Pit Propagation Research Articles

ON THE PROPAGATION OF OPEN AND COVERED PIT IN 316L STAINLESS STEEL

Pitting corrosion on stainless steel has been widely studied during the last decades, but since it is a stochastic process, it remains difficult to analyze experimentally such a phenomenon. In this work, reproducible single pits were performed on 316L steel by using an experimental setup based on the use of a glass microcapillary to locally supply chloride ions on the steel surface in order to characterize the pit propagation. This original approach allowed obtaining new results about pit propagation. Indeed, it was possible to control the presence of a metallic cap covering the pit by adjusting the experimental parameters (potential – chloride to sulfate ratio – temperature). The presence of this cover was shown to be an important issue concerning the propagation mechanism. It was also possible to study the evolution of both the pit depth and the pit diameter as a function of various parameters. Then, based on the simulation of the current densities at the pit bottom and at the pit aperture, a special attention has been paid for the investigation of the local propagation mechanism.

Pit Propagation at the Boundary between Manganese Sulfide Inclusions and Austenitic Stainless Steel 303 and the Role of Copper

Pitting corrosion around manganese sulfide (MnS) inclusions in stainless steel 303 (SS 303) were investigated using in situ Atomic Force Microscopy (AFM), Scanning Kelvin Probe Force Microscopy (SKPFM) and Scanning Electron Microscopy. In situ AFM experiments in 0.1 M sodium chloride solution at voltages near the pitting potential combined with post immersion energy dispersive X-ray spectroscopy (EDS) found copper (Cu) deposition on MnS inclusions but no pitting corrosion. SKPFM images before and after Cu deposition found that Cu ennobled the inclusion. Damage evolution at MnS inclusions was imaged in real-time with AFM at an applied anodic potential. During pit propagation the MnS inclusion remained “passivated” while a trench formed in the bulk SS 303 at the MnS/SS boundary. Focused Ion Beam (FIB) cross sectioning of trenches after this immersion period found no damage to the MnS and Cu deposition on the portion of the inclusion that was inside the trench. FIB images of unexposed samples found preexisting sub-surface trenches at MnS inclusions below the plastic deformation zone at the surface. From these results a proposed mechanism of corrosion propagation at the MnS / SS boundary is described.

Pulsed Potential Dissolution Reduces Anode Residue Formation during Nickel Electroplating

The electrodissolution of nickel pellets with different controlled electrical variables was investigated in an effort to mitigate anodic residue in Watts bath based electroplating cells. During galvanostatic dissolution the anode potential was seen to oscillate between passive and transpassive (pitting) potentials. In-situ microscopy and ex-situ characterization has shown that lacy-pit formation and large, partially perforated, pit covers formed during galvanostatic potential decay and contributed significantly to anode residue. Constant potential measurements at high potentials reduced the anode residue but require large power consumption. Current transients during potentiostatic pulses were analyzed and the critical time where pit propagation changed from superficial to quasi-hemispherical was determined. An alternative method of anode dissolution was devised using potential pulses and duty cycles that allowed average current densities comparable to industrial plating conditions to be achieved. By restricting the time spent at large positive potentials to 50 ms, anode residue was reduced more than six-fold in comparison to an equivalent galvanostatic dissolution run at a comparable current density.

The Influence of pH on Localized Corrosion Behavior of X65 Carbon Steel in CO2-Saturated Brines

Pitting and localized corrosion of carbon steel is considered to be a complex process influenced by a wide range of parameters such as temperature, bulk solution pH, and chloride ion concentration. Solution pH is known to influence corrosion product characteristics and morphology in CO2- and H2S-containing corrosion systems. However, from the perspective of pitting corrosion in CO2-saturated environments, the extent to which bulk pH of solutions and the presence of corrosion products influence localized attack is still not clearly understood. This paper presents an investigation into the role of pH on the characteristics of corrosion product and pitting corrosion behavior of X65 carbon steel (UNS K03014) in CO2-saturated brine. Pitting corrosion studies were conducted over 168 h at 50°C in 3.5 wt% NaCl solutions at different bulk pH (buffered to pH values of 6.6 and 7.5 in some cases) in order to understand and correlate the role of pH on corrosion product morphology, chemistry, initiation, and propagation of pits within each distinct environment. Corrosion product composition and morphology are identified through a combination of electrochemical and surface analysis techniques, which include scanning electron microscopy and x-ray diffraction. The extent of corrosion damage of the carbon steel is evaluated through the implementation of surface interferometry to study discrete pit geometry, namely the size, depth, and aspect ratio. Results indicate that the process of pit initiation and propagation of carbon steel in CO2 corrosion environment is different depending upon bulk solution pH. At low pH (pH values starting at 3.8), pitting initiates faster and propagates steadily along with significant uniform corrosion resulting from the formation of an “amorphous” form of FeCO3. At higher pH, uniform corrosion is significant, while pitting initiates with increasing protection from crystalline FeCO3. At a pH value of 7.5, pitting corrosion initiation occurs after and/or during pseudo-passivation is achieved as a result of the formation of a “protective and pseudo-passivating” FeCO3 film.

Understanding Pitting Corrosion Behaviour of X-65 Carbon Steel in CO2 Saturated Environments: The Temperature Effect

A systematic study examining the pitting corrosion behavior of X65 carbon steel (UNS K03014) in CO2-saturated 10 wt% NaCl brine is presented. This paper examines the impact of changes in a key environmental parameter (temperature) on the pitting corrosion processes of carbon steel. Pit propagation studies were conducted in static conditions at different operational temperatures. The evolution of corrosion products on carbon steel was followed over 168 h of immersion and their characteristics were related to initiation and growth of pits on the steel surface. The developed corrosion products were studied through the use of x-ray diffraction and scanning electron microscopy. The extent of pitting was evaluated through the implementation of surface interferometry to study discrete pit geometry, namely, the size and depth, while the general corrosion rate was evaluated using linear polarization resistance measurements. The results indicate that pitting corrosion dominates the general thickness loss of the mat...

In-Situ Critical Pitting Temperature on Duplex Stainless Steels

The Critical Pitting Temperature (CPT) of 25Cr Duplex Stainless Steel (DSS) in chloride containing environments was obtained by using a mini-autoclave. The work presented here shows, for the first time, simultaneous In-situ micro-visualization and electrochemical monitoring of DSS. A real time video of the surface of the sample was recorded while electrochemical measurements were taken as the temperature was slowly increased to obtain the CPT. This methodology allowed the observation of metastable pitting, pit initiation and pit propagation in DSS.

Passivity degradation of alloy 800 in simulated crevice chemistries

Passivity degradation of Alloy 800 in simulated crevice chemistries was systematically investigated using cyclic polarization curve, electrochemical impedance spectroscopy(EIS), Mott-Schottky analysis, Auger electron spectroscopy(AES)and atomic absorption spectrometry(AAS). Cyclic polarization showed that the pitting potential in a thiosulfate solution was much lower than in either a chloride solution or a sulfate-chloride solution. Mott-Schottky results revealed that passive films showed n-type semiconductivity, and the presence of thiosulfate in chloride solution led to an increased donor density in the passive film. EIS spectra indicated that thiosulfate enhanced the film dissolution rate in chloride solutions. Moreover, thiosulfate enhanced the pitting propagation rate in chloride solution by stabilizing the metastable pits and forming sulfide within the pits.

Corrosion of martensitic stainless steel in ethanol-containing gasoline: Influence of contamination by chloride, H2O and acetic acid

Corrosion of stainless steel DIN 1.4035 (Fe-13%Cr) was studied in ethanol-containing gasoline mixtures as a function of H2O, chloride and acetic acid concentrations. H2O and Cl− are primary corrosion triggering factors in EtOH/gasoline mixtures. The critical water content depends on the EtOH/gasoline-ratio. Pitting corrosion occurred in EtOH/gasoline blends at extremely low chloride-concentrations, which did not lead to passivity breakdown in a purely aqueous solution. An increasing chloride-concentration accelerates pit propagation, with little influence on pit densities. On the other hand, higher concentrations of acetic acid lead to a larger attacked area, with minor influence on the pit propagation in depth.

Corrosion measurement on shear strength of Cu/Sn–9Zn/Cu lap joints

Corrosion of the Cu/Sn–9Zn/Cu lap joints in 3.5wt.% NaCl solution showed reduction in the ultimate shear strength with increasing immersion time. The highest ultimate shear strength value obtained was at 122.67MPa before immersion while the lowest value was 85.32MPa after immersion for 28days. Corrosion products consist of tin (II) chloride and zinc hydroxide chloride had been obtained due to the active dissolution of tin. The formation of pits observed on the lap joints marked as stress concentration points which are detrimental toward the shear strength. Upon longer immersion period, the pit coalescence and propagation led to the existence of long, deep cracks. The fracture paths nucleates from the cracks clearly established the mechanism of the fracture formation for the Cu/Sn–9Zn/Cu lap joints. The fracture analysis together with the presence of the corrosion by-products fully reiterated that corrosion activities was the main factor for the decrease in shear strength of Cu/Sn–9Zn/Cu lap joints.

Electrochemical evaluation of sodium metabisulfite as environmentally friendly inhibitor for corrosion of aluminum alloy 6061 in a chloride solution

Inhibition properties of sodium metabisulfite (Na2S2O5) on pitting corrosion of 6061 aluminum alloy, in 5×10−2 M NaCl solution of pH near 7.2 at 298K, are characterized using open circuit potential, polarization resistance, cyclic and chrono-amperometric polarization measurements. In addition, scanning electron microscopy coupled with energy dispersive spectroscopy and X-ray photoelectrons are employed. Sodium metabisulfite, which is well compatible with environmental requirements, seems to act as a cathodic-type corrosion inhibitor. The passivation range and the polarization resistance increase with Na2S2O5 concentration. The inhibition effects are also reflected through the substantial reduction of both the rate of pit nucleation and the current rise characterizing the pit propagation progress. The SEM–EDS and XPS analyses reveal the formation of a passive film, which contains sulfur atoms.

Initiation and propagation of a single pit on stainless steel using a local probe technique.

Experiments about single pit initiation and propagation were performed on 316L stainless steel with the aim to determine the pitting corrosion behaviour for nuclear waste containers. The experimental setup permits to control the pit development at will and to create reproducible single pits in three dimensions. Radial and deep evolutions of a disc shaped pit were studied for propagation times of up to 10 hours. These evolutions were used to determine what limiting mechanism takes place during long-term pit propagation. Special attention has been paid to the chloride ion action on pitting. A minimum chloride concentration was found to be necessary in the bulk electrolyte to support pit propagation. The existence of a critical pit depth of 230 μm was also underlined.

A High-Throughput Artificial Pit Technique to Measure Kinetic Parameters for Pitting Stability

A cyclic electrochemical technique was developed which enables rapid, high-throughput extraction of important kinetic parameters from one-dimensional artificial pit (lead-in-pencil) electrode experiments. This approach combined methods for pit initiation and propagation with kinetic measurements at a series of pit depths to measure two key kinetic parameters – the pit stability product under a salt film (i⋅x)saltfilm and the repassivation potential Erp – from a single experiment. The entire sequence was cycled and automated for efficient, high-volume data collection and analysis for the case of stainless steel in neutral chloride solution. This method offered advantages in the extraction and interpretation of data from a wide range of pit depths, permitting statistical analysis of the kinetic parameters required for determining the critical conditions for pit stability and repassivation.

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.

Slow strain rate testing and stress corrosion cracking of ultra-fine grained and conventional Al–Mg alloy

Abstract Stress corrosion cracking susceptibility was investigated for an ultra-fine grained (UFG) Al–7.5Mg alloy and a conventional 5083 H111 alloy in natural seawater using slow strain rate testing (SSRT) at very slow strain rates between 1E−5 s−1, 1E−6 s−1 and 1E−7 s−1. The UFG Al–7.5Mg alloy was produced by cryomilling, while the 5083 H111 alloy is considered as a wrought manufactured product. The response of tensile properties to strain rate was analyzed and compared. Negative strain rate sensitivity was observed for both materials in terms of the elongation to failure. However, the UFG alloy displayed strain rate sensitivity in relation to strength while the conventional alloy was relatively strain rate insensitive. The mechanical behavior of the conventional 5083 alloy was attributed to dynamic strain aging (DSA) and delayed pit propagation while the performance of the UFG alloy was related to a diffusion-mediated stress relaxation mechanism that successfully delayed crack initiation events, counteracted by exfoliation and pitting which enhanced crack initiation.

Modelling pitting corrosion of stainless steel in atmospheric exposures to chloride containing environments

We describe work in modelling pitting corrosion of 300 series stainless steel during exposure to chloride containing environments, with a focus on atmospheric conditions. It is well known that, under certain conditions of temperature and humidity, pitting corrosion can initiate on stainless steel if sufficient quantities of chloride containing salts are deposited on its surface. One of the key hypotheses that this work intended to test is the existence of bounds on maximum pit depths in the presence of fixed and possibly limited amounts of cathodic current. In the presence of the relatively high cathodic currents associated with high relative humidity and high surface contamination by hygroscopic salts, which are most likely to be representative of outdoor environments, maximum pit depths are predicted to be of the order of 150–300 μm, developed over periods of several months. In conditions more likely to be representative of indoor conditions with corrosion occurring under thin moisture layers, pit propagation is severely inhibited and very high chloride concentrations are required that favour the development of shallow, dish shaped pits.

Electrochemical Behavior and Nonlinear Mott-Schottky Characterization of a Stainless Steel Passive Film

The electrochemical behavior of 304-stainless steel in solutions with different pH values and chloride concentrations was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and a nonlinear Mott-Schottky analysis method. The results showed that the corrosion behavior of 304-stainless steel was affected by various factors, such as pH, chloride ion concentration, and dissolved oxygen. The pit initiation and propagation stage were observed in EIS plots through the addition of sodium chloride. In addition, the effect of pH on the passive film dominated one of the film growth kinetic reactions. The effect of chloride ion concentration was caused by the generation of more cation vacancies, leading the passive film more susceptible to pitting, which was also demonstrated by X-ray Photoelectron Spectroscopy. The results are consistent with the point defect model.

Numerical modeling of corrosion pit propagation using the combined extended finite element and level set method

A sharp-interface Eulerian formulation for modeling the propagation of localized pitting corrosion is presented. This formulation allows for an accurate representation of the corrosion front indepe...

Role of Manganese Sulfide Inclusions in Steel Rebar in the Formation and Breakdown of Passive Films in Concrete Pore Solutions

This study examined manganese sulfide (MnS) inclusions in steel rebar exposed to a simulated concrete pore solution to understand their role in passive film, corrosion, and pit propagation behavior. The passive film was characterized using x-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and electrochemical techniques. The results showed that MnS inclusions adversely affected the nature of the passive film and accelerated corrosion and pit formation. A schematic model was developed to explain the deterioration of passive film formed on steel containing MnS inclusions, and was validated by Raman spectroscopy of the resulting rust formed on such steel.

Mechanism of MnS-mediated pit initiation and propagation in carbon steel in an anaerobic sulfidogenic media

In a saline anaerobic sulfidogenic environment, pitting on 1018 carbon steel was initiated within a 20–30 nm zone at the MnS inclusion boundary. Nanoscale analysis was performed using scanning electron microscopy and a scanning Auger nanoprobe. The pitting was more pronounced in the presence of a biofilm of sulfate-reducing bacteria than in abiotic sulfide medium. It is proposed that initiation of an anodic reaction leading to dissolution of Fe matrix and subsequent pitting of steel in MnS inclusion boundary regions is due to disorder and strain exerted on the Fe matrix by MnS contamination of the interface from metallurgical processes.

The Features of 304 Nitrogen Controlled Stainless Steel during Slow Strain Rate Test by Electrochemical Noise

An investigation was carried out on the features of electrochemical noises of 304 nitrogen controlled stainless steel (304NGSS) in solution 4mol/LNaCl +0.01mol/LNa2S2O3under slow strain rate testing (SSRT) conditions. Electrochemical voltage and current noise were measured simultaneously in the Zero Resistance Ammeter (ZRA) mode. After experiment, there are several cracks on the edge of pit on the specimen. In order to characterize the stress corrosion cracking (SCC) processes of 304NGSS, time analysis of data alongwith the spectral estimation using FFT method, the results show SCC processes consisted of breakdown of the oxide film, the formation of pitting, crack initiation and crack propagation. According to the varieties of spectrums in time and frequency, the different state can be distinguished. The work in this paper will contribute to the research of indicator for real-time detection of SCC in plant.