Corrosion potential, protein solutions

Cyclic Voltammetry—5 V/min. Figures la-e present the j-U behavior for the first cycle of copper-2% zinc in phosphated saline and in protein solutions. The corrosion potentials (ia—ic) during the forward scans were between -0.35 to -0.40 V in all cases. Two anodic peaks were observed for all protein solutions at —0.25 to —0.10 V and +0.10 to +0.30 V. The first peak in the supporting electrolyte was also observed, whereas j continued to increase, never reaching a peak up to the reversal potential of +0.5 V. Two main cathodic peaks were observed at —0.30 to —0.45 V and —0.65 to —0.75 V in all cases. A prepeak inflection also occurred at -0.2 to -0.3 V for both the albumin and globulin systems, and a small peak at —1.1 to —1.2 V for most systems. Cathodic currents increase sharply below about —1.5 V. Figures 2a-b represent the surface appearances after the first cycle of polar-... 

Because of zinc s high electronegativity, this element must have participated in some manner with the corrosion processes. About the only possible indications from the electrochemical and x-ray evaluations made, are the small reduction peaks observed at about — 1.15 V for protein solutions on the 5 V/min cyclic voltamograms. These cathodic peaks for use as evidence in showing zinc corrosion may just as well be reduction of copper products, since cathodic peaks are shifted negatively with respect to their redox potentials at faster sweep rates. 

Concentration polarization as reflected by the limiting diffusion current is observed for protein-free solutions at U s slightly negative to the corrosion potential, and at potentials lower than about —0.5 V for both protein-free and protein-containing solutions. The activation polarization region with a Tafel slope of beta = 0.22 V is higher by almost a factor of 2 from the beta = 0.12... 

All protein solutions shifted the corrosion potential (ia=ic) in the negative direction due to the adsorption process. For albumin and globulin systems, an enhancement in the anodic current density was observed at potentials positive to the passivation potentials. However, fibrinogen solutions exhibited an inhibition in the polarization profiles. 

We have thus at least a mathematical argument for the hypothesis that a potential change can be caused by a second protein layer adsorbed on the first layer. If one agrees with the idea above, there are several questions which have to be answered. Are the potential changes due to polarisation changes at the metal surface, changes in corrosion potential, redox levels etc. induced by the protein Are the protein molecules in the second protein layer physically different from the molecules in the solution ... 

Chromium is so called because of the brightness of many of its salts, hence the use of the Greek word for colour. Chromium can occur in every one of the oxidation states from -2 to +6, but the ground states 0, +2, +3, and +6 are common (Love 1983). Chromium metal itself does not act as an allergen and must do so in combination with a protein. Only the trivalent and hexavalent salts are able to act as haptens that is, they form potentially antigenic bonds with proteins. The metal is highly resistant to corrosion in the atmosphere and many aqueous solutions and is an unlikely cause of contact allergy. 

Thus, one obtains indirectly with Equations 5.5,5.7, and 5.8 an expression for the potential drop A(p2 3 for stationary and nonstationary conditions, which requires the measurement of the related corrosion current densities ic and i s- This situation is similar to the charging of colloid particles, like a protein or an oxide particle, with a potential drop at their surface, which is established by the pH of the contacting solution with a reaction involving hydrogen ions. For a protein, the dissociation equilibrium of -NH3/-NH2 and -COOH/-COO groups is established and, at oxides surfaces, the formation of (or OH ) is described by Equation 5.2. 

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