Resistencia electrica

Páginas: 5 (1036 palabras) Publicado: 2 de noviembre de 2010
In some cases etching can be accomplished by simply reducing the voltage to aproximeattely one tenth the potential required for polishing and continuing electrolysis for a fewseconds
Electropolishing is frequently useful in electron metallography since the higher resolution puts a premium on clean torted metal surfaces. In is sometimes combeniend to electropolish so as to leave a residual etchof suitable depth. The very clean, slightly undercut surface may cause difficulties in replica stripping.
Metallographic preparation by electropollishing is subjet to a number of limitation which should be recognized to prevent misapplication 0f the method and disappointment in the results. The principal disadvantages include the following:
Many of the chemical mixtures used inelectropolishing are poisonous or dangerous If not properly hannled. These hazards are similar to those involved in the mixing and handling of etchants.
The conditions and electrolytes required to obtain a satisfactorily polished sur face are different for different alloys, hence considerable time may be required develop a procedure for a new alloy, of it can be developed at all. Thislimitation does not aplly if appropriate procedures exist.
In multiphase alloys the rate of pollushing of different phases may not be the same; al times this phenomenon may be an advantage.
A large number of electrolytes maybe needed to polish the variety of metals encountered by a given laboratory.
Plastic or metal mounting naturials may react with the electrolyte.
Electropolished surface exhibit anundulating rather than a plane surface and may not in some cases be suited for examination at all magnifications.
Edge effects limit applications involving small samples, surfaces, and cracks.
Attack around nonmetallic practices and adjacent metal, voids, and various inhomogeneities may not be same as that of the matrix, exaggerating the size of the voids and inclusions.
The electropolishedsurfaces of certains materials may be passive and difficult to etch.
Artifacts may occasionally be produced by electropolishing .
For thwe electropolishing of metal speceiments in apppropiate electrolyte, a suitable electrolysis cell, and a controllable power supply are needed. Simple laboratory aooaratus can be assembled to perform the function and many such arrangements are sescriben in theliterature. There also several commercial models of electropolshing apparatus.
Whenever an attemopt is made to polish large surface areas, the problems of obtaining sufficient areas, the problems of obtainting sufficient current density and cooling of the specimen and electrolyte become very troublesome. An adequate mass of electrolyte should be used sothet local overheathing does not occur.Supplementary cooling may be required. In general electropolshing of areas larger than I cm2 is not recommended for difficulties encountered, but it is usually possible to polish larger areas.
Many variations are possible in the actual desing of a cell for elcetroysis. A simple arrangement is shown in fig.2. Since the current density is critical. It is advantageous to predetermine the approximatearea to be polished so that the conditions for optimum results can be achieved. This variable can be controlled by the use of protecting masks of insulating paints of tapes.
Suitable power spurces may be batteries, rectifier power supplies, or direct-current generations. Since As much as 150 V may be required for the polishing of some metals. Precautions must be taken to avoid electrical shock. Theideal power source would be a true direct-current potential wit a closely controlled voltage.
One of the best ways of obtaining the power for electropolishing is to rectify the output of a variable-voltage transformer.
When a single-phase, full-wave bridge-rectifier circuit is used, the output current is not satisfactory for all electropolishing unless the ripple voltage is reduced to some...
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