Electrochemistry of non-aged 90-10 copper-nickel alloy (UNS C70610) as a function of fluid flow: Part 1: Cathodic and anodic characteristics

The polarisation behaviour of freshly polished UNS C70610 (CN 102) 90-10 copper-nickel has been examined in fully characterised seawaters using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE) geometries. The charge and mass transport controlled responses of both the cathodic and anodic reactions are presented as a function of both laminar and turbulent undisturbed fluid flow. At low values of polarisation applied over short exposure periods (<1 h), the anodic behaviour of non-aged material is controlled by the selective dissolution of the copper component of the alloy. Under conditions of complete mass transport control, oxygen reduction proceeds via the irreversible direct four-electron reduction to the hydroxide ion. Above a critical range of Reynolds numbers the rate of both the reduction and oxidation reactions tended towards a more reversible character. This change in mechanism, however, was not observed for unalloyed copper and was attributed to a convective-diffusion-based modification of the corrosion product film.     

Electrochemistry of active chromium: Part II. Three hydrogen evolution reactions on chromium in sulfuric acid

It was shown that chromium in deaerated sulfuric acid exhibits two stable corrosion potentials, depending whether the metal had previously been in contact with air or subjected to activation by cathodically evolving hydrogen. Electrochemical polarization measurements, as well as measurements of the actual metal dissolution rate at the corrosion potentials, anodic or cathodic polarization, using the analytical determination of Cr ions in the solution, weight-loss of metal, or volumes of hydrogen evolved, showed that hydrogen can evolve on chromium by three different reaction mechanisms. The first one is the electrochemical hydrogen evolution reaction from H⁺ ions at the bare chromium surface obtained by cathodic activation. This reaction and the active anodic dissolution of chromium determine one stable corrosion potential. The second reaction is the reaction of H⁺ ions on the oxidized chromium surface which, coupled with the anodic dissolution of passivated chromium determines the other stable corrosion potential. The third one is the “anomalous” or chemical reaction of chromium with water molecules and hydrogen ions whereby hydrogen is liberated. This is a potential independent reaction, occurring on the bare metal surface, and which is at pH <2 several times faster at the corrosion potential than the electrochemical hydrogen evolution reaction. The consequence is that the overall corrosion rate is several times faster than that determined by the usual electrochemical methods. The applicability of the different methods of measuring electrochemical corrosion rates and cathodic current efficiencies in chromium plating is discussed. Also, a possible role of the “anomalous” chromium dissolution in corrosion fatigue and stress corrosion cracking of stainless steels is considered.     

Cation participation during the redox switching of poly(vinylferrocene) films in aqueous 0.05 M perchlorate solutions: Part 1: Cyclic voltammetry and the EQCM

Mobile species transfer accompanying the redox switching of poly(vinylferrocene) (PVF) in aqueous perchlorate solutions of four cations was investigated using the electrochemical quartz crystal microbalance (EQCM). Cyclic voltammetry in 0.05 M solutions containing hydronium, sodium, rubidium or tetraethylammonium cation was employed. The mass transients could not be explained solely in terms of anion and solvent transport processes. EQCM transient data showed that the cation as well as perchlorate and water transport participated in the redox switching process in the four bathing solutions. Results and interpretation of the experimental approach are presented.     

Corrosion inhibitors : Part II: Quantum chemical studies on the corrosion inhibitions of steel in acidic medium by some triazole, oxadiazole and thiadiazole derivatives

The corrosion inhibition efficiencies of some triazole, oxadiazole and thiadiazole derivatives for steel in presence of acidic medium have been studied by using AM1, PM3, MINDO/3 and MNDO semi-empirical SCF molecular orbital methods. Geometric structures, total negative charge on the molecule (TNC), highest occupied molecular energy level (E_HOMO), lowest unoccupied molecular energy level (E_LUMO), core-core repulsion (CCR), dipole moment (μ) and linear solvation energy terms, molecular volume (V_i) and dipolar-polarization (π^*), were correlated to corrosion inhibition efficiency. Four equations were proposed to calculate corrosion inhibition efficiency. The agreement with the experimental data was found to be satisfactory; the standard deviations between the calculated and experimental results ranged between ±0.03 and ±4.18. The inhibition efficiency was closely related to orbital energies (E_HOMO and E_LUMO) and μ. The correlation between quantum parameters and experimental inhibition efficiency has been validated by single point calculations for the semi-empirical AM1 structures using B3LYP/6-31G^** as a higher level of theory. The proposed equations were applied to predict the corrosion inhibition efficiency of some related structures to select molecules of possible activity from a presumable library of compounds.