cleaning, pretreatment & surface preparation

CLEANING AND SURFACE PREPARATION

BY BRAD GRUSS

PRETREATMENT & PROCESS INC., ASHBY, MINN.

The quality of coatings, regardless of the type, or process of application, varygreatly in terms of quality offered. One statement about pretreatment isbecoming more true every day: “You can make a poor coating perform withexcellent pretreatment, but you can’t make an excellent coating perform withpoor pretreatment!” The point is that today, with the emphasis on qualitycoupled with new technology in coatings (powder, electrocoat, water-based,and high solids), the shift of burden of performance is pointed directly atpretreatment. Having a solid process, which meets or exceeds expectations,must be all encompassing to address soils, metals, water quality, and the process,control, and maintenance of the pretreatment system. Here we provide abroad stroke on the basics, and hope to promote further investigation by our

readers depending on the specifics of each product and their needs. The bestway to begin pretreatment is with a series of questions designed to promoteboth specifics and generalities that have impact on the process.

FINISHER FACT-FINDING QUESTIONNAIRE

1. What base metals are pretreated?

2. What soils are on incoming metals?

3. What soils are applied to metals in-house?

4. What is the production flow of the products?

5. What production assemblies are premanufactured and stored? Dothey corrode in storage? Do the soils age or become more difficult to

remove?

6. What are the physical size limitations of your products? Can they beclassified as to percentage of small or light, medium or large, and heavy

or bulky?

7. How many of what part must be finished per shift?

8. During welding and fabricating are soils entrapped or sandwichedbetween metals?

9. Do you preclean prior to welding? If not, how much carbonaceousresidue is left on or near weldments?

10. Do you physically abrade via wire brush, grind, steel shot, or sandblastthose corroded, carbonaceous areas?

11. If you have weld spatter, does it interfere with finishing?

12. What paint specifications are in-house?

13. What paint specifications do your customers have?

14. Do you currently meet your expectations on production parts?

15. Do your currently purchased coatings meet your specifications onproduction parts?

16. Do your currently purchased coatings meet your specifications on testpanels?

17. What is your current pretreatment process? Does it provide a qualitybase for adhesion and salt spray?

18. What are your current process controls for pretreatment and finishing?Do they get done? Are they logged, recorded, and reviewed?

19. What preventative maintenance steps are taken? Is it by poundage, hours,weeks, or need?

20. What space limitations do you have for expanding pretreatment andfinishing?

21. What are the local, state, and federal laws and regulations for effluentemissions? Do you currently meet these?

22. What safety program do you have? What products can be replaced?What energy sources do you have? What are the limitations?

23. What manpower resources are available?

24. What type of training do you or your vendors offer?

25. What are your financial resources or limitations?

26. What is your competition doing in the marketplace and where do youfit in the market niche? Where do you want to be? What do you have toaccomplish to be there from a finishing perspective?

THE MECHANISMS OF AQUEOUS CLEANING

Wetting: Cleaners contain surface-active agents (surfactants) that “wet out”the soil. This loosens the soil-surface bond by a reduction of surface tension.Wetting is actually the first requirement for soil removal.Emulsification: This occurs following wetting. Simply stated, emulsification isthe dispersion of two mutually immiscible liquids (i.e., oil and water). Primaryfactors affecting emulsification are type of oil encountered and choice of surfactantsused in th cleaner. Secondary factors include pH, temperature, andconcentration of the cleaning solution.Neutralization (saponification): A reaction where in fatty acid soils (oils) are neutralizedin the presence of alkali. The result is generation of water-soluble soapsthat assist in cleaning and rinsing. Examples of fatty oils are vegetable (corn),animal fats (lard), and marine (whole).Solubilization: “Like dissolves like.” This simply means that the solubility of waterinsolublesoils (oil) is increased in the presence of surfactants.Displacement: Soil is displaced from the surface as a result of select surfactantactivity. This is particularly desirable in spray applications where the soil can beremoved using oil-skimming techniques.Mechanical Action: This can greatly increase the speed and efficiency of soil removalin aqueous cleaning systems. It can be accomplished by solution movementor movement of the part itself. Examples include air, impeller, ultrasonic, spray,and gas scrubbing (electrolytic).Sequestration: Water must be properly conditioned or softened in order for effectivecleaning and rinsing to occur. Hard water consists of divalent calcium, magnesium,and iron ions that must be complexed to avoid generation of insolublesthat would otherwise interfere with cleaning and rinsing. In effect, cleaners withadequate sequestering ability obtain better surfactant performance.Deflocculation: A cleaning mechanism whereby soil is peptized or broken downinto very fine particulates and maintained in a dispersive phase to preventagglomeration (coming together).

DETERMINING A CLEAN SURFACE

A clean surface is one that is free of oil and other unwanted contaminants. Thedegree of cleanliness required is dependent on the operation or process towhich the part or product must pass. Manufacturers utilizing the cell cleaningconcept or workstation cleaning are typically cleaning between process steps.Situations like these usually do not require the degree of cleanliness needed forfinal prepaint preparation.

A water break-free surface tells you that you have removed all organic soils. Theparts exiting the last pretreatment or rinse stage prior to drying will show auniform sheeting of the rinse water indicating an organically clean surface.The water break-free surface has been the long-standing test for cleanliness.The key to this test is using fresh uncontaminated rinsewater. Detergent additivesor rinse aids used in a final rinse may hide poor cleaning. Additionallycontaminated rinses due to poor overflow may also mask poor cleaning dueto the surfactant’s wetting ability.A water break surface tells you that you have not sufficiently cleaned and thatorganic soils are still present. The part will exhibit a surface that resembles afreshly waxed car surface after a good rain. There will not be uniform sheetingof the water but beading. Normally, poor cleaning is most often found on ornear weldments, or in areas that receive poor spray impingement to the part.Another test of a clean surface is the white towel test. Wiping a white towel across cleanand dry surfaces will indicate the effectiveness of inorganic soil removal. Check flatsurfaces and those areas most likely not to receive direct spray impingement.In the tape pull test, apply scotch tape to a clean and dry surface, then remove the tapeand place on a white piece of paper. This will also indicate the effectiveness of inorganicsoil removal as the contrast allows for easy identification of remaining soils.The ultraviolet (UV) detection requires soiling with a fluorescent oil, cleaning, and

inspecting under ultraviolet light. The degree of cleanliness can be quantified bya numbering system. This is accomplished through photoelectron emission orreflectance. The higher the reflectance, the cleaner the surface is.

THE IMPORTANCE OF CLEANING

Cleaning of metals and other finishing-related substrates is the single mostimportant consideration to successful coating application. Achieving cleansurfaces has applications throughout a manufacturing facility: for corrosionprotection, for welding operations, for part handling, for part inspection, andfor metal finishing. All of these cleaning applications can and should have aquantitative degree of cleanliness required. The degree can vary from grosssoil removal to a high degree of cleanliness, which surpasses the standardwater-break-free test of cleanliness. The ultimate requirement is dictated bythe requirements of the part, the process, the chemical type, and control ofprocess parameters. With today’s new coatings, a greater emphasis is placedon achieving a totally clean surface.

FACTORS THAT AFFECT AQUEOUS CLEANING

The success of a cleaner relies on more than just the functional chemistry thatcomprises it. Effective cleaner-to-surface contact must be made. A numberof factors must be considered, understood, and properly implemented andmaintained for effective results. Failure to utilize a workable combinationof these factors will often produce marginal results and render the cleaningsystem less effective.

There are several factors that directly impact aqueous cleaning. Because of theirsignificance, each should be addressed: (1) application methods and equipment,(2) history and configuration of part, (3) soil, (4) type(s) of substrate(s), and (5)cleaner selection and operation parameters.

APPLICATION METHODS AND EQUIPMENT

Several questions must be answered in conjunction with the equipment andthe application of the cleaner. The method of and amount of agitation mustbe determined. Chemicals must be selected in either the high-, medium-, controlled-,or low-foam category. More severe agitation or pressure at the nozzle,for example, would place your chemical choice in the least foaming category toprevent excessive foaming.

The temperature range of the process equipment should be known. Cleanerstend to be formulated with surfactants and detergents that offer optimalcleaning within a given temperature range. Typically, low temperature rangesfrom 90¡F to 120¡F, medium temperature ranges from 120¡F to 140¡F, andhigh temperature ranges from 140¡F to 160¡F. The trade-off becomes this. Ifyou are using a cleaner designed for high temperatures, but the equipmentcan only maintain process heat at 120¡F, the chances for poor cleaning andfoaming are present. On the other hand if your chemistry is designed for lowtemperature and the process heat cannot be lowered to that range, you mayexperience stratification of the solution, and in severe cases oiling out of thecleaner’s detergent package.The length of time that the solution is in contact with the part must alsobe decided. Pretesting the parts with the cleaner for the allotted time is alwaysadvised. The process equipment, based on the length of each stage and thespeed of travel, will yield a total contact time. Typically these times for cleanersrange from 60 to 120 seconds; however, many coil lines operate in a range from3–15 seconds. The chemical choice for cleaning should be made only when theprocess contact time is known.History and configuration of the part play a key factor in not only cleanerchoice, but also the application. Multiconfigured parts, for example, may bebest suited for immersion cleaning rather than spray. Usually, machined castings,or parts with ports, threads, extensions, blind holes, etc., are very difficultto clean because the part positioning is typically fixed. In these cases, immersion,or immersion spray combinations, or rotating fixtures may be required.In addition to the configuration of the part, what is the history of the part?Is it a component, finished product, or subassembly? Will it be cleaned once,twice, or more before leaving the factory as a finished product?

Finally, how long may the part be staged, or stored? Will the surface corrodeor tarnish, and will the in-process soil or rust inhibitor adequately protectwithout becoming more difficult to remove if the part is not in a just-in-time,or on a first-in, first-out inventory schedule?

SOIL AND SUBSTRATE AUDIT

Soils

There are many different types of soils used in a manufacturing facility. It isoften assumed that all soils will be easily cleaned.The cleaning operation wouldbe less difficult if all the individual soils were understood more completely.Soils are generally shop dirt, smut, oils, metal chips, and drawing, stamping,and buffing compounds.Upon completion of a soil audit, and the determinationof a suitable cleaner, every effort should be made not to introduce newsoils without pretesting.Soils can be classified as organic or inorganic. Organic soils are oily, waxy films

such as mill oils, rust inhibitors, coolants, lubricants, and drawing compounds.Alkaline cleaners should be used to clean organics. Inorganic soils include rust,smut, heat scale, and inorganic particulate, abrasives, flux, and shop dust. These

inorganic soils are most easily removed by acidic cleaners.Soils can also be classified by the degree of difficulty present in cleaning. Soils

that are very difficult soils to remove include chlorinated lubricants, sulfurizedlubricants, heavy-duty rust-inhibiting compounds, honey oils, buffing compounds,stearates, diecast release agents, and oxidized soils. Those that presenta moderate degree of difficulty include fatty oils, waxy oils, heavy-duty hydraulicoils, mill oils, lapping compounds, and water-displacing rust inhibitors. Lastly,those soils that are relatively easy to clean are soluble oil-cutting fluids, syntheticcutting fluids, spindle oil, lightweight machine oils, mill oils, water-soluble andrust inhibitors, and vanishing oils.The very difficult soils tend to be heat sensitive. Soils falling into the napthenic,paraffinic, chlorinated paraffin blends, or those containing waxes aregenerally heat sensitive to some degree. When you encounter this type of soil,

it limits the variable of temperature. A heat sensitive soil of say 160¡F requires

you to adjust upward accordingly.Specially formulated low-temperature cleaners rely on both soil displacementand slight emulsification. The blend of detergent systems built into thelow-temperature cleaners is designed to reduce surface tension at the soil–metal

interface. This unique factor enables removal of soils sensitive to heat at a lowtemperature; lower than the melting point of the waxes of that soil. This fact alsoproduces less contamination if properly skimmed, resulting in longer tank life.

SUBSTRATES

The composition or chemistry of the base metal is one of the key limiting factorsin cleaner choice. The cleaner must be chosen so as to be compatible withthe metal being processed. In multimetal cleaning lines, nonferrous metalsare typically the limiting factor. With these metals it is important to choose acleaner that either does not attack or overetch the metal and where the attackis controllable or desirable.

A common mistake by both chemical vendors and manufacturers is when abase metal audit is made for cleaner selection, but not done completely. Mostaluminum and zinc alloys with slightly different alloy content can vary widely intheir ability to withstand either alkaline or acidic cleaner attack. In some cases,where minute etch is desirable, slightly more or less is unacceptable.Substrates should be classified to make cleaner choice easier.

1. Ferrous or Iron Bearing: Cold-rolled steel, hot-rolled steel, stainless steel,and ferrous castings.

2. Nonferrous: Aluminum, sheet, coil, castings, extrusions, zinc castings,galvanized, terne plate, and zinc plated.

3. Yellow Metals: Copper and brass.

4. Mixed Metals: Combinations of the above.

5. Composites: Mixtures of metals with other materials.

CLEANER SELECTION AND OPERATING PARAMETERS

Cleaner selection is codependent on the other aqueous cleaning factors discussed.Table I provides alkaline cleaner characteristics and the typical choicerelationship.

POSTCLEANING RINSING STAGES

The purpose of rinsing is to remove or flush the remaining peptized soil, toneutralize the remaining alkaline salts, and to maintain a wet surface priorto entering the subsequent chemical stage. Initial rinsewater quality is veryimportant as the part or product in question will only be as clean as the wateris pure. Deionized or reverse osmosis treated water is used where a high degreeof surface cleanliness is required.

Part configuration, drain vestibules, and adequate time are important considerationsin reducing overall water usage. Table II shows typical cleaner dragoutthat can be expected from various part configurations. Many improvements inrinse stages have been tested and employed to reduce the volumes of effluentto be treated. A common practice is the backflow rinses in a conventional fiveor more stage pretreatment system. The process is as follows:

1. Clean

2. Rinse

3. Phosphate

4. Rinse

5. Seal

Finishing system organizations have introduced unique design improvementsto utilize rinsewater more efficiently and to assist in maintaining rinse cleanliness.Counterflow rinsing provides the cleanest possible water as the last contactwith the part, and allows for multiple use rinse effectiveness.The major control mechanisms for rinse tanks remain the control of pH andtotal dissolved solids (TDS). These tools have been automatically incorporatedinto washers, which allow sensing devices to either increase the overflow rateor reduce or drop TDS by automatic draining, thus maintaining consistency in water quality without regard to part shape, drag-in or drag-out