Corrosion steady-state

For many electrode processes of interest, the rates of electron transfer, and of any coupled chemical reactions, are high compared with that of steady state mass transport. Therefore during any steady state experiment, Nernstian equilibrium is maintained at the electrode and no kinetic or mechanistic information may be obtained from current or potential measurements. Apart from in a few areas of study, most notably in the field of corrosion, steady state measurements are not therefore widely used by electrochemists. For the majority of electrode processes it is only possible to determine kinetic parameters if the Nernstian equilibrium is disturbed by increasing the rate of mass transport. In this way the process is forced into a mixed control region where the rates of mass transport and of the electrode reaction are comparable. The current, or potential, is then measured as a function of the rate of mass transport, and the data are, then either extrapolated or curve fitted to obtain the desired kinetic parameters. There are basically three different ways in which the rate of mass transport may be enhanced, and these are now discussed. 

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

The critical current and primary passivation potential will not appear on an anodic polarisation curve when the steady-state potential already is higher than In such a case the potentiostat is unable to provide direct data for constructing the full polarisation curve. If that portion of the curve below the steady-state potential is desired, then the potential has to be held constant at several points in this range and corrosion currents calculated from corrosion rates as determined from solution analyses and/or weight losses. 

Steady-state potential comparable with Type 1 reversible electrode Metal in a solution of electrolyte in which ions are produced by a corrosion reaction in an VAf exchange that determines the potential. Zn in NaCI solution Zn in dilute HCI... 

When the load on a utility boiler changes from a steady state to cycling up to a peak load or down to a reduced load, the concentration of hydrogen in steam can easily triple, indicating a significant increase in corrosion in FW heaters and boiler section components. 

The anodic oxidation of sheet aluminum has been used for a long time to protect aluminum against corrosion by a well-adhering oxide layer. Porous oxide layers are formed if acid electrolytes are used that can redissolve the aluminum oxide (mostly sulfuric or phosphoric acid). A compact oxide layer is formed at the beginning of the electrolysis (Fig. 20.3). Simultaneously, the current decreases, due to the electric resistance of the oxide. Subsequently follows a process in which the oxide is redissolved by the acid, and the current increases until it reaches a steady state. The electrochemical oxidation continues to take place with formation of pores. At the end of a pore, where it has the largest curvature, the electric field has its largest gradient and the process of redisolution is fastest. 

Adsorption versus Polymerization. It is instructive to examine further the time dependence of the corrosion inhibition. In acid corrosion inhibition tests, steady state is customarily assumed to be reached within 10 to 20 min after initial exposure of the metal specimen. Since the inhibitors function by reducing the available active surface area, we expect an increase in and a corresponding decrease in P. The degree of corrosion protection the inhibitor provides is given by... 

Non-steady state corrosion rate behavior appears to be a general phenomenon and is associated with polymerization reactions. The latter, which results in formation of a film on the sorbed monolayer, provides a smaller increment of protection than does adsorption and occurs at the expense of inhibitor loss from the solution. In some cases, however, the increased protection provided by the film is substantial and merits further investigation. 

Pt on TiC and TiN has also been reported to show good cycle stability (to 1.2 V), especially when mixed with uncatalyzed carbon.i° Steady-state corrosion tests of Pt/TiC at 1.2 and 1.4 V showed gradual oxidation to Ti02, although this was not dependent on potential. TiC has also been used by 3M as a support for a series of non-PM catalysts and has showed very stable performance over 1,000 h at 0.6 V. However, the overall activity was four times lower than that of an equivalent catalyst on carbon. 

Copper alloys are used extensively in power plant condensers, and as a result, copper can usually go into a corrosion product film or directly into solution as an ion or as a precipitate in the initial stages of condensation by tube corrosion. As corrosion products form and increase in thickness, the corrosion rate decreases until a steady state is achieved. Studies indicate that copper release is a function of flow rate more so than of the salt content of the makeup water. 

Fig. 11. Evolution of the apparent normalized release rates of various species as a function of the duration of corrosion. Although the initial rates (i.e., after one day) are different from sample to sample and from element to element, the rates measured after 10 days tend to a limiting value independent of the HT material. Indeed, steady state would be reached after a longer period of corrosion.

Medieval and antique glasses have an observed durability greater than 103 years (Gillies Cox 1988 Macquet Thomassin 1992 Sterpenich 1998), whereas the extrapolated durability of nuclear HLW glasses extends above 104 years (Jollivet et al. 1998), as determined from corrosion experiments performed under conditions of long-term rate and steady state. On this basis, and by comparison of our results (one day corrosion) and the ones obtained by the MCC-1 test, the durability of our HT materials is estimated to range at least between 103 years and 104 years. 

This can be elucidated by a corrosion diagram (Fig. 12), which shows in semilogarithmic coordinates current-voltage characteristics for two conjugated reactions. Using condition (43) and neglecting ohmic potential drop in the system, one can find from the intersection of those characteristics the steady state corrosion current icorr and corrosion potential [Pg.283]

Current and potential distributions are affected by the geometry of the system and by mass transfer, both of which have been discussed. They are also affected by the electrode kinetics, which will tend to make the current distribution uniform, if it is not so already. Finally, in solutions with a finite resistance, there is an ohmic potential drop (the iR drop) which we minimise by addition of an excess of inert electrolyte. The electrolyte also concentrates the potential difference between the electrode and the solution in the Helmholtz layer, which is important for electrode kinetic studies. Nevertheless, it is not always possible to increase the solution conductivity sufficiently, for example in corrosion studies. It is therefore useful to know how much electrolyte is necessary to be excess and how the double layer affects the electrode kinetics. Additionally, in non-steady-state techniques, the instantaneous current can be large, causing the iR term to be significant. An excellent overview of the problem may be found in Newman s monograph [87]. 

---