Hexavalent Chromium-Free Hard Coat Maskant

The present disclosure relates generally to hardcoat anodized components and, more particularly, to hardcoat anodized components having corrosion protection applied to non-wear surfaces.

As known, hardcoat anodize is used to provide wear-resistance to a variety of components, including aerospace components. Hardcoat anodize is a hard, thick, and wear resistant anodize coating that is often applied to functional surfaces that are susceptible to wear. Hardcoat anodize is not a desired coating for non-functional surfaces because it carries a significant fatigue debit and may create loosely adhered hard particles on the non-functional surfaces that can break off and create foreign object damage (FOD) that can be detrimental to the wear system.

One aspect of this disclosure is directed to a part including corrosion- and wear-resistant regions having a base alloy with a plurality of surfaces, a corrosion-resistant coating deposited on at least one surface, and a wear-resistant coating deposited on at least one surface. The corrosion-resistant coating includes a thin film sulfuric acid anodize sealed with sequentially applied dipotassium hexafluorozirconate, lanthanum nitrate hexahydrate plus hydrogen peroxide, and hydrothermal seal systems. The wear-resistant coating comprises hardcoat anodize.

Another aspect of this disclosure is directed to a method for forming corrosion- and wear-resistant regions on a part. A part of a base alloy with a plurality of surfaces is provided. The part is degreased and deoxidized. A thin film sulfuric acid (TFSA) anodize is applied to the deoxidized part and the TFSA anodized part is sealed with an aqueous solution of dipotassium hexafluorozirconate, an aqueous solution of lanthanum nitrate hexahydrate plus hydrogen peroxide, and a hydrothermal seal. The combination of TFSA anodize, dipotassium hexafluorozirconate seal, lanthanum nitrate hexahydrate seal, and hydrothermal seal form corrosion-resistant regions on the part. The combination of TFSA anodize, dipotassium hexafluorozirconate seal, lanthanum nitrate hexahydrate seal, and hydrothermal seal is machined from regions of the part to receive a wear-resistant treatment to expose base alloy and a hardcoat anodize is applied to base alloy in regions of the part to receive a wear-resistant treatment, thereby forming the wear-resistant treatment.

Hardcoat anodize and corrosion-resistant anodize treatments are typically applied to parts made from aluminum and aluminum alloys. Such treatments can also be applied to other metals, including magnesium. Hardcoat anodize and corrosion-resistant anodize treatments are frequently used with aerospace parts that operate in high-wear and high-corrosion environments, such as environments characterized by high-velocity air and high temperatures. For example, wrought, forged, or cast aluminum alloys such as 2000 series (e.g., 2024 (Al—Cu—Mg)) alloys, the 6000 series (e.g., 6061 (Al—Mg—Si), C355 (Al—Si—Cu), A356 (Al—Si—Cu) alloys, and the 7000 series (e.g., 7075 (Al—Zn—Mg)) are often used for aerospace applications and are candidates for hardcoat anodize and corrosion-resistant anodize treatments.

shows a flow chart for a prior art method of applying a hardcoat anodize maskant to a part. The part can be any part that can benefit from application of both hardcoat anodize and corrosion-resistant anodize treatments and can be made from any material that can benefit from such coatings. First, the part is subject to a degreasing process at step. The degreasing processcan by any such process that is effective to remove residual surface grease from the part. For example, the degreasing processmay use Turco 4215 NC-LT, a granular degreasing product available from Henkel (also known as Bonderite® C-AK 4215 NC-LT Aero) as the degreasing agent. In an exemplary process, the Turco 4215 NC-LT product can be added to cold water at a ratio of 45 g/liter to 60 g/liter of water using air or mechanical agitation to form an immersion bath. The part can be immersed in the bath at a temperature of 40° C. to 55° C. (104° F. to 131° F.) for 5 min to 10 min. For example, the part can be immersed in the bath at a 40° C. (104° F.) 10 min. Alternately, the degreasing processcan be performed using a spray system, an ultrasonic system, a combination of these systems or any other suitable system. Following the degreasing process, the part can be rinsed by any suitable method, for example with cold to warm water by spray or overflowing immersion methods, at step.

The part is then subject to a deoxidizing process at step. The deoxidizing processcan by any such process that is effective to remove oxides, alkaline etching smut, and discoloration from the part. For example, the degreasing processmay use Oakite® Deoxidizer LNC, available from BASF Chemetall. In an exemplary process, the part can be immersed in a 10 vol % to 20 vol % solution of Oakite® Deoxidizer LNC product at ambient temperature for 1 min to 5 min. The solution should be air agitated. For example, the part can be immersed in the solution at 21° C. (70° F.) for 10 min. Following the deoxidizing process, the part can be rinsed thoroughly in clean, ambient temperature water by any suitable method at step.

After degreasing and deoxidizing, the part is subject to a chromic acid (HCrO) anodize process at step. The chromic acid anodize processcan be any suitable process known in the art. For example, the part is connected to a positive terminal (anode) of an electrical circuit, a negative terminal (cathode) is connected to a metal electrode (e.g., a sheet or wire), and the part and metal electrode are placed into a chromic acid bath (e.g., at 21° C. (70° F.) for 10 min). When an electrical current flows through the circuit, the chromic acid in the bath dissociates or ionizes into positive and negative ions and the part draws negative oxygen ions from the solution to form an aluminum oxide coating on the part. Aluminum ions from the part migrate to the cathode. Depending on the process parameters the aluminum oxide coating can be on the order of 0.0000508 mm to 0.00254 mm (0.00002 in to 0.0001 in) thick or any other thickness deemed appropriate for a particular application. It may also be desirable for the aluminum oxide coating to have a minimum weight of 2.15 g/m(200 mg/ft) or any other weight deemed appropriate for a particular application. Following the chromic acid anodize process, the part can be rinsed thoroughly in clean, ambient temperature water by any suitable method at step.

After the chromic acid anodize process at stepand subsequent rinsing step, the part is sealed with a nickel acetate (CHNiO) sealing process at step. The a nickel acetate scaling processcan be any suitable process known in the art. For example, the anodized part can be immersed in a nickel acetate solution (e.g., Ni concentration of 1.4 g/L to 1.8 g/L; pH of 5.5 to 6.0) at a temperature of 85° C. to 99° C. (185° F. to 210° F.) for 10 min to 30 min (e.g., 99° C. for 20 min). The sealing can block open pores in the aluminum oxide layer with a deposition of bochmite ((γ-AlO(OH)) and nickel hydroxide (Ni(OH)) to improve the corrosion-resistance of the aluminum oxide layer.

After the nickel acetate sealing processlocations on the part that are to be coated with hardcoat anodize are machined to remove the aluminum oxide layer that was formed during the chromic acid anodize processto provide a bare metal surface to support the hardcoat anodize. The portions of the part with the remaining aluminum oxide layer will be masked from deposition of the hardcoat anodize. The hardcoat anodize processcan be any suitable process known in the art. For example, the part is connected to a positive terminal (anode) of an electrical circuit, a negative terminal (cathode) is connected to a metal electrode, and the part and metal electrode are placed into a temperature-regulated acidic electrolyte solution (for example, sulfuric acid). The anodizing cycle is initiated with an applied voltage of about 25 VDC that increases to about 120 VDC near the end of the process to compensate for decreasing conductivity of the part as the deposited aluminum oxide layer thickens. The thickness of the aluminum oxide layer is controlled by the electrolyte temperature, voltage, acid concentration, and time. For example, the aluminum oxide layer formed as a result of the hardcoat anodize process can be from 0.0127 mm to 0.0762 mm (0.0005 inches to 0.0030 inches) thick or any other thickness deemed appropriate for a particular application. For some applications, the electrolyte temperature can be about −18° C. (0° F.) and the process can run for about 40 min. Following the hardcoat anodize process, the part is inspected for hardcoat breakthrough at step. Hardcoat breakthrough is deposition of the hardcoat anodize in undesirable locations that can result in a fatigue debit and may create loosely adhered hard particles that could break off and create foreign object debris (FOD) that could be detrimental to the hardcoat anodize wear system.

Processes like those ofhave been used successfully for many years. The chromic acid anodize sealed with nickel acetate has been an effective maskant that is also serves as a functional anodize coating for corrosion protection. This coating is desirably thin so that it provides a much smaller fatigue debit compared to the hardcoat anodize. This process, however, requires hexavalent chromium, which is a material of concern and restricted for use in Europe under Registration, Evaluation, Authorisation and Restriction of Chemicals (REACh) regulations. As a result, processes that are substantially hexavalent chromium free (i.e., result in a chromium (VI) oxide concentration of below 0.1 wt %) are desired for certain applications.

shows a flow chart for a method for applying a corrosion-resistant coating that uses an organic maskant (such as wax or lacquer) to prevent hardcoat breakthrough. The first four steps, degreasing, rinsing, deoxidizing, and rinsingcan be the same as steps degreasing, rinsing, deoxidizing, and rinsingas discussed with regard toabove. The thin film sulfuric acid (HSO) anodize (TFSAA) processcan be any suitable process known in the art. For example, the part is connected to a positive terminal (anode) of an electrical circuit, a negative terminal (cathode) is connected to a metal electrode (e.g., a sheet or wire), and the part and metal electrode are placed into a sulfuric acid bath (e.g., at 21° C. (70° F.) for 20 min). When an electrical current flows through the circuit, the sulfuric acid in the bath dissociates or ionizes into positive and negative ions and the part draws negative oxygen ions from the solution to form an aluminum oxide coating on the part. Aluminum ions from the part migrate to the cathode. Depending on the process parameters the aluminum oxide coating can be on the order of 0.00254 mm to 0.0254 mm (0.0001 in to 0.001 in) thick or any other thickness deemed appropriate for a particular application. It may also be desirable for the aluminum oxide coating to have a minimum weight of 2.15 g/m(200 mg/ft) to 10.77 g/m(1000 mg/ft) or any other weight deemed appropriate for a particular application. Following the TFSAA process, the part can be rinsed thoroughly in clean, ambient temperature water by any suitable method at step.

After the TFSAA process at stepand subsequent rinsing step, the part is sealed with any suitable sealing process, such as the nickel acetate (CHNiO) sealing processdescribed above with respect toto improve the corrosion-resistance of the aluminum oxide layer. An organic maskant, such as wax, lacquer, or other suitable maskant, can be applied to the part at stepto limit hardcoat breakthrough. It can, however, be difficult and time consuming to apply the organic maskant to the part due to complex geometries on the part. If the maskant is not applied uniformly and completely to the part, there is a significant potential for hardcoat breakthrough.

After the organic masking process, locations on the part that are to be coated with hardcoat anodize are machined at stepto remove the aluminum oxide layer that was formed during the TFSAA processto provide a bare metal surface to support the hardcoat anodize in the same way as discussed for stepin. The hardcoat anodize processcan be the same hardcoat anodize processdescribed with regard to. Following the hardcoat anodize process, the organic maskant can be removed at stepusing any appropriate process. Finally, part is inspected for hardcoat breakthrough at step.shows locationsandon partthat exhibit hardcoat breakthrough. As discussed above, these locations can exhibit a fatigue debit and may create loosely adhered hard particles that could break off and create foreign object debris (FOD) that could be detrimental to the hardcoat anodize wear system

As discussed above, neither the process of(due to the use of hexavalent chromium) nor the process of(due to the high potential for hardcoat breakthrough) provide the desired wear and corrosion protection for the part.shows another method that addresses the downsides of the processes of. The first six steps, degreasing, rinsing, deoxidizing, rinsing, TFSAAand rinsingcan be the same as steps degreasing, rinsing, deoxidizing, rinsing, TFSAA, and rinsingas discussed with regard toabove.

The Socusurf TCS seal processuses a product known as Socusurf TCS (and its companion product known as Socusurf PACS described below) for chemical conversion and sealing parts after an anodizing process, such as TFSAA processdescribed above. Both Socusurf TCS and Socusurf PACS, which are used together as a Cr (III)-based aluminum surface treatment solution, are available from Socomore S.A.S of Vannes CEDEX, France and various global distributors, including Dysol Inc. of Fort Worth, TX. The Socusurf TCS seal processincludes immersing the part in the Socusurf TCS solution (an aqueous solution of >=1% to <3% of dipotassium hexafluorozirconate) at 35° C. to 45° C. (95° F. to 113° F.) for 10 minutes to 40 minutes, for example at 40° C. (104° F.) for 10 minutes. The parts are then rinsed at stepwith water. Rinsing can be done by immersion in a tank of water with either tank or part agitation. Rinsing also can be done with sprayed water that impinges on and drains from all part surfaces. The Socusurf PACS seal processincludes immersing the part in the Socusurf PACS solution (an aqueous solution of >=1% to <3% lanthanum nitrate hexahydrate) plus hydrogen peroxide (e.g., by adding 5 vol % to 7 vol % of 35% hydrogen peroxide or 6 vol % to 8 vol % hydrogen peroxide of 30% hydrogen peroxide) at 15° C. to 30° C. (59° F. to 86° F.) for 3 minutes to 10 minutes, for example at 15.5° C. (60° F.) for 5 minutes, without agitation. The parts are then rinsed at stepwith water using the same procedure as described for rinsing stepabove.

Following treatment with the Socusurf TCS and Socusurf PACS solutions as described above, the parts are subject to a hydrothermal seal process. In the hydrothermal seal processthe part is immersed in de-ionized water at 96° C. to 99° C. (200° F. to 210° F.) for 20 to 22 minutes, for example at 99° C. (210° F.) for 20 minutes, without agitation. The combination of the Socusurf TCS process, Socusurf PACS process, and hydrothermal seal processon a part that has had a TFSAA treatmentcreates a smooth surface without cracking that functions as a physical barrier (i.e., maskant) that blocks build-up of new anodize build-up during later processing, such as hardcoat anodize processing. As a result, surfaces treated with the combination of the Socusurf TCS process, Socusurf PACS process, and hydrothermal seal processwill not be available for formation of new hardcoat anodize to form. These surfaces will exhibit residual lanthanides as a result of treatment with the Socusurf PACS process.

Locations on the part that are to be coated with hardcoat anodize are machined at stepto remove the seal coatings applied with the combination of the Socusurf TCS process, Socusurf PACS process, and hydrothermal seal processto provide a bare metal surface to support the hardcoat anodize in the same way as discussed for stepinand stepin. The hardcoat anodize processcan be the same hardcoat anodize processdescribed with regard toand hardcoat anodize processdescribed with regard to.

show comparative partsand, each having anodized maskant surfacesand, respectively, and hardcoat anodized surfacesand, respectively. Partwas processed using the prior art method of(i.e., chromic acid anodize with nickel acetate seal) as a control article. The partexhibits anodized maskant surfaceswith no hardcoat breakthrough and hardcoat anodized surfacesthat have a continuous, flaw-free hardcoat anodize. Partwas processed using the method of(i.e., TFSAA with Socosurf TCS/Socosurf PACS/hydrothermal seal) to demonstrate the process that is the focus of the present disclosure. The partexhibits anodized maskant surfaceswith no hardcoat breakthrough and hardcoat anodized surfacesthat have a continuous, flaw-free hardcoat anodize.provides visual evidence that the process that is the focus of the present disclosure is a direct replacement for the prior art chromic acid anodize with nickel acetate seal process of. Combining the TFSAA process with Socosurf TCS/Socosurf PACS/hydrothermal seal process results in corrosion-resistant coatings and wear-resistant coatings that are both comply with the REACh regulations regarding hexavalent chromium. As such, these coatings can be described as having a concentration of chromium (VI) oxide (i.e., chromium trioxide: CrO) of below 0.1 wt % and being substantially hexavalent chromium free.

Table 1 below reports data from a series of corrosion resistant tests performed on test panels (two panels per system) coated with the processes of. The panels were coated as described above and then subject to 336 hours of a salt spray test according to ASTM B117. Following the test the panels were examined to identify and count pits formed during the testing. The success criterion was a count of 5 or fewer pits.

The test demonstrated that the corrosion resistance of Type IIb Anodize, TFSAA with Socosurf TCS/Socosurf PACS/hydrothermal seal as shown in, as measured by the salt spray test reported above is fully equivalent to the prior art Type I Anodize, chromic anodize with nickel acetate seal as shown in. These test results are consistent with the visual assessment of partsanddiscussed with regard toabove.

The method of the present disclosure (i.e., TFSAA with Socosurf TCS/Socosurf PACS/hydrothermal seal as shown in) provides a substantially hexavalent chromium-free (i.e., corrosion protection system with a chromium (VI) oxide concentration of below 0.1 wt %) substitute for the standard prior art method (i.e., chromic anodize with nickel acetate seal as shown in) that relies on hexavalent chromium. By using a substantially hexavalent chromium-free corrosion protection system, the corrosion protection system of this disclosure complies with the European Union REACh regulations regarding hexavalent chromium and can be used in Europe without relying on the difficult to use organic masking process (i.e., the process of), which has a high potential for failure due to hardcoat breakthrough.

The following are non-exclusive descriptions of possible embodiments of the present invention.

A part including corrosion- and wear-resistant regions comprises a base alloy with a plurality of surfaces, a corrosion-resistant coating deposited on at least one surface, and a wear-resistant coating deposited on at least one surface. The corrosion-resistant coating comprises a thin film sulfuric acid anodize sealed with sequentially applied dipotassium hexafluorozirconate, lanthanum nitrate hexahydrate, and hydrothermal seal systems. The wear-resistant coating comprises hardcoat anodize.

The part of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional elements:

The part of the preceding paragraph, wherein the base alloy is aluminum or an aluminum alloy.

The part of the preceding paragraph, wherein the base alloy is an aluminum alloy selected from the group consisting of 2000 series, 6000 series, and 7000 series aluminum alloys.

The part of any of the preceding paragraphs, wherein the thin film sulfuric acid anodize comprises aluminum oxide coating between 0.00254 mm to 0.0254 mm (0.0001 in to 0.001 in) thick.

The part of any of the preceding paragraphs, wherein the hardcoat anodize comprises aluminum oxide coating between 0.0127 mm to 0.0762 mm (0.0005 inches to 0.0030 inches) thick.

The part of any of the preceding paragraphs, wherein the corrosion-resistant coating and wear-resistant coating are substantially hexavalent chromium free with a chromium (VI) oxide concentration of below 0.1 wt %.

The part of any of the preceding paragraphs, wherein the corrosion-resistant coating comprises residual lanthanides.

The part of the above paragraph, wherein the base alloy is an aluminum alloy selected from the group consisting of 2000 series, 6000 series, and 7000 series aluminum alloys; the thin film sulfuric acid anodize comprises aluminum oxide coating between 0.00254 mm to 0.0254 mm (0.0001 in to 0.001 in) thick; the hardcoat anodize comprises aluminum oxide coating between 0.0127 mm to 0.0762 mm (0.0005 inches to 0.0030 inches) thick; the corrosion-resistant coating and wear-resistant coating are substantially hexavalent chromium free with a chromium (VI) oxide concentration of below 0.1 wt %; and the corrosion-resistant coating comprises residual lanthanides.

A method for forming corrosion- and wear-resistant regions on a part, wherein the method comprises providing the part with a plurality of surfaces, wherein the part comprises a base alloy; degreasing the part; deoxidizing the degreased part; applying a thin film sulfuric acid (TFSA) anodize to the deoxidized part; sealing the TFSA anodized part with an aqueous solution of dipotassium hexafluorozirconate; sealing the TFSA anodized part with an aqueous solution of lanthanum nitrate hexahydrate; sealing the TFSA anodized part with a hydrothermal seal. The combination of TFSA anodize, dipotassium hexafluorozirconate seal, lanthanum nitrate hexahydrate seal, and hydrothermal seal form corrosion-resistant regions on the part. The combination of TFSA anodize, dipotassium hexafluorozirconate seal, lanthanum nitrate hexahydrate seal, and hydrothermal seal is machined from regions of the part to receive a wear-resistant treatment to expose base alloy and hardcoat anodize is applied to base alloy in regions of the part to receive a wear-resistant treatment, thereby forming the wear-resistant treatment.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional elements:

The method of the preceding paragraph, further comprising rinsing the part with water after each of the degreasing step, the deoxidizing step, the TFSA anodize step, the dipotassium hexafluorozirconate sealing step, and the lanthanum nitrate hexahydrate sealing step.

The method of the preceding paragraph, further comprising inspecting the corrosion-resistant region for hardcoat breakthrough following the hardcoat anodize step.

The method of any of the preceding paragraphs, wherein the aqueous solution of dipotassium hexafluorozirconate comprises between >=1% and <3% dipotassium hexafluorozirconate.

The method of any of the preceding paragraphs, wherein the aqueous solution of lanthanum nitrate hexahydrate comprises between >=1% and <3% lanthanum nitrate hexahydrate.

The method of any of the preceding paragraphs, wherein the base alloy is aluminum or an aluminum alloy.

The method of the preceding paragraph, wherein the base alloy is an aluminum alloy selected from the group consisting of 2000 series, 6000 series, and 7000 series aluminum alloys.

The method of any of the preceding paragraphs, wherein the thin film sulfuric acid anodize comprises aluminum oxide coating between 0.00254 mm to 0.0254 mm (0.0001 in to 0.001 in) thick.

The method of any of the preceding paragraphs, wherein the hardcoat anodize comprises aluminum oxide coating between 0.0127 mm to 0.0762 mm (0.0005 inches to 0.0030 inches) thick.

The method of any of the preceding paragraphs, wherein the corrosion-resistant coating and wear-resistant coating are substantially hexavalent chromium free with a chromium (VI) oxide concentration of below 0.1 wt %.

The method of any of the preceding paragraphs, wherein the corrosion-resistant coating comprises residual lanthanides.

The method of the above paragraph, wherein: the base alloy is an aluminum alloy selected from the group consisting of 2000 series, 6000 series, and 7000 series aluminum alloys; the thin film sulfuric acid anodize comprises aluminum oxide coating between 0.00254 mm to 0.0254 mm (0.0001 in to 0.001 in) thick; the hardcoat anodize comprises aluminum oxide coating between 0.0127 mm to 0.0762 mm (0.0005 inches to 0.0030 inches) thick; the aqueous solution of dipotassium hexafluorozirconate comprises between >=1% and <3% dipotassium hexafluorozirconate; the aqueous solution of lanthanum nitrate hexahydrate comprises between >=1% and <3% lanthanum nitrate hexahydrate; the corrosion-resistant coating and wear-resistant coating are substantially hexavalent chromium free with a chromium (VI) oxide concentration of below 0.1 wt %; and the corrosion-resistant coating comprises residual lanthanides.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.