Basics of Alkaline Cleaning

Cleaning is the most important phase of metal working, fabricating and finishing. A clean surface is the critical foundation for subsequent operations.  Alkaline cleaning is the mainstay of industrial cleaning. Cleaners are many in number and vary in type and formulation.  Working solutions of alkaline cleaners may employ both physical and chemical actions. Proprietary alkaline cleaners are specifically formulated chemical blends.  These products typically consist of alkaline builder salts, surfactants, emulsifiers and chelators.

Alkaline builder salts can vary considerably and must be chosen judiciously with respect to the metal substrate being cleaned.  Caustic Soda and Caustic Potash are highly alkaline materials that impart a high alkalinity to the cleaning solution.  They are particularly effective because these materials saponify fats and oils (Saponification is the process of creating a soap from the interaction of alkaline materials with fatty acid lubricants) while neutralizing any acidic soils.  However highly alkaline cleaners based upon caustic building salts are not safe on soft metals such as aluminum, zinc based die-castings and zinc alloys such as brass.  In order to safely clean these substrates, a silicate building salt imparts medium alkalinity and exhibits good detergency.  Furthermore while having the ability to neutralize acid soils, the milder alkaline building salts inhibit alkaline attack on soft metals.  Despite these benefits, cleaners based upon mild silicate alkaline builders tend to form scale at higher temperatures and can leave dried on films if not rinsed quickly after the cleaning process.  Another mild alkaline building salt is phosphate based cleaners.  They exhibit good detergency and are generally safe on soft metals.  Phosphate based alkaline building salts further function to condition hard water.  Despite these benefits, care should be taken because phosphatized cleaners are subject to environmental control depending upon local regulations.  As a result their use can be restricted.

Surfactants (which is short for “surface active agents”) are specialized chemicals which are soluble in both oil and water.  This unique ability allows the cleaner to lift the oil from the surface being cleaned and holds the oil in solution.  The resulting oil in water emulsion is then easily rinsed away.  Wetting agents, soaps and synthetic detergents act as emulsifiers and allow oil to be removed as the oil is displaced from the surface of the part being cleaned.  This results in an emulsion in which the oil droplets are easily dispersed in water.  Heat can greatly accelerate this emulsification process by reducing the oil viscosity and increasing the activity of the surfactant.

Sequestering agents and chelators are the final piece in the construction of a cleaner.  Metal ions which are present in water, such as calcium, magnesium iron or manganese interfere with the detergency of a cleaner by using up the surfactants.  This makes the cleaner inefficient and reduces its useful life.  Chelators are chemicals which combine or “tie up” these metals to allow the cleaner to function efficiently and to extend the cleaner’s useful life.

Alkaline cleaners owe their detergency to the displacement of soils by surface active materials (surfactants) and alkaline builders to create emulsions that are easily rinsed away.   The correct blend of alkaline builders, surfactants, emulsifiers and chelators provide the alkaline cleaners to meet any need.  It is important to note that a cleaner should be selected that is consistent with the method by which the parts will be processed.  An immersion cleaner will be formulated quite differently than a spray cleaner.

The correct cleaner is determined by consideration of a number of factors.  First, it is necessary to determine the base metal undergoing the cleaning process.  This is the most critical step to assure that the alkaline builders chosen will not attack the basis metal.  Soft metals such as aluminum, zinc and zinc alloys such as brass require a milder (lower pH) cleaning solution.

Once the base alkalinity is determined, a cleaner should be selected that is formulated to effectively remove the soils most commonly encountered on the basis metal. Heat can greatly accelerate this process by reducing the oil viscosity and increasing the activity of the surfactants.  Drawing and stamping compounds typically require higher heat to saponify the animal fat usually encountered with this type of soil.  While simple rust preventive oils and water soluble coolants are most easily removed with less aggressive (lower pH) containing products and lower temperatures.  Waxes, heavy oil rust preventives and heavy corrosion resistant compounds usually require a combination of an aggressive cleaning compound (higher pH), strong emulsifiers and dispersing agents along with higher temperatures in order to effective clean the metal substrate.

Finally, it is necessary to decide if a liquid cleaner or powdered product is preferred.  Liquid products can be easily automated and are typically safer to handle.  Powdered products may offer some economies, however the total operating costs may favor liquids.

Alkaline cleaners are effective in the removal of a wide variety of soils.  There is not a universal cleaner, because there is no universal soil. The proper cleaner will produce a clean surface from which to base subsequent operations.