Chromium-free conversion coating

Abstract

A chromium-free conversion coating is prepared by the addition of inorganic metallic salts and one or more silanes to dispersions of conducting polymers which are then exposed to alloys of aluminum or other metals. Advantageously, the performance of the coating is comparable to that of conventional chromium-based methods for a number of aluminum alloys having particular significance in the manufacture of aircraft.

Claims

1. A conversion coating composition for the treatment of metallic surfaces, the conversion coating composition comprising: a conducting polymer dispersion containing a conducting polymer selected from the group consisting of polyethylenedioxythiophene (PEDOT) and polypyrrole (PPY), one or more silanes comprising (3-Glycidoxypropyl) trimethoxysilane (GPMS), 1,2-Bis(trimethoxysilyl)ethane (TMSE), 1,2-Bis(Triethoxysilyl) Ethane (BTSE), Bis[3-(trimethoxysilyl)propyl]amine (BAS), Vinyltriacetoxysilane (VTAS), and combinations of two or more thereof; and an inorganic metallic salt of at least one of, molybdenum, magnesium, zirconium, titanium, vanadium, cerium, hafnium, silicon, aluminum, boron, cobalt, and zinc, wherein a concentration of the inorganic metallic salt is between 2.0 g/L (grams per liter) and 20 g/L (grams per liter), and a pH value of the conversion coating composition is between 1 and 6.0.

2. The conversion coating composition of claim 1, wherein the one or more silanes are water soluble.

3. The conversion coating composition of claim 1, wherein the one or more silanes are present in an amount of from 0.01 v % (volume percent) to 1.0 v % (volume percent) of the conducting polymer dispersion.

4. The conversion coating composition of claim 1, wherein the conducting polymer dispersion is one selected from the group consisting of polyethylenedioxythiophene (PEDOT) and polypyrrole (PPY), and the inorganic metallic salt is hexafluorozirconate.

5. The conversion coating composition of claim 1, wherein the concentration of the inorganic metallic salt is between 2.0 g/L (grams per liter) and 8.0 g/L (grams per liter).

6. The conversion coating composition of claim 1, wherein the inorganic metallic salt comprises a salt of zirconium, and the concentration of the inorganic metallic salt of zirconium is produced with K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH is adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

7. The conversion coating composition of claim 1, for the treatment of metallic surfaces, wherein the metallic surfaces are ones selected from the group consisting of aluminum, copper, iron, and alloys thereof.

8. The conversion coating composition of claim 7, for the treatment of metallic surfaces, wherein the metallic surfaces are ones selected from the group consisting of 2024-T3 and 7075-T6 aluminum alloys.

9. The conversion coating composition of claim 1, wherein the one or more silanes are of a formula:
YSiX.sub.(3-a)Z.sub.a wherein, X are independently selected hydrolysable groups, Y is non-hydrolysable and includes a functional group, Z is independently selected from H or alkyl, and a is 0, 1 or 2.

10. The conversion coating composition of claim 9, wherein the hydrolysable groups are selected from the group consisting of methoxy and ethoxy.

11. The conversion coating composition of claim 9, wherein: the functional group is selected from the group consisting of vinyl, amino, epoxy and mercapto; and/or the functional group is linked to Si (the silicon atom) by an alkyl group, an alkyl ether group, or an alkyl amine group.

12. The conversion coating composition of claim 9, wherein Y is RSiX.sub.3-aZ.sub.a, and wherein R is an alkyl group, an alkyl ether group, or an alkyl amine group.

13. A conversion coating composition for the treatment of metallic surfaces, the conversion coating composition comprising: a conducting polymer dispersion consisting of: a conducting polymer selected from the group consisting of polyethylenedioxythiophene (PEDOT) and polypyrrole (PPY); one or more silanes selected from the group consisting of (3-Glycidoxypropyl) trimethoxysilane (GPMS), 1,2-Bis(trimethoxysilyl)ethane (TMSE), 1,2-Bis(Triethoxysilyl) Ethane (BTSE), Vinyltriacetoxysilane (VTAS), and combinations of two or more thereof; and inorganic metallic salts selected from at least one of molybdenum, magnesium, zirconium, titanium, vanadium, cerium, hafnium, silicon, aluminum, boron, cobalt, and zinc, in concentrations of the inorganic metallic salts between 2.0 g/L (grams per liter) and 20 g/L (grams per liter), and a pH value of the conversion coating composition is between 1 and 6.0.

14. The conversion coating composition of claim 13, wherein the one or more silanes are water soluble.

15. The conversion coating composition of claim 13, wherein the one or more silanes are present in an amount of from 0.01 v % (volume percent) to 1.0 v % (volume percent) of the conducting polymer dispersion.

16. The conversion coating composition of claim 13, wherein the inorganic metallic salts comprise a salt of zirconium, and the concentration of the inorganic metallic salt of zirconium is produced with K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH is adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

17. A conversion coating composition for coating pretreated metallic surfaces, the conversion coating composition comprising: a conducting polymer dispersion consisting of: a conducting polymer selected from the group consisting of polyethylenedioxythiophene (PEDOT) and polypyrrole (PPY); one or more silanes selected from the group consisting of (3-Glycidoxypropyl) trimethoxysilane (GPMS), 1,2-Bis(trimethoxysilyl)ethane (TMSE), 1,2-Bis(Triethoxysilyl) Ethane (BTSE), Vinyltriacetoxysilane (VTAS), and combinations of two or more thereof; and inorganic metallic salts selected from at least one of molybdenum, magnesium, zirconium, titanium, vanadium, cerium, hafnium, silicon, aluminum, boron, cobalt, and zinc, in concentrations of the inorganic metallic salts between 2.0 g/L (grams per liter) and 20 g/L (grams per liter), and the conversion coating composition having a pH value between 1 and 6.0.

18. The conversion coating composition of claim 17, wherein the one or more silanes are water soluble.

19. The conversion coating composition of claim 17, wherein the one or more silanes are present in an amount of from 0.01 v % (volume percent) to 1.0 v % (volume percent) of the conducting polymer dispersion.

20. The conversion coating composition of claim 17, wherein the inorganic metallic salts comprise a salt of zirconium, and the concentration of the inorganic metallic salt of zirconium is produced with K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH is adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the disclosure may be realized by reference to the accompanying drawings in which:

(2) FIG. 1 is a process flow chart depicting the steps associated with the chromium-free conversion coating of the disclosure;

(3) FIG. 2 is a table that shows the properties of several conducting polymers used in the disclosure according to data provided by suppliers of the polymers;

(4) FIG. 3 is a table that shows the experimental conditions for PEDOT/Zr for both tested alloys according to the disclosure;

(5) FIG. 4 is a table that shows the experimental conditions for PPY/Zr for both tested alloys according to the disclosure;

(6) FIG. 5 is a table that shows the measured corrosion of alloys treated with PEDOT/Zr according to the disclosure;

(7) FIG. 6 is a table that shows the measured corrosion of selected alloys treated with PPY/Zr according to the disclosure;

(8) FIG. 7 is a table that shows the molecular and structural formula of several silanes used in the disclosure;

(9) FIG. 8 is a table that shows the experimental conditions for PEDOT/Zr/silane for both tested alloys according to the disclosure;

(10) FIG. 9 is a table that shows the experimental conditions for PPY/Zr/silane for both tested alloys according to the disclosure;

(11) FIG. 10 is a table that shows the measured corrosion, adhesion and surface contact electrical resistance of alloys treated with PEDOT/Zr according to the disclosure; and,

(12) FIG. 11 is a table that shows the measured corrosion, adhesion and surface contact electrical resistance of alloys treated with PPY/Zr according to the disclosure.

DETAILED DESCRIPTION

(13) As can be appreciated by those skilled in the art, chemical conversion surface treatments/coatings generally involve the process of immersion or other contact of a metal (e.g., aluminum and/or alloys of aluminum) with an active bath or spray thatthrough a redox reaction at the metallic surface or chemical deposition at the metallic surface due to physicochemical changes in the treatment bathform a superficial adhered protective coating. Such conversion coatings typically exhibit quite low solubility andin the case of aluminuma thickness of approximately 20 nm (nanometer) to 1 mm (millimeter), depending upon the process parameters and the alloy treated, while the substrate thickness lost is quite small or minimal. The color of the resulting conversion coating obtained depends upon the base material and the bath/spray parameters.

(14) Advantageously, the conversion coating may be prepared in a single-step immersion process. Accordingly, parts, e.g., panels, to be coated are bathed in a conducting polymer dispersion in which different inorganic salts and silanes along with other additives that affect the bath and/or resulting coating, e.g., bath dispersion agents, wetting agents, or polymeric film formation agents.

(15) FIG. 1 depicts an overview of the steps involved in the process. More particularly, the process includes three general phases or steps namely, pretreatment, conversion, and drying. And while the discussion herein is concerned primarily with aluminum and certain specific alloys of aluminum, the disclosure is not so limited. In particular, different metal compositions and alloys, as well as additional applications, e.g., automotive, industrial, etc., would benefit from the disclosed process or method and resulting coating as well.

(16) Returning now to FIG. 1, it may be observed that pretreatment begins with step 110 by degreasing the panels to be coated. Degreasing may be performed using any of a variety of known detergent solutions and/or organic solvents. Additionally, such degreasinglike all of the process stepsmay be performed by spray application or bath/immersion, or a mixture of the two techniques.

(17) Once the panel(s) to be coated is degreased, it is then cleaned/washed with an alkali solution with step 120 (see FIG. 10 of alkaline cleaning. Such alkali solutions are commercially available under various trade names e.g., TURCO (4215NCLT), and this alkali cleaning/washing is advantageously performed for approximately 10 (ten) minutes at a modest elevated temperature, e.g., 50 C. (degrees Celsius). After cleaning/washing, the panel is rinsed with water and then deoxidized with step 130 (see FIG. 1) of deoxidizing, with, for example, TURCO Smut Go NC for approximately 5 (five) minutes at ambient temperature(s), and then rinsed. Advantageously, other pickling or desmutting steps can be used depending on the treated substrate material and surface material or thickness to be removed.

(18) As can be now appreciated, the process employs commercially available pretreatment steps which are well known and understood. Advantageously, such pretreatment is compatible with a variety of alloys and their application is widely understood.

(19) In an exemplary embodiment, step 140 (see FIG. 1) of conversion treatment includes immersion of aluminum alloy panels in a bath for a period of time followed by direct (no rinse) step 150 (see FIG. 1) of drying of the treated panels. Generally, the conversion treatment bath is prepared by an initial stirring of a conducting polymeric dispersion. Advantageously, the polymer dispersion(s) used may be commercially available water-based ones and exhibit satisfactory formulation(s), including solid content, pH, and dispersive additives. Consequently, only a minimal amount of stirring is required for these commercial dispersions. Of further advantage, the conversion treatment in the bath is only a 2 (two) minute process.

(20) Such conducting polymeric dispersions include Polyanaline (PANI), Polyethylenedioxythiophene (PEDOT), and Polypyrrole (PPY), among others. The particular conducting polymeric dispersions used in the examples and their physical properties are shown in FIG. 2 which illustrates a table. While the discussion herein is limited to those conducting polymeric dispersions exhibiting superior performance in the experiments, it should be noted that a number of dispersions may be suitable, depending upon the particular application requirements. More specifically, dispersions of polyphenylene, polyphenylene vinylene, polyethylenesulfide, and derivatives of all the mentioned conducting polymers should produce satisfactory results.

(21) In addition, other polymeric components, such as acrylics, polyurethanes, epoxies, amino resins, phenolics, vinylics, polyesters, etc., may be added to enhance particular characteristics of the coating.

(22) Returning now to the description of the method, after stirring the conducting polymeric dispersion (and any polymeric components), a quantity of inorganic salt(s), or mixtures thereof, are added to the conducting polymeric dispersion and subsequently mixed until the added salts are suitably dissolved. Example salts include the inorganic salts of molybdenum, magnesium, zirconium, and titanium. More particularly, sodium molybdate, potassium permanganate, potassium hexafluorozirconate, and potassium hexafluorotitanate have been used with success. Final concentrations of the added salts in the bath solution(s) may vary over a wide range, e.g., 2 g/L (grams per liter) to 20 g/L (grams per liter).

(23) After the inorganic salt(s), or mixtures thereof, are added to the conducting polymeric dispersion and subsequently mixed until the added salts are suitably dissolved, a quantity of silane(s), or mixtures thereof, are added to the conducting polymeric/salt(s) dispersion and subsequently mixed until the added silanes are suitably dissolved. The particular silanes used in the examples and their molecular and structural formula are shown in FIG. 7. (3-Glycidoxypropyl)trimethoxysilane (GPMS), 1,2-Bis(trimethoxysilyl)ethane (TMSE), 1,2-Bis(Triethoxysilyl) Ethane (BTSE), Bis[3-(trimethoxysilyl)propyl]amine (BAS), and Vinyltriacetoxysilane (VTAS) have been used with success. The final concentrations of the added salts in the bath solution(s) may vary over a range, e.g., 0.01 v % (volume percent) to 1.0 v % (volume percent).

(24) Finally, the polymeric dispersion/inorganic salt/silane solution is subsequently pH adjusted using alkaline compounds, such as ammonia or phosphate or acidic compounds, including hexafluorozirconic acid and fluorhydric acid.

EXAMPLES

(25) A number of samples of two particular aluminum alloys, namely 2024-T3 and 7075-T6 aluminum alloys were subjected to the chromium-free conversion process and evaluated. Those showing superior characteristics in salt spray fog corrosion tests (SSFCT) were obtained using PPY and PEDOT in combination with hexafluorozirconic. The particular experimental conditions are shown in the tables of FIGS. 3 and 4 for PEDOT/Zr, PPY/Zr, based compositions and the tables of FIGS. 5 and 6 show the obtained results, respectively. For all of the samples shown in these tables of FIGS. 3-6, the drying conditions were substantially room temperature and pressure, for a period of time of at least 24 hours.

(26) More specifically, the table of FIG. 3 shows the experimental conditions for PEDOT/Zr. In this set, the [Zr] (zirconium) concentration was effected by varying the amounts of K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH was adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

(27) The table of FIG. 4 shows the experimental conditions used for a PPY/Zr set of samples. In this particular set, the [Zr] (zirconium) concentration was effected by varying the amounts of K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH was adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

(28) Turning now to the table of FIG. 5, there it shows the corrosion resistance for the PEDOT/Zr conversion coating on both 2024-T3 and 7075-T6 aluminum alloys. The results obtained were after 168 hours of salt spray fog corrosion testing (SSFCT) and the hexavalent chromium based commercial ALODINE 1200S exhibited the best corrosion performance, with a corrosion score of 10.0. The corrosion score values go from 0 (zero) for the worst corrosion performance to 10 (ten) for best corrosion performance.

(29) Similarly, the table of FIG. 6 shows the corrosion resistance for the PPY/Zr coating on the 2024-T3 and 7075-T6 aluminum alloys, as well as the ALODINE 1200S treated alloys.

(30) A number of samples of two particular aluminum alloys, namely 2024-T3 and 7075-T6 aluminum alloys, were subjected to the polymeric dispersion/inorganic salt/silane chromium-free conversion process and evaluated. Those showing superior characteristics in salt spray fog corrosion tests (SSFCT) were obtained using PPY and PEDOT in combination with hexafluorozirconate and GPMS, TMSE and BTSE silanesadded either alone or in combination. Those showing superior characteristics in the scribed wet tape paint adhesion tests of a subsequently applied organic coating were obtained using PPY and PEDOT in combination with hexafluorozirconate and GPMS, TMSE, BTSE, BAS and VTAS silanesadded either alone or in combination. Some of the proposed treatments provided combined superior characteristics in salt spray fog corrosion tests and in the scribed wet tape paint adhesion tests of a subsequently applied organic coating. Additionally, some of those treatments providing combined superior characteristics in salt spray fog corrosion tests and in wet tape paint adhesion tests of a subsequently applied organic coating also provided superior characteristics in surface contact electrical resistance measurements. The particular experimental conditions are shown in the tables of FIGS. 8 and 9 for PEDOT/Zr/silane and PPY/Zr/silane based compositions and the tables of FIGS. 10 and 11 show the obtained results, respectively. For all of the samples shown in these tables of FIGS. 8-11, the drying conditions were substantially room temperature and pressure, for a period of time of at least 24 hours.

(31) More specifically, the table of FIG. 8 shows the experimental conditions for PEDOT/Zr/silane. In this set, the [Zr] (zirconium) concentration was effected by varying the amounts of K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH was adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

(32) The table of FIG. 9 shows the experimental conditions for a PPY/Zr/silane set of samples. In this particular set, the [Zr] (zirconium) concentration was effected by varying the amounts of K.sub.2ZrF.sub.6 (potassium hexafluorozirconate), and the pH was adjusted with H.sub.2ZrF.sub.6 (fluorozirconic acid) and/or NH.sub.4OH (ammonium hydroxide).

(33) Turning now to the table of FIG. 10, there it shows the corrosion resistance for the PEDOT/Zr/silane conversion coating on both 2024-T3 and 7075-T6 aluminum alloys. The results obtained were after 168 hours of salt spray fog corrosion testing (SSFCT) and the hexavalent chromium based commercial ALODINE 1200S exhibited the best corrosion performance, with a corrosion score of 10.0. The corrosion score values go from 0 (zero) for the worst corrosion performance to 10 (ten) for best corrosion performance. The table in FIG. 10 also shows the adhesion performance of a subsequently applied organic coating on both 2024-T3 and 7075-T6 aluminum alloys. The paint adhesion performance was measured according to a wet tape paint adhesion test. Once dried (after 14 (fourteen) days air curing), the corresponding conversion coating coated panels were painted with an epoxy primer according to the MIL-PRF-85582 standard. The epoxy primer used was a water-reducible epoxy primer system made of 10PW20-4 base and ECW-104 hardener according to MIL-PRF-85582 Type 1 Class 2, provided by Akzo Nobel Aerospace Coatings, BV. Two parallel, 2 (two) inch long scratches, to 1 (one) inch apart through the coating and to the substrate were made on the panels. The parallel scratches were joined with two intersecting lines, or an X pattern. The primed and scribed panels were immersed in deionized water during 24 hours, prior to carrying out the wet paint adhesion tests. Within 2 (two) minutes after removing test panels from water adhesive tape was applied and pressed against the test surface with firm hand pressure and then removed. The hexavalent chromium based commercial ALODINE 1200S exhibited the best pant adhesion performance, with an adhesion score of 10.0. The adhesion test score values were from 0 (zero) for the worst adhesion performance (total detachment of the primer) to 10 (ten) for best adhesion performance (no detachment of the primer). The table in FIG. 10 also shows the surface contact electrical resistance for the PEDOT/Zr/silane conversion coating on 2024-T3, 7075-T6, and 6061-T6 aluminum alloys. The surface contact electrical resistance of the coatings was measured as described in the MIL-DTL-81706-B standard. The applied load was within one percent of the calculated 200 psi (pounds per square inch) applied pressure. The contacting electrodes were copper or silver-plated copper with a finish not rougher than that obtained by the use of 000 metallographic abrasive paper. The electrodes were flat enough so that when the load was applied without a specimen between them, light was not visible through the contacting surface. The area of the upper electrode was one square inch (25 square mm (millimeters)) and the area of the lower electrodes was larger. The maximum electrical resistance values allowed by aeronautical standards for 6061 T6 alloy are of 5000 /square inch (5 m/square inch) before salt spray exposure test. The hexavalent chromium based commercial ALODINE 1200S exhibited the lowest surface contact electrical resistance for the 2024-T3 and 7075-T6 aluminum alloys. The values for the PEDOT/Zr/silane treatments were also well below the 5 m/square inch.

(34) Similarly, the table of FIG. 11 shows the corrosion resistance, the paint adhesion performance and the surface contact electrical resistance measurements for the PPY/Zr/silane coatings on the 2024-T3, 7075-T6, and 6061-T6 alloys, as well as the ALODINE 1200S treated alloys.

(35) At this point, it should be noted that in addition to the Zr (zirconium) salts used in these exemplary tests, other saltseither alone or in combinationmay produce satisfactory results as well. In particular, salts of vanadium, cerium, hafnium, silicon, aluminum, boron, cobalt, magnesium, and zinc may be employed. Additionally, other bath components such as pH adjusting compounds, solvents, non-aqueous dispersion media, other silanes, dispersing agents, surfactants and coalescing solvents may be used to provide various degrees of coating effectiveness. Further, while the method and resulting coating(s) have been described in the context of immersion bath(s), it is understood that alternative coating, i.e., spray coating, may be used as well. Lastly, other metallic substrates, such as steel, aluminum, copper, and/or iron, and/or their alloys, will benefit from the disclosed method and coating(s).

(36) While the disclosure discusses and describes herein some specific examples, those skilled in the art will recognize that the disclosed teachings are not so limited. More specifically, it is understood that the method and coating may be used in virtually any application requiring corrosion protection, and/or adhesion of subsequently applied organic coating(s) and/or low electrical surface contact resistance, and in particular, those applications concerned with the problems associated with hexavalent chromium. Accordingly, it is understood that the method and coating may be applicable to any automotive, marine, construction, industrial, or household use in addition to aeronautical applications and therefore should be only limited by the scope of the claims attached hereto.