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Surface treatments and finishes

PCP Aluminium cast plate can be used in many applications without further surface machining. While the plate offers good corrosion resistance, the customers may desire to change plate appearance for particular applications, improve corrosion resistance or increase surface hardness. PCP Aluminium plates respond well to a variety of finishing treatments, including grinding, polishing, painting, anodizing, hard coating, electroplating, etching and chemical treatments.

 

Mechanical

Generally, some type of mechanical finishing is required on the surface of the aluminum plate prior to subsequent finishing treatments. Grinding and polishing with coated abrasives in the form of wheels, belts and discs are suitable for this purpose.

Aluminum oxide of 80 to 400 mesh grit size, operating at 1800 m/min with suitable lubrication, produces directional textured surfaces ranging from coarse to fine. Buffing with cloth wheels and a fine abrasive compounds will produce a high luster by eliminating surface marks produced by grinding and polishing.

 

Chemical

Solvents applied by hand or in vapor degreasers are satisfactory for removing soils and lubricants from the surfaces. Subsequent cleaning in alkaline or acidic solutions result in even cleaner surfaces and acts as pretreatment for anodizing, painting and other finishing treatments.
Etching in heated sodium hydroxide solutions after degreasing is often used to prepare a surface for subsequent treatments. In limited cases, etching is used as a lower-cost means of removing large quantities of aluminum chemically rather than by mechanical means. Chemical conversion of the chromium chromate, chromium phosphate and oxide types also may be applied to PCP Aluminium cast plates. They are generally used as a base for organic coatings. When applying these coatings or other treatments, PCP Aluminium recommends the use of proprietary materials available from many chemical suppliers.

 

Electroplating

Electroplated coatings of nickel, chromium, silver, tin and other metals may be applied to PCP Aluminium cast plate, to obtain a specific properties such as metallic appearance, improved resistance to wear and corrosion, increased electrical conductivity, improved solderability, and frictional properties. Because of the natural oxide film on its surface, aluminum will respond properly to procedures usually employed when other metals are prepared for plating. Different conditioning treatments must be used to obtain a satisfactory base for plating aluminum.

The pretreatment most often used commercially is the zinc immersion process. The Alstan Process*, involving activation of the surface with tin, is also widely used. Since cast plate is producrd with DC casting technology, it can exhibit porosity which may affect the appearance.

*Trademark of Atotech, USA

 

Anodizing and Hard Coatings

PCP Aluminium Cast Plate has been used in a variety of anodizing processes for commercial applications under several trade names. Anodizing processes generally employ sulfuric or chromic acid as the electrolyte. However, other acid electrolytes produce anodic coatings with specific characteristics. Anodized finishes are both decorative and functional. Hardness can be controlled by the thickness and density of the anodizing finish. Coatings formed in a 15% (wt) sulfuric acid electrolyte maintained at 21°C, 1.1 amp/dm2 current density can vary in thickness from 0,0025 mm to 0,020 mm.

Due to alloy composition and cast structure, the anodized finish can exhibit a non-uniform gray appearance. However, this will not affect its corrosion or wear resistance. Alca5® will respond slightly differently to this anodizing practice depending, on the selected process. By altering the voltage and/or current density, the difference in rate of coating growth for Alca5® may be modified to achieve the desired coating thickness.

 

ALCA 5® — Hard Coatings

Hard anodic coatings can be applied to PCP Aluminium cast plate to improve its wear characteristics. This is related to the increased coating thickness and density. Surface roughness will be increased as a result of anodizing.

 

ALCAMAX® — Anodizing

ALCA MAX® can be anodized in a 20°C, 20% sulfuric acid solution at 12 amperes per 0,1 sqm, «floating» voltage, for one hour. Current should be ramped over 15 seconds. Initial voltage should be 5.5V, slowly increasing over the one-hour time period to 10V. Sealing can then be accomplished in a standard nickel acetate sealing solution. Hold in this solution at pH 6.5 and 100°C for 15 minutes.

 

ALCAMAX® — Hard Coatings

ALCA MAX® can be hard coated in a 5°C, 25% sulfuric acid solution for 40 minutes. Current density should be 36 amperes per 0,1 sqm «floating» voltage. Current should be ramped to operating current density over 10 minutes. Initial voltage should be 4V, slowly rising to 22V over 40 minutes. Sealing can then be carried out in a standard nickel acetate sealing solution. Hold in the solution at pH 6.5 and 100°C for 15 minutes.