Chromium electrolytes
Chromium electrolytes are aqueous solutions based on chromic acid (H 2 CrO 4 ), which are used in electroplating to create a chrome coating on metal and plastic objects . While most other metal coatings can be produced with various electroplating processes as bulk material, with individual contacts or in a continuous strip process, chrome coatings are almost always produced with individual contacts. Chromium electrolytes are very toxic and highly corrosive.
application
The “classic” chrome bath is based on a patent by Erik Liebreich from 1920, which he further developed for Elektro-Chrom-Gesellschaft mb H. Berlin . Another patent from 1924 is considered the key patent for chrome plating. According to this, a chromium electrolyte consists of about 250 g of CrO 3 with an addition of 1% = 2.5 g of sulfuric acid (H 2 SO 4 ) in a liter bath. The acid not only acts as a catalyst , but also takes part in the reduction. At 50 A / dm², about 18% current yield (= material efficiency , i.e. 82% of the current is generated by hydrogen ) is achieved. With the same electrolyte, over 50% current yield can be achieved in high-speed processes, but the electrolyte must be moved very quickly. Piston rods for shock absorbers in the automotive industry are coated in platinum tubes at over 1000 A / dm² in just 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 classic sulfuric acid (1% of CrO 3 ), formerly hydrofluoric acid or hexafluorosilicic acid , so-called mixed acid catalysts, or, with current fluoride-free electrolytes, sulfonic acids . The most common are baths with methanesulfonic acid (brand name e.g. HEEF 25) as a catalyst, these achieve a current yield of 25% in use. Both hard and bright chrome can be deposited from these baths.
Insoluble lead alloys serve as anodes . The anode material is important here because the trivalent chromium that is generated at the cathode has to be oxidized back to hexavalent chromium. This happens particularly well on the lead anodes, which are covered with a layer of lead dioxide. The deposited chromium is added back to the electrolyte as chromic anhydride (CrO 3 ).
In order to avoid toxic lead sludge, lead is replaced by platinum-coated titanium (element) in modern systems . The use of anodes made of titanummanteltem copper ( English titanium clad copper busbars ) with the micrometer range platinized titanium surfaces is chromium plating in a sulfuric acid electrolyte a special advantage. Increasingly stringent environmental regulations and expensive waste disposal require methods that allow a more environmentally friendly hard and chromium plating in fluoride-free electrolyte .
Bright chrome plating
With gloss chrome plating (decorative chrome plating), a very thin chrome layer of usually 0.2 to 0.5 µm is deposited. Because of the small thickness of such chrome layers, the gloss of the finished chrome-plated workpiece is determined not only by the chrome layer itself, but also by the layer underneath (usually nickel ). If a nickel layer under the chrome is matt, the workpiece is still matt after the (thin) bright chrome plating. Such a matt surface is desired in some cases and is then perceived as very high quality (silk matt). A bright chrome layer must be run in a certain window of temperature and current density. Outside of this window, the chrome layer does not become shiny, but matt and gray. This can occur especially in the high current density range. If the gray area on the edges of the workpiece is relatively small, it can be brought back to a high gloss with a special polishing paste and a cloth polishing disc.
An optimal coating on steel is z. B. cyanide copper , acid copper, bright nickel, chromium; z. In part, the acidic copper layer is polished before nickel-plating. A special feature in this case is stainless steel ; this can be polished to a high gloss and chrome-plated without an intermediate layer.
Hard chrome plating
The term "hard chrome plating" is a misleading expression because it suggests that a hard chrome layer is harder than a (thin) bright chrome layer. In reality, the hard chrome layer is just as hard as the (thin) bright chrome layer. However, the bright chrome layers are usually so thin (see above) that the test tip does not measure the hardness of the chrome-plated layer during a hardness measurement; hardness measurements on the chrome layer with the underlying ground are not meaningful. For hardness measurements, a piece is therefore cut out of a chrome-plated part, rod, or the like. Then the hardness is measured on the chrome layer. An appropriate term would be “thick chrome plating”, but the term “hard chrome plating” is common. Sometimes chrome layers above about 1 µm are referred to as hard chrome, but there are also hard chrome layers of several millimeters before z. B. in printing cylinders. There are hardships to 1200 HV accessible
A normal glossy or hard chrome layer contains a dense network of very fine cracks that are not visible to the naked eye and cannot be felt. The formation of these cracks is closely related to the hydrogen that is released during deposition. Part of the hydrogen is temporarily stored in the chromium layer in the form of chromium hydride. When the chromium hydride subsequently decays, the chromium layer shrinks and the resulting stresses lead to the cracks. The cracking of chrome layers makes it understandable that a bright chrome layer alone, despite the excellent properties of chrome, does not provide good corrosion protection . Corrosion protection is only created in connection with suitable intermediate layers (usually nickel or copper and nickel). This crack structure is even advantageous for some special cases, since z. B. an oil film can adhere better.
See also : Chroming
Black chrome plating
Black chrome plating is another special case. Due to an increased current density in conjunction with special additives, chrome layers are deposited in a deep black color. The black chrome layer is one of the few deep black surfaces that are electrically conductive. Some black chrome layers have only moderate abrasion resistance. This effect can be improved a little by subsequent oiling. A black chrome electrolyte must be cooled during operation. Black chrome plating must not be confused with black chrome plating .
Surface qualities
Using special electrolyte additives, crack-free, micro-cracked or microporous chrome layers can be deposited. The crack-free chrome layers are not very important in practice, because they usually become cracked later under everyday conditions. For the corrosion resistance (in connection with the intermediate layers) it is beneficial if the crack structure is finer. A normal bright chrome layer has about 1 to 20 cracks per centimeter. At 300 to 800 cracks per centimeter one speaks of a micro-cracked chrome layer. Another way to improve corrosion resistance is to create microporous chromium layers. Like the cracks, the micropores are also invisible to the naked eye. Double chrome plating is also possible under certain conditions.
If the workpiece is exposed to corrosion (e.g. hydraulic ram) it must be coated beforehand (e.g. with chemical nickel plating ). An alternative to this is mechanical post-treatment by honing or polishing , in this case the surface is smoothed and the cracks are smeared (e.g. in piston rods for motor vehicle shock absorbers )
Environmental aspects
Due to the poor efficiency and the high currents, a lot of oxyhydrogen , i.e. hydrogen and oxygen, is generated during chrome plating . The bathroom foams. When the gas bubbles burst on the surface of the bath, the chrome bath is finely atomized. Because of the high risk of cancer posed by chromium (VI), the chromium baths must therefore be suctioned off and the formation of chromium aerosols suppressed, which is possible with a foam carpet formed by surfactants .
Chromium electrolytes based on the non-toxic chromium (III) are currently being researched, but they are currently hardly suitable for production or are limited to special cases. They usually consist of solutions of ammonium salts and contain strong complexing agents . Here, too, insoluble anodes (mostly graphite ) are used.
Lately there have been contract electroplating shops, especially in England, that use chromium (III) electrolytes. The area of application is mainly the fittings industry , since chromium (III) electrolytes attack the base material less and spread it better. Good throwing power means that metal is deposited even in places that are disadvantaged by electricity. The color tone of a layer deposited from a chromium (III) electrolyte differs from that deposited from a chromium (VI) electrolyte and is strongly influenced by foreign metals.
Individual evidence
- ↑ Patent DE398054 : Process for the electrolytic deposition of metallic chromium. Registered on March 9, 1920 , published on July 1, 1924 , applicant: Erik Liebreich.
- ↑ Patent DE448526 : Process for producing a solution suitable for the electrolytic deposition of metallic chromium. Registered on July 22, 1924 , published on July 28, 1927 , applicant: Elektro-Chrom-Gesellschaft mbH Berlin.
- ↑ Stéphane Itasse: From chrome shine to the blind spot . 09/14/17.