Testing Safe Alternative Conversion Coatings

Conversion coatings are commonly used on aluminum alloys to improve subsequent primer adhesion. However, these coatings sometimes contain hexavalent chromium compounds, which are toxic to humans with negative health effects from acute and chronic exposure. Chronic exposure to hexavalent chromium is associated with lung cancer, nasal and sinus cancers (IARC 2012).

Regulatory mandates have accelerated a global effort to replace hexavalent chromium-containing materials. In 2011, the Defense Federal Acquisition Regulation Supplement mandated that applicable U.S. Department of Defense contracts must not include specifications that result in a maintenance material containing more than 0.1% hexavalent chromium for which acceptable substitutes are available. (Federal Acquisition Regulations System 2016).

In 2018, the Hex Chrome-Free Aerospace Consortium started to identify, test and evaluate various safer commercially available conversion coating materials. The consortium includes representatives from the following organizations: Blue Origin, Boeing, Lockheed Martin, National Aeronautics and Space Administration (NASA), RTX - Raytheon, Textron Aviation, and the Toxics Use Reduction Institute (TURI) at the University of Massachusetts Lowell.

A joint test protocol was developed by the consortium members to best evaluate the most recent commercially available alternative conversion coatings. The conversion coating materials listed in Table 1 were selected for inclusion in the evaluation.

Supplier	Product	Corrosion inhibitor
Confidential Supplier A	Confidential Product A	Hexavalent chromium (baseline for comparison)
Chemeon	eTCP	Trivalent chromium
Confidential Supplier B	Confidential Product B	Not hexavalent chromium
Confidential Supplier C	Confidential Product C	Not hexavalent chromium
Socomore	Socosurf TCS/PACS	Trivalent chromium

Table 1. Conversion coating materials and corrosion inhibitors. 

The aluminum alloys used for the test panels were 2024, 6061, 2219 and 7075, and their copper content is shown in Table 2. In general, the higher the copper content, the more challenging it is to meet corrosion requirements.

Alloy	Minimum copper concentration (%)	Maximum copper concentration (%)
6061	0.15	0.4
7075	1.2	1.6
2024	3.8	4.9
2219	5.8	6.8

Table 2. Copper content of aluminum alloys.

Conversion coating application

The Chemeon, Socomore and hexavalent chromium-based conversion coatings were applied in a production environment at a contracted metal finishing facility. The hexavalent chromium-based conversion coating and the Chemeon eTCP conversion coatings were applied at Poly-Metal Finishing (Springfield, Massachusetts). The Socomore conversion coating was applied to test panels at International Hardcoat (Detroit, Michigan). The chemical tanks were prepared as recommended by the Technical Data Sheet provided by the conversion coating manufacturer and the MIL-DTL-5541 standard for chemical conversion coatings on aluminum.

The same non-hexavalent chromium primer and topcoat were applied to the panels. The non-hexavalent chromium primer used was PPG 02GN097, that might be a future candidate to qualify for MIL-PRF-23377 Type 1 Class N. The non-hexavalent chromium topcoat used was PPG 03W127BF urethane color 17925 White that meets MIL-PRF-85285 Type 1 Class H. The primer was spray applied per MIL-PRF-23377 Section 4.4.1, and the topcoat was spray applied per MIL-PRF-23377 Section 4.4.2 to one side of the panel. The topcoat was applied on the same day as primer application and within 5 hours of the primer application (MIL-PRF-23377 2012).

Beachfront corrosion testing

The beachfront long-term corrosion testing was conducted at the NASA Beachside Atmospheric Corrosion Test Site at Kennedy Space Center in Florida. Visual inspections were conducted by NASA personnel at periodic intervals, and the rate of the maximum creep from the scribe was determined non-destructively as outlined in ASTM D1654 Procedure A. These inspections occurred at six months, nine months, 12 months, 18 months, 24 months, 30 months, 36 months, and 42 months. The ratings used for the creep distance from the scribe are shown in Table 3.

The beachfront corrosion testing included 120 panels in total, with six panels per aluminum alloy and conversion coating combination. All six panels for Product B, Product C and Socomore TCS/PACS had both primer and topcoat. For the hexavalent chromium-based and Chemeon eTCP coatings, three panels had primer only and three panels had primer and topcoat. A representative photograph of the beachfront corrosion test after 42 months is provided in Figure 1. The photograph is of the eTCP conversion coating with primer and topcoat on 2219 alloy test panel #3 with a rating of 7, indicating that corrosion had crept between 1 to 2 mm from the scribe area.

Table 4 provides the visual ratings for each of the test panels at the 42-month visual inspection.  Yellow indicates results with a non-passing rating less than 10, and green indicates test panel combinations that did pass the beachfront corrosion testing requirement with a rating of 10.

Testing duration impact

The average rating of the panels decreased as the duration of testing time increased as shown in Figure 2. For test panels with both primer and topcoat, the average test panel rating was 8.6 after six months, and the average test panel rating was 5.5 after 42 months. Statistical significance indicates that the observed data is not the result of chance, but rather can be attributed to a specific cause. For this article, a P value of less than 0.05 was used to determine statistical significance.

Figure 2. Corrosion ratings over time.

Conversion coating impact

For test panels with both primer and topcoat for all test durations, the hexavalent chromium-based conversion coating, Chemeon eTCP, Product C and Socomore TCS/PACS conversion coatings all had average ratings between 8.0 to 8.2, with no statistically significant difference observed between these four different conversion coatings. However, the Product B conversion coating had an average rating of 2.3 and was statistically different from the other four conversion coatings. The results for all five conversion coatings are shown in Figure 3.

Figure 3. Corrosion test results by type of conversion coating.

Finish impact

All six panels for Product B, Product C and Socomore TCS/PACS conversion coatings had both primer and topcoat. For the hexavalent chromium-based and Chemeon eTCP coatings, three panels had primer only and three panels had both primer and topcoat. Therefore, in this section, we compared the results just for the panels with the hexavalent chromium based and Chemeon eTCP conversion coatings for all test durations and the results are shown in Figure 4. First, there is no statistical difference between these two types of conversion coatings. However, there is a statistical difference between the panels with primer only (average rating of 10) and panels with both primer and topcoat (average rating of 8.1) for all test durations combined. Therefore, the primer-only coupons provided better corrosion resistance than the panels with both primer and topcoat.

Figure 4. Finish results for panels with primer and primer/topcoat.

Aluminum alloy impact

The four aluminum alloys used for this evaluation have varying levels of copper content. In general, the higher the copper content, the more challenging it was anticipated to meet corrosion requirements. The results for this section are based only on the test panels with both primer and topcoat for all test durations. For the 6061 panels, the lowest level of corrosion was observed.  The 6061 panels had the highest average rating of 9.0. The 2219 panels had the highest levels of observable corrosion with the lowest average rating of 4.2 as shown in Figure 5. These results are consistent with the anticipated greater difficulty of corrosion protection resulting from increasing amounts of copper within the aluminum alloy.

Figure 5. Corrosion test results by aluminum alloy.

Progress is made

The long-term corrosion test results for the conversion coatings without hexavalent chromium were encouraging. Overall, for the test panels with both primer and topcoat, the Socosurf TCS/PACS, Product C and Chemeon eTCP conversion coatings were the best performing hexavalent chromium-free conversion coatings across all four types of aluminum alloys with results comparable to the hexavalent chromium-based conversion coating. As the duration of the beachfront exposure increased, the test results were a better identifier for distinguishing the corrosion resistance among the different conversion coating alternatives.

No single evaluation will completely resolve the long-standing issue of replacing hexavalent chromium in conversion coatings with a safer alternative. Given the number of hexavalent chromium-free conversion coating that passed long-term corrosion testing for various aluminum alloys, the results of this evaluation provide significant progress to achieving that goal of identifying a replacement material for a traditionally hexavalent chromium material application on aluminum.

References

Federal Acquisition Regulations System (2016) Title 48. United States of America: Defense Acquisition Regulations System, Department of Defense. Part 222, p. 159-160.

International Agency for Research on Cancer (IARC) (2012) Monographs on the evaluation of carcinogenic risks to humans. United States of America: IARC. v. 100C.

Morose, Gregory, Humphrey, Chandler, DeFranco, Kent (Lockheed Martin) David Pinsky (Raytheon), “Safer Coatings without Hexavalent Chromium for Aerospace and Defense Industry Applications”, Journal of Aerospace Technology and Management, January, 2022.