Patent Application
20210189580
The invention relates to a novel electrolytic process to control the deposition of chemical conversion coatings on metal substrates. The process could be adjusted for different coating-substrate to achieve optimal coating formation and corrosion prevention. The process could be used in immersion applications, or made portable by a handheld device. The process involves the passage of current through a conversion coating electrolyte in which the work surface is either the cathode or anode. The cathodic or anodic current density is equal to or less than 3.0 A/FT2, (3.0 amperes per square foot) and the immersion time is equal to or less than 60 minutes. A novel feature of this invention is the application of electric current to the metal work surface during the coating formation. Prior art demonstrates diffusion-controlled coating growth, whereas the electrolytic process alters the reaction kinetics to promote faster coating growth. The electrodeposited coatings afford superior corrosion resistance and improved coating thicknesses compared to coatings prepared using traditional diffusion-controlled processes. The coatings produced with impressed current do not worsen paint adhesion, as determined by ASTM D3359 testing. The process produces trivalent chromium (Cr(III))-containing conversion coatings that exhibit equal corrosion performance to hexavalent chromium (Cr (VI))-containing conversion coatings without the health and environmental risks associated with the use of Cr(VI) chemistry. The process similarly improves the corrosion performance of hexavalent chromium and non-chromium conversion coatings.
More specifically, the use of chromate conversion coatings on aircraft aluminum alloys is the need for excellent corrosion resistance and to serve as a base for paint. Baths used to develop these coatings contain hexavalent chromium, and residual chromates in the coating are largely responsible for the high degree of corrosion inhibition. These same chromates are hazardous and their presence in waste water is severely restricted. It would be desirable to provide a coating for aluminum and its alloys, utilizing trivalent chromium as an alternative to the hexavalent chromates. Trivalent chromium has been used in conversion coatings instead of hexavalent chromium to produce replacements for hexavalent chromium-containing coatings.
The development of chromate (CrO4) conversion coatings (known commercially by names such as (Alodine, Iridite or Chromital) for the corrosion protection of high-strength, aerospace aluminum alloys occurred between 1945 and the 1950's. In the United States, these coatings are qualified to MIL-DTL-81706 for military use, while MIL-DTL-5541 provides guidance for the quality control of these processes. An effort to eliminate hexavalent chromium resulted in the development and optimization of trivalent chromium (Cr(III)) conversion coatings in the early 2000's. The Cr(III) coatings are also qualified to the MIL-DTL-81706 specification, though their adoption is limited by their lesser corrosion performance and a near-colorless appearance.
Currently available non-chromium conversion coatings fail to meet MIL-DTL-5541 requirements. Although the Cr(III) conversion coatings offer numerous advantages over the legacy Cr(VI) products, the absence of the chromate anion in the Cr(III) coatings reduces the pitting resistance in ASTM B117 neutral salt fog exposure for a duration of at least two weeks. Some of the current commercial Cr(III) coatings can meet the minimum two-week requirement, albeit with greater difficulty. Given the known health hazards of chromate conversion coatings, there is a strong desire to develop an alternative, safer process that affords similar levels of protection against localized corrosion. Therefore, it is desirable to have coatings that are free of hexavalent chromium, but capable of imparting corrosion resistance and paint bonding which are comparable to hexavalent chromium coatings.
The invention is directed to a process wherein an electric current is passed through a conversion-coating electrolyte at a cathodic or anodic current density not exceeding 3 amperes per square foot (A/ft2) for a duration equal to or less than 60 minutes. The conversion coating electrodeposits at the cathodic or anodic metal surface, while oxygen or hydrogen are evolved at the counter electrodes.
More specifically, prior to processing, the test panels are affixed to a rack made of suitable, conductive material. A bus bar made from suitable, conductive material is suspended over the chemical conversion coating tank. Two or more counter electrodes are attached to the bus bar, such that the counter electrodes are immersed in the solution, no more than 1 foot from the work surfaces. The counter electrodes are immersed at a depth that is more shallow than the parts to avoid high current density areas on the parts. The bus bar is electrically connected to the rectifier to impart the desired polarity to the counter electrodes, either cathodic or anodic.
Once the parts are cleaned and deoxidized, the rack is immersed in the conversion coating solution. Next, the rack is electrically connected to the rectifier so that the rack is the opposite polarity of the counter electrodes. The rectifier is then powered for the full duration of the conversion coating process. At the conclusion of the conversion coating immersion, the rectifier is powered down, and the rack is disconnected from the rectifier and removed from the conversion coating solution. Alternatively, the rack and bus bar may be electrically connected to the rectifier, and the rectifier powered prior to the immersion of the rack in the conversion coating solution. Similarly, the rack may be removed at the conclusion of the conversion coating process while the rectifier is still on. Entering and exiting the solution while “hot” describes this process.
Several 3″×6″×0.025″ (AA2024-T3) test panels were procured from Q-Lab Corporation, cleaned with ASTM-D329 acetone, and placed on a titanium alloy rack. The panels were immersed in BONDERITE C-AK 6849 AERO, an alkaline, non-etch, non-silicate cleaner until a water-break-free surface was achieved in a 120-140° rinse water bath. The test panels were then immersed in BONDERITE C-IC SmutGo AERO ACID, a non-chromium deoxidizer for one minute. The racked test panels were immersed in SurTec 650 conversion coating solution. A suitable DC power supply was connected to the titanium alloy rack and two 1000-series aluminum counter electrodes. The power supply was then operated in a constant-current mode to provide a cathodic or anodic current density not exceeding 3.0 A/ft2. The duration of coating formation was equal to or less than 60 minutes, at which point the power supply was turned off, and the test panels were removed from the conversion coating solution.
Trials in which the panels were immersed and removed from the chemical conversion coating while the power supply was on were also conducted. The test panels were rinsed with deionized water, and then air-dried. Two panels were set aside from each experimental condition for coating weight determination and analysis using electrochemical impedance spectroscopy (EIS).
Coating performance was evaluated with a comparative analysis of ASTM B117 neutral salt fog exposure results. Control panels were processed under diffusion-controlled conditions without the electrolytic process, while the experimental panels were processed with the passage of current through the conversion coating electrolyte. Both the experimental and the control test panels were allowed to air dry for at least 24 hours (at approximately 77F and 40% RH) in order to allow the coatings to dehydrate before neutral salt fog exposure. All test panels were placed in the same neutral salt fog test cabinet to minimize variation associated with the method. During neutral salt fog exposure, the test panels were removed, imaged, analyzed, and returned to the same test chamber at one week intervals until the experimental panels displayed extensive localized corrosion.
Tables 1 through 6 and 8 contain coating weight measurements (mg/ft2) for control (without impressed current) and experimental (with impressed cathodic or anodic current) panels. Control conditions are italicized. It is clear that the impressed current increases the mass of the deposited conversion coating (coating weight). Furthermore, the results below demonstrate the process's ability to improve conversion coating weight on multiple alloys, and with two different conversion coating solutions (Cr(III)-containing and Cr-free).
Purpose: To directly compare the performance of a trivalent chromium conversion coating with and without impressed current.
Table 1 shows the effect of cathodic impressed current in a Cr(III) conversion coating solution on the coating weight. The coating weight at 0-3 ASF applied current is up to 112% greater than the control (AA2024-T3).
Purpose: To examine the electrodeposited trivalent chromium conversion coating on a “repaired” surface.
Table 2 shows the effect of cathodic impressed current in a Cr(III) conversion coating solution on the coating weight in a repair/depot-type situation. The control and impressed current panels were abraded with ScotchBrite 7447 to simulate rework. The coating weight at 0-3 ASF applied current is 54% greater than the control.
Purpose: To repeat the direct comparison the performance of a trivalent chromium conversion coating with and without impressed current.
Table 3 shows the effect of cathodic impressed current in a Cr(III) conversion coating solution on the coating weight. The coating weight at 0-3 ASF applied current is 126% greater than the control (AA2024-T3).
Purpose: To evaluate the performance on an electrodeposited trivalent chromium conversion coating with a dye additive.
Table 4 shows the effect of cathodic impressed-current in a dyed Cr(III) conversion coating solution on the coating weight. The coating weight at 0-3 ASF applied current is 105% greater than the control (AA2024-T3).
Purpose: To evaluate the performance of an electrodeposited trivalent chromium conversion coating on a copper-rich alloy, AA2219-T87.
Table 5 shows the effect of cathodic impressed current in a Cr(III) conversion coating solution on the coating weight. The coating weight at 0-3 ASF applied current is 308% greater than the control (AA2219-T87).
Purpose: To evaluate the performance of an electrodeposited non-chromium conversion coating.
Table 6 shows the effect of cathodic impressed current in a non-chrome conversion coating on the coating weight. This experiment was conducted with a 10-minute immersion time. The coating weight at an applied current of 0-3 ASF is 21% greater than the control.
Purpose: To evaluate the performance of an electrodeposited trivalent chromium conversion coating from a non-commercial conversion coating bath, TCP-S. For this example, a conversion coating bath was made using 3.0 grams per liter chromium sulfate basic, 4.0 grams per liter potassium hexafluorozirconate and 0.12 grams per liter potassium tetrafluoroborate. The composition and use of TCP-S is documented in U.S. Pat. No. 6,511,532 and others as awarded to the United States Navy. Test panels of AA2024-T3 aluminum were first immersion cleaned in Bonderite C-AK 6849 (an alkaline cleaner), rinsed in hot tap water, immersion treated in Bonderite C-IC (a ferric sulfate based deoxidizer), rinses in cold tap water, and then processed in the composition described above either with or without applied current. After the conversion coating step, panels were immersion cleaned in cold tap water followed by a final deionized water spray rinse, and allowed to air dry in the lab for 12 hours.
The performance of coatings made using the impressed current process compared to standard conversion coating are shown in
Electrical resistance data collected in accordance with MIL-DTL-81706 indicates that the electrodeposited coating does not exceed the maximum limit of 5000 μOhm PSI for Class III Coatings.
In summary, the data show that the electrodeposited conversion coatings achieve excellent corrosion resistance without the use of hexavalent chromium. This is in line with United States Navy and international initiatives to eliminate the use of hexavalent chromium. The process enables the deposition of a conversion coating on several series of aluminum alloys, a titanium alloy and silver and improves the corrosion resistance of difficult-to-protect aluminum alloys by up to 200-300% as determined by accelerated corrosion testing in neutral salt fog (ASTM B117). Moreover, the process allows the user to select a desired coating weight through the adjustment of cathodic current density and amp-hours and provides a shorter immersion with a high-current density to produce a thicker coating than the diffusion-controlled process with heavier coating weights. The process allows for a scalable coating weight between 2-7× the average coating weight for a typical diffusion-controlled immersion process. The resultant thicker coatings are stable, lack the “powdery” appearance of thicker-coatings achieved using the current art, and perform well in neutral salt fog exposure.
The electrolytic process of this invention also allows the user to dope existing conversion coating solutions with organic dyes to achieve highly visible, easily-detectable conversion coatings without significant loss in performance. The addition of organic dyes to a standard, diffusion-controlled process typically results in corrosion performance degradation. Aluminum alloys treated with the electrolytic process of this invention develops a slight opacity that provides improved contrast for the detection of localized corrosion. Easier detection of corrosion allows system maintainers to readily find and treat problem areas. The electrolytic process also works well on abraded surfaces. Corrosion performance of simulated rework surfaces suggested that the electrolytic process out performs diffusion-controlled processes and complements the technological transition of the aerospace industry to non-hexavalent chromium replacements with simple modifications to conversion coating facilities.
While this invention has been described by a number of specific examples, it is obvious that there are other variations and modifications which can be made without departing from the spirit and scope of the invention as particularly set forth in the appended claims.
The invention described herein was made by employee(s) of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
