Corrosion Corner
Corrosion testing for
new sunscreen formulas
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 (https://www.fda.
gov/NewsEvents/Newsroom/PressAnnouncements/
ucm631736.htm).
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 bleachingissue,
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.
W. Stephen tait, ph.D.
Chief Science Officer & principal Consultant,
pair O Docs professionals, LLC
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 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 provides a graph of the corrosion rate as a function of
temperature for a spray package with an internal polymer coating.
40 Spray May 2019