Pit Initiation Research Articles

Effect of calcium on X65 carbon steel pitting in saturated CO2 environment

Micro-pits could propagate during in-service life in facilities such as crude oil storage tanks in which calcium salts are present. Calcium is known to influence pit initiation and propagation but explanation on pitting kinetics on low carbon steel remains debatable. More so, knowledge is lacking on how different CaCO3 polymorphs influence pitting in low carbon steel. The effect of calcium on pitting in UNS K03014 (X65) carbon steel was investigated using potentiostatic induced pits in an environment typical of crude oil storage tanks and liquid aquifers. SEM, EDX and XRD investigations were performed to profile formed corrosion products followed by surface profilometry to determine the extent of pit growth. It was revealed that Ca2+influence general and pitting corrosion while the kinetics of pit propagation depends on the nature of the calcium carbonate polymorph that forms on steel surface. Pit propagation was high when amorphous CaCO3 deposit formed at pH 3.8 but pit growth retarded as aragonite started to grow on the steel surface.

The Effects of Pits Size and Shape on Fatigue Crack Initiation

Generate and monitor the growth of corrosion pits on the surface of 316L stainless steel using the scanning droplet cell technique and assess the effects of varies factors such as pits size, pits shape, flow rate, and applied stress on pit initiation and growth. Configure of a test system to measure the pit and short fatigue crack growth behaviour, to investigate each stage of the corrosion fatigue damage process occurring at the material surface and compare this with the modified film surface using alternating voltage passivation process (AVPP). Understand the effects of corrosion on cracking behaviour during the different stages of fatigue crack growth.

Effect of Accelerated Cooling on Linepipe Steel Mill Scale and Resulting Localized Corrosion Susceptibility

The structure and composition of mill scale on linepipe steel formed with and without accelerated cooling conditions (ACC) was investigated and correlated to localized corrosion susceptibility. The mill scale structure/composition was investigated using scanning electron microscopy equipped with x-ray energy dispersive spectroscopy and electron backscatter diffraction, as well as x-ray diffraction. Localized dissolution of the mill scale was investigated using electrochemical techniques including open-circuit potential measurements, electrochemical impedance spectroscopy, and electrochemical noise measurements in a corrosive phase solution. The various surface analytical and electrochemical techniques indicated that the mill scale formed without ACC consists of a relatively crack-free, thick inner wüstite layer with a thinner magnetite outer layer. However, the mill scale formed with ACC comprised a magnetite layer containing islands of retained wüstite, with some evidence of magnetite/iron eutectoid formation and which exhibited a relatively high density of through-scale cracks. These cracks can provide direct paths that connect the corrosive solution to the steel substrate, leading to a more rapid breakdown of the mill scale. Additionally, the cracks can form a crevice between the mill scale and the steel surface, providing sites for pit initiation and growth. Coefficient of thermal expansion mismatch thermal stress calculations indicate that a magnetite-based scale is more susceptible to cracking/spalling than a wüstite-based scale, resulting in the ACC plate being more susceptible to localized corrosion.

Experimental study on passivation and corrosion performances of pre-rusted steel in simulated concrete pore solution

ABSTRACT The influences of the rust layer on the passivation and corrosion of Q235b carbon steel in simulated concrete pore solution were studied. The differences between the inner and outer rust layer were characterized. Results show that the pre-rusted samples could still reach passive state at high pH condition free of chloride ions. However, the quality of the passive film formed on the pre-rusted steel were dramatically lower than that formed on the fine polished steel. The pitting corrosion resistance of the pre-rusted steel was significantly higher than that of the fine polished steel in the chloride-containing pore solution. The pitting corrosion potential would be further promoted after the elimination of the outer rust layer. The reaction between the hydrogen ions and the rust layer could inhibit the pitting initiation. Furthermore, the consumption of the corrosive agents induced by the rust layer could further retard the area expansion of the pits.

The pitting to uniform corrosion evolution process promoted by large inclusions in mooring chain steels

The corrosion affects the service life of mooring chain steels in marine environments. The evolution of the corrosion process on mooring chain steels in simulated seawater was investigated through in-situ experiments, and the effect of sulfide-oxide complex inclusions on the pitting initiation is discussed. The matrix surrounding sulfide-oxide complex inclusions is enriched in iron sulfide using secondary ion mass spectrometry. The local dissolution of matrix enriched in iron sulfide triggers pitting corrosion. With the immersion time increasing, the corrosion is developed on the matrix around pits to form corrosion rings, resulting in uniform corrosion when corroded areas are connected. Under corrosion product film, the matrix around inclusions is more severely corroded than other areas to form pits. The corrosion behavior of mooring chain steels is a repeating cycle of pitting to uniform corrosion transition. Large inclusions more promote the process of pitting corrosion initiation than small inclusions and increase the rate of transition from pitting to uniform corrosion. • The corrosion evolution process of mooring chain steels is a repeating cycle of pitting to uniform corrosion transition; • The matrix around inclusions is enriched in iron sulfide, which preferentially dissolves and triggers pitting corrosion; • The average size of sulfide-oxide complex inclusions is larger due to high Mn content in material; • Large inclusions more promote the process of pitting corrosion initiation.

Investigation of the Microstructure Dependence of Critical Pitting Temperature and Pitting Potential in a 2205 Duplex Stainless Steel

Dependence of 2205 duplex stainless steel pitting resistance on its microstructure was investigated using microstructural examinations and potentiodynamic and potentiostatic polarization techniques. An as-received hot-rolled alloy was subjected to different heat treatments to understand how microstructural characteristics individually affect the corrosion resistance in terms of critical pitting temperature (CPT), pitting potential (E pit), and metastable pitting characteristics. Results revealed that there is a transition temperature interval for the 2205 DSS. By promoting pit initiation and pit propagation, the Widmanstätten structure significantly deteriorated both CPT, by 10 °C, and E pit, compared to the as-received hot-rolled specimen. The σ phase appearance was also associated with a minor drop in CPT, but a considerable reduction in E pit as it increased the occurrence frequency of metastable pits. The specimen having equiaxed grains had the highest CPT, underwent stable pitting at 47 °C, but still it had a low E pit compared to the hot-rolled specimen due to its higher occurrence frequency of metastable pits. The formation of the Widmanstätten austenite needles weakened the ferrite phase in terms of the pitting resistance by reducing the contents of Cr and Mo in it and made it more susceptible to pit propagation.

Effect of Mg on Intergranular Corrosion of Al-Cu Alloy

Due to high strength and light weight, 2000 series Al alloys (Al-Cu-Mg) are widely utilized in several applications such as aircrafts and automobiles. However, 2000 series alloys have limitations to use for some applications because corrosion resistance of these alloys is relatively low in commercial Al alloys. The localized corrosion is sometimes initiated at intermetallic particles. Kakinuma et.al. studied the pitting corrosion initiation at Al-Fe-Si intermetallic particles using a micro-scale electrochemical method. They clarified the effect of pH on pitting initiation 1. Also, they found that open circuit potential was changed by pit initiation and propagation 2. In addition to pitting, intergranular corrosion (IGC) should be concerned in the case of 2000 series alloys. It has been known that compositional heterogeneity related to precipitations at grain boundaries affects the occurrence of IGC.It has been known that Mg in 2000 series alloys improves IGC resistance. However, it is not clear whether Mg inhibits the initiation of IGC or the propagation of IGC. In this study, the comparison of IGC on Al-Cu and Al-Cu-Mg alloys was performed.To clarify the effect of Mg on corrosion behavior Al-Cu alloy, open circuit potential of Al-Cu and Al-Cu-Mg alloys in 0.1 M NaCl (pH 6.0) were measured for 1 h. After that, the total number of IGC and length of IGC at each site in electrode area (ca. 1 cm × 1 cm) was measured using an optical microscope. Total number of IGC on Al-Cu-Mg alloy was almost same as that on Al-Cu alloy. On the other hands, total length of IGC on Al-Cu-Mg alloy was two times shorter than that on Al-Cu alloy. It was suggested that Mg effectively suppresses propagation of IGC.Reference Kakinuma, I. Muto, Y. Oya, Y. Kyo, Y. Sugawara, N. Hara: J. Electrochem. Soc., 166(2) (2019), C19-C32Kakinuma, I. Muto, Y. Oya, T. Momii, Y. Sugawara, N. Hara: J. Electrochem. Soc., 168 021504 (2021)

Reexamining the Role of Mns Inclusions in Pitting of Stainless Steel

Most models of pit initiation in austenitic stainless steels involve the anodic dissolution of manganese sulfide (MnS) inclusions. This work reexamined the role of MnS inclusions and looked at whether they are anodic or cathodic to the matrix metal surrounding them. Specifically, pit formation and the initial stages of growth were investigated using potentiodynamic polarization paired with microscopy.Stainless steel (SS) 304 samples were exposed to 0.5 M NaCl and slowly polarized from 0.2 VSCE to 0.4 VSCE. Polarization scans showed evidence of metastable pits and, in some cases, pit growth. Microscopy of sample surfaces revealed many small open pits with an occasional larger covered pit. The small open pits were associated with inclusions of various types that had fallen to the bottom (Figure 1a). The larger covered pits had MnS inclusions that remained attached to the sample surface (Figure 1b). The attached inclusions acted as a pit cover during initial stages of growth. Growth directions indicated by the perforated pit covers usually showed asymmetric enlargement that is likely controlled by local metallurgical conditions such as crystal orientation and grain boundaries.Examination of MnS inclusions in both open and covered pits showed no evidence of significant corrosion of the inclusion. The electrochemical behavior of MnS samples was analyzed subsequently using potentiodynamic polarization scans. MnS was passive in 0.5 M NaCl up to the maximum potential applied, 0.85 VSCE. The open circuit potential of MnS was approximately -0.26 VSCE, which is slightly more active than that for bulk SS 304, and the passive current density ranged from slightly less than to slightly greater than that of SS 304 up to the pitting potential of SS 304.Based on the results from this study, the following model for pit initiation and initial growth is proposed for SS 304 in NaCl solutions: Pits initiate at cathodic inclusions (MnS, oxides, etc.) with corrosion occurring in the matrix metal at the inclusion / metal interface.When corrosion occurs around the entire interfacial area of the inclusion, the inclusion falls into the pit and repassivation can occur.When corrosion does not occur evenly around the inclusion due to local metallurgical conditions (e.g., composition, crystal orientation, grain boundaries, etc.), the inclusion acts as a cover and pit growth can occur. Figure 1

Beneficial Effects of Cerium Addition to Sulfide Inclusions on Pitting Corrosion Resistance of Stainless Steels

The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. The chemical composition of the inclusions affects the pitting corrosion resistance of stainless steels. It is known that the inclusion solubility in aqueous solutions affects pitting corrosion resistance. The dissolution of sulfide inclusions produces S species and exposes the bare surface of the steel matrix, resulting in high pitting susceptibility near the inclusions.1 Cerium is expected to be an additive element for insolubilizing water-soluble inclusions because Ce tends to form sulfides and Ce-sulfides exhibit low solubility in aqueous solutions. Additionally, Ce ions have been shown to be effective in the corrosion inhibition of stainless steel. In this study, a microelectrochemical technique was applied to analyze the effect of Ce addition to sulfide inclusions on the pitting corrosion resistance of stainless steels.2,3 To analyze the effect of Ce addition on water solubility of CaS inclusions, spark plasma sintering was used in the facile fabrication of stainless steel specimens containing artificial Ce-added and Ce-free inclusions.3,4,5 The pit initiation resistance at the Ce-added CaS inclusions was higher than that at the Ce-free CaS inclusions. The surface of the Ce-free CaS inclusions largely dissolved during immersion in 0.1 M NaCl, while a large part of the Ce-added CaS inclusions remained undissolved. Thermodynamic calculation showed that CeS is less soluble than CaS.To analyze the effect of Ce ions on pitting at sulfide inclusions, a type 304 stainless steel (Fe-18Cr-8Ni-1.5Mn-0.02S) was prepared by vacuum induction melting. Manganese sulfide inclusions existed in the specimens. Polarization curves were measured in 3 M NaCl and 2.97 M NaCl-0.01 M CeCl3. Pitting occurred at the MnS inclusions in the Ce3+-free solution, whereas no stable pit was generated in the Ce3+-containing solution even when metastable pitting events were observed.It was found that Ce addition inhibits the dissolution of water-soluble inclusions and improved pitting corrosion resistance. Moreover, the Ce3+ ions are expected to inhibit the pitting even after the dissolution of Ce-added inclusions.

Micro-electrochemical Analysis of Pit Initiation at Inclusions of Martensitic Stainless Steel

Martensitic stainless steels are widely used for various machine parts because of their excellent mechanical properties. However, pitting corrosion sometimes occurs in chloride solutions such as seawater. Understanding the mechanism of pit initiation is required. In this study, a micro-electrochemical system equipped with an optical microscope [1] was applied to the real-time observation of pit initiation of a martensitic stainless steel.Re-sulfurized AISI 420 (Fe-13% Cr-0.3% C-0.02% S) stainless steel was prepared by vacuum melting. To obtain a martensite structure, a heat-treatment was conducted at 1273 K for 5 h in Ar atmosphere, followed by air-cooling. The details of the experimental setup and procedure for micro-electrochemical measurements were described in our previous paper [1-3]. Electrochemical measurements were performed in boric acid-borate buffer solutions (pH 8.0) with and without NaCl at 298 K.Polarization curves of a small working electrode with a MnS inclusion were measured in the boric-borate buffer solution without NaCl. A broad current peak was observed around 0.75 V (vs. Ag/AgCl, 3.33 M KCl). In this potential region, discoloration occurred on the inclusion surface. In the case of a small area without inclusion, no current increase was seen around 0.75 V. Therefore, the increase in current around 0.75 V was due to the dissolution of MnS. During micro-scale polarization in the boric-borate buffer solution with 10 mM NaCl, the inclusion surface was observed to dissolve around 0.73 V, and then the current increased. It was observed that pitting corrosion was generated at the same time as the current increased. The dissolution potential of MnS in AISI 420 was determined to be around 0.7 V in the boric-borate buffer solution at pH 8.0, and it was concluded that the MnS dissolution results in pit initiation.

Pitting corrosion behavior of friction-surfaced 17-4PH stainless steel coatings with and without subsequent heat treatment

Improvement in pitting corrosion resistance of the friction-surfaced (FSed) 17-4PH stainless steel coatings was mainly attributed to fragmentation of sulfide inclusions during friction surfacing (FS). Formation of vermicular δ-ferrite and grain refinement of martensitic matrix around the δ-ferrite affected the distribution of pits. FS parameters had a negligible effect on the pitting corrosion resistance of the FSed 17-4PH coatings. However, the decrease in pitting corrosion resistance of the FSed coatings after aging (H900) was due to the higher dislocation density for facilitating the metastable pit initiation and promoting the transition of metastable pits to the stable ones.

Effect of Sensitization on Pitting Corrosion at MnS and CrS in Type 304 Stainless Steel

The effect of sensitization on the pitting potentials at MnS and CrS inclusions in Type 304 stainless steels was assessed. For Type 304 containing MnS inclusions, MnS dissolution was observed approximately 0.3 V, and the pitting potential at MnS was in the passive region and decreased by sensitization. Based on the pit morphology, the pit initiation at MnS inclusions is thought to be caused by the transition from passive to active state of the steel matrix. In contrast, the pitting potential at CrS inclusions was located in the transpassive region, and no decrease in the pitting potential was observed due to sensitization. These mechanisms were analyzed based on the effect of the Cr concentration on the active dissolution and transpassive dissolution of the steel.

A study on localized corrosion behavior of 304 stainless steel in the presence of Allium Sativum extract inhibitor using electrochemical noise analysis

The present work aimed to investigate the Allium Sativum (garlic) extract as a localized corrosion inhibitor for AISI 304 stainless steel in hydrochloric acid medium. The Allium Sativum extract was characterized by FTIR and UV–visible spectroscopy. The corrosion resistance of the 304 stainless steel was studied using electrochemical noise (EN), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). According to the EN analysis, it was demonstrated that the addition of the inhibitor up to the optimum amount (8 cc L−1) increased the spectral noise resistance. The shot noise results illustrated that the distribution of the frequency of events shifts to a higher frequency region in the presence of Allium Sativum inhibitor, which is an indicator of less localized corrosion of stainless steel. Stochastic analysis for pit initiation and growth showed that although the pits initiation rate increased in the presence of the Allium Sativum extract because of its antioxidant nature, the formation of stable pits with a radius larger than 30 μm was significantly inhibited due to the filling of the metastable pits with phosphate compounds. Microscopic corrosion morphology confirmed the idea of the formation of phosphate compounds on the pits nuclei and re-passivation of the steel surface.

Pitting at inclusions of the equiatomic CoCrFeMnNi alloy and improving corrosion resistance by potentiodynamic polarization in H2SO4

The pitting initiation site of an arc-melted equimolar CoCrFeMnNi alloy was investigated, and a method to improve the pitting corrosion resistance of the alloy was proposed. Cr–Mn–O inclusions with S-containing segments on the outer sections were observed; these S-containing segments served as the pitting initiation sites. The S-containing inclusions were removed by potentiostatic polarization and immersion in H2SO4, but pitting corrosion resistance after treatment did not improve. By potentiodynamic polarization in H2SO4, the S-containing inclusions were removed and the passive film on the alloy matrix became Cr enriched, resulting in a significant improvement in the pitting corrosion resistance.

Stability of Current Collectors Against Corrosion in APC Electrolyte for Rechargeable Mg Battery

Rechargeable magnesium batteries (RMBs) are highly attractive due to their high volumetric capacity, relatively low cost, and enhanced safety. Significant progress in the development of RMBs was the introduction of nonaqueous electrolyte solutions that enable reversible electrodeposition of Mg metal. These solutions contain a unique mixture of organo−aluminum and chloro−aluminum species. While these solutions are shown to be stable during cathodic polarization, the presence of chlorine anions in the solution can promote the corrosion process during the anodic polarization. Among all the cell components, the cathode current collector is most prone to corrosive processes. In this study, we characterize the corrosion behavior of different metallic current collectors in standard APC (All-Phenyl Complex) electrolyte solutions by following their electrochemical response and surface morphology changes during anodic polarization. In addition, we investigate the influence of carbon coating on the corrosive behavior of Ni metal. The study shows the efficacy of carbon coating on the current collector. We demonstrate that Ni@C current collector possesses higher corrosion pit initiation potential (2.25 V vs Mg/Mg2+) with respect to the uncoated metallic foils. Improving the anodic stability of the current collectors is a crucial step in the development of a practical rechargeable Mg battery.

Correlation between hyperfine structure of inclusion and localized corrosion mechanism of DSS2101 with Ce microalloying in simulated marine environment

The discrepancies in inclusion characteristics and localized corrosion behaviour and mechanism induced by inclusions in DSS2101 are contrastively investigated. We found that the two types of inclusions consisted of (Al, Mn, Cr)–O–S and (Ce, Al, Mn)–O–S. Numerous twins and denser entangled dislocations were distributed near the (Al, Mn, Cr)–O–S/matrix. The distorted and normal BCC phase exhibited the {1‾10}N∥{1‾12}D orientation relationship, whereas severe lattice expansion existed along the {01‾2} and {1‾20} crystallographic directions, presumably resulting in the priority initiation of pitting. Besides, the (Al, Mn, Cr)–O, (Al, Mn)–O core and CeAlO3 shell had especially poor electrical conductivity, making formation of the galvanic couple difficult. However, the deformed matrix served as an anode relative to the normal matrix (as a cathode), facilitating pitting rate of Ce-free steel. In contrast, the electrochemical reaction of MnS (as an anode) and the chemical reaction of the CeAlO3 shell protected the adjacent matrix (as a cathode) from erosion by self-dissolution in Ce-modified steel. The synergistic effect of the differential aeration cell (DAC) and occluded corrosion cavity (OCC) was supposed to deteriorate the microcrevice corrosion and galvanic corrosion.

Establishing first order correlations between pitting corrosion initiation and local microstructure in AA5083 using automated image analysis

A rapid image segmentation and correlation-based approach to investigating the local microstructural effects on pitting corrosion susceptibility is introduced. The impact of four different microstructural features, consisting of grain orientation, proximity to grain boundaries, proximity to intermetallic particles, and local geometrically necessary dislocation (GND) density, on pitting initiation in the 5083 aluminum alloy was directly investigated using automated pit detection processing in combination with electron microscopy-based characterization. Three different thermomechanical treatments were used to vary the system microstructure and samples were immersed in 3.5 wt% NaCl aqueous solution for 24 h to induce pitting. GND density and proximity to intermetallic particle both showed positive correlations, while grain orientation and proximity to grain boundary showed no correlation with pit initiation sites.

In-situ monitoring of pitting corrosion of AZ31 magnesium alloy by combining electrochemical noise and acoustic emission techniques

Pitting corrosion of AZ31 magnesium (Mg) alloy was in-situ monitored by combining electrochemical noise (EN) and acoustic emission (AE) techniques. The EN signals were analyzed by transient shape observation, Hilbert spectrum and recurrence quantification analysis (RQA) based on recurrence plot (RP). AE characteristic parameters including rise time, amplitude, absolute energy and peak count were adopted to classify the AE signals. According to the mutual information obtained by EN and AE measurements, three corrosion stages including film-forming process, pit initiation and pit propagation can be identified. The detailed relationship between the AE and EN parameters and different corrosion mechanisms is discussed. The main contribution of present study is to provide a feasible method to effectively monitor the self-corrosion process of Mg alloy.

Temperature-dependent corrosion behaviour of the amorphous steel in simulated wet storage environment of spent nuclear fuels

Temperature dependence of corrosion behaviour in simulated wet storage conditions for spent nuclear fuels were investigated systematically by comparing SAM2X5 amorphous steel and 316 L SS. Results indicated that with elevating temperature, the passive current for the amorphous steel increased distinctly but without pitting corrosion. The sensitivity of the passive current with temperature for the amorphous steel was related to the reduction of Cr2O3 content and altered semiconductor characteristics for its passive film, and the prominent pitting resistance at higher temperatures was associated with the remarkably inhibited pit initiation by the uniform passive film and the suppressed stable growth of metastable pits.

Electrochemical study and molecular dynamics (MD) simulation of aluminum in the presence of garlic extract as a green inhibitor

The aim of this study was to investigate the effect of garlic extract as a green inhibitor on the uniform and localized corrosion of 1050 aluminum alloy in aerated 3.5 wt% NaCl solution. In this regard, electrochemical methods such as linear polarization, electrochemical impedance spectroscopy (EIS), and electrochemical noise analysis (EN), were carried out to study the corrosion behavior. The theoretical approaches based on molecular dynamics simulation were also applied to obtain precise insight into the interfacial interactions and adsorption mechanism. EN measurements revealed that the addition of garlic extract increases the pit initiation rate by forming an antioxidant layer on the Al surface. However, the extract's chemical adsorption enhances over time, thereby the propagation of corrosion damage reduces up to one-tenth. EIS results showed that, the garlic extract with 8 mL·L−1 concentration induces the most effective inhibition by enhancing the corrosion resistance from 2917 Ω·cm2 to 14,557 Ω·cm2. The outcomes of MD simulation indicated that the pyridoxal phosphate has not only the parallel adsorption on the surface but also the most negative bonding energy. These phenomena are the factors behind the anticorrosive behavior of garlic extract on the Al surface.