After the industrial hard chrome plating, the work pieces need to pass from the finisher for the surface preparation in order to achieve the desired roughness.
Centreless metal surface finishing is a certified machining process that uses fine abrasive belts to achieve the right finished surface on the work piece; this metal coating process doesn’t remove chrome from the work piece. (just some tenth of micron).
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The importance of centreless finishing
The Chromed tube finishing line has 2 or 4 pressure controlled belt-grinding stations and 2 or 4 adjustable angle buffing stations. The wet grinding belt and the mirror finish buffing with automatic compound system in one pass.
When the layer of hard chrome is under 35µm, the roundness of tubes is very limited, so the centreless finishing is the perfect technology to maintain the high performance characteristics of hard chrome and at the same time to have a good geometrical tolerance.
For higher thickness of hard chrome (60-100µm) stone grinding is necessary, but when the chrome grinding plate as too large a cut or pressure can cause excessive heat and can induce cracking in the part from propagation of the microcracks.
The importance of maintaining the micro cracks is essential, seen as the chromium electro deposits are not homogenous, because they contain micro cracks formed during the chromium plating, born from the pressurized reduction during the treatment. These micro cracks are filled with material containing more oxide than metal chrome
The micro cracks are not empty, they are instead a three-dimensional area of one of the most electrochemical active areas compared to the chrome total mass.
The pressurized deposit is inversely proportioned to the micro cracks number. the crack free deposits are very pressurized and softer than the chrome micro crack deposit.
These micro cracks are not noxious for the corrosion resistance as we could expect, for these two reasons:
- Firstly, the micro cracks are not empty because the structure and the composition is different from the deposit mass; in addition, they contain more oxide than chrome, increasing the micro hardness of the deposits.
- Secondly, the micro cracks divide and increase the corrosion to a larger area, diminishing the corrosive penetration force on the coating.