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

Sugawara

et al. 2017

Self Cite

The pitting corrosion resistance of AISI 1045 carbon steel with as-quenched, tempered, and low-carbon martensitic microstructures was investigated in boric-borate buffer solutions with and without NaCl. Analysis by micro-scale polarization found that tempering and decarburizing treatments decreased the pitting corrosion resistance of as-quenched martensite. The high corrosion resistance of the as-quenched martensite was likely due to the large amount of interstitial carbon. The pitting corrosion resistances of asquenched martensite, primary ferrite, and pearlite were compared using micro-scale polarization measurements. It was determined that the pitting corrosion resistances of the typical steel structures were ordered as follows: (high) as-quenched martensite > tempered martensite ≈ primary ferrite > pearlite (low). The pitting corrosion resistance of steel was shown to depend on its microstructure. © The Author ( Background.-Quenched and tempered martensitic carbon steels are known for their high strength.1-5 The supersaturation of interstitial carbon and high dislocation density provide martensitic steels with their high strength. 4 However, the toughness of as-quenched martensitic steels is relatively low. To optimize the balance between strength and toughness, the steels are subjected to tempering.1 Tempering involves heating the steels so that the non-equilibrium microstructure can return to near-equilibrium conditions. 4 However, tempering also lowers the dislocation density due to the recovery and recrystallization processes.2-4 It has also been shown that the precipitation of carbides results in a decrease in the amount of interstitial carbon.1 While tempered martensitic carbon steels have excellent mechanical properties, the effect of tempering on the pitting corrosion behavior of carbon steels is unclear. While such steels can be successfully protected from corrosion by coating and/or painting, localized corrosion is readily initiated at cut edges and in coating defect areas in atmospheric environments. To prolong their service life and improve their reliability, it is necessary to elucidate the mechanism of pitting corrosion and assess the pitting corrosion resistance of as-quenched, and quenched and tempered martensitic steels.Fundamental research on the corrosion mechanisms of carbon steels started during the 1950s. Stern found that the addition of carbon (0.11 mass% C) increased the corrosion rate of pure iron in 4% NaCl at pH 1 and 2. 6 This increase in the corrosion rate was explained by the decrease in the hydrogen overvoltage due to the addition of carbon. It is known that iron carbides (Fe 3 C) act as cathodic sites.

Section: Resultsmentioning

confidence: 99%

Pitting Corrosion Resistance of Martensite of AISI 1045 Steel and the Beneficial Role of Interstitial Carbon

Kadowaki

Sugawara

et al. 2017

Self Cite

“…No cathodic treatment was performed, and polarization began in the passive region at −0.1 V to inhibit the active dissolution of steel matrix. 36 The cathodic currents were initially measured. The open circuit potentials (OCPs) of the as-quenched specimen, 1 h tempered specimen, 15 h tempered specimen, and 10 h tempered specimen were −0.07 V, −0.09 V, 0.00 V, and 0.21 V, respectively.…”

Section: Journal Of the Electrochemical Societymentioning

confidence: 99%

“…It appears that the corrosion resistance of the steel matrix was lower than that of the carbides. Kadowaki et al conducted in situ observations during micro-scale polarization of the pearlite structure (ferrite and cementite lamellae) in a boricborate buffer solution containing 100 mM NaCl 36 and found that the cementite (iron carbide) acts as a barrier against the growth of the pits initiated in the ferrite lamellae. In addition to the above explanation, although there were many small carbides in the specimens tempered for at most 10 h, no pit was initiated on these specimens.…”

Section: Journal Of the Electrochemical Societymentioning

confidence: 99%

Improving Pitting Corrosion Resistance at Inclusions and Ductility of a Martensitic Medium-Carbon Steel: Effectiveness of Short-Time Tempering

Kadowaki

Sugawara

et al. 2018

Self Cite

The optimal tempering conditions for a martensitic medium-carbon steel (0.47 mass% C) were investigated in terms of balancing ductility and pitting corrosion resistance. By tempering the as-quenched martensite, its Vickers hardness drastically decreased within 0.1 h, suggesting that ductility was sufficiently recovered by short-time tempering. Based on the results of micro-scale polarization in boric-borate buffer solutions with NaCl (pH 8.0), stable pits were initiated at non-metallic inclusions in the specimens tempered for at least 1 h; however, no stable pit was generated on the 0.1 h tempered and as-quenched specimens. Short-time tempering of martensite was suggested to be a feasible approach to striking an optimal balance between facilitating pitting corrosion resistance and achieving the desired mechanical properties of martensitic carbon steels. To reduce the weight of automobiles and other transportation vehicles, ultra-high strength steels with excellent ductility are necessary. It is desirable that excellent mechanical properties be attained without deteriorating the corrosion resistance. Even if organic or inorganic coatings are used in the actual applications of the steels, bare steel surfaces are exposed at cut edges, scratched surfaces, and other damaged areas of the coatings. Therefore, high corrosion resistance remains a fundamental requirement for ultra-high strength steels.A martensitic structure with fine grains is favored for strengthening steels, and interstitial carbon imparts high strength to martensite. The martensitic structure is formed during shear-dominated transformation via quenching.1 In the phase transformation from austenite to martensite in carbon steels, interstitial carbon atoms in the fcc (facecentered cubic) crystal structure of austenite remain as supersaturated carbon in the bct (body-centered tetragonal) martensitic structure. 1-5Supersaturation of interstitial carbon, a fine structure, and a high dislocation density are introduced by this transformation. 5 To optimize the balance between ductility and strength, the as-quenched martensite is subjected to tempering because the ductility of as-quenched martensitic steels is relatively low.1 During this process, the strength of the steel decreases and the ductility recovers, supplying an optimum balance of strength and ductility. In the case of carbon steel, precipitation of carbides also occurs during tempering. As a result, the concentration of interstitial carbon in martensite decreases, which is associated with changes in both the chemical and mechanical properties. Kadowaki et al. demonstrated that the pitting corrosion resistance of as-quenched martensite (0.47 mass% C) is higher than that of low-carbon martensite prepared by quenching after decarburization treatment.6 Despite the optimum balance between strength and ductility achieved by quenching and tempering treatments, the pitting corrosion resistance decreased during tempering. This seems to be because of the decrease of the interstitial carbon concentration. An...

“…[21][22][23] Because localized corrosion can be stopped at an early stage in micro-scale polarization, a very small pit was obtained. 24 Analyzing the area near the pit initiation point allows the roles of inclusions and segregation of alloying elements in pit initiation to be elucidated.To gain a better understanding of the pitting mechanism at inclusions, the target area for microelectrochemical measurements must be selected carefully. The pit initiation behavior can be successfully analyzed by making a small electrode area with one or a few inclusions.…”

mentioning

confidence: 99%

mentioning

confidence: 99%

Pitting at the δ/γ Boundary of Type 304 Stainless Steel in NaCl Solution: The Role of Oxide Inclusions and Segregation

Otake

Chiba

et al. 2017

Self Cite

The role of inclusion compositions and the pit initiation mechanism at the δ/γ boundary of cast Type 304 stainless steel was investigated in 1 M NaCl. Three types of inclusions were detected: large CaO-SiO 2 -MgO-Al 2 O 3 slag inclusions (Type I) and small SiO 2 -Al 2 O 3 -MnO-Cr 2 O 3 -TiO 2 complex oxide inclusions that were located at the δ/γ boundaries (Type II) or found in the γ phases (Type III). In Types II and III, a small amount of Ca was occasionally detected. In micro-scale polarization, the size of the electrode area was changed from 0.01 to 10 mm 2 , and the number densities for the stable and meta-stable pit initiation sites were estimated to be 0.1 and 1 site mm −2 , respectively. Type II inclusions acted as the pit initiation sites at the δ/γ boundary. At the δ/γ boundary without an inclusion, a meta-stable pit was initiated. The segregation of P and S was detected at the boundary where the pit was initiated, but no accumulation of Si was observed. At a randomly selected boundary of δ/γ on the steel, no clear segregation of P and S was observed, but the Si concentration was two to three times higher than the average Si concentration in the steel. Cast austenitic stainless steels such as ASTM CF8 (18Cr-8Ni, equivalent to Type 304) are widely used in chloride environments due to their superior corrosion resistance.1 However, austenitic cast stainless steels are susceptible to solidification cracking. The primary ferrite solidification mode is required to prevent cracking. 2,3 In the casting process, solidification begins with the δ phase (ferrite), and the δ phase transforms to the γ phase (austenite) after solidification. At room temperature, the retained δ phase generally accounts for approximately 10 vol.% in the γ-matrix. Localized corrosion is known to occur at the δ/γ boundary in the form of intergranular corrosion, stress corrosion cracking, or pitting. 1,4 In cast austenitic stainless steels, pits are known to be preferentially initiated at the δ/γ boundaries. This is attributed to the segregation and/or depletion of alloying elements at the phase boundaries. Gill et al. 5 demonstrated that precipitation of M 23 C 6 (M = Fe, Cr, Mo, etc.) occurs at the δ/γ boundary even in low-carbon stainless steels. It was indicated that the pitting potential of as-welded Type 316L stainless steel in 0.5 M H 2 SO 4 -0.5 M NaCl decreased with increasing heat-input when welding. This was assumed to be the result of the depletion of Cr and Mo in the adjacent regions to the carbides as the heat-input increased. Manning et al. 6 investigated the pitting corrosion of Type 304L stainless steel in 1 M NaCl at pH 4 and found that the most susceptible site for pit initiation is at the δ/γ boundary. They proposed that the P-and/or S-segregation at the boundary increases the dissolution tendency of the passive film on the boundary and that the repassivation kinetics are degraded by the presence of these impurities. Kato et al. 7,8 studied the pitting corrosion behavior of cast 18Cr-8Ni and 19Cr-10Ni-2Mo steels in ...