Anode contamination processes

Although most corrosion systems can be described by the limiting models presented above, there are instances where control of the corrosion system is a combination of both types, viz., activation controlled anodic partial process with two cathodic partial processes - one under activation control and another under transport control. Examples are iron corrosion in acid solution with inorganic contaminants (, 18) and oxygen ( ). The corrosion current density in such systems is... 

The metal at the edge of the blister is therefore anodic. The process is usually initiated at preferential sites, such as a particle of contaminant from the cleaning procedure or a scratch on the surface. 

Contaminants/impurities at the anode are mainly brought in by the fuel feed stream. Impurities in the hydrogen fuel, such as CO, H2S, NHj, organic sulfur-carbon, as well as carbon-hydrogen compounds, are primarily from the manufacturing process, in which natural gas or other organic fuels are reformed to produce hydrogen. In this section, several major anode contaminants, such as CO H2S, and NH3, will be discussed. 

The membrane electrode assembly (MEA) in a proton exchange membrane (PEM) fuel cell has been identified as the key component that is probably most affected by the contamination process [1]. An MEA consists of anode and cathode catalyst layers (CLs), gas diffusion layers (GDLs), as well as a proton exchange membrane, among which the CLs present the most important challenges due to their complexity and heterogeneity. The CL is several micrometers thick and either covers the surface of the carbon base layer of the GDL or is coated on the surface of the membrane. The CL consists of (1) an ionic conductor (ionomer) to provide a passage for proton transport ... 

A general anode contamination model was developed by Zhang et al. (2005) and is capable of describing the effect of various contaminant species. St.-Pierre s generalized contamination model that is applicable to both the cathode and anode (St.-Pierre, 2009) has already been discussed briefly in the Section 8.3.1.2. All of the above models use similar reactions to describe tbe kinetics of HOR and the contamination reactions, along with Butler-Volmer expression to determine the PC s overpotential or current density. The reaction network and rate constants for a contamination process involving a general contaminant P is shown below ... 

In electrogalvanizing, copper foil, and other oxygen-evolving appHcations, the greatest environmental contribution has been the elimination of lead-contaminated waste streams through replacement of the lead anode. In addition, the dimensionally stable characteristic of the metal anode iatroduces greater consistency and simplification of the process, thus creating a measure of predictabiUty, and a resultant iacreased level of safety. 

Carbon, present in iron or remaining after inadequate degreasing, can form CO or CO2. Carbon particles may occur in the chlorate if graphite anodes were used in the production process. Additionally, barium peroxide contains carbonate as a contaminant. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

In addition to inspecting for possible contamination, it usually is also of interest to determine whether the chemical etching or anodization process has actually produced the desired oxide. For this purpose, anodization has somewhat of an advantage over etching (FPL, for example) because the thicker oxide developed... 

An alternative to the direct anodic oxidation of organic contaminants are the methods of indirect oxidation with the aid of oxidizers formed electrochemically in situ. These oxidizers (or mediators) can be obtained in both anodic and cathodic processes. Anodic agents are the salts of hypochloric acid (hypochlorites), the permanganates, the persulfates, and even ozone. 

An account of cell features should make a reference to the diaphragm. The diaphragm used in some electrolytic processes is essentially constituted of a separator wall, though this allows the free passage of the electric current. It performs the important function of preventing the products of electrolysis formed at the anode from coming into contact with those formed at the cathode so as to avoid, as far as feasible, either secondary reactions which would lower the current efficiency, or contamination of the products which would diminish their value. 

The fourth factor is the current density. At an inert anode and for 100% Faradaic efficiency for water oxidation, the density of the current controls the flux of H+ ions. The cathodic current density and the species available in its vicinity establish the efficiency of the reduction processes (Pb2+ —> Pb). These vary to a greater extent than the anode process, because the pH and the species reaching the cathode vary with processing time. Thus, control of the current density is critical to ensure optimal EO efficiency and contaminant removal. 

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