Chrome plating

Chromium electrolytes are aqueous solutions of chromic acid based ( H2CrO4 ) used in electroplating to produce a chromium coating on metal and plastic objects. While leaving most other metal coatings produce optionally with different electroplating process as a bulk, with Single wire or continuous belt process, chromium coatings are almost always made ​​with Single wire. Chromium electrolytes are highly toxic and highly corrosive.

Application

The "classic " chrome bath based on a patent Erik Liebreich of 1920, he for the electrical chrome Society m. b. H. Berlin evolved. Another patent from 1924 is considered the key patent for chromium plating. Then there is a chromium electrolyte from about 250 g CrO3 with an addition of 1% = 2.5 g of sulfuric acid (H2SO4 ) as catalyst in a liter of bath. Here, ( = efficiency, i.e., 82% of the current generates hydrogen ) at 50 A / dm ², approximately 18% current efficiency achieved. With the same electrolyte can be achieved at high speed process over 50 % current efficiency, however, the electrolyte must be moved very quickly. Piston rods for shock absorber of the automobile industry are coated in platinum tubes at about 1000 A / dm ² in a few seconds.

Further attempts to increase the efficiency, were only moderately successful. For the deposition of metallic chromium foreign acids are added as catalysts, such as sulfuric acid Classic (1% of CrO3 ) earlier hydrofluoric acid or hexafluorosilicic called mixed acid catalysts or in current fluoride- free electrolyte, sulfonic acids. The most common are bathrooms with methanesulfonic acid (brand name eg HEEF 25 ) as a catalyst, they reach in use a current yield of 25%. From these baths both hard and polished chrome can be deposited.

As anodes are insoluble lead alloys. The anode material is important here, because the trivalent chromium which is generated at the cathode, it must be oxidized back to the hexavalent chrome. This is done particularly well in the lead anodes, which become covered with a lead dioxide. The deposited chromium is the electrolyte as chromic anhydride ( CrO3 ) was added again.

To avoid poisonous lead sludge, in modern plants lead replaced by platinized titanium. The use of anodes made titanummanteltem copper (english titanium clad copper busbars ) with platinized in the micrometer range titanium surfaces is chromium plating in sulfuric acid electrolyte a special advantage dar. Increasingly stringent environmental regulations and expensive waste disposal require methods that allow a more environmentally friendly hard and bright chrome in fluoride- free electrolyte.

Bright chrome

The bright chrome ( decorative chrome plating ) a very thin chromium layer of mostly 0.2 to 0.5 microns is deposited. Because of the small thickness of such layers of chrome shine of the chrome finished workpiece is determined not only by the chromium layer itself, but also on the underlying layer (usually nickel). If a nickel layer underlying the chrome is dull, then the workpiece after the ( thin ) bright chrome is still dull. Such a matte finish is desired in some cases and is then perceived as very high quality ( satin finish). A polished chrome layer has to be driven in a certain window of temperature and current density. Outside this window, the chrome layer is not shiny, but dull and gray. This can occur especially in the high current density range. Is the gray area at the edges of the workpiece relatively small, it can be brought back to a high gloss by means of special polishing paste and a cloth polishing wheel.

An optimal coating on steel, for example, cyanide copper, acid copper, bright nickel, chromium; in some cases the acid copper layer is polished prior to nickel plating. A special feature in this case is stainless steel; this can be polished to a high gloss and chrome without an intermediate layer.

Hard chrome plating

Hard chromium plating is a misleading term because it suggests that a hard chromium layer is harder than a (thin ) polished chrome layer. In reality, the hard chrome layer is approximately as hard as the (thin ) polished chrome layer. However, the gloss chrome layers are usually so thin ( see above) that the high hardness has no effect in a conventional hardness measurement, because the probe is pierced in the softer underlayers. An apt name would be " thick chrome plating ", but the term " hard chrome plating " is in general use. Sometimes chromium layers are referred to in excess of about 1 micron as a hard chromium, but there are also hard chromium layers of several millimeters, for example, in front of pressure cylinders.

A normal gloss or hard chrome layer contains a dense network of very fine cracks, but they are not visible to the naked eye and you also can not palpate. The formation of these cracks is closely related to the outgassing during the deposition of hydrogen. A portion of the hydrogen is temporarily stored in the form of Chromhydrid in the chromium layer. During the subsequent disintegration of the Chromhydrids there is a shrinkage of the chromium layer and the resulting tensions lead to tears. The cracking of chromium layers makes it clear that a polished chrome layer alone, despite the excellent properties of chromium, yet causes no good corrosion protection. The corrosion protection only occurs in conjunction with suitable intermediate layers ( usually nickel or copper and nickel). This plan structure is even advantageous for some special cases, as for example, an oil film can adhere better.

See also: Aufchromen

Schwarzverchromen

Another special case is the Schwarzverchromen dar. by an increased current density in connection with special additives to chromium layers are deposited in a deep black color. The black chromium layer is one of the few deep black surfaces that are electrically conductive. Some black chrome layers have only a moderate abrasion resistance. This effect can be improved by subsequent oiling something. A black chrome electrolyte must be cooled during operation. The Schwarzverchromen should not be confused with the black chromating.

Surface qualities

By special electrolyte additives can be deposited crack-free, micro-cracked or microporous chromium layers. The crack-free chromium coatings have no very great importance for the practice because they mostly still be cracked under everyday conditions later. For the corrosion resistance (in conjunction with the intermediate layers ) it has a positive effect when the crack structure is fine. A normal gloss chrome layer has about 1 to 20 cracks per centimeter. At 300 to 800 cracks per centimeter is called a micro- cracked chromium layer. Another possibility for improving the corrosion resistance is the production of chromium layers microporous. Such as the cracks and micropores with the naked eye are not visible. Also a Doppelverchromung is possible under certain conditions.

For corrosion resistance, the workpiece must be coated (eg hydraulic ram ) before ( eg with a chem. Nickel plating). An alternative to this is a mechanical treatment by honing or polishing, in this case the surface is smoothed and the cracks are smeared (eg piston rods for passenger car shock absorber )

Environmental aspects

Due to the low efficiency and the high currents produced when chroming much oxyhydrogen gas, ie hydrogen and oxygen. The bath foam. When bursting of the gas bubbles on the surface of the bath, the chromium bath is finely atomized. Because of the high cancer risk posed by chromium (VI ), chromium baths must therefore aspirated and the emergence of chromium aerosols are suppressed, which is possible with a formed by surfactants foam carpet.

Chromium electrolytes based on the non-toxic chromium (III ) are currently being researched, they are for production at present but hardly suitable or limited to special cases. They usually consist of solutions of ammonium salts and contain strong complexing agents. Again (usually graphite) is working with insoluble anodes.

Recently there especially in England wage electroplating using chromium (III) electrolytes. The field of application forms predominantly the valve industry, as chromium (III) electrolytes, the base material attack less and better spread. Good throwing power means that even at current disadvantaged places metal is deposited. The hue of a deposited from a chromium (III ) electrolyte layer differs from the deposited from a chromium (VI ) electrolyte layer and is strongly affected by introduced foreign metals.