Corrosion carbonate

My mother still thiirks I m very smart. Forty-eight years ago, I removed rust stains from our bathtub with Coca-Cola. It is the carbonic acid in Coke that removed the iron oxide rust stains from the tub. This is an illustration of the corrosive nature of carbonic acid brought into contact with ferric compounds. The carbotric acid is formed from CO2 dissolved in water. 

I mentioned an example of how CO2 becomes concentrated in steam systems (see Fig. 12.6). Anytime CO2 starts to dissolve in water, carbonic acid will form, which has a relatively high pH as compared to other common minerals acids (sulfuric, hydrochloric, nitric acid). Actually, carbonic acid is aggressively corrosive to carbon steel at a pH of 5.5. Sulfuric acid at a pH of 5.5 is otrly mildly corrosive to carbon steel. 

CO2 is often produced by the chemical breakdown of calcium carbonate or sodium carbonate. CO2 is generated as a by-product of the production of hydrogen and ammonia. Processing of natural gas for pipeline sales is another common source of CO2. The effluent from a sulfur plant or any combustion process contains wet CO2. 

---

Figure 4.15 As in Fig. 4.14 but with silt removed to reveal bluish-white copper carbonate corrosion products. 

In acid electrolytes, carbon is a poor electrocatalyst for oxygen evolution at potentials where carbon corrosion occurs. However, in alkaline electrolytes carbon is sufficiently electrocatalytically active for oxygen evolution to occur simultaneously with carbon corrosion at potentials corresponding to charge conditions for a bifunctional air electrode in metal/air batteries. In this situation, oxygen evolution is the dominant anodic reaction, thus complicating the measurement of carbon corrosion. Ross and co-workers [30] developed experimental techniques to overcome this difficulty. Their results with acetylene black in 30 wt% KOH showed that substantial amounts of CO in addition to C02 (carbonate species) and 02, are... 

Similar ECA loss phenomena have been observed in PEMECs. Understanding ECA loss and carbon corrosion mechanisms may help with designing more durable... 

Carbon corrosion, 300 Carbon monoxide adsorption, 248,250,255, 325-327, 347, 386-391,528-532 Carbon monoxide oxidation... 

Pt particle agglomeration is due to carbon support corrosion. Electrochemical carbon corrosion is known to occur above 0.9 V. It has been suggested that loss of carbon causes Pt particle agglomeration and electrical isolation, leading to loss in activity. 

The mechanism of carbon corrosion has been investigated in MEAs and in liquid electrolytes. Carbon itself is thermodynamically unstable toward oxidation at higher potentials, but this oxidation is kinetically limited ... 

It is generally thought that carbon corrosion proceeds in three steps ... 

Ball et al. investigated the effect of carbon surface area on carbon corrosion at 1.2 V for 24 h and found that, for commercial carbon blacks, cumulative carbon corrosion correlated with carbon BET (Brunauer Emmett Teller) area, although when analyzed as specific carbon corrosion (weight of carbon corroded per unit of carbon area), some variation was observed. The effect of Ft on carbon corrosion has also been studied and conflicting results have been reported. Roen, Paik, and Jarvi found that Ft did increase carbon corrosion... 

J. E. Owejan, P. T. Yu, and R. Makharia. Mitigation of carbon corrosion in microp-orous layers in PEM fuel cells. ECS Transactions 11 (2007) 1049-1057. 

Mechanisms Symptoms Carbon corrosion (air-air start) Gas impurities (e.g., CO, H2S, Sp2, ) Contaminants (e.g. some transition metal cations, anions) Catalyst instability (pt sintering, dissolution, re-crystallization) GDL loss of wet-proof (flooding) Seal failure (gross leaking) Membrane failure (pinholing, and tear)... 

Consequently, since graphitized carbon-supports have lower carbon corrosion rates, the use of cathode catalysts with graphitized supports significantly reduces H2/air-front start-stop damage.12,22 Furthermore, if the ORR activity of the anode electrode is reduced by lowering anode Pt loading, H2/air-front start-stop degradation is decreased.22,23... 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

Figure 3. Carbon corrosion rate versus carbon weight loss for both conventional and graphitized KB-supported Pt catalysts. The carbon corrosion rate (in units of A/g( ) is based on the measured CO2 concentration at the exit of a 50 cnr cell using a GC, assuming 4e /( (T molecule. The carbon weight loss is obtained by integrating the measured CO2 evolution rate over time. The cell is operating on neat H2/N2 (95 °C, 80% RIIjn, and 120 kPaa, s) with potential held at 1.2 Volts versus RHE.

Equation (8) states the fact that the sum of carbon corrosion current (f co, = /cor,call,) and oxygen evolution current (/o2 = /oER.cath) on the cathode must be equal to the oxygen reduction current on the anode determined by the 02 crossover rate ... 

The kinetic model described by Eq. (8) can be used to evaluate the material impact on the carbon corrosion rate. As shown for 50% Pt/Vulcan in Fig. 4 and Table 3 (first row), its carbon corrosion rate is essentially equal to the 02 crossover current. Since the... 

Figure 4. Polarization curves of carbon corrosion and oxygen evolution reactions based on measured carbon corrosion kinetics for Pt/Vulcan and Pt/Graphitized-Vulcan and oxygen evolution kinetics for Pt/C catalysts. The upper horizontal dotted line denotes a current density equivalent to oxygen crossover through membrane from cathode to anode.

Figure 5. Graph illustrating reduction of cathode carbon corrosion during start-stop with (a) corrosion-resistant carbon support (Gr-Vulcan) and (b) lower anode catalyst loading (0.05 mgpt/cm2). The base case is Vulcan and 0.40 mgpt/cm2 anode loading.

Carbon corrosion current density can be further reduced by controlling the cell voltage at a low level during the start-stop process.25,27 Neglecting /qer, Eq. (12) is simplified to... 

Equations (18-20) are discretized by the control volume method53 and solved numerically to obtain distributions of species (H2, 02, and N2) concentration, phase potential (solid and electrolyte), and the current resulting from each electrode reaction, in particular, carbon corrosion and oxygen evolution currents at the cathode catalyst layer, with the following initial and boundary conditions ... 

We use the coupled kinetic and transport model to predict when local H2 starvation occurs and how it affects carbon corrosion rate. 

Figure 6 shows in a cylindrical coordinate (radial/thm-plane) how H2 depletes and N2 pressure builds up with time. At the center of the H2-starved region, the N2 pressure becomes higher than 80 kPaabs after 1 s. The radius of this N2 bubble grows to 7 mm after 10 s and covers 80% of the liquid-water-blocked region at 100 s (near steady-state). The H2 pressure drops quickly to zero at the edge of the N2 bubble, within which the cathode potential rises sharply, and the carbon corrosion rate starts to increase, as shown in Fig. 7. Conventional DM has a value of permeability ranging from 1 to 10 Darcy. There is no impact of DM permeability within its realistic range.14 N2 crossover through the membrane results in N2 pressure build-up in the H2-starved anode region. As a result, convective...

Figure 7. Evolution of cathode potential (defined as rj>s 4>e at cathode electrode) and carbon corrosion current distributions corresponding to H2 depletion and N2 bubble buildup as shown in Fig. 6.

---