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What are DoD-approved alternatives to the hexavalent chromium in aerospace coatings for use on government/military aircraft (aluminum) and/or ground support equipment (steel)?
The Defense Systems Information Analysis Center (DSIAC) received a technical inquiry about approved hexavalent chrome replacements for U.S. Department of Defense (DoD) applications. DSIAC subject matter experts performed a study and identified several works that provide lengthy discussions about the available options. In addition, they identified a National Aeronautics and Space Administration database that focuses on disseminating screening test results for hexavalent chrome-free coatings that have either been approved for use within the DoD or private sector on aerospace equipment or have shown promise in previous testing by one or more of the project stakeholders. However, based on the searches, it is likely that a general-purpose replacement for Cr+6-based coatings may not be possible and that each alternative may better be selected to match the application. In the end, this would result in better products for many different applications, each with a specific coating, but it would mean that there is no general-purpose coating for all applications.
Hexavalent chromium is NOT a material. It is the oxidation state of element Cr., i.e., a +6 valence. There are many compounds based on Cr, such as CrO3. These are used as pigments in paints and mostly as the metal source for electroplating Cr onto metal surfaces. In air, the surface converts to CrO3, which is self-passivating, hard, and clear. It is a very good protective coating and resists both acids and bases, as well as mechanical wear. Unfortunately, chromium is a heavy metal. In its organic form, it is a serious health risk. So, the problem is not the completed chromium metal coating, but the chemicals used to create the plating.
Hexavalent chromium (Cr6+) conversion coating (CCC) serves as a corrosion inhibitor in metals, such as zinc and its alloys, magnesium and its alloys, aluminum, and cadmium. The corrosion inhibition is attributed to an inert barrier created on the metal’s surface. The conversion coating process involves immersion of a metal part in chromic acid. Chromate conversion coating is distinct from chromic acid anodizing because anodizing involves an electrochemical method whereby the metal surface is converted into an oxide. The anodizing coating provides a harder and thicker coating as well as better corrosion protection on zinc surfaces as compared to the chromate conversion coatings, but issues such as cost, complexity, and aerosolization of Cr6+ are problematic.
The reason why hexavalent chromium has been so popular for hard chrome plating is because of its favorable corrosion resistance properties, high level of hardness, low friction, and resistance to abrasive wear. Additionally, it can be applied in a wide range of thicknesses—an important element for reducing fatigue. These properties suit a whole range of applications. Decorative chromium plating gives an attractive glossy finish to otherwise dull metals. Chromium plating is used to increase hardness and abrasion resistance of industrial tools, such as hydraulic cylinders and rollers. The plating also facilitates easy cleaning of components. Hexavalent chromium is also fast, simple, and cost-effective to produce. Essentially, it works, and it works well.
Even though hexavalent CCC is a good corrosion inhibitor, there are some disadvantages of its use because it is carcinogenic. These disadvantages are based on environmental, health, and safety concerns, as well as the legislation governing its usage. Exposure to hexavalent chromium may result in the following health problems:
- Lung cancer.
- Lung, nose, and throat irritation if hexavalent chromium is breathed at high levels.
- Skin or eye irritation at high concentrations, such as 20 to 25 ppm of hexavalent chromium.
Workers who inhale the mist of hexavalent chrome released into the air during the deposition process have reported health problems, such as pulmonary cancer, nasal septum, asthma, and skin conditions. For these reasons, the U.S. Environmental Protection Agency defined hexavalent chromium as hazardous in 2006, and it is slowly being phased out of use.
Omari et al.  discuss several alternatives to Hex-Cr. Some of these alternatives include cerium oxide, phosphate, tungstate, zirconate, titanate, molybdate, and vanadate coatings, as well as trivalent chromium compounds. Cerium chloride and cerium nitrate solutions were effective corrosion inhibitors when used on a zinc metal surface. Salt spray testing was conducted per the American Society for Testing and Materials (ASTM) B117 standard to evaluate the corrosion performance of cerium oxide and chromate conversion coating using magnesium alloy as a substrate. They reported that after 70 hr, white patches were formed on the substrate coated with chromate, whereas for the substrate coated with cerium oxide, white patches were apparent after 28 hr. This result implies that hexavalent chromium coatings have better corrosion resistance than cerium oxide coatings.
4.1 Phosphate Coatings
Phosphate conversion coatings are composed of crystalline salts of the substrate undergoing treatment (e.g., the formation of a zinc phosphate coating when zinc metal is used as a substrate). Phosphates are widely used to improve the corrosion resistance of a metal when employed as a base for organic coatings; but when used without organic coatings, hexavalent chromium coating is applied over phosphate coating for better corrosion resistance.
4.2 Zirconate/Titanate Coatings
Zirconate- and titanate-based coatings are used as adhesives and corrosion inhibitors on aluminum metal surfaces; but without the use of a paint finish, their corrosion resistance is poor when compared with hexavalent chromium coating. Vanadate coating also provides moderate corrosion protection to metals, such as zinc, and is relatively toxic but not as much as hexavalent chromium.
4.3 Molybdenum Coatings
In 1944, a U.S. patent described the use of molybdenum-based systems in the hexavalent state as a conversion coating material capable of replacing hexavalent CCC. However, subsequent work compared the coating efficiency of sodium molybdate and sodium dichromate on aluminum alloys and found that dichromate coatings were better than the molybdate coatings due to the strong oxidizing power of dichromate.
4.4 Tin/Zinc Coatings
Conversion coating treatments for tin-zinc alloy using tungstate systems are another option. The corrosion resistance of tungstate coatings is analogous to molybdate coatings. However, when compared with hexavalent coatings in a corrosion test using salt spray, tungstate coatings provided less protection.
In addition, salt spray testing was conducted to evaluate the corrosion performance of molybdate, tungstate, vanadate, and hexavalent chromium coatings on a zinc substrate. They reported that it took 350 hr for 10% red rust to occur on the zinc substrate coated with hexavalent chromium coating; but for molybdate, tungstate, and vanadate coatings, it took only 85 hr, 26 hr, and 30 hr, respectively, for the same level of rusting. This proved that hexavalent chromium coating has a better corrosion resistance than molybdate, tungstate, and vanadate coatings.
4.5 Trivalent Chromium Coatings
Trivalent chromium conversion coating has been used most prevalently as a replacement for hexavalent chromium systems since being developed for zinc metal surfaces. Trivalent chromium-based conversion treatments are preferred over hexavalent chromium-based systems in that they are not carcinogenic. The trivalent chromium conversion coating solutions are primarily employed in the form of chromium nitrate. Previous work has compared trivalent coatings with hexavalent coatings in relation to their corrosion resistance, reporting that after a thermal shock, the hexavalent coatings diminish more significantly than the trivalent coatings. However, in a salt spray test, the corrosion resistance of hexavalent coatings is better than trivalent coatings.
4.6 Thermal Sprays
Thermal sprays like high-velocity oxygen fuels are well-suited to applications requiring maximum hardness, such as landing gears for aircraft, industrial rolls, and most hydraulic rods. They offer a high level of hardness and good wear resistance and are suitable for rebuilds. Thermal spray is the most commonly used method to replace hard chromium plating as it can be applied to plenty of industrial processes using a wide range of coating materials and equipment.
4.7 Electroless Nickel-Based Coatings
Electroplating and electroless plating use similar technology to the original chrome plating process and have a wide range of application types. Electroless nickel-based plating is particularly good for internal components.
4.8 Vacuum Coating Methods
Vacuum coating methods like physical vapor deposition (PVD) are expensive, but they do offer the best level of hardness and wear resistance. Due to the cost and complexity, they are most often applied to small items with a high worth. Heat treatments are limited to heat-resistant materials only but can be applied in just about any form and size. They are not suitable for delamination or rebuilding. Finally, weld coatings and laser coatings are ideal when a thick coating is required, but they are limited to heat-resistant materials only and require refinishing. Welding is mostly applied to external components.
A lengthy review of the many alternatives (including advantages and disadvantages) is provided in a paper in Nature – Materials Degradation . NASA has also developed a database of alternatives . The National Aeronautics and Space Administration database focuses on disseminating screening testing of hexavalent chrome-free coatings that have either been approved for use within the DoD or private sector on aerospace equipment or have shown promise in previous testing by one or more of the project stakeholders.
There are several articles available via a Google search. One of them makes a point that a general-purpose replacement for Cr+6-based coatings may not be possible. Instead, each alternative may be better selected to match the application. In the end, this would result in better products for many different applications, each with a specific coating, but it would mean that there is no general-purpose coating for all applications.
 Omari, I., J. Penafiel, and J. S. McIndoe. “Chromate Conversion Coating and Alternatives as Corrosion-Resistant Treatments for Metal Parts.” ChemRxiv, Cambridge: Cambridge Open Engage, doi:10.26434/chemrxiv.12451208.v1., 2020.
 Gharbi, O., S. Thomas, C. Smith, and N. Birbilis. “Chromate Replacement: What Does the Future Hold?” Nature – Materials Degradation, vol. 2, article 12, 12 April 2018.
 Data.gov. “Hexavalent-Chrome Free Coatings.” https://catalog.data.gov/dataset/ hexavalent-chrome-free-coatings, updated 28 February 2019.
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