The Scheme of Electropolishing
Before the aluminum be proceed by anodizing, the material must treat use electropolishing first, this process will open the surface film above the aluminum part, because in the store or travel after the aluminum be produced will react with free air build an aluminum oxide that cover the surface of aluminum. To open the surface pore can't use just dipped in water but must use electric current. Even if dipped in hot water the pores can be closed.
Electropolishing Definition
Electropolishing is an excellent preparation of metals for electroplating, this is a kind of metal surface treatment before anodizing process or other kind of process. In addition to providing a chemically and physically clean surface, electropolishing removes mechanically surface damage. As a result, subsequent electroplates have the best adhesion possible and a decreased tendency toward developing pits and voids that lower corrosion protection. Metal surfaces that have been machined, ground, picked and rolled, abrasive polished, and buffed, have asperities that are detrimental to uniform and pit-free electroplates, adhesion of the plate, and strength of the interface region of the plate basis metal.
Electropolishing is a process of producing a polished and bright limit on the surface of metals. In this process the metal part or the device to be coated with a layer of suitable metal is immersed, as the anode in a solution having such a composition that when the metal is made the anode, there occur unstable conditions of passivity which manifest themselves by an anomalous change in current strength is adjusted to give a current density, previously determined by observation, which is characteristic for the given metal and the given solution, at which there occurs the transformation of the Matt surface into a bright polished one.
Electropolishing Bath:
The most common used in electropolishing bath are based on sulfuric and phosphoric acids in about 50/50 weight proportion at about 54 to 105 oC for ferrous alloys, 85% phosphoric acid for copper at about 43 to 60 oC; and phosphoric acid with about 5 to 7% chromic anhydride (CrO3) at about 60 oC for brass.
Current density of practical electropolishing range from 5 to 40 A/dm2, depending on the metal being finished, type of bath, and temperature. The time required usually is 2 to 7 min for current densities of 15 to 30 A/dm2 and temperature of 54 to 82 oC.
The solution used for the polishing of aluminum can be divided into two groups. The first of these includes the phosphate, alkaline and fluoboric acid solution. They are characterized by a low rate of dissolution of the metal and are used as a finishing treatment for mechanically polished parts with a view to obtain a very high reflectivity. They are employed for treatment of reflectors where a high coefficient of reflection can’t be obtained by mechanical polishing alone. The fluoboric acid and the phosphate electrolytes are suitable only for treatment of aluminum of high purity. The second group includes solutions containing phosphoric acid, sulfuric and chromic acid. They are characterized by a higher rate of dissolution of the metal than the solutions belonging to the first group, and they enable the smoothing out of microasparities and the production of a bright, but not a mirror finished surface. These solution are used in place of mechanical surface finishing operations for parts of aluminum of different purity grades and for certain wrought aluminum alloys. In addition to the above, solutions of perchloric acid and acetic anhydride have also been recommended for the polishing of aluminum. The use of these electrolytes, however, involves taking certain precautions because of the risk of explosions.
The purity of aluminum is an important factor in electropolishing of the metal. An appreciable increase in reflectivity can achieved only in able amount of admixtures is much less satisfactory. Very often polishing becomes impossible because of the non-uniform dissolution of the surface of the anode. This factor renders difficult the polishing of aluminum alloys, especially those containing silicon. For the treatment of castings of an aluminum alloy containing 5 per cent silicon, an electrolyte comprising 13 per cent hydro-fluoroboric acid and 52 percent glycerin is recommended. The solution is used at 25 0C with a current density of 200 amp./sq.ft., the time of polishing being 10 minutes. During polishing the solution is stirred.
Related Articles:
- Aluminum Anodizing
- Formulation
- Operating Condition of Electroplating
- Fluoboric Acid Solution
- Alkaline Solution
- Phosporic Sulfuric Chromic Acid Solution
- Anodizing in Oxalic Acid
- Sealing Oxide Film
- Aluminum Anodized Color
No comments:
Post a Comment