Separator plate corrosion

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

Corrosion of steel separator plate on the anode side... 

Austenitic stainless steels like 31 OS, 316, or 316L are typically used for the construction of cathode and anode current collectors and bipolar separator plates. Corrosion of these steel components is a major lifetime-limiting factor in MCFC. The corrosion behavior of stainless steel components in molten carbonate conditions has been studied extensively during the past decade. Research is being aimed at increasing the corrosion resistance of these components by altering the alloy composition or by surface modification techniques. ... 

To prevent short circuiting between the separator plates the electrolyte matrix made of LiA102 extends to the outer edge of the separator plates. The molten carbonate electrolyte penetrates the matrix tile up to the edges, thus providing the necessary wet seal. This sealing is necessary to take care of small pressure differences (some tens of mbar) between gas chambers and the ambience and also between the fuel and oxidant gas chambers. From this short description of the geometry of the bipolar separator plates, it becomes clear that we have three different situations to take care of if we are to prevent corrosion of the separator plates ... 

J.P.T. Vossen Ph. D. Thesis Corrosion of Separator Plate Constituents in Molten Carbonate , 1994, Delft University of Technology, The Netherlands. 

FJ. Perez, M.P. Hierro, D. Duday, C. Gomez, M. Romero, L. Daza, Hot-corrosion studies of separator plates of AISI-310 stainless steels in molten carbonate fuel cells, Oxid. Met. 53 (2000) 375-398. 

Low current densities - Low partial load toleration - System dimensions and complexity (footprint) - Extensive gas purification - Corrosive liquid electrolyte - Corrosive environment ( acidic membrane) - High investment costs due to costly components (catalysts, current collectors, separator plates) - Laboratory and test phase - Long-term stability (mechanical) - Heat management... 

Perez F J, Duday D, Hierro M P, Gomez C, Agtlero A, Garcia M C, Muela R, Sanchez Pascual A and Martinez L (2002), Hot corrosion study of coated separator plates of molten carbonate fuel cells by slurry aluminides , Surf Coat Technol, 161,293-301. 

Separator Plate Corrosion The stainless steel separator plate used can suffer severe corrosion in the high-temperature environment at the anode and the cathode. Corrosion with the electrolyte reduces contact efficiency and also consumes electrolyte. This problem can be mitigated with a nickel or alternative metal coating on the anode [35] and reduction of the separator plate surface area on the cathode. 

In an effort, to reduce acid loss by absorption in separator plates, better materials with low open porosity are used. Additionally, acid loss can be reduced by low temperature operation of PAFC. Some groups including the author s laboratory is adopting a temperature range that is around 150-170°C. This reduces component corrosion and acid loss. However, for anode catalyst, Pt/Ru alloy catalyst has to be used for better CO tolerance with reduced current density. Additionally, the upstream reformer system has to be tuned to ensure CO level at < 1%. However, this design is found to be quite acceptable for small capacity power plants. 

Poor preparation of the substrate can result in loss of adhesion, pitting, roughness, lower corrosion resistance, smears, and stains. Because electroplating takes place at the exact molecular surface of a work, it is important that the substrate surface be absolutely clean and receptive to the plating. In the effort to get the substrate into this condition, several separate steps may be required, and it is in these cleaning steps that most of the problems associated with plating arise. 

Plate and Frame Composed of metal-formed thin plates separated by gaskets. Compact, easy to clean. Viscous fluids, corrosive fluids slurries. High heat transfer. Not well suited for boiling or condensing limit 350-500°F by gaskets. Used for Liquid-Liquid only not gas-gas. 0.8-1.5... 

The actual number of trays needed for a particular separation duty depends on the efficiency of the plate, and the packings if they are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the colunm. Tray efficiencies are affected by such factors as fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus the proper materials of construction must be selected for tray construction. 

Heat exchangers that utilize copper coils are potential candidates for galvanic corrosion due to dissolved copper salts interacting with the galvanized steel shell. This problem can be avoided by nickel plating the coils. The coils then can be separated from direct contact with the vessel via insulation. Also, it is preferable to conduct the water on the tube side of heat exchangers. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

Tubesheets form the end barriers to separate the shell-side and tube-side fluids. Most exchangers use single plates for tubesheets. However, for hazardous or corrosive materials such as chlorine, hydrogen chloride, sulfur dioxide, etc., where the intermixing due to leakage from shell- to tube-... 

To check the efficacy of grease removal, the alkali solution is rinsed away or neutralised by dipping in dilute acid. If, after removal from the acid, the draining metal surface remains wetted evenly all over for 30-60 s (or until it dries by evaporation), hydrophobic soils have been removed. Traces of grease cause the surface to de-wet, and surface tension draws the water into separate droplets. This is the water-break test. Traces of grease which remain when the work is plated do not prevent electrodeposition, but are detrimental to adhesion and corrosion resistance. 

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