Carbon corrosion technique

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... 

Carbon steel reinforcement corrosion rates are determined using in situ electrochemical corrosion techniques. These techniques have advantages and disadvantages, and are complementary to some extent. Electrochemical impedance spectroscopy (EIS) is an AC method particularly suited for coated metal corrosion rates. 

Figure 23.30. Comparison of the cumulative carbon corrosion following a 24-h 1.2 V potentiostatic hold at 80 °C in 1 M H2SO4 for two commercial carbons COl, C02 and one heat-treated carbon C03, and platinum and Pt/Co alloy catalysts on these carbons [95]. (Reprinted from Journal of Power Sources, 166(1), Prasanna M, Cho EA, Kim H-J, Oh I-H, Lim T-H, Hong S-A, Performance of proton-exchange membrane fuel cells using the catalyst-gradient electrode technique, 18-25, 2007, with permission from Elsevier.)...

The issue of carbon corrosion has received considerable attention in recent years. There are several drivers for this (1) the cost drivers for commercialization require the use of high performance catalysts with less durable carbon catalyst supports, (2) the need for system simplification and low cost prevents additional control systems to be implemented to avoid the carbon corrosion conditions, and (3) the use of the fuel cells subjected to "real world" conditions as opposed to carefully controlled demonstration projects, with very dynamic duty cycles and many start-up/shutdown cycles. This increased attention has resulted in new or improved measurement techniques and several studies and reviews on the high cathode potential and associated carbon corrosion mechanism [39,40,48-51]. 

Other online techniques can be used to examine the change in electrode or electrolyte materials. If parts of the electrode or electrolyte are lost due to reaction, finite vapor pressure, or other reasons, analysis of the effluent product can often be correlated to the particular loss mechanism. For example, in PEFCs, one mode of physicochemical electrolyte degradation is accompanied by loss of the fluorine ion, which can be detected by measurement of the effluent condensed water fluorine content. Also, if carbon corrosion is occurring, carbon monoxide or carbon dioxide gas is produced, and this can be measured with a sufficiently sensitive device. In general, if there is a chemical reaction causing the degradation, the product species from this can be detected in the effluent and correlated with the measured loss. 

In an ideal cyclic voltammetric (CV) set-up, one would choose the appropriate scan rate and a potential window for scanning. In an aqueous electrolyte, one can scan the 1.5 V potential window to get mechanistic information, but in a real fuel cell, potentials above 0.75 V (vs. SHE) is not advised if the electro-active Pt is supported on carbon. At these elevated potentials, the carbon corrosion current density increases significantly and such a useful technique could impact the cell performance post diagnostics. Scan rates are typically... 

Techniques for handling sodium in commercial-scale appHcations have improved (5,23,98,101,102). Contamination by sodium oxide is kept at a minimum by completely welded constmction and inert gas-pressured transfers. Residual oxide is removed by cold traps or micrometallic filters. Special mechanical pumps or leak-free electromagnetic pumps and meters work well with clean Hquid sodium. Corrosion of stainless or carbon steel equipment is minimi2ed by keeping the oxide content low. The 8-h TWA PEL and ceiling TLV for sodium or sodium oxide or hydroxide smoke exposure is 2 mg/m. There is no defined AID for pure sodium, as even the smallest quantity ingested could potentially cause fatal injury. 

The Dilex Process utilises a molten lead bath as transfer medium and is applicable to diflfusion coatings of Cr, Al, Ti, Mo, Ni and Co. Finally, a Japanese fused borate bath process produces carbide coatings (Cr, V, Nb or Ta) on carbon and tool steels. The coatings are wear and corrosion resistant. The TD Process uses this technique. 

Cihal and Prazak determined the resistance of 18/8 stainless steel to this type of corrosion. They claimed that the technique could be used on steels which are difficult to test by other methods, including steels of low carbon content, and steels in which stabilising elements are present. By means of potentiostatic curves and light etching at constant potential they confirmed that the extent of intergranular corrosion depended upon the amount of precipitated chromium carbide. 

Corrosion under insulation is a major problem with carbon steels and must be taken into consideration in the RBI analysis of any plant to avoid serious failures. Several techniques are available to minimize this attack including painting, spraying with aluminum, and/or wrapping with aluminum foil. 

Electrochemical impedance, weight loss, and potentiodyne techniques can be used to determine the corrosion rates of carbon steel and the activities of both sulfate-reducing bacteria and acid-producing bacteria in a water injection field test. A study revealed that the corrosion rates determined by the potentiodyne technique did not correlate with the bacterial activity, but those obtained by electrochemical impedance spectroscopy (EIS) were comparable with the rates obtained by weight loss measurements [545]. 

Portland cement is susceptible to corrosion by CO2 and H2S. The chemical attack by CO2 is called carbonation. A microsample technique has been developed to study the CO2 corrosion in cements, because the corrosion is difficult to monitor with common test procedures [264]. This technique is also advantageous as an accelerated testing method. A polymer-modified cement has been tested in field studies [694]. The addition of silica also improves chemical resistance [146], in particular brine corrosion. 

Metallic electrodes may dissolve as a result of electrolysis and may introduce corrosion products into the solid mass. However, if the electrodes are made of carbon or graphite, no residue will be introduced in the treated soil mass as a result of the process. The energy expenditure for Pb removal has been estimated to in the range 30 to 60 kWh/m1 2 3 4 of soil. The EO method also provides an advantage over conventional pumping techniques for in situ treatment of contaminated finegrained soils. 

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