April 2013

Is aluminum really more corrosion resistant than steel?

 

Hello everyone. There are several types of metals used to fabricate spray package components. There’s also a common corrosion myth that aluminum is more corrosion resistant than steel. Let’s briefly review the types of metals used to fabricate your spray packages and aerosol valves and then discuss why the concept of superior aluminum corrosion resistance is a corrosion myth.

Corrosion Basics: metals used to fabricate spray packages

The two metals used for fabricating spray packages are aluminum and steel. Aluminum, steel and stainless steel are used in the fabrication and construction of aerosol valves. The aluminum used for spray packages is commercially pure aluminum—purities range from around 99% to 99.7% aluminum. Aluminum aerosol containers are fabricated by extruding an aluminum puck into the final container shape. Bags inserted into aerosol containers are formed with aluminum foil that has one or more layers of polymer film laminated on both sides of the foil. Bags are formed by welding sections together and welding the valve to the top of the bag.

Steel is the common designation for iron alloys. The steel used for spray packages could be one of three iron alloys (type D, type L and type MR). Steel alloys are typically coated either with tin metal, topped with a thin layer of chromium (commonly called tinplate), or a thin layer of chromium metal (commonly called tin-free steel) to prevent atmospheric rusting. In many instances, the alloy used for the container body is different from the alloy used to fabricate the container tops and bottoms.

Steel aerosol containers are formed in one of two ways:

  • The body, tops and bottoms are formed from separate sheets then seamed together
  • The body and top or the body and the bottom are forged into the container shape—without the bottom or top, respectively. The top or bottom is subsequently seamed to the top or the bottom—depending on which part is not forged with the body

Aerosol valves are stamped from aluminum or steel sheet metal in multiple stamping stages. Aerosol valve check-balls and springs are fabricated from a variety of stainless steel alloys.

 Are aluminum containers more corrosion resistant than steel containers?

Aluminum is a more chemically reactive metal than iron and thus should be less corrosion-resistant than steel. However, the actual answer to the above question on which metal is more corrosion resistant is: it depends on your formula chemistry.

Steel typically corrodes very rapidly when the solution (formula) pH is below approximately five. Conversely, steel corrosion could be significantly slower when the solution (formula) pH is approximately above 10.

Generally, aluminum corrosion rates are the lowest when the solution pH is somewhere between four and seven. However, aluminum also corrodes rapidly when the solution pH is below approximately four, and aluminum dissolves in very high pH solutions (e.g., a pH of 12).

Two examples will help illustrate the compatibility of aluminum and tinplated steel containers with different types of formulas.

A hair mousse formula with a pH of four is usually compatible with aluminum containers, but is usually not compatible with steel containers (in the early 1990s most of us unsuccessfully attempted to find a coated steel aerosol container for hair mousse formulas)

An aluminum container is usually not compatible with a high pH oven cleaner, while steel containers are typically used for aerosol oven cleaners.

There are also many formulas, such as hair sprays, air fresheners and insecticides that can be packaged in either aluminum containers, or steel containers, depending on the formula’s chemical composition.

There are a few rules-of-thumb about formula pH as a guide for when to use aluminum or steel containers. These are:

  • Aluminum containers with internal coatings are most likely more corrosion resistant with low pH formulas (around 4–6)
  • Steel containers are most likely more corrosion resistant when formula pH is above 10
  • Aluminum and steel containers could be compatible with formulas whose pH is around seven

However, other formula ingredients could invalidate these pH rules-of-thumb. For example, organic halogens (most notably carbon tetrachloride) could violently react with aluminum. (Please don’t confuse chloro-organic molecules with inorganic chloride ions.)

Ultimately corrosion testing is needed for both types of spray package metals to determine if they are compatible with a given type of formula. In addition, a properly optimized corrosion inhibitor system will significantly increase the corrosion resistance of either aluminum, or steel. However, different types of corrosion inhibitors are typically needed for aluminum and for steel.

Please send your questions/comments/suggestions to [email protected] Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in June. SPRAY