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Spray May 2015

W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC Don’t fall into the “temperature accelerates corrosion” trap. May 2015 Spray 29 Corrosion Corner Hello, everyone. Every year, we receive multiple requests to determine the cause of spray package failure from corrosion (leaking packages). In many of these instances the spray product was commercialized after a few months (e.g., three months) of storage testing at a high temperature, such as 100°F. It is well known in corrosion and materials science that temperature typically does not accelerate metal and polymer corrosion. Figure 1 on the following page provides an example of the corrosion rate for polymer coated tinplate as a function of temperature. Notice that the corrosion rate actually decreases between 20°C (approximately room temperature) and 60°C. Thus, increasing temperature in this range does not accelerate coated tinplate corrosion. It is well known that the glass transition temperature of a wet polymer is significantly lower than the glass transition temperature of the dry polymer. Polymer coatings lose their properties above their glass transition temperature, including their ability to provide corrosion protection. Consequently, it is reasonable to conclude that above 60°C, the increasing corrosion rate in Figure 1 is caused by temperatures above the wetpolymer glass transition temperature. In other words, exceeding the coating glass transition temperature could cause coated tinplate corrosion. Figure 1 also illustrates why coated metals sometimes have significantly more corrosion at higher storage temperatures. There are four conditions needed for higher temperatures to accelerate material corrosion: 1. Material corrosion is present at all storage temperatures • Metal corrosion • Polymer coating corrosion or laminate film corrosion 2. The corrosion rate for each higher temperature is larger than the corrosion rate for each previous lower temperature 3. A temperature-corrosion rate graph is linear for the entire storage test temperature range (you will need to measure the corrosion rates at each temperature) 4. The energy of activation for the corrosion reaction has a


Spray May 2015
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