Chromium plating

The operation of chromium plating consists in covering a part a more or less large thickness, of 0.5 µm (decorative chromium) to 1/10 mm rectified (hard chromium plating). Not to confuse with the “chromic completion” or “chromatization”, which is a chemical treatment of conversion in a bath at base chromium plates +VI or +III after zinguage or cadmium-coating.

One distinguishes usually two kinds of chromium plating, the qualities requested from the surface layer not being the same ones according to the applications considered. the purpose of

the decorative chromium plating is to give at piece-rates the aspect shining characteristic of polished chromium surfaces. One uses it for example for bumpers and handles of door.
the hard chromium plating is used in the mechanical applications where the conditions of Frottement are severe like anti-wear Revêtement. One uses it for example for landing gears, systems of doors of planes, etc

Galvanic processes

Since work of Bunsen (1854) and Genther (1856), the use of the electrochemical chromium deposits was largely spread in all the fields of mechanics. Hardness, the wear resistance and to the corrosion of chromium made it possible to cover with this metal of the segment S of Piston, of the cable guides, the stems of Amortisseur S or Vérin S, of the ices of distribution of Pompe S, of the tools of all kinds.

The galvanic deposits present many disadvantages however:

the baths of chromium plating are containing chromic acid and of sulphuric acid with a ratio of 1/100. The bath is containing Cr+6 which is reduced in Cr+3, and into final chromium metal settles on the part.

the chromium deposit can be deposited directly on the part if the series of treatments is well applied. For the jacks it is advised to carry out a nickel underlayer of 20 µm to limit corrosion.
It should be known that the chromium deposit to cathodic poor yield (15%) and to cathode occurs a release of Hydrogène which weakens steel and decreases its resistance to the tiredness. A treatment of degasification is to be made to find the mechanical characteristics of the part.

To increase the fatigue strength, a shot-blasting of prestressed can also be carried out before chromium plating.

Improvements were made to the process, to avoid the appearance of microscopic cracks in the deposits, to obtain a certain microporosity favorable to the lubricant retention, to replace hexavalent chromium by less toxic trivalent chromium, etc

The electrodeposition under pulsated currents improves considerably the speed of deposition and the cathodic output.

Processes of vacuum deposit

One can carry out chromium plating by physical Dépôt in vapor phase (DEVELOPING COUNTRY).

The most promising technique is undoubtedly the Cathode sputtering magnetron (PCM).

This operation occurs in a vacuum chamber containing from the Argon or another inert gas under very low pressure (some hundredths of Torr). A target made up of a metal M is carried to a sufficiently high negative tension so that the electric field thus created can ionize the atmosphere in its vicinity. The ions argon which result from this field are attracted by the target, which they strike violently, tearing off from the atoms of metal M and projecting them in the environment. These atoms then will condense on the surface of the objects placed opposite the target. One obtains “cathode sputtering then diode”, which is very slow. It is necessary between 10 min and 1:00 to obtain a thickness of deposit about the μm. While applying in more one magnetic field perpendicular to the electric field, one increases to a significant degree the density of the plasma which is formed around the target, the speed of deposition can then reach an industrially acceptable value of about 1 μm/min.

The deposits carried out by PCM are much fissured and much more compact than those which are carried out by galvanic way but they are also definitely less hard. The presence in the enclosure of traces of other elements as oxygen, the nitrogen, carbon, makes it possible to correct this defect and to even reach hardnesses much higher. One can show by Diffraction X-rays that these elements do not combine however with chromium, so that their action is not easy to interpret.

The tests of friction carried out on the layers obtained by PCM show that the fatigue strength of the parts thus treated is quite higher than what one obtains by galvanic way, and naturally there is no more no hydrogen embrittlement.

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