Recycling of Water-Based Paint by F. Wilhelm, Eisenmann, Guringen, Germany

gainst the background of the reduction of solvent emissions, A water-dilutable paints have

found large-scale applications world- wide in the painting of car bodies. The fact that these paints are water dilutable offers the additional prospect of recov- ering the paint overspray from the wastewater of the spray booths by the removal of water or the extraction of the paint particles.

PRECONDITIONS FOR WATER- BASED PAINT RECYCLING

Ready-to-use water-based paints consist of approximately 50% solid matter (resins and paint pigments), 5-15% organic solvents, and 3545% water. During spray painting, the over- spray first enters the outlet air, from which it is washed out with the aid of water. The wash water should not have a solid matter content of more than approximately 1%, so as not to exceed the maximum legal limit for outlet air from spray booths of 3 mg of solid paint matter/Nm3. As indicated in Figure 1, this means that the original water-based paint is diluted in the booth washout process to approxi- mately 1% solid paint matter, 3% organic solvents, and 96% water, as well as dirt particles that get into the wash fluid.

During recycling, a concentrate is produced that is largely similar to water-based paint in concentration: approximately 50% solid matter, 2% organic solvent, and 48% water. Some dirt content cannot be excluded.

Water-Based Paint Requirements

The paint must meet the following requirements so that the material re- leased by the application equipment that does not reach the car bodies is once again usable at the end of the recovery process:

This article is based on a presentation at SurCar in Cannes, France in June, 1993 sponsored by Surfaces.

1.

2.

3.

The paint must not be altered to an impermissible degree due to contact with air on the way from the paint application equipment to the water. It must be possible for the paint to be infinitely dilutable and subse- quently thickened in the booth water, without irreversible changes to the paint characteristics occur- ring. The paint itself must not be imper- missibly altered during the respec- tive thickening process, and it should influence the function of the thickening installation as little as possible.

Spray Booth Requirements The quality of the recovered paint

concentrate is affected to a large extent by the foreign material that enters the wash water in the spray booth. The most common impurities are the fol- lowing:

Partially dried paint particles, such as flakes from booth elements and conveyor systems Rough dirt particles, such as cloths, lever stoppers, and masking material Cleaning fluid from application de- vices Grease/oil from the conveyor system Dust from inlet air and the environ- ment Salts from coagulants and supple- mentary water

Partially dried paint and rough dirt particles can be separated from the booth water by means of suitable filter equipment. Retroactive removal of cleaning agents, oil, dust, and salts from the wash water is only possible, if at all, with considerable effort and expense and results in a reduction in paint quality.

Salts from the coagulant can be avoided by doing without the coagu- lant in the first place. Salts from the supplementary water do not occur if the accretion and supplementary water of the spray booth wash water are in the form of desalinated water.

ResindPigments OrganicSolvents Wafer ~

Water Paint

-Based

Figure 1. Composition of water-based paint, spray-booth wash water, and recycling concentrate.

To keep cleaning agents, greases, and oils out of the recycling process as much as possible, a system was devel- oped wherein the spray booth has two water circuits, as shown in Figure 2. A water-flooded surface is located under the entire working level in the spray booth. In circuit A, the water is conducted through a circuit containing a cleaning system and a groove in which no material can be deposited. All solid impurities and those with a high specific weight fall into the water, located under the work level, and are separated by it.

The liquid can flow over these surfaces at right angles to the booth, which means that the existing division of the surfaces will cause the water to flow from the upper to the lower part of the surface. This has a special weir that divides the water curtain into individ- ual strands of liquid. Naturally, these surfaces can also be aerated in the longitudinal direction of the spray

from the upper onto the lower part of the surface. The fine components of the spray booth exhaust air (i.e., paint vapor) enter via the gap between the two parts of the surface of circuit A, a Venturi washout, in which the paint

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booth to prevent water overflowing ~~~

METAL FINISHING OCTOBER 1993 0 Copyright Elsevier Science Publishing Co., Inc. 23

Exhaust Air <=

Inlet Air

~~

Spray Booth

Figure 2. Spray booth with two water circuits.

components are separated. An important requirement of correct

recycling booth design is the elimina- tion of dead zones in which undesira- ble paint deposits can collect. It must also be possible for the booth to completely empty itself and be sprayed clean with deionized water after an operation has ended.

The spray booth with two water circuits described here fulfills ideally all of the preconditions for paint recovery. Furthermore, division into a

Permeate

dirt circuit A and a Venturi circuit B generally increases operational secu- rity and reduces maintenance. Rough dirt particles cannot get into the Ven- turi washout and compromise its effec- tiveness (e.g., by separating the water film).

Measurements were carried out on an actual booth segment and revealed a maximum paint content in the booth exhaust air of 2 mg/Nm3. It was also shown that approximately 80% of the paint overspray reached circuit B.

Membrane Paint-water mixture \ Permeate

1

Met i m =

4- Circulating Tank Pump Module Concentrate

Figure 3. Ultrafiltration as a recycling process for water-based paint: installation diagram (left) and process principle (right).

RECYCLING PROCESSES FOR

ULTRAFILTRATION, ELECTROPHORESIS, AND EVAPORATION

Ultrafiltration, electrophoresis, and evaporation have been successfully ~

used in the recovery of paint concen- trate from spray booth wash water in both production and pilot plants. These processes are described briefly and ~

their principal advantages and disad- vantages listed.

WATER-BASED PAINT:

Ultrafiltration Figure 3 shows the installation dia-

gram and process principle of ultrafil- tration for the recovery of water-based paint. In the ultrafiltration process, the painuwater mixture is passed through a semipermeable membrane under high pressure in the ultrafiltration module. The paint particles are retained, whereas the water passes through the membrane. This permeate is largely free of solid paint.

The painuwater mixture is pumped continuously in a circuit through the module, as indicated in the installation diagram (Figure 3). The solid matter content in the circulation tank is in- creased through separation from the permeate in the module. The water, which is carried away by the permeate, is supplemented by the inlet water. In this way, the solid matter concentration in the circulating tank gradually rises to the desired level. The method of transport in the charging process then removes the concentrate, and the pro- cedure is repeated with new material. In the case of a continuous operation, concentrate is continuously drawn from the circulation tank. A constant concentration level is established in the circulation tank in relation to the solid matter concentration in the inlet and the drawn-off quantities of permeate and concentrate.

Ultrafiltration is in use in numerous production installations in Germany, Switzerland, and Austria, although not in the automotive industry.

Advantages

Proved process from electrocoating technology Can be operated using both continu- ous operation and charging (batch) procedure

24 METAL FINISHING OCTOBER 1993

Paint

Inlet I I

Process container Coagulate accumulator

Figure 4. Electrophoresis as a recycling process for water-based paint: installation diagram (left) and process principle (right).

Relatively low energy consumption

Disadvantages

Strong mechanical demands placed on the paint material due to frequent pumping around the circuit at rela- tively high pressure Membranes must be cleaned or replaced periodically Paint must be ultrafilterable in all thinning stages Dissolved salts and resins are par- tially carried out with the permeate

0 Conducted dirt particles remain in the concentrate

Electrophoresis In the area of electrophoretic coat-

ing, this process is well known for the coagulation of solid paint onto metallic surfaces by creating an AC current from a painuwater mixture. Figure 4 shows the installation diagram and process principle as applied to water- based paint recycling. A rotating elec- trode that is connected to the positive pole and located in a work container with diluted water-based paint coagu- lates soli:! paint maiicr ta its siirface by forming hydrogen ions. A character- istic of this process is the fact that the water content of the paint concentrate or coagulate gained can be influenced only to a limited degree, generally between 40% and 60%.

Advantages

Proved process from electrocoating technology

0 Low mechanical demands placed on the paint material

0 Low level of maintenance 0 Produces a concentrate with a high

solid matter content in one step from a strongly diluted paidwater mix- ture

0 Simple apparatus 0 Paint concentrate is separated with a

low impurity content

Disadvantages

0 Paint must not be irreversibly changed by means of coagulation

0 Higher costs involved with posttreatment of paint concentrate

0 Relatively high energy consumption 0 Lack of functional security in the

case of very low concentrations

Evaporation Evaporation is a well-established

process for the concentration of water- diluted mixtures. Figure 5 shows the installation diagram and process prin- ciple for an evaporation plant of this type.

To avoid damaging the paint, it should not be heated above approxi- mately 35°C. It is therefore necessary to operate in a vacuum. The use of vapor compression means that the process principle is similar to that of the open heat pump. This obviates the need for additional heating and cool- ing. As with ultrafiltration, it is possi- ble to use both batch processing and continuous operation.

An installation that operates on the evaporation principle has successfully been in operation in Germany for several years for the recycling of UV-hardened water-based paint.

Advantages

0 Process operates independently of paint characteristics

0 Can be operated using both continu- ous operation and batch procedure

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~

Disadvantages

0 Relatively high energy consumption 0 High cleaning costs likely 0 Strong withdrawal of organic sol-

Evaporator Process Princii.de

Non-evaporated contents

Figure 5. Vaporization as a recycling process for water-based paint.

METAL FINISHING OCTOBER 1993 25

1 .J

I ?

[TI I 1

10

9

Figure 6. Specific power consumption for the continuous thickening process and diluted paint with 1% solids.

vents from the concentrate 0 Conducted dirt particles remain in

the concentrate

SELECTING THE PROCESS TECHNIQUE

The task of recycling water-based paint in the automotive industry in- volves producing a concentrate with a solid paint content of approximately 50% from booth wash water with a solid content of approximately 1%. At first glance, the simplest, most favora-

ble technique would appear to be a continuously operating installation in which the 1% paint dilution is sepa- rated into a solid-matter-free liquid and a 50% concentrate. Figure 6 shows the relevant specific power requirement with continuous operation for the dif- ferent processes as a function of the thickening concentration. The inlet is assumed to have a solid content of 1 %.

In the case of ultrafiltration, the specific power consumption per kil- ogram of recovered solid matter gener- ally increases as the solid concentra- tion rises. The results obtained from numerous experiments involving vari- ous types of modules, membrane mate- rials, and paint types indicated that power consumption can vary consider- ably in the high-solid-matter concen- tration range. Thus, for a 50% final solid matter concentration, ultrafiltra- tion requires between 3 and 12 kWh/kg of solids, whereas it consumes 0.5 kWhJkg at 5%.

The level of power consumption is not only an indication of the costs involved in the process. In the case of ultrafiltration, it also indicates the mechanical demands that will be placed on the paint during thickening owing to the pumping action.

In the case of the vacuum evaporator using vapor compression, the specific

Figure 7. Specific power consumption for thickening using charging process up to 50% solids.

power consumption rate also increases with the intended final concentration, although on a decreasing scale. The consumption level is above that for ultrafiltration up to a 40% solid matter concentration and only approaches its range above this percentage. However, because power is consumed mainly by the compressor in the case of this compression process, the mechanical demands placed on the paint are con- siderably lower than for ultrafiltration. On the other hand, the risk of paint sticking to the heat exchanger surfaces

Fl Spray booth

^. . . I I I I

Concentration Stage 2

I Solution I: UF installation I

Paint UF module concentrate 2

I Solution 11: Electrophoresis I

Figure 8. Spray booth with two water circuits and a two-stage paint recovery process. DI, deionized; UF, ultrafiltration.

26 METAL FINISHING OCTOBER 1993

TABLE 1. Calculation Data

Power costs Coagulating chemical costs Water content in the paint sludge Disposal costs for paint sludge Painting line capacity Exhaust air quantity for primer spray booth Additional costs for deionized water compared with city Paint application efficiency for primer paint Overspray quantity

water

0.25 DMlkWh 1 .OO DWkg of solids 50% 500 DMltonne of sludge or 1000 DMltonne of solids 50 Bodies/hr 420,000 m3/hr 0.30 DWm3 __ 65% 0.7 kg of solidslbody

is more problematic when numerous types of paints are used at higher solid concentrations. Electrophoresis is not shown because a final solid concentra- tion of only approximately 4040% can be achieved.

Because the power required to achieve a final concentration of 50% solid matter content in a continuously operating process is necessary for paint recycling, the power consumption for thickening using the charging process was also determined. Figure 7 shows the results, as a function of solid matter concentration, at the start of the thick- ening process for all three processes.

In the case of ultrafiltration, the power consumption for an initial con- centration of 1% solids, which is then thickened to 50% solids, is approxi- mately 0.5 kWhkg of solids, with this figure decreasing to very low levels as the initial concentration rises. The vacuum evaporator requires approxi- mately 4 kWh/kg of solids to concen- trate a 1 % paint dilution to a 50% paint concentrate. This level drops rapidly as the initial concentration increases.

The ratios for electrophoresis are shown in shaded form (Fig. 7) in the range 1-2% because reliable operating results are not available for this area. Furthermore, the power consumption for electrophoresis is even higher than that for vacuum evaporation using a vapor compressor, but it then drops to approximately 1 kWh/kg of solids at a 15% solid matter content at the hegin- ning of the concentration stage.

The high power consumption re- quired in a continuous operation, in the range of the desired final concentration of 50%, and in the batch process, with a very low initial concentration, would suggest the advisability of combining the processes of continuous thickening and charge concentration. The most favorable combination would then in- volve using ultrafiltration to initially increase the solid matter content from 1% to approximately 15% in the

continuous operation and then from 15% to 50% in a second plant stage.

Figure 8 shows the process diagrams for the three possible versions of this two-stage concentration process, with ultrafiltration, electrophoresis, and an evaporator as the second stage.

Today, a whole host of positive operational experiences with ultrafil- tration exist, and electrophoresis has been successfully tested in pilot plants. Vaporization can pose problems in the case of certain paint types with regard to the soiling of deposits and their sticking to heat exchanger surfaces, thereby entailing a degree of operational insecu- rity.

COST-EFFECTIVENESS To shed some light on the economic

aspects of water-based paint recovery, the costs of paint disposal from a primer booth by means of coagulation and waste disposal were compared with those involved in paint recycling. Table I shows the model calculation data used. We assumed a throughput rate of 50 car bodieshr and an overspray of ap- proximately 35 kg of solidshr.

Figure 9 illustrates disposal in a conventional spray booth based on the principle of sedimentation, with a decanter as the final thickening stage, or flotation, with thickening by means of a drainage container. In both cases it can be assumed that the solid matter content nf the sepaated paint s!udge is approximately 50%, thereby corre- sponding to the level required in paint recycling.

In the case of paint recovery, all versions were examined in accordance with Figure 8. It is assumed that the circulating water in the spray booths contains 1% solid paint, that 15% is achieved in the first concentration stage, and that 50% solid paint content is secured in the second stage.

To apply the data to installations with only slightly deviating throughput

rates and overspray quantities, the costs refer to 1 kg of solid overspray. Based on these figures, conventional paint disposal, as shown in Figure 9, returned coagulating and disposal costs of DM 2.25kg of solids. On the other hand, the recovery costs for paint using two-stage ultrafiltration (i.e., solution I in Figure 8) were DM 0.18kg of solid matter. This includes costs for cooling and the use of desalinated water in the spray booth, which represents a cost advantage for two-stage ultrafiltration compared with paint sludge disposal of DM 2.07kg of solids. The cost advan- tage is enhanced by the value of the recovered paint concentrate.

The comparisons are illustrated in Figure 10. Two cases are shown: (1) when 100% of the overspray is added to the recycling process, and (2) when 80% of the overspray is added. The previously mentioned disposal costs apply to the proportion of overspray that is not recovered.

The curves in Figure 10 show that the potential profit associated with the value of the recovered paint concen- trate increases enormously. Even at DM 1Okg of solids (Le., if the costs of processing to produce paint are DM 10 lower than a new quantity of paint with 1 kg of solids), the profit adds up to DM 9.65 or DM 12.07kg of solids, respectively.

The result is basically the same if electrophoresis or evaporation is used 2s the second stage of the precess. In the case of electrophoresis, the profit would be reduced in a linear fashion by DM 0.24kg of solids and by DM 0.02kg of solids for evaporation. This means that the choice of the second stage of the process can be made solely on the basis of plant and process considerations.

Today, the choice would seem to be between ultrafiltration and electro- phoresis. Ultrafiltration is already in use in production installations and is a proved process; however, it requires

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METAL FINISHING OCTOBER 1993 27

.t

* 4

Disposal according to the Sedimentation Principle

,,.::- .:, :.:+. .:.:.:.:... ..... y.:.:.:.:.:. i .................................................. :Decanter! ,+, , i

...............................

Paint sludge

Disposal according to the Flotation Principle

II n 9 II

Figure 9. Spray booth disposal by coagulation.

25

discontinuous operation during the second stage, which necessitates a greater degree of control. Furthermore, all dirt particles from the booth wash water remain in the paint concentrate during co~centratior, hlr -3 ultrafiltration, thereby compromising the quality of

10 With electrophoresis, the installa- tion can be operated continuously (Le.,

1 with a low degree of extra control). 8 5 Furthermore, with the electrophoresis i process, essentially only solid paint is r 1 I ~ 1 extracted, which means that a low dirt B -D content in the concentrate can be

Figure 10. Profit per kilogram of paint solids It can therefore be expected that both as a function of the value of the recovered processes will become co"onPlace paint concentrate. in practice. The type of paint used will

P g 2o

k

2 B :: b . s d B 1 the recovered paint.

5

IDMOl expected. Value of recoveredpaint solid malfer

determine the appropriate process in each case. If one assumes a rate of 50 car bodies/hr, an overspray of 0.7 kg of solidshody, 15 hr of production/day, and 220 workdaydyear, this will result in a total annual overspray solid quan- tity of 115,000 kg. This corresponds to costs of approximately DM 260,000 in the case of coagulation waste disposal.

If coagulation were to be replaced by a two-stage ultrafiltration process, and if the processing of concentrate into paint were to cost the same as new paint (value of the recovered solid paint matter is DM O/kg, an 80% or 100% recovery of the overspray would yield annual savings of DM 190,000 and DM 240,000, respectively.

If processing per kilogram of solids were to cost DM 5 less than the corresponding quantity of new paint (value of the recovered solid paint is DM 5kg of solids), the savings would be DM 650,000 and DM 815,000, respectively. If the value of the recy- cled paint concentrate were 10 DMkg of solids, the figures would be DM 1,114,000 and DM 1,394,000, respec- tively.

Against these savings must be weighed the, additional investment for paint recovery using a two-stage ul- trafiltration process compared with that for paint coagulation with (approxi- mately DM 220,000) and without (ap- proximately DM 440,000) a decanter stage for the size of painting installa- tion described.

If electrophoresis or evaporation were used in the second stage instead of ultrafiltration, the corresponding additional investment costs would be approximately DM 450,000 and DM 670,000, respectively. If we then com- pare these additional investment costs with the profit from paint recovery, the system would pay for itself in only approximately two years even in the least favorable case. The payhack pe- riod drops below one year as the value of the recovered solid paint matter increases.

What do these figures look like if a new painting installation is built but the processing of recovered water paint concentrate cannot be introduced be- cause, for example, there are still quality problems or obstacles to reuse due to the range of paints being used, as is the case with base coats? Even assuming the most unfavorable case, in which the costs involved in disposing

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28 METAL FINISHING OCTOBER 1993

I

of the paint concentrate are as high as those associated with the paint coagu- late, the savings would still be DM l.O7/kg of solids, or approximately DM 125,00O/year. This means that, even in this case, the additional invest- ment costs would pay for themselves within approximately two to four years. Thus, as soon as paint quality is ready for production for paint recov- ery, the savings mentioned above can be realized without additional invest- ment. This clearly suggests that new paint installations should be equipped immediately with appropriate paint particle concentration equipment.

Eisenmann has equipped three pro- duction plants with paint recovery installations in various areas outside the automotive industry. Although the quantities of paint overspray are con- siderably lower in these plants, it was possible to achieve payback periods of two years and less.

Eisenmann has contracts to build pilot plants for two-stage paint recy- cling by means of ultrafiltration and electrophoresis. Ope1 was scheduled to

commence paint recovery by means of ultrafiltration in Eisenach this summer, which means that operational experi- ence will soon also be available from the automotive industry.

There are still misgivings associated with water-based paint recovery, in- volving in particular the concern that reuse would be made impossible owing to the multiplicity of paint colors. Experiments at Mercedes Benz in the area of primer plants have shown that it is possible to produce a gray paint color from the mixture of recycled paints, which can then be reused in production. With regard to base coats, experiments are not yet quite so advanced because in this case the range of paints is still considerably greater, and recycling would only be conceivable if the recov- ered concentrate can be used in other areas, such as in the primer.

The problems of a broad range of paints do not exist in the area of clear coat spraying, but sensitivity to accu- mulated dirt particles is particularly acute. This would tend to suggest that the use of a two-stage ultrafiltration

process plus electrophoresis promises particular success.

CONCLUSIONS Water-based paint recycling is al-

ready a reality in numerous production plants outside the automotive indusq, with ultrafiltration being the most popular process. Based on the require- ments of the automotive industry, the use of two-stage processes is recom- mended, the first stage being an ul- trafiltration installation and the second stage an ultrafiltration or electro- phoresis installation for reasons of energy and operational security.

The existing potential gains as a result of the recovery of water-based paints are so high that the recycling processes described should be installed even if materials recycling has not yet been tested in production in the actual painting installations of the automotive industry. The additional investment costs would pay for themselves in approximately two to four years, even if the accompanying concentrates were to be disposed of as waste. MF

-

you need it done quickly!

PAUL MUELLER COMPANY P.O. Box 828 / Springfield, Missouri, U.S.A. 65801-0828

Facsimile: (41 7) 831 -6642 200

Circle 025 on reader information card

METAL FINISHING OCTOBER 1993 29

reappear at any time in the future . I'm a freshman su- unless all possible sources are exam Cratering and Fish Eyes

pervisor at a very

cratering in wet paint. A base spots started to

Recently, I had my i

There are many possibilities, the finishing

Q coat o-f an alkyd primer. A-fter a quick inspection of a number of painted parts, I was at a loss to explain the cause of the problems. Our senior spray painter was on an extended vacation, and none of the other painters had enough experience to offer any help. Reacting to the situation, we decided to scuff sand the craters and spray on the topcoat, which is an alkyd enamel. The small spots in the primer seemed to enlarge in size with the top coating. After a little research with our paint vendor, we corrected the immediate problem with a solution of fish eye additive. We were able to salvage several hours of production, and, strange as it seems, the problem went away as quickly as it first appeared. We were unable to find the cause of the fish eye craters, and I have been told that it may reoccur at any time unless we find the source of the problem. Realizing that there may be any number of areas to consider, I thought you could help with some suggestions as to where to start investigating.

J.V. A Craters, or fish eyes, as they are

ok identical, subtle

through the cloth for several and observe the cloth for any

Use this procedure sev- the day. Even if the

Another possible source to check is fie sometimes called, have been a / the maintenance department. Have nemesis for painters for many years. Automobile and furniture refinishers head :he !ist ~f thme with the m ~ s + experience in correcting or resolvin the fish eye problem, since they expe t to see craters and fish eyes in most I f their painting and refinishing work. The cause of the problem is silicone, which is an element found in many cleaners and polishes that develop the shiny look so desirable on our cars and furniture. However, in the production paint shop, a completely different set of circumstances exists. First, consider yourself lucky that the problem was short lived. Second, the problem may

they changed any materials or proce- dures? Sometimes a material, such as grease or !u?xiczting oi!, wi!l be changed with all good intentions and will eventually be found to be a problem in the paint shop. Employees using hand cream can transfer contami- nants to the parts prior to painting. A smudge or two of some brands of hand cream have been known to shut down paint shops for days before the cause was eliminated.

Parts cleaning can be suspect if there has been any change in the process or materials. Is it possible that hand wiping with a contaminated solvent or

incompatible cleaner was used on any parts that needed extra cleaning?

The reasons for cratering and fish eyes in the paint shop are numerous. Because I feel that the subject deserves more attention, I'd like to invite our readers to share their experiences in trouble-shooting the causes of crater and fish eye problems and the correc- tive actions taken to resolve them. Fill out and return the enclosed Shop Problem card in this issue with your comments.

Poor Degreasing Can Cause Adhesion Failures

Our painting and finishing op- Q e eration utilizes both liquid and powder coatings. When our regular production was affected by a business downturn, we took on a couple of contract finishing jobs. One contract was to coat several thousand steel parts with polyester powder coating. We had been servicing a customer who had given us

\ some michining and welding iobs in the the first timethat we had

coating for them, and

by our customer. The finished parts were wrapped and shipped by our customer to a warehouse, where they were to be used in the assembly of electronic devices.

that 75% of the finished parts were showing coating failure. The powder was flaking off the surface and showing rusty metal where the coating had failed. This was not an overall failure. Some parts had one or two spots; others had

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After a few weeks in storage we found ~~

30 0 Copyright Elsevier Science Publishing Co., Inc. METAL FINISHING OCTOBER 1993