How to Select the Best Ultrasonic Cleaning Solution

Choosing the correct ultrasonic cleaning solution chemistry is arguably the most critical step in the entire ultrasonic cleaning process, directly determining the effectiveness, safety, and efficiency of the operation.

That’s because the wrong chemistry can fail to remove contaminants, damage the parts being cleaned, or even pose a hazard to the operator. This post will guide you through the key considerations and categories of solutions to help you select the ideal chemistry for your application.

Understanding the Role of Chemistry in Ultrasonic Cleaning

Ultrasonic cleaning uses high-frequency sound waves above the range of human hearing, about 20,000 cycles per second (20 kHz).  These sound waves generate microscopic bubbles that implode with immense energy in the ultrasonic cleaning solution – a process called cavitation – creating a scrubbing action that removes contaminants from the surfaces of submerged objects. For more details on this see our post How Ultrasonic Cleaners Work.

While cavitation provides the mechanical action, the cleaning solution chemistry provides the chemical action. The chemistry’s job is to:

1. Lower Surface Tension: This allows the solution to penetrate small crevices and holes where contaminants hide, enhancing the reach of cavitation.
2. Solubilize or Emulsify Contaminants: It breaks down or surrounds soils (like oils, greases, or fluxes) so they can be carried away by the liquid.
3. Prevent Redeposition: It holds the soils in suspension so they don’t settle back onto the cleaned parts.
4. Neutralize or React: It can chemically react with certain soils (e.g., neutralizing acids or dissolving oxides).

The ideal solution is non-toxic, biodegradable, non-flammable, and effective at its recommended operating temperature.

Key Factors in Chemistry Selection

Selecting the right chemistry requires a comprehensive assessment of three main factors: the soil, the substrate, and the process requirements.

1. The Soil (Contaminant)

The type of contamination is the primary driver for chemistry selection. What are you trying to remove?

2. The Substrate (Material Being Cleaned)

The material of the part must be protected from damage. The chemistry that cleans the soil must not attack the base material.

• Aluminum, Magnesium, and Zinc: These softer, reactive metals are highly susceptible to corrosion and etching from strong alkaline (high pH) solutions and, to a lesser extent, strong acids. Neutral or mild alkaline (pH 8-10.5) solutions are essential, and formulation with corrosion inhibitors may be warranted.
• Stainless Steel, Ferrous Metals (Steel, Iron): Generally robust, but strong acids (especially hydrochloric acid) can cause pitting or hydrogen embrittlement. Mild acids for descaling or strong alkaline solutions are typically safe.
• Plastics and Composites: Solvent-based solutions can cause swelling, softening, or cracking. Aqueous (water-based) detergents are almost always the safest option.
• Delicate Components (e.g., optics, electronics): Require ultra-gentle, residue-free chemistries, often neutral pH or precision electronic cleaning formulations.

3. Process and Environmental Requirements

These factors govern the operational and regulatory limits of your choice.

• Temperature: Solutions are typically formulated to operate optimally within a specific temperature range, often 120 to 180⁰F (49C to 82⁰C). Too high can cause flash-off of volatile components; too low may render the surfactants ineffective.
• Safety/Environmental: Regulations (e.g., VOC restrictions, wastewater discharge limits) are pushing industry toward aqueous (water-based), biodegradable, non-hazardous, and phosphate-free chemistries.
• Rinsing: For precision parts, the solution must be easily and completely rinsed away. Low-foaming and free-rinsing formulas are preferred.
• Bath Life: The solution should maintain its cleaning power for a reasonable period before needing disposal or filtration.

Major Categories of Cleaning Solutions

Demulsifying vs. Emulsifying Cleaning Solutions

This distinction deals with what happens to the contaminants, particularly greasy contaminants, as they are removed by the cleaning process.

Demulsifying Ultrasonic Cleaner Solutions

Demulsifying ultrasonic cleaner solutions cause oily contaminants to float to the surface of the cleaning solution. This makes them easy to skim off either manually or via spray bars and weirs for storage and later disposal.

The step serves two purposes:

1. To reduce the potential for floating contaminants to adhere to the product while being removed from the bath.
2. To help prolong the useful life of the ultrasonic cleaning solution. Demulsifying formulations are typically used in high-volume cleaning operations.

Emulsifying Cleaning Formulations

Emulsifying cleaning formulations hold contaminants in suspension.

These ultrasonic cleaner solution formulations are used for low-volume cleaning operations. 

Because contaminants remain in the solution rather than float to the surface they build up over repeated cleaning cycles. In doing so they inhibit cleaning efficiency and, perhaps more important, may remain on the surfaces of cleaned products. This could necessitate post-cleaning rinsing operations.

Hard particles such as chips, fines, shavings and plain old dirt fall to the bottom of the cleaning tank. These must be removed each time the solution is changed, otherwise they can damage the tank bottom.  Tank cleaning recommendations are provided in user manuals.

Ultrasonic cleaning solutions also fall into two broad categories: Aqueous (water-based) and Solvent-based. The vast majority of industrial and commercial applications today use aqueous solutions due to safety and environmental considerations.

Aqueous (Water-Based) Solutions

These solutions use water as the primary solvent, combined with specialized additives (surfactants, builders, chelators, inhibitors). They are generally the safest and most environmentally friendly choice.

Alkaline Cleaners

These are the workhorses of industrial cleaning, highly effective at removing heavy organic soils like oils, greases, waxes, and heavy cutting fluids.

• Mechanism: Relies on a combination of chemical and physical actions that break down organic soils like greases and oils into simpler, more soluble components then lift and carry them away. 
• Caveat: Check before using with light metal alloys as they can cause rapid etching. May require thorough rinsing.

A popular alkaline cleaning solution is elma tec clean A4. Diluted to 2 to 5% with water it removes grease, oils, soot, wax, combustion residues, and organic contaminants from:

• engine parts
• all metals
• glass, ceramics
• plastics and rubber

A milder alkaline formula is elma tec clean A1.  With recommended dilutions to 3 to 10% with water, use it for removing light oils, fluxing agents, dust, grease, light polishing suspensions and fingerprints from:

• PCBs
• electromechanical devices
• electronics
• fine optics 

Alkaline, ammonia-containing elma tec clean A2 diluted to 5 to 10% with water removes oil, fat, grease, sweat, oxides, grinding, polishing and lapping media from

• nonferrous metals
• precious metals
• brass, copper and bronze, where its ammonia content leaves a bright shiny surface

A Note on Rust Protection: 

Protecting ferrous products from rust when cleaning in water-based ultrasonic cleaning solutions can be accomplished by adding elma KS Rust Inhibitor to the cleaning solution or rinse tank. It covers metals with a protective coating on a molecular level which inhibits corrosion and does not impede subsequent surface treatments.

Some alkaline formulations, such as elma tec clean A4, contain silicates which prevent flash rusting.

A Note on Phosphates:

Phosphates act as a buffer to maintain alkalinity of the cleaning solution when acidic contaminants are removed.  They also soften hard water by sequestering metal ions (only when used at pH 9.5 or above).  This enables use of a less concentrated cleaning solution.  Phosphates help to maintain dirt in suspension and prevent redeposition on surfaces.  Phosphates are freely rinsed off with water. However, discharge of phosphates into the environment causes algae overgrowth. 

Neutral Cleaners

The best general-purpose cleaners, ideal for light oils, fingerprints, dust, and general maintenance. They are the safest choice for sensitive materials like aluminum, copper, brass, and delicate plastics.

• Mechanism: Rely heavily on surfactants (detergents) and emulsifiers to lift and suspend soils.
• Applications: Electronics, aerospace, optics, and general-purpose metal finishing.

An example of a neutral cleaning solution is elma lab clean N10. With a pH of 7.1 and diluted to 2% with water it is widely used for cleaning aluminum, light metal alloys, glass, plastic, ceramics, medical instruments and labware. 

Use to remove:

• emulsions
• marking and label residues
• lime soaps
• light greases and oils
• fingerprints and dust

Another example of a neutral cleaning solution formulation is foam-inhibited elma clean 260 dip & splash that can be used either in an ultrasonic bath diluted to 2%, or for spray cleaning (1%) with tap or deionized water.

This formula is suitable for metallic surfaces including aluminum and light metal alloys as well as for glassy, ceramic, plastic and mineral surfaces.  It removes:

• aqueous cooling emulsions
•  deposited lime soaps
• greases, oils
• fingerprints and dust

Acidic Cleaners

Used for specialty applications where the soil is inorganic, such as rust, scale, hard water deposits, and oxide layers.  May also be used to passivate steel surfaces.

• Mechanism: Low pH chemically dissolves metal oxides and scale.
• Caveats: Corrosive to metals and hazardous to handle. May require robust corrosion inhibitors to protect the base metal. Post-cleaning neutralization may be required.

An excellent example of a mildly acidic cleaning solution is elma tec clean S1. This is typically diluted to 1 to 5% with water before use.

Important Safety Note: Using corrosive, inorganic, low-pH solutions like nitric, sulfuric, formic, or hydrofluoric acid requires special precautions. The stainless steel ultrasonic cleaner tank must be protected. You can accomplish this by using an acid-resistant tub, which we offer in several sizes. For more detailed safety information, please read our post on safely cleaning with acid.

Enzymatic Cleaners

A specialty sub-category, often used in medical, dental, and surgical device cleaning.

• Mechanism: Enzymes (proteases, lipases) specifically break down organic bio-soils (blood, tissue, protein) at lower temperatures, preserving delicate instruments.
• Benefit: Highly effective on bio-films without the use of harsh chemicals.

An example is MedClean C7 medical and surgical instrument cleaner concentrate.  It removes blood, tissue, ointments, light oils, and other contaminants prior to sterilizing or disinfecting.

Refer to this post for a more extensive list of ultrasonic cleaning solutions and their applications.

Solvent-Based Solutions

Ultrasonic Cleaning with Volatile Solvents

In certain instances a volatile solvent such as IPA, acetone or toluene is recommended for ultrasonic cleaning operations. In such cases you must be in conformance with NEC and local regulations regarding operations in a hazardous location.   Some  industries (medical, aerospace, defense) have validated cleaning processes that mandate a specific flammable solvent for consistency and compliance.

If your operations require cleaning with volatile solvents we invite you to check out our post on selecting an explosion-proof ultrasonic cleaner.

Practical Steps for Cleaning Solution Selection

Follow this systematic approach to narrow down your selection:

Step 1: Define the Soil and Substrate

Identify the primary contaminant (e.g., “heavy mineral oil and metal fines”) and the material being cleaned (e.g., “stainless steel and aluminum parts”).

Step 2: Determine the Chemical Action Needed

Based on the soil, determine the best chemical approach:

• Oil/Grease → Alkaline (Surfactant Action/Emulsification/Saponification)
• Rust/Scale → Acidic (Dissolution/Surfactant Action)
• General/Light → Neutral (Surfactant Action/Emulsification)

Step 3: Validate Temperature and Process Requirements

Ensure the product can operate effectively at the available tank temperature and that the rinsing and disposal requirements can be met. A Safety Data Sheet (SDS) is supplied with all Elma ultrasonic cleaning solutions.

Step 4: Contact Tovatech’s Cleaning Experts for Advice and Help

We’ll provide expert and unbiased recommendations on selecting ultrasonic cleaning solutions, equipment and operating procedures.