CONVERSION COATING SOLUTIONS AND METHODS OF USE THEREFOR

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 63,672,842 (DBC 0257 MA/69607.727) filed on Jul. 18, 2024, which is hereby incorporated herein in its entirety.

FIELD

Embodiments of the present disclosure generally relate to conversion coating solutions and methods of use therefor, and particularly to conversion coating solutions that do not include fluoride.

BACKGROUND

In the art of treating metal surfaces, it is common practice to improve the corrosion resistance characteristics and adhesion/bonding qualities of a metal surface to subsequent materials by depositing a conversion coating or the like thereon. In conversion coating, the substrate is typically submerged in or otherwise exposed to a chemical solution (most commonly containing zirconium and/or phosphate) that reacts with the material of the substrate, thereby forming a coating layer on the surface of the substrate.

SUMMARY

However, typical chemical solutions used for imparting zirconium conversion coatings often utilize fluoride and/or fluoridized zirconium compounds in the process. Particularly, fluoride-containing acids have traditionally be used to great effect in carrying the substance ultimately deposited on the substrate. However, fluoride-containing compounds are subject to increasingly stringent environmental restrictions in wastewater streams due to their negative toxicological and environmental effects. Accordingly, it may be desired to transition from fluoride-containing conversion coating solutions to non-fluoride-containing conversion coating solutions. Further, it would be advantageous if these non-fluoride solutions perform comparable to fluoride-containing solutions, such that motivation for use is not entirely regulation based.

Accordingly, provided herein are conversion coating solutions that do not incorporate fluoride, such that the environmental and toxicological concerns are mitigated. The coatings also exhibit high levels of reactivity with a metal substrate to form a conversion coating, corrosion resistance, and surface adhesion benefits comparable to fluoride-containing solutions.

In at least one embodiment, a zirconium conversion coating solution may comprise an aqueous solution; a non-fluoride containing zirconium compound; sulfuric acid; and an organic acid.

In another embodiment, a method of coating a substrate may comprise exposing the substrate to the zirconium conversion coating solution to form a coated substrate, wherein the zirconium conversion coating solution comprises an aqueous solution; a non-fluoride containing zirconium compound; sulfuric acid; and an organic acid.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

FIG. 1A illustrates a substrate exposed to a zirconium conversion coating solution having a pH of approximately 4.5 and including sulfuric acid, according to embodiments herein, the substrate showing signs of conversion coating deposition;

FIG. 1B illustrates a substrate exposed to a zirconium conversion coating solution having a pH of approximately 4.5 and including methane sulfonic acid instead of sulfuric acid, according to embodiments herein, the substrate not showing signs of conversion coating deposition;

FIG. 1C illustrates a substrate exposed to a zirconium conversion coating solution having a pH of approximately 4.5 and including nitric acid instead of sulfuric acid, according to embodiments herein, the substrate not showing signs of conversion coating deposition;

FIG. 2 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D1 as shown in the depletion table test, according to embodiments herein;

FIG. 3 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D2 as shown in the depletion table test, according to embodiments herein;

FIG. 4 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D3 as shown in the depletion table test, according to embodiments herein;

FIG. 5 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D4 as shown in the depletion table test, according to embodiments herein;

FIG. 6 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D5 as shown in the depletion table test, according to embodiments herein;

FIG. 7 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D6 as shown in the depletion table test, according to embodiments herein;

FIG. 8 illustrates four painted substrates subjected to a salt spray test under ASTM D1654, the substrates pre-treated with a zirconium conversion coating solution corresponding to D7 as shown in the depletion table test, according to embodiments herein

FIG. 9A illustrates a painted substrate subjected to a salt spray test under ASTM D1654, the substrate pre-treated with a fluoride-free zirconium conversion coating solution according to embodiments herein;

FIG. 9B illustrates a painted substrate subjected to a salt spray test under ASTM D1654, the substrate pre-treated with a second fluoride-free zirconium conversion coating solution according to embodiments herein with a higher zirconium concentration than the pre-treatment solution used for the panels in FIG. 9A;

FIG. 9C illustrates a painted substrate subjected to a salt spray test under ASTM D1654, the substrate pre-treated with a commercially available fluoride-containing zirconium conversion coating solution; and

FIG. 9D illustrates a painted substrate subjected to a salt spray test under ASTM D1654, the substrate pre-treated with a commercially available phosphate-based conversion coating used for testing paint durability.

DETAILED DESCRIPTION

As previously stated, embodiments herein are generally relate to conversion coating solutions and methods of use therefor, and particularly to zirconium conversion coating solutions that do not include fluoride and/or phosphate. Accordingly, at least some of the embodiments of the conversion coating solution herein may not comprise fluoride or phosphate.

In embodiments, the conversion coating solution may comprise an aqueous solution; a zirconium compound, sulfuric acid, and an organic acid. Accordingly, the conversion coating solution described herein may be regarded as a zirconium conversion coating solution. As described in detail herein, the zirconium conversion coating solution may also comprise an accelerator, a nonionic surfactant, or both.

Accordingly, as described herein, a method of coating a substrate may comprise exposing the substrate to the zirconium conversion coating solution, such as any zirconium conversion coating solution described herein, thereby forming a coated substrate. The method may further comprise exposing the coated substrate to one or more coatings of paint, thereby forming a painted substrate. The one or more coatings of paint may be multiple coatings of the same paint, coatings of a unique first paint followed by coatings of a unique second paint (such as colored paint followed by clear coat), or combinations thereof. As used herein, the substrate may comprise a metallic or non-metallic medium, such as, but not limited to steel, galvanized steel, aluminum, magnesium, or combinations thereof.

As previously stated, the zirconium conversion coating solution may comprise an aqueous solution. The aqueous solution may comprise fresh water, such as, but not limited to, potable water, mineral water, deionized water, tap water, distilled water, or combinations thereof. In one embodiment, the aqueous solution may be deionized water. The zirconium conversion coating solution may comprise a balance aqueous solution, in other words, the remaining weight percent (wt. %) of the zirconium conversion coating solution after including the zirconium compound, the sulfuric acid, the organic acid, the accelerator, the nonionic surfactant, any other component, or combinations thereof may be aqueous solution.

As previously stated, the zirconium conversion coating solution may also comprise a zirconium compound, as previously stated. The zirconium compound may not comprise fluoride, such that the zirconium compound may be regarded as a non-fluoride containing zirconium compound. The zirconium compound may comprise zirconium acetate. The zirconium compound may additionally or alternatively comprise ammonium zirconium (IV) carbonate, zirconium (IV) carbonate, zirconium (IV) dinitrate, zirconium (IV) nitrate, zirconium (IV) sulfate, zirconium carbonate, zirconium dinitrate, zirconium n-propoxide, zirconium oxide, or combinations thereof.

Additionally or alternatively, the zirconium compound may comprise a zirconium carboxylate, such as but not limited to zirconium acetate, zirconium formate, zirconium oxylate, zirconium methanetetracarboxlate, or any other zirconium carboxylate salt. In at least one embodiment, the zirconium compound may be zirconium acetate.

The zirconium conversion coating solution may comprise from 0.5 wt. % to 8 wt. % zirconium acetate, such as from 0.5 wt. % to 1 wt. %, from 1 wt. % to 2 wt. %, from 2 wt. % to 4 wt. %, from 4 wt. % to 6 wt. %, from 6 wt. % to 8 wt. %, or any combinations of the previous ranges or smaller range therein, such as from 2 wt. % to 6 wt. %. However, without being limited by theory, the zirconium conversion coating solution may also be diluted depending on customer/user preference.

Accordingly, the weight percent zirconium acetate in the zirconium conversion coating solution (as well as any other component of the zirconium conversion coating solution) may be diluted by a factor of 10, 100, 1,000, etc., such that the ranges above also contemplate, for example, 0.0005 wt. % to 0.0008 wt. %, from 0.005 wt. % to 0.008 wt. %, from 0.05 wt. % to 0.08 wt. %, from 0.0005 wt. % to 8 wt. %, and so on. As previously stated, this dilution factor, and the modified ranges therefrom, may apply to all of the components of the zirconium conversion coating solution, with the remaining balance being the aqueous solution.

As previously stated, the zirconium conversion coating solution may also comprise sulfuric acid. Without being limited by theory, sulfuric acid may be preferred over other acid species, such as but not limited to nitric acid, alkylsulfonic acid, hydrochloric acid or similar, due to sulfuric acid having a greater ratio of free acid to total acid than the other species. As described in further detail herein, when acid species without sufficiently great ratios of free acid to total acid are used, lower pHs (less than 4) of the zirconium conversion coating solution are required to begin depositing the conversion coating on the substrate. However, this lower pH increases the risk of the substrate flash rusting when exposed to the zirconium conversion coating solution. This rust may then reduce the adhesion corrosion resistance of the subsequently applied paint layer, thereby reducing the effectiveness of the conversion coating as a whole when non-sulfuric acids are utilized.

The zirconium conversion coating solution may comprise from 0.5 wt. % to 10 wt. % sulfuric acid, such as from 0.5 wt. % to 1 wt. %, from 1 wt. % to 2 wt. %, from 2 wt. % to 4 wt. %, from 4 wt. % to 6 wt. %, from 6 wt. % to 8 wt. %, from 8 wt. % to 10 wt. %, or any combinations of the previous ranges or smaller range therein, such as from 2 wt. % to 6 wt. %, as well as any dilution factors of the previous ranges, such as 10, 100, 1,000, etc.

As previously stated, the zirconium conversion coating solution may also comprise an organic acid, such as but not limited to a carboxylic acid, a hydrocarboxylic acid, or both. For example, and in embodiments, the hydrocarboxylic acid may comprise an alpha, beta, or omega hydroxylic acid, such as, but not limited to, acetic acid and/or lactic acid. The zirconium conversion coating solution may comprise from 0.5 wt. % to 8 wt. % organic acid, such as from 0.5 wt. % to 1 wt. %, from 1 wt. % to 2 wt. %, from 2 wt. % to 4 wt. %, from 4 wt. % to 6 wt. %, from 6 wt. % to 8 wt. %, or any combinations of the previous ranges or smaller range therein, such as from 2 wt. % to 6 wt. %, as well as any dilution factors of the previous ranges, such as 10, 100, 1,000, etc.

Without being limited by theory, the organic acid may act as a stabilizing agent for the zirconium conversion coating solution, such that the zirconium may remain solubilized in the solution during adjustment to an end-use concentration and pH, i.e., prior to deposition onto the substrate. It is contemplated that the organic acid may allow the zirconium conversion coating solution to remain stable and useable over many iterations of coating a substrate.

As previously stated, the zirconium conversion coating solution may also comprise an accelerator, such as but not limited to a nitroaromatic sulfonate salt (such as sodium nitrobenzene sulfonate), hydroxylammonium sulfate, ammonium dimolybdate, hydroxylamine hydrochloride, or combinations thereof. In at least one embodiment, the accelerator may be sodium nitrobenzene sulfonate. The zirconium conversion coating solution may comprise from 0.5 wt. % to 5 wt. % accelerator, such as from 0.5 wt. % to 1 wt. %, from 1 wt. % to 2 wt. %, from 2 wt. % to 3 wt. %, from 3 wt. % to 4 wt. %, from 4 wt. % to 5 wt. %, or any combinations of the previous ranges or smaller range therein, such as from 3 wt. % to 4 wt. %, as well as any dilution factors of the previous ranges, such as 10, 100, 1,000, etc. Without being limited by theory, the accelerator may act to increase the rate of oxidation and/or reduction occurring on the substrate during the coating process.

As previously stated, the zirconium conversion coating solution may also comprise a nonionic surfactant, such as an ethoxylated copolymer surfactant, a propoxylated copolymer surfactant, an alcoxylated alcohol, an ethoxylated alcohol, or combinations thereof. For example, and in embodiments, the nonionic surfactant may include, but may not be limited to, alkyl phenyls, fatty amines, fatty alcohols, fatty acids, vegetable oils, block/random copolymers, or combinations thereof.

In at least one embodiment, the nonionic surfactant may be (INCI name) Meroxapol 174, CAS 9003-11-6. The zirconium conversion coating solution may comprise from 0.1 wt. % to 5 wt. % nonionic surfactant, such as from 0.1 wt. % to 1 wt. %, from 1 wt. % to 2 wt. %, from 2 wt. % to 3 wt. %, from 3 wt. % to 4 wt. %, from 4 wt. % to 5 wt. %, or any combinations of the previous ranges or smaller range therein, such as from 3 wt. % to 4 wt. %, as well as any dilution factors of the previous ranges, such as 10, 100, 1,000, etc. Without being limited by theory, the nonionic surfactant may operate to decrease the surface tension of the substrate and/or zirconium conversion coating solution, thereby increasing a rate of transfer of the zirconium conversion coating solution onto the substrate. Further, the nonionic surfactant may also provide a dispersive effect, increasing stability of the concentrate by minimizing the precipitation of the accelerator ingredients from the solution.

EXAMPLES

As previously stated, sulfuric acid may be preferred over other acid species due to the relatively greater ratio of free acid to total acid, which may increase the operable pH of the conversion coating solution, reduce the risk of flash rusting, and thereby increase the adhesion of the subsequent paint layer as well as the effectiveness of the conversion coating solution as a whole. This may be shown with respect to Tables 1-2 below, as well as with reference to FIGS. 1A-1C.

Particularly, Table 1 shows the composition of various fluoride-free zirconium conversion coating solutions, wherein Samples A, B, C, and D include sulfuric acid, hydrochloric acid, methane sulfonic acid, and nitric acid respectively as the acid included.

TABLE 1
Conversion Coating Compositions for Acid Testing

Sample

A

B

C

D

Acid Used

			Methane
	Sulfuric Acid	HCl	Sulfonic Acid	Nitric Acid

Wt. %

Deionized Water	87.35	82.69	84.21	80.81
m-Nitrobenzene Sulfonate	1.5	1.5	1.5	1.5
Lactic Acid 88%	4	4	4	4
Sulfuric Acid 93%	4
Hydrochloric Acid 32%		8.66
Methane Sulfonic Acid 70%			10.54
Nitric Acid 67%				7.14
Zirconium Acetate 22%	2.85	2.85	2.85	2.85
Pluronic 17R4 (Surfactant)	0.3	0.3	0.3	0.3

Table 2 shows the procedure for exposure of the conversion coating solution to the steel substrate.

TABLE 2
Procedure for Conversion Coating for Acid Testing

Provided 800 g of 3% solution with Deionized water for each variation

Added NaOH until each soln' had pH of 3, 4.5, or 5

Heated each solution up to 37.8° C. (100° F.)

Sprayed 3 inch by 6 inch steel substrate with a warm 3% alkaline

cleaning solution (GF Clean 1052M)

Rinsed substrate with room temperature (20° C./68° F.) tap water

Dipped substrate in and out of solution for 1 minute to simulate

a treatment stage

Rinsed substrate with room temperature tap water

Air dried substrate

Table 3 shows the subsequent observed deposition, or lack thereof, of the conversion coating solution on a substrate for various pHs of the conversion coating solution. FIGS. 1A-1C show the visual results for 4.5 pH conversion coating solution for samples A, C, and D. As shown, samples C and D included no deposition, whereas sample A did show deposition.

TABLE 3
Conversion Coating Deposition Results for Acid Testing

Sample	pH 3.0	pH 4.5	pH 5.0
A	Deposition Observed	Deposition Observed	Deposition Observed
B	Deposition Observed	No Deposition Observed	No Deposition Observed
C	Deposition Observed	No Deposition Observed	No Deposition Observed
D	Deposition Observed	No Deposition Observed	No Deposition Observed

As shown in Table 3, deposition occurred only for a pH of 3 for the conversion coatings not including sulfuric acid, whereas deposition occurred for all pHs for the sulfuric acid conversion coating, according to embodiments herein.

The zirconium conversion coating solutions described herein were also tested to determine their performance in at least four aspects, stability over time, ability to continually supplement with additional solution, paint adhesion, and corrosion resistance.

Initially, the zirconium conversion coating solutions were subjected to a depletion test where a bath of the zirconium conversion coating solution was alternatively exposed to steel substrates and supplemented with additional zirconium conversion coating solution. The substrates were first cleaned then rinsed before exposure to the bath, followed by another rinsing, ambient drying, then oven drying at 48.9° C. (120° F.). The aqueous solution used for the depletion test was tap water. The results for the depletion testing are shown below in Table 4, and indicate that the zirconium conversion coating solution can be supplemented as necessary to allow continued treatment of substrates.

TABLE 4
Depletion Test of Zirconium Conversion Coating Solution

Day of Test

		Sept 28	Sept 29	Oct 2	Oct 3	Oct 4	Oct 5	Oct 6
Solution	Units	D1	D2	D3	D4	D5	D6	D7

Conversion solution added	mL		15	18	24	27	30	35
Starting pH (after addition)	—	4.1	4.1	4.25	4.22	4.24	4.24	4.13
Beginning Titration, 20 mL	mL	0.5	0.8	1.0	1.4	1.6	1.8	2.2
sample
Ending pH	—	4.49	4.66	4.67	4.71	4.72	4.71	4.64
Beginning Titration, 20 mL @	mL	0.6	0.6	0.8	1.0	1.4	1.6	1.9
0.1N NaOH
Sulfate	ppm	993	1066	1051	1106	1271	1303	1314
Iron	ppm	2.09	74.3	94.9	125	171	189	261
Potassium	ppm	972	1036	991	991	1043	952	1089
Zirconium	ppm	85.7	88.3	81.7	84.0	92.6	883	103
Total # of panels treated	—	42	84	126	168	210	252	294
Total ft2 Substrate Treated	ft2	18.7	37.33	56.00	74.67	93.33	112.00	130.67
Surface Area treated per	ft2/gallon	3.73	7.47	11.20	14.93	18.67	22.40	26.13
volume of solution

To test the corrosion resistance of the zirconium conversion coating, select substrates from D1-D7 (Systems 1-7 corresponding to D1-D7) were painted and subjected to a salt spray under ASTM B117-16, as well as a subsequent scribe measurement under ASTM D1654, the results of which are shown below in Table 5.

TABLE 5
Salt Spray Corrosion Testing of Subsequently
Painted Substrates hours)

Salt Spray ASTM B117-16
(Vertical Scribe, 1000 hours)	Scribe

		Dry Film		Mean
		Thickness	Exposure	Creepage	ASTM D1654
System #	Part #	(mm)	hours	(mm)	Rating

1	1	2.3	840	7.4	3
	2	2.3	840	7	4
	3	1.9	840	15.6	1
	4	2.3	1000	1.6	7
2	5	2.2	624	3.6	5
	6	2.3-2.8	624	3.6	5
	7	2.1	624	5.8	4
	8	2.5	624	3.6	5
3	9	2.2	1000	3.8	5
	10	2.3	1000	5.6	4
	11	2.4	1000	3.2	5
	12	2.4-2.8	840	5.4	4
4	13	3	840	5.2	4
	14	2.2-2.6	624	3.4	5
	15	2.4-2.8	840	5	5
	16	2.7	840	4.6	5
5	17	2.7	1000	3.4	5
	18	2.1	624	4.2	5
	19	2.9	1000	3	6
	20	2.6	840	4.2	5
6	21	2.3	1000	3	6
	22	2.3	1000	3.4	5
	23	2.4	1000	4.8	5
	24	2.7	1000	5	5
7	25	2.3	624	4	5
	26	2.8	840	4.2	5
	27	2.7	840	4.2	5
	28	2.1	840	7.8	3

To test the adhesion properties of the zirconium conversion coating, select substrates from D1-D7 were again chosen and painted before being subjected to a paint adhesion tape test under ASTM D3359-17, the results of which are shown below in Table 6 and FIGS. 2-8. Again, systems 1-7 and FIGS. 2-8 correspond to D1-D7, respectively.

TABLE 6
Paint Adhesion Tape Test of Subsequently Painted Substrates

			Dry Film
			Thickness	ASTM D3359-17
	System #	Part #	(mm)	Rating

	1	1	2.1	5B
	2	2	2	5B
	3	3	2.1	5B
	4	4	2.3	5B
	5	5	2	5B
	6	6	2	5B
	7	7	2	5B

As shown in the above examples, the zirconium conversion coating solution exhibited significant stability, ability to supplement, adhesion benefits, and corrosion resistance. Further, as shown in Tables 5 and 6, the substrates exhibited adhesion benefits and corrosion resistance across the supplementation of the bath.

Finally, the fluoride-free zirconium conversion coating solution (A, B) was tested as compared to a commercially available fluid containing a zirconium conversion coating solution (C) and a commercially available iron phosphate coating solution (D) as shown below in Table 7. As shown in Table 7, the fluoride-free zirconium conversion coating solutions performed similar or better than those containing fluoride. This can also be shown with respect to FIGS. 9A-9D, illustrating Systems A, B, C, and D, respectively.

TABLE 7
Fluoride-free Zr Conversion Coating vs. Fluoride Containing Zr Alternatives

Salt Spray ASTM B117-16

(Vertical Scribe, 500 Hours)

Scribe

		ASTM D7091-	Mean		Adhesion
	Part	13 DFT	Creepage	ASTMD1654	ASTM D3359-17
System #	#	(mm)	(mm)	Rating	Rating (0B-5B)

A: Inventive	1	2.9	1.2	7
Example	2	3.3	0.5	9
	3	2.8	1.2	7
	4	3	1.4	7
	5	2.9			5B
B: Inventive	6	2.9	0.5	9
Example, higher	7	2.5-3.0	1	8
zirconium content	8	2	1.4	7
	9	2.5	1.4	7
	10	2.9			5B
C: DuraTec 602	11	2.7	1.4	7
	12	2.8	1	8
	13	2.8	1	8
	14	3	1.4	7
	15	3			5B
D: B-1000	16	2.8	2.8	6
	17	2.9	2.8	6
	18	2.7	4.4	5
	19	2.8	2.4	6	5B

Having described the subject matter herein in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Further, it will be apparent that modifications and variations are possible without departing from the scope herein, including, but not limited to, embodiments defined in the appended claims. More specifically, although some aspects herein are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects.

It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.” It is noted that the use of the terms “having” or “including”, or grammatical variations thereof, in this disclosure should also be interpreted in like manner as the more commonly used open-ended preamble term “comprising”.