Diffusion layer corrosion studies

Metal foams have been used in the past in the development of FF plates. However, Gamburzev and Appleby [53] used Ni foams as both a DL and a flow field plate with an MPL layer on one of its surfaces. They observed that such a design had high contact resistance between the nickel foam and the MPL and also increased gas diffusion resistance due to the required MPL thickness. Arisetty, Prasad, and Advani [54] were able to demonstrate that these materials can also be used as potential anode diffusion layers in DMFCs (see Figure 4.10). In fact, the nickel foam used in this study performed better than a carbon cloth (Avcarb 1071HCB) and a stainless steel mesh. However, it was recognized that a major drawback for these foams is their susceptibility to corrosion. 

It has been shown from corrosion studies (97, 98) that bulk sulfide formation involves metal cation diffusion through the sulfide layer to the surface with the formation of a new metal sulfide layer on the outer surface beyond the original metal. Apparently the formation of multilayer sulfides occurs slowly at room temperature and at Ph2s — 1 atm, cation diffusion through the sulfide layer controlling the rate (9, 96). Presumably this step would also be rate limiting at higher temperatures and at H2S concentrations as low as 10-100 ppm. However, in most catalytic processes H2S concentrations are below those needed for bulk sulfide formation. 

Metal ions that diffuse to the surface react with atmospheric gases to form hydroxides in a thin layer of high pH [36]. Fowkes improved adhesion of a basic polymer to glass with an acid wash [6, 7] a similar treatment also improved adhesion to epoxy [37], an acidic polymer. The former effect was attributed to replacement of sodium silicate with silanols by ion exchange the latter may result from removing a corrosive layer of metal hydroxides from the interface. IGC and PTD measurements in this study demonstrate a measurable basicity on bare fibers, which other investigators also noted [6,14,17],... 

The response to the applied perturbation, which is generally sinusoidal, can differ in phase and amplitude from the applied signal. Measurement of the phase difference and the amplitude (i.e. the impedance) permits analysis of the electrode process in relation to contributions from diffusion, kinetics, double layer, coupled homogeneous reactions, etc. There are important applications in studies of corrosion, membranes, ionic solids, solid electrolytes, conducting polymers, and liquid/liquid interfaces. 

In reality, however, salts dissolved in water find or create paths into a hydrophobic polymer matrix and cause the breakdown of an insulating layer or the corrosion of the substrate metal. In contrast to the diffusion process described above, the process of salt going into polymer matrix could be termed salt intrusion because the breakdown of the surface state does not occur with water that does not contain salts. The salt intrusion starts with the breakdown of the surface state. In a study of the electric insulation characteristics of LDPE film, it was found that salt ions intrude into the polymer matrix by different mechanisms [3,4]. The exact mechanisms for salt intrusion are not known, but the phenomenological salt intrusion found can be summarized as follows, in an effort to explain the nature of salt intrusion that causes the breakdown of the surface state. 

Although there is little evidence for auto-catalysis in dechlorination by Fe , it is still possible that localized corrosion contributes to the remediation of contaminants in environmental applications. Various investigators have postulated that localized corrosion contributes through increased surface area (44) and creation of corrosion cell domains (49-51). The corrosion cell model works on the same principle as the electrochemical model described above (Figure 3), but invokes additional effects such as the reduction of protons as the major cathodic reaction, and the creation of an electrical double layer between the anode and cathode that permits transport due to electrical migration as well as diffusion. Although many aspects of these models are plausible, there are not yet any data that specifically support them, and a study that systematically addresses the role of localized corrosion in remediation applications of Fe remains to be done. 

---