Ruben Hernandez Peña
Commercial Projects Adviser
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Basic Principles of Aqueous Cleaning

By Ruben Hernandez Peña | Mon, 07/18/2022 - 10:30

Cleaning is defined as the removal of unwanted soils from the surface of a part. Mechanical components must be cleaned as part of the manufacturing process or during maintenance and overhaul operations.

Proper cleaning during remanufacturing and overhaul is also critical. Cleaning mechanical parts will reveal defects, which may lead to a decision to scrap some of them. Cleaning also contributes to worker safety, as much of the contamination will be removed from components before they are dismantled and overhauled.

There are four factors that affect the cleaning process: chemistry; temperature; mechanical action; and time. These factors are generally synergistic. For example, in most cases, adding a little heat (temperature) makes the cleaning solution (chemistry) more effective.

Chemistry

Most chemical agents used in aqueous cleaning fit into one of three categories according to pH.

Neutral cleaners, the kindest of cleaners, are preferred for light soils. Generally, they are environmentally friendly. Depending on local codes and the composition of the soils that become part of the mix during use, spent cleaning solutions can sometimes be disposed of without further treatment.

Alkaline cleaners are the most used for removing oils, grease and general soils. There are numerous chemical substances available depending on the type and degree of contamination, the material to be cleaned, the type of cleaning equipment used and the subsequent use of the cleaned material.

Acidic cleaners are primarily used to remove tarnish and oxides and to brighten non-ferrous metals. Although some limited cleaning of organic soils (such as oils) is possible with some acidic chemicals, they are rarely used for general cleaning. It is not unusual, however, to use an acidic chemical substance to brighten work previously processed with an alkaline cleaner and a final rinse.

Temperature

Temperature has a significant effect on cleaning efficiency in most applications. Generally, the hotter the cleaning solution is, the faster cleaning occurs. One application that illustrates the effect of temperature is the removal of buffing compounds. Most buffing compound formulations contain a significant portion of fats. Try to remove it at 48°C and you are in for a long day. At about 65°C cleaning becomes possible. At 82°C cleaning is much easier. This is because the higher temperature softens, or pre-conditions, the contamination and allows the chemistry to work more efficiently.

There are other considerations concerning temperature. In a buffing compound removal application with brass parts, using 82°C could cause the parts to tarnish as soon as they are removed from the cleaning solution. For this application, there is more than one right answer. You can back off on the temperature and accept a longer cleaning cycle. Alternatively, you can add another wash step (and rinse) using an acidic chemical step to remove the tarnish. This trade-off is typical of many cleaning applications, where the advantages and disadvantages of various alternatives need to be balanced.

Mechanical Action

Mechanical action is sometimes described in more technical articles as "impingement," which refers to the application of mechanical force.

A simple method of applying mechanical action is agitation. The most common type of part agitation machine uses vertical agitation. The repeated up and down motion is effective in cleaning simple parts, such as parts without blind holes and/or deep grooves. Another type of part agitation machine uses a rotating basket, typically for small parts that tumble through the cleaning solution as the basket rotates. There is also part-on-part contact with a rotating basket that adds some mechanical scrubbing into the process.

For industrial parts cleaning, there is practical equipment that uses liquid agitation. Methods for moving the liquid include impellers, eductors and air agitators. Liquid agitation can also be an effective way of cleaning simple parts without blind holes and deep grooves. It's not unusual to add liquid agitation to other types of equipment to increase effectiveness. For example, it is not unusual to combine a rotating basket with liquid agitation.

Ultrasonic cleaning depends on a phenomenon called "cavitation."

In ultrasonic cleaning, a sound wave is created in water in a manner similar to the way an audio speaker creates sound in air, by vibrating a diaphragm. As the sound wave passes through the water, alternating areas of high and low pressure are created. As the frequency of the sound becomes higher, the sound waves are closer together. When the frequency approaches the limit of human hearing and beyond, the alternating high- and low-pressure areas occur fast enough that cavitation can occur on a microscopic scale if the intensity (amplitude) of the sound wave is high enough.

The mechanical energy in ultrasonic cleaning occurs when the cavitation bubble implodes (collapses rapidly). Water rushes in at high velocity, although not exactly evenly, to fill the space formerly occupied by the vapor bubble. The result is "micro-jetting" that provides the mechanical energy for cleaning.

Cavitation occurs everywhere throughout the liquid, including deep grooves and blind holes, or any area that the liquid can reach. Ultrasonic cleaning is effective on parts that are not cleanable by other methods.

A quick note about ultrasonic cleaning and chemistry is needed here. Not all cleaning chemical agents are suitable for use in ultrasonic cleaning equipment. For good cavitation, the cleaning solution should have high surface tension, low vapor pressure and low viscosity.

Time

The fourth factor, time, is an important consideration in cleaning applications. For a given set of process conditions, longer exposure generally results in cleaner parts.

No discussion of aqueous cleaning is complete without mention of the importance of adequate rinsing. Depending on the material being cleaned, the chemical agents used, and the cleanliness specifications, anything from a simple immersion or spray rinse to multiple rinses, including an ultrasonic rinse, may be required. Rinsing can be especially important with parts made from more than one metal. Chemical residue between the different metals can act like a battery and promote long-term corrosion.

How Clean Is Clean?

Before beginning to develop a cleaning process, it's best that you are able to answer an often-overlooked question: How clean is clean? The answer can vary widely. For example, a machined aluminum bracket for a farm tractor accessory may need to be only clean enough for the assembler to handle and attach it without making a mess. On the other hand, an aluminum valve body for an ABS brake system may need to be 100 percent free of any particles greater than 10 microns.

Nowadays, the residue-free cleaning of surfaces plays a vital role in virtually every industrial manufacturing process. Perfect cleaning enhances the quality of your products.

The introduction of quality management systems and increasingly stringent environmental legislation has led to more and more attention being focused on aqueous cleaning techniques. This technology is now firmly established in many fields. 

Precision parts cleaning and ultrasonic parts cleaning in the metal-processing industry meet the highest demands.

In the precision metal industry, the requirements that have to be met for the aqueous cleaning of parts are enormous. Besides factors such as material compatibility and the contaminants themselves, the success of aqueous cleaning processes depends above all on taking the customer’s existing cleaning system fully into account in the cleaning proposal.

Decoating is the process of removing the old coating on the workpiece before the coating. The workpiece can be repaired after the initial coating is used and the coating can be used again. It can also be repaired and repeated several times. However, the continuous accumulation of multiple coatings will have a negative impact on the service life of the workpiece and the accuracy of the workpiece, so there is a need for repainting.

The decoating process is mainly divided into chemical decoating and electrolytic decoating. Both decoating processes utilize the chemical reaction between the decoating solution and the coating to oxidize and decompose the coating. 

During the decoating process, regardless of the methods, the decoating solution may also damage the substrate while oxidizing and dissolving the coating because some steel contains the same alloy components as the coating, such as Ti, W and Al, especially P-type hard alloy, which contains about 15 percent TiC and is more sensitive to the corrosion solution. In addition, cobalt is used as a bonding agent in cemented carbide and cobalt is easily lost in a chemical solution. After cobalt loss, the cemented carbide composition around it loses its binding force, making it easy to fall off, and the surface of the workpiece is destroyed. Therefore, for the workpiece that needs to be decoated, be fully prepared before decoating and understand the characteristics of the workpiece substrate material. In addition, after the workpiece is decoated, apply some necessary post-treatment, such as sandblasting or polishing, to reprocess and repair the workpiece surface.

 

T5DC SA de CV is the exclusive distributor in Mexico of Borer Chemie AG and the deconex® brand, which stands for quality, efficiency and safety and excellent value for money. For more than 50 years, Borer Chemie has been the best manufacturer of Aqueous cleaning agents for different applications. In the following tables, you can see all Borer products and their main features. For detailed information, please contact us: contacto@t5dc.com

Photo by:   Ruben Hernandez Peña

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