CA1134725A - Non-chromate conversion coatings - Google Patents

Abstract

RC-1456-ri011 NON-CHROMATE CONVERSION COATINGS

ABSTRACT

Improved color, brightness and corrosion resistance are imparted to metal surfaces such as zinc plated surfaces, by treat-ment with a non-toxic solution comprised of sulfuric acid, hydro-gen peroxide, a silicate and at least one cationic triarylmethane dye. Certain organophosphorus compound additives further enhance the corrosion resistance.

Description

~1347Z5 BACKGROUND OF THE INVENTION

The formation of chromate conversion coatings on surfaces of various metals, such as zinc and cadmium, is presently the most common technique of imparting increased brightness and S corrosion resistance to the metal. In a typical process, the metal work pieces are immersed in an acidic solution containing hexavalent chromium compounds, which react with the metal causing the precipitation of a complex gel-like coating or film of trivalent chromium and entrapped soluble he~avalent chromium compounds onto the metal surface. The coated work pieces are then rinsed and dried under controlled conditions.
There are several serious disadvantages common to all chromate conversion coating processes. One of these is the relatively short life of the process bath expressed in terms ofunit surface area coated per unit volume of bath. The main reason for the short life if the continuous build-up in the bath of dissolved 347'ZS
trivalent chromium resulting from the oxidation-reduction :
reactions that occur between the metal and the hexavalent , chromium. Trivalent chromium is a contaminant in the process affecting the coating efficiency. Thus, when reduced coating activity is noted, or when the contaminants have built up to a certain predetermined level, a process solution of this type is at least partially replaced with freshly prepared solution, and ultimately completely discarded in favor of a fresh bath.
The disposal of the spent process solution is wasteful, as the solution still contains considerable quantities of hexavalent chromium. Not only does the loss of these values contribute significantly to the overall cost of the coating process, but disposal also adds to this coat in that the solution present a substantial waste treatment problem. Hexavalent chromium is highly toxic and must be reduced to the trivalent form, e.g. by reaction with sodium hydrosulfite or sodium bisulfite, and is thereafter precipitated from solution by addition of alkalies, such as sodium carbonate or lime. After dewatering of the precipitate by settling or filteration, the concentrated sludge of trivalent chromium hydroxide must be disposed of in specially designated areas, since trivalent chromium is still too toxic to be used as landfill. Substantial waste treatment requirements of spent rinse waters are also created due to dragout of toxic chemicals from the process bath into Z5 subsequent rinse waters. Although there are integrated processes for the reoxidation and regeneration of spent chromate solutions and rinse water, the small processor usually finds that the refined and sophisticated techniques involved are neither practical nor economically feasible for solving his waste treatment problems.
Applicant has developed a non-toxic conversion coating solution which is comprised of sulfuric acid, hydrogen peroxide and a soluble silicate and optionally .

Ij 113~'7'Z5 ¦Icontaining additives, e.g. certain or~anophos~horus compounds for further enhancement of corrosion resistance of metal surfaces Itreated with the solution.

Ii Although the acidic silicate 'solution" may or may not S ¦Ibe a true solution but rather in the form of a hydrosol, for the ii ¦purpose of this application, the term "solution" is intended to cover a hydrosol as well as a true solution.
In addition to the formation of conversion coatings of excellent properties, there are many other important advantages ~ of the invention described in the aforementioned application.
One of these is the extremely long life of the conve~sion coating Isolution before it is discarded in favor of a fresh ~olution. It has been found that the solutions are capa~le of treating up to !approximately 185 m2 of surface area per liter, which is far superior to the typical value of approximately 20 m2/1 obtained llwith conventional chromate conversion coating baths.
I! Another and related advantage is that, apart from some build-u~ of dissolved metal in the solution, there are no detri-mental by-products forming and accumulating therein during use, !las is the case with conventional chromate conversion coating ;solutions, in which trivalent chromium rapidly builds up.
The most important advantage, however, is the non-toxic , nature of the system, which greatly facilitates waste disposal of ! spent solutions from the conversion coating process. Rinse waters can usually be disposed of without any treatment reouired. SPent conversion coating baths are merel~ treated with lime for neutral-ization and removal of diss~lved metal ions and phosphorus (when , organophosphorus nromoters are used) as a precipitate. After !:
settling or other separation, the liquid phase may be disposed of 11 ~afely in co~mon sewers, while the dewatered sludge mainly com-l¦posed of silicate can be dumped in munici~al landfill areas.

!l One disadvantage, however, iY that the resulting con~
j~version coatings, although brigh~ and corrosion resistant, lack ¦Ithe decorative color, usually blue, which is characteristiC of l~chromate conversion coated parts. The incornoratiOn into the !¦silicate conversion coating solution of conventional dyes ; 1134725 - recommended and used in the industry for coloring chromate con-version coatings failed to impart any lasting color to the coated surfaces, even at very high dye concentrations and prolon~ed treatment times. Attempts to use these dves in a water solution as a post-dip treatment after formation of the conversion coatings on the work pieces also failed to impart any color.
In addition, it was found that many of the conventional metal dyes were unstable in the sulfuric acid-hydrogen peroxide-silicate environment resulting in excessive hydrogen peroxide consumption, complete loss of color or a drastic color change of the bath solution.
It is therefore an object of the present invention to iprovide an improved, stable conversion coating solution which imparts a lasting color to the coated surface.
Another object of the invention is a process for the formation of bright, corrosion resistant, colored conversion coatings onto metallic surfaces.
Still another object is to provide decorative bright corrosion resistant work pieces.

THE INVENTION

We have now discovered that certain dyes which heretofore E)redominantly have been used in the dyeing of natural fibers such as paper, cotton, wool, silk, etc., when incorporated into a Sll]-furic acid-hydrot~en peroxide-silicate conversion coating solution, unexpectedly impart pleasing and lasting colors to the coated work ~ieces without detrimentally affecting the corrosion resis-tance ~uality of the coating or the stability of the coating solution. The dyes, which are ùseful in the present invention are cationic triarylmethane dyes.

Thus in accordance with the present invention there is rrovided a dyeable conversion coating solution which comprises an ; aqueous solution of from about 0.2 g/l to about 45 g/l of free sulfuric acid, from about 1.5 ~/1 to about 58 g/l of H2O2, from about 3 ~/1 to ahout 33 g/l of SiO2 and an effective amount of at least one cationic triarylmethane dye.

4'~25 The SiO2 component is conveniently Provide~ in the form of a soluble silicate, e.g. sodium silicate or potassium silicate, of predetermined contents of SiO2 and Na2O or K2O. The mole ratios of SiO2 to either ~a2O or K2O generallv range between 1 and l4, and it is preferred to use those silicates wherein the mole ratio is at least about 1.8 and most preferably at least about 2.2.
IAMmonium or lithium silicates are also useful in providing the ;ISiO2 component.
The triarylmethane dyes used in this invention are well known in the art and are recognized as a separate generic group of dyes having a Colour Index (C.I.) in the range from 92,000 to 44,999. They are commercially available in a wide variety of ,colors both in solid form or as aqueous solution concentrates with solids contents ty~ically in the 40 - 50~ range. The amount of dye to ~e added to the conversion coating solution depends obviously on the desired depth of color.
The solution is easily prepared, e.g. by first adding sufficient sulfuric acid to at least a major portion of the makeup water under agitation to provide the desired free H2SO4 ~o content and ta~ing into account that some o the free acid will be ; subsequently neutralized by the Na2O or K2O ~ortions introduced '; ~ with the silicate. The silicate is added under agitation to the ; cooled acidic solution until it is completely dispersed. The peroxide is added and then the dye, prefera~ly in the form of a dilute solution in a minor portion of the water used in the ~rep-aration of the conversion coating solution. The se~uence of addition can be changed, however, without any detrimental effect, rovided that the silicate is acidified with sulfuric acid prior to mixing with the hydroyen peroxide, or peroxide decompositiOn will occur.
The preferred concentrations of the components in the aqueous solution are from about 1.8 g/l to about 18 g/l of free O4~ from about 7 g/l tO about 29 g/l of ll2O2, from about 8 g/l , to about 18 g~l of SiO2 and from about 0.05 to about 0.3 g/l of ~the triarylmethane dye or mixture of dyes.
5_ l~ 1134725 The solution is useful for forming conversion coatings on various metallic surfaces, such as those of zinc, cadmium, silver, copper, aluminum, magnesium, and zinc alloys.
The rnost common application is, however, in the forma-tion of conversion coatings on zinc plated articles such as zinc ,plated steel articles. The zinc plate provides the steel with ! cathodic protection against corrosion, and the conversion coating further improves the corrosion resistance, reduces the suscepti-~lbility to finger markings and enhances the appearance by chemical polishing of the article and by the color imparted by the dye.
IIt is important that the zinc plate deposit is relatively smooth and fine-grained prior to coating, and that the thickness of the l'plate deposit is at least 0.005 mm since some metal removal occurs ,when the fil~ is formed. The preferred plate thickness is between jjabout 0.005 ~n and about 0.02 mm.
I' Usually the formation of the conversion coating follows ¦limmediately after the last rinse in the plating cycle. Thus, the ' 'freshly plated articles are immerse~ for a period of from about 5 seconds to about 300 seconds into the solution which is main-tained at ambient temperatures. For best results, the immersion treatment is carried out for a duration of from about 20 seconds to about 50 seconds in a bath maintained at temperatures not less than about 20C and not more than about 35C. The coated articles are suksequently rinsed, first in cold water and then briefly in ~5 warm water to aid drying of tlle films. The hot water rinse ty~ically has a tem~erature in the range of from about 60 to about 70C. The final step of the coating process is a drying step, which is carried out by any means that will neither a~rade the soft and then rather fragile film, nor expose it to excessive temperatures, i.e. temperatures higher than about 70C. The use of circulating warm air or an airblast are examples of suitable means in the drying operation. After drying, the conversion ~' coatings are quite resistant to damage from abrasion and generally Il do not require the 12-24 hour a~in~ necessary with conventional 1, chromate conversion coatings.
il - 6 -The resulting conversion coatings have very good resistance to corrosion as determined by the accepted accelerated ~orrosion test AsTr~ 117-64. By the use of one or more of certain organic promoters as additives to the solution of sulfuric ;acid-hydrogen peroxide-silicate the corrosion resistance of the coatings can be further enhanced. The organophosphorus compounds sE~ecified hereinafter have been found to be especially useful in this respect.
i These promoters are organic phosphorus compounds having the general formula:

lX(Rl)m]n [R2]p lX(Rl)m]q ',wherein X is a group of the formula Z1 ~ P ~ Z2 in which O
Zl and Z2 independent from each other are hydrogen, sodium or potassium;
m is either O or l;

p is either O or l;

n -~ q is either (a) 1 when p = O, or

2() (b) equal to the number of available bonds provided by R2 when p = 1;

Rl is a (a) Cl-C4 alkyl or a Cl-C4 hydroxy-substituted alkyl and p = O; and (b) Cl-C4 alkylene or a Cl-C4 hydroxy-substituted alkylene and p = l;

R2 is selected from (a) N- , m = 1 (b) =N(CH2)rN= , m = 1 and r is an integer from 2 to 6 (c) =N(CH2)2 N (CH2)2 (Rl) X
~ .

--` 11347Z5 and (d) a Cl-C4 alkylene or a Cl-C4 hydroxy-substituted alkylene, m = 0 or l.

,', ,' Examples of these organophosphorus compounds include IICl-C4 alkyl phosphonic acids, Cl-C4 hydroxyalkalenephosphonic ¦iacids, amino tri-Cl-C4 alkylene phosphonic acids, C2-C8 alkylene diamine-tetra (Cl-C4 alkylene phosphonic acid), diethylenetri-llamine-Penta (Cl-C4 alkylene phosphonic acid) as well as the acid !or neutral sodium or potassium salts of any of the above-listed ¦~phosphonic acids. l-hydroxyethylidene-l,l-diphosphonic acid is a preferred compound.
The organophosphorus compound or mixture of such com-lipounds is added to the conversion coating solution to provide a Iconcentration therein of from about 0.15 g/l to about lO g/l, !, preferably from about 0.5 g/l to about 2 g/l.
During the course of the coating process, the coating solution becomes depleted in both free sulfuric acid and hydrogen peroxide values and must be replenished. lherefore, monitorin~
of these values should be carried out on a re~ular basis to assure ~ha~ the respective concen~rations have not fallen below their minima and to assess the arnounts needed for replenishment.
Free sulfuric acid can be determined by conventional titration methods using sodium hydroxide or ~y pl{ determinations. In order to maintain the free sulfuric acid within the broad ranges of about 0.2 to about 45 g/l the pH should be controlled between about 0.5 and about 3.5 and preferably between about l.0 and about

3.0 which approximately corresponds to a free sulfuric acid con-centration of from about 1.8 to about 18 g/l. The hydrogen perox-, ~ ide concentration levels are advantageously monitored by conven-j tional titration with ceric ammonium sulfate. The silicate (SiO
,jconsumption is relatively small compared to the consumptions of either the free sulfuric acid or the hydrogen peroxide, and --` 113~7Z5 generally neither monitoring (which can be carried out using e.~.
colorimetric ~rinciples involving the reaction of silicate with ammonium molybdate to form a yellow-colored molybdo silicate l,solution) nor replenishment is required during the practical life llof the conversion coating bath. The rate of consumption (i.e.
Ilpercent decrease in concentration per unit time) of organophos-jlphorus additives has been found to ~e approximately of the same ¦,order as that of the hydrogen peroxide consumption. Therefore, l¦replenishments of the solutions with these additives are suitably !Icarried out at the time of hydrogen peroxide replenishment in j;amounts proportional to the h~drogen peroxide addition. The dye ! generally does not need to be replenished during the practical j,lifetime of the conversion coating bath. Monitoring of the color ',depth quality of the coating is easily carried out by visual l,inspection of the coated article and comparison against a refer-ence color.
1'he following examples are provided to illustrate but not to limit the invention.
lhe general procedures used in t~le examples for pre-paring the conversion coating solutions, test specimens and forming the conversion coatings are described below.
The aqueous conversion coating solutions were each prepared to contain 2.4 g/l free ll2SO4 , 16.2 g/l SiO2 , 11.7 ~/l ~22 I'he SiO2 ingredient was added in the form of sodiuM
silicate (SiO2 = 33.2~ w/w; Na2O = 13.85~ w/w) and a sufficient excess of sulfuric acid was provided to result in the indicated free ll2SO4 content after neutralization of the Na2O in the sodiuM silicate.
I Standard Hull cell steel panels (10 cm x 6.8 cm x 0.03 cm) were p]ated with ~inc using a cyanide electrolyte. After thorouyh rinsing and drying, the samples were then immersed for 20 seconds (unless otherwise noted) in the conversion coating , solution maintained at room temperature. The treated samples were then rinsed in water and then dried with a hot air gun.

, I _ g _ ~` ii3~'725 A number of conversion coating solutions containing various blue dyes were prepared and tested for color and hydrogen peroxide stability after 24 and 90 hours storage. Table 1 below identifies the dyes, shows the dye concentrations and the results of the stability testing. Of the seven dyes tested in this series , only those of Examples 4 and 8 did not appear to promote peroxide consumption of the bath nor undergo an undesired color change.
These dyes were therefore used for conversion coating trials to determine if they would impart a desired blue color to zinc plates -~ treated with the respective solutions. Results of 20 second immersion in each of the two baths were that no permanent color was imparted to the surface of the test panels.

: - ~! ol 113~725 O ~:
~a) a~ o~ ~D O~ ) 2 al 1~ ~ d I ~
I o ~ I
~ o 3 o ~
o ~ ~ ~ U~ o ¢ S ~ 3 :~ h ~ m m ~ o ~ ~ ~

o ;' ~
O ~ u~
u~ ~ o~
. 0~
~c ~:r ~ o ~ m a~ ~ ~
~ ~C
I m ~ ~ h m c~

~1 o o z ~ u ~ ~ o ~ ~ m u O
~J -- ::~ h ~ h :; h CJ
, a~ n~ h ~
I P~ m E~ u m a m a u ~ ~
11 ~ ~ ~
V~ o -- O O O O O o o U O
_ ~ h ~: \ N~1 !! U C~ ~~O~ ~
P~
~ -- O
_ ,~ O
L~ ^ ~h O
3 ~ al --I l _ Q~
~ u~ . a I ~ . ~ o ~ ~ ~ m Q ::1 h ~O
E~ O
~ o o I ~ ~
I ~ ~ o o o I G) O O
I t~ K Z
O .C ,C _1 ~1 0 m u~
.
0 o o Z ~ o O ~ ~ N ~ ~ u .) X

11;~4'~5 il EXAMPI.ES 9~
i' ~
In Exam~le 9 the procedures of the t~revious comparison examples were followed exactly except that the dye was Basic Violet 3, which is a cationic triarylmethane dye having a Colour ¦Index of 42555. The particular dye used in this example was ¦Paper Blue R solution obtained from E.I. DuPont de N~mours pro-vided in the form of an aqueous acetic acid solution o~ a~out 1.115 sp.~r. and a solids content of about 50 wt. ~. When 0.5 ml/?
of the dye solution was added to the bath there resulted a dark blue color which after 90 hour~ of storage did not change. The peroxide concentration was not significantly affecte~ afte~ con- !
clusion of the testing (92% retention vs 94% without anY dye).
Results of 20 second immersion coatin~ test8 in the dyed bath of zinc plated te~t panelg showed a permanent reddish blue ¦color to the surface. ~epeatinq the im~ersion coating tests with ¦varYing concentration8 of the dye from 0.1 to 0.5 ml/l showed that any desired ~epth of color could be imparted to the surface merely b~ changing the dye concentration.
Examples 10 and ll, in which the triarylmethane dves were respectively a Basic Blue 7 (C.I. 42595) and Basic ~,reen 4 (C.I. 42000), showed the same successful coloration in the col-ce~
tration range used in Example 9.

ll In this example the conversion coating solution con-1I tained 0.85 ~/1 (dry basis) of l-hydroxyethylidene-l,l-diphos-Il phonic acid as a further promoter for corrosion resistance. The ¦I dyes used were a mixture of Ba6ic Blue 7 and Basic Violet 3 2 ml/l ~uPont Victoria Pure Blue BOP Solution, and 0.l ml/1 il DuPont Paper Blue R Liquid~. ~ull cell panels plated in a small ¦I scale as well as commercially plated clamps and elbow brackets I ~ I
~ -12-11347;~

,1 served as zinc-plated specimens for conversion coating, which was , carried out for 20 seconds.
;I Visual examination of the coated specimens showed a , ¦Idesirable blue color of excellent uniformity and shade, closely S Ijnlatching those obtained with conventlonal blue c~lromate treatment.

1~

1~ , , ,, ' -13-

Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A conversion coating solution which comprises an aqueous solution of from about 0.2 g/l to about 45 g/l of free H2S04, from about 1.5 g/l to about 58 g/l of H202, from about 3 g/l to about 33 g/l of SiO2 and an effective amount of at least one cationic triarylmethane dye.

2. The solution of claim 1, wherein the free H2S04 con-centration is between about 1.8 g/1 and about 18 g/1.

3. The solution of claim 1, wherein the H202 concentration is between about 7 g/1 and about 29 g/1.

4. The solution of claim 1, wherein the SiO2 concentration is between about 8 g/1 and about 18 g/1.

5. The solution of claim 1, in which the SiO2 is provided in the form of sodium silicate or potassium silicate.

6. The solution of claim 5, wherein the molecular ratio of SiO2 to either Na20 or K20 in the sodium silicate or potassium silicate is maintained between about 1 and about 4.

7. The solution of claim 6, wherein said molecular ratio is at least about 2.2.

8. The solution of claim 1, containing from about 0.15 g/1 to about 10 g/1 of a promoter additive or mixtures of promoter additives selected from organophosphorus compounds having the general formula:
[X(R1)m]n . [R2]p . [X(R1)m]q , wherein X is a group of the formula Z10 - IMG - 0Z2 in which Z1 and Z2 independent from each other are hydrogen, sodium or potassium;
m is either 0 or 1;

p is either 0 or 1;

n + q is either (a) 1 when p = 0, or (b) equal to the number of avail-able bonds provided by R2 when p = 1;

R1 is a (a) C1-C4 alkyl or a C1-C4 hydroxy-substituted alkyl and p - 0; and (b) C1-C4 alkylene or a C1-C4 hydroxy-substituted alkylene and p _ l;

R2 is selected from (a) N? , m = 1 (b) =N(CH2)rN- , m = 1 and r is an integer from 2 to 6 (c) IMG , m = 1, and (d) a C1-C4 alkylene or a C1-C4 hydroxy-substituted alkylene m = 0 or 1.

9. The solution of claim 8, containing from about 0.5 to about 2 g/l of said organophosphorus compound.

10. The solution of claim 8, wherein the organophosphorus compound is a hydroxy alkylene diphosphonic acid.

11. The solution of claim 10, wherein the organophorphorus compound is 1-hydroxyethylidene-1,1-diphosphonic acid.

12. The solution of claim 1 in which the dye concentration ranges between about 0.05 to about 0.3 grams/liter on a dry basis.

13. The solution of claim 1, in which at least one of the dyes is Basic Violet 3 having a Colour Index of 42555.

14. The solution of claim 1, in which at least one of the dyes is Basic Blue 7 having a Colour Index of 42595.

15. The solution of claim 1, in which at least one of the dyes is Basic Green 4 having a Colour Index of 42000.

16. In a process for the formation of corrosion resistant conversion coating onto metallic surfaces selected from zinc, cadmium, silver, copper, aluminum, magnesium and zinc alloys, wherein the metallic surfaces are immersed in a conversion coating solution, and subsequently rinsed and dried, the improvement which comprises:
immersing the metallic surfaces into the con-version coating solution of claim 1.

17. In a process for the formation of corrosion resistant conversion coating onto metallic surfaces selected from zinc, cadmium, silver, copper, aluminum, magnesium and zinc alloys, wherein the metallic surfaces are immersed in a conversion coating solution, and subsequently rinsed and dried, the improvement which comprises:
immersing the metallic surfaces into the con-version coating solution of claim 8.

18. A metallic surface coated by the process of claim 16.

19. A metallic surface coated by the process of claim 17.

20. The metallic surface of claim 18, wherein the metal is zinc plate.

21. The metallic surface of claim 19, wherein the metal is zinc plate.