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May 15, 2008

Alkaline Cleaning

Emulsion cleaning in agitated emulsions of soap, kerosene, or other hydrocarbon and water is effective for removing non saponifiable oil and grease when heated to at least 54 oC, or up to 70 oC. Time cycles range from 2 to 4 min, as a rule. Emulsion spraying is sometimes adopted for other surface indentations. Thus power-spray equipment might include both emulsion and alkaline washing. Emulsion cleaning must be followed by alkaline soak cleaning or thorough water spray rinsing.

Alkaline soak cleaning is frequently used in place of solvent degreasing, power-spray alkaline washing, or emulsion cleaning especially for unbuffed castings. A typical soak cleaning solution contains 25 to 35 g/l of sodium tripolyphosphate and 2 to 3 g/l of surface-active agents. To avoid prolonged contact with the alkaline solution, cleaning periods usually are restricted to 1 or 2 min. Alkaline cleaners with a high pH (> 10.5) preferentially attack the aluminum phase of zinc alloys.

Aluminum phase corrosion coinciding with subsurface porosity induces cavities that tend to entrap corrosive solution which subsequently causes blistering when the solution entrapped under the electroplate reacts with the zinc alloy to produce hydrogen gas. Loosely adherent gelatinous aluminum compounds formed on the surface of the aluminum rich phase have also been cited as a cause for blistering. Operating temperatures for cleaning zinc die castings are 70 to 82 oC. Ultrasonic agitation will facilitate the softening and removal of buffing compounds from buffed surfaces. A soak and spray rinse, preferebly with warm water, is recommended after alkaline soak cleaning to remove the bulk of the alkali, surfactant, and soil before electrolytic alkaline cleaning.

After precleaning, anodic cleaning is usually selected for zinc alloy in preference to cathode cleaning. Anode current density usually ranges from about 1.6 to 3.2 A/dm2. Time cycles vary from 25 to 45 sec. A typical solution contains 30 to 40 g/l of mixed alkali such as sodium tripolyphosphate and sodium metasilicate, 0.5 g/l of surfactant, and not more than 0.5 g/l sodium hydroxide, and is heated to 70 to 82 oC. Solution with lower concentration of alkaline compounds or lower temperatures are adopted if time cycles must be prolonged for more than 45 sec, or if the transfer time from the electro-cleaner to the rinse is more than about 30 sec.

A cycle including a hot water overflow rinse, a cold water overflow rinse, and water spray rinse is recommended after alkaline electro cleaning. The overflow rinses should be agitated with air to dilute as much as possible the concentration of alkaline solution in blind holes, groove, and other surface indentations and cavities.