Potential ranges

For larger systems N> 1000 or so, depending on the potential range) anotlier teclmique becomes preferable. The cubic simulation box (extension to noncubic cases is possible) is divided into a regular lattice of n x n x n cells see figure B3.3.7. These cells are chosen so that the side of the cell = L/n is greater than the... 

In tire potential range anodic to stable pit growtli occurs. The value of is shifted to lower anodic potentials... 

In the potential range catliodic to one frequently observes so-called metastable pitting. A number of pit growtli events are initiated, but tire pits immediately repassivate (an oxide film is fonned in tire pit) because the conditions witliin tire pit are such that no stable pit growtli can be maintained. This results in a polarization curve witli strong current oscillations iU [Pg.2728]

FIGURE 8.1 Approximate potential ranges in nonaqueous solvents. 

Anodic Protection On the reverse anodic scan there will be a low current region (LCB) in the passive range. The passive potential range of the LCB is generally much narrower than the passive region seen on a forward slow scan. In anodic protection (AP) work the midpoint of the LCB potential is the preferred design range. This factor was verified for sulfuric acid in our laboratory and field studies. 

One must always bear in mind that potential dependence is not the same in different types of corrosion. Thus critical potential ranges for different kinds of corrosion can overlap or run counter to one another. This is particularly important... 

The principle of electrochemical corrosion protection processes is illustrated in Figs. 2-2 and 2-5. The necessary requirement for the protection process is the existence of a potential range in which corrosion reactions either do not occur or occur only at negligibly low rates. Unfortunately, it cannot be assumed that such a range always exists in electrochemical corrosion, since potential ranges for different types of corrosion overlap and because in addition theoretical protection ranges cannot be attained due to simultaneous disrupting reactions. 

To discover the effective potential ranges for electrochemical protection, the dependence of the relevant corrosion quantities on the potential is ascertained in the laboratory. These include not only weight loss, but also the number and depth of pits, the penetration rate in selective corrosion, and service life as well as crack growth rate in mechanically stressed specimens, etc. Section 2.4 contains a summarized survey of the potential ranges for different systems and types of corrosion. Four groups can be distinguished ... 

Fig. 2-18 J U) curves and critical potential range for intergranular stress corrosion (hatched) for a hardened 10 CrMo 9 10 steel (ASTM P21) in boiling 35% NaOH — potentio-dynamically measured with +0.6 V h - - potential change after every 0.5 h At/ = +0.1 V x-x-x potential change after every 0.5 hAf/ = -0.1 V.

In this section a survey is given of the critical protection potentials as well as the critical potential ranges for a possible application of electrochemical protection. The compilation is divided into four groups for both cathodic and anodic protection with and without a limitation of the protection range to more negative or more positive potentials respectively. 

Protection potentials and protection potential ranges for important systems... 

The adjustment of a protection station or of a complete protection system where there is stray current interference is made much easier by potential control. Potential control can be indispensable for electrochemical protection if the protection potential range is very small (see Sections 2.4 and 21.4). This saves anode material and reduces running costs. 

Cathodic protection of different materials in installations of dissimilar metals is only possible if the protection potential ranges of the individual materials overlap. Section 2.4 gives information on the protection potential ranges of various systems. If there is no overlapping, then insulating couplings must be installed. This is also appropriate and even necessary if the protection current densities are very different. 

The protection potentials for seawater are described in Section 2.4. In pipelines and harbor installations, there is no limiting negative potential t/ for uncoated earbon steel or for steel provided with thiek eoatings over 1 mm, with yield points up to 800 N mm". With dynamieally highly loaded structures, the protection potential ranges in Table 16-2 should be adhered to as in the regulations [1-3] because of the risk of hydrogen-induced stress corrosion (see Section 2.3.4). 

The protection current requirement for aluminum ships is considerably less because of the dense adherent oxide films. The necessary protection current requirement is being clarified in current investigations [24] but good results have been obtained by assuming a figure of 10% of that for steel. With aluminum there is only a very narrow permissible potential range [25] (see Section 2.4) so that impressed current protection cannot be used because of the anodic voltage cone and only selected anode materials can be considered. 

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

Anodic protection is particularly suitable for stainless steels in acids. Protection potential ranges are given in Section 2.4. Besides sulfuric acid, other media such as phosphoric acid can be considered [13,21-24]. These materials are usually stable-passive in nitric acid. On the other hand, they are not passivatable in hydrochloric acid. Titanium is also a suitable material for anodic protection due to its good passivatability. 

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