Chromium electrolytic

Li B, Lin A, Gan F (2006) Preparation and characterization of Cr-P coatings by electrodeposition from trivalent chromium electrolytes using malonic acid as complex. Surf Coat Technol 201 2578-2586. doi 10.1016/j.surfcoat.2006.05.001... 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

Solvent for Electrolytic Reactions. Dimethyl sulfoxide has been widely used as a solvent for polarographic studies and a more negative cathode potential can be used in it than in water. In DMSO, cations can be successfully reduced to metals that react with water. Thus, the following metals have been electrodeposited from their salts in DMSO cerium, actinides, iron, nickel, cobalt, and manganese as amorphous deposits zinc, cadmium, tin, and bismuth as crystalline deposits and chromium, silver, lead, copper, and titanium (96—103). Generally, no metal less noble than zinc can be deposited from DMSO. 

Titanium Sulfates. Solutions of titanous sulfate [10343-61-0] ate readily made by reduction of titanium(IV) sulfate ia sulfuric acid solutioa by electrolytic or chemical means, eg, by reduction with ziac, ziac amalgam, or chromium (IT) chloride. The reaction is the basis of the most used titrimetric procedure for the determination of titanium. Titanous sulfate solutions are violet and, unless protected, can slowly oxidize ia coatact with the atmosphere. If all the titanium has been reduced to the trivalent form and the solution is then evaporated, crystals of an acid sulfate 3 Ti2(S0 2 [10343-61-0] ate produced. This purple salt, stable ia air at aormal temperatures, dissolves ia water to give a stable violet solutioa. Whea heated ia air, it decomposes to Ti02, water, sulfuric acid, and sulfur dioxide. 

Fig. 2. Flow sheet for production of electrolytic chromium by the chrome alum process at the Marietta Plant, Flkem Metals Co., Marietta, Ohio.

Fig. 3. Principal electrolytic ceU reactions and electrolyte concentrations in chromium production by the chrome alum process.

At the end of the 72-h cycle, the cathodes are removed from the cells, washed in hot water, and the brittie deposit, 3—6 mm thick, is stripped by a series of air hammers. The metal is then cmshed by roUs to 50-mm size and again washed in hot water. The metal contains about 0.034% hydrogen and, after drying, is dehydrogenated by heating to at least 400°C in stainless steel cans. Composition limits for electrolytic chromium are shown in Table 4. 

Chromic Acid Electrolysis. Alternatively, as shown in Figure 1, chromium metal may be produced electrolyticaUy or pyrometaUurgicaUy from chromic acid, CrO, obtained from sodium dichromate by any of several processes. Small amounts of an ionic catalyst, specifically sulfate, chloride, or fluoride, are essential to the electrolytic production of chromium. Fluoride and complex fluoride catalyzed baths have become especially important in recent years. The cell conditions for the chromic acid process are given in Table 7. 

The vacuum melting process can upgrade chromium at a modest cost the other purification processes are very expensive. Thus iodide chromium is about 100 times as expensive as the electrolytic chromium and, therefore, is used only for laboratory purposes or special biomedical appHcations. 

R. Winer, Electrolytic Chromium Plating, Finishing PubUcations Ltd., 1978. 

Ghromium(II) Compounds. The Cr(II) salts of nonoxidizing mineral acids are prepared by the dissolution of pure electrolytic chromium metal ia a deoxygenated solution of the acid. It is also possible to prepare the simple hydrated salts by reduction of oxygen-free, aqueous Cr(III) solutions using Zn or Zn amalgam, or electrolyticaHy (2,7,12). These methods yield a solution of the blue Cr(H2 0)g cation. The isolated salts are hydrates that are isomorphous with and compounds. Examples are chromous sulfate heptahydrate [7789-05-17, CrSO 7H20, chromous chloride hexahydrate... 

This conversion is normally accompHshed by immersion, but spraying, swabbing, bmshing, and electrolytic methods are also employed (178) (see Metal SURFACE treatments). The metals that benefit from chromium surface conversion are aluminum, cadmium, copper, magnesium, silver, and 2inc. Zinc is the largest consumer of chromium conversion baths, and more formulations are developed for 2inc than for any other metal. 

Since World War 11, the U.S. space program and the military have used small amounts of insoluble chromates, largely barium and calcium chromates, as activators and depolarizers in fused-salt batteries (214,244). The National Aeronautics and Space Administration (NASA) has also used chromium (111) chloride as an electrolyte for redox energy storage cells (245). 

The engineering properties of electroless nickel have been summarhed (28). The Ni—P aHoy has good corrosion resistance, lubricity, and especiaHy high hardness. This aHoy can be heat-treated to a hardness equivalent to electrolytic hard chromium [7440-47-3] (Table 2), and the lubricity is also comparable. The wear characteristics ate extremely good, especiaHy with composites of electroless nickel and silicon carbide or fluorochloropolymers. Thus the main appHcations for electroless nickel are in replacement of hard chromium (29,30). 

The process consists of pre-etching, etching, etch neutralization, catalyst appHcation, catalyst activation, and plating. Most commercial appHcations, except REl/EMl shielding, use the initial copper or nickel deposit as a base for subsequent electrolytic plating of electrolytic copper, nickel, or chromium. The exact types and thicknesses of metal used are determined by part usage, eg, automotive exterior, decorative, plumbing, and others (24). 

The Electrolytic Corrosion Test. Also developed for use on nickel—chromium and copper—nickel—chromium decorative automobile parts is the electrolytic corrosion (EC) test (44). Plated specimens or parts are made anodic in a corrosive electrolyte under controlled conditions for 2 min, and then tested for penetration to the substrate. 

Spray liberated from vessels at which an electrolytic chromium process is carried on, except trivalent chromium Every 14 days while the process is being carried on... 

Other hydrides with interstitial or metallic properties are formed by V, Nb and Ta they are, however, very much less stable than the compounds we have been considering and have extensive ranges of composition. Chromium also forms a hydride, CrH, though this must be prepared electrolytically rather than by direct reaction of the metal with hydrogen. It has the anti-NiAs structure (p.. 555). Most other elements... 

The main use of the chromium metal so produced is in the production of non-ferrous alloys, the use of pure chromium being limited because of its low ductility at ordinary temperatures. Alternatively, the Cr203 can be dissolved in sulphuric acid to give the electrolyte used to produce the ubiquitous chromium-plating which is at once both protective and decorative. 

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