METAL PREPARATION

Metal Preparation for Product Finishes

WHY METAL PREPARATION

HAS BECOME INCREASINGLY IMPORTANT

Many factors contribute toward producing the optimum and results from industrial coatings. One of the most important of these is surface preparation. This brochure deals with the most common metals used in today’s industrial products and covers the most generally accepted metal cleaning processes of industrial finishers.

It is almost impossible to read trade and finishing journals which do not contain at least one article on the subject of Metal Preparation. It may be asked-“Why?” Products have been painted for years and it is comparatively recent that such emphasis has been made placed on the subject.

An obvious cause is the trend toward increased and varied metal fabrication. Greater use of aluminum, magnesium, zinc, and other metals has created new problems. The fundamental reason is continued demand for better paint performance. Not only has the fact, in itself, necessitated higher standards in each step of a finishing process, but it has also brought about many new basic changes in paint formulations.

In the era of long-oil, slow-drying type paints, oil on the surface was often compatible with the paint and absorbed by the paint film without any appreciable loss of adhesion. The newer, fast –drying resins unfortunately are, in general, incompatible with rustproofing oils, drawing compounds, etc. If these paints are applied over such foreign substances, proper adhesion is impossible.

Metal Preparation covers two steps in the finishing process, cleaning and treatment – cleaning being the removal of foreign substances, and treatment – that operation performed to improve adhesion and ultimate durability by etching, depositing other substances, or otherwise changing the surface character of the metal. In some instances, both of these steps are performed in one operation, though generally to a limited degree.

Metal Cleaning

Purpose – To remove dirt, rust, rustproofing and drawing oils and compounds, fingerprints, mill scale and many other foreign substances, both visible and invisible, thereby improving adhesion and reducing the tendency to blister and corrode on exposure.

METHODS

1. SOLVENT CLEANING

A. Wiping. This consists of simply washing off the surface with petroleum or aromatic

solvents, using rags or sponges. The method is widely applicable, but because rags and solvent are immediately contaminated with grease and dirt, a complete cleaning is practically impossible, and the method is recommended only as a last resort. Unless noninflammable solvents are used, fire hazard is an additional objection.

B. Solvent-Jet Cleaning. In its simplest form, this method consists of spraying solvent,

generally naphtha or mineral spirits, or Xylol or mixtures thereof, on the surface in a washing action. A spray gun with the fan turned down makes a good “jet”. The method can be likened to

This method is applicable on large units not easily put through washing machines. Partially concealed surfaces are more readily reached. Solvents are not re-used, eliminating re-contamination. The volume of solvent used is generally not large. Excessively heavy grease deposits can be removed, preceding other cleaning methods. The chef objection is the fire hazard.

C. Vapor Degreasing. This is a common, solvent-type cleaning method. The use of chlorinated solvents such as trichlorethylene and perchlorethylene in closed-cycle degreasing systems has found widespread application where further treatment is not necessarily, although occasionally a vapor degreaser will precede a phosphatizing process. These degreasers may employ vapor or liquid-phase, cleaning with or without spray scrubbing action.

Most commonly, the cold article to be cleaned is placed in the vapors which condense and run off until the metal has reached the temperature of the vapors. On the renewal, it is dry and grease-free.

The solvents used are not particularly effective in removing soaps, oxidized lanolin, tightly adhering dirt, rust, etc.

Costs are generally somewhat higher than alkaline cleaning.

2. ALKALINE CLEANING

The alkaline cleaners are the most common cleaners in use. They are low in cost and generally effective. Available under numerous proprietary names, they consist of alkaline salts, soaps, detergents, etc. and vary in composition depending on the type of equipment used and cleansing requirements. They are used both in power washers and in soak or dip tanks. Sometimes an electric current is added for faster and more complete cleaning. The method, much more common in the plating industry, is known as electrolytic cleaning and can be either cathodic or anodic.

Alkaline cleaners are generally used as hot solutions, at strengths ranging from 1/8 oz. to 16 oz. per gallon Temperatures of the solution run from 170-212°F.

Cleaning action results from emulsification or saponification of oils and dirt. Efficiency is dependent on agitation in dip or soak tanks and on the scouring action of the sprays in power washers.

Rinsing is of extreme importance in using alkaline cleaners. Alkali left on the surface attacks the paint film, causing early failure by loss of adhesion and blistering, In extreme cases, particularly where pigments (some reds, blues and greens) susceptible to alkali are used, actual discoloration occurs.

Since the rinse water is rapidly contaminated by the alkali being “dragged-over”, it is necessary to overflow the rinse tank by continuous addition of fresh water. As a rule, the rinse water should be hot – 180-212°F. – both to obtain more efficient rinsing, and also faster drying off of cleaned parts.

In practice, sufficient heat is not always available to keep the tank hot and overflowing rapidly. In such cases, as well as where “drag-over” is abnormally high, it may be desirable to use two rinses- the first cold and the second hot. The cold tank can then be overflowed more rapidly, the hot tank at a slower rate.

In many plants, the wate4r may be very hard. As there are numerous objections to excessively hard water, softened water or steam condensate may be necessary. The calcium and magnesium salts in hard water may break down at the high temperature and leave deposits on the work.

Reducing the temperature of the water may trend to eliminate these deposits, but even when in solution, some of the salts will remain on the metal. If these are subsequently painted over, early failure by blistering can be expected.

Since aluminum is readily attacked by alkali, use alkaline cleaning on this surface with caution and always in accordance with the manufacturer’s directions.

3. EMULSION CLEANERS

These are made up of soaps, detergents and grease solvents. When mixed with water – from 10-100 parts of water to 1 part cleaner – they form neutral to slightly alkaline emulsions. Some are mixed with higher boiling naphthas.

Emulsion cleaners are generally used in the same way as alkali cleaners – in power washers or soak tanks followed by the rinsing. It is claimed that they are more efficient in removing certain contaminates like carbon smut, drawing compound filers, etc. They may also be faster with certain oils. Being more nearly neutral, they are somewhat less hazardous to use.

Alkali cleaners will attack such metals as tin, aluminum and zinc. Where this is not permissible, less alkaline emulsion cleaners may be successfully used.

Some emulsion cleaners are claimed to have rust inhibitive properties to prevent rust formation after cleaning and before painting. These are not usually rinsed off, or a small amount of the cleaner is added to the rinse tank.

Where adhesion and good durability are critical, this method of rutproofing is not satisfactory, since it is dependent of a film of rustproofing oil or other equally objectionable matter.

4. ACID CLEANERS

A. Pickling. This method is used primarily to remove rust and scale. If the metal is greasy,

pickling generally follows another form of cleaning, such as alkaline cleaning.

Pickling acids vary in strength with the severity of rust and scale. An 8-10% solution of

sulphuric acid, at 140 - 160°F., will remove heavy deposits. Because of hazards involved in the use of this and other acid solutions, metal-cleaning and treatment specialists must be consulted for proper instructions.

Proprietary pickling solutions contain unhibitors to limit the attack on the base metal. After

pickling, thorough rinsing is necessary. Acid pickled steel is in a very active state and rusts readily; painting or other treatment should follow immediately. This flash rusting can be controlled by rinsing in weak phosphoric and/or chromic acid solution. Fumes from hot sulfuric and hydrochloric acids are very corrosive and surrounding equipment must be properly protected. Non-fuming proprietary pickling solutions are available.

B. Phosphoric Acid Type Cleaners. Proprietary cleaners based on phosphoric acid and grease

solvents are available which will remove moderate amounts of surface contamination. These are actually combination cleaners and treatments, since they etch and form protective iron-phosphate salts, giving improved paint adhesion and corrosion resistance.

The most common these of these are the “wipe-on” cleaners which are very effective when properly used. Others are used in similar fashion to alkali and emulsion cleaners. These are often the most effective cleaners for use in the two-stage washers, although, for best results, 3 stages are recommended.

Under this caption Met-L-Mateä or Dual Etch may be used.

5. BURN-OFF

By passing the article through an oven, usually heated with open gas burners to relatively high temperature, residual oils are driven off or ignited. The method, at one time quite common, has been mostly replaced by chemical cleaning processes. Some forms of soil, particularly pigmented drawing compounds are not removed and, as used in the past, the metal was in a rather active state and rusted readily unless further treated or immediately painted.

A modern version of the burn-off process was recently introduced. By using special burners, the metal is heated to the blue stage. The steel under these conditions does not rust readily and, it is claimed, makes a good painting surface.

6. FLAME CLEANING

This method is used chiefly on structural steel to remove scale. A flame is directed at the steel, causing rapid heating and cooling when the flame is removed. Differences in expansion cause the scale to break away. Flame cleaning may also be used to remove condensed moisture.

7. MECHANICAL CLEANING

Scraping wire brushing or sanding by hand or with power tools can be effective if properly done. As a rule, only rust and loose mill scale are so removed. Complete removal of mill scale by these methods often increases the activity and tendency to corrode.

8. BLASTING

Blasting with sand, shot or grit is a very common method for cleaning large machines, railroad equipment, structural steel, etc. Blasting produces a chemically clean surface and, because of the surface roughness, gives excellent adhesion. Blasted surfaces rust very rapidly and painting should be done immediately. If this is not possible, washing with weak chromic acid solution will control rusting for moderated periods.

GENERAL PRECLEANING

Where the article is to be cleaned is very dirty, it is often possible to effect definite operation economy by including a precleaning stage to remove most of the dirt before going into the normal cleaning cycle. This avoids excessive contamination of the final cleaning bath and often permits use of lower cost chemical cleaners in the first stage.

RUSTPROOFING

In many finishing cycles, there is a lapse of time between cleaning and subsequent painting. It is an unfortunate circumstance that clean metal often rusts more rapidly than dirty metal. As a result, some means of controlling flash rusting is necessary. Where cleaning is followed by a metal treatment in a continuous system, no problem exists, but if no subsequent treatment is used, it is necessary to passivate or protect the metal.

A frequent method following alkali or emulsion cleaning is to add chromic and phosphoric acid to the second rinse. Whether one or both are used will depend somewhat on the water condition involved.

Not only is the metal temporarily passivated, but corrosion resistance of the paint film is definitely improved.

There are numerous proprietary compounds available for this purpose. Many contain emulsifiable oils which, while effective in preventing flash rust, do impair adhesion. They are recommended only for less critical uses where long life of the paint is not expected.

CHEMICAL TREATMENTS

It is now standard practice to chemically treats metals after cleaning to further improve adhesion, corrosion and humidity resistance. The most common chemical treatments are based on phosphoric acid and may be classified into two groups – Crystalline and Amorphous.

1. CHRYSTALLINE PHOSPHATE

There are several patented systems employing the phosphate principle which include the proper cleaning followed by chemical treatment. The phosphating solutions consists of phosphoric acid and metal phosphates which react with and deposit complex crystals on the metal.

The crystalline, somewhat porous surface is excellent as a paint base, giving improved adhesion, corrosion, and corrosion creep resistance.

Most commonly, the treatment consists of three stages:

A. Phosphating solution

B. Hot water rinse

C. Hot, dilute chromic acid rinse

The three stages may be either spray or dip with spray being preferred since it permits better control of the crystal size. Large crystals reduce gloss of the finishing paint and may be excessively brittle.

2. AMORPHOUS PHOSPHATE

Except as already discussed under “acid cleaners”, the amorphous types are used in much the same way as the crystalline types.

While generally they do not contribute quite as much to the ultimate durability of the paint finish, they do not have a number of advantages. They are generally lower in cost and, being noncrystalline, they have less effect on gloss.

The amorphous chemical treatments are recommended for use both with and without the final chromic acid rinse. For best results, the chromic acid rinse is desirable since it has been proved that where the final rinse is neutral (clean water), the resistance to corrosion is much lower. In fact, under certain conditions, a cleaning cycle with a chromic acid rinse is preferable to using a cleaning-phosphating cycle without the final acid rinse.

TREATMENT OF VARIOUS METALS

ZINC

Paint adheres poorly to new zinc, possibly due both to the smooth surface and to chemical reaction between the metal and the paint. This is sometimes known as “alkaline peeling”.

Weathering, mechanical roughening, or simple etching of zinc surfaces improves paint adhesion. However, for added corrosion resistance, a chemical treatment is desirable. Best results are generally obtained with the crystalline phosphate types, although the amorphous types are ample for many uses.

Zinc-coated steels with phosphate treatment are marketed under various trade names. These usually give good paint performance without the need for additional cleaning or treatment unless contaminated during fabrication.

In cleaning zinc, since it is reactive to alkalis and acids, care must be exercised in choosing the proper cleaner to avoid excessive attack of the metal.

ALUMINUM

Aluminum is, in itself, very resistant to corrosion since on exposure a tight film of oxide forms which protects the base metal from further corrosion. Intermediate treatments are unnecessary except under extreme humidity or salt aid conditions.

Most paints, however, do not adhere well to aluminum and treatments are generally necessary to prevent embrittling and loss of adhesion.

For many purposes, phosphoric acid type treatments are satisfactory. These may be either wipe-on, spray or dip type. Hot, dilute chromic acid gives similar results.

Where exposure conditions are more severe, anodizing or alkaline dichromate give better results. These processes form protective coatings which are excellent bases for paint. Both processes are slow, requiring up to thirty-minute immersions. Since aluminum is readily attacked by alkali, use alkaline cleaning on this surface with caution and always in accordance with the manufacturer’s directions.

Proprietary aluminum treatments are available which give about equal results with much shorter reaction times. Costs usually are considerably lower.

When aluminum is assembled in contact with steel, it is generally recommended that both surfaces that both surfaces be primed before the final operation.

Unless so insulated, electrolysis takes place at the joint, causing galvanic corrosion of the aluminum.

MAGNESUIM

Paint applied to magnesium is subject to “alkaline peeling” similar to that experienced with zinc.

Most magnesium products are finished with a protective chemical coating, even before final fabrication of the product. This coating is intended to protect the metal during storage, shipment and manufacture. The chrome pickle process, known as Dow No. 1, is used for this purpose. The treatment is a short immersion in a solution of nitric acid and sodium dichromate.

The coating is suitable as a paint but cannot be used where close tolerances are required, due to etching of the metal. The dichromate process, known as Dow No. 7, is the best for corrosion resistance and is widely used as a base for painting. Owing to certain hazards involved, manufacturer’s recommendations must be followed implicitly. Under no circumstances should the process be undertaken without proper investigation.

Magnesium can also be anodized, producing a coating which has excellent corrosion resistance.

CLEANING-TREATMENT SYSTEMS

FACTORS AFFECTING CHOICE

The proper choice of cleaning and treatment must be made by the manufacturer, of the articles to be processed. Not only do the various methods vary in efficiency, but the number of stages through which the work is put will affect the final results.

Some of the many factors that must be considered are:

1. The kind of metal (steel, aluminum, etc.). 2. The kind of soil (rust, dirt, grease, drawing

compounds, etc.). 3. Ultimate durability required of the finish. a. Will the article be exposed to high humidity, salt atmospheres, etc.? b. Is it a short or long life article? c. Is the finish purely decorative, or must it also protect? 4. Initial cost. 5. Operation space available. 6. Competitors’ methods.

These points are of such scope and involve basic policy to such an extent that only those responsible for the manufacture and sale of the article in question can given them proper weight and consideration.

TYPES OF CLEANING-TREATMENT SYSTEMS

It would be impossible within the scope of this Brochure to consider all the possible cleaning treatment systems. They must consist of simply one stage or operation or 10 to 12 stages. Only some of the more commonly encountered are considered below:

ONE STAGE OPERATION – TYPES

1. Burn-off. 2. Blasting. 3. Mechanical cleaning. 4. Solvent cleaning. 5. Acid cleaning (occasionally).

Within the scope of their applicability, these systems give satisfactory results. Single-stage alkali, emulsion and some acid cleaners cannot generally be considered satisfactory pre-painting preparations. Deposits left on metal are often more harmful than the soil they remove.

TWO STAGE OPERATION

Two stage power washers are quite common in industry. Usually an alkali or emulsion cleaner is used in the first stage, followed by a rinse.

One of the major objections to a two-stage washer is the lack of the control of flush rusting if the work is not immediately painted. Since steel in an alkaline state will rust more slowly than when neutral, and since some emulsion cleaners have rust inhibitors added, it is all too common practice to allow the rinse tank to become contaminated by the cleaner. Most often the rinse could then just as well have been omitted entirely.

If the soil is such that it can satisfactorily be removed by one of the available acid cleaners, these are generally preferable in two stages. (Heavy grease and certain drawing compounds are effectively removed.) By using these acid cleaners, moderate contaminations of the rinse tank will have less adverse effect on adhesion, etc. then will even slight alkali contaminations.

THREE STAGE OPERATION

Three stages should be considered the minimum for general steel fabrication.

1. Cleaning – alkali, emulsion or acid. 2. Rinse. 3. Rinse with dilute chromic acid (1/16-

1/10%) (For certain water conditioning some phosphoric acid may be used).

The first rinse must be thorough the prevent any cleaner, particularly alkali, from being

dragged into the final rinse and neutralizing the acid.

This system rustproofs the steal sufficiently to prevent flash rust for short periods. The final

acid rinse adds some corrosion resistance, but since, at the strength used, there is no acid etching, adhesion is promoted only to the extent that the metal is clean.

FOUR-STAGE OPERATION

A. 1. Alkali or emulsion cleaning. 2. Rinse. 3. Phosphating solution. 4. Rinse.

A moderately good system, with improved adhesion due to the phosphating solution which may be either the crystalline or amorphous type. The main objection is the lack of a final acid rinse. Normal tap water is usually slightly alkaline and some of the corrosion and blister resistance gained from the third operation is lost in the fourth.

B. 1. Vapor degrease. 2. Phosphating solution (crystalline or amorphous). 3. Rinse.

4. Chromic acid rinse.

Where the soil present is such that vapor degreasing ill remove it sufficiently, so that the phosphating action can proceed without hindrance, this system is equal to the more generally applicable five and six-operation systems.

FIVE AND SIX-STAGE OPERATIONS

1. Clean – (alkali or emulsion). 2 or 2 and 3. One or two rinses. 3 or 4. Phosphating – (crystalline or amorphous). 4 or 5. Rinse. 5 or 6. Chromic acid rinse.

Whether one or two rinses must be used after cleaning will depend on local conditions, efficiency of the rinse, drag-over, etc. This type of system is generally used – particularly with the crystalline phosphate – for such items as washing machines and refrigerators, where maximum paint performance is required.

Additional modification of this system could include precleaning where excessively heavy soil, rust, etc. exists.

Common Metal Preparation Problems

1. MECHANICAL CLEANING

Problem	Cause	Remedy
A. Variation in blast profile.	A. Fresh abrasive media added at irregular intervals in batch process.	A. Add fresh abrasive media at regular intervals based on average product flow.
B. Too much or too little blast profile.	B. Wrong mesh size of media.	B. Verify abrasive mesh size. Consult abrasive supplier for proper size, mixture, grit, type, etc.
C. Adhesion or cratering.	C. Crease or oil imbedding in substrate.	C. Remove grease, oil, etc. prior to mechanical cleaning.
D. Rusting.	D. Process time between cleaning and coating application.	D. Apply coatings to mechanically cleaned parts as soon as possible.

11. SOLVENT CLEANING

Problem	Cause	Remedy
A. Inconsistent paint adhesion on part.	A. Wipe rags.	A. Change wipe rags more frequently or modify wipe procedure.
B. Adhesion failure on entire part.	B. Solvent combination.	B. Change wipe solvent solution.
C. Adhesion failure with automatic cleaning system.	C. Inadequate overflow and solvent turnover. Impingement. Solvent contamination.	C. Increase overflow or fresh solvent addition. Increase spray pressure. Verify by testing with virgin solvent.

1. MECHANICAL

CLEANING

Problem	Cause	Remedy
A. Variation in blast profile.	A. Fresh abrasive media added at irregular intervals in batch process.	A. Add fresh abrasive media at regular intervals based on average product flow.
B. Too much or too little blast profile.	B. Wrong mesh size of media.	B. Verify abrasive mesh size. Consult abrasive supplier for proper size, mixture, grit, type, etc.
C. Adhesion or cratering.	C. Grease or oil imbedding in substrate.	C. Remove grease, oil, etc. prior to mechanical cleaning.
D. Rusting.	D. Process time between cleaning and coating application.	D. Apply coating to mechanically cleaned parts as soon as possible.

111. ALKALINE WASH/ ACID ETCH/ PHOSPHATIZING

Problem	Cause	Remedy
A. Inadequate cleaning.	A. Cleaning chemical. Die lubricant, rust. Cleaning time. Temperature. Testing. Spray Pressure.	A. Verify with chemical supplier and test concentration. Check to see if metal supplier has changed process. Replace die lubricant, rust preventative, etc. Operate cleaning system at proper line speed. Operate cleaning system at recommended temperatures. Water-break-free surface (water will bead on surfaces with oil or grease present). White handkerchief test (wipe with light cloth over clean surface will not stain. Verify spray pressure. Check for plugged or misaligned nozzle. Change nozzle type.
B. Water spotting.	B. Rinse water.	B. Use deionized water for final rinse.
C. Rusting prior to painting.	C. Low cleaner concentration. Rinse water contamination. Low Chemical concentration. Excess free acid. Low accelerator concentration. Low chromic acid rinse concentration. Low solution temperature. Accumulated water in recessed areas.	C. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Drain parts adequately. Dry parts completely in several minutes after cleaning/phosphatizing.
D. Low coating weight.	D. Low chemical concentration. Rinse water contamination. Low chemical concentration. Excess free acid. Low solution temperature. Low accelerator concentration.	D. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications.
E. High coating weight. F. Excess sludge.	E. Low chemical concentration. High solution temperature. High accelerator concentration. F. Excess free acid. Low accelerator concentration. High solution temperature. Fluctuating solution temperature. Poor filtration.	E. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. F. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications. Check and maintain filters.
G. Low salt spray resistance.	G. High cleaner concentration. Low accelerator concentration. Low chromic acid rinse concentration.	G. Adjust per operating specifications. Adjust per operating specifications. Adjust per operating specifications.
H. White powder.	H. Chemical concentration. Rinse time. Cleaning time.	H. Adjust per operating specifications. Increase rinse time. Reduce cleaning time.
I. Foreign substances in coating – i.e. sealers, marker’s mark, rust, temper discoloration, drippage after phosphate, etc.	I. Foreign objets not cleaned off in pre-clean operation. Surplus sealers washed off and redeposited on exterior surfaces. Drippage of condensate and oils after phosphate stage	I. Wipe all exterior surfaces to loosen up and/or remove all foreign particles. Mechanically grind or wipe with organic acids to remove remaining foreign particles. Improve filtration in all stags. Apply sealers carefully to avoid contamination of exposed material surfaces. Check and adjust drip rails and drip pans. Wipe off contamination before priming.
J. Heavy streaks; bluish and/or yellowish streaks and spots.	J. Poor cleaning. Metallurgical conditions: heat scale, dried-on oils and poor steel quality. Defective spray nozzles in washer, resulting in poor spraying pattern. Drippage of trapped acidic condensate on the surface while in the rinse area prior to the phosphate stage. Excessive ferrous iron in bath. Acid mist or back spray of phosphate on surface.	J. Improve cleaning. Check cleaner’s operation parameters and change cleaner if necessary. Remove through mechanical buffing or by wiping with organic acids. Check and adjust nozzles all stages. Repair and adjust drip rails to divert the drippage away from processed surface. Adjust nozzles in phosphate stage to eliminate any out-stage spray. Lower pressure in the phosphate stage to avoid undesirable atomization. Check for iron content and add accelerator. Adjust nozzles or airflow to production direction.
K. Thin coating; shiny and sparkling coating.	K. Temperature too low in the phosphate stage. Concentration of the phosphate stage is too low. Free acid concentration in phosphate bath is too high or phosphate bath is in balance (coating is usually thin and shiny, but dark in color) Temperature too high in the titanated cleaning stage. Low pressure in phosphate stage. Concentration of the non-titaneted cleaning stage. Excessive bath ingredients, aging of particularly titanated cleaners and titanated rinse conditioners. Partial or complete pump failure in the phosphate stage. Metallurgical conditions: heat scale, dried-on oils, and poor quality steel. <