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The characteristics of a true Tafel slope
No full text2005 is the 100th anniversary of the two original publications of the Tafel equation [1,2]. The international corrosion community is currently celebrating [3] the use of the corresponding Tafel slope (?), which is one of the most frequently used parameters in electrochemical corrosion. Even now, with the use of modern research and testing techniques, both electrochemists and corrosion engineers are frequently encountering this ‘Tafel constant’ in the technical literature and in instrumentation manuals (you can’t use an LPR meter without assuming beta values). Unfortunately, Tafel slopes are commonly misused and measured completely out of context. So, what is a Tafel slope? Perhaps most importantly, when is an apparent ‘Tafel’ slope a true Tafel slope
Electrochemical corrosion of unalloyed copper in chloride media––a critical review
No full textThe literature dealing with the electrochemical corrosion characteristics of unalloyed copper in aqueous chloride media is examined. The enormous quantity of polarisation and mixed/corrosion potential data that has been made available in the literature over the last 50 years has been compiled and discussed in a comprehensive review. For a wide range of electrode geometries, the importance of the chloride ion and the mass transport of anodic corrosion products on the corrosion behaviour of copper are made clear for both freshly polished and ‘filmed’ surfaces
Corrosion and impressed current cathodic protection of copper-based materials using a bimetallic rotating cylinder electrode (BRCE)
No full textA bimetallic rotating cylinder electrode (having individual electrode areas of 10 cm2 and rotating at 200–1400 rpm) has been used to examine the corrosion and protection characteristics of copper/nickel aluminium bronze and 90–10 copper–nickel/nickel aluminium bronze galvanic couples in filtered seawater at 25 °C. The flow-influenced electrochemistry of the systems was determined using zero resistance ammetry, corrosion potential measurements and a potential step current transient technique. In each case, the galvanic corrosion potential and corrosion rate displayed a Reynolds number dependency where mass transport control of the anodic dissolution reaction partially controlled the reaction rate. Bimetallic impressed current cathodic protection (ICCP) has also been demonstrated for a range of applied protection potentials and Reynolds numbers. A comparison has been made between the directly measured bimetallic ICCP current demand and that determined from independent, single-metal rotating cylinder electrode measurements. In this case, the mixed charge and mass transport controlled rate of oxygen reduction was examined
Electrochemistry of non-aged copper-nickel (UNS C70610) in fully developed fluid flows. Part 1: Cathodic and anodic characteristics
No full textThe 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
Electrochemical corrosion behaviour of 90-10 Cu-Ni alloy in chloride-based electolytes
No full textThe corrosion of 90–10 copper–nickel alloy in aqueous chloride electrolytes has received considerable attention in the literature due to its widespread use in seawater and saline environments. From an analysis of the electrochemical behaviour of the alloy, it is clear that both the polarization and mixed/corrosion potential characteristics show a close comparison to unalloyed copper. Important differences arise, however, due to the semi–conducting nature, composition and overall protectiveness of the corrosion products on the 90–10 copper–nickel alloy. In this work the metallurgy, electrochemistry and mechanism of passivation of the alloy are reviewed to provide a focused source of data regarding the electrochemical characteristics of the alloy in saline media
Flow influenced electrochemical corrosion of nickel aluminium bronze – Part II. Anodic polarisation and derivation of the mixed potential
No full textThe mixed charge and mass transfer influenced anodic current response of CA 104 nickel aluminium bronze (NAB) is presented as a function of both laminar (rotating disc electrode) and fully turbulent (rotating cylinder electrode) fluid flow. At low values of positive polarisation, the overall behaviour of the freshly polished material in filtered and artificial seawaters is closely related to that of unalloyed copper. The primary anodic reaction in this case is the selective dissolution of the copper component via a cuprous di-chloride complex anion. At large positive polarisation, the solid solution mole fraction for the production of a discrete film of protective alumina (Al2O3) is examined as a function of Reynolds number and discussed in terms of a new mechanism for the passivation NAB in seawater. The polarisation data is used to replicate experimental Reynolds number dependent, corrosion potentials and corrosion current densities over a wide range of electrode angular velocities
Strategies for the determination of the convective-diffusion limiting current from steady state linear sweep voltammetry
No full textThe limiting current is an important parameter for the characterization of mass transport in electrochemical systems operating under convective-diffusion control. Four methods to determine the limiting current from current (I) vs. potential (E) plots are considered. Strategies to determine the limiting current values include: 1) direct measurement from I vs. E curves, 2) estimation from the current value at EL =DE/2 where DE is the length of the limiting current plateau), 3) evaluation of the first derivative dI/dE in the I vs. E curve and 4) from plots of E/I vs. I-1. The electrode reactions chosen to demonstrate the different strategies are: Cu(II) ? Cu(I) and Cu(I) ? Cu(0) in 1.5 mol dm-3 NaCl (pH 2) at a platinum rotating disc electrode and K3Fe(CN)6 ? K4Fe(CN)6 in 1 mol dm-3 NaOH at a 60 ppi reticulated vitreous carbon electrode (RVC)
Flow influenced electrochemical corrosion of nickel aluminium bronze – Part I. Cathodic polarisation
No full textThe cathodic polarisation behaviour of CA 104 nickel aluminium bronze (NAB) has been examined in fully characterised seawaters (filtered and artificial) using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE). Linear sweep voltammetry and a potential step, current transient technique were used to examine the charge transfer and mass transfer controlled cathodic response as a function of both laminar and turbulent fluid flow. For freshly polished surfaces, the rate of irreversible charge transfer controlled oxygen reduction is controlled by the exchange of a single electron and hydrogen evolution is only significant at potentials more negative than approximately –1.0 V vs. the saturated calomel electrode (SCE)
Electrochemical study of UNS S32550 super duplex stainless steel corrosion in turbulent seawater using the rotating cylinder electrode
No full textThe cathodic and anodic characteristics of freshly polished and pre-reduced UNS S32550 (ASTM A479) super duplex stainless steel in a filtered and conductivity-adjusted seawater have been investigated under controlled flow conditions. A rotating cylinder electrode was used together with both steady and non-steady-state voltammetry and a potential step current transient technique to investigate the electrode reactions in the fully characterized electrolyte. Both oxygen reduction and hydrogen evolution were highly irreversible and the material exhibited excellent passivation and repassivation kinetics. Relative corrosion rates were derived and the corrosion mechanism of the alloy was found to be completely independent of the mass-transfer effects, which can contribute to flow-induced corrosion
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