Wednesday, February 13, 2008

Porous Chromium Plate

This name have been given to modified chromium deposits with oil-retaining properties, used on internal combustion engine cylinders and piston rings. Such deposits were used especially during the war on aircraft and diesel engine cylinders for salvage and to make engines last longer. Three main of porous chromium plate have come into common use.

The first is the mechanical type, produce by grit blasting the basis metal, chromium plating, and finally finishing to size by grinding, honing, or polishing. The second and third types of plate are those with pitted and channel porosity. Both of the later are obtained by treating the chromium deposit in an etching solution. The type of porosity obtained depends on careful control and regulation of the condition of deposition. Numerous publications and patents describe the production of all these types of porous chromium plate and the result obtained with them.

Another variation is to etch pits into the surface of the deposit through a plastic mask or photoresist. Etching by ion bombardment trough a screen and with alternating current are proposed. Plate process at extremely high current densities from 232 to 1160 A/dm2 at 30 to 55 oC to produce a porous deposit directly. Still another variation is to impregnate the porosity with Teflon to provide the possibility of dry lubrication, but this does not seen to have become important commercially.

The "pit" type of porous chromium may be produced, for example by plating under ordinary hard chromium plating conditions, such as in a bath containing 250 g/l chromic acid and 2.5 g/l sulfate, at 50 oC and to 46 to 54 A/dm2 for a minimum of 2 to 3 hr to get a deposit at least 10 μm thick, and treating the resultant deposit as anode or cathode in a suitable etching solution or by simple immersing in acid. A typical anodic treatment is about 150 A-min/dm2, but this may prolonged or repeated if it is desired to removed more metal or to obtain deeper porosity. After the deposit has been heavily attacked, numerous crack are found to be eaten away, and a surface crust of undermined metal remains. When this crust is ground? Honed, or polished away to the extent of 25 to 50 μm numerous pits remain in the chromium plats.

Good condition for producing the "channel" type of porous chromium plate are 60 oC and the ratio of chromic acid to sulfate of 115:1. The usual current densities of 46 to 62 A/dm2 are employed for a deposit thickness of at least 100 or 125 μm. After treatment in the etching solution, the deposit does not have a loose surface crust but only a network of fissures, so that grinding, polishing, or honing off about 25 μm leaves channels, with dense chromium "plateau" or "land" between.

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