Chromium plating applications

Applicable local, state, and federal environmental laws require that the waste generated by the nickel-chromium plating process be pretreated to provide a discharge acceptable to the public wastewater treatment system. 

Ferrous sulfide acts as a reducing agent at pH 8 to 9 for reduction of hexavalent chromium and then precipitates the trivalent chromium as a hydroxide in one step without pH adjustment.5162 So, the hexavalent chromium in the nickel-chromium plating wastewater does not have to be isolated and pretreated by reduction to the trivalent form. The new process is applicable for removal of all heavy metals. All heavy metals other than chromium are removed as insoluble metal sulfides, M(II)S. 

In the application of the previous chemical methods, a certain amount of steady-state continuity has been built into the system. To accomplish this, initial concentrated alkaline and acid rinse wastewaters are retained after dumping in the waste holding tank [T-91] (Figure 6.4) and acid chromium plating wastewater is stored in the waste holding tank [T-51], Extremely concentrated chromium plating wastewater from rinse step No. 1 is sent to an evaporation tank [T-40] for... 

A dense and electronically insulating layer of LiA102 is not suitable for providing corrosion resistance to the cell current collectors because these components must remain electrically conductive. The typical materials used for this application are 316 stainless steel and chromium plated stainless steels. However, materials with better corrosion resistance are required for longterm operation of MCFCs. Research is continuing to understand the corrosion processes of chromium in molten carbonate salts under both fuel gas and oxidizing gas environments (23,25) and to identify improved alloys (29) for MCFCs. Stainless steels such as Type 310 and 446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (29). 

Careful control of temperature, pH, process chemical concentrations, and other process parameters is important in obtaining the maximum lifetime from baths. In some applications, such as in trivalent chromium plating systems, it is essential to keep anolyte solutions contained in anode boxes strictly segregated from the electrolytes in the rest of the bath. Mixing of the two chemistries can ruin the effectiveness of the baths. 

Replacement of hexavalent chromium with trivalent chromium offers important environmental advantages. Trivalent chromium is considerably less toxic than hexavalent. Trivalent systems use chromium concentrations that are typically two orders of magnitude less than in hexavalent systems. Thus, far less chromium enters the waste stream. Trivalent systems also generate few toxic air emissions, while hexavalent systems involve a reaction that produces hydrogen bubbles which entrain chromium compounds and carry them out of the baths. Trivalent chromium is readily precipitated from wastewater, while hexavalent chromium solutions must go through an additional step in a treatment system in which the chromium is reduced to its trivalent form before precipitation. It has been shown that trivalent chromium systems can successfully replace hexavalent ones for decorative chrome applications. Trivalent chromium systems are not suitable for hard chrome applications. More information regarding trivalent chromium plating can be obtained from Roy (1984), Robison (1978), Chementator (1982), and Smart (1983). 

The zinc-base dic-casting alloys are the mast widely used. A typical composition is 1.0% copper. 3.9% aluminum, 0.06% magnesium, balance zinc. This alloy has a strength of about 43.000 psi (3.061 atmospheres) with 3% elongaliun in 2 inches (3 centimeters). Typical applications are carburetors, fuel pumps, tools, typewriter frames, instrument cases, and hardware often finished by chromium plating. 

Chromium. Applications of chromium plating can be separated into two areas hard chromium, also called functional, industrial, or engineering chromium, and decorative chromium. The plating bath compositions may be the same for both. In most cases, the differentiating factor is plate thickness. Decorative chromium is usually less than about 1 Jim hard chromium can be from about 1 Jm to 500 Jm or more. 

Specialty Chromium-Plating Baths. Chromic acid baths using sodium chromate and sodium hydroxide to form a tetrachromate (92) have had limited use. Porous chromium is used in lubricated wear applications, and is made by chemically etching regular chromium plate, sometimes with light grinding after the etch. Black chromium is used on solar collector surfaces (see PHOTOVOLTAIC CELLS Solarenergy). Baths are sulfate-free, and include fluosilicic acid or acetic acid (91). 

Another application where the reduction of metallic impurities is possible is in the purification of plating solutions containing negatively charged metal oxides, e.g., chromium plating baths [28]. In this application, negative metal oxide ions migrate away from the cathode where the positive impurities (Cu, Zn, Fe ) are reduced. 

Exchange resins are also employed for the concentration of ions present in very dilute solutions instances are the recovery of silver from photographic residues, chromate from the waste liquor of chromium plating and magnesium from sea water. They have also been used for the separation of rare earths (p. 426), and of uranium, plutonium and radio-active fission products (p. 437), and for plutonium and uranium-233 purification. A striking application was the historic separation of single atoms of mendelevium on a sulphonated polystyrene resin and their elution therefrom, at 87 , with a-hydroxyisobutyrate (Seaborg, 1955). 

Chromium trioxide is used in the following applications in chromium plating, in copper stripping, in aluminum anodizing, as an anticorrosive, in photography, in hardening microscopic preparations, and in purifying oil and acetylene. 

Chromium is a refractory metal having a melting point of 3375°F (1857°C). Neither chromium metal nor chromium-based alloys are widely in the hydrocarbon or chemical industries. Chromium plating is useful for aesthetic purposes, and hard chromium plating finds some use in hardface applications. It is extensively used as an alloy addition to low-alloy steels (usually for the purpose of stabilizing carbides) and in cast irons (to produce wear-resistant products) and nickel alloys (for increased corrosion resistance). Chromium is the main alloying addition in the 400-series stainless steels and is used extensively in the 200- and 300-series stainless steels. 

Detroit and New Orleans. Frequency of chromium dermatitis was highest in constmc-tion workers using cement. Other occupational exposures associated with chromium sensitivity include chromium plating, tanning of leather, application of anticorrosive agents, and printing. Oral ingestion of chromium com-poimds can sometimes lead to skin reactions in sensitive people. Hexavalent chromium com-poimds are more potent inducers and elicitors of skin sensitivity than trivalent chromium compoimds, probably because Cr+ com-poimds can penetrate die skin more readily than Cr+ compoimds. 

ABS. A thermoplastic, a terpolymer of acrylonitrile, butadiene, and styrene. ABS products are characterized by toughness and rigidity as well as by a higher thermal stability and scratch resistance than those of many other thermoplastics. Glass fibre reinforced, fiame-retardant, and other modified ABS grades are also available. ABS components can be metallized, for example, chromium-plated. Some applications panels and other car accessories casing and various parts of business machines, appliances, radio, TV sets, tape recorders pipes. Trade names Cycolac (USA), Kralastic (USA), Novodur (FRG), Techster (F), Terluran (FRG). 

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