Cathod fouling

Ratajewicz and co-workers have also studied the effect of a ultrasonic field on electrode polarization and activation energy in zinc electroplating [96], The conclusion from their work was that the zinc electrochemical reduction reaction was influenced by the presence of organic additives, especially CN ions, and that in the presence of ultrasound the cathode fouling is reduced. 

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

C02 is produced as the primary product and this precludes the use of alkaline electrolytes due to the precipitation of CO - in the pores of the anode and consequent electrode fouling. Acid electrolytes lead to problems of corrosion and slow kinetics for the reduction of 02 at the air cathode. 

Figure 6.15 Effect of membrane surface charge on ultrafiltration flux decline. These membranes were used to ultrafilter cathodic electrocoat paint, which has a net negative charge. Electrostatic repulsion made the negatively charged membrane significantly more resistant to fouling than the similar positively charged membrane [13]...

A separator is needed to prevent mixing of the very highly oxidizing anolyte with the reduced species formed at the cathode. Problems inherent to separators are diffusion of mediators, fouling and rupture, leakage of separator seals, and an increase in the electrical cost of the process. 

Under-deposit corrosion is a particular type of corrosion caused by differential aeration. If sparingly soluble salts, loosely adherent corrosion products, algal, or other fouling, is deposited on a metal surface, then these areas become depleted in oxygen. Unfouled or less fouled areas have a greater supply of oxygen and hence become cathodic to the fouled areas. Thus, the anodic under-deposit areas will corrode preferentially. 

As described in the discussion of heat exchanger fouling elsewhere in this encyclopedia, anodic and cathodic reactions occur. Chemicals may be added to prevent these reactions they are termed anodic and cathodic inhibitors. Cathodic inhibitors form a barrier at the cathode reducing or eliminating H" " or O2 transport to the cathode. They include nitrites, silicates, tannins, and orthophosphates. Anodic inhibitors prevent or restrict electron transfer and include polyphosphates, polyphosphonates, and molybdates. Some of these chemicals represent nutrients for aquatic life and may encourage the growth of microorganisms. 

Some researchers [121, 126, 139] have demonstrated that intermittent application of AC or DC electric fields across MF or UF membranes during filtration can often promote displacement of polarization- or adherent fouling layers, with significant improvements in sustained permeation flux. The electrode is installed on either side of the membrane with the cathode on the permeate side and the anode on the feed side. Usually, the membrane support is made of stainless steel or the membrane itself is made of conductive materials to form the cathode. Titanium coated with a thin layer of a noble metal such as platinum could be one of the best anode materials [140], The electromagnetic field reduces fouling and biofilm development. 

Although the reactions at the cathode are not severe in terms of electrode deterioration, fouling problems can be severe at the cathode. The presence of pH-sensitive salts, e.g., Ca(HC03)2 in the feedwater is a matter of concern, because the OH ions generated at the cathode can lead to precipitation. 

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