Cathode surface area

The crevice shape markedly affects corrosion. Crevices so tight that water may not enter are entirely immune to attack. In misting environments or alternately wet-diy environments, the crevice holds water and may allow continued attack even when neeu by surfaces eire dry. In sea water, the severity of attack in stainless steel crevices depends on the ratio of the crevice area to the cathodic surface area outside the crevice. If the cathodic area is large relative to crevice eirea, corrosion is promoted. 

Stainless steels tend to pit in acid solutions. Pits form local areas of metal loss associated with breakdown of a protective oxide layer. Breakdown is stimulated by low pH as well as by the decrease of dissolved oxygen in occluded regions. Small, active pit sites form and remain stable because of the large ratio of cathodic surface area (unattacked metal surface) to the pit area. Active corrosion in the pit cathodically protects immediately adjacent areas. If conditions become very severe, pitting will give way to general attack as more and more of the surface becomes actively involved in corrosion. 

Considering the first condition, the cathode surface area should be above a certain size depending on the initial analyte concentration in order to collect the limiting current id during the electrolysis the current will decrease rapidly as shown in Fig. 3.84, and this occurs, as we know from coulometry (see later), exponentially with time. 

Maximum yield was obtained when the applied voltage was 13.1 V, the cathode surface area was 4 cm2 and the total CH3I concentration was 35.2 pmol. The C-14 labelled tetramethyllead was isolated by the extraction procedures developed for the Grignard route (vide supra). It was next converted to 14CH3(CH3)2PbCl by controlled oxidation with HC1. 

Several basic principles well known to the electroplating industry are employed in electrolytic recovery expanded cathode surface area, close spacing between cathode and anode, and recirculation of the rinse solution. Electroplaters can design their own units by... 

Other considerations affecting the efficiency of the plating process were the voltage used, the agitation of the bath and the cathode surface area. 

Avci [19] developed a rotating tubular-bed reactor with extended cathode surface areas to improve mass transfer for nickel recovery from industrial Watts plating... 

The influence of the surface area of cathode on the plasma deposition rate is shown in Figure 15.27. It is clear that the plasma deposition rate decreases with the increase of cathode surface area. This is because plasma deposition rate is proportional to current density in cathodic polymerization as described in Chapter 8. The patterns of distribution due to magnetron configuration are similar, but the trends are magnified as the deposition rate increases with smaller cathode area. 

The glass tube is fixed on a stand by an aluminum support frame. A glass rod is used to fix the distanee between the eleetrodes, as they may be pulled each other and attract together under the strong magnetic field, if a cold-rolled steel (CRS) panel is used as the cathode. The gas is fed through a small hole placed in the center of the anode. The cathode is made of a 7 x 7 in CRS plate. The ratio of cathode surface area to anode surface area is 4.5 1. 

Figure 17.35 The dependence of the sputtering rate distribution on cathode surface area 7 //stmax= 1550G, a= 1.5cm, 6 = 8.0cm, p= lOmtorr.

The sputtering rate does not change much with the increase of cathode surface area at low system pressure p= lOmtorr). The plasma treatment is localized and effectively confined. However, at a relatively high system pressure (/) = SOmtorr), the sputtering rate is almost zero for the larger cathode. This case reiterates the importance of system pressure in AMT Ar sputtering treatment. 

In rare cases, a relatively small area near the weld will be an anode to the relatively large cathodic surface area of the parent metal. In moderately corrosive media, this zone may corrode much faster than either the weld metal or the parent metal. Postweld heat treatment is usually helpful. In some instances, normalizing (or even solution annealing in the case of an austenitic stainless steel) the weldment is necessary, a measure that can cause significant distortion problems. In most cases, the weld metal, HAZ, and parent metal do not have significant galvanic differences. 

Electrolysers with a fluidized bed cathode are constantly under investigation, since the possibility of attaining a large cathode surface area in a relatively small volume is attractive, however nothing is known about their technical exploitation. This has several reasons. During electrolysis, the particles get into contact with the auxiliary cathode for a rela-... 

A mixture of 250 g. each of KCl and NaCl is melted, and 30 g. of KThFg is added. When the melt is homogeneous, electrolysis proceeds, with the above Mo cathode, at a temperature of 775°C. A current of 18-20 amp. is required if the submerged cathode surface area is about 20 cm . After 20 minutes the cathode is carefully removed from the liquid and replaced with a new piece of Mo, 30 g. of KThFs is added, and the electrolysis is continued for 20 minutes more. This procedure may be repeated several times. 

Passivator ions acting as oxidizers are adsorbed on the substrate and reduced easily, thus enlarging the cathode surface area. An optimum passivator solution concentration should exceed some critical value and, the higher the passivator concentration, the easier it is adsorbed, and the smaller the anodic areas on the substrate will be. This promotes an increase in anodic polarization and total passivation of the substrate. If the passivator concentration is lower than some critical value, it initiates local corrosion of the substrate. 

Finally, inhibition may also be (partially) caused by the presence of ohmic potential drops (e.g. because of the formation of poorly conducting films) between anodic and cathodic surface areas. Flere the rates of the partial reactions are additionally reduced due to the opposite negative and positive potential shifts in the anodic and... 

The solution resistivity should be very low. The overall system resistivity should be controlled by the cathode surface area. 

Assume that a crevice is exposed to a differential in acidity between the bottom of the crevice and its outer surface in the absence of chlorides or other dissolved oxidizers. The crevice bottom acts as an anode, consuming hydrogen ions through active metal corrosion. The anodic reaction depletes acid concentration in the crevice. The outer crevice suffice is passivated and acts as a cathode. On the cathode, hydrogen evolution occurs by hydrogen reduction in solution. Assume the cathode surface area is 10 times larger than the anode suffice area. 

We can see from the above expression that the anodic current density depends upon the ratio of the surface area of the cathode to the surface area of the anode. If the anode surface area is very small in comparison to the cathode surface area, then it results in very high corrosion current densities at the anodic site, leading to accelerated corrosion. 

In common culture, there is a tendency to protect the part that corrodes, for example, by painting the surface. However, painting the part that corrodes means that the anodic surface area is decreased and, when equal to the cathodic surface area, thus increases the rate of corrosion penetration in correspondence with the defects of the painted coating. A more appropriate solution would be instead to remove thermal oxides and to protect or, even better, to electrically isolate the old part, which is the cathodic one. 

FIGU RE 12.26 Effects of both the environmental relative humidity (a) and the nature of the particle (h and c) on the extent of the cathodic surface area. 

Examples illustrating the influence of the difference between anodic and cathodic surface areas are given in section 1.4.1.3. 

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