Chromium surface treatment

Electroplating. Chromium is electroplated onto various substrates in order to realize a more decorative and corrosion- or wear-resistant surface (24—32). About 80% of the chromium employed in metal treatment is used for chromium plating over 50% is for decorative chromium plating (see Metal surface treatments). Hard chromium plating differs from decorative plating mostiy in terms of thickness. Hard chromium plate may be 10 to several 100 p.m thick, whereas the chromium layers in a decorative plate may be as thin as 0.25 p.m, which corresponds to about two grams Cr per square meter of surface. 

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. 

Several immersion treatments using solutions containing chromates have been developed for aluminium. It is not always clear to what extent the films formed can properly be called chromate films, i.e. films containing a substantial amount of a slightly soluble chromium chromate, but even if the film consists largely of aluminium oxide or hydroxide or other salt with chromate physically absorbed, it will still provide a reservoir of soluble chromate at the metal surface. Treatments fall into two classes alkaline and acid. The latter are of more recent development. 

The process can be used to immobilize heavy metals such as Cd, Zn, Cu, Pb, Ni and Co. Cr(VI) can be reduced by some metal-reducing bacteria to the less toxic and less soluble form Cr(III). Arsenate [As(V)] can be reduced to the more mobile arsenite [As(III)] which precipitates as AS2S3, and is insoluble at low pH. Several laboratory-scale tests (batch and column) are currently available to study the feasibility of this process. However, only a few field tests have been performed to date. Two such tests have been conducted in Belgium, one at a non-ferrous industrial site, where the groundwater was contaminated with Cd, Zn, Ni and Co, and the other which was treated by injection of molasses in order to reduce chromium (VI) to chromium (III). A third demonstration in The Netherlands has been performed at a metal surface treatment site contaminated by Zn. The outcomes of a batch test of a groundwater heavily contaminated by Zn, Cd, Co and Ni are presented in Table 5. The initial sulphate concentration was 506mg/l. With the addition of acetate, a nearly... 

Stainless steel 316L material used for piping and equipment shows considerable corrosion resistance because of the beneficial effect of molybdenum on the surface properties. It is also observed that the surface treatment (pre-reduced, polished, passivated and chemically treated surfaces) of stainless steel equipment and piping reduces the corrosion process in seawater applications. The corrosion resistance of stainless steel in seawater applications can also be enhanced by bulk alloying the stainless steel with nitrogen, chromium, molybdenum and nickel by converting the stainless steel into super austenitic stainless steel. From leaching studies it is also observed that the release of iron, chromium and nickel from the super austenitic stainless steel to seawater is considerably... 

Chromium and chrome-plated alloys can be etched in a 50% solution of concentrated hydrochloric acid for 2 to 5 min at 90°C. Zinc and galvanized metal parts can be similarly immersed for 2 to 4 min at room temperature in such a solution at 15% concentration. In both cases, the part should be primed or the adhesive applied as soon as possible after surface treatment. 

Table II presents the results of some wettability measurements on adsorbed monolayers prepared from molten Compound D on pure, polished, clean, chromium surfaces after solvent treatment had been used to remove all surplus solidified acid. A series of successive solvent treatments was applied to each coated specimen (see first four columns of Table II) using liquids which have been shown to be good solvents for Compound in the bulk (9). These liquids were either absolute ethyl alcohol or benzene at or above 20°C., or n-hexane at or above 60°C. In the remaining columns are listed the average values of the slowly advancing contact angles measured by the drop-buildup method on from three to five different drops. Measurements were made on sessile drops of water, thiodiglycol, and methylene iodide. These three diagnostic liquids were chosen because of their high surface tensions (72.8, 54.0,...

Surface Treatments. Certain inorganic chemicals (especially hexavalent chromium compounds), when applied as dilute aqueous solutions to wood surfaces, provide the following benefits (70, 135, 139—141) (1) retarding degradation of wood surfaces by UV radiation ... 

Williams and Feist (149) described the application of electron ESCA techniques to evaluate wood and cellulose surfaces that had been modified by aqueous chromium trioxide treatment. ESCA data showed at least 80% Cr(VI) to Cr(III) reduction on all substrates. Leaching experiments confirmed this reduction to a highly water-insoluble or fixed chromium complex on both wood and on filter paper (cellulose). Similar oxidation products were observed with wood and filter paper. These experiments indicate that chromium-cellulose and chromium-lignin interactions are involved in the mechanism of chromium(VI) stabilization of wood surfaces. 

Desai and Clarke (149) reported successful results using zinc-containing compounds. Sell et al. (ISO, 151) described surface treatment with chromium-copper-boron salts. Field weathering tests, leaching tests, and electron-probe microanalysis showed that this treatment was resistant to leaching and weathering. Water-soluble extractives in redwood and redcedar were fixed by treating the wood surface with dilute solutions of copper and chromium salts. This treatment permits direct application of latex paints to these woods (146). 

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