in practical applications, pitting and stress corrosion cracking occur in stainless steels in near-neutral pH environments. NaCl solutions are used to evaluate the pitting corrosion resistance and the susceptibility of sensitized stainless steels to stress corrosion cracking (SCC). Cheng et al. studied the pitting corrosion of sensitized Type 304 stainless steel under wet-dry cycling and demonstrated that both the probability of pitting and the average size of the pit increased as a result of sensitization. 4 It was suggested that the observed change in the pit morphology from round to an irregular shape could be attributed t...

“…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.…”

mentioning

confidence: 99%

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

Chiba

Koyama

Akiyama

et al. 2018

Self Cite

Five Fe-33Mn-xC steels, referred to as 0 C, 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels according to their carbon content in mass%, were prepared to clarify the effect of interstitial carbon on the dissolution behavior of steel. The 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels indicated a fully austenitic structure with no carbide precipitate. The lattice parameters of the 0.6 C, 0.8 C, and 1.1 C steels calculated from the γ(111) and γ(200) diffraction peaks increased by up to around 0.8% over that of the 0.3 C steel, suggesting that the added carbon was present as interstitial carbon in the steels. The 0.6 C, 0.8 C, and 1.1 C steels were passivated during the anodic polarization measurements in 0.1 M Na 2 SO 4 solution at pH 12.0, whereas the 0 C and 0.3 C steels actively dissolved. The anodic polarization measurements in a buffer solution at pH 10.0 demonstrated a lower dissolution current density for the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels with higher amounts of interstitial carbon. The dissolution current density at 0.3 V vs. Ag/AgCl (3.33 M KCl) of the 1.1 C steel was reduced to approximately 1 × 10 −2 A m −2 , which was one hundredth that of the 0.3 C steel. The dissolution current density of the steels was not inhibited by the presence of 0.1 M CO 3 2− ions, which is an expected dissolution product of interstitial carbon, implying that the interstitial carbon improved the electrochemical property of the steels themselves. The work function of the 1.1 C steel, which showed improved corrosion resistance with interstitial carbon, was 0.12 eV lower than that of the 0 C steel. Interstitial carbon has been reported to successfully improve the corrosion resistance of austenitic stainless steels. Low-temperature carburizing treatments have been typically applied to introduce a substantial amount of interstitial carbon at the surface region of austenitic stainless steels to form a carbide precipitation free carbon super saturated solid solution layer. [1][2][3][4][5][6][7][8][9] The carburizing treatments are known to dramatically improve the pitting corrosion resistance of austenitic stainless steels in chloride-containing environments. 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, 10-21 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. 350 nm)19,22 during the microscopic anodic polarization. The microscopic polarization measurements revealed that the carbur...