Pit Generation Research Articles

Synergistic effects of CO2 and H2S on stress corrosion cracking of stainless steel 254SMo in extremely aggressive oilfield environment

Synergistic effects of CO2 and H2S on stress corrosion cracking of stainless steel 254SMo in extremely aggressive oilfield environments were explored through experiments and water chemistry modelling calculations. Fracture morphology reveals transition from ductile fracture to plastic fracture characteristics. Initiation of fracture was attributed to pitting corrosion, and depth of the pits was consistent with the Gumbel distribution. Presence of CO2 and H2S exhibits a coupling effect on stress corrosion cracking of stainless steel 254SMo. Ionization of H2S promoted by CO2 produced more HS− and S2− with increasing temperature and pressure, accelerated pit generation on the surface, and increased susceptibility to stress corrosion cracking.

Experimental study on the axial compression behavior of circular steel tube short columns under coastal environmental corrosion

This study adopts experimental, numerical, and algorithmic methods to investigate the compressive behavior and failure mode of corroded circular steel tube short columns. The study establishes an equivalent relationship between electrochemical accelerated corrosion and natural coastal corrosion environments. The effects of current intensity, sodium chloride concentration, and energizing time on corrosion equivalent duration and mechanical properties of specimens are analyzed. The results demonstrate that the ultimate bearing capacity and energy absorption capacity of the specimens are significantly reduced with increasing current intensity and energized time. However, the NaCl concentration has minimal effect on the mechanical properties of the specimens. After natural corrosion in a coastal area with medium salinity for 19.2 years, the ultimate bearing capacity, ductility, initial stiffness, and energy absorption capacity of circular steel tube short columns decreased by 49.5 %, 63.1 %, 48.9 %, and 85.6 %, respectively. A novel corrosion pit generation algorithm is suggested to accurately simulate the distribution of corrosion pits and determine the ultimate bearing capacity of circular steel tube short columns. This algorithm demonstrates improved accuracy and stability compared to traditional simplified methods. Subsequently, a corrosion pit random generation model is developed using this method, and the accuracy of the finite element model is validated against experimental results. Furthermore, the impact of the diameter thickness ratio on the ultimate bearing capacity of corrosion specimens is thoroughly analyzed.

Comparative research of rock breaking by heat effect of continuous and pulse laser with experiment and simulation

Laser rock breaking is a novel rock breaking technology to raise the rock breaking efficiency in deep and ultra-deep strata. Continuous and pulse lasers are two types of laser that are usually used in laser rock breaking research at present. However, the present studies’ main focus is on the breaking mechanism and character optimization of continuous laser. Studies to pulse laser and the differences between the two types of laser are rarely reported. The pulse and continuous laser with 800 W power were used in the laser rock breaking experiment, and the morphology of ablation pits was measured and compared. Then, a FEM simulation model was established by the results of the experiment. The distribution condition and the evolution process of temperature, ablation pit, yield condition, and equivalent strain at test points generated by two types of laser were measured and compared. The experiment results show a thick enamel layer covering the ablation pit generated by the continuous laser, which referring a higher ablation ability but with a shallower ablation pit in measurement. And the simulation results not only represent the higher ablation ability of continuous laser at the end of simulation but also show the circular heating–cooling process, stepping expansion process of ablation pit, faster ablation pit generation speed (0.03 s earlier), higher yield volume, and higher equivalent plastic strain (about three times higher) in radiated part with a pulse laser. Referring that it has higher ablation ability in short radiation time, stronger thermal stress effects on the radiated part, and less molten materials generated in the laser rock breaking process than continuous laser. This research can offer some references to the laser type selection in laser rock breaking and explain the rock breaking theory more comprehensively.

Clathrin-mediated endocytosis cooperates with bulk endocytosis to generate vesicles

SummaryClathrin-mediated endocytosis, the most prominent endocytic mode, is thought to be generated primarily from relatively flat patches of the plasma membrane. By employing conventional and platinum replica electron microscopy and super-resolution STED microscopy in neuroendocrine chromaffin cells, we found that large Ω-shaped or dome-shaped plasma membrane invaginations, previously thought of as the precursor of bulk endocytosis, are primary sites for clathrin-coated pit generation after depolarization. Clathrin-coated pits are more densely packed at invaginations rather than flat membranes, suggesting that invaginations are preferred sites for clathrin-coated pit formation, likely because their positive curvature facilitates coated-pit formation. Thus, clathrin-mediated endocytosis closely collaborates with bulk endocytosis to enhance endocytic capacity in active secretory cells. This direct collaboration between two classically independent endocytic pathways is of broad importance given the central role of both clathrin-mediated endocytosis and bulk endocytosis in neurons, endocrine cells, immune cells, and many other cell types throughout the body.

Phase Planning for Open Pit Coal Mines through Nested Pit Generation and Dynamic Programming

This paper presents a phase planning method specially designed for coal deposits with nearly horizontal, bedded coal seams. The geology of this type of deposit is modeled into a column model, instead of a block model, to avoid coal-rock mixing in blocks. A nested pit generation algorithm is developed for producing a series of nested, least-strip ratio pits with a column model as its input. The algorithm completely overcomes the troublesome gap problem. Taking the least-strip ratio pits as possible phase states, a dynamic programming formulation is proposed to simultaneously optimize the number of phases, the phase-pits, and the ultimate pit, with an objective of maximizing the net present value. The merits and capability of the proposed method are demonstrated through a case study on a large coal deposit.

Effect of Molybdenum on Pit Initiation Rate and Pit Growth Using Electrochemical Noise and Its Correlation with Confocal Laser Scanning Microscopic Studies

The effect of Mo concentration on pit initiation rate and pit growth in austenitic stainless steels (SS) were investigated by electrochemical noise (EN) and confocal laser scanning microscopy (CLSM) techniques for the first time. We used 304LN, 316LN and 317LN containing different concentrations of Mo (0.02, 2.53 and 3.58 wt.%) for our studies. Using EN technique, initiation of pits and growth of pits were analyzed using Weibull and Gumbel distribution function, respectively. Pit depth was obtained using CLSM and the correlation between pit aspect ratio and Mo concentration were studied. Weibull probability plots showed that the Mo present in the alloy reduces the pit generation rates and improves the passivity. The plot for the pit size distribution using Gumbel distribution function showed the lowest metastable pit in 317LN and the highest in 304LN, suggesting the improvement in the pitting corrosion resistance due to Mo addition. The CLSM imaging showed maximum pit depth for specimen 304LN and minimum for specimen 317LN SS. Specimen 316LN SS showed intermediate pit depth. The depth of the pits observed in 304LN, 316LN and 317LN ranged from 80-100 µm, 30-40 µm and 20-30 µm, respectively. Alloy 317LN containing highest Mo concentration (3.58 wt.%) showed the lowest pit aspect ratio values followed by alloy 316LN with 2.53 wt.% Mo. These results indicate that Mo present in the stainless steel helps in arresting the pit growth and improve the resistance to pitting corrosion.

Influence of the individual microstructural features on pitting corrosion in type 304 austenitic stainless steel

The individual role of microstructural features viz. grain boundary character distribution (GBCD), grain size and residual strain on pitting corrosion is assessed in type 304 stainless steel employing potentiodynamic and potentiostatic polarization tests. Greater fraction of Σ3n (n ≤ 3) boundaries and J3-type triple junctions have resulted in better pitting resistance in the optimized GBCD specimen. The presence of higher dislocation density in the deformed specimen has resulted in greater pit generation rate. The fine-grained microstructure has exhibited inferior pitting resistance due to its higher random high angle boundary surface area which serve as preferential sites for nucleation and growth of pits.

Corrosion of Al xCoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior

The present work investigates the influence of Al-content and potential-scan-rate on stable/metastable pitting of Al xCoCrFeNi high-entropy alloys in a 3.5wt.% NaCl solution. The increased Al content causes the volume fraction of the Cr-depleted phase to increase, resulting in thicker/dispersive passive films that degrade the localized corrosion resistance. The varied stable/metastable pitting behavior under different potential scan rates indicates that pit generation is influenced by the properties of passive films and the interactions between chloride ions and the active sites. Furthermore, a mean-field theory is employed to predict the scan-rate dependent, power-law distribution of current fluctuations.

Corrosion behavior of Al–3.0 wt%Mg alloy in NaCl solution under magnetic field

The influences of applied magnetic field on the corrosion behavior of Al–3.0 wt%Mg alloy in 3.5 wt% NaCl solution were investigated by electrochemical measurements, scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Stochastic analysis was applied to investigate the influences of applied magnetic field. The results indicate that the application of horizontal magnetic field of 0.4 T would increase the pitting corrosion potential (E pit), decrease the corrosion current density (i corr), prolong the pit initiation time, slow down the pit generation rate and inhibit the growth of pitting of the tested alloys in 3.5 wt% NaCl solution. The applied magnetic field would also change the mechanism of pit initiation of Al–3.0 wt%Mg alloy from A3 model (without magnetic field) to A3 + A4 model (with magnetic field). The intermediate product Al (ad) + is the paramagnetic ion that would be influenced by magnetic field sensitively.

In Situ Monitoring of Pit Nucleation and Growth at an Iron Passive Oxide Layer by using Combined Atomic Force and Scanning Electrochemical Microscopy

AbstractGeneration of single corrosion pits and in situ monitoring of pit growth on iron exposed to 0.5 m NaCl solution was achieved by using combined atomic force and scanning electrochemical microscopy (AFM–SECM). Pits as small as 2.7 μm in diameter were formed at arbitrary locations on the substrate through the local generation of highly concentrated nitric acid in the vicinity of the AFM–SECM probe. Addition of nitrite ions to the environment, which act as corrosion inhibitors for iron, ensures passivation of the metal and hinders metal corrosion, despite exposure to the chloride‐containing media. Localized acidification was achieved by oxidizing nitrite ions at the probe. Acidification in combination with the high chloride content in the solution led to a local rapid attack at the surface and pit generation below the AFM–SECM probe. Besides improved spatial resolution and precise control of the pit nucleation site, combined AFM–SECM allows simultaneous imaging of the generated pits by the AFM tip.

Stability Criteria for Intergranular Corrosion of AA5083-H131

The high strength-to-weight ratios, excellent uniform corrosion resistance, and weldability of Al-Mg alloys make them lightweight substitutes for steel in marine structures. The vulnerability of these alloys becomes apparent after extended periods in service environments, making the alloys sensitized due to the preferential grain boundary precipitation of the more anodic β-phase (Al3Mg2), satisfying an electrochemical condition for an intergranular corrosion (IGC) path. The increasing body of research on IGC of AA5083, resulted in several established characteristics such as the IGC spreading potential, the IGC propagation threshold potential, and the critical IGC propagation current density.[1-3] However, the roles of these parameters on IGC stability remain unexplored. The general consensus definition for stable localized corrosion of depassivated acidic sites calls for steady state anodic dissolution in the presence of a critical chemistry, an appropriate potential inside the corrosion cavity, or the combination of both.[4-6] There are two known electrochemical thresholds for localized corrosion stability to determine whether these conditions are sustained: the repassivation potential, Erp, and Galvele's pit stability product, (i*x)crit.[4, 6] When the potential is sustained to be more positive than Erp, or when the product of the dissolution current density and propagation depth exceeds a critical value, localized corrosion can propagate stably. Although these stability criteria have been utilized in pit growth studies across a number of metal systems, there are currently no equivalent data for Al-Mg alloys. An added complication is the heterogeneous grain boundary microstructure consisting of α and β phases. Thus, the objective of this work is to develop a quantitative understanding of the IGC propagation stability criteria for sensitized Al-Mg alloys. The pit stability products, i*x, were determined for 99.99% Mg, and sensitized and unsensitized AA5083-H131 wires of 127, and 112 µm diameter, respectively. A series of potentiostatic holds were conducted on lead-in-pencil electrodes exposed to 0.6 M NaCl, pH 8.3. Artificial pits of controlled depths were generated by polarizing the electrodes to 1 VSCE for 1200 s. The potential was then rapidly scanned down and held at a lower potential for 100 s. The i*x at a given potential (Figure 1) is the product of the actual pit depth and the respective average current densities at each potential. These findings showed the influence of both alloying and sensitization on i*x, which could be attributed to the solution saturation at the pit bottom. To investigate the effect of these pit solutions, polarization scans were conducted on composite α-phase and β-phase in electrolytes of varying compositions of AlCl3 and MgCl2. Potentiodynamic scans were conducted following artificial pit generation in electrolytes of varying AlCl3 and MgCl2saturations to measure pit bottom dissolution kinetics and to identify critical pit chemistry. Select experiments were also conducted on bulk AA5083-H131 exposed to the critical chemistry. Erp measurements were conducted on unsensitized and sensitized AA5083-H131 in 0.6 M NaCl, pH 8.3. For wire specimens, the method of Srinivasan, et al.[7] was employed wherein lead-in-pencil electrodes were held at a potential above the pitting potential to generate artificial pits of certain depths, followed by a downward potentiodynamic scan at 0.5 mV/s. To ensure that no uniform corrosion occurred in bulk specimens, IGC were generated at the spreading potential (-0.73 VSCE), followed by the same downward potentiodynamic scan. The Erp was identified at which the current density was just below 250 µA/cm2. The Erp for AA5083-H131 was determined as the plateau potential on a potential versus accumulated charge density plot. To verify the applicability of Erpas a stability criterion, current decay measurements and long-term potentiostatic holds were conducted. The correlation between the two stability criteria presented aids in formulating a physical mechanism for stable IGC propagation in Al-Mg alloys in the context of threshold potential, critical applied current density, and sensitization. Additionally, this knowledge will provide information for corrosion mitigation strategies in this alloy class. Acknowledgements Office of Naval Research under the contract ONR: N00014-08-10315 References [1] S. Jain, M.L.C. Lim, J.L. Hudson, J.R. Scully, Corros. Sci., 59 (2012) 136-147. [2] D. Mizuno and R.G. Kelly, Corrosion 69 (2013) 580-592. [3] M.L.C. Lim, R.G. Kelly, J.R. Scully, “Critical Electrochemical Conditions for Intergranular Corrosion in Sensitized AA5083-H131”, in NACE DoD Virtual Corrosion Conference 2013. [4] J.R. Galvele, J. Electrochem. Soc., 123 (1976) 464-474. [5] H.W. Pickering, K.Y. Cho, E. Nystrom, Corros. Sci. 35 (1993) 775-783. [6] N. Sridhar and G.A. Cragnolino, Corrosion 49 (1993) 885-894. [7] J. Srinivasan, M.J. McGrath, R.G. Kelly, ECS Trans., 53 (2014) 1-11. [8] Y. Shi, “Experimental Evaluation and Modelling for Galvanically-Induced Localized Corrosion of AA7075-T6”, (Ph.D. diss., The University of Virginia: Charlottesville, VA, 2015). Figure 1

Simultaneous pit generation and visualization of pit topography using combined atomic force–scanning electrochemical microscopy

Combined atomic force microscopy–scanning electrochemical microscopy (AFM–SECM) is for the first time used to generate single corrosion pits on passivating iron surfaces in the micrometer range. The AFM–SECM probe locally generates nitric acid during the oxidation of nitrite ions with the release of protons at selected sites on the surface of the otherwise passive metal. High confinement of passive film breakdown is achieved from the combination of a small probe size and the inhibiting properties of non-reacted nitrite ions on the surrounding passivated surface. Simultaneous visualization of pit nucleation and propagation can be obtained in the same solution without changing the probe by AFM.

Numerical and experimental investigations on micro-detonation of striking arc machining of alumina

Numerical and experimental investigations on micro-detonation of striking arc machining of alumina ceramics were performed to ascertain the material removal mechanism and study the effects of the process parameters on the machining performance. The shape of the micro-detonation plasma jet and the ablation pit generation process were observed by high-speed video and machining test. A three-dimensional finite element model for temperature simulation during a single-pulse micro-detonation of striking arc machining process was developed and validated by corresponding experiments. The effects of the five process parameters, that is, working current, gas pressure, pulse width, nozzle stand-off distance, and muzzle diameter, on the highest temperature and material removal rate were studied by employing the Taguchi method. The findings indicated that the simulated results were in good agreement with the experiments. The material removal mechanism for alumina machined by micro-detonation of striking arc machining was determined based on high-speed video observation, numerical simulation, and experimental study. The alumina ceramics were mainly removed by a combination of sublimation, melting, and phase transformation under the synergistic effect of high temperature and high impact force of the micro-detonation plasma jet. The highest temperature and the material removal rate were significantly affected by the process parameters. Both temperature and material removal rate increased with increase in the working current, gas pressure, and pulse width, but decreased with the increase in the nozzle stand-off distance and nozzle diameter. Based on the analysis of variance and the effects of parameters on the highest temperature and material removal rate, the optimal combination of parameters was presented. This study may serve as a reference in understanding the machining mechanism of micro-detonation of striking arc machining and presents a method that can be used for efficient machining of alumina.

Pitting corrosion of 304ss nanocrystalline thin film

Pitting corrosion behavior of coarse crystalline (CC) 304ss and its nanocrystalline (NC) thin film have been investigated by electrochemical measurement and in situ AFM observation in 3.5% NaCl solution. Results show two effects of nanocrystallization on pitting corrosion behavior: (1) more frequent occurrence of metastable pits, but with lower probability of transition to stable pits, which is attributable to differences in morphologies of sulfur and manganese as well as outstanding repassivation ability of NC thin film; (2) nanocrystallization decreases stable pit generation rate and its propensity to form larger pit cavities, and modifies the morphology of stable pit cavity.

Markov Chain Models for the Stochastic Modeling of Pitting Corrosion

The stochastic nature of pitting corrosion of metallic structures has been widely recognized. It is assumed that this kind of deterioration retains no memory of the past, so only the current state of the damage influences its future development. This characteristic allows pitting corrosion to be categorized as a Markov process. In this paper, two different models of pitting corrosion, developed using Markov chains, are presented. Firstly, a continuous-time, nonhomogeneous linear growth (pure birth) Markov process is used to model external pitting corrosion in underground pipelines. A closed-form solution of the system of Kolmogorov's forward equations is used to describe the transition probability function in a discrete pit depth space. The transition probability function is identified by correlating the stochastic pit depth mean with the empirical deterministic mean. In the second model, the distribution of maximum pit depths in a pitting experiment is successfully modeled after the combination of two stochastic processes: pit initiation and pit growth. Pit generation is modeled as a nonhomogeneous Poisson process, in which induction time is simulated as the realization of a Weibull process. Pit growth is simulated using a nonhomogeneous Markov process. An analytical solution of Kolmogorov's system of equations is also found for the transition probabilities from the first Markov state. Extreme value statistics is employed to find the distribution of maximum pit depths.

Pit distribution in the equatorial region of Titan

Cassini radar images of Titan's surface reveal numerous dark circular features in the equatorial region. These may be related to methane sublimation and have been interpreted as pits. In the T8 equatorial swath, we identify 199 individual pits and estimate their diameters. We analyze the observed spatial distribution and size to characterize these features and understand their formation. Chi-square analysis confirms a significant deviation from a random distribution and shows clustering over the entire swath. However, analysis of the densest cluster of pits, a group of 50, shows a more random distribution. Fractal analysis and comparison with a same-sized random set find only a hint of linearity. A Poisson distribution fits the observed pit-sizes, although resolution limits size determination for the smallest pits, those less than 1km in diameter. Models for random pit generation and evolution simulate the observed Poisson distribution of pit sizes, with larger pits forming by the coalescing of smaller, overlapping ones. Pits, estimated to cover 0.5% of the equatorial T8 swath, could be an erosion mechanism that significantly contributes to the negative skew that has been observed in Titan's hypsography.

Failure probabilistic analysis of steam generator heat-transfer tubing with pitting corrosion

Pitting corrosion damages seriously steam generator (SG) heat-transfer tubing of nuclear power plants. To estimate accumulative failure probability of the SG heat-transfer tubing, a failure probabilistic analysis method with pitting corrosion of stochastic processes is proposed in this paper. Pitting corrosion can be considered as a combination process of pit initiation and growth. The pit generation process is built as a non-homogenous Poisson process. The pit growth process is modeled by a stationary gamma process. A cumulative failure probability model is derived from the stationary gamma process. The proposed model is evaluated with the maximum likelihood estimation method according to the experiment data. Finally, Monte Carlo method is applied to simulate the cumulative failure probability model. The results illustrated that the proposed method is feasible and practical to estimate the accumulative failure probability of the SG heat-transfer tubing.

Lifetime Distribution Model of Port Facilities with Pitting Corrosion of Stochastic Processes

Pitting corrosion damages seriously the structure strength of port facilities. To estimate the lifetime of port facilities, a new lifetime distribution model of port facilities with pitting corrosion of stochastic processes is proposed. Pitting corrosion can be considered as a combination of two physical processes: pit initiation process and pit growth process. The pit generation process is modeled as a non homogeneous Poisson process. The pit growth process is represented by a gamma process. A lifetime distribution model is derived based on the combined stochastic processes. Finally the proposed models are tested and simulated. The results indicate that the proposed model is feasible and valid for estimating the lifetime of port facilities.

A stochastic analysis of the effect of magnetic field on the pitting corrosion susceptibility of pure magnesium

AbstractThe effect of magnetic field on the pitting corrosion susceptibility of pure magnesium was investigated by stochastic approaches. Stochastic models had been applied to simulate pitting corrosion as the combination of two physical processes: pit initiation and pit growth. The results revealed that magnetic field increased the pitting corrosion susceptibility of pure magnesium. For the pit initiation process, the presence of magnetic field changed the mechanism of pit initiation from the parallel birth and death stochastic model (B1) to the parallel birth stochastic model (A3). Magnetic field increased the pit generation rate λ while decreased the repassivation rate µ, which indicated that magnetic field accelerated the pit initiation process of pure magnesium. For the pit growth process, magnetic field didn't change the pit growth mechanism, but it could decrease the ability of repassivation of pitting corrosion resulting in a great probability for the stable pitting corrosion to grow up with a higher growth rate and finally develop into larger pit cavity.

Study of the electrochemical behaviour of nanocrystalline zinc by statistical methods

Pulse electrodeposition was used to synthesize nanocrystalline (NC) zinc coatings from citric acid bath. The electrochemical behaviour of the NC zinc coatings was investigated by using potentiostatic and potentiodynamic polarization methods in 0.5 mol/L NaCl (pH = 12) solution and compared with that of cast zinc. Pitting corrosion behaviour was characterized by pitting potential, induction time and stable pit growth rate which were analyzed according to statistical method. The results showed that nanocrystallization increased the sensitivity of E pit refer to potential sweep velocity, changed the type of the pit generation from B1 (parallel) to B2 (series), accelerated the pitting initiation process and inhibited the stable pit growth process of NC zinc.