Influence of nanocrystallization on pitting corrosion behavior of an austenitic stainless steel by stochastic approach and in situ AFM analysis

The pitting corrosion behavior of an austenitic stainless steel nanocrystalline (NC) coating, fabricated by magnetron sputtering as well as that of the conventional polycrystalline (PC) alloy have been investigated in 3.5% NaCl solution by stochastic approach and in situ atomic force microscopy (AFM). The results indicate that the pitting corrosion resistance of the NC coating was much higher than that of the PC alloy with higher prevalence of metastable pits and lower rates of stable pit nucleation and growth. The influence of nanocrystallization on the pit initiation and pit growth processes has been discussed according to the in situ AFM observations.     

Pitting corrosion behavior of stainless steel in ultrasonic cell

Under potentiostatic conditions on stainless steel electrodes in chloride solution, a novel electrochemical cell equipped with an ultrasonic transducer has been designed to evaluate the metastable and stable pitting corrosion behaviors. The advantage of the cell is that the rupture of pit cover can be controlled during the metastable or stable pitting growth periods, and in which the effect of the cover on pitting stability is affirmed. Some shallower stable pits, which were formed at higher anodic potential, were found to repassivate after ultrasonic cover rupture due to their shorter diffusional length. In terms of the relative susceptibility to localized corrosion, repassivation potential (E_r) determined by the cyclic potentiodynamic polarization curve was found to describe more properties of the occluded pit cover than the others.     

Cluster and discriminant analysis of electrochemical noise data

In a typical pitting system of Q235 low carbon steel in 0.50 mol/L NaHCO₃ + NaCl solutions, a new method is presented to analyze electrochemical noise (EN) data, including potential (E) and current (I) noise, and identify its corresponding pitting states. The proposed method is based on cluster analysis (CA) and discriminant analysis (DA). Firstly, E and log|I| were determined as the variables for clustering. Then, data points (E, log|I|) of the EN groups from different pitting states were classified by CA to two clusters, which relate to the metastable state (Cluster 1) and stable state (Cluster 2), respectively. When a group of (E, log|I|) data points were dispersed stochastically into two clusters, it relates to the intermediate state that was defined to describe the transformation from the metastable pitting to stable pitting. Based on the obtained clustering results, a discriminant function(s) was established to discriminate the ungrouped EN data from the similar pitting processes and thus its corresponding pitting state could be determined by the cluster distribution result. The validity of the cluster/discriminant analysis has been proved in the studied pitting system.     

Electrochemical noise analysis of the corrosion of AZ91D magnesium alloy in alkaline chloride solution

The corrosion behavior of AZ91D magnesium alloy in alkaline chloride solution was investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and power spectral density (PSD), had been employed to analyze the EN data. It was revealed that there exist three different stages of corrosion for AZ91D magnesium alloy in alkaline chloride solution, including an anodic dissolution process companying with the growth, absorption and desorption of hydrogen bubbles, a development of pitting corrosion, an inhibition process by protective MgH₂ film. The results demonstrated that energy distribution plot (EDP) was a powerful tool to provide useful information about the dominant process for the different corrosion stages.     

Influence of nanocrystallization on passive behavior of Ni-based superalloy in acidic solutions

The electrochemical corrosion behaviors of Ni-based superalloy nanocrystalline coating (NC) fabricated by a magnetron sputtering technique have been investigated in comparison with cast alloy in 0.25 M Na₂SO₄ + 0.05 M H₂SO₄ and 0.5 M NaCl + 0.05 M H₂SO₄ solution, respectively. Compared with cast alloy, the NC coating had a little higher passive current density in Na₂SO₄ acidic solution, while it had superior resistance to pitting corrosion in NaCl acidic solution. The semiconductive type of passive film of the NC coating was p-type in both acidic solutions, while, that of cast alloy changed from p-type in Na₂SO₄ acidic solution to n-type in NaCl acidic solution. XPS results indicated that Cr₂O₃ was the main component for the passive films of the NC coating as well as those of the cast alloy. No chloride ion was found in the passive film of NC coating while it was in the passive film of cast alloy. The chloride ions adsorbing on the surface of cast alloy incorporated into the passive film, which induced the formation of n-type oxide film. The nanocrystallization of Ni-base superalloy obviously weakened the adsorption of chloride ions on surface, which decreased the susceptibility of pitting corrosion in acidic solution.     

In-situ AFM and EIS study of a solventborne alkyd coating with nanoclay for corrosion protection of carbon steel

A solventborne alkyd composite coating containing modified montmorillonite (MMT) nanoclay was made on carbon steel, and its corrosion protection was investigated by in-situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) measurements in 3 wt.% NaCl solution. X-ray diffraction (XRD) analysis indicated intercalation of the MMT sheets in the composite coating. Thermo-gravimetric analysis (TGA) demonstrated improved thermal stability of the composite coating due to the modified nanoclay. Scanning electron microscopy (SEM) and AFM examination revealed dispersion and also some aggregation of the nanoclay particles in the coating. In-situ AFM images show a stable coating surface at nano-scale during relative long time exposure in the NaCl solution, indicating an enhanced stability of the composite coating. The EIS results confirmed that the composite coating provides an enhanced barrier type corrosion protection for carbon steel in the corrosive solution, which could be attributed to the intercalated lamellar MMT sheets in the coating that block the defects and decrease the transport of water and corrosive species.     

A stochastic analysis of the effect of hydrostatic pressure on the pit corrosion of Fe-20Cr alloy

The effect of hydrostatic pressure on the pit corrosion behavior of Fe-20Cr alloy was investigated in 3.5% NaCl solution by means of potentiodynamic polarization and potentiostatic technology, and the experiment data was analyzed based on stochastic theory. With the increase of hydrostatic pressure, the pit corrosion resistance of Fe-20Cr alloy was deteriorated, which was distinguished by the decrease of critical pit potential (E_cirt) and the increase of passive current density. The results also demonstrated that there exist two effects of hydrostatic pressure on the corrosion behavior of Fe-20Cr alloy: (1) the pit generation rate was evidently increased compared to that under lower hydrostatic pressure, and the metastable pits become faster and larger. However, it seemed that pit generation mechanism shows no hydrostatic pressure dependence; (2) the probability of pit growth increased with the increase of hydrostatic pressure, which implied that the metastable pit on Fe-20Cr alloy exhibited higher probability to become larger pit cavity during shorter time interval than that under lower hydrostatic pressure.     

Corrosion initiation of stainless steel in HCl solution studied using electrochemical noise and in-situ atomic force microscope

An in-situ atomic force microscope (AFM), optical microscope and electrochemical noise (ECN) techniques were applied to the investigation of corrosion initiations in an early stage of 1Cr18Ni9Ti stainless steel immersed in 0.5 M HCl solution. The electrochemical current noise data has been analyzed using discrete wavelet transform (DWT). For the first time, the origin of wavelet coefficients is discussed based on the correlation between the evolution of the energy distribution plot (EDP) of wavelet coefficients and topographic changes. It is found that the occurrence of initiation of metastable pitting at susceptive sites is resulted from the reductive breakdown of passive film of stainless steel in the diluted HCL solution. The coefficients d₄-d₆ are originated from metastable pitting, d₇ represents the formation and growth of stable pitting while d₈ corresponds to the general corrosion.     

Integrated AFM and SECM for in situ studies of localized corrosion of Al alloys

Rolled 3xxx series Al alloys, e.g., EN AW-3003, are generally used as fin or tube material in heat exchangers for automobiles. With reducing fin thickness, maintaining fin material integrity is of increasing importance. This study aimed at exploring the differences in intrinsic corrosion properties between EN AW-3003 and a newly developed Al-Mn-Si-Zr fin alloy using state-of-the-art local probing techniques. Volta potential mapping of both alloys by scanning Kelvin probe force microscopy (SKPFM) indicates a cathodic behaviour of constituent intermetallic particles (>0.5 μm) relative to the alloy matrix. Compared to EN AW-3003, the Al-Mn-Si-Zr alloy has a smaller number of particles with large Volta potential difference relative to the matrix. In situ atomic force microscopy (AFM) measurements in slightly corrosive solutions showed extensive localized dissolution and deposition of corrosion products on EN AW-3003, and only a small number of corroding sites and “tunnel-like” pits on Al-Mn-Si-Zr. Probing the ongoing localized corrosion process by integrated AFM and scanning electrochemical microscopy (SECM) revealed more extensive local electrochemical activity on EN AW-3003 than on Al-Mn-Si-Zr. In all, the lower corrosion activity and smaller tunnel-like pits resulted in lower material loss of the Al-Mn-Si-Zr alloy, a beneficial property when striving towards thinner fin material.     

A characterization of the inhibiting effect of Cu on metastable pitting in dilute Al-Cu solid solution alloys

Copper additions to aluminum decrease susceptibility to pit initiation provided that Cu is retained in solid solution. This can be observed as an increase in pitting potential with increasing Cu content in an alloy. To further understand this effect, metastable pitting of high purity Al, Al-0.2Cu and Al-2.0Cu exposed to 0.1 M NaCl solutions has been examined in detail. Results show that 0.2 wt.% Cu additions decrease the metastable pit initiation rate by more than an order of magnitude and slow the pit growth rate mainly by decreasing the peak pit current attained. In an Al-2.0 wt.% alloy, metastable pitting events were too rare for rigorous study. Repassivation of metastable pits occurs by a two-stage process in Al-0.2 wt.% Cu alloy. The repassivation rate during the first stage is identical to that of high purity Al and appears to be completely unaffected by Cu in the alloy or in the pit solution. In the second stage, repassivation is slow, but is not believed to affect ultimate pit stability. Overall, Cu additions decrease the probability of stable pit formation by decreasing metastable pit initiation and growth rates.     

Effects of hydrogen on the corrosion of pure magnesium

Electrochemical measurements and capacitance measurements were performed for better understanding of the effects of hydrogen on the corrosion of pure magnesium. Anodic polarization curves, activation energy (E_a), pitting initiation time and electrochemical noise (EN) were carried out, which showed that hydrogen had a strong influence on the corrosion of magnesium. There existed a highest corrosion resistance of magnesium, when a series of cathodic charging current density were applied to the specimens due to the optimum hydrogen concentration in the hydride coating (MgH₂) on the surface. Mott-Schottky results confirmed that there was a hydride coating on the charged magnesium. Hydrogen ionized as H⁻ and depleted donor/electron, which induced the inversion of semi-conductivity from N-type to P-type.     

Preparation and characterization of a double-layer coating on magnesium alloy AZ91D

A double-layer coating was prepared on AZ91D alloy by plasma electrolytic oxidation (PEO) plus electroless plating (EP). The plasma eletrolytic oxidation film was prepared in a silicate bath as an inner layer of the coating. Electroless plated Ni-P layer grew from the pores of the PEO film in a nickelous acetate bath and formed as the outer layer of the coating. The microstructure and crystallographic structure was observed with FESEM and XRD. The corrosion resistance of the double-layer coating was evaluated by means of chronopotentiometric (E-t), potentiodynamic polarization (E-i), neutral salt spray test and electrochemical impedance spectroscopy (EIS) test. Compared with the data of as-cast AZ91D magnesium, the open circuit potential of the double-layer coated AZ91D alloy increased by 1.1815 V, while the self-corrosion current density decreased by two orders of magnitude. E-i, EIS result showed that the corrosion resistance of magnesium alloy AZ91D was improved by the double-layer coating. The salt spray test and polarization test results show that the pitting corrosion resistance of AZ91D alloy was improved greatly. An equivalent circuit was proposed to fit the impedance diagrams of AZ91D alloy with the coating.     

Electrochemical noise studies of the corrosion behaviour of lead anodes during zinc electrowinning maintenance

Following anodic polarisation of lead anodes with different silver contents (0.5-0.7%) during Zn electrowinning, the discharge and corrosion of lead anode during the decay period at open circuit potential in sulphuric acid bath have been studied by electrochemical noise analysis (ENA) method. Potential and current decay behaviour over 16 h shows 4 and 2 main characteristic regions, respectively. The wavelet transform has been employed to analyse the ENA data. The results of wavelets have demonstrated that energy distribution plot “EDP” was a powerful tool to provide useful information about the dominant process for different discharge and corrosion stages and mechanisms. The discharge is predominant than corrosion for all anodes for the regions I, II and III, and ENA feature of events changes from large timescale to low timescale, diffusion control or exfoliation to metastable pitting and pit growth, respectively. Region IV, only observed for 0.6% and 0.7%Ag, corresponds to general and metastable pitting corrosion under diffusion control. It has been found that anode containing lower silver content have more corrosion products even after 16 h of decay (discharge and attack dominated by pitting corrosion type with the highest corrosion due to the longest period of pitting during the 16 h).     

Study on the corrosion behavior of reinforcing steel in cement mortar by electrochemical noise measurements

The corrosion behavior of reinforcing steel in cement mortar has been studied by electrochemical noise (EN) compared with the electrochemical impedance spectroscopy (EIS). The wavelet transform, as well as the statistical methods including the standard deviation of current noise (σ_I) and noise resistance (R_n), has been employed to analyze the EN data of reinforcing steel in mortar. It is revealed that there exist three different corrosion stages of reinforcing steel in cement mortar, including a competition process between breakdown and repassivation of passive film, a pitting development and an active corrosion during the 20 cyclic immersion and drying tests. The energy distribution plot (EDP) is able to provide useful information about the dominant process for the different corrosion stages.     

The role of inhibitors on the repassivation of pitting corrosion of carbon steel in synthetic carbonated concrete pore solution

The semiconductive properties and pitting susceptibility of passive films formed on carbon steel in synthetic carbonated concrete pore solution containing Cl⁻ ions were studied by Mott–Schottky (M–S) plots and electrochemical noise (EN). The results show that both nitrite and tetraethylenepentamine (TEPA) can reduce the donor density in passive film, leading to a more positive film breakdown potential. Moreover, nitrite can rehabilitate metastable pits and passivate the broken passive film more quickly than TEPA. Polarization curves also show that nitrite can increase the pitting potential more than TEPA at same concentration. Atomic force microscopy (AFM) imaging shows that Cl⁻ ions could induce large metastable pits on passive film, but the size of pits decreases and the density of pits increases with addition of nitrite. However, AFM imaging fails to detect the metastable pits under TEPA adsorbed film in spite of a smooth-like image on the steel surface is observed. The force curves of AFM indicate a stiffness (elasticity) order of films: nitrite oxidation film > original passive film > enamine adsorbed film.     

Influence of grain size on the corrosion behavior of a Ni-based superalloy nanocrystalline coating in NaCl acidic solution

The effects of grain size on the electrochemical corrosion behavior of a Ni-based superalloy nanocrystalline (NC) coating fabricated by a magnetron sputtering technique, has been investigated in 0.5 M NaCl + 0.05 M H₂SO₄ solution. Coatings with grain sizes 10 nm, 50 nm and 100 nm were fabricated on glass and the superalloy substrates. The results indicate that a passive film with porous property, n-type semiconductive property and incorporation of chloride ions formed on the NC coating with 100 nm grain size, which increased the susceptibility to pitting corrosion. The NC coatings with 10 nm and 50 nm grain size formed compact, non-porous and p-type passive films without chloride ions, which improved resistance to pitting corrosion. The smaller grain size of the material decrease the amount of chloride ions adsorbed on the surface and promoted the formation of compact passive film, which significantly increased the material's resistance to pitting corrosion in acidic solution.     

Poly(N-ethylaniline) coatings on 304 stainless steel for corrosion protection in aqueous HCl and NaCl solutions

Poly(N-ethylaniline) (PNEA) coatings were grown by potentiodynamic synthesis technique on 304 stainless steel (SS) alloy from 0.1 M of N-ethylaniline (NEA) in 0.3 M oxalic acid solution. Characterization of adhesive and electroactive PNEA coatings was carried out by cyclic voltammetry, FT-IR spectroscopy and scanning electron microscopy (SEM) techniques. The protective properties of PNEA coatings on SS were elucidated using linear anodic potentiodynamic polarization, Tafel and electrochemical impedance spectroscopy (EIS) test techniques, in highly aggressive 0.5 M HCl and 0.5 M NaCl solutions. Linear anodic potentiodynamic polarization test results proved that PNEA coating improved the degree of protection against pitting corrosion in HCl and NaCl solutions. Tafel test results showed that PNEA coating appears to enhancement protection for SS in 0.5 M NaCl and 0.5 M HCl solutions. However, according to long-term EIS results, PNEA coating is better for the protection of SS electrodes during the long immersion period in NaCl compared to that in HCl medium.     

Interactions among metastable pits on heterogeneous electrodes

A previously developed spatiotemporal model of the interactions among metastable pits on homogeneous electrodes is extended to heterogeneous surfaces with randomly distributed inclusion sites in an otherwise passive electrode. The inclusions are treated as favored sites for pit initiation events. The effect of the spatial density of inclusions on susceptibility to explosive growth in pitting corrosion is investigated. It is shown that there exist critical values of the pit density for the explosive growth of metastable pits that leads to a transition to pitting corrosion. Convective mass transport is shown to alter the density required for explosive growth. The effect of a limitation on the number of times that an inclusion can initiate a pit on the transition to the pitting corrosion is also considered.     

AC impedance spectroscopy study of the corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution

The corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution at the corrosion potential (E_corr) was investigated using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results showed that when the immersion time was less than 18th, general corrosion occurred on the surface and the main corrosion products were hydroxides and sulfates. The film coverage effect was the main mechanism for the corrosion process of AZ91 alloy. At this stage, the matrix had a better corrosion resistance. With the increasing immersion time, pitting occurred on the surface. At this stage, the corrosion process was controlled by three surface state variables: the area fraction θ₁ of the region controlled by the formation of Mg(OH)₂, the area fraction θ₂ of the region controlled by the precipitation of MgAl₂(SO₄)₄·2H₂O, and the metastable Mg⁺ concentration C_m.     

Pitting corrosion of AZ91D and AJ62x magnesium alloys in alkaline chloride medium using electrochemical techniques

Pitting corrosion of AZ91D-DC (die cast), AZ91D-ESTC (electromagnetically-stirred billets; thixocast), AZ91D-SFTC (billets solidified freely; thixocast) and AJ62x-DC (die cast) specimens was studied in alkaline chloride medium (0.1 M NaOH + 0.05 M NaCl + 2 ml H₂O₂) at 25 °C and pH 12.3. Electrochemical noise (EN) measurements have confirmed to some extent the polarization results (passive zone, pitting current and average corrosion rate). AZ91D-ESTC specimens have shown the best corrosion resistance followed by AZ91D-SFTC and AZ91D-DC. Intense corrosion rate was observed at the beginning of experiment and it decreased with immersion period. Localized corrosion with dense pitted areas was observed during a 16 h immersion period for AZ91D-SFTC and AZ91D-ESTC specimens. The best passive zone was observed for AJ62x-DC because of the corrosion products formed at the surface. After a 6 h of immersion, EN analyses in the frequency domain indicated a change in the sub-mode of pitting, becoming a classical pitting type, for AJ62x and AZ91D die cast specimens. Analysis with the scanning reference electrode technique (SRET) has showed that AJ62x specimen presented the biggest potential difference between the most active anode and the most active cathode and more numerous zones of intense localized corrosion. It was also found that the lifetime of the pit appeared after 8:20 h of immersion was longer for AJ62x and AZ91D die cast specimens being associated to a classical pitting.