materials Article Title Synergistic Effect of Al2O3 Inclusion and Pearlite on the Localized Corrosion Evolution Process of Carbon Steel in Marine Environment Chao Liu 1, Xuequn Cheng 1,*, Zeyu Dai 1, Ryan Liu 2, Ziyu Li 1, Liying Cui 1, Mindong Chen 1 and Le Ke 1 1 Institute of Advanced Materials & Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (C.L.); [email protected] (Z.D.); [email protected] (Z.L.); [email protected] (L.C.); [email protected] (M.C.); [email protected] (L.K.) 2 The Calverton School, Huntingtown, MD 20639, USA; [email protected] * Correspondence: [email protected]; Tel.: +86-189-0138-8796 Received: 28 September 2018; Accepted: 9 November 2018; Published: 14 November 2018 Abstract: The initiation and evolution of the localized corrosion in carbon steel were investigated in a simulated marine environment of Xisha Island in the South China Sea. In the initial stage, localized corrosion occurred in the form of corrosion spot. The localized corrosion morphology and electrochemical information during corrosion process were tracked by field emission scanning electron microscopy energy dispersive spectrometry (FE-SEM-EDS), scanning vibrating electrode technique (SVET) and scanning Kelvin probe force microscopy (SKPFM). Localized corrosion was induced by the microcrevices around Al2O3 inclusions. The occluded cells and oxygen concentration cell formed in the pits could accelerate the localized corrosion. Pearlite accelerated the dissolution of the inside and surrounding ferrite via the galvanic effect between Fe3C and ferrite. Overall, the localized corrosion was initiated and evaluated under a synergistic effect of crevice corrosion, occluded cells, oxygen concentration cell and the galvanic couple between FeC3 and ferrite. Keywords: carbon steel; localized corrosion; inclusion; pearlite 1. Introduction High strength steels have been used globally for the construction of road installations, electricity posts, utility towers, guide rails, ornamental sculptures and facades and oil and gas pipelines in the marine environment. During the service processes of steel, localized corrosion is one of the most serious problems, which causes catastrophic failure of metallic structures, especially in the harsh marine environment [1–4]. As a typical form of localized corrosion, pitting has been described as a random, sporadic and stochastic process, making it extremely difficult to predict and control [5]. Pitting has long been explained as a result of the variation of electrochemical homogeneity in materials, such as the appearance of inclusion [2,6–8]. In the initial stage of pitting process, some corrosion spots are formed in the localized corrosion areas on metal surface due to the dissolution of nanoscale secondary ion sites in the stainless steel, such as MnS [9]. A similar phenomenon has also been observed in ultra-low-carbon bainitic steel [10]. A galvanic couple is generally considered to exist between the steel matrix and the MnS inclusions, which is a common inclusion type [10,11]. In carbon steel, MnS can accelerate the dissolution of the matrix as a cathodic phase [10]. In carbon steel, cementite (Fe3C) is cathodic to ferrite [12], thus pitting is observed in the adjacent ferrite [13]. The widespread distribution of Fe3C might result in multiple localized corrosion with limited depth, which is regarded as a kind of localized general corrosion [14]. It has been reported that there is an interactive effect between Materials 2018, 11, 2277; doi:10.3390/ma11112277 www.mdpi.com/journal/materials Materials 2018, 11, 2277 2 of 15 cementite and MnS inclusion on corrosion. Both MnS and cementite are cathodic phase compared to the matrix, and localized corrosion area can form around them as an anodic area. Meanwhile, for each anodic area, there is an associated cathodic area (and current) that supports the anodic reaction [15]. The interactive effect between cementite and MnS is reported to be closely related to the ratio of carbon to sulfur content (C/S) in the steel [11]. Deeper pitting possesses lower C/S ratio in carbon steel. With the development of desulfurization technology during smelting process, the content of sulfur decreased significantly. Hence, MnS inclusion with smaller amount and size is negligible for corrosion effect. Al2O3 inclusion is the most common inclusion in Al-killed steel. We preciously clarified the mechanism of pitting initiation process by Al2O3 inclusion [16]. However, the synergistic effect of Al2O3 inclusion and pearlite on the localized corrosion evolution process of carbon steel remains unclear. Therefore, we thoroughly explored the synergistic effect of Al2O3 inclusion and pearlite on the localized corrosion evolution process of Q460NH steel under a simulative marine environment. Scanning vibrating electrode technique (SVET) and scanning Kelvin probe force microscopy (SKPFM) were used to investigate the localized electrochemical information during corrosion. Field emission—scanning electron microscopy–energy dispersive spectrometry (FE-SEM-EDS) was employed to characterize the microstructures. 2. Experimental 2.1. Specimen Preparation All specimens were made from a carbon steel with a composition (wt %) of 0.03 C, 0.25 Si, 0.1 Mn, 0.011 P, 0.002 S, 0.4 Cu, 1.2 Cr, 0.3 Ni, 0.024 Al, 0.08 Nb and Fe for balance. The specimen with a size of 10 mm × 10 mm × 6 mm was mechanically ground with silicon–carbide papers down to 4000 grit and then polished with a 0.5 µm diamond. The specimens were then ultrasonically rinsed in ethanol. Prior to the microstructure observation, the specimens were mechanically polished and then etched by 4% nital solution. The microstructures of the specimen before and after etching were observed with a JEOL JSM-7100F Field emission scanning electron microscopy (FE-SEM) before and after etching the specimens. The elemental distributions of the inclusions were identified by the energy dispersive spectrometer (EDS). An accelerating voltage of 30 kV, a probe current of 10 nA, and a working distance of 10 mm were fixed for both secondary electrons images and EDS analysis. 2.2. In-Situ Micro-Electrochemical Measurements SKPFM was used to investigate the electrochemical nature of the inclusions in Q460NH steel; in particular, the Volta potential was investigated with respect to the matrix. The SKPFM measurements were conducted at room temperature in air using a commercial atomic force microscope (Park Systems XE-100). Cr/Au-coated tips on conductive cantilevers (NSC36/Cr-Au) with a nominal resonant frequency of about 65 kHz and a nominal spring constant of about 0.6 N/m were used. The thickness of coating on both sides was ~20 nm, and the radius of curvature was ~50 nm. A single-pass methodology was employed, in which the topography and corresponding contact potential difference between the probe and the sample were simultaneously measured. The samples were scanned at a rate of 0.1 Hz. The contact potential signals were inverted to reflect the Volta potential of the surface. SVET (Scanning Vibrating Electrode Technique) measures the local ionic current by scanning about 100 µm above a sample surface with a vibrating probe. It picks up small potential variations between the probe and a reference electrode at certain frequencies with a lock-in amplifier filtering out electrical noise and subsequently converts the potential difference into an ionic current density by using Ohm’s law. The SVET instrument was manufactured by Applicable Electronics, LLC (Sandwich, MA, USA) and controlled by the ASET 2.10 program developed by Science Wares, Inc (Sandwich, MA, USA). A 20 µm diameter platinum black sphere was electrodeposited on the tip. The microelectrode vibrates in two directions, one parallel (y axis) and the other normal (z axis) to the sample surface, sensing the electric field in the two directions; however, in corrosion, the signals for the x vibration Materials 2018, 11, 2277 3 of 15 Materials 2018, 11, x FOR PEER REVIEW 3 of 15 Materials 2018, 11, x FOR PEER REVIEW 3 of 15 the x vibration are seldom used. Further details can be found in the literature [17,18]. In the SVET arethe seldom x vibration used. are Further seldom details used. canFurther be founddetails in can the be literature found in [the17, 18literature]. In the [17,18] SVET. In test, the an SVET about test, an2 about 1–4 mm2 area was selected to track the micro-electrochemical signal. To ensure the 1–4test, mm an areaabout was 1–4 selected mm2 area to trackwas selected the micro-electrochemical to track the micro-electrochemical signal. To ensure signal. the conveniencesTo ensure the of conveniences of measurement, the sample surface was covered with paraffin to isolate unmeasured measurement,conveniences the of samplemeasurement surface, the was sample covered surface with was paraffin covered to isolate with paraffin unmeasured to isolate face unmeasured and solutions. face and solutions. face and solutions. 2.3. Characterization of Corrosion Morphology 2.3. Characterization of Corrosion Morphology 2.3.Immersion Characterization test is of an Corrosion effective Morphology method to observe the corrosion morphology on the material Immersion test is an effective method to observe the corrosion morphology on the material surfaceImmersion [19–21]. An test immersion is an effective solution method consisting to observe of 0.1 wtthe %corrosion NaCl, 0.05 morphology wt % Na2 SOon4 theand material 0.05 wt % surface [19–21]. An immersion solution consisting of 0.1 wt % NaCl, 0.05 wt % Na2SO4 and 0.05 wt % CaClsurface2 was [19 employed–21]. An immersion as a simulated solution solution consisting of theof 0.1 thin wt electrolyte% NaCl, 0.05 film wt % that Na is2SO known4 and 0.05 to formwt % in CaCl2 was employed as a simulated solution of the thin electrolyte film that is known to form in the theCaCl humid2 was atmosphere employed as on a Xishasimulated Island solution in the of South the thin China electrolyte Sea [22 ].film The that corrosion is known morphology to form in the was humid atmosphere on Xisha Island in the South China Sea [22].