Y.F. Cheng

Electronic structure and pitting susceptibility of passive film on carbon steel

Abstract The semiconductive properties, electronic structure and pitting susceptibility of passive films formed on A516-70 carbon steel in chromate solutions with and without chloride ions are studied by capacitance measurements, Mott–Schottky analysis and the electrochemical noise technique. The passive film shows features of an amorphous or highly disordered n-type semiconductor. The donor concentration is in the range of 10 26 –10 27 m −3 and decreases with the film formation potential (prepassivation potential). The thickness of the space charge layer ranges from 1 to 6 A. The flat band potential increases with an increase in the film formation potential. The nonlinearity of Mott–Schottky plots, depending upon the film formation potential, indicates the existence of two donor levels in the space charge layer. When the film formation potential is below −50 mV, the two levels are revealed by two slopes in the Mott–Schottky plots. For a film formed at a prepassivation potential of −50 mV, only the shallow donor level can be ionized due to increase in the stability of the passive film. The onset potential and the transitional potential of metastable pitting, determined by electrochemical noise measurements, coincide with the flat band potential of the film formed at the corrosion potential and the critical potential at which both deep and shallow donor levels are ionized. It is suggested that the divalent iron cation vacancies generated by the ionization of the deep donor level are mainly responsible for pitting corrosion.

The role of chloride ions in pitting of carbon steel studied by the statistical analysis of electrochemical noise

Abstract The role of chloride ions in metastable pitting of A516-70 carbon steel was studied based on the statistical analysis of recorded current and potential noise. It is shown that the amplitude and the occurrence frequency of current and potential transients, indicating the initiation of metastable pits, increase with the concentration of chloride ions. The time dependence of pit initiation rate ( λ ) is well described by equations of λ ( t )= λ (0)exp( at ), 0≤ t τ and λ ( t )= λ ( τ )exp(− bt ), t ≥ τ . The interaction between chloride ions and passive film has the reaction order of 3. The average peak potential depends upon the chloride concentration by a logarithm law. An analysis of the probability distribution of pit repassivation time vs. chloride concentration shows that the main role of chloride ions in pitting is to increase the chance of the breakdown of passive film, rather than to inhibit the surface repassivation.

A comparison of the pitting susceptibility and semiconducting properties of the passive films on carbon steel in chromate and bicarbonate solutions

Abstract The semiconducting properties and pitting susceptibility of the passive films formed on A516-70 carbon steel in chromate and bicarbonate solutions were studied by polarization measurements, electrochemical noise (EN) and Mott–Schottky analysis. A stable passivity is established in 0.01 M CrO 4 2− solution with a potential range of more than 1000 mV and the pitting potential of 900 mV (Ag/AgCl). Similar passive properties are obtained in bicarbonate solution only when the concentration of HCO 3 − reaches 0.5 M. Upon the addition of Cl − , metastable pitting is initiated and indicated by the typical current transients, which have the shape of a quick current rise followed by a slow recovery. There is a higher pit-initiation rate and a smaller noise resistance in the bicarbonate solution than in the chromate solution. The strong frequency dependence of the capacitance behavior shows that the passive films formed in both solutions are n-type semiconductors with a highly disordered nature. There is a thicker space-charge layer, a lower donor density and a more positive flat-band potential for the passive film formed in the chromate solution. Electrochemical results show that the passive film that formed in the chromate solution is more stable than that formed in the bicarbonate solution.

Spectral analysis of electrochemical noise with different transient shapes

Abstract The noise spectra under different transient shapes were calculated and the relationship between the spectral parameters (roll-off slope and roll-off frequency) in the frequency domain and the transient variables in the time domain were analyzed. The influence of corrosion types on noise spectra was considered. It is shown that a ‘white’ noise always appears in the low-frequency range of the noise spectrum and the roll-off slope of the spectrum in the high-frequency range depends on the transient shape, not the corrosion type. During pitting, the roll-off frequency of the noise spectrum is directly related to the time constant of the exponential function contained in the noise transient in the time domain and reflects the repassivation or growth rate of the metastable pits. For the triangular- and delta-shaped transients generally generated during passivity or general corrosion, the roll-off frequency is related to the time interval of the current transient during the current rise or drop stage. Although spectral analyses can extract useful information from the noise transients, the spectra cannot display the difference of any two transients that are symmetrical in shape in the time domain.

Metastable Pitting of Carbon Steel under Potentiostatic Control

The metastable pitting of A516-70 carbon steel was studied under potentiostatic control in solutions containing chloride ions. It was shown that there were different current fluctuation patterns and spectral slopes, that is, roll-off slopes, in passivity, general corrosion, and metastable pitting. Pits were often covered by a deposit which played an important role in the current fluctuation, with a quick current rise followed by a slow drop. There was a transitional potential (about 0 mV vs Ag/AgCl electrode) below which the metastable pitting initiation rate increased with the potential, because more sites would be activated. Above the transitional potential, the decay of the pitting occurrence rate with increased potential was due to the elimination of available pit sites. When the applied potential was between {minus}50 and 100 mV, pit growth kinetics was controlled by the potential drop through the deposit over the pit mouth. The potential dependence of repassivation time was mainly due to the effect of applied potential on the deposit over the pit mouth. There seemed to be good agreement between the calculated pit size and the measured values by optical microphotography. The assumption of hemispherical pit geometry was reasonable in calculating the pit radii.

Recent advances in electrocatalysts for electro-oxidation of ammonia

The electro-oxidation of ammonia has attracted much attention in recent years since it addresses both the clean energy supply free of COx and environmental protection. However, the large-scale applications of ammonia electro-oxidation technologies have been strongly hindered by the insufficient performance and the high cost of the electrocatalysts related to the usage of high Pt loadings. Therefore, considerable effort has been devoted to developing electrocatalysts for ammonia electro-oxidation with improved activity and lower Pt loading over the past 10 years. From the initial focus on pure metal electrocatalysts, a wide range of Pt-based and Pt-free electrocatalysts for ammonia electro-oxidation have been investigated. This review begins with a brief discussion on the mechanism of the ammonia electro-oxidation with particular attention on the latest findings on the active intermediates and poisoning species, which is important for the fundamental understanding of the principles of improving the electrocatalyst performance for ammonia electro-oxidation. Typical types of electrocatalysts for ammonia electro-oxidation are next described, including pure metal, Pt alloy and Pt-free electrocatalysts. The various preparation methods are discussed and related to the resulting structures and electrocatalytic properties for ammonia electro-oxidation. The factors influencing the performance of the electrocatalysts are also highlighted for gaining fundamental insight into the optimization of the electrocatalyst performance. Based on the results achieved in this area, several future research directions are finally proposed and discussed.

Analysis of the role of electrode capacitance on the initiation of pits for A516 carbon steel by electrochemical noise measurements

The fluctuations of potential and current of A516 carbon steel were monitored in chloride solution. Different noise patterns were observed during the incubation and initiation periods of pitting. During the incubation of pits, the fluctuations of potential were in phase with the current fluctuations, indicating that the faradaic current plays a major role in pit incubation. The initiation of pitting was characterized by sharp fluctuations of potential and current. The slower recovery of potential always exceeded the time for the recovery of the current. This was attributed to the slow discharging of the capacitance on the electrode surface. The capacitance plays a major role on potential fluctuations generated during pitting of carbon steel.

Stress corrosion cracking of 2205 duplex stainless steel in H2S-CO2 environment

Stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in H2S–CO2 environment was investigated by electrochemical measurements, slow strain rate test (SSRT), and scanning electron microscopy (SEM) characterization. Results demonstrated that the passive current density of steel increases with the decrease of solution pH and the presence of CO2. When solutions pH was 2.7, the steel SCC in the absence and presence of CO2 is expected to be a hydrogen-based process, i.e., hydrogen-induced cracking (HIC) dominates the SCC of the steel. The presence of CO2 in solution does not affect the fracture mechanism. However, the SCC susceptibility is enhanced when the solution is saturated simultaneously with H2S and CO2. With elevation of solution pH to 4.5, the hydrogen evolution is inhibited, and dissolution is involved in cracking process. Even in the presence of CO2, the additional cathodic reduction of H2CO3 would enhance the anodic reaction rate. Therefore, in addition to the hydrogen effect, anodic dissolution plays an important role in SCC of duplex stainless steel at solution pH of 4.5.

Features of electrochemical noise generated during pitting of inhibited A516-70 carbon steel in chloride solutions

The features of electrochemical noise generated during pitting of A516-70 carbon steel in chloride solutions containing Na2CrO4 were investigated. Polarization curves showed that the threshold Cl− concentration in 0.01m Na2CrO4 solution to initiate pitting and the threshold CrO42− concentration in 0.1m NaCl solution to maintain passivation for A516-70 carbon steel were approximately 0.1m and 0.01m, respectively. Noise measurements indicated that above the critical Cl− concentration, the stochastic current fluctuations appeared after 5h of immersion, which was an indication of pit initiation. Increasing CrO42− concentration to 0.05m led to the production of symmetrical current spikes at the start of immersion. The noise pattern changed to frequent fluctuations with small amplitude during immersion. Noise analysis in the frequency domain showed that pitting initiation was characterized by the slope of the PSD plot approaching -2. Extending the immersion time decreased the pitting initiation rate, as indicated by the decreased PSD slope and increased PSD frequency.

Hydrogen Permeation and Electrochemical Corrosion Behavior of the X80 Pipeline Steel Weld

In this work, the microstructure of an X80 pipeline steel weld was characterized by optical and scanning electron microscopy. The hydrogen permeation and electrochemical corrosion behavior were investigated by various electrochemical measurements and analysis. It was found that there is the smallest hydrogen permeation rate, but the largest hydrogen trapping density at heat-affected zone, while the base steel has the lowest hydrogen trapping. These results are associated with the typical microstructure of the individual zone. Moreover, the accumulation of hydrogen atoms would result in an enhanced corrosion locally.