Removal of Pigmented Drawing Compounds

All pigmented drawing lubricants are difficult to remove from metal parts. Consequently, many plants review all aspects of press forming operations to avoid the use of pigmented compounds. Pigmented compounds most commonly used contain one or more of the following substances: whiting, lithopone, mica, zinc oxide, bentonite, flour, graphite, white lead (which is highly toxic), molybdenum disulfide, animal fat, and soaplike materials. Some of these substances are more difficult to remove than others. Because of their chemical inertness to acid and alkali used in the cleaners and tight adherence to metal surfaces, graphite, white lead, molybdenum disulfide, and soaps are the most difficult to solubilize and remove.

Certain variables in the drawing operation may further complicate the removal of drawing lubricants. For example, as drawing pressures are increased, the resulting higher temperatures increase the adherence of the compounds to the extent that some manual scrubbing is often an essential part of the subsequent cleaning operation. Elapsed time between the drawing and cleaning operations is also a significant factor. Drawing lubricants will oxidize and loosely polymerize on metal surfaces over time, rendering them even more resistant to cleaning.

Table 2 indicates cleaning processes typically selected for removing pigmented compounds from drawn and stamped parts such as Parts 1 through 6 in Fig. 1.

Table 2 Metal cleaning processes for removing selected contaminants

Type of production

In-process cleaning

Preparation for painting

Preparation for phosphating

Preparation for plating

Removal of pigmented drawing compounds(a)

Occasional or intermittent

Hot emulsion hand slush, spray emulsion in single stage, vapor slush degrease

Boiling alkaline blow off, hand wipe

Hot emulsion hand slush, spray emulsion in single stage, hot rinse, hand wipe

Hot alkaline soak, hot rinse (hand wipe, if possible) electrolytic alkaline, cold water rinse

Vapor slush degrease, hand wipe

Type of production

In-process cleaning

Preparation for painting

Preparation for phosphating

Preparation for plating

Acid clean(c)

Continuous high production

Conveyorized spray emulsion washer

Alkaline soak, hot rinse alkaline spray, hot rinse

Alkaline or acid(d) soak, hot rinse, alkaline or acid(d) spray, hot rinse

Hot emulsion or alkaline soak, hot rinse, electrolytic alkaline, hot rinse

Removal of unpigmented oil and grease

Occasional or intermittent

Solvent wipe

Solvent wipe

Solvent wipe

Solvent wipe

Emulsion dip or spray

Vapor degrease

Emulsion dip or spray, rinse

Emulsion soak, barrel rinse, electrolytic alkaline rinse, hydrochloric acid dip, rinse

Vapor degrease

Phosphoric acid etch

Vapor degrease

Cold solvent dip

Alkaline spray

Alkaline dip, rinse, dry or dip in rust preventative

Continuous high production

Automatic vapor degrease

Automatic vapor degrease

Emulsion power spray, rinse

Automatic vapor degrease, electrolytic alkaline rinse, hydrochloric acid dip, rinse(e)

Emulsion, tumble, spray, rinse, dry

Vapor degrease

Acid clean(c)

Removal of chips and cutting fluid

Occasional or intermittent

Solvent wipe

Solvent wipe

Solvent wipe

Solvent wipe

Alkaline dip and emulsion surfactant

Alkaline dip and emulsion surfactant

Alkaline dip and emulsion surfactant®

Alkaline dip, rinse, electrolytic alkaline*8-1, rinse, acid dip, rinse®

Stoddard solvent or trichlorethylene

Solvent or vapor

Solvent or vapor

Steam

Continuous high production

Alkaline (dip or spray) and emulsion surfactant

Alkaline (dip or spray) and emulsion surfactant

Alkaline (dip or spray) and emulsion surfactant

Alkaline soak, rinse, electrolytic alkaline*8-, rinse, acid dip and rinse(h)

Type of production

In-process cleaning

Preparation for painting

Preparation for phosphating

Preparation for plating

Removal of polishing and buffing compounds

Occasional or intermittent

Seldom required

Solvent wipe

Solvent wipe

Solvent wipe

Surfactant alkaline (agitated soak), rinse

Surfactant alkaline (agitated soak), rinse

Surfactant alkaline (agitated soak), rinse, electroclean(i)

Emulsion soak, rinse

Emulsion soak, rinse

Alkaline spray

Continuous high production

Seldom required

Surfactant alkaline spray, spray rinse

Surfactant alkaline spray, spray rinse

Surfactant alkaline soak and spray, alkaline soak, spray and rinse, electrolytic alkaline, rinse, mild acid pickle, rinse

Agitated soak or spray, rinse®

Emulsion spray, rinse

(a) For complete removal of pigment, parts should be cleaned immediately after the forming operation, and all rinses should be sprayed where practical.

(b) Used only when pigment residue can be tolerated in subsequent operations.

(c) Phosphoric acid cleaner-coaters are often sprayed on the parts to clean the surface and leave a thin phosphate coating.

(d) Phosphoric acid for cleaning and iron phosphating. Proprietary products for high-and low-temperature application are available.

(e) Some plating processes may require additional cleaning dips.

(f) Neutral emulsion or solvent should be used before manganese phosphating.

(g) Reverse-current cleaning may be necessary to remove chips from parts having deep recesses.

(h) For cyanide plating, acid dip and water rinse are followed by alkaline and water rinses.

(i) Other preferences: stable or diphase emulsion spray or soak, rinse, alkaline spray or soak, rinse, electroclean; or solvent presoak, alkaline soak or spray, electroclean.

(j) Third preference: emulsion spray rinse

Fig. 1 Sample part configurations cleaned by various processes. See text for discussion.

Emulsion cleaning is one of the most effective methods for removing pigmented compounds, because is relies on mechanical wetting and floating the contaminant away from the surface, rather than chemical action which would be completely ineffective on such inert materials. However, emulsions alone will not do a complete cleaning job, particularly when graphite or molybdenum disulfide is the contaminant. Emulsion cleaning is an effective method of removing pigment because emulsion cleaners contain organic solvents and surfactants, which can dissolve the binders, such as stearates, present in the compounds.

Diphase or multiphase emulsions, having concentrations of 1 to 10% in water and used in a power spray washer, yield the best results in removing pigmented compounds. The usual spray time is 30 to 60 s; emulsion temperatures may range from 54 to 77 °C (130 to 170 °F), depending on the flash point of the cleaner. In continuous cleaning, two adjacent spray zones or a hot water (60 to 66 °C, or 140 to 150 °F) rinse stage located between the two cleaner spraying zones is common practice.

Cleaning with an emulsifiable solvent, a combination of solvent and emulsion cleaning, is an effective technique for removing pigmented compounds. Emulsifiable solvents may either be used full strength or be diluted with a hydrocarbon solvent, 10 parts to 1 to 4 parts of emulsifiable solvent. Workpieces with heavy deposits of pigmented compound are soaked in this solution, or the solution is slushed or swabbed into heavily contaminated areas. After thorough contact has been made between the solvent and the soil, workpieces are rinsed in hot water, preferably by pressure spray. Emulsification loosens the soil and permits it to be flushed away. Additional cleaning, if required, is usually done by either a conventional emulsion or an alkaline cleaning cycle.

Most emulsion cleaners can be safely used to remove soil from any metal. However, a few highly alkaline emulsion cleaners with pH higher than 10 must be used with caution in cleaning aluminum or zinc because of chemical attack. Low alkaline pH (8 to 9) emulsion cleaners, safe on zinc and aluminum, are available. Emulsion cleaners with a pH above 11 should not be used on magnesium alloys.

Alkaline cleaning, when used exclusively, is only marginally effective in removing pigmented compounds. Success depends mainly on the type of pigmented compounds present and the extent to which they have been allowed to dry. If the compounds are the more difficult types, such as graphite or white lead, and have been allowed to harden, hand slushing and manual brushing will be required for removing all traces of the pigment. Hot alkaline scale conditioning solutions can be used to remove graphite and molybdenum disulfide pigmented hot forming and heat treating protective coatings. The use of ultrasonics in alkaline cleaning is also highly effective in removing tough pigmented drawing compounds.

The softer pigmented compounds can usually be removed by alkaline immersion and spray cycles (Table 2). The degree of cleanness obtained depends largely on thorough mechanical agitation in tanks or barrels, or strong impingement if a spray is used. A minimum spray pressure of 0.10 MPa (15 psi) is recommended.

Parts such as 1 to 6 in Fig. 1 can be cleaned effectively by immersion or immersion and spray when the parts are no longer than about 508 mm (20 in.) across. Larger parts of this type can be cleaned more effectively by spraying. Operating conditions and the sequence of processes for a typical alkaline cleaning cycle are listed in Table 3. This cycle has removed pigmented compounds effectively from a wide variety of stampings and drawn parts. Energy saving low-temperature solventized-alkaline cleaners are available for soak cleaning. Similarly low-temperature electro-cleaners also are effectively employed in industry, operating at 27 to 49 °C (80 to 120 °F).

Table 3 Alkaline cleaning cycle for removing pigmented drawing compounds

Process sequence

Concentration

Time, min

Temperature

Anode current

Remarks

g/L

oz/gal

°C

°F

A/dm2

A/ft2

Alkaline soak clean

Barrel1-3-1

65 to 90

9 to 12

3 to 5

Boiling

Boiling

Rack(b)

65 to 90

9 to 12

3 to 5

Boiling

Boiling

Hot water rinse, immersion, and spray

Barrel(a)

3(c)

43

110

Spray jet if barrel is open type

Process sequence

Concentration

Time, min

Temperature

Anode current

Remarks

g/L

oz/gal

°C

°F

A/dm2

A/ft2

Rack(b)

2(c)

43

110

Spray rinse, immerse, and spray rinse

Electrolytic alkaline clean

Barrel(a)

55 to 65

7 to 9

2

82 to 99

180 to 210

4 to 6

40 to 60

Rack(b)

65 to 90

9 to 12

2

82 to 99

180 to 210

4 to 6

40 to 60

Hot water rinse, immersion, and spray(d)

Barrel(a)

3(c)

43

110

Spray jet if barrel is open type

Rack(b)

2(c)

43

110

Spray rinse, immerse, and spray rinse

Cold water rinse, immersion, and spray(e)

Barrel(a)

2(c)

Spray jet if barrel is open type

Rack(b)

1(c)

Spray rinse, immerse, and spray rinse

(a) Rotate during entire cycle.

(b) Agitate arm of rack, if possible.

(c) Immersion time.

(d) Maintain overflow at approximately 8 L/min (2 gal/min).

(e) Clean in cold running water.

Electrolytic alkaline cleaning is seldom used as a sole method for the removal of pigmented compounds. Although the generation of gas at the workpiece surface provides a scrubbing action that aids in removal of a pigment, the cleaner becomes contaminated so rapidly that its use is impractical except for final cleaning before plating (Table 2).

Copper alloys, aluminum, lead, tin, and zinc are susceptible to attack by uninhibited alkaline cleaners (pH 10 to 14). Inhibited alkaline cleaners (pH below 10), which have reduced rates of reaction, are available for cleaning these metals. These contain silicates and borates.

Acid Cleaning. Acid cleaners, composed of detergents, liquid glycol ether, and phosphoric acid have proved effective in removing pigmented compounds from engine parts, such as sheet rocker covers and oil pans, even after the pigments have dried. These acid compounds, mixed with water and used in a power spray, are capable of cleaning such parts without hand scrubbing.

A power spray cycle used by one plant is given in Table 4. A light blowoff follows the rinsing cycle. Parts with recesses should be rotated to allow complete drainage. This cleaning procedure suitably prepares parts for painting, but for parts to be plated, the acid cleaning cycle is conventionally followed by electrolytic cleaning which is usually alkaline, but sometimes done with sulfuric or hydrochloric acid. Phosphoric acid cleaners will not etch steel, although they may cause some discoloration.

Table 4 Power spray acid cleaning for removing pigmented compounds

Steel parts cleaned by this method are suitable for painting, but electrolytic cleaning normally follows if parts are to be electroplated; solventized, phosphoric acid-based, low-temperature (27 to 49 °C, or 80 to 120 °F) products are successfully used for power spray cleaning.

Cycle

Phosphoric acid

Solution temperature

Cycle time, min

g/L

oz/gal

°C

°F

Wash

15-19

2-2.5

74-79

165-175

3-4

Aluminum and aluminum alloys are susceptible to some etching in phosphoric acid cleaners. Chromic acid or sodium dichromate with either nitric or sulfuric acid is used to deoxidize aluminum alloys. Nonchromated deoxidizers are preferred environmentally. Ferric sulfate and ferric nitrate are used in place of hexavalent chromium. However, nonchromated deoxidizers tend to produce smut on the workpiece, especially 2000- and 7000-series alloys, when the deoxidizer etch rate is maintained (normally with fluoride) above 0.003 ^m/side per hour (0.1 ^in./side per hour). For more information on removing smut from aluminum, see the article "Surface Engineering of Aluminum and Aluminum Alloys" in this Volume.

Vapor degreasing is of limited value in removing pigmented compounds. The solvent vapor will usually remove soluble portions of the soil, leaving a residue of dry pigment that may be even more difficult to remove by other cleaning processes. However, modifications of vapor degreasing, such as slushing, spraying, ultrasonic, or combinations of these, can be utilized for 100% removal of the easier-to-clean pigments, such as whiting, zinc oxide, or mica.

The latter practice is often used for occasional or intermittent cleaning (Table 2). However, when difficult-to-clean pigments such as graphite or molybdenum disulfide are present, it is unlikely that slush or spray degreasing will remove 100% of the soil.

Vapor degreasing of titanium should be limited to detailed parts and should not be used on welded assemblies that will see later temperatures in excess of 290 °C (550 °F) because degreasing solvents are known to cause stress-corrosion cracking of titanium at these temperatures. Subsequent pickling in nitric-fluoride etchants may relieve this concern.

Solvent cleaning, because of its relatively high cost, lack of effectiveness, rapid contamination, and health and fire hazards, is seldom recommended for removing pigmented compounds, except for occasional preliminary or rough cleaning before other methods. For example, parts are sometimes soaked in solvents such as kerosene or mineral spirits immediately following the drawing operation to loosen and remove some of the soil, but the principal effect of the operation is to condition parts for easier cleaning by more suitable methods, such as emulsion or alkaline cleaning.

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