Stable Pitting Research Articles

Development of a New Microelectrochemical Measurement System for In Situ Optical Microscopic Observation of Pit Initiation Processes

The previous microelectrochemical measurement system based on the micro-electrode technique with a glass capillary dramatically advanced our understanding of pitting corrosion over the past ten years. However, by using this system, it was impossible to take in-situ and real-time observations of dissolving surfaces. Here we developed a measurement system for in-situ and real-time observations (lateral resolution: ca. 350 nm, time resolution: 30 images/s) of the initiation processes of pitting corrosion on stainless steel by combining an optical microscope with a microelectrochemical cell. The results of the observations on pit initiation at the manganese sulfide (MnS) inclusions clearly indicated marked differences in the initiation processes of metastable and stable pits. The experimental technique developed here is applicable to a wide range of the local electrochemical analysis of electrode surfaces.

Potentiostatic Electrochemical Noise Analysis of 2101 Lean Duplex Stainless Steel in 1 mol/L NaCl

The critical pitting temperature (CPT) of 2101 lean duplex stainless steel (DSS) in 1 mol/L NaCl was investigated. Potentiostatic electrochemical noise (EN) technique coupled with scanning electron microscopy (SEM) was used to study the corrosion process of 2101 DSS at different temperatures (15, 25, 28, and 35 °C) from below CPT to above CPT. The EN data were analyzed by wavelet technique based on orthogonal db4 wavelet. The results showed that, the patterns of current noises were different at different temperatures. The characteristics of energy distribution plot (EDP) obtained from wavelet analysis technique can be used as “fingerprints” of EN signal and can be used to interpret the corrosion process. With increasing solution temperature, the maximum relative energy defined in EDP changed from the region of crystal series coefficients with smaller scales through middle scales to larger scales, which indicated the process of initiation, propagation and growth into stable pitting of metastable pitting. The results were well confirmed by the corresponding morphologies.

Analysis of Pit Stability in Type 316L Stainless Steel Exposed to H2S-Saturated Dilute Chloride Solutions Above the Critical Pitting Temperature

Abstract This study examines the effect of hydrogen sulfide (H2S) on the transition from metastable to stable propagating pits using Type 316L (UNS S31603) steel in brines with Cl− concentrations of 10−3 M to 2 × 10−2 M at 75°C and 150°C. The potentiostatic tests were performed at discrete intervals of 50 mV from the recorded open-circuit potential (OCP) to 300 mV above the OCP. The results based on analysis of the chrono-amperometric transients suggests that the transition to stable pit growth is hindered by H2S saturation at both temperatures. The comparison of the experimental results with similar tests in an acidified control brine (without H2S saturation) indicates that the effect of H2S in the promotion of pit metastability is associated with the diffusion of H2S to the bottom of the cavity. This increases the net anodic current by irreversibly forming metallic sulfides, which, in turn, affect the pit's anolyte by reducing the metal salt concentration. The consequence of this effect is that small pi...

A Microelectrochemical System for In Situ High-Resolution Optical Microscopy: Morphological Characteristics of Pitting at MnS Inclusion in Stainless Steel

A microelectrochemical system for in situ high-resolution optical microscopy was fabricated and applied to the real-time observation of pit initiation at MnS inclusion in type 304 stainless steel in NaCl solutions. It was directly observed that the metastable and stable pits were initiated at the MnS/steel boundaries, and that deep trenches were generated at these boundaries during anodic polarization. The initial rounded form of metastable and stable pits became polygonal in shape within 1 s. After that, the dissolution proceeded in the depth direction with no change in the appearance of the pit as observed externally. In the case of the metastable pitting, the duration of this stage was ca. 1.5 s, and then the pit repassivated, and the polygonal metastable pit remained on the electrode surface. The in-depth growth stage for stable pitting was relatively longer (ca. 3.5 s), and the pit grew deeply into the steel matrix and wrapped beneath the inclusion, leading to the formation of a large occluded cavity, in which the corrosivity considerably exceeded the critical conditions for autocatalytic pit growth. Chloride ions were shown to increase the probability of metastable pit initiation and affected the surface and cross-sectional morphology of stable pits.

Relating the Early Evolution of Microstructure with the Electrochemical Response and Mechanical Performance of a Cu-Rich and Cu-Lean 7xxx Aluminum Alloy

In this work AA7150 and AA7079 were studied as examples of 7xxx aluminum alloys with a high and low Cu content. The alloys were carefully examined to determine the evolution (incl. density and size) of precipitates during heat-treatment using small angle X-ray scattering (SAXS). Such characterization made it possible to relate the evolution of microstructure to specific properties such as electrochemical response (including metastable pitting analysis), hardness and crack growth rate (from SCC testing). The results revealed several important findings, indicating that there is a critical precipitate size above which a transition in electrochemical response occurs; in addition it is seen that Cu has an important role in both metastable pitting and crack propagation. The corrosion initiation and propagation phases are distinct in their mechanisms, and this is a function of the nature and state of the precipitates. Increased aging times increased the metastable pitting rate and SCC velocity in the Cu-lean alloy, whereas increased aging times decreased the metastable pitting rate the Cu-rich alloy. A correlation between metastable pitting rate and stable pitting is also presented.

Analysis of the composition of the passive film on iron under pitting conditions in 0.05 M NaOH/NaCl using Raman microscopy in situ with anodic polarisation and MCR‐ALS

The composition of the surface film formed on pure iron was investigated in a solution of 0.05 M NaOH and 0.05 M NaCl. Raman spectra of the film were recorded in situ during anodic polarisation over the passive region after addition of the NaCl to the electrolyte, under conditions of preresonance enhancement using excitation at 636.4 nm. Multivariate curve resolution with alternating least squares analysis was applied to the spectra to measure the relative amounts of different iron oxide and oxyhydroxides in the film at different potentials. The water content was also determined in this way from Raman spectra recorded using excitation at 514.5 nm. It was found that the composition of the film and the amount of incorporated water were influenced by the applied anodic potential. The results show that stable pitting can occur when the composition changes from the primary constituents β‐FeOOH and Green Complex (a hydrated, amorphous magnetite) with smaller amounts of γ‐Fe2O3 and γ‐FeOOH, to δ‐FeOOH and Green Complex, simultaneously with a reduction in water content. These changes result in conditions that favour the rate of localised breakdown of the film by Cl− ions over the rate of repassivation by water in the passive film. Copyright © 2011 John Wiley & Sons, Ltd.

Interpretation of electrochemical noise by wavelet transform and fractal analysis

Electrochemical noise(EN) was measured during pitting and stress corrosion tests in 3.5 % NaCl solutions for type 403 stainless steel tempered at 520 °C and 610 °C. The energy contributions of smooth crystal and detail crystals in three frequency divisions were analyzed after wavelet transform and compared with the change in the Hurst parameter based on the rescaled range analysis. The contribution of smooth crystal to the total energy is greater than that of low-frequency detail crystal in general corrosion and stable pit formation, but it is less in both meta-stable pitting and growth of stable pits. Crack growth rate can be estimated qualitatively by the evaluation of the energy contributions of smooth and detail crystals, and correlation was found between the change in the Hurst parameter and that in the energy contribution of smooth crystal or one of detail crystals in three frequency divisions.

Effect of boron addition on pitting corrosion resistance of modified 9Cr–1Mo steel: Application of electrochemical noise

9Cr–1Mo steels indigenously melted with the addition of boron (Alloy B) and without it (Alloy D) along with Alloy C (without boron addition with minor changes in the trace element concentrations) were studied for their pitting corrosion resistance in 0.001 M, 0.01 M, 0.05 M and 0.1 M sodium chloride solutions using electrochemical noise (EN) technique. Weibull probability plots were used to determine the pit embryo generation rates. Gumbel extreme value analysis was conducted to determine the maximum metastable as well as stable pit radii. The analysis of the data showed superior pitting corrosion resistance of the Alloy B compared to Alloy C as well as Alloy D.

Effect of Inhibitors on the Critical Pitting Temperature of the High-Entropy Alloy Co1.5CrFeNi1.5Ti0.5Mo0.1

The purpose of this study is to investigate the effect of inorganic/organic inhibitors on the pitting potential (Epit) and critical pitting temperature (CPT) of the high-entropy alloy Co1.5CrFeNi1.5Ti0.5Mo0.1 in chloride solutions using potentiodynamic polarization tests. The results indicate that six anions all retard the occurrence of pitting corrosion; the inhibitive effect decreases in the order: nitrates≒ benzonates>acetates≒ perchlorates>citrates>molybdates. The SEM micrographs show that perchlorates and citrates inhibit propagation of stable pits; acetates and molybdates restrain nucleation of stable pits.

Galvanic corrosion of gas tungsten arc repair welds in 17-4PH stainless steel in 3·5% NaCl solution

In this paper, the galvanic corrosion of individual components of a 17-4PH repair welded stainless steel in 3·5% NaCl solution was investigated using various dc electrochemical measurements and microscopy. Open circuit potential measurement of the regions in the vicinity of a repair weld [i.e. parent metal, weld metal and heat affected zone (HAZ)] in 17-4PH stainless steel in 3·5% NaCl solution indicated that the most likely galvanic couple was between HAZ and weld with the HAZ acting as the anode and weld metal as the cathode. Slow scan rate potentiodynamic polarisation measurement of pitting potentials revealed a lower passive current density and a higher pitting potential in the weld region, while the HAZ showed the highest passive current density and the lowest pitting potential. Observation of the material after applying an anodic potential close to the pitting potential of the individual weld parts also confirmed the formation of several stable pits in the HAZ but only a few metastable pits in the weld and parent metal zones. Galvanic coupling using a zero resistance ammeter also showed a higher current density in the weld metal/HAZ as compared with the parent metal/HAZ and parent metal/weld galvanic couples. Although, the current densities in all measurements were in the range of a few to tenths of nanoampere per square centimetre, it can still be concluded that the weld metal/HAZ couple has the highest risk of galvanic corrosion among the three individual galvanic couples.

Localised dissolution of iron in buffered and non-buffered chloride containing solutions

Pitting corrosion of pure iron was studied by using conventional samples, and artificial pit electrodes. Experiments were conducted in solutions of 0.01, 0.1 and 1 mol dm −3 NaCl at pH values of 7, 10, 11 and 12; and in borate buffer solutions with the same chloride concentrations and pH 8.7 and 9.2. Four times higher concentration of borate salt was required to reach inhibiting capacity of the hydroxyl anions, as determined via pitting potentials. From measurements of solution resistance, the increase in local conductivity due to dissolved corrosion products exuded from the pit was calculated for each bulk solutions. For the artificial pit electrodes, contribution of the borate species to the internal pit solution conductivity in low chloride solution was associated with the difference between transition potential, E T , values for buffered and non-buffered solutions, and this contribution was also used to explain the non-linear dependence of E T on [Cl −]. Further analysis was conducted using the concept of the critical i.x parameter for stable pit propagation.

Microstructure and corrosion behaviour aging at 750 °C in 25% Cr duplex stainless steel

This work concerns the effect of solution temperature on the microstructure and corrosion property aged at 750 °C in 25% Cr duplex stainless steel (DDS). The evolution of microstructure and phase compositions were studied by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Precipitation of sigma phase initially occurred rapidly aging for 80 min, subsequently became slower with further aging for 330 min corresponding to the decomposition of δ-ferrite. The precipitation of sigma phase was suppressed by increasing solution temperature from 1060 to 1230 °C. The EDS analysis combined with microstructure evolution indicates an increase of sigma phase precipitation is attributed to the decrease of δ/γ interfaces and Cr concentration in the ferrite. The potentiodynamic anodic polarization test confirms that the transition from metastable pitting to stable pitting occurs easily for aging at 750 °C due to more precipitation of sigma phase, the better resistance to pitting corrosion was attributed to higher solution temperature in initial high temperature aging.

Evaluation of 316L Stainless Steel Corrosion Resistance in Solution Simulating the Acid Hydrolysis of Biomass

The corrosion resistance of 316L stainless steel in 65 wt % ethanol, 35 wt % water, 1 wt % H2SO4 solution containing different NaCl concentrations and at different temperatures was investigated using open circuit potential measurements, polarization curves, and electrochemical impedance spectroscopy. At 25°C, the minimum NaCl concentration required for occurrence of pitting was 0.58 wt %. In 0.35 wt % NaCl solution, localized corrosion was observed at temperatures higher than 40°C and, depending on the potential, metastable or stable pits were observed. No pits were observed below 40°C in 0.35 wt % NaCl solution.

Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values

Reinforcement steel embedded in six different concrete mixes was exposed to chloride by wetting/drying cycles. Various parameters were continuously monitored during more than 1 year. Cement replacement with fly ash had beneficial long-term effects regarding chloride penetration resistance. Concerning corrosion performance, by far the most dominant influencing parameter was the steel/concrete interface since corrosion initiated on the lower side of the rebar (with respect to casting direction) regardless of binder type and w/b ratio. In many cases, after the first signs of depassivation, a marked increase in chloride content was required to prevent repassivation and to enable stable pit growth.

Initiation and repassivation of pitting corrosion of carbon steel in carbonated concrete pore solution

The initiation, propagation, repassivation of metastable pits of Q345 carbon steel in Cl − containing concrete pore solution were studied by electrochemical noise. The initiation rate of metastable pits increases exponentially up to a constant value with time. After stable pit occurs, the nucleation of metastable pits is mainly induced by the rust layer, and mainly takes place in the outskirt of the rust layer. Stable pits incline to expand horizontally to produce a shallow pit. In addition, high concentration of nitrite can inhibit the nucleation of metastable pits around the rust cover and accelerate the repassivation of stable pits.

The electrochemical corrosion behavior of K38G nanocrystalline thin film in 3.5% NaCl solution

The electrochemical corrosion behavior of a K38G nanocrystalline (NC) thin film fabricated by magnetron sputtering technique has been investigated in comparison with cast K38G alloy in 3.5% NaCl solution. The results indicated that compared with the cast K38G alloy, the NC thin film had a higher occurring frequency of metastable pits and lower rates of stable pitting nucleation and growth. The influence of nanocrystallization on the pit initiation and pit growth processes has been discussed, respectively.

Inhibition of Ethyleneamine on the Pitting Corrosion of Rebar in a Synthetic Carbonated Concrete Pore Solution

Tetraethylenepentamine (TEPA) was surveyed to show strong inhibition on pitting corrosion of Q345B carbon steel in a carbonated synthetic pore solution (SPS) by electrochemical noise (ECN), electrochemical impedance spectroscopy (EIS), and polarization curves. The pitting corrosion potential shifts positively with an increase in the TEPA concentration. ECN data show that the low content of TEPA can increase the nucleation rate of the metastable pits slightly and can effectively reduce their lifespan. The high content (0.10 mol·L - 1 ) of TEPA accelerates the repassivation of metastable pitting and this is accompanied by an increase in noise resistance and a decrease in the nucleation rate and average pitting charge until the noise current transients disappear completely. Both the baseline current and the amplitude of the current transients decrease with an increase in the TEPA concentration, indicating that TEPA retards not only the pitting corrosion but also the general dissolution of the passive film. Micrographs show that the metastable pits mainly initiate and develop around the stable pits because of the induction of corrosion products, which causes the pitting cavity on carbon steel to grow generally in a planar rather than in a perpendicular direction.

Uniform and pitting corrosion events induced by SCN− anions on Al alloys surfaces and the effect of UV light

The influence of the alloying elements on the uniform and pitting corrosion processes of Al-6061, Al–4.5%Cu, Al–7.5%Cu, Al–6%Si and Al–12%Si alloys was studied in 0.50M KSCN solution at 25°C. Open-circuit potential, Tafel polarization, linear polarization resistance (LPR) and ICP-AES measurements were used to study the uniform corrosion process on the surfaces of the tested alloys. Cyclic polarization, potentiostatic current-time transients and impedance techniques were employed for pitting corrosion studies. Obtained results were compared with pure Al. Passivation kinetics of the tested Al samples were also studied as a function of applied potential, [SCN−] and sample composition by means of potentiostatic current transients. The induction time, after which the growth of stable pits occurs, decreased with increasing applied potential and [SCN−]. Regarding to uniform corrosion, alloyed Cu was found to enhance the corrosion rate, while alloyed Si suppressed it. Alloying elements of the tested samples diminished pitting attack to an extent depending on the percentage of the alloying element in the sample. Among the investigated materials, Al–Si alloys exhibited the highest corrosion resistance towards uniform and pitting corrosion processes in KSCN solutions. The passive and dissolution behaviour of Al was also studied under the conditions of continuous illumination (300–450nm) based on cyclic polarization and potentiostatic techniques. The incident photons had a little influence on pit initiation and a marked effect on pit growth. These explained in terms of a photo-induced modification of the passive film formed on the anode surface, which render it more resistant to pitting. The effects of UV photons energy and period of illumination on the morphology of the pitted surfaces were also studied.

Electrochemical noise analysis of localized corrosion by wavelet transform

Electrochemical noise (EN) was measured during pitting and stress corrosion tests in NaCl solutions for type 403 stainless steel tempered at 390 °C for 2 h. The EN data were analyzed mainly by wavelet transform in terms of the energy contribution of smooth crystal and detail crystals in three frequency regions. In the early stage of the pitting tests, general corrosion occurs, and contribution of smooth crystal to the total energy was greater than that of low frequency detail crystal, but less with the development of meta-stable pitting. A similar trend was observed in the early stage of stress corrosion cracking (SCC) tests. The contribution of low frequency detail crystals to the total energy decreased as stable pits were formed, and it was less than the energy contribution of smooth crystal when crack propagation occurred in the later stage of SCC. In addition, the contribution of energy of high frequency detail crystal is shown to be indicative of the occurrence of plastic deformation. These results demonstrate that the onset of detailed processes in pitting and SCC can be sensitively detected by wavelet analysis in terms of the energy contribution of smooth crystal and low and high frequency detail crystals, but not with that based on the change in position of peak in energy contribution of detail crystals.

Corrosion of AA2024-T3 Part III: Propagation

Optical and electron microscopies and EBSD were used to study the early stages of corrosion propagation during stable pit formation on AA2024-T3. Polished AA2024-T3 developed large scale rings of corrosion product, typically a few hundred microns in diameter, within 2 h of exposure to 0.1 M NaCl at room temperature. These features were sectioned using diamond ultramicrotomy and substantial subsurface attack, in the form of intergranular corrosion was observed beneath these sites with virtually no grain etchout. A model is proposed for the mechanism of stable pit progression which involves extensive grain boundary attack, followed by grain etchout leading to open pit formation.