WO1991019837A1

WO1991019837A1 - Electrodeposition coating baths and processes for controlling iron ion levels in the bath

Info

Publication number: WO1991019837A1
Application number: PCT/US1991/004303
Authority: WO
Inventors: Rodney W. Stockstad; James A. Prevost; Michael J. Bourdeau
Original assignee: The Valspar Corporation
Priority date: 1990-06-14
Filing date: 1991-06-14
Publication date: 1991-12-26
Also published as: AU8189391A

Abstract

The concentration of contaminant iron in an electrodeposition bath may be controlled by incorporating in the bath an iron ionization inhibitor such as an azole. Ultrafiltration, with optional removal of iron from the ultrafiltrate by ion exchange or reverse osmosis, may concurrently be used to control iron contamination of the bath. The latter procedure can be made more effective by use of an iron complexing agent such as 1,10-phenanthroline in the bath to increase the permeation of iron through the ultrafilter.

Description

ELECTRODEPOSITION COATING BATHS AND PROCESSES FOR CONTROLLING IRON ION LEVELS IN THE BATH

FIELD OF THE INVENTION

The invention relates to the field of electrodeposition coating baths of the type employed to provide organic protective coatings upon metal cathode workpiece surfaces.

BACKGROUND OF THE INVENTION

Electrodepositable coating compositions of the type known to the art commonly comprise a water dispersion of a cathodic, ionized resin which, under the influence of an electric field, is drawn to and deposits upon a cathodic workpiece surface. Cathodic electrodeposition coating compositions are described, for example, in U.S. patents 3,663,403 and 4,218,296. The electrodeposition bath commonly includes not only the ionizable resin, but also may include such ingredients as pigments, leveling and flow agents, additional resins and the like which are to be incorporated into the coating that is formed upon the workpiece cathode. It has generally been found that electrodeposition coating compositions are rather fragile when compared to paint that is adapted to be sprayed or brushed onto a surface. The coating baths employed for electrodeposition coating procedures commonly are carefully formulated and maintained; changes in such conditions as temperature and pH and the addition of even small amounts of unwanted •materials to the bath can result in poor coating properties.

Iron ions are common contaminants of electrodeposition coating compositions employed in baths into which objects to be painted are immersed. Iron ion sources include equipment such as pump housings and shafts, piping, process tanks and the like, or, more often, iron parts or iron turnings, powder or scrap which have inadvertently dropped into the bath during the coating procedure, and incompletely removed metal pretreatment chemicals such as iron phosphate. Iron ions that are derived from these ferrous surfaces can be detrimental to the coating that ultimately is formed. One such problem, identified in U.S. patent 4,218,296 (Gilchrist) is that of staining of the resulting coating. Another problem involves the ability of coatings to withstand weathering; coatings derived from cathodic electrodeposition paint baths having substantial concentrations of iron (e.g., above about 40 ppm) do not withstand weathering well.

Several partial solutions to the problem of iron contamination of cathodic electrodeposition baths have been advanced. In U.S. patent 4,218,296 (Gilchrist), particular quantities of phosphoric acid are incorporated in the bath to provide H₂PO^ ions. U.S. patents 3,663,398 and 3,663,403 refer to a method of removing deleterious ions such as iron from an electrodeposition bath through a process in which a portion of the bath is first subjected to ultrafiltration to separate the dispersed resin from an aqueous filtrate which contains deleterious ions to be removed, and then treating the filtrate with an ion exchange resin to remove the deleterious ions. Both anionic and cationic ion exchange resin columns may be used. The ultrafiltrate, after passing through the ion exchange resin, may be returned to the bath. Further compositions such as 1,10-phenanthroline may be incorporated in cathodic electrodeposition coating compositions, as shown in Anderson, et al. U.S. patent 4,511,692, and materials of this type form complexes with iron ions and permit them to permeate more readily through ultrafilter membranes so as to increase the iron ion-removing efficiency of the ultrafilter/ion exchange processes as shown in U.S. patent 3,663,403. The use of 1,10-phenanthroline, in combination with ultrafiltration/ion exchange processes to remove iron ions from a cathodic electrodeposition bath is helpful in controlling the level of iron in the bath. However, this process by itself requires frequent and expensive regeneration of the ion exchange resin columns. Moreover, this process has only been marginally effective in maintaining low concentrations of iron in cathodic electrodeposition baths.

SUMMARY OF THE INVENTION

We have found that the level of ionized iron in an electrodeposition bath can be substantially reduced by incorporating in the bath an iron ionization-inhibiting material such as an azole. In this manner, the rate of iron ionization in the bath can be controlled, and known ultrafiltration and ion exchange procedures can be employed to maintain the iron ion level of the bath at an acceptable level, eg., below about 40 ppm and preferably below about 25 ppm.

Thus, in one embodiment, the invention relates to a process for controlling the concentration of iron ions in an electrodeposition coating bath which includes a synthetic resin ionically dispersed in the bath, comprising, in combination: A. Incorporating in the bath a composition capable of significantly reducing the rate of ionization of iron from iron surfaces in the bath, and

B. Removing iron ions from the bath by subjecting a portion of the electrodeposition bath to ultrafiltration.

The ultrafiltrate comprises an aqueous, resin-free solution containing iron ions. Desirably, iron ions are removed from the ultrafiltrate, as by the use of ion exchange or reverse osmosis procedures, and the ultrafiltrate is then returned to the bath.

Preferably, the method also includes the step of incorporating in the bath an iron ion complexing agent such as a chelating agent capable of increasing the rate of iron ion permeation from the bath through the ultrafilter.

In another embodiment, the invention relates to a water-based electrodeposition bath comprising a cathodic, ionized resin dispersed in water, and an effective concentration (desirably from about 0.01% to about 1% based on the weight of the bath) of an iron ionization inhibiting composition capable of significantly inhibiting the rate of iron ionization from an iron surface exposed to the bath.

The iron ionization inhibiting compositions of the invention preferably are azoles, that is, an organic compound with a 5-membered N-heterocycle that contains in its heterocyclic ring two double bonds, one or more carbon atoms, and which may also contain a sulfur atom. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrodeposition coating baths which may be improved in accordance with the present invention can be any electrodeposition composition that employs one or more cathodic resins which, in aqueous dispersion, provide resin cations that are attracted to a workpiece cathode upon application of an electric current. One such composition is shown in Anderson, et al. U.S. patent 4,511,692 which discloses an alkyd-based cationic resin. "Cationic resin" refers to a resin which includes cationic sites (e.g., from the presence of protonated amine groups or onium groups), and which may include, as an anion, any of a variety of acids such as acetic, formic, dimethylolpropionic, lactic, and propionic acids. Presently, the most widely used cathodic electrodeposition vehicle resins are synthetic, polybasic, modified epoxy resins containing amino functional groups, such as are described in U.S. patents 4,639,299; 4,714,531; 4,742,097; 4,769,420, and 4,788,260, which are incorporated herein by reference. Other cathodic electrodeposition vehicle resins derived from butadiene or acrylic based resins and which may contain amino, sulfonium or phosphonium functional groups are described in U.S. patents 4,231,907; 4,265,795; 4,714,532, and 4,812,215 which are also incorporated herein by reference.

Electrodeposition coating compositions of the type disclosed herein may include such additional ingredients as pigments, organic solvents, antioxidants to enhance stability over extended periods of time, pigment dispersing aids, anti-crater aids, bactericides and anti-foaming agents.

In addition to the one or more electrodepositable vehicle resins, there may be present in the electrodepositable cathodic coating composition of the invention other resinous materials which do not contain amine or onium groups. Examples include special pigment dispersion vehicles or crosslinking materials such as amine-aldehyde condensation products or blocked isocyanate based resins. The iron ionization-inhibiting compositions used in the present invention contain one or more compounds which, in effective concentrations of less than about 1 % (and particularly 0.01% to 1.0%) by weight based upon the weight of the bath, are effective in significantly inhibiting the rate of iron ionization. These compounds preferably are azoles, and the azoles that are employed herein are organic compounds which have a 5-membered N-heterocycle containing two double bonds. The azoles employed in the invention include in the heterocyclic ring one or more nitrogen atoms and one or more carbon atoms and may include a sulfur atom. Desirably, azoles employed in the invention exhibit only limited solubility in the electrodeposition baths. Further, those azoles having high electropositivity (the ability to donate electron pairs) appear to yield the best results and are preferred. The triazoles and thiazoles are preferred, and a particularly preferred azole of the invention is benzotriazole. Other triazoles include 5,6-dimethyl benzotriazole, 5-methyl benzotriazole, and 1,2,4-triazole. Thiazoles that can be employed in the invention include benzothiazole, 2-amino thiazole, thiazole, and 2,4-thiazolidinedione.

It has been found that certain compounds, particularly certain azoles, are excellent in retarding iron ionization when used in accordance with the invention. The ionization- inhibiting compounds employed in this invention are those which have a significant iron ionization inhibiting effect, and candidate compounds may be screened by their ability to inhibit iron ionization from contaminating iron surfaces in stirred cathodic electrodeposition coating baths. The ability of a compound to significantly inhibit iron ionization is demonstrated by its ability to limit iron ion concentration in an iron-contaminated bath to a value which is at least 20% less than that which would have been attained had the candidate compound not been incorporated in the bath, after 28 days with moderate stirring. The iron ionization rate from a contaminating ferrous source in an electrodeposition coating bath depends upon various parameters among which may be listed the surface area of iron exposed to the bath and the volume of the bath, and the intensity with which the bath is agitated, ionization tending to increase as the ratio of ferrous surface to volume of the bath increases and also as the rate of agitation increases. The ability of a candidate ionization inhibitor to inhibit the production of iron ions may be determined utilizing the following testing technique:

A rectangular polyethylene container (24.5cm in height x 12.3cm x 12.3cm) is equipped with a 1550 RPM electric stirrer motor to which a polyethylene stirring blade is attached.

An alkyd-based electrodeposition bath as described in Example I (3000 g) at room temperature is added to the polyethylene container, the stirring blade is positioned about 2.5 cm from the container bottom and the agitation started. To the container is added 3.0 g of a candidate iron ionization inhibitor and the bath is mixed uniformly.

An emery cloth cleaned and acetone-rinsed 24 gauge steel coupon measuring 29mm x 29mm is suspended 3 cm below the bath surface. Bath samples are withdrawn periodically and analyzed for iron via normal atomic absorption or direct current plasma analytical techniques. Iron concentration levels (in ppm) of the bath containing the candidate inhibitor are measured at 7, 14, 21, and 28 days and compared to the iron levels of a control bath containing no candidate iron ionization inhibitor. Although differences in bath iron levels of about 20% after 28 days are deemed significant, differences of 50% or greater after 28 days are preferred.

The "ultrafilters" which are used are filters of known type which utilize semi-permeable membranes having large surface areas exposed to flow of paint from the bath. In a typical situation, a small portion of the bath is continuously pumped into contact with the ultrafilter membrane, the paint portion that does not pass through the ultrafilter being returned to the bath. Under a pressure differential, the ultrafilter membrane separates from the paint flow a watery ultrafiltrate which may include solvent and low molecular weight materials (including iron ions and iron complexes) which are dissolved in the ultrafiltrate. The ultrafiltrate itself is generally transparent. Ultrafilters of the type described are known in the field and are commercially available from, for example, Romicon Corporation, Koch Membrane Systems, Inc., Osmonics, Inc., Rhone-Poulenc, and others.

The ultrafilters are desirably continuously supplied with a flow of paint from the bath, and the pressure differential across the ultrafilters is kept substantially constant. As the filters become fouled or clogged with paint solids (pigment, resin solids and the like), they may be readily regenerated. It should be understood that the ultrafilter membranes commonly are permeable to and pass many anions and cations, including those which are desired to be retained in the paint bath. With respect to desirable bath ingredients •that are removed from the paint bath by the ultrafilter and are not returned to the bath, make-up quantities of these ingredients may be periodically added to the bath. The electrodeposition paint baths of the invention also desirably contain one or more iron ion complexing agents that are capable of complexing with iron to improve the permeation of iron through the ultrafilters that are employed. Chelating agents are preferred, such as 1,10-phenanthroline, 2,4-pentanedione and 2,2'-bipyridyl. One of these agents, 1,10-phenanthroline, is commercially available from Vanderbilt Chemical Co. as an ingredient of its "Activ-8" product and has long been used as an ingredient in electrodeposition coating baths as shown in Anderson, et al. U.S. patent 4,511,692. Such materials preferably are used in concentrations of from about 0.025% to about 1.00% by weight of the bath. It has been found that the ability of such materials to increase permeation of iron through an ultrafilter is a function of the concentration of these compositions in the bath. Concentrations of above about .02% by weight, and preferably from .025% to .5% by weight are preferred. The ability of an iron complexing composition such as 1,10-phenanthroline to significantly increase permeation of iron ions through an ultrafilter can be readily determined by incorporating a small concentration - for example, about 0.1% by weight, of a candidate iron complexing agent in an electrodeposition paint bath, subjecting the bath to ultrafiltration, measuring the concentration of iron ion in the ultrafiltrate, and comparing that value to the value obtained from the same procedure but in which no candidate iron complexing agent was employed. Iron permeation may be said to be significantly increased by a candidate iron -complexing agent if the agent has the effect of increasing the concentration of iron ion in the ultrafiltrate by at least 50%. and desirably has the effect of decreasing the difference in iron ion concentration between the bath and filtrate by at least 10%.

Iron ions may be removed from the ultrafiltrate by any of several known ion-removal methods, one of which employs an ion exchange procedure using one or more ion exchange resins through which the ultrafiltrate is passed and another of which employs reverse osmosis. Ion exchange resins that are employed in the present invention may be any of those that are commercially available and are useful for removing iron ions from an aqueous solution. Ion exchange resins of the type described are commercially available from such suppliers as Dow Chemical Corporation, Biorad Corporation, Omnitech and Rohm and Haas, and the Dow Chemical M-33 ion exchange resin has given good results. It is appropriate, when using ion exchange resins, to monitor the iron ion concentrations upstream and downstream from the ion exchange resin column; when the downstream iron ion concentration rises, the ion exchange resin column can be readily regenerated by known means.

The known reverse osmosis process involves the use of semi-permeable membranes which reject the passage of charged chemical species such as iron ions and complexes under pressure differentials. Appropriate reverse osmosis membranes are commercially available, as from Osmonics Corporation as their product designations SF 10, ST 10 and SS 10. In use, ultrafiltrate from the ultrafilter is conveyed under pressure, as by pumping, to one side of a large surface reverse osmosis membrane, and charged ions to be removed from the ultrafiltrate are retained by the membrane while water and neutral, water-soluble species pass through the membrane, all in a known manner. If desired, both ion exchange and reverse osmosis could be used, in parallel or in series, to remove iron ions from the ultrafiltrate, but preferably only one or the other of these ion-removal systems is employed at one time.

The ultrafiltrate from the ultrafiltration step may, if desired, simply be discarded, if this is feasible, in which case iron ions need not be removed from the ultrafiltrate. If, as is preferred, the ultrafiltrate is subjected to a subsequent iron removal step, as by ion exchange or reverse osmosis, the resulting largely iron-free liquid may be returned directly to the bath or may be employed to rinse the painted articles after they have emerged from the paint bath. This preferred procedure has apparent pollution control benefits, and serves as well to return to the bath those desired bath ingredients that would be lost were the ultrafiltrate to be discarded.

The invention may be more readily understood by reference to the following non-limiting examples:

Example 1

This example illustrates the use of an electrodeposition coating composition to evaluate the efficacy of various candidate iron ionization inhibiting materials. The coating composition was that described in Example XII of U.S. patent 4,914,139, incorporated herein by reference. Aliquots (3000 g.) of the composition were placed in each of several containers, and the previously mentioned screening procedure was followed except that, where noted, the concentration of iron ionization inhibitor was varied. All candidate materials were post-added to the •agitating bath as either a 20% wt/wt solution in l-propoxy-2-propanol or in a 1/1 wt/wt blend of l-propoxy-2-propanol/acetone. The "background" concentration of iron ions in an aliquot of the coating composition containing no iron coupons nor ionization inhibiting compositions was measured at 6 ppm. The iron ion concentrations reported in the following tables represent increases in iron ion concentration above this "background" concentration.

Cone. PPM Iron Bath Level Wat

Candidate Material Wt .% 7 Days 14 Days 21 Days 28 Days Solubili

42 54 68 85

3 4

1 2

<1 <1

15 21

<1 <1

11 14

36 45

42 55

3 1

3 0

13 12

11 15

5 10

* Added as a powder

**Added to the anhydrous paint prior to inversion to bath with deionized wate . "d" - sparingly soluble "v" - very soluble Example 2

The level of iron ionized into a cathodic electrodeposition bath from exposed iron surfaces is a function of the exposed surface area of the iron articles in the bath and the agitation rate, among other variables. This example illustrates the effect of exposed surface area of a source of iron ions. Four baths were prepared according to the composition of

Example I and exposed to either the standard surface area of 29mm x 29mm, 5 x standard, or 10 x standard.

In this example, one candidate iron ionization inhibitor, benzotriazole, was also evaluated at 0.1% and 0.2% by weight on the bath.

Benzotriazole PPM Bath Iron Level Area Cone. , % 3 Days 7 Days 16 Days 21 Days

2 0 1 <1

Example 3

This example illustrates the effect of agitation.

The bath of Example 1 was placed in a polyethylene container equipped with a magnetic drive teel pump

(Model No. 1P676A, 1/55 hp. , 1630 RPM) connected to inlet/outlet holes in the container via inert polypropylene tubing and PVC nipples. The shear condition and flow rates created by such a pumping cell are considerably greater than those experienced by the exposed bath in the aforedescribed stirring apparatus.

For this study, the exposed iron surface area was 4xStd.

Benzotriazole, Mixing PPM Bath Iron Level % Method 3 Days 7 Days

Example 4 The following candidate ionization inhibitors were evaluated for effectiveness in the electrodeposition bath of Example 1, measurement of iron concentration being made after 28 days in accordance with the earlier described testing technique. Candidate PPM Bath Iron Level 0 28 Days Water Sol.

d d d unk. v

d

Example 6 An acrylic copolymer was prepared as shown in Example No. 4 of U.S. patent 3,922,212 (Gilchrist, incorporated herein by reference) . This resin was combined as hereafter described with a melamine crosslinker resin and a pigment dispersion prepared using a non-functional alkyd resin and rutile titanium dioxide pigment. This blend was subsequently neutralized with lactic acid and reduced with deionized water according to the formula below to give a cathodic electrodeposition bath at 10% bath solids (non-volatile matter), a pigment/binder ratio of 0.37, a base resin/crosslinker solids ratio of 75/25 and an 85% theoretical neutralization level. The bath so prepared was split into two 3000 g baths and evaluated with and without benzotriazole (0.1 wt. % of bath) using the previously described stirring test procedure. Component Parts by Wqt Comments

Acrylic copolymer 366.0 Add ingredients in order with good Cymel 1130* 115.0 agitation. Pigment dispersion 253.0 Lactic acid (88%) 25.0 Deionized water 5541.0 Reduce slowly with good agitation.

Total 6300.0

*American Cyanamid Company Benzotriazole PPM Bath Iron Level § 7 Days

Without 95

With 31

EXAMPLE 7 A paint preparation was prepared according to Example 1. Each of four aliquots of the preparation was exposed to a steel coupon for varying periods of time to provide varying concentrations of iron. Paint films were electrocoated over iron phosphated cold rolled steel panels to a 1.0-1.2 mil film thickness and were cured for 30 minutes at 250°F. The panels were aged at room temperature for one week and then were exposed to the weather in Florida, tilted at a 45° angle to the horizontal, for a period of nine months. The gloss of the painted surfaces was measured before weathering and thereafter at 3-month intervals. The gloss readings at an angle of 20° are reported as follows. This example shows the deleterious effect upon weathering of iron ions in paints.

Florida Weathering 20 De ree Gloss Measurements

EXAMPLE 8 To a commercially available electrodeposition paint bath of the type described above in Example 1 was added a candidate complexing agent capable of chelating iron ions in the bath. A portion of the bath was continuously pumped to an ultrafilter, and the iron ion levels of the bath and of the ultrafiltrate were measured. Two candidate chelating agents were tested. One of these, 1,10-phenanthroline, was tested at bath concentrations of 0.025%, 0.05% and .25%. This material is commercially available from Vanderbilt Chemical Company as the principal component of its "Activ-8" product. A second candidate compound was 2,4-pentanedione, and this material was tested at bath levels of 0.033%, .1%, .5% and 1.0%. The data indicate that these compounds were effective in increasing the permeability of iron through the ultrafilter at all concentrations that were tested, but that the effectiveness of the candidate compounds increased as their concentration increased.

EXAMPLE 9 An aliquot was taken from a commercial electrodeposition paint bath which had acquired an iron ion concentration of about 46 parts per million. The aliquot was subjected to ultrafiltration on a continuous basis. For a period of approximately 8 hours each day, the ultrafiltrate stream was processed through a reverse osmosis unit employing a commercial reverse osmosis membrane (Osmonics SF 10) . Iron ion levels were measured in the bath and in the ultrafiltrate at the beginning of each 8-hour period, and were measured in the reverse osmosis ("R.O.") permeate and the reverse osmosis ("R.O.") retentate at the beginning and the end of each 8-hour period. The reverse osmosis permeate was returned to the bath. Retentate was discarded at the end of each 8 hour period. Ultrafiltrate was subjected to reverse osmosis only during the 8-hour periods; the rest of the time the ultrafiltrate was returned directly to the bath.

Over the approximately 1-week period, the iron ion concentration in the bath dropped from 46 parts per million to 28 parts per million.

Iron Levels, PPM

While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

Claims

CLAIMS :

1. A water-based cathodic electrodeposition coating composition comprising a cathodic, ionized resin dispersed in water, and an effective concentration of an iron ionization-inhibitor capable of significantly inhibiting ionization of iron from an iron surface exposed to the bath.

2. The coating composition of claim 1 wherein the ionization-inhibitor comprises an azole containing in its heterocyclic ring one or more carbon or sulfur atoms.

3. The coating composition of claim 2 in which the azole is present in a concentration of from about 0.01% to about 1.0% based on the weight of the bath.

4. The coating composition of claim 2 in which the azole is a triazole or a thiazole.

5. The coating composition of claim 2 in which the azole is a benzotriazole.

6. The coating composition of claim 1 in which the iron ionization-inhibitor is benzotriazole.

7. The coating composition of claim 2 in which the azole is 5,6-dimethylbenzotriazole.

8. The coating composition of claim 2 in which the azole is 5-methylbenzotriazole.

9. The coating composition of claim 2 in which the azole is a benzothiazole.

10. The coating composition of claim 2 in which the azole is benzothiazole.

11. The coating composition of claim 2 in which the azole is thiazole.

12. The coating composition of claim 2 in which the azole is 2-aminothiazole.

13. The coating composition of any of claims 1-12 including an iron ion complexing agent.

14. The coating composition of claim 13 wherein the iron ion complexing agent is at a concentration of at least about 0.025% based on the weight of the bath.

15. The coating composition of claim 1 including an iron ion chelating agent.

16. An electrodeposition coating system comprising a bath containing a water-based cathodic electrodeposition coating composition comprising an ionized resin dispersed in water and an effective concentration of a iron ionization-inhibitor capable of significantly inhibiting ionization of iron from an iron surface in the bath, and an ultrafilter for removing iron from the bath, the coating composition including an iron ion complexing agent capable of significantly increasing the rate of iron permeation through the ultrafilter.

17. The coating system of claim 16 wherein the iron ion complexing agent is at a concentration of at least about 0.025% by weight of the bath.

18. The coating system of claim 16 wherein the iron ion complexing agent is a chelating agent.

19. The coating system of claim 16 wherein the iron ion complexing agent is 1,10-phenanthroline, 2,4-pentanedione or 2,2'-bipyridyl.

20. The coating system of any of claims 16-19 wherein the iron ionization inhibitor is an azole.

21. An electrodeposition process for applying a coating to a workpiece cathode while restraining the ionization of iron from ferrous surfaces in the bath, the process utilizing a water-based electrodeposition bath including a cathodic resin dispersed in water, the process comprising incorporating in the bath from about 0.01% to about 1% based on the weight of the bath of an iron ionization-inhibitor capable of significantly inhibiting the ionization of iron from an iron surface in the bath.

22. The method of claim 21 wherein the iron ionization-inhibitor is an azole containing in its heterocyclic ring one or more carbon or sulfur atoms.

23. The method of claim 21 including the step of removing iron from the bath by subjecting a portion of the bath to ultrafiltration.

24. The method of claim 23 including the step of incorporating in the bath an iron ion complexing agent capable of significantly increasing the rate of permeation of iron through an ultrafilter during ultrafiltration to produce an iron-containing ultrafiltrate.

25. The method of claim 23 wherein the iron ion complexing agent is at a concentration of at least 0.025% based upon the weight of the bath.

26. The method of claim 24 wherein the iron ion complexing agent is a chelating agent.

27. The method of claim 26 wherein the chelating agent is 1,10-phenanthroline, 2,4-pentanedione or 2,2'-bipyridyl.

28. The method of claim 23 including the step of removing iron from the ultrafiltrate and returning the resulting composition to the bath.

29. The method of claim 28 wherein iron is removed from the ultrafiltrate by subjecting the ultrafiltrate to ibn exchange.

30. The method of claim 28 wherein iron is removed from the ultrafiltrate by subjecting the ultrafiltrate to reverse osmosis.

31. An electrodeposition process for applying a coating to a workpiece cathode from an electrodeposition bath and including the step of subjecting the bath to an ultrafilter to remove from the bath iron that has ionized from an iron source in the bath, the process including incorporating in the bath (a) an iron ionization inhibitor capable of significantly inhibiting the ionization of iron from said source, and (b) an iron complexing agent capable of significantly increasing the rate of iron permeation through the ultrafilter.

32. The process of claim 31 wherein the iron complexing agent is at a concentration of at least 0.025% based on the weight of the bath.

33. The process of claim 31 wherein the iron ionization inhibitor is an azole containing in its heterocyclic ring one or more carbon or sulfur or oxygen atoms, the azole being present in a concentration of from about 0.01% to about 1.0% based on the weight of the bath.

34. The electrodeposition process of claim 33 in which the azole is benzotriazole.

35. The electrodeposition process of claim 33 in which the iron ion complexing agent is a chelating agent.

36. The electrodeposition process of claim 35 in which the chelating agent is 1,10-phenanthroline,

2,4-pentanedione or 2,2'-bipyridyl.