Corrosion Corner 30 Spray April 2014 W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC What about stainless steel corrosion in spray package components? H ello, everyone. The metals used to fabricate spray packages include: l Steel alloys (both tinplated and tin-free) l Aluminum alloys l Polymers l Laminated aluminum alloy foils l Stainless steel alloys Stainless steel is the least visible of all packaging materials, but is used in all types of spray packaging. Stainless steel is used to fabricate the springs in spray package valves and the check balls in some types of valves. Figure 1 shows a cutaway of a spray package valve. The cutting process slightly distorted the spring, which would not normally be so affected. The most common type of stainless steel is 304 (UNS S30400), which is composed of 18% chromium and 8% nickel in the alloy; trace amounts of other alloying elements (e.g., carbon) with iron making up the balance. In other words, stainless steel is an iron alloy with chromium and nickel as the major alloying elements. The presence of chromium and nickel in stainless steel gives this iron alloy a unique surface property—the ability to form a very thin, complex chromium oxide/nickel oxide barrier layer or film. This oxide film protects the underlying metal from corrosion by a wide variety of environments, such as consumer formulas. Please note that the steel alloys and aluminum alloys also used to fabricate spray packages do not have the requisite amount of chromium and nickel in them. Consequently, spray package steel alloys and aluminum alloys do not form a protective surface barrier film of chromium oxide with nickel oxide. The mechanism by which corrosion and pitting corrosion attacks stainless steel is significantly different from the mechanism for corrosion of other spray package materials. There are several competing theories on how stainless steel pitting corrosion occurs. Each theory has its merits, but the ion exchange mechanism is the most plausible theory to this author. Halogen ions, such as chloride ions, are more electronegative than the oxygen ions in the chromiumnickel oxide barrier film on stainless steel. Consequently, halogen ions are able to displace the oxygen ions in Figure 1. Partial cutaway of a spray package valve showing the internal stainless steel spring the oxide barrier film and
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