Alkaline Cleaning Guide
An overview of the alkaline cleaning process…
This guide provides finishers an overview of cleaners, the differences between them and discusses what to look for in choosing an effective alkaline cleaner. Alkaline cleaning blends alkalinity sources with a balanced amount of surfactants to constitute a highly effective metal cleaner. The concentrate is dissolved in water and, in combination with mechanical action and temperature, generally removes most contaminants. This method is cost effective, and disposal of the spent material is easy.
Cleaning is an essential part of the finishing process. The cleaning method can vary depending on the contaminant to be removed, amount of material involved, possible need for an automated process and overall effect on subsequent operations.
Alkaline cleaners are specifically formulated chemical blends consisting of alkaline salts, wetting agents and sequestrant (chelating) agents. They owe their detergency, or cleaning ability, to the displacement of contaminants by surface-active materials and alkaline builders. These constituents remove the contaminants and allow them to be easily rinsed away.
The term "alkaline builders" covers a broad group of chemicals: caustic soda (NaOH), caustic potash (KOH), phosphates, silicates and carbonates. These chemicals supply the alkalinity for the cleaner. High-alkalinity products saponify fats and vegetable oils into soluble soaps. These alkaline salts also neutralize acidic contaminants and aid in dispersing oils.
Caustics (KOH, NaOH), the most common type of alkalinity builders, are highly alkaline (pH 12 to 14). They saponify fats and work with surfactants to disperse contaminants. This type of builder is not safe for use on soft metals like aluminum and zinc.
Silicates provide medium alkalinity (pH 11 to 12.5) and contribute to detergency. They inhibit attack on soft metals, but they become insoluble at a pH less than 10.
Phosphates have slightly lower alkalinity values (pH 9.5 to 11.5) and provide more detergency than the other builders listed. They provide some protection for soft metals and tie up hard water ions, preventing them from interfering in the cleaning process.
Mildly alkaline carbonates (pH 9 to 9.5) are mainly used to neutralize acidic contaminants. They also buffer solutions to maintain a specific pH range.
Wetting agents (surfactants and synthetic detergents) aid in removing contaminants by lowering the surface tension of the solution, allowing the cleaner to get under the contaminant and displace it from the metal surface. Once the contaminant is in solution, the wetting agent creates an emulsion, preventing redeposition onto the part being cleaned. Surfactants have one end that is soluble in water (hydrophilic) and one end that is soluble in oil (hydrophobic). This allows the surfactant molecule to create an oil-water emulsion that is easily rinsed away. Cleaning principles. Soil is defined as matter out of place. Regardless of the type or category, all cleaners remove contaminants from a substrate by one or more of the following principles:
- solvent action - enables the cleaner to dissolve oils present on the metal surface;
- saponification - chemically converts drawing compounds (organic oils and fatty acids) into water-soluble soaps that can add to cleaning efficiency;
- detergency - surface active agents, or surfactants, reduce the interfacial tension between solution and contaminant, enabling cleaning solutions to better penetrate and displace contaminants from the metal surface;
- emulsification - surfactants in the cleaning solution suspend contaminants in the aqueous phase for easy rinsing; and
- deflocculation - disperses contaminants into very fine particles that are suspended in the cleaning solution. Materials to be removed are classified into two general
categories: oil and particulate matter. Oil, by definition, is a petroleum-based product. However, for discussion purposes, simple waxes, vegetable oils and animal fats, which may have been applied to facilitate processing operations or rust prevention, may be included. Particulate matter is finely divided contaminants present on the surface of the substrate to be cleaned, including smut, pigments, drawing materials and shop dirt.
To select a cleaner it is important to consider which metal(s) will be processed to prevent attack of the substrate. This is particularly important with aluminum, zinc and certain exotic metals.
The cleaner must be formulated to effectively remove the contaminants it will encounter. Simple, light-rust preventive oils and water-soluble coolants are easily removed with mildly alkaline cleaners at moderate temperatures. Waxes, heavy-oil rust inhibitors and other durable corrosion prevention compounds require a more aggressive product. Typically, a high-alkaline product with a good oil-solubilizing surfactant package is needed in conjunction with high temperatures.
The cleaner must also be suitable for the mechanics of the operating system. Immersion cleaners normally require different surfactant systems than spray cleaners. Also, the use of chelates is often necessary to counteract the undesirable effects of hard water salts.
Concentrate form also needs to be considered. Liquids are easy to use and can be automated. Powders are usually added manually, but they are more cost effective for most operations.
An important part of the cleaning process is the rinse stage. As the substrate leaves the cleaner stage it carries spent cleaner, emulsified soils and other contaminants. If not immediately rinsed, these contaminants can redeposit on the part and become difficult to remove. The rinse must remove these unwanted materials and not interfere with subsequent operations. Typically, a multi-stage rinse is used to ensure all contaminants have been removed.
Caustics and silicates, two major ingredients of cleaners, are poor rinsers. On the other hand, phosphate and phosphate blends along with blends of caustics and silicates are relatively easy to rinse. Hot water can assist in rinsing; however, care must be exercised so that drying does not occur just before the rinse tank. Double rinsing is common, using either deionized water for spot-free parts or adding a corrosion inhibitor to the final rinse if rusting is a problem.
Single stage: clean only, no rinse. This method should contain inhibition if parts are ferrous steel. It should be skimmed frequently to remove floating oils, extending cleaner life and preventing redeposition.
An immersion tank should be used for low-volume or batch work. An automated cabinet spray washer is more efficient for low-volume batch work, while a conveyorized spray is more efficient for high-volume, continuous production work. Another equipment option is ultrasonic.
Two stage: clean and rinse. With this method, frequent skimming to remove oils is needed in stage one. This extends cleaner life and prevents redeposition. The rinse should also be skimmed and changed frequently, and a rust inhibitor may be added as well.
An immersion tank, automated spray cabinet equipment with rinse cycle, conveyorized spray, auger washer or ultrasonic equipment may be used.
Three stage: clean, rinse, rinse. This system can produce the cleanest part. Stage one should be skimmed, while the stage two rinse should be kept clean by overflowing. Stage three can use deionized water for spot-free parts and/or incorporate a water-soluble rust inhibitor if needed.
Equipment options include immersion tanks, automated cabinet spray equipment with three indexed cycles (wash, rinse, and conditioned rinse), conveyorized spray, auger wash and ultrasonic.
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