cleaning & pretreatment aqueous ultrasonic cleaning...

CLEANING & PRETREATMENT

Aqueous Ultrasonic Cleaning and CorrosionProtection of Steel ComponentsBy Sami B. Awad, Ph.D.Crest Ultrasonics,1 Scotch Rd., P.O. Box 7266, Trenton, NJ 08628; www.crest-ultrasonics.com

Contaminants are soils or impurities either gener-ated during the forming process of new surfaces ordeposited foreign matter from surrounding environ-ments. Contaminants adhered to the surface underhigh mechanical pressure, or byproducts of chemicaladditives or chemical protective films are commonin metal forming processes and are difficult toremove.

The degree of required cleanliness can range frompractical cleaning needed in in-process operations toprecision or critical cleaning required prior to coat-ing or final assembly.

Critical or precision cleaning is defined as thecomplete removal of undesirable contaminants to apredetermined high standard and without introduc-ing new contaminants in the process.

CONTAMINANTSContaminants may be categorized as follows:

Organic ContaminationsExamples include: lubricating oils, cutting, machin-ing fluids and oils, fingerprints, carbon, organic vehi-cles in buffing and lapping compounds, waxes, siliconeoils, mold release compounds, coolants, polymers,adhesives, photo resist compound, lacquers, paints,inks, antifoam additives, and residual biocides.

Inorganic ContaminantsExamples include: various metal oxides in buffingand lapping compounds, polishing compounds, inor-ganic salts, dust, metal fines, slivers, and othermetal oxides.

Surface preparation of steels and other metalsand alloys is essential prior to most finishingprocesses, particularly coating and vacuum

coating. Otherwise, yields will suffer. Aqueous andsolvent ultrasonic processes have been developedwith the specific objectives of achieving the highestquality surfaces without inflicting any damage tocomponents.

For steels, the major concern when componentsare to be cleaned aqueously is flash rusting, whichoccurs when clean active steel surfaces are exposedto water and oxygen (Fig. 1). Ironically, some halo-genated (nonaqueous) solvents used to clean ferrouscomponents have periodically, for other reasons,manifested flash rusting problems.

Aqueous ultrasonic cleaning offers excellent clean-ing results. The method is preferred over solvent-based methods for well-known environmental rea-sons. The challenge always has been on how to cleansteels aqueously without having flash rusting orworst pitting occur.

FLASH RUSTINGFlash rusting is the first phase in a corrosionprocess that can be visually observed on iron or ironalloy surfaces. Corrosion can be broadly defined asmaterials deterioration that is caused by chemicalor electrochemical attack. For ferrous surfaces,water and oxygen are sufficient to initiate the elec-trochemical attack and to generate the surfaceoxide.

Corrosion processes are electron transfers, oxida-tion-reduction processes that can occur when thesurface of a metal is in contact with a humid atmos-phere. The oxidation-reduction reactions involved inthe flash rusting process include generation of fer-rous oxides, which is further oxidized by oxygen intoferric oxide with the familiar color, ranging fromlight yellow to deep brown (Fig. 1).

Examples of rustable steels include mild steels,high carbon steels, silicone core steels, iron compos-ites, cast iron, and some ferrite stainless steels.

SURFACE CLEANINGCleaning by general definition is freeing the surfacefrom contaminants that are adhered chemically,physical or mechanically to that surface.

September 2004 Metal Finishing

Figure 1. Flash rust of ferrous metals in presence of oxygen.

ParticlesParticles are insoluble individual or aggregates ofmicro solid contaminants, which tenaciously adhereto the surface with various physical forces.Obviously, smaller size particles are the most diffi-cult to remove. Particles down to 10 microns can beseen under high intensity light, those down to 2microns require dark field illumination. Particlesdown to 0.1 micron can be detected by laser opticalscattering technique (Profilometer) or by AFM(atomic force microscopy). Particle diameters below0.1 micron (100 nm) need scanning electronmicroscopy (SEM) for detection and recording.

AQUEOUS ULTRASONIC PROCESSAqueous cleaning is a widely acceptable alternativeto the use of halogenated solvents. Decontaminationof surfaces with water can be done universally,except in rare cases where the substrate itself iswater sensitive or reactive or very difficult to dry.

A typical ultrasonic aqueous batch cleaningprocess essentially consists of four steps: ultrasonicwash, ultrasonic overflowing rinse, second ultrason-ic overflowing rinse, and a drying station. The sec-ond rinse overflows to the first one and is known asreverse cascade rinsing. Spray rinsing may also beutilized to assist the removal of the cleaning chem-istry. The actual number of stations, tank sizes, andprocess parameters are determined and verified bytesting upon examining the parts, contaminants,required throughputs, cleanliness, and dryingrequirements. Additional mechanical assistingdevices such as rotating baskets, oscillation, roboticsarms, or transport mechanisms may be used. Theultrasonic cleaning system may also include exter-nal water heaters, closed loop water system forwater preservation, and auxiliary process monitor-ing devices such as pH and resistivity meters.

CLEANING CHEMICALS AND ULTRASONICCAVITATIONSCleaning chemicals are essential in removing, dis-persing, or emulsifying the contaminants and thenpreventing contaminants from redeposition on thesurfaces. Cleaning chemicals work in synergy withultrasonically generated cavitations to provide therequired levels of cleanliness. The cavitations pro-vide the necessary scrubbing forces through contin-uous surface impact with the generated wave shocksand acoustic streaming. Wave micro-shock pressurecan reach 5,000 psi and fluid micro-streams up to250 mph.

The removal of contaminants may appear simple,however, it is a very complex process. Cleaning

depends mainly on two concurrent steps: displace-ment and scrubbing. The displacement can takeplace through different mechanisms including wet-ting to lower the interfacial tension and the surfacefree energy followed by encapsulation, emulsifica-tion, dispersion, or solubilization. Other mecha-nisms involve changing the soil nature throughbreaking its bonds first with the surface followed bythe same other steps.

Cleaning chemistry can be divided into two maincategories: solvent-based and water-based. The sol-vent-based, known as semi-aqueous chemistries,includes pure organic nonhalogenated solvents e.g.alcohols, hexanes, heptanes, N-methyl pyrolidone,acetone, methyl ethyl ketone, esters, etc. The semiaqueous chemistries may include formulated prod-ucts with bases chosen from medium-to-high flashpoint petroleum hydrocarbons, natural terpenes,hydrocarbons, and natural esters e.g. soy esters,cyclic alcohols, or cyclic amides. Water-based clean-ing chemistry is essentially based on anionic, cation-ic, and nonionic surfactants and various tailoredadditives. Surfactants have unique propertiesbecause of their chemical structures.

In aqueous cleaning the surfactant’s first functionis to interact with and wet the soiled surfaces. Thisis followed by one or more mechanisms, which caninclude displacement, dispersal, dissolution, seques-tering, assisted hydrolysis, or emulsification of vari-ous soils. Dispersal or suspension of soils takes placeby encapsulating suspended contaminants to pre-vent their redeposition. The chemistry can be tai-lored to fit the requirement of a certain soil. Themolecular structures of surfactants and additiveshave a significant impact on their properties andtheir behavior in the cleaner. Therefore, not allcleaning chemistries are equal.

The second step is rinsing with water. The waterrinse steps are essential to provide surfaces freefrom contaminants and from cleaning chemicalresidues. Ultrasonic cavitations greatly assist inspeeding up and completing the removal of residualsurfactants. Without ultrasonics it may take longeror be incomplete. Lack of good rinsing of a detergentfilm or using poor quality water to rinse alwaysresults in residual detergents or salts left on sur-faces. These in turn become new contaminants.

Cleaning process parameters and parts handlingmust be thought of as one integral process.Therefore, compatible chemistries, operating tem-perature, quality of rinsewater, effective removal ofsuspended contaminants through filtration, and theproper drying technique are all indispensable for asuccessful operation.



ULTRASONIC RINSING AND DRYING WITH-OUT FLASH RUSTINGRinsing of steel components with water requiresthat the water must be inhibited to stop any elec-trochemical corrosion reaction. Multiple propertiesmust be exhibited in a good inhibitor. The inhibitormust be water soluble and active enough to protectthe surfaces during the water rinse step(s) and alsothrough the hot air drying step. Also, the inhibitorfilm residue must not interfere with any subsequenttreatment such as vacuum coating. The inhibitoralso must not affect precision or gauging measure-ments of clean components. A good inhibitor will notstain the surface and will be easy to remove. Most ofthese inhibitors are proprietary formulations. Inprinciple they work by depleting the available oxy-gen or by forming a very thin organometallic protec-tive film on the steel surface.

There are two effective approaches to rinse and airdry steel components, which are prone to rusting.The first is two ultrasonic reverse cascade rinsingwhere the inhibitor is injected into the second rinsestation at a low rate and is overflown into the firstrinse. The next step is air-drying of the components ina circulated filtered heated air dryer. HEPA filtrationand nitrogen or helium injection can be utilized to drycritically cleaned components in a clean-room envi-ronment. The second process is good for the practicalcleaning level (Fig. 2). Protection from flash rustingcan be achieved through rinsing the components in asimilar system using two individual rinse tanks. Tapwater is fed into the first overflowing ultrasonic rinseand the inhibitor is added periodically to the secondstationary ultrasonic rinse.

ADVANTAGES AND CHARACTERISTICS OFULTRASONICSUniformity and consistency of the cleaning resultsare two main characteristics of cleaning with ultra-sonics. An optimized ultrasonic process is character-ized by selecting the appropriate frequency and theappropriate power amplitude suitable for the type ofparts to be cleaned. One fact to remember is that notall industrial high-power ultrasonics are equal. Theprocess of generating the ultrasonic waves involvesspecially designed PZT transducers and high-fre-quency power generators. For example, selecting theright materials and devising the design and dimen-sions for the transducer assemblies involves scienceand art as well. The main objective is to maximizefrequency matching and lower the impedance tomaximize energy transfer into the liquids. This wasaccomplished at Crest Ultrasonics by a new ceram-ic transducer design (Fig. 3). Recent developments

include using two or more frequencies simultane-ously in the same tank. Another important recentdevelopment is the development of wide range ofrates for the sweeping frequency. Currently, sweepfrequency is a standard feature in all generators.Without sweep frequency (+/- 2 kHz) the fluid in atank will be split into active and inactive lateralzones. The new feature allows contamination controlengineers to control the process and specificallydetermine what is the best range for their sensitivecomponents.

Countless micron-sized vapor cavities are generat-ed by high-power ultrasonic sound waves through-out the liquid, even in recesses, holes (blind or not),and internal cavities in the workpieces. These cavi-ties are very short lived and they violently implodein the cleaning media (Fig. 4) giving rise to highlocalized temperatures (of around 20,000°F) plusthe wave pressure impact (about 5,000 psi) and anacoustic streaming (250 mph) effect that causes thescrubbing action (Fig.5).

The mechanism for producing the cavities is as fol-lows: the alternating rarefaction and compression ofthe sound wave, generated during the half-cycles,stretches the liquid molecules beyond their naturalbonding forces to form the micro cavities ( cavitationthreshold) that grow to macro cavities, proportion-ate to the ultrasonic energy applied and its frequen-


Figure 2. Ultrasonic cleaning system for rustable steels.

Figure 3. Ultrasonic transducer design.


DRYER


cy. The instantaneous vaporization that follows tofill the cavities has a relatively shorter life com-pared with the very rapid violent implosions, whichcharacterize the ultrasonic activity in liquids.

Maximum cavitation intensities can be attained attemperatures of 130 to 160°F. Lowering the surfacetension of water results in faster degassing and bet-ter distribution of cavitation. The size of cavities andtheir number are a function of the ultrasonic fre-quency. Larger size cavities in a relatively smallernumbers, with intense implosion impacts, are gen-erated at frequencies in the range of 20 to 30 kHz.Moderate-size cavities, in greater numbers andeffective if more moderate implosion impact, occurat frequencies of 40 kHz. Frequencies greater than100 kHz are utilized to generate finer implosionswith fine to moderate impact. The cavitations at 132to 192 kHz are numerous and effectively remove submicron particulates. At these frequencies the cavita-tions have a relatively milder impact, which makesthem ideal for use in rinse stations where it is desir-able for detergent films to be entirely diffused andremoved into the water, without inflecting erosiondamage to sensitive components. Most current com-

mercial ultrasonic cleaning equipment operates inthe range from 20 to 200 kHz.

Other cleaning process parameters are also equal-ly important such as compatible chemistries, oper-ating temperature, quality of rinsewater, effectiveremoval of suspended contaminants through filtra-tion, and the proper drying technique.

CONCLUSIONSelecting the proper cleaning ultrasonic equipmentis equally important as selecting the appropriatecleaning chemistries and process parameters toachieve two goals when dealing with steel compo-nents. The first is to accomplish the desired cleanli-ness level and the second is to protect the steel com-ponents from any potential for flash rusting orcorrosion. Both goals can be successfully achievedwith properly designed ultrasonic aqueous process.

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Figure 4. Ultrasonic frequency and cavitation size and population.

Figure 5. Scrubbing action.


ABOUT THE AUTHORSami B. Awad, Ph.D., is Vice President of Technology at Crest Ultrasonics Corp. with 23 years of industrial expe-rience in developing new chemistries and processes for general and ultrasonic precision cleaning, metal surfacespreparation, and treatment. Awad has authored more than 15 US and international patents and more than 25academic research papers in chemical reaction mechanisms, and has published numerous technical articles.Awad received his Ph.D. in Organic Chemistry and served as a Professor of Chemistry on the faculties of DrexelUniversity, Philadelphia, and Cairo University, Egypt. He has served as R&D Scientist with Atochem and as aPrinciple Scientist with Henkel Corp. Surface Technology Group. Awad is a member of the American ChemicalSociety (ACS), Ultrasonic Industry Association (UIA), Society of Manufacturing Engineers (SME), InternationalDisk Drive Equipment and Materials Association (IDEMA), and ASM International.

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