Evaluation of metastable pitting on titanium by charge integration of current transients

The metastable pitting of titanium has been studied under potentiostatic control in solutions containing chloride ions. An approach based on the charge integration of current transients was proposed for a quantitative determination of metastable pitting. A pit density (d_mpit) was defined as the number of metastable pits per unit area per unit time (cm⁻² h⁻¹) with a typical size, instead of a size distribution. The calculated d_mpit of titanium at 0.5 V_SCE in 0.6 M NaCl was about 1.0 × 10³ cm⁻² h⁻¹ with a typical radius of 0.12 μm. An exponential potential dependence of d_mpit was obtained through the integration approach.     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

Galvanic corrosion of titanium coupled to welded titanium in LiBr solutions at different temperatures

The galvanic corrosion generated between the titanium-welded titanium pair has been studied in heavy brine LiBr solutions at 25, 50 and 100 °C under open circuit conditions using a zero-resistance ammeter (ZRA). The results showed that welded titanium was the anode of the pair, so that its corrosion resistance decreases due to the galvanic effect. However, the extremely low galvanic current densities registered by the pair reveal the poor severity of the coupling under the studied conditions. Furthermore, it was observed that the electrodes were in the passive state, increasing the probability of localized corrosion with temperature.     

The effect of oxidation on the corrosion resistance and mechanical properties of a Zr-based metallic glass

The oxidation kinetics of the Zr₆₄Cu₁₆Ni₁₀Al₁₀ bulk metallic glass (BMG) roughly follows a two-stage rate law at both 433 and 593 K in air. An oxide film of 940 nm can be formed by oxidation at 593 K, which is ZrO₂-enriched but Cu-depleted on the outer surface. The oxide film leads to a superior passivity in 0.5 M NaCl and great corrosion resistance improvements in other solutions. The oxidation effect on mechanical properties were characterized by nanoindentation, wedge indentation and compression tests. The Zr-based BMG still keeps the amorphous nature and its good mechanical properties are retained after oxidation.     

Critical pitting temperature dependence of 2205 duplex stainless steel on dichromate ion concentration in chloride medium

In this study, the influence of various concentrations of dichromate and chloride ions on critical pitting temperature (CPT) of duplex stainless steel 2205 (DSS 2205) is investigated by employing potentiodynamic and potentiostatic CPT measurement methods. Potentiostatic results indicate that by adding 0.01 M to 0.1 M NaCl solution the CPT raised by 12 °C. Based on potentiodynamic CPT measurements in the solutions with ratio equal to one for solutions containing 0.1 M NaCl + 0.1 M and 0.01 M NaCl + 0.01 M , no CPT was detected up to 75 °C.     

Influence of the anion specificity on the anodic polarization of titanium

The anodic polarization of titanium in the presence of selected electrolytes at 0.1 mol/L concentration has been investigated. The results were interpreted according to the categorization of anions based on their cosmotrope/chaotrope character. The aggressiveness of the anions is found to be related for a part to their proneness to loose their hydration shell during the penetration of the passive film, provided they can access sufficiently to the passive surface.     

Correlation between critical pitting temperature and degree of sensitisation on alloy 2205 duplex stainless steel

In this study effect of different ageing conditions on intergranular corrosion, pitting corrosion and relation between critical pitting temperature (CPT) and degree of sensitisation (DOS) was investigated by potentiostatic polarisation and double loop-EPR methods. The results showed by increasing sensitisation time, DOS increased and measured CPT value decreased. In addition the values of DOS and CPT of specimens aged at 650 °C showed almost liner relation while this correlation was diminished for the specimens aged at 800 °C. The results may be attributed to the further formation of precipitates on specimens aged at temperature of 800 °C.     

Understanding the effects of electrode inhomogeneity and electrochemical heterogeneity on pitting corrosion initiation on bare electrode surfaces

The effects of electrode inhomogeneity (EI) and electrochemical heterogeneity (EH) on pitting corrosion initiation have been analysed by revisiting research findings reported in the literature and experimental evidences obtained in our laboratories using the wire beam electrode (WBE) method. Two mechanisms of pitting corrosion initiation have been identified on bare metal surfaces exposed directly to electrolytes. For WBE surface under free corrosion or low anodic polarisation conditions the initiation of pitting corrosion was found to be due to the disappearance of minor anodes, leading to accelerated dissolution of a few remaining major anodes. The nucleation stage of pitting corrosion appeared to be controlled by EI, while the propagation stage appeared to be determined by EH. For WBE surface under large anodic polarisation the initiation of pitting corrosion was found to be due to the formation of active new anodic sites, which is in agreement with the conventional mechanism of pitting nucleation.     

Modeling pitting corrosion by means of a 3D discrete stochastic model

Pitting corrosion is difficult to detect, predict and design against. Modeling and simulation can help to increase the knowledge on this phenomenon as well as to make predictions on the initiation and progression of it. A cellular automaton based model describing pitting corrosion is developed based on the main mechanisms behind this phenomenon. Further, a sensitivity analysis is performed in order to get a better insight in the model, after which the information gained from this analysis is employed to estimate the model parameters by means of experimental time series for a metal electrode in contact with different chloride concentrations.     

Inhibition effect of chromate on the passivation and pitting corrosion of a duplex stainless steel in LiBr solutions using electrochemical techniques

The electrochemical behavior of duplex stainless steel (DSS) in LiBr media was investigated by anodic cyclic polarization curves and AC impedance measurements. The effect of bromide concentration and the presence of chromate in the solutions on the corrosion behavior of AISI 2205 was studied. Cyclic polarization curve analyses showed that there was different pitting susceptibility of passive films depending on the LiBr concentration. Pitting potential decreases with LiBr concentration in a semilogarithmic scale following two different slopes. Chromate presence displaces pitting potentials towards more positive values at low LiBr concentrations but it has no effect when LiBr concentration increases.The comparative analysis carried out in LiBr and LiBr chromate-containing solutions at two different concentrations, 0.016 M and 0.032 M, verifies the assumption that halogen ions facilitate inhibitor adsorption. The addition of halides strongly increased the inhibition efficiency of chromate. The passive film becomes more resistant when bromide concentration increases, although film thickness decreases.     

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.     

Critical pitting temperature (CPT) assessment of 2205 duplex stainless steel in 0.1 M NaCl at various molybdate concentrations

In this study, the influence of various concentrations of molybdate () on critical pitting temperature (CPT) of duplex stainless steel 2205 (DSS 2205) in 0.1 M NaCl solution has been investigated by employing potentiodynamic and potentiostatic CPT measurements methods. No significant increase in CPT was observed in the presence of 0.0001 and 0.001 M molybdate. However, addition of 0.01 M increases 10 °C in CPT and pitting corrosion was not observed in solution containing 0.1 M molybdate up to 85 °C. Potentiostatic CPT measurements showed CPT values of 68 and more than 85 °C in 0.01 and 0.1 M molybdate concentrations, respectively.     

Identifying the role of nanoscale heterogeneities in pitting behaviour of Al-based metallic glass

To clarify the correlation of nanoscale heterogeneity with corrosion in Al-based metallic glasses, three model alloys with a single nanoscale α-Al, Al₃Ni or Al₁₁Ce₃ phase embedded in amorphous Al-Ni-Ce alloy matrix were obtained directly by melt quenching. The results indicated that the high pitting corrosion resistance of AM alloys was not deteriorated by nanocrystalline α-Al precipitation; whereas the pitting potential was slightly decreased and considerably reduced relative to their amorphous state due to the precipitation of nanocrystalline Al₃Ni or Al₁₁Ce₃ respectively. Such a pitting sensitivity of different types of heterogeneities attributes to the nano-scale pit initiation events.     

Effect of microcrystallization on pitting corrosion of pure aluminium

A microcrystalline aluminium film with grain size of about 400 nm was prepared by magnetron sputtering technique. Its corrosion behaviour was investigated in NaCl containing acidic solution by means of potentiodynamic polarization curves and electrochemical noise (EN). The polarization results indicated that the corrosion potential of the sample shifted towards more positive direction, while its corrosion current density decreased compared with that of pure coarse-grain Al. The EN analysis based on stochastic model demonstrated that there existed two kinds of effect of microcrystallization on the pitting behaviour of pure aluminium: (1) the rate of pit initiation is accelerated, (2) the pit growth process was impeded. This leads to the enhancement of pitting resistance for the microcrystallized aluminium.     

Influence of cold working on the pitting corrosion resistance of stainless steels

This paper addresses the influence of cold rolling and tensile deformation on the pitting corrosion resistance of AISI 304 and AISI 430 stainless steels, investigated using some electrochemical techniques specifically designed for the different pitting stages to be analyzed separately. Cold work is shown to act differently depending on the pitting stage under consideration. (i) The pit initiation frequency shows a maximum after 20% cold-rolling reduction or 10% tensile deformation. This maximum is also observed on the ferritic grade, contradicting the hypothesis of a direct effect of strain-induced martensite, and is more likely related to the dislocations pile-ups. (ii) The pit propagation rate increases monotonously with cold rolling reduction, and pit repassivation ability decreases (leading to a larger number of stable pits), suggesting that the overall dislocation density is the controlling factor in these stages. Last, the significance of pitting potential measurements is discussed in the light of the effect of the cold-rolling reduction on the measured values.     

Corrosion mechanisms of steel concrete moulds in contact with a demoulding agent studied by EIS and XPS

The behaviour of E24 mild steel was studied by XPS analysis and electrochemical impedance spectroscopy (EIS) in a filtered solution of cement (pH 13), and an alkyl N-aminodiphosphonate aqueous solution called Aquadem^® (7?pH?13). XPS results showed that the corrosion products developed in both media consisted of Fe₂O₃, covered by a very thin layer of goethite. The thickness of this oxide layer was estimated to be 3 nm. XPS analysis also demonstrated the adsorption of Aquadem^® on the outer layer of FeOOH for pH lower than the zero charge pH of goethite (7.55). From XPS and EIS results, physical models of the E24 steel/electrolyte interface are proposed as a function of pH. For 11?pH?13, the steel is covered by a passive film, while for pH?10, pitting corrosion takes place. At pH 7, an additional mass transport phenomenon must be taken into account. The fitting procedure provided values for several physical parameters (electrolyte resistance, passive film resistance), from which the film capacitance and the dielectric constant of the oxide layer were calculated.     

The influence of the cathodic process on the interpretation of electrochemical noise signals arising from pitting corrosion of stainless steels

The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.     

The nucleation and growth of metastable pitting on pure iron

The nucleation and growth of metastable pitting on pure iron surface in NaNO₂ + NaCl solution were investigated by potentiodynamic tests. The current fluctuations appear as cluster and overlapped. The active sites for nucleation depend mainly on the surface geometry. The growth behaviour of pure iron is different from stainless steels and carbon steels. On pure iron surface, many pits pile up together to form huge damage area. Pits are shallow, do not grow deeply and pits array one by one along the direction of abrasion grooves. The growth of stable pitting is different from metastable pitting.     

Employing electrochemical frequency modulation for pitting corrosion

By using the electrochemical frequency modulation (EFM) technique, the non-linear behavior of a corroding system is measured. This non-linear behavior is likely to be different for a system undergoing uniform or pitting corrosion. The implementation of the EFM technique to detect pitting corrosion has been investigated by observing the fluctuations in the so-called causality factors. These causality factors, resulting from an EFM test and in the ideal case having values of 2 and 3, respectively, are normally used for quality and data validation purposes. While investigating pitting corrosion, they show different behavior leading to the CPT (critical pitting temperature) detection.     

Corrosion behaviour and galvanic coupling of titanium and welded titanium in LiBr solutions

Corrosion resistance and galvanic coupling of Grade 2 commercially pure titanium in its welded and non-welded condition were systematically analyzed in LiBr solutions. Galvanic corrosion was evaluated through two different methods: anodic polarization (according to the Mixed Potential Theory) and electrochemical noise (using a zero-resistance ammeter). Samples have been etched to study the microstructure. The action of lithium chromate as corrosion inhibitor has been evaluated. Titanium and welded titanium showed extremely low corrosion current densities and elevated pitting potential values (higher than 1 V). The results of both methods, anodic polarization and electrochemical noise, showed that the welded titanium was always the anodic element of the pair titanium-welded titanium, so that its corrosion resistance decreases due to the galvanic effect.