Corrosion testing for new sunscreen formulas

Written on: May 1, 2019 by W. Stephen Tait

Hello, everyone. I recently read several news articles about a few sunscreen chemicals that might have alleged adverse environment effects; I also read of a new U.S. Food & Drug Administration (FDA) proposal for sunscreen regulations (link here).
The news articles discussed a possible link between two sunscreen chemicals—oxybenzone and octinoxate—with reef coral bleaching. These articles also mentioned that a few local governments have passed laws banning these two sunscreen chemicals on their beaches. I was unable to locate scientific papers discussing the alleged bleaching-issue, so the scientific research on this phenomenon is either fairly new, or probably incomplete and thus without broad scientific consensus at this time.


















The FDA Fact Sheet for the proposed new regulations list three categories for sunscreen ingredients:
1. GRASE (generally recognized as safe and effective)
2. Not GRASE
3. Insufficient data for use in sunscreen products.

Oxybenzone and octinoxate appear in the proposed “insufficient data” category. Para-aminobenzoic acid (PABA) and trolamine salicylate are in the proposed “not GRASE” category.
Discussing these issues is beyond the scope of Corrosion Corner. However, I want to emphasize the need for corrosion testing as an integral part of reformulating, particularly if news articles, proposed new regulations and local legislation instigate reformulating efforts to find substitute sunscreen ingredients.

Corrosion and sunscreen ingredients
I observed an instance where PABA caused rapid stainless steel pitting corrosion within 12 hours after initial exposure to a product containing PABA. In this instance, a corrosion inhibitor was developed to prevent the stainless steel corrosion.
I mention this example to emphasize that some sunscreen ingredients are capable of causing rapid metal corrosion. Hence, there is the need for corrosion testing when replacing particular sunscreen ingredients in spray products and products that use non-spray packaging with pumps incorporating internal stainless steel springs. Skipping corrosion tests on metal packaging, coated metal packaging and laminated metal foil packaging results in what is typically considered to be an unacceptable high risk.
Two things to remember when conducting corrosion tests to qualify substitute ingredients: conduct tests for the appropriate length of time to reduce the corrosion risk, and don’t use higher temperatures to accelerate corrosion in order to reduce test length.

Figure 1: Risk versus corrosion test time

















Figure 1 provides empirical estimations for risk as a function of test time. Notice in Figure 1 that the no-testing-risk (percent at 0 days) is above 60% for aerosol containers and above 20% for internal laminated foil bags with attached aerosol valves. Notice also in Figure 1 that after one year of storage testing, the risks decrease to approximately 7% for aerosol containers and approximately 3% for laminated metal foil bags. The risk for electrochemical corrosion testing is less than 1% in less than 100 days of testing when Aristartec technology is used for aerosol containers and laminated metal foil bags.
It’s tempting to use a higher storage test temperature to accelerate spray package corrosion when timetables are short. However, trying to make corrosion proceed faster by raising the test temperature often produces unexpected corrosion that could either delay production on a new product or a reformulated product or cause a very costly product recall.

Figure 2: Corrosion rate for a coated metal as a function of temperature















Figure 2 provides a graph of the corrosion rate as a function of temperature for a spray package with an internal polymer coating. The shape of this curve also applies to laminated metal foil bags with attached aerosol valves.
Notice in Figure 2 that the measured corrosion rate is decreasing with increasing temperature—instead of increasing with increasing temperature—until around 60°C (140°F) and the corrosion rate subsequently increases with increasing temperatures above 60°C. In other words, corrosion rates are not linear functions of temperature and increasing temperature does not increase the corrosion rate in a linear fashion.
The minimum inflection at approximately 60°C in Figure 2 occurs because the storage temperature equals or exceeds the coating’s wet glass transition temperature (Tg). Polymer coatings lose their physical properties, such as barrier properties, when the storage temperature equals or exceeds the wet glass transition temperature.
The dry polymer glass transition temperature for this example is 100°C (212°F) as noted in Figure 2. In other words, the Tg for the wet coating is approximately 40°C (104°F) lower than the Tg for the dry coating.

Corrosion testing is needed to have low-risk decisions when modifying the chemical compositions of formulas—such as finding substitute sunscreen ingredients. It takes at least one year to achieve low risk with a storage test (approximately 3% or 7% for laminated foils and aerosol containers, respectively) and raising the storage temperature does not shorten test time while lowering risk. Electrochemical testing can both lower risk and significantly shorten test time when the appropriate technology is used.
Please visit for more information. Thanks for reading and I’ll see you June.  SPRAY