Coupling active, passive

Note that Reference" draws attention to the possibility of an increase of anodic polarisation of the more negative member of a couple leading to a decrease in galvanic corrosion rate. There can also be a risk of increased corrosion of the more positive member of a couple. Both these features can arise as a result of active/passive transition effects on certain metals in certain environments. 

A simple variant of the COSY experiment is COSY-35 (sometimes called COSY-45), in which the second 90° pulse is reduced from a 90° pulse to a 35° or 45° pulse (Fig. B.3). The result is that the fine structure of crosspeaks is simplified, with half the number of peaks within the crosspeak. This makes it much easier to sort out the coupling patterns in both dimensions and to measure couplings (active and passive) from the crosspeak fine structure. A more important variant of the COSY experiment is the DQF (double-quantum filtered)— COSY (Fig. B.4), which adds a short delay and a third 90° pulse. The INEPT transfer is divided into two steps antiphase I spin SQC to I,S DQC, and I,S DQC to antiphase S spin SQC. The filter enforces the DQC state during the short delay between the second and third pulses either by phase cycling or with gradients. DQF-COSY spectra have better phase characteristics and weaker diagonal peaks than a simple COSY, so this has become the standard COSY experiment. 

Figure 14 Evans diagrams for active-passive metal when coupled to (a) a metal that holds corr in a passive region, (b) a metal that holds Ecm above pitting or transpassive potential, and (c) a metal that causes a passive-active transition.

The major effect is on the distal tubules of nephrons, where aldosterone promotes sodium retention and potassium excretion. Under the influence of aldosterone, sodium ions are actively transported out of the distal tubular cell into blood, and this transport is coupled to passive potassium flux in the opposite direction. Consequently, intracellular [Na" ] is diminished and intracellular [K+] is elevated. This intracellular diminution of [Na+] promotes the diffusion of sodium from the filtrate into the cell, and potassium diffuses into the filtrate. Aldosterone also stimulates sodium reabsorption from salivary fluid in the salivary gland and from luminal fluid in the intestines, but these sodium-conserving actions are of minor importance. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Na extrusion from plant cells is powered by the operation of the plasma membrane H -ATPase generating an electrochemical gradient that allows plasma membrane Na /H antiporters to couple the passive movement of inside the cells, along its electrochemical potential, to the active extrusion of Na [21]. Recently, AtSOSl from Arahidopsis thaliana has been shown to encode a plasma membrane Na /H antiport with significant sequence similarity to plasma membrane Na /H antiporters from bacteria and fungi [32]. The overexpression of SOSl improved the salt tolerance of Aro-hidopsis, demonstrating that improved salt tolerance can be attained by limiting Na accumulation in plant cells [33] (Table 10.1). 

E4.8. An active-passive alloy corrodes in the active state in an acidic solution in the presence of the redox couple Fe Fe. The corrosion potential and corrosion current are Ecorr= 0.35 V vs. SHE and Jcorr=10 iA/cm. If the reduction occurs under activation control with bc = — 0.1 V/decade, concentration of Fe of 10 M, and the exchange current density of the redox couple p 2+ = 0.01 pA/cw , calculate the concentration of Fe " oxidizer necessary to passivate the alloy. Recalculate the oxidizer concentration for an exchange current density of 0.1 and 1 ihJcrr for the redox couple. 

Mixed potential theory is used to estimate the galvanic current and the galvanic potential in an active-passive metal that passivates at potentials less noble than the reversible hydrogen potential. A galvanic couple between titanium and platinum of equal area of 1 cm is exposed to 1 M HCl. The electrochemical parameters for the active-passive alloy are eeq xi = —163 V vs. SHE anodic Tafel, b Ti = 0.1 exchange current density, ixi= 10 A/cm passivation potential, pp= —0.73 V passivation current, 7pass= 10 A/cm transpassive potential, = 0.4 V vs. SHE and activity of dissolved species [Ti ] = 1 M. The exchange current densities, i°, on platinum and titanium... 

Fig. 6.10 Galvanic couple between platinum and active-passive metal Fe in air-free acidic...

D.W. Urry, Entropic Elastic Processes in Protein Mechanism. II. Simple (Passive) and Coupled (Active) Development of Elastic Forces. J. Protein Chem., 1,81-114,1988, especially Figure 12. 

FIGURE 15.3 Schematic Evans diagram for the behavior of an active-passive metal. (m/m )> reversible potential of the couple o(m/m+)> exchange current density Epp, passivation potential trio critical anodic current density ip, passive current density Ef, transpassive potential. 

FIGURE 15.12 Schematic Evans diagram illustrating the influence of the rate of the reduction reaction (dotted lines) on active-passive behavior of a metal (solid line). ,ed> reversible potential for the reduction reaction oi, 02, 03, increasing exchange current densities for the reduction reaction (m/m+)> reversible potential for the M/M couple corr(i) and corr(2) are stable corrosion potentials. Concentration polarization is assumed to be absent. 

An interesting variation of the effect of galvanic coupling occurs with metals that exhibit active-passive transitions. When noble metals such as platinum, which are good catalysts for hydrogen reduction, are coupled to a metal with an active-passive transition below the reversible proton-hydrogen potential, spontaneous passivation may ensue (Fig. 7). Thus, a porous coating of noble metal on titanium, chromium, or stainless steels will result in anodic protection of the substrate. 

RG. 7—Spontaneous passivation of an active-passive metal, such as titanium, by galvanically coupling to a noble metal such as platinum. The noble metal has a high rale constant for the proton-hydrogen reaction thus, the corrosion potential of the system Is near to the reversible potential for this reaction [7]. 

Multiple-quantum coherences do not exhibit scalar couplings between their active spins. Their scalar couplings to passive spins are linear combinations of those of their individual active spins to the passive spin concerned. Thus a double-quantum coherence between spins k and / would exhibit a scalar coupling constant of (Jkm Jim) to a passive spin m the corresponding zero-quantum coherence scalar coupling constant would be (Jkm JlJ-... 

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