Corrosion product cations

Once the plate starts to corrode, many problems appear to affect performance and durability, even serious failure, of fhe fuel cells. For example, fhe interface contact resistance between the corroded metal plates and GDL will increase to reduce the power output. The corrosion products (mainly various cations) will contaminate the catalyst and membrane and affect eir normal functions because the polymer membrane essentially is a strong cation exchanger and the catalyst is susceptible to the ion impurity. Hence, adding a corrosion-resistant coating to the metal plate will almost inevitably assure the performance and long-term durability of a sfack. 

The distribution of cation concentration across the cut-edge is in agreement with studies of a Fe/Zn galvanic couple [13], Diffusional effects are in evidence in figure 2(b) where the highest iso-contour of cation concentration can be seen to exist within a pit formed by dissolution from a much smaller dendrite. A reduced zinc loss can be seen to be offset by the more tortuous route from the pit, impeded further still by closer proximity of cells containing corrosion product. 

With a cation-selective rust layer, the anodic ion transport across the mst layer carries mainly hydrated protons migrating outward from an occluded solution into the solution bulk as shown in Figure 22.32b. No accumulation of chloride ions and hydrogen ions is thus expected to occur in the occluded solution. Instead, the outward migration of hydrogen ions leads to the basification of the occluded solution, where the precipitation of corrosion products will then be accelerated. 

Reference has been made to the observation that both anionic and cationic species in the environment can influence the anodic polarization of active-passive types of metals and alloys. Specific examples have related to the effect of pH as it influences the stability and potential range of formation of oxide and related corrosion product films. The effect of pH, however, cannot be treated, even with single chemical species, independent of the accompanying anions. For example, chloride, sulfate, phosphate, and nitrate ions accompanying acids based on these ionic species will influence both the kinetics and thermodynamics of metal dissolution in addition to the effect of pH. Major effects may result if the anion either enhances or prevents formation of protective corrosion product films, or if an anion, both thermodynamically and kinetically, is an effective oxidizing species (easily reduced), then large changes in the measured anodic polarization curve will be observed. 

In view of this sequence, the crevice geometry parameters of gap width and depth become important. If the gap is sufficiently wide and shallow, oxygen depletion and chloride-ion influx will decrease and metal-ion buildup will be less due to increased diffusion of corrosion products from the crevice. The pH decrease due to hydrolysis of cations will be less, the passive film may be preserved, and if so, crevice corrosion will not occur. These factors are reversed for deep, narrow crevices, and at some critical geometry, crevice corrosion will occur. As with pitting, increased concentration of chloride ions in the environment will increase chloride-ion concentration in the crevice and increase the probability of initiating crevice corrosion. 

The penetrability of plasticized PE films containing MTet VCI towards iron ions was estimated in [5,6] visually judging by the color of 0.01 N aqua solution of Na2S04 with immersed steel electrodes sealed with the inhibited film. The samples were kept in the electrolyte at room temperature for three months. The Na2S04 solution did not change its color from the yellow-green hues characteristic of bivalent and trivalent iron solutions during the specified time. The film material averted the removal of iron cations from the zone of the electrochemical reaction and small amounts of corrosion products in the form of brown iron oxides remained beneath the film. 

Pourbaix diagrams for metals in aqueous solutions can be generated in order to visualize the stabiKty regions for the metal and its various corrosion products. In order to construct a metal Pourbaix diagram, the possible reaction products in an aqueous solution must be known. In general, a metal will oxidize to form a soluble cation, a soluble anion, or a metal oxide or hydroxide. For a generic metallic element M, the electrochemical half reactions that form these various products are ... 

In all, the question of which process is rate limiting in atmospheric corrosion depends on many factors, including metal ion and cation transport properties through the corrosion products, and the access of oxygen through the aqueous phase. 

It is commonly accepted that the basic driving force underlying the FFC process is a differential aeration cell. Filaments are normally quite thin and shallow but can reach a length of several hundred millimeters. Two different regions of the progressing filaments can be observed the liquid filled active head and a tail of corrosion products. In their active head, filaments carry an acidic solution of the metal cations and the initiating anions [170]. 

Chloride ions attack oxide layers on iron, aluminum, and magnesium. Subsequently, the metal is electrochemically dissolved. The hydration of Fe " ", AP" ", or Mg " " releases protons and thereby leads to an acidification of the tip of the filament. At the cathodic site, the primary cathodic reaction, the reduction of oxygen to hydroxyl ions takes place. In between the anode and the cathode a potential gradient is estahhshed, which forces anions to migrate to the front and cations to the back. As the distance from the anode increases, the pH also increases on the basis of the dilution of hydronium ions and the migration of hydroxyl ions from the cathodic site. When favorable conditions are reached, the corresponding hydroxides of the cations are formed as gels. As the head advances, these hydrated corrosion products lose their water and convert to the dry corrosion products that fill the tail see Ref. [168] and references therein. 

Eq. (10.12), may oxidize the dissolved SO2 to sulfate in the outer layers. The species within rectangular boxes represent the solution constituents and those in ovals represent the corrosion products. Dotted ovals represent reactions or chemical compounds for which there is no evidence by laboratory or field studies. The chemical reactions that have been described and confirmed by laboratory studies are presented as sohd arrows. The dotted arrows represent the mechanisms that are uncertain. TMI in the upper part of Fig. 10.3 stands for transition metal ions and soot catalyzes the S(IV) to S(VI). The ferrous cations react with reduced sulfur to produce several insoluble sulfides. Multistep processes including Fe(II), Fe(III), and OH produce hydroxysulfate mixed salts [5]. 

When metals react with gases, the main corrosion products are ionic compounds that can be stoichiometric or nonstoichiometric. Generally, only defect ions (ion condnctors) arise in stoichiometric componnds snch as silver chloride (AgCl) and NaCl. Four border cases of imperfections are possible When cation vacancies in the lattice and cations at interstitial lattice sites are found in an undisturbed anion lattice, the cations are mobile. Alternatively, the anions are mobile. In compounds with anion and cation vacancies, both can migrate, as they can when an equal number of cations and anions are present at interstitial lattice sites. 

Assuming that only ions and electrons migrate in the scale and not neutral atoms, Wagner (1933, 1936) established a formula whereby the parabolic scale constant of a pure metal can be calculated from the free formation enthalpy of the corrosion product, the electrical conductivity of the protective layer, and the transport nnmbers of cations, anions, and electrons in the film ... 

From the results achieved in the DECODE project, it can be concluded that the accumulation of cationic stainless steel corrosion products in the electrolyte membrane can be suppressed by proper design of the MEA, particularly by avoiding any contact of free electrolyte membrane with liquid water originating from the coolant or from condensate accumulated in the active area. Furthermore contact of corrosion inducing contaminants such as for example chloride ions with the metallic bipolar plates must be prevented. 

---