W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner Are aluminum spray packages more corrosion-resistant than steel spray packages? 26 Spray April 2016 The graphs in Figure 1 help to explain why low pH formulas can often be packaged in aluminum aerosol containers, but not in steel aerosol containers and why very high pH formulas are often packaged in steel packages but not in aluminum. There are also many formulas, such as hair sprays, air fresheners and insecticides, that can be packaged in either aluminum or steel containers, depending on the formula’s chemical composition. The graphs in Figure 1 provide a few rules-of-thumb that serve as a guide for when to use aluminum or steel packages: • 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 nine Anhydrous formulas could be packaged with either type of package metals. Please remember that other formula ingredients could negate these rules-of-thumb. For example, organic halogens could violently react with aluminum (most notably carbon tetrachloride). 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 aluminum and steel. However, aluminum and steel often use different types of corrosion inhibitors. We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestions/questions/comments to rustdr@ pairodocspro.com or visit www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in May. Spray Hello everyone. Aluminum and steel alloys are the predominant metals used to fabricate spray packages. The question often arises as to whether one metal is more corrosion-resistant than the other. It depends! Let’s discuss this ambiguous answer. Spray package metals The aluminum used for spray packages is commercially pure aluminum with purities ranging from around 99% to 99.7%. Aluminum aerosol containers are fabricated by extruding an aluminum disc into the final container shape. The internal laminate bags used in aerosol containers are fabricated from aluminum foil with one or more layers of polymer film on both sides. Steel is an iron alloy, and the three alloys typically used to fabricate spray package components are specified in ASTM 623. Tinplate is steel coated with tin metal topped with a thin layer of chromium. Spray package steel without tin and only a thin layer of metal chromium on the steel is called tin-free steel. In many instances, the alloy used for the container body is different from the alloy used for the container tops and bottoms. Polymer coatings could also be added to one or both sides of the tinplate or the tin-free-steel. Steel aerosol containers are formed in one of two ways: • The body, top and bottom are formed from separate sheets then seamed together • The body and top, or the body and bottom, are forged into the container shape and the top or the bottom is subsequently seamed to the bottom or the top Aerosol valves are stamped from aluminum or steel sheet metal in multiple stages. The aluminum or steel valve could have a variety of polymer coatings on the metal. Aerosol valve springs are typically fabricated from type 302 stainless steel. However, other stainless steel alloys are available. Is aluminum more corrosion-resistant than steel? Aluminum is more chemically reactive than steel and thus should have less corrosion resistance. However, the actual corrosion resistance is determined by the chemical composition of your formula. For example, Figure 1 illustrates how solution (formula) pH could affect the corrosion rates for aluminum and steel. A higher corrosion rate corresponds to a lower corrosion resistance and vice versa. Notice in Figure 1 that: • Aluminum has a lower corrosion rate (higher corrosion resistance) when the pH range is approximately 4–7. • Steel has a lower corrosion rate (higher corrosion resistance) when the pH is approximately below two and above nine. Figure 1: Steel and aluminum corrosion rates as a function of solution pH
Spray April 2016
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