magnitude between 18,000 to 70,000 calories per mole per degrees Kelvin (°K) The activation energy for a corrosion reaction is calculated using the Arrhenius equation: Ea = R (T2T1)(Ln(CR2/CR1)) (T2-T1) • Ea is the activation energy of a corrosion reaction in calories per mole per °K • R is the universal gas constant in calories per mole °K • T1 is the lower storage temperature in °K • T2 is the higher storage temperature in °K • CR1 is the corrosion rate (in moles/second) at the lower temperature Figure 1: Corrosion rate-temperature graph to polymer coated tinplated steel 30 Spray May 2015 • CR2 is the corrosion rate (in moles/second) at the higher temperature in °K Temperature is not accelerating coating, laminate film or metal corrosion when all four of the criteria are not fulfilled. In other words, product thermal instability is causing corrosion when the four criteria are not fulfilled. Should testing at a high temperature be included in a storage stability test? Yes, but not to accelerate spray package and aerosol valve corrosion. Higher temperature testing is needed to determine if: • The product is thermally stable in the package at higher temperatures • Product thermal instability contributes to or causes package corrosion The exposure time for higher temperatures should be determined by the actual weather in the areas where commercial spray packages are stored. For example, if the hottest weather to which your products are exposed is 90°F (32°C) and above for approximately five hours each day and there are 90 days of this weather per year. These weather conditions translate to a total of 19 days per year where temperatures are at or above 90°F (32°C). Consequently, in this example, storage testing at 100°F for 19 days should simulate approximately one year and approximately three years when tested for 60 days. In summary, we recommend using higher temperatures to evaluate product thermal stability in a spray package and not to accelerate coating, laminate film and metal corrosion. The four temperature-acceleration criteria can be used to verify if higher temperature is accelerating package material corrosion. Product instability is most likely causing higher temperature package corrosion when all four of the criteria are not fulfilled. We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. 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 June. SPRAY Corrosion Corner
Spray May 2015
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