cleaning, pretreatment & surface preparation

METAL CLEANING

BY ROBERT FARRELL AND EDMUND HORNER

HUBBARD-HALL INC., WATERBURY, CONN.; WWW.HUBBARDHALL.COM

Simply stated, the function performed in metal cleaning is removal of material,collected in the previous operations, from the metal’s surface to prepareit for subsequent operations.Cleaning metals involves not only the selection of the type(s) of cleaners,

but also the proper cleaning cycle and process equipment necessary to generateacceptable parts at a given rate (parts/hour).

An equation for such a cleaning operation may be illustrated as follows:Process Equipment + Process Cycle + Cleaner(s) = Acceptable Parts/HourEach of the terms in the equation shares the burden in providing acceptableparts at a given rate for an economical operation. The equation also notes thatin certain operations more than one cleaner may be required, as per a line forelectroplating.

Process equipment is the equipment selected—rack line, barrel line, spiralwasher, ultrasonic, etc.

SOILS

Soils are the materials left on the metal’s surface from the previous operation(s)or the surface condition of incoming metal stock. Examples of the variety ofsoils that are encountered in metal cleaning are listed below. There may beinstances where more than one soil is present on the part.Rust Scale (weld or heat)Tarnish Oxides that inhibit subsequent finishingSmuts Carbonaceous soils

Drawing compounds Machining oilsStamping oils Spinning lubricantsFingerprints Buffing compounds

Polishing compounds Metallic compoundsGlove prints Corrosion-preventive compoundsFluxes from brazing operations Phosphate coatings impregnated withforming lubesCorrosion products Stenciling inksBurnishing-compound residues Brightener residues left on the surface

from previous plating stepsGeneral shop soils that accumulate during storageThese soils may generally be divided into three categories:

Organic soils are typically the lubricants used in metal forming, rolling,and machining operations. The lubricants may be based upon petroleum orsynthetics (water-soluble) formulations. Soaps, lard oils, and wax bases arealso encountered.Inorganic soils include rust, heat and weld scale, smuts, and oxides (tarnish).Miscellaneous soils include shop dirts, glove prints from handling the parts,fluxes from brazing operations, and burned-on soils from quenching operations.As a rule soil removal is not a simple reaction, e.g., lard oil reacts with causticsoda to form a soap. The reactions can be somewhat more complex.An important concern, which adds to the cleaning problem, is the age of

the soil. Soils that are allowed to age on the metal surface for an extended timebecome increasingly difficult to remove. A classic example is aged buffingcompounds on zinc die castings. It is important, therefore, to clean parts soonafter they arrive from their last operation.

PROPRIETARY CLEANERS

To begin with the concept of one cleaner for all soils and all metals does notexist, although chlorinated solvents have come close to achieving that honor.Proprietary cleaners fall into the following groups: alkaline (mild to strong);neutral (pH 7.0); acidic (mild to strong); emulsion; and solvent. The physicalforms of cleaners on the market may be powder, liquid, or gel.In the development of a cleaner not only are the soils a consideration but alsothe base metal to be cleaned. As a rule the cleaner’s function is to remove the

soil and not have any detrimental effect on the metal’s surface. For example, aproduct containing caustic soda would be satisfactory for cleaning ferrous metalsbut not for cleaning aluminum alloys, zinc die castings, galvanized stock, oryellow brass. Such a product would attack these nonferrous surfaces. The properselection of a cleaner for the metal substrate to be cleaned is thus paramount.

The bulk of the proprietary cleaners used in industry fall into the alkalinegroup. Many solvent cleaners are being phased out because of environmentalconsiderations and other hazards. Acid cleaners generally are used for cleaningstainless steel alloys, wrought aluminum alloys, copper, and brass alloys.Proprietary alkaline soak and spray cleaners are generally formulated to clean

a variety of metal-forming lubricants from a metal’s surface, and may also findapplication for cleaning a variety of metals, i.e., ferrous metals, aluminum alloys,brass, and magnesium alloys.Within the last three years the development of a new cleaner formulation has

become further complicated by environmental restrictions imposed by federaland local regulations and by corporations themselves. For example, a specificrequirement given for a spray cleaner was that the product must be safe onmost metals within the specific allowed cleaning time, but must also be free ofphosphates, silicates, chelators, and nitrites; have a low COD/BOD; and havean operating pH between 8 and 9.If, for example, some of the restrictions encountered when formulating acleaner were applied, it would have to have the following characteristics:Chromate free NoncyanidePhosphate free NoncausticSilicate free Solvent freeFluoride free No foam productsChelator free High flash point solventsNitrite free Powders or liquidsAmine free Low BODLow COD Emulsifier freeBorate free pH of 7.0-9.0Some of the reasons for these restrictions are self-evident such as cyanide free,solvent free, chelator free, and phosphate free.From the collection of restrictions given one may readily note that the productsof the future must be not only safe to the environment, but also relativelysafe to use in the work area, and provide a cleaner that will allow the separationof the soil from the cleaning solution.

PROCESS ALTERNATIVES

The method or combination of methods selected to clean parts is critical and willdepend upon the type of final finish (plated, painted, anodized, etc.) and whetherit is an in-process cleaning operation, or just a final cleaning. And, of course, theother paramount factor to be considered is the volume of work processed per hour.The cleaning methods given may be used independently or in conjunctionwith one another, as in a preplate cleaning cycle, where three to four modes ofcleaning are used.

Immersion (Rack or Barrel)

Rack immersion may utilize air agitation or work agitation to improve or shortenthe cleaning time. In barrel lines the movement of the barrel will provide thenecessary agitation to flush cleaner solution through the parts.

Power, Spray Cleaning

Spray washers (spiral spray, belt washer, spray strip line, cabinet, and monorailtypewasher) provide reduced cleaning time by utilizing impingement to cleanparts that may not respond to conventional soak cleaning. Spray pressures mayrange from 14 to 200 psi, depending upon the type of machine used. Somecontinuous strip spray washers may also utilize rotating brushes along with thespray cleaning solution.

Ultrasonic Cleaning

Ultrasonic energy is utilized in conjunction with aqueous cleaners or hydrocarbonchlorinated solvents to clean parts. Areas of application are small precisionparts, parts with complex configurations, removal of tightly adhering orembedded particles from parts, or cleaning parts for hermetically sealed units.One of the key factors in a successful ultrasonic cleaning operation, besidesselecting the proper cleaner, is proper racking.

Alkaline ElectrocleaningAlkaline electrocleaners should not be used as the initial cleaner to remove thebulk of soils such as drawing compounds, stamping oils, buffing compounds,machining oils, heavy rust, and weld scales. This function should be reserved foran alkaline soak cleaner, emulsion cleaner, spray cleaner, acids or combinationsthereof, which precede the electrocleaner. The electrocleaner is the last alkalinecleaning process performed on the metal surface prior to electroplating. At thepoint entering the electrocleaner the only soils present should consist of smuts(carbon or iron oxides), light flash rust, light oxides, and residues of soils andcleaners left on the metal surface from the prior cleaning operations.Alkaline electrocleaning, prior to electroplating, is one of the most reliablecleaning methods available. The cleaning action not only depends upon thecleaner formulation but also utilizes the liberation of oxygen or hydrogen(depending on polarity) formed during electrolysis to scrub residues from thesurface.Of the three electrocleaning methods anodic cleaning is most frequently used,

especially for steels and brass and zinc die castings.Periodic reverse cleaning is usually confined to cleaning ferrous metals, which

have as their soils heat scales (weld scale), rust, and smuts.Cathodic cleaning must be used when electrocleaning nickel, nickel alloys,

pewter, lead and lead alloys, and stainless steels.Maintaining the proper current density, besides operating concentrations and

temperature, is a paramount factor that must be maintained in order to obtaithe desired results. Each metal is electrocleaned using a different current densityrange. The ranges for the particular metals are given in Table I.

Cathodic Pickling

Cathodic electrolytic pickling in an electroplating line is usually confinedto the pickling of ferrous metals and activation of nickel-plated surfaces.Electrolytic pickling is another form of electrocleaning, but on the acid side.The principal functions of an acid solution in an electroplating cycle areremoval of rust, scale, tarnish, light oxides, metal slivers, and in some cases,smut. Electrolytic pickling is usually restricted to difficult tasks where thesoils are quite heavy or where the allotted pickling time is short. The picklingaction, in an electrolytic process, is assisted by the evolution of hydrogenor oxygen on the work. These gases aid in prying off scale during pickling.Electrolytic pickling’s advantages over chemical pickling can be stated as follows

1. Pickling time is reduced.

2. The rate of pickling is affected to a lesser extent by changes in the acidconcentration and by the iron salt accumulation in the solution.

3. Ferrous metals, including alloy steels, can be pickled much more readilythan by a conventional acid pickle.The acids used as an electrolytic pickle could be sulfuric acid, with or withoutfluoride additions, or proprietary acids (dry acid salts or liquid).In cathodic pickling the work is made the cathode, and during picklinghydrogen is evolved on the work surface. Cathodic pickling would be selectedwhen any of the following conditions are present:

1. Dimensionally accurate fabrication or machine parts.

2. Highly finished steel (#3 finish).

3. Fabrications having deep recesses.

4. Soils consisting of light oxides or smut.

5. Activating metals.In any pickling operation there is always the problem of hydrogen embrittlement,and in cathodic pickling this danger is increased because of the evolutionof hydrogen on the work surface. Usually the pickling time is rather short—30sec to 1 min—thus the embrittlement factor is minimized.

Anodic Pickling

In anodic pickling oxygen is evolved on the metal surface. The oxygen formedon the work surface merely performs a scrubbing action in that it aids inloosening and removing the scale, rust, and smut. Consequently, all of thepickling action is accomplished by the acid solution. The advantage of anodicpickling over cathodic is that removal of heavy layers of scale and rust may beaccomplished.Anodic pickling does a better job in removing scales, rust, and embedded soilsby attacking the base metal. In this type of pickling one must expect some metalloss and, in some cases, pitting of the surface. This loss of metal may be reducedor stopped with the use of 70% by volume sulfuric acid. The problem associatedwith using 70% by volume sulfuric acid is the danger of smut formation. Thiswould be prevalent in high carbon steels.

MECHANISMS OF CLEANING

The removal of objectionable contaminants from metallic surfaces can beaccomplished by mechanical processes, chemical processes, or a combinationof both.

Mechanical Processes

1. The physical removal of surface layers by means of aggressive mechanicalaction.Shot blasting with glass, aluminum oxide, sand, or dry ice (CO2) pellets.Mass finishing via vibratory mill or part-on-part burnishing.Grinding.Abrasive pad buffing.

2. The physical removal of surface contaminants with minimal base metalremoval.Gas scrubbing by electrocleaning, cathodically, anodically, or periodicreverse.

Impingement by high- or low-pressure spray.Turbulence, such as that supplied by slosh washers or turbo washers, in whichcleaning solution is turbulently flowed over the surfaces to be cleaned.Cavitation supplied by ultrasonics.Abrasive brushing in solution.Abrasive slurry.Chemical ProcessesSolubilization is the cleaning method in which surface contaminants becomesoluble in the cleaning solution. Examples are the dissolution of iron oxide in acidsor acrylic coatings in alkaline aqueous solutions.Emulsification is the process by which a normally insoluble soil becomesuniformly dispersed in an incompatible solvent. The most common emulsionencountered by people is milk, in which insoluble fats and proteins are dispersedin water. Emulsification is accomplished by a combination of proper surfactants,cosolvents, and coupling agents.

Saponification is the reaction of oils containing reactive fatty acids with alkalito yield soluble soaps. An example of this mechanism would be the cleaning of alard oil lubricant from stamped steel by the use of an aqueous cleaning solutioncontaining significant amounts of sodium or potassium hydroxide.Wetting is the method by which a soil is displaced from the substrate surfaceby the use of wetting agents that have a greater affinity for the substrate surfacethan for the soils present. The wetting agent can work by having the same actionon the soil particles present. In both cases the attraction between soil and substratesurface is eliminated and soils are removed.Complexing is accomplished by the use of chelators, sequesterants, or complexors.The soils—typically metal soaps or lubricants—form soluble complexeswith those agents and are thereby removed from the substrate. Complexors alsofunction to prevent redeposition of insoluble or suspended soils on the substrate

surface, particularly in hard water locations. Examples of complexors are organicacids and their salts, polyphosphates, and zeolites.

Defloculation is the process by which soils are broken down into finelydivided particles that become suspended in the cleaning solution. Defloculationis normally accomplished in combination with mechanical action.

Other Processes

The surface cleaning of metallic substrates can be accomplished by use of heatand high vacuum to volatilize soils and oxides.

TESTS FOR CLEANLINESS

The degree of cleaning required for the surface of a part is a function of, and dictatedby, operations to follow cleaning. The cleanliness of a part can be describedas a function of the removal of a specific surface contaminant such as oil andgrease, oxides, or particulate matter.

Test methods used to determine the cleanliness of a surface range from crudeto highly sophisticated. A summary of several tests follows.

Water Break Test

The water break test involves examination of a surface for the presence of a continuouswater film that has “no water breaks.” If a water-break-free film of wateris present it is indicative of the absence of hydrophobic surface contaminants.Oils, greases, and water-insoluble organiccompounds would be examples ofhydrophobic contaminants. The water break test does not confirm the presenceor absence of hydrophilic particulate contaminants or oxides.

White Glove Test

The white glove test is used to show the presence of particulate and, to a certainextent, organic contaminants on a surface after cleaning. The part maybe tested while still wet from rinsing or after drying. The surface of the partto be tested is wiped with a white glove, cotton swab, or lens tissue. The materialused to wipe the surface is then examined for the presence of black, gray,or off-white residue or oil staining. If contaminants are found to be present,microscopic examination or advanced chemical or surface analysis can be performedon the part surface or the item used to wipe the surface to determinethe nature of the contaminants.

Other Methods

Sophisticated physical and chemical analytical methods can be applied to testfor residual contaminants on surfaces that have been cleaned. Samples of partsthat have been cleaned and dried can be immersed in a turbulent solutionof a solvent. The solvent can then be analyzed for organic contaminants andinsoluble particulate matter. The amount of contaminant found in the solventis indicative of the degree of ceaning.

Surfaces of parts that have been cleaned can be subjected to special analysisto determine the presence of oxides, organics, and particulate contaminants.Specifications can be written for the allowable presence and concentrations ofcontaminants in critical cleaning operations.

Analytical techniques such as infrared microprofiling (developed by SandiaNational Laboratories), X-ray photoelectron spectroscopy (Oak Ridge NationalLaboratory), and light reflective technology (Dow Chemical Inc.) have been usedto analyze for residual contaminants in critical cleaning operations.If soils are doped with compounds that exhibit fluorescence exposure of cleanedparts to ultraviolet light will confirm the presence or absence of residual soils.Tests based on surface tension have been used to determine the cleanlinessof surfaces. Care must be taken to ensure the use of test solutions specific tothe substrate surface.

SAFETY

Unfortunately, one of the most overlooked aspects of industrial cleaning is safety.The use of chemicals for industrial cleaning exposes the user to potentialinjury if proper safeguards are not employed. The potential problems are welldocumented in Material Safety Data Sheets, books, and articles that have beenwritten over the years. The warnings are of little value unless they are read,understood, and acted upon by those handling, using, or working in areas inwhich the chemicals are used.A summary of the safety aspects of chemical cleaning are as follows.

Acids

The use of acidic cleaners containing appreciable amounts of sulfuric acidcan expose the worker to potential splashing due to exothermic reactions thatcan result in localized boiling. Additions should be made in a slow, controlledmanner to prevent splashing and localized boiling. Acids should be added towater. Water should not be added to concentrated acids.Addition of acid cleaner concentrates to replenish working solutions shouldalways be made to cool solutions (<100oF). Without exception, acid-resistant goggles, face shield, boots, apron, and gloves should be worn by workers tominimize the potential for body contact with the acidic solutions.Acid cleaners containing fluoride compounds can result in severe tissuedamage. Precautions should be taken to avoid any contact with fluoridecontainingsolutions. If fluoride compounds are used, personnel should betrained in the treatment of fluoride burns.The use of acid cleaning compounds can result in the evolution of flammableand explosive hydrogen gas. Care should be taken to avoid uncontrolledrelease of pressure when parts are cleaned in sealed cleaning chambers. Sourcesof spark or flame that can ignite accumulated hydrogen should be identifiedand removed from the area in which acid cleaning is conducted.The use of acidic cleaners for cleaning sulfurized steel or parts that havesulfurized oil present can result in the generation of poisonous and flammablehydrogen sulfide gas. The same precautions employed for dealing withhydrogen evolution should also be employed for the potential evolution ofhydrogen sulfide.Acid cleaner tanks and equipment should be properly vented to keep worker

exposure to mists and vapors below OSHA limits. The reactivity of substratematerial should be evaluated prior to acid cleaning to prevent violent reactiondue to incompatibility and/or part damage.

Alkaline Cleaners

The use of alkaline cleaners containing strong alkaline compounds, such assodium hydroxide and potassium hydroxide, can result in strong exothermicreactions when working solutions are made or replenished. Additions shouldbe made to water or working solutions in a cautious, controlled fashion toprevent splashing and localized boiling. Additions should only be made to coolsolutions (<100oF). Water should not be added to alkaline powders.The use of alkaline cleaners for etching or cleaning reactive metals, suchas aluminum, zinc, or magnesium, will result in the evolution of flammablehydrogen gas that can accumulate in foam blankets or in enclosed cleaningequipment. Equipment should be ventilated and ignition sources should beremoved from areas in which reactive metals are cleaned.The use of alkaline electrocleaners will result in the evolution of hydrogenand oxygen. Tanks should be well vented to remove these gases. Foam blanketson electrocleaners should be sufficient to prevent misting without excessiveaccumulation of hydrogen and oxygen. Disconnect current prior to removingwork from electrocleaning tanks to prevent hydrogen explosions. Personnelusing, handling, or working in areas where exposure to alkaline cleaners is

possible must wear alkaline-resistant personal protection consisting of safetygoggles, face shield, gloves, apron, and boots. Respiratory protection shouldbe worn when dust or mist is a problem.

Neutral Cleaners

Although the destructive effect of neutral cleaners on body tissues may be minimal,in many cases these cleaners are used hot and the possibility of thermalburns from splashing may exist. Additions to working solutions should be madeslowly and cautiously in a controlled manner to prevent splashing. Additionsshould only be made to cool solutions (<100oF)

Equipment

A preventive maintenance schedule should be implemented for the inspectionand repair of defective cleaning equipment. Hoist systems, exhaust systems,heating systems, and tank systems should be inspected on a routine basis forproper function and equipment integrity. Worn, corroded, or damaged equipmentshould be repaired or replaced upon discovery.

General

1. Never work alone when working with chemicals.

2. Read and understand Material Safety Data Sheets, technical bulletins, anddrum labels for the materials you handle and work with.

3. Always wear the personal protective equipment specified in the MaterialSafety Data Sheets, technical bulletins, or drum labels.

4. If you are injured notify the appropriate personnel and get medical attentionas soon as possible.

5. If a spill occurs notify the individuals in your facility who are properly trained to respond to chemical spills