Stable Pitting Research Articles

(Olin Palladium Award Address of the Electrochemical Society) Pitting Corrosion Retrospective

Pitting corrosion has captured the attention of corrosion scientists for many decades because it embodies so many interesting aspects and phenomena. Passive film breakdown and pit growth stability are the two critical phenomena in pitting, either one of which can be controlling. The pitting processes involve reaction and transport on the passive film surface, inside the passive film, on the actively dissolving pit surface and inside the pit electrolyte. This talk will summarize different studies that have shed insight into pitting corrosion. The analysis of metastable pits, very small pits that initiate, grow for a period, and then repassivate at potentials below the pitting potential or at times before the onset of stable pitting, has illuminated pit growth kinetics and the conditions for pit stability. So called 2-D pitting, the pitting of thin metallic films on inert substrates clarified the behavior of very small pits because 2-D pits have depths that are fixed at the depth of the film thickness, unlike pits in bulk materials, which deepen very quickly. Specimens with another configuration and dimensionality, artificial pit electrodes or 1-D pits, provide insight on the behavior of deeper pits including pit growth kinetics and pit growth stability. Finally, the application of a technique that makes use of the lateral sensitivity of atomic force microscopy, Scanning Kelvin Probe Force Microscopy, provides visualization of the microstructural heterogeneities that drive pitting corrosion and other forms of localized corrosion, particularly in Al alloys, which are controlled by the behavior of intermetallic particles.

Corrosion propagation monitoring using electrochemical noise measurements on carbon steel in hydrogenocarbonated solution containing chloride ions

The propagation of localized corrosion on API 5L X65 carbon steel in hydrogenocarbonated solution containing chloride ions was monitored during long immersion times by electrochemical noise (EN) measurements in ZRA mode. Monitoring corrosion propagation from the EN amplitude is possible but interpreting the potential and current transients after long immersion times is made difficult by the different corrosion processes (metastable or stable pits, crevices, corrosion products detachment…) occurring simultaneously. The noise impedance at 0.1 Hz, directly derived from the EN measurements, is shown to decrease and stabilize in the same time scale as the coupled potential of the electrodes.

Corrosion resistance and passivation behavior of 3004 AlMnMg and 4044AlSi aluminum alloys in acid-chloride electrolytes

Corrosion resistance of 3004 and 4044 aluminium alloys (3004Al and 4044Al) in neutral chloride (0.5%–4.5% concentration), sulphate (0.00625M-0.1 M concentration), and chloride-sulphate (0.00625 M H2SO4/0.5%–4.5% chloride concentration) solutions was studied with potentiodynamic polarization, open circuit potential, cyclic polarization, and optical microscopy. Results show 4044Al exhibited higher resistance to general corrosion while 3004Al was more resistant to localized corrosion. Corrosion of 4044Al decreases with increase in chloride concentration while 3004 Al increases. Corrosion rate values for 3004Al and 4044Al in sulphate solution were generally similar between 0.061–0.395 mm y−1 and 0.168–0.213 mm y−1, respectively. In chloride-sulphate solution, corrosion rate of 3004Al increased from 0.130 mm y−1 to 1.563 mm y−1 at peak chloride concentration whereas the corrosion rate of 4044Al is near constant. The passive film on 4044Al is found to weaken significantly with increase in chloride concentration. Passivation values varied from 0.39 V at 0.5% chloride concentration to 0.01 V at 4.5% concentration while the potential at which stable pitting occurred increased. The passivation range values for 3004Al are relatively stable with respect to chloride concentration. Results from cyclic polarization experiments show the deterioration rate of both alloys in NaCl solution is subject to chloride concentration. The results show the alloys corrode at all NaCl concentrations (0.5%–4.5%) with the lowest pitting corrosion risk in 0.5% and 1.5%NaCl solutions. The highest pitting corrosion risk of the alloys occurred in 3.5% NaCl solution. Significant localized morphological deterioration is visible throughout the entirety of 4044Al relative to the adjacent Al alloy matrix compared to total surface deterioration on 3004Al.

Pitting behavior of Ti-15-3 titanium alloy with different surface in salt spray studied using electrochemical noise

Ti-15-3 titanium alloy has been widely used as a connecting material for aerospace structural parts. However, the non-uniform pitting caused by salt spray corrosion and the limitations of the conventional electrochemical techniques in predicting localized corrosion seriously prevent the practical application of Ti-15-3 alloy. Herein, the electrochemical noise based on discrete wavelet transform and Hilbert-Huang transform is employed for studying pitting corrosion of both the untreated and polished Ti-15-3 alloy under different neutral salt spray time. Combining with the further analysis of the electrochemical impedance spectroscopy, a complementary evaluation is used to analyze the effect of surface roughness on pitting corrosion behaviors of Ti-15-3 alloy systematically. It is found that pitting forms of both surface roughness electrodes exposed to different salt spray time are successfully distinguished, and the polished Ti-15-3 alloy shows better corrosion resistance. Microstructural characterization confirms the inferences from electrochemical noise analyses, the results indicate that the smooth surface of the polished Ti-15-3 alloy is less vulnerable to the coexistence of metastable pitting and the growth of stable pitting, and effectively improves the resistance to pitting corrosion of Ti-15-3 alloy. Furthermore, a possible mechanism for the triggering of pitting corrosion of Ti-15-3 alloy with different roughness in salt spray environment is proposed. This work will provide significant insights for rational design of Ti-15-3 alloy used in the tropical ocean facilities to avoid the attack of pitting corrosion.

Analysis and Comparison of the Corrosive Behavior of Nickel-Based and Cobalt-Based Dental Alloys.

Nickel-based and cobalt-based metal alloys are frequently used in dentistry. The introduction of various elements in the alloy changes its characteristics, and a thorough study of each alloy should be completed to determine its appropriate corrosion resistance and biocompatibility in contact with physiological fluids. There are scarce investigations on these widely used dental alloys in Ringer solution, and findings in this research bring new experimental data and information. The present study evaluated and compared the corrosion behavior of six NiCr- and two CoCr-based dental materials in Ringer solution, using the following techniques: potentiostatic polarization curves (chronoamperometry), microstructural analysis, and EIS (electrochemical impedance spectroscopy). The results obtained in this investigation showed that in the NiCr-based specimens Ni4, Ni5, and Ni6 the stability of the passive layer was destroyed after polarization and a development and growth of stable pits was found in the microstructural analysis after electrochemical treatment. In terms of susceptibility to corrosion, two different groups of specimens were derived from this investigation. A first group which included the two CoCr (Co1 and Co2) and three of the six NiCr alloys studied (Ni1, Ni2, and Ni3). A second group with the other NiCr alloys investigated Ni4, Ni5, and Ni6.

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.

Correlation between active/inactive (Ca, Mg, Al)-Ox-Sy inclusions and localised marine corrosion of EH36 steels

The correlation between (Ca, Mg, Al)-Ox-Sy inclusions and localised marine corrosion of EH36 steels was investigated. According to their ability to induce localised corrosion, the inclusions were categorised as ‘active’ or ‘inactive’, and the main difference between them was the ratio of CaO/Al2O3. The residual stress at the active inclusion/matrix interface was higher than the yield strength of the steel; therefore, a local plastic deformation region was preferentially formed, triggering localised corrosion initiation. Moreover, the occluded cell accelerated the localised corrosion propagation, and finally the dissolution of the inclusions and the matrix resulted in a stable pit.

On the Critical Factors for Estimating the Pit Stability Product under a Salt Film

The pit stability product of 316L stainless steel (SS) under a salt film was examined by experimental techniques, analytical methods, and numerical modeling. Both analytical and numerical results suggested that electromigration had a measurable contribution to the dissolution current during stable pit growth under a salt film, preventing the use of the 1D Fick’s law of diffusion to obtain the pit stability product under such conditions. Moreover, the numerical results indicated that migration contributed to almost ⅔ of the mass transport limiting current. Although the diffusion coefficient of metal cations decreased with an increasing concentration inside the pit, it could be replaced by a constant diffusion coefficient, defined as an equivalent diffusion coefficient. When the complexation reaction was cconsidered, the modeling results agreed with the experimental data, indicating that a 4.2 M FeCl2 could be used as a simplified pit-like electrolyte to estimate the pit stability product under a salt film for 316L SS.

Effect of NH4+ on the pitting corrosion behavior of 316 stainless steel in the chloride environment

• The effect of NH 4 + on pitting growth of 316 stainless steel are clarified. • Pitting parameters are statistically analyzed in NH 4 + -containing environment. • The high applied potential can promote the rupture of the residual passive film. • NH 4 + can promote the metastable pitting proliferation and transition process. • The pitting growth is controlled by diffusion. Statistics analysis method is used to study the corrosion process of 316 stainless steel (316 SS) in NH 4 + -containing environments. The pitting corrosion develops through three stages: nucleation, metastable pitting propagation and stable pitting propagation. The research results show that NH 4 + does not significantly promote the nucleation process and the number of nucleation sites. However, the hydrolysis of NH 4 + makes the solution weakly acidic and further slightly increases the degree of acidification of the solution, NH 4 + can significantly promote the growth process of metastable pitting corrosion that has been formed. Positive shift potential can stimulate more active points on the surface of 316 SS, promote pitting initiation, and increase the nucleation number of metastable pitting. In the growth and propagation process of metastable pitting, the high applied potential can promote the rupture of the residual passive film above pits. In the process of stable pitting growth and propagation, the pit growth is controlled by diffusion and has nothing to do with applied potential and solution ions, and the pit growth is mainly along the depth direction.

Effects of rare earth modifying inclusions on the pitting corrosion of 13Cr4Ni martensitic stainless steel

• ASPEX PSEM, SKP, SEM, TEM and electrochemical methods are employed to study corrosion behavior of 13Cr4Ni steel modified with rare earth metals. • Rare earth metals change the composition, size and quantity of inclusions in steel. • The addition of rare earth metals has a significant effect on both metastable and stable pitting processes, the optimum content of rare earth elements (La, Ce) added is 0.021%. • The mechanism of pitting corrosion induced by rare earth inclusions is revealed. In this study, the pitting corrosion behavior of 13Cr4Ni martensitic stainless steel (BASE) and that modified with rare earth (REM) in 0.1 mol/L NaCl solution were characterized. Techniques such as automatic secondary electron microscope (ASPEX PSEM detector), scanning electron microscope (SEM), transmission electron microscope (TEM), scanning Kelvin probe force microscope (SKP), potentiodynamic and potentiostatic polarizations were employed. The results obtained indicate that BASE steel contains Al 2 O 3 /MnS, Al 2 O 3 and MnS inclusions, while REM steels contain (La, Ce, Cr, Fe)-O and (La, Ce, Cr, Fe)-O-S inclusions. Compared with BASE steel, REM steel is more susceptible to induce the metastable pitting nucleation and repassivation, whereas it restrains the transition from metastable pitting to stable pitting. Adding 0.021% rare earth element to BASE steel can reduce the number and area of inclusions, while that of 0.058% can increase the number and enlarged the size of inclusions, which is also the reason that pitting corrosion resistance of 58REM steel is slightly lower than that of 21REM steel. In the process of pitting corrosion induced by Al 2 O 3 /MnS inclusions, MnS is preferentially anodic dissolved, and also the matrix contacted with Al 2 O 3 is subsequently anodic dissolved. For REM steels, anodic dissolution preferentially occurs at the boundary between inclusions and matrix, while (La, Ce, Cr, Fe)-O inclusions chemically dissolve in local acidic environment or are separated from steel matrix. The chemically dissolved substance (La 3+ and Ce 3+ ) of (La, Ce, Cr, Fe)-O inclusions are concentrated in pitting pits, which inhibits its continuous growth.

Study of the Passivity Limits of Austenitic Stainless Steel in H2S-Containing Brines Using Mott-Schottky and Point Defect Model Analysis

The objective of the study is to correlate the effects of H2S and Cl− concentration on the passivity limits and the onset of localized corrosion in UNS S31603 stainless steel by evaluating the changes in the semiconducting behavior exhibited by the passive layer. The study is accomplished experimentally by using a combination of direct and alternate current electrochemical methods, to study the passive layer formed by the stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H2S (bal. CO2) at 25°C. The results obtained using the Mott-Schottky analysis indicate that the decrease of the passive layer stability formed on the UNS S31603 stainless steel is consistent with the increase in the electron donor carrier density. This is observed as the consequence of the effect of Cl− and H2S. In this context, the Cl− content in the brine was found to exert a larger effect than the H2S activity. The correlation with the evaluation of the passive layer using the Point Defect model suggest that both the polarizability and the rate of annihilation of the cation vacancies at the metal/film interface increase with the H2S content in the environment. This behavior can explain the increased content of Cl− and sulfides as main electron donor species, also the observable increase in the passive layer susceptibility to both stable and metastable pitting.

Pitting resistance of 316 stainless steel after laser shock peening: Determinants of microstructural and mechanical modifications

• Grain refinement was avoided through modified LPwC. • Compressive residual stress after LSP reduces the metastable pitting rate. • Microdefects induced by LSP dominate the growth of metastable pits. • Adjacent metastable pits combine easily to transform into a stable pit after LSP. Laser shock peening (LSP) was performed on a 316 stainless steel for microstructural and mechanical modifications. To identify the dominant factor controlling pitting corrosion resistance, a single LSP impact was performed and various characterization techniques, such as scanning electron microscopy, X-ray diffraction, transmission electron microscopy, electron backscattered diffraction, microhardness and residual stress analysis were conducted. The results show that a considerable increase in the microdefects, microhardness, and compressive residual stress occurred at a depth of ∼200 μm from the surface, while no evident grain refinement was observed. The compressive residual stress reduced the metastable pitting nucleation rate. However, microdefects after LSP treatment resulted in larger metastable pits, which are more likely to combine to transform into a stable one. It is summarised that the substantial microdefects can deteriorate the superiority of compressive residual stress induced by LSP in the pitting resistance of 316 stainless steel.

Morphological Change and Open-circuit Potential of Single Metastable Pit on AA1050 Aluminum in NaCl Solution

The relationship between the change in the open-circuit potential (OCP) and the morphology of metastable pitting at Al–Fe–Si particles of AA1050 aluminum in 0.1 M NaCl was clarified. First, a metastable pit grew in the depth direction as the OCP decreased suddenly. Second, the pit started to grow on the Al surface, and the OCP remained low. Finally, the pit was repassivated with the reincrease of the OCP. In the early stage of OCP measurements of the electrode area of 1 cm 2 , the potential oscillations and values were found to be associated with the growth of many metastable pits rather than the initiation and repassivation of each metastable pit. The amplitude of the OCP oscillation decreased with time and disappeared. Then, the OCP decreased slowly and became constant. The transition from metastable to stable pitting was determined to occur gradually.

Geotechnical prerequisites for safe and efficient accessing and mining of thick coal seam in highlands

Bel-Alma deposit is located in the Batken region in Kyrgyzstan. Orographycally, the deposit occurs on the south shoulder of the Kichik–Alai ridge, 1–1.5 km away from the cognominal river. The local relief is high-mountain, rocky and heavily broken. Geologically, the deposit is composed of the Upper Paleozoic rocks, which underlay the stratified Meso-Cenozoic deposits, and amorphous loose overburden. The productive strata is the bottom range of the Sogul series of the Jurassic system. The total thickness of the series in the cross-section is 102.7 m. The Sogul series rocks feature erosion and distinct structural unconformity in the Paleozoic residuum. The overlying strata are represented by red conglomerates and sandstone of the Upper Cretaceous age. The geotechnical estimation of the properties of rocks composing the study area in Bel-Alma deposit is performed. The rock mass structure is determined using geological exploration cores. The parameters of stable pit wall and benches are preliminarily optimized and will be included in the ultimate pit limit design.

The reliability of metastable pit sizes estimated from dissolution current in aluminium alloys

By treating each current transient as a signal arising from single discrete pit events, the pit sizes of the locations associated with recurring and co-occurring pit events could be underestimated or overestimated. In the present study, the error introduced in the estimated pit sizes by this hypothesis and other factors such as the hydrogen evolution occurring within pits and pit geometry were investigated in-situ by mapping the pit locations. Further, with the insights derived, a criterion was proposed to identify the transients associated with recursive pit events that eventually developed into stable pits.

Enhanced pitting resistance through designing a high-strength 316L stainless steel with heterostructure

A high-strength 316L stainless steel with heterostructure (HS) was used to explore its corrosion properties in 3.5wt.% NaCl solution at 25 °C. Compared with a conventional coarse-grained (CG) sample, the HS sample with the characteristic of that the recrystallized grains (RGs) were surrounded by ultrafine/nano-structures (UFS/NSs) exhibited an enhanced pitting resistance. This was ascribed to interrupted propagation of the stable corrosion pits in RGs by the surrounded UFS/NSs. The results in current work may provide a new strategy to improve corrosion properties by heterostructural designing.

Addition of an Eco-Friendly Corrosion Inhibitor in Self-Assembled Monolayers Coatings to Improve Pitting Corrosion Resistance of Stainless Steel

In automotive industry and equipment manufacturers, treatment by phosphatation [1] is used both as an adhesion primer before painting and as a lubricant coating [2]. This treatment also offers a good protection against corrosion. However, phosphatation is quite harmfull for the environment, producing a lot of waste to be treated. With regards to toxicity, it is composed of concentrated acids, nitrates and nitrites. Eventually, OEM are working to reduce the thickness of this coating which is typically in the range 2-30 μm. The objective of our research is to propose an alternative to steel/phosphate systems by thin organic coating on 304 stainless steel, a low carbon content substrate free of Molybdene available at affordable costsTo fit with the required specification while being as attractive as possible, this alternative coating must present a thickness reduced to a few nanometers, be environmentally friendly, non-toxic and easy to use. It must keep the main properties of phosphatation which consist in adhesion or anti-wear. Self-assembled Monolayers (SAMs) [3] coatings were chosen with addition of Aloe Vera as an ecological corrosion inhibitor [4][5]. SAMs are tunable by adjusting either their terminal group or the length of the carbon chain to obtain the desired surface properties and corrosion resistance. Three different terminal groups combined with four carbon chain lengths have been tested to find the molecule providing the best protection against corrosion.An electrochemical study is carried out to quantify protection against pitting corrosion in a chloride-containing medium. The substrate used being a 304 stainless steel. As pitting corrosion mechanism involves both stable and unstable pits, two types of electrochemical methods are used. Pulsed potentials technic will highlighted unstable pits, while stable pits are analysed by cyclic voltammetry. These measurements are supported by surface analysis (SEM, EDS).[1] T. W. COSLETT, Treatment of iron or steel for preventing oxidation or rusting, British patent 8667 and US patent 870937, 1906 et 1907[2] J. BOGI, Phosphate conversion coatings on steel, J Mater Sci (1977) 12, 2235-2240[3] A. ULMAN, Formation and structure of self-assembled monolayers, Chem. Rev. 1996, 96, 1533-1554[4] Devarayan, Kesavan, Green Inhibitors for Corrosion of Metals: A Review, Chem Sci Rev Letts, (2012)[5] M. MEHDIPOUR, Electrochemical noise investigation of Aloe plant extract as green inhibitor on the corrosion of stainless steel in 1M H2SO4, J. Ind. Eng. Chem, (2015), 21, 318-327 Figure 1

(Invited) Micro-Electrochemistry of Pit Initiation at Non-Metallic Inclusions of Stainless Steels and Roles of Alloying Elements in Improving Corrosion Resistance

A micro-electrochemical system for in situ high-resolution optical microscopy was developed and applied to the real-time observation of pit initiation at MnS inclusions in Type 304 stainless steel in NaCl solutions. Metastable and stable pits were found to be initiated at the MnS/steel boundaries during potentiodynamic polarization. After polarization, it was confirmed that deep trenches were generated at the boundaries. Therefore, metastable and stable pits were determined to be initiated in the trenches. From the real-time observation, it was found that 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 approximately 1.5 s, and then the pit repassivated, and the polygonal metastable pit remained. The in-depth growth stage for stable pitting was relatively longer (approx. 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. The anodic dissolution behavior of CrS, TiS, and Ti4C2S2 inclusions was compared with that of MnS. The dissolution potential of CrS inclusions was in the transpassive region of stainless steels. Stable pits were initiated in this potential region. In contrast, TiS and Ti4C2S2 inclusions did not dissolve in the passive or the trans-passive regions of stainless steels, and no pits were initiated at these inclusions in NaCl solutions. The calculations made from the potential-pH diagrams for CrS, TiS, and Ti4C2S2 systems indicated that Cr-oxide or Ti-oxide enriched surface films would form, and this was confirmed by surface analysis. The Cr-oxide or Ti-oxide enriched layers on the inclusions inhibited the anodic dissolution of the inclusions in the passive or trans-passive regions of stainless steels. In contrast, MnS inclusions dissolved in NaCl solutions, and pits were initiated at the inclusions. While MnS inclusion surfaces were covered by Mn-oxides, the corrosion resistance of Mn-oxides was not sufficient to prevent inclusion dissolution and subsequent pit initiation. The oxide enriched surface layers on the sulfide and carbo-sulfide inclusions were shown to significant enhance pit initiation resistance at the inclusions. In addition to the above findings, the roles of the addition of Ce and Mo were studied. From the results of micro-scale polarization, thermodynamic calculations, and scanning electron microscopy, it was suggested that the Ce3+ ions inhibit trench formation at the inclusion/steel matrix boundary, resulting in improved pitting corrosion resistance at sulfide inclusions. And also, the active dissolution rate of the steel was suppressed by Mo alloying. This suggests that, even after trenching at high potentials, Mo alloying inhibits the initiation of pitting inside the trenches.

Early identification of stress corrosion cracking of P110 low alloy steel in downhole fluid by electrochemical noise measurement

ABSTRACT Electrochemical noise (EN) measurements were applied to detect the stress corrosion cracking (SCC) of API P110 low alloy steel in simulating CO2-containing downhole fluid during the slow strain rate test (SSRT). Several statistical parameters of potential and current transients were defined, and they were proven to be indicative of the metastable and stable pitting, as well as the initiation and propagation processes of microcracks. The EN results showed that the amplitude and lifespan of current transients increase with the CO2 concentration. The increasing SCC susceptibility of P110 steel is mainly attributed to the decreasing pH at the crack tip, which not only accelerated the rupture of passive film on crack walls but also prolonged the lifespan of cracks advance. EN measurement may be a promising technique for the early diagnosis of SCC.

Crystallization-dependent transition of corrosion resistance of an Fe-based bulk metallic glass under hydrostatic pressures

We reported a crystallization-dependent transition of corrosion resistance of an Fe-based bulk metallic glass (BMG) under hydrostatic pressures. When the crystallized volume fracture (Vf) is below a critical value of 30 %, the corrosion resistance remains almost unaffected as that in the as-cast BMG; when the Vf is higher than this critical value, the corrosion resistance decreases, and the passivation dynamics changes significantly. The underlying mechanism is understood by the competitions among the passivation, pitting kinetics, and micro galvanic effects. When Vf was above 30 %, a tri-phase structure formed such that micro galvanic effect induced stable pitting starts to dominate the corrosion.