W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner 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 40 Spray May 2013 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 checkballs 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 corrosionresistant 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
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