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Basic Analitical Skills
General Metal Finishing
Plating Solution Analysis
The Hull Cell
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chemical conversion coating
Preparatory Steps of Plating
Since quality plating is primarily dependant on reactions which take place at the cathode during deposition, It is important that the plating bath be properly prepared and adjusted in order for these reactions to be favorable. However, even if conditions are optimum both in the bath and at the cathode, there is no guarantee that the deposit will be satisfactory. Preparatory steps are dependant on the surface and on the plating bath to be used. The preparation of steel for example, is greatly different than that of aluminum. General steps applicable to many plating cycles include:
(1) cleaning, (2) descaling, (3) activation, (4) plating.
Clean & Active Surfaces
No phase is more essential to successful plating than a thorough respect for the principles of activity and passivity during preparation of the surface and plating of the first layer of metal. The surface to be plated needs to be free of grease, soil and scale.
An alkaline cleaning agent or solvent can be used to remove grease and soil from the surface. Scale can be removed by chemical means such as pickling, or by mechanical means such as abrasive blasting. After cleaning and rinsing, the part should be examined for absence of loose dirt and oil. If the part is not clean or shows water breaks it should be recleaned.
The surface is activated by the complete removal of all surface films. Once rendered active, the surface must be kept in this state until it is covered by the metal being deposited.
Here are examples of some of the metal finish processes.
Sealing of Anodized Aluminum
Sealing of anodized aluminum is the last step in a normal production process and attributes to the finish maximum corrosion resistance. The anhydrous aluminum oxide formed in anodizing contains pores which, if left open, would permit corrosive environments to contact the aluminum base. Typically, seal coatings are deposited onto the anodized coating at elevated temperatures and are usually applied by immersion or spray processes. Sealing converts the anhydrous aluminum oxide to a hydrate. This hydrate, occupying greater molecular volume than in the anhydrous form, expands the surface of the anodic film with resultant "sealing" of the pores. Post treatments are usually called out by the military or commercial specifications that govern each coating being treated. As such, there is not a unique "post treatment" specification for all anodize aluminums like there is for "conversion coating" aluminums.
Anodized aluminum(s) are generally sealed or post-treated after anodizing by processes employing a variety of sealing compositions. One method of sealing anodized parts is accomplished by immersing a thoroughly rinsed object in water operated at 206° to 210°F and a pH of 5.5-6.5 for a minimum of fifteen minutes. Current high-performance post treatments or sealers for anodized aluminum are based on hexavalent chromium chemistry. Small amounts of dichromates may be added to pure water to promote superior corrosion resistance to the sealed anodic film. However, sometimes a slight chromate color is imparted to the anodic film and this limits its usefulness in applications where appearance is important. Hexavalent chromium is highly toxic and a known carcinogen. As a result, the solutions used to deposit these protective coatings and the coatings per se are toxic. These films do, however, yield outstanding paint adhesion, abrasion, and corrosion resistance to the anodized aluminum.
Chemical Conversion Coatings
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