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Question

Why before electroplating ,a brass spoon is first cleaned and then is washed in the hot solution of caustic soda followed by dilute hydrochloric acid and water after which it is thoroughly wash with detergent and several times with distilled water?


Solution

Cleaning should enhance the surface of the part to be cleaned. If not used properly, however, it can impair the surface. You may remove a soil and, in the process of doing this, oxidize or make the surface partially or completely inert. In this sense, you have soiled the surface with a new contaminant! But applied correctly, cleaning can enhance the acceptance of the subsequently applied coating.

A third reason for cleaning is to remove stains, oxide films, etc. Sometimes the process simply enhances cosmetic appeal. Cleaning also can facilitate handling, quality checking, visual acceptance, proper fixturing, etc.

Degreasing is another primary reason for use of any cleaning operation. But for any further surface finishing to be done properly, removal of the film left by degreasing is required. At the same time, you should recognize that, while degreasing is an excellent in-process step and while you may get proper surface preparation in the alkaline cleaner station, it is likely that another step will be required to complete pretreatment for electroplating. The limitations on use of chlorinated solvents for degreasing have increased the value and potential of electrocleaning.

Basis Metals That Can Be Cleaned

It makes life a little easier if we have some general rules of thumb as to what can and cannot be cleaned with various types of alkaline cleaners.

Any alkaline cleaner. Standard alkaline cleaners can be used on steels, stainless steels, tool steels, alloy steels, copper, nickel, nickel alloys, titanium, zirconium and lead/tin without causing a “cure worse than the disease” situation.

Inhibited alkaline cleaners. These solutions contain chemicals that prevent unwanted reactions with metals or alloys in the presence of the caustic -(OH) radical. Such chemistry is required for cleaning brass, bronze, zinc and its alloys, aluminum and its alloys, and tin—the metal itself or as plated.

Acid. There are many acid cleaners that work well in cleaning such metals as magnesium. Cleaning processes must be tailored to handle the type of soils expected on a given part. Table I lists many soils that finishers must remove. By no means is this a complete list. But suppliers of cleaning chemicals must know what soils you are trying to remove.

Soils are generally classified as either organic or inorganic. Organic soils include oils and waxes, and redeposited soils. Inorganic soils may range from heat-treat scale, oxide films and pickling smut to polishing compounds and abrasives to shop dust.

Alkaline cleaners commonly contain carbonates, borax, sodium metasilicates, phosphates, synthetic detergents and surfactants. These chemicals have specific functions, as noted in Table III. Carbonates are added for pH control and buffering. Sodium metasilicate, as a dispersant. Phosphates, for water softening and sequestering. Borax, for buffering and dispersing. Synthetic detergents (“mini soil magnets”), for wetting, emulsifying, complexing, chelating and as biocides. Surfactants come with positive charges (cationic), negative charges (anionic) and no charges (non-ionic).

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