W. STEPHEN TAIT, PH.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Spray Package Metallurgy & Corrosion
Part I: Aluminum
Aluminum corrosion behavior with chlorinated
hydrocarbons & inorganic chlorides
Aluminum has been observed to react violently with certain chlorinated
hydrocarbons, such as chloroform. However, there are also
chlorinated hydrocarbons, such as biocides and insecticides, that
do not react violently with aluminum.
Please remember that chlorinated hydrocarbons are not the
same as inorganic chlorides, such as sodium chloride or potassium
chloride. Inorganic chlorides typically do not cause aluminum
Other ingredients in your particular formula could actually
inhibit the reaction between chlorinated hydrocarbons and aluminum,
thereby producing a formula that is not corrosive. Therefore,
corrosion testing is needed to determine if chlorinated hydrocarbons
in your formula will cause or contribute to aluminum spray
package, aerosol valve and laminated foil bag/pouch corrosion.
Aluminum corrosion behavior with ethanol
Anhydrous ethanol could be corrosive toward aluminum when
there is absolutely no water in the formula. However, adding only
a few ppm of water to the ethanol typically makes it non-corrosive
Aluminum corrosion behavior as a function
of formula pH
The corrosion rate of aluminum in water is a function of pH as
illustrated in Figure 1. Notice the corrosion rate is very low when
the solution pH is between approximately four and seven. Outside
of this range, the corrosion rates are extremely high when the pH
is lower than approximately four and exceeds seven. Aluminum
corrosion behavior is often referred to as amphoteric because it
corrodes at high and low pH.
Other ingredients in a formula might shift the range of pH
that produces low corrosion rates and change the shape of the
pH-corrosion rate curve. The surface treatment on the aluminum,
such as a polymer coating or laminate film, will also change the
pH-rate curve shape and low corrosion rate pH range. In other
words, Figure 1 cannot be used to predict the corrosivity of all
formulas with aluminum spray packaging.
Coated aluminum corrosion behavior
Coating research has demonstrated that organic coatings usually
bond more strongly to aluminum than to steel and tin. In other
words, a coating on aluminum will bond more strongly than the
same coating on steel or tinplated steel. Stronger coating-metal
Hello, everyone. Several types of metals are used to fabricate
2. Steel with a thin layer of tin
3. Steel with a very thin coating of chromium/
chromium oxide—referred to as tin free steel or TFS
4. Various stainless steel alloys.
Aerosol valves are fabricated from either aluminum or steel
and the valve springs and check balls in aerosol valves are typically
made from stainless steel.
The corrosion resistance for each of these metals is significantly
different. As a general rule, the corrosion resistance for a pure
metal is typically higher than the corrosion resistance for the
corresponding metal alloys. However, pure metals typically don’t
have the strength needed for packaging, so metal alloys are used
instead of the pure metals.
An alloy is a mixture of metals and non-metals. For example,
steel is a mixture of iron, carbon, silicon or aluminum, sulfur,
manganese, phosphorous, copper, nickel, chromium and molybdenum.
The non-metals, such as carbon and sulfur, are present at
less than 1%, as are most of the other metals.
This month I’m starting a four-part series that discusses the
corrosion properties of the various metals used to fabricate spray
packaging, internal metal foil bags/pouches and aerosol valves.
We’ll start the series with a discussion of aluminum in spray
Aluminum spray package and aerosol
Aluminum aerosol containers and aerosol valves are typically
fabricated from a 1000 series aluminum alloy. For example,
type 1070 aluminum (universal numbering system A91070) has
a minimum concentration of 99.1% aluminum and type 1050
aluminum (A91050) has a minimum concentration of 99.5%
aluminum. The balance of the materials in type 1050 and type
1070 is a range of maximum concentrations below a fraction of
1% each for magnesium, copper, manganese, silicon, titanium,
vanadium, zinc and iron.
The aluminum foil used for the polymer laminated foils to
fabricate internal bags is also a type 1000 series aluminum.
The chemical composition of your formula determines the
corrosion resistance of an aluminum alloy. Let’s review several
chemistries and factors that determine the corrosion resistance of
32 SPRAY September 2017