Metal pretreatment composition containing zirconium, copper, and metal chelating agents and related coatings on metal substrates

Abstract

Disclosed is a zirconium-based metal pretreatment coating composition that includes a metal chelator that chelates copper in the metal pretreatment coating composition and thereby improves adhesion of paints to a metal substrate coated with the pretreatment coating composition and the chelating agent is present in a sufficient amount to ensure that in the deposited pretreatment coating on the metal substrate the average total atomic % of copper to atomic % of zirconium is equal to or less than 1.1. The pretreatment coating composition is useful for treating a variety of metal substrates.

Claims

1. An article of manufacture comprising a coated metal substrate which has been coated according to a method comprising steps of: a) optionally cleaning a metal substrate; b) applying to the metal substrate a zirconium-based metal pretreatment coating composition comprising water and dissolved Zr, a source of fluoride, dissolved copper, optionally including added dissolved Cu, a copper chelating agent, and optionally materials comprising one or more of silicon, boron and yttrium, wherein the composition has a pH that ranges from neutral to pH 4, thereby forming a pretreatment coating on the metal substrate; with the proviso that the copper chelating agent is the only organic molecule present in the zirconium-based metal pretreatment coating composition and is present in said coating composition in an amount sufficient to result in an average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating deposited on the metal substrate is equal to or less than 1.1; and c) optionally applying a paint to the metal pretreatment coated metal substrate thereby producing a painted coated metal substrate; wherein the pretreatment coating on the metal substrate comprises Zr and Cu present in amounts such that the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating deposited on the metal substrate is equal to or less than 1.1.

2. The article of manufacture according to claim 1, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating deposited on the metal substrate that is in a range of 0.0001 and 1.1.

3. The article of manufacture according to claim 1, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating on the metal substrate is about 0.9 to 0.02.

4. The article of manufacture according to claim 3, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating on the metal substrate is about 0.3 to 0.1.

5. The article of manufacture according to claim 1, wherein atomic % of Cu in said pretreatment coating measured at a series of depths from an outer surface of the pretreatment coating to the metal substrate does not exceed 33 atomic % Cu at any of said depths.

6. The article of manufacture according to claim 1, wherein the metal substrate comprises cold rolled steel and the pH is acidic.

7. The article of manufacture according to claim 6, further comprising at least one paint applied to the pretreatment coating resulting in a painted coated substrate that achieves at least 95% paint remaining when tested according to ASTM 3330M (Revised Oct. 1, 2004).

8. The article of manufacture according to claim 6, wherein optional step c) is present and the painted coated substrate achieves 1.9 mm or less average corrosion creep when tested according to ASTM B117 (Revised Dec. 15, 2007) for 500 hours.

9. The article of manufacture according to claim 1, wherein the pH of the zirconium-based metal pretreatment coating composition ranges from 4.0 to 4.2.

10. An article of manufacture comprising a coated metal substrate which has been coated according to a method comprising steps of: a) optionally cleaning a metal substrate; b) applying to the metal substrate a zirconium-based metal pretreatment coating composition comprising water and: i. dissolved Zr, ii. a source of fluoride, iii. a copper chelating agent, iv. mineral acid, organic acid and/or an alkaline source such that the composition has an acidic pH, v. dissolved copper, optionally including added dissolved Cu; and vi. optionally materials comprising one or more of silicon, boron and yttrium, and optional added dissolved Cu, thereby forming a pretreatment coating on the metal substrate; wherein the copper chelating agent is present in said zirconium-based metal pretreatment coating composition in an amount of at least 10 ppm to at most 2000 ppm; and c) optionally applying a paint to the metal pretreatment coated metal substrate thereby producing a painted coated metal substrate; wherein the pretreatment coating on the metal substrate comprises Zr and Cu present in amounts such that the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating deposited on the metal substrate is equal to or less than 1.1.

11. The article of manufacture according to claim 10, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating on the metal substrate is about 0.9 to 0.02.

12. The article of manufacture according to claim 10, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating on the metal substrate is about 0.3 to 0.1.

13. The article of manufacture according to claim 10, wherein atomic % of Cu in said pretreatment coating measured at a series of depths from an outer surface of the pretreatment coating to the metal substrate does not exceed 33 atomic % Cu at any of said depths.

14. The article of manufacture according to claim 10, wherein the metal substrate comprises cold rolled steel.

15. The article of manufacture according to claim 14, further comprising at least one paint applied to the pretreatment coating resulting in a painted coated substrate that achieves at least 95% paint remaining when tested according to ASTM 3330M (Revised Oct. 1, 2004).

16. The article of manufacture according to claim 14, further comprising at least one paint applied to the pretreatment coating resulting in a painted coated substrate that achieves 1.9 mm or less average corrosion creep when tested according to ASTM B117 (Revised Dec. 15, 2007) for 500 hours.

17. The article of manufacture according to claim 10, wherein the pH of the zirconium-based metal pretreatment coating composition ranges from 4.0 to 4.2.

18. An article of manufacture comprising a coated metal substrate comprising: a cold rolled steel metal substrate; and deposited on the cold rolled steel metal substrate, a pretreatment coating comprising metal from said substrate, zirconium, oxygen, copper, and optional elements fluorine and carbon; wherein the pretreatment coating on the cold rolled steel metal substrate has an average total ratio of atomic % of Cu to atomic % of Zr that is equal to or less than 1.1.

19. The article of manufacture according to claim 18, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating deposited on the metal substrate that is in a range of 0.0001 and 1.1.

20. The article of manufacture according to claim 18, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating on the metal substrate is about 0.9 to 0.02.

21. The article of manufacture according to claim 18, wherein the average total ratio of atomic % of Cu to atomic % of Zr in the pretreatment coating on the metal substrate is about 0.3 to 0.1.

22. The article of manufacture according to claim 18, wherein the element fluorine is present in the pretreatment coating.

23. The article of manufacture according to claim 18, wherein the element carbon is present in the pretreatment coating.

24. The article of manufacture according to claim 18, wherein atomic % of Cu in said pretreatment coating measured at a series of depths from an outer surface of the pretreatment coating to the metal substrate does not exceed 33 atomic % Cu at any of said depths.

25. The article of manufacture according to claim 18, further comprising at least one paint applied to the pretreatment coating resulting in a painted coated substrate that achieves at least 95% paint remaining when tested according to ASTM 3330M (Revised Oct. 1, 2004).

26. The article of manufacture according to claim 18, further comprising at least one paint applied to the pretreatment coating resulting in a painted coated substrate that achieves 1.9 mm or less average corrosion creep when tested according to ASTM B117 (Revised Dec. 15, 2007) for 500 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings that accompany the detailed description are described below. FIGS. 1A, 1B, 1C, and 1D are scanning electron microscope (SEM) images of pretreatment coatings on cold rolled steel;

(2) FIG. 2A is an SEM image of the sample shown in FIG. 1A with several circled areas of interest, FIG. 2B is a graph of the chemical composition of the areas circled in FIG. 2A;

(3) FIG. 3A is an SEM image of the sample shown in FIG. 1B with several circled areas of interest, FIG. 3B is a graph of the chemical composition of the areas circled in FIG. 3A;

(4) FIG. 4 is a graph of an X-ray photoelectron spectroscopy analysis of the pretreatment coating of FIG. 1A according to the invention; and

(5) FIG. 5 is a graph of an X-ray photoelectron spectroscopy analysis of the pretreatment coating of FIG. 1B.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(6) The present invention is directed to a metal pretreatment coating composition, and a method for applying the same, as well as to articles of manufacture comprising coatings according to the invention. The invention provides surprising improvements in performance in zirconium-based conversion coating pretreatments such as, by way of non-limiting example, zirconium-based conversion coatings deposited on a metal substrate by contact with a working bath containing dissolved zirconium in the coating compositions. These conversion coating compositions are exemplified by aqueous coating baths comprising dissolved zirconium and free fluoride that form coatings comprising zirconium and oxygen. The baths are typically aqueous, neutral to acidic, and comprise dissolved zirconium, dissolved copper, either as an additive or as a trace element from water or metal substrates, and a source of fluoride. Optional components may be present including materials comprising one or more of silicon (e.g. silica, silicates, silanes), boron, yttrium, particular embodiments of which have no phosphates and no zinc, nickel, cobalt, manganese, and chromium.

(7) Many zirconium-based coating baths contain copper, either as an additive or as a trace element from water or from metal workpieces being coated. Regardless of its source, the present inventors have discovered that copper from the zirconium-based coating bath that is deposited in the coating can negatively affect performance of the coated metal substrate, if present in amounts such that undesirable morphologies in the coating arise and/or in amounts above desirable levels.

(8) Many zirconium-based pretreatment coating compositions may benefit from the invention. The coating baths typically are aqueous, neutral to acidic, and comprise dissolved zirconium, dissolved copper, a source of fluoride and counter ions for the dissolved metals, for example sulfates and/or nitrates. Optional components may be present including materials comprising one or more of silicon (e.g. silica, silicates, silanes), boron, yttrium. The zirconium-based pretreatment coating compositions may contain acid, generally a mineral acid, but optionally organic acids; and/or an alkaline source. The acid and/or alkali may be a source of other components in the composition, may be used to control pH or both. The zirconium-based pretreatment coating compositions according to the invention may likewise, consist essentially of or consist of the materials described herein.

(9) The coating composition according to the invention provides zirconium-based coatings having improved paint adhesion and maintained corrosion resistance. These and other benefits are achieved by adding to a zirconium-based coating composition, either a bath or the concentrate, a chelating agent, preferably a copper metal chelating agent, to control the amount of copper deposited onto the metal substrate by the zirconium-based pretreatment coating composition. This chelating agent can be added to the zirconium-based pretreatment coating composition even where no copper is present in the unused zirconium-based pretreatment coating composition, as a protective agent to prevent later copper deposition as the bath ages and copper is incorporated into the bath as a trace element from water, such as from prior cleaning or rinse steps, and/or from metal workpieces being coated. The inclusion of the chelating agent also extends the pot life of the pretreatment coating bath because it allows for a wider range of immersion times without negative effects on paint adhesion or corrosion protection.

(10) In one embodiment of the invention, a zirconium-based pretreatment coating composition is provided comprising 50 to 300 ppm of dissolved Zr, 0 to 50 ppm of dissolved Cu, 0 to 100 ppm of SiO.sub.2, 150 to 2000 ppm of total Fluoride, 10 to 100 ppm of free Fluoride and a chelating agent. That is, the composition may comprise amounts within the disclosed ranges, such as: 50, 60, 70, 80, 90, 100, 120, 130, 140 or 150 ppm to 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290 or 300 ppm of dissolved Zr; 0, 5, 10, 15, or 20 ppm to 25, 30, 35, 40, 45, or 50 ppm of dissolved Cu; 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50 ppm to 60, 65, 70, 75, 80, 85, 90, 95 or 100 ppm of SiO.sub.2; 150, 170, 190, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 700, 800, 900, or 1000 ppm to 1150, 1170, 1190, 1200, 1225, 1250, 1275, 1300, 1325, 1350, 1375, 1400, 1425, 1450, 1475, 1500, 1525, 1550, 1575, 1600, 1700, 1800, 1900, or 2000 ppm of total Fluoride; 10, 15, 20, 25, 30, 35, 40, 45, or 50 ppm to 60, 65, 70, 75, 80, 85, 90, 95 or 100 ppm of free Fluoride and a chelating agent.

(11) In another embodiment of the invention, a zirconium-based pretreatment coating composition is provided comprising 100 to 300 ppm of dissolved Zr, 0 to 50 ppm of dissolved Cu, 0, 25, 50, 75, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 or 2000 ppm to 2500, 3000, 4000, 4500 or 5000 ppm of SO.sub.4, 100 to 1600 ppm of total Fluoride, 10 to 200 ppm of free Fluoride and a chelating agent.

(12) The chelating agent may be any chelating agent capable of reducing the amount of copper deposited in the zirconium based coating. The chelating agent may be a copper metal chelator. A partial list of exemplary chelating agents, many of which are molecules having multiple carboxylic and/or phosphonic functional groups, that can be used in the present invention includes the following: adenine, adenosine, alanine, aminosalicylic acid, ascorbate/ascrobic acid, aspartate/aspartic acid, benzoic acid, citrate/citric acid, cyanuric acid, cysteine, cuprizone, diethanolamine, diethylenetriamine, diethylenetriamine-pentamethylene phosphonic acid, dihydroxybenzoic acid, dimethylenediamine, dimethylenetriamine, dimethylenetriaminepentaacetate (DTPA), dimethylglycine, dimethylglyoxime, ethylenediaminetetraacetate (EDTA), ethyleneglycol, gluconate/gluconic acid, glutamate/glutamic acid, glycerol, glycine, guanine, guanosine, histadine, histamine, hydroxyacetic acid, hydroxyethylidene diphosphonic acid (HEDP), hydroxyglutamic acid, hydroxylamine, iminodisuccinate, kojic acid, lactate/lactic acid, leucine, malonic acid, mannitol, methylglycine, molybdate, nitrilotriacetate, nitrosalicylic acid, ornithine, oxalic acid, polyacrylates, polyaspartates, phenylalanine, salicylic acid, salicylaldoxime, sodium nitrite, sodium nitrobenzenesulfonate, tartrate/tartaric acid, triethanolamine (TEA), triethylenetriamine (TETA), tris (2-aminoethyl)amine (diethylenetriamine), or thioacetamide.

(13) These chelating agents may be utilized according to the following methods: they may be incorporated into a pre-rinse applied prior to contacting the metal substrate with a zirconium-based pretreatment coating composition; the chelating agents may be incorporated into a zirconium-based pretreatment coating composition as discussed above; the chelating agents may also be applied as a post-rinse applied after the metal substrate has been contacted with a zirconium-based pretreatment coating composition.

(14) The chelating agents are used a level sufficient to ensure that in the deposited pretreatment coating the average total ratio of the atomic % of Cu to the atomic % of Zr in the pretreatment coating on the metal substrate is equal to or less than 1.1, preferably from 0.9 to 0.02, and most preferably from 0.30 to 0.10.

(15) The amount of chelating agent in the coating composition may range from 10 ppm to 2000 ppm. The amount required is affected by, for example, the amount of copper present in the coating composition, the temperature of the coating bath, the substrate being coated, whether the composition is a concentrate or the working bath and the particular chelating agent being used. Chelating agents with multiple coordination sites may be used at lower levels. In one embodiment the chelating agent is present in an amount ranging from 25-100 ppm in the coating bath. More chelating agent may be added provided the concentration does not adversely affect bath performance. Desirably, the amount of chelating agent in the pretreatment coating composition is an amount sufficient to achieve a desired Cu:Zr ratio in the deposited coating and preferably the chelating agent amount is at least, in increasing order of preference 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70 or 75 ppm and is at most, in increasing order of preference, 2000, 1500, 1000, 900, 800, 700, 600, 500, 400, 300, 200, 100 ppm.

(16) The average total ratio of atomic % of Cu to atomic % of Zr may range downward from, in order of increasing preference 1.10, 1.05, 1.0, 0.95, 0.90, 0.85, 0.80, 0.75, 0.70, 0.65, 0.60, 0.55, 0.50. For some zirconium-based pretreatment coating compositions, copper is a desirable part of the composition and the coating. For some such coating compositions, the ratio of copper to zirconium is desirably not less than, in increasing order of preference 0.0001, 0.0005, 0.0010, 0.0050, 0.010, 0.050.

(17) Zirconium-based pretreatment coatings of the invention may have a variety of components in the coating provided that the amount of copper in the coating is not such that undesirable coating morphology and performance failures result.

EXAMPLES

(18) In a standard industrial coating process, the immersion bath time for a pretreatment coating step is about 120 seconds, but during an assembly line stoppage this time can be 10 minutes or longer. To simulate a line stoppage and to test various parameters an alternative protocol was developed by the present inventors. The process used in the experiments described in the present specification is as shown in TABLE 1 below.

(19) The standard pretreatment process for all of the data, unless otherwise noted, is as described below in TABLE 1. The Parco Cleaner 1533R is an alkaline cleaner available from Henkel Adhesive Technologies. The Ridosol 1270 is a basic nonionic surfactant and is available from Henkel Adhesive Technologies. The weight ratio of Parco to Ridosol used was 8.33 to 1. Aging of the cleaner was simulated by adding the oil Tirroil 906 available from Tirreno Industries, to age the cleaner at 4 grams/liter. The base pretreatment composition was a zirconium-based pretreatment. The electrodeposited paint coating used in all of the paint adhesion tests was BASF Cathoguard 310X available from BASF. This is a standard coating used in the automotive industry.

(20) TABLE-US-00001 TABLE 1 Treat- Appli- Time, Temp Stage ment Product cation seconds C. 1 Clean Parco Cleaner Spray 70 60 1533R/Ridosol 1270 fresh or aged 2 Clean Parco Cleaner Immersion 150 60 1533R/Ridosol 1270 fresh or aged 3 Rinse City water Spray 60 28 4 Rinse Deionized water Spray 60 25 5 Pre- zirconium-based Immersion 600 25 treatment pretreatment bath 6 Pre- zirconium-based Spray 30 25 treatment pretreatment bath 7 Rinse Deionized water Spray 60 25 8 Electro- BASF immersion 120 32 (230 V) deposited Cathoguard coating 310 X 9 Rinse Deionized water Spray 30 25 10 Bake 1200 350 F. or electro- 375 F. deposited paint

Example 1

(21) The zirconium-based pretreatment bath used for Example 1 included 180 parts per million (ppm) of zirconium, 30 ppm of copper, 35 ppm of free and 400 ppm of total fluoride, 42 ppm of SiO.sub.2; the zirconium-based pretreatment bath pH was set at 4.2. Two different batches of commercially available, cold rolled steel (CRS 1 and CRS 2), as is typically used in automobile manufacture, were processed according to Table 1. The zirconium coating weight in milligrams Zr per square meter was determined for each sample.

(22) In addition for each sample, the paint adhesion of the BASF Cathoguard 310 X was determined using the following protocol. A sample area was cross hatched down to the level of the substrate with a razor using a line spacing of 1 millimeter and 6 lines for each direction. Then a 75 millimeter long strip of adhesive tape 20 millimeters wide was applied to the cross hatched area. The tape adhesively bonds to steel according to ASTM 3330M (Revised Oct. 1, 2004) with a 180 degree peel strength value of 430 N/m. After 5 to 10 seconds of adhesion, the tail end of the tape was grasped and pulled upward with a rapid jerking motion perpendicular to the paint. The percent paint remaining attached to the substrate (indicative of paint adhesion) was determined as a percentage of the area covered by the tape. The results of Example 1 are reported below in TABLE 2.

(23) TABLE-US-00002 TABLE 2 Zr coating Bake Sample CRS weight temperature % paint No. sample Cleaner mg/m.sup.2 F. remaining 1 CRS 1 1533/1270 143 350 100 fresh 2 CRS 1 1533/1270 203 350 100 aged 3 CRS 1 1533/1270 143 375 99-100 fresh 4 CRS 1 1533/1270 203 375 100 aged 5 CRS 2 1533/1270 165 350 95-98 fresh 6 CRS 2 1533/1270 182 350 99-100 aged 7 CRS 2 1533/1270 165 375 60-70 fresh 8 CRS 2 1533/1270 182 375 80 aged

(24) The results demonstrated a bake temperature effect on the electrodeposited coating adhesion. When the bake temperature of the electrodeposited paint was raised from 350 F. to 375 F., there was a reduction in paint adhesion, especially on the CRS 2 substrate. The results for CRS 2 were also quite different than for CRS 1. Further examination of samples from each CRS revealed striking differences in the deposited pretreatment coating composition. FIGS. 1A and 1B are at a magnification of 10,000 and 1C and 1D are a magnification of 30,000.

(25) Sample 3:

(26) FIGS. 1A and 1C are scanning electron microscope (SEM) photographs of CRS 1 coated with a pretreatment coating composition according to Example 1, using fresh 1533/1270, a Zr coating weight of 143 mg/m.sup.2 and a bake temperature of 375 F. as described above (Sample 3).

(27) Sample 7:

(28) FIGS. 1B and 1D are SEM photographs of CRS 2 coated with a pretreatment coating according to Example 1, using fresh 1533/1270, a Zr coating weight of 165 mg/m.sup.2 and a bake temperature of 375 F. as described above (Sample 7). Sample 7, the CRS 2 sample exhibited poor paint adhesion.

(29) The photographs show that the deposited pretreatment coating of Sample 3 in FIGS. 1A and 1C was composed of much smaller substructures than that found in the pretreatment coating surface of Sample 7 in FIGS. 1B and 1D. The surface in FIGS. 1B and 1D had larger and more clumped looking substructures.

(30) Sample 3:

(31) FIGS. 2A and 2B are a further analysis of the Sample 3 surface shown in FIGS. 1A and 1C. FIG. 2A shows an SEM photograph of the pretreatment coating at a magnification of 15,000 and also shows three circles labeled 1, 2, and 3. Each of these areas was subjected to Auger Emission Spectroscopy (AES) to identify the elements and their levels found in each area of analysis. The results were evaluated by looking at the deviation from the baseline for each area, to make comparison possible the baselines were offset as can be seen. The units on the y-axis in FIG. 2B are (counts/second)10.sup.5, that is, the y-axis amounts were increased by a factor of 100,000. The results show differences in the levels of copper between the three areas. The iron, zirconium and carbon levels were all very similar in the three areas. The largest substructure, area 1, had the highest level of copper. By way of contrast area 2, a very small substructure, had very little copper in it. Finally, area 3, which was taken between two larger substructures, showed a copper level that was between areas 1 and 2. The actual levels of copper were as follows: area 1 had a copper level of 27 atomic percent; area 2 had a copper level of 5 atomic percent; and area 3 had a copper level of 6 atomic percent. This represents a pretreatment coating (Sample 3) that led to good paint adhesion as shown in TABLE 2.

(32) Sample 7:

(33) FIGS. 3A and 3B show a further analysis of the surface shown in FIGS. 1B and 1D (Sample 7). FIG. 3A shows an SEM photograph of the pretreatment coating at a magnification of 15,000 and also shows two circles labeled 4 and 5. Each of these areas was subjected to AES to identify the elements and their levels found in each spot of analysis. The results were evaluated by looking at the deviation from the baseline for each area, to make comparison possible the baselines were offset as can be seen. The units on the y-axis of FIG. 3B are (counts/second)10.sup.4, that is, the y-axis amounts were increased by a factor of 10,000, therefore 1 unit in FIG. 3B is equal to 10 units in FIG. 2B. Area 4 is of a large substructure and the AES analysis showed that it had a very high level of copper, much higher than that found in the large substructure shown in FIGS. 2A and 2B, area 1. In addition, area 5, a small substructure showed lower levels of copper than area 4, but much higher than even area 1 of FIGS. 2A and 2B considering the differences in the units. The actual values for Sample 7, FIG. 3B, were as follows: area 4 had a copper level of 31 atomic percent and area 5 had a copper level of 25 atomic percent, much higher on average than those of Sample 3, which had good paint adhesion. These results show that excess copper in the deposited pretreatment coating caused poor paint adhesion and led to formation of larger substructures which was also not beneficial for paint adhesion. Pretreatment coatings with good paint adhesion tended to have smaller and fewer substructures and less deposited copper.

(34) FIGS. 4 (Sample 3) and 5 (Sample 7) are graphical representations of the results from X-ray photoelectron spectroscopy (XPS) depth analysis of the two sample pretreatment coatings described in FIGS. 2 (Sample 3) and 3 (Sample 7), respectively. In this analysis an argon beam was used to penetrate the coating and as it moved through the coating the atomic percentages of the coating components were determined at a series of depths from the outer surface of the coating. The spot size for analysis was approximately 22 millimeters. Once the atomic percentage of iron (Fe) exceeded 50% the beam had reached the underlying CRS substrate. Turning to FIG. 4 (Sample 3), the box outline represents the pretreatment coating, it can be seen that the coating was approximately 145 nanometers thick while the coating of FIG. 5 (Sample 7) was approximately 220 nanometers thick. The Figures further confirmed that higher copper levels in the coating correlated to poor paint adhesion: FIG. 5, showing a graph of Sample 7, the sample exhibiting poor paint adhesion, showed that the copper levels in the deposited pretreatment coating were much higher than in Sample 3, the pretreatment coating exhibiting good paint adhesion, whose graph is shown in FIG. 4. Both the atomic percentage and the area under the curve for the copper were much greater in FIG. 5 (Sample 7) compared to FIG. 4 (Sample 3). The peak atomic % Cu in FIG. 4 (Sample 3) was 33 atomic %. The peak atomic % Cu in FIG. 5 (Sample 7) at any depth was 42.73 atomic %.

(35) In further testing it has been determined that enhanced paint adhesion is seen when the pretreatment coating composition has sufficient chelating agent to ensure that the deposited pretreatment coating on the metal substrate has an average total ratio of the atomic % of copper to the atomic % of zirconium equal to or less than 1.1, more preferably the ratio is from 0.9 to 0.02, and most preferably from 0.3 to 0.1. This ratio is determined from the average overall atomic percentages of the Zr and Cu in the coating not from the ratio at a single depth. As can be seen in the data from FIGS. 4 (Sample 3) and 5 (Sample 7), as one moved through the coating composition down to the metal substrate the atomic percentage of the coating components varied by depth until one reached the metal substrate, so it is the total overall average atomic % ratio that must be determined. By way of contrast, the overall average total ratio of atomic % of Cu to atomic % of Zr seen in the deposited pretreatment coating composition shown in FIGS. 1B, 1D, 3, and 5 (Sample 7) was 2.73. The results led the inventors to develop the hypothesis that controlling the amount of deposited copper in the pretreatment coating, in the presence of copper in the pretreatment bath, could improve paint adhesion and also extend the pot life of zirconium-based pretreatment coatings baths, which was tested in Example 2 below.

Example 2

(36) In Example 2, the control pretreatment coating composition was a zirconium-based coating bath, wherein the Zr level was 180 ppm, Cu was 30 ppm, total Fluoride was 400 ppm and free Fluoride was 35 ppm, the level of SiO.sub.2 was 42 ppm. The test pretreatment coating composition was the same as the control and further comprising a chelating agent, tartrate introduced as tartaric acid at 50 ppm. The pH of the pretreatment coating compositions was adjusted to 4.0. The substrate was CRS that had been pre-cleaned with fresh Parco 1533 and rinsed as described in TABLE 1 above. The immersion time in the control and the test zirconium-based coating baths was either 4 minutes or 10 minutes, simulating a shorter and a longer line stoppage. A portion of each set of samples were then further coated with BASF Cathoguard 310X as described above and baked at 375 F. The baked samples were then tested for paint adhesion as described above. In addition, the coating weights of Zr in mg/m.sup.2 were determined for the samples. Finally the average atomic percentage of Zr and Cu in the pretreatment coatings was determined for each sample. The results are present below in TABLE 3.

(37) TABLE-US-00003 TABLE 3 Average Average Paint Pretreatment Immersion Zr coating atomic atomic Ratio adhesion % Example coating bath time minutes wt. mg/m.sup.2 % Cu % Zr of Cu/Zr remaining Comp. zirconium-based 4 166 3.8 3.3 1.15 90 Ex. 2-1 coating bath Comp. zirconium-based 10 340 9.1 7.6 1.20 50 Ex. 2-2 coating bath Ex. 2-3 zirconium-based 4 115 2.5 2.6 0.96 100 coating bath, plus 50 ppm tartrate Ex. 2-4 zirconium-based 10 182 4.0 5.2 0.77 100 coating bath, plus 50 ppm tartrate

(38) The results of Table 3 showed that an increased immersion time led to an increase in Zr coating weight, amount of Zr deposited, and the amount of Cu deposited. Inclusion of the tartrate at 50 ppm reduced the Zr coating weight, the amount of Zr deposited, and the amount of copper deposited in the pretreatment coating. More significantly, the presence of tartrate enhanced the pot life of the zirconium-based coating bath. This is seen by the fact that with tartrate present in the coating bath, the paint adhesion remains at 100% even after a 10 minute immersion, whereas in the absence of tartrate, the paint adhesion was significantly reduced to 90% or 50% of the applied paint coating. This tends to show that too much copper, relative to zirconium, deposited during the zirconium-based pretreatment coating bath can reduce paint adhesion and shorten pot life of the coating bath and that chelating agents, particularly copper metal chelators can improve paint adhesion and pot life.

Example 3

(39) In a next series of experiments, the effect of inclusion of the metal chelator tartrate on corrosion performance was tested. Again the substrate was CRS. The CRS was treated as described below in TABLE 4. The 2 minute treatment in the zirconium-based coating immersion bath is a standard time used in the industry.

(40) As a separate control samples of the CRS were also treated with the pretreatment coating Bonderite 958 and sealer Parcolene 91, both available from Henkel Adhesive Technologies per the manufacturer's directions. As a final control CRS samples were simply cleaned with Parco Cleaner 1533R/Ridosol 1270 fresh and rinsed with no pretreatment coating. Then all the samples were coated with the BASF Cathoguard 310X, rinsed and baked.

(41) TABLE-US-00004 TABLE 4 Temper- Treat- Appli- Time, ature Stage ment Product cation seconds C. 1 Clean Parco Cleaner Spray 70 60 1533R/Ridosol 1270 fresh 2 Clean Parco Cleaner Immersion 150 60 1533R/Ridosol 1270 fresh 3 Rinse City water Spray 60 28 4 Rinse Deionized water Spray 60 25 5 Pre- zirconium-based Immersion 120 or 25 treatment coating bath 600 with or without 50 ppm tartrate 6 Pre- zirconium-based Spray 30 25 treatment coating bath with or without 50 ppm tartrate 7 Rinse Deionized water Spray 60 25 8 Electro- BASF immersion 120 32 (230 V) deposited Cathoguard coating 310 X 9 Rinse Deionized water Spray 30 25 10 Bake 1200 375 F. electro- deposited

(42) Samples were then scribed to the CRS substrate and subjected to one of two corrosion performance tests. The first test was according to ASTM B117 (Revised Dec. 15, 2007) for 500 hours. In a second test, a 31 cycle test, the sample panels were subjected to 31 cycles of a 24 hour testing protocol using a salt misting spray. The salt misting spray comprised 0.9% by weight sodium chloride, 0.1% by weight calcium chloride, and 0.075% by weight sodium bicarbonate at pH 6 to 9. The first 8 hours the panels were kept at 25 C. and 45% Relative Humidity (RH) and misted 4 times during the 8 hours at time 0, 1.5 hours, 3 hours and 4.5 hours. The panels were then put at 49 C. and 100% RH for the next 8 hours with a ramp up from 25 C. to 49 C. and 100% RH over the first hour. The final 8 hours were at 60 C. and less than 30% RH with a ramp to the new conditions of 3 hours. The cycle was carried out for a total of 31 times. The panels were then evaluated for average creep and maximum creep in millimeters from the scribe line. The results for the ASTM B117 test are presented in TABLE 5. The results for the 31 cycle corrosion test are presented in TABLE 6.

(43) TABLE-US-00005 TABLE 5 ASTM B117 Maximum Average Corrosion creep, Corrosion creep, Pretreatment millimeters millimeters zirconium-based coating bath, 9 3.9 2 minutes zirconium-based coating bath, 3.5 2.5 10 minutes zirconium-based coating bath, 6 3 2 minutes, 50 ppm tartrate zirconium-based coating bath, 3 1.9 10 minutes, 50 ppm tartrate Bonderite 958/Parcolene 91 2 1.3

(44) TABLE-US-00006 TABLE 6 31 Cycle Corrosion Test Maximum Average Corrosion creep, Corrosion creep, Pretreatment coating millimeters millimeters Clean only 11 10.4 zirconium-based coating bath, 3.8 3.1 2 minutes zirconium-based coating bath, 5.3 4.3 10 minutes zirconium-based coating bath, 4.2 3.6 2 minutes, 50 ppm tartrate zirconium-based coating bath, 4.5 3.8 10 minutes, 50 ppm tartrate Bonderite 958/Parcolene 91 2.2 2.2

(45) The results indicate that inclusion of the tartrate did not have a negative effect on the ability of the zirconium-based pretreatment coating to provide corrosion resistance to the CRS. Under the ASTM B117 500 hour test the results of using the tartrate were at least as good as the standard zirconium-based coating bath and were slightly better for extended dwell time of the CRS in the bath evidencing the improved pot life from the chelator. The longer immersion times did not reduce the corrosion protection and may even increase it. In the 31 cycle test the benefit of using a pretreatment coating was shown, in the clean only sample there was much more corrosion than in any of the pretreatment coating examples. The presence or absence of the tartrate did not seem to affect the corrosion protection ability of the pretreatment coating. These results are important because if the presence of a chelating agent, such as the tartrate, was detrimental to the corrosion protection then one would have to balance that negative effect against the beneficial effect on paint adhesion.

Example 4

(46) In a next series of experiments the effects of another chelating agent, triethanolamine (TEA), were tested. The substrate was CRS and the pretreatment coating and BASF Cathoguard were applied as described below in TABLE 7. Again the zirconium-based coating bath included 180 ppm of Zr, 30 ppm of Cu, 35 ppm of free and 400 ppm total Fluoride and 42 ppm of SiO.sub.2. Samples were then tested for Zr coating weight in mg/m.sup.2, paint adhesion, and corrosion protection under ASTM B117 for 500 hours. As a control samples were also prepared with a pretreatment coating of Bonderite 958 and Parcolene 91 as described in Example 3. The results are presented below in TABLE 8. Only a single concentration of TEA was tested, the same level as used for tartrate of 50 ppm.

(47) TABLE-US-00007 TABLE 7 Temper- Treat- Appli- Time, ature Stage ment Product cation seconds C. 1 Clean Parco Cleaner Spray 70 60 1533R/Ridosol 1270 fresh 2 Clean Parco Cleaner Immersion 150 60 1533R/Ridosol 1270 fresh 3 Rinse City water Spray 60 28 4 Rinse Deionized water Spray 60 25 5 Pre- zirconium-based Immersion 240 or 25 treatment coatings bath with 600 or without 50 ppm TEA 6 Pre- zirconium-based Spray 30 25 treatment coating bath with or without 50 ppm TEA 7 Rinse Deionized water Spray 60 25 8 Electro- BASF immersion 120 32 (230 V) deposited Cathoguard coating 310 X 9 Rinse Deionized water Spray 30 25 10 Bake 1200 375 F. electro- deposited paint

(48) TABLE-US-00008 TABLE 8 Zr Maximum Average coating Paint creep creep Pretreatment weight adhesion % milli- milli- (ND = not determined) mg/m.sup.2 remaining meters meters zirconium-based coating 122 60 9 3.9 bath, 4 minute immersion zirconium-based coating 202 50 3.5 2.5 bath, 10 minute immersion zirconium-based coating 115 98 11.8 8.8 bath, plus 50 ppm TEA, 4 minute immersion zirconium-based coating 231 90 4.8 3.3 bath, plus 50 ppm TEA, 10 minute immersion Bonderite 958/ 0 ND 2.0 1.3 Parcolene 91

(49) The results again demonstrate the benefit of including a chelating agent, in particular a copper metal chelator, in the pretreatment coating on the paint adhesion. In the presence of 50 ppm of TEA the paint adhesion was significantly enhanced, even with a long immersion of 10 minutes. The results show that at this level of TEA there was a negative effect on corrosion protection. Clearly, the optimum level of copper metal chelator is dependent on the identity of the chelator. There was also no reduction of Zr coating weight with TEA at 50 ppm.

(50) The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.