W. Stephen tait, ph.D. Chief Science Officer & principal Consultant, pair O Docs professionals, LLC Corrosion Corner How does concentration affect spray package corrosion? Part 2 30 Spray February 2016 The concentration of electrochemically active chemicals in your formula must be high enough to both initiate and sustain corrosion. Thus, there is a critical concentration for electrochemically active ingredients below which corrosion cannot be sustained enough to reduce package service lifetime. The chemical composition of your formula determines the critical concentration for specific electrochemically active ions and molecules. Copper ions Copper ions have been known to cause spray package pitting corrosion. The minimum concentration of copper ions needed to cause pitting corrosion is very small, ranging from one part per billion to two parts per million. Some types of formula ingredients, such as EDTA, could complex copper ions and reduce or eliminate the potential for pitting corrosion by copper ions. Therefore, the actual concentration of copper ions that cause package pitting corrosion is specific for each type of formula or formula family. Chloride ions Figure 1 contains an example of the effect of chloride ion concentration on aluminum aerosol containers with internal polyacrylamide (PAM) coatings. Notice that the container service life actually increases as the chloride ion concentration increases from 0.14% to 0.27%. In other words, the chloride ions are not causing spray package corrosion. However, negative ions, such as chloride ions, could support pitting corrosion. Nitrates and sulfates Nitrates and sulfates are electrochemically active and are sometimes suspected of causing spray package corrosion. However, these ionic molecules typically cause package corrosion when the solution pH is around four. Hello, everyone. Last month, I started a discussion on how formula ingredient concentrations could affect spray package corrosion. This month I will complete that discussion, starting with surfactants. Surfactants Surfactants have critical concentrations, such as those for micelle formation. Surfactants play a role in how a formula interacts with internal package surfaces. Thus, critical concentrations and concentration ranges that determine surfactant activity and performance could also contribute to or cause spray package corrosion. Electrochemically active formula ingredients Electrochemically active ions and molecules are those that remove electrons from metals, such as water molecules and hydrogen ions. Unsaturated organic molecules and rings are also potentially electrochemically active. Figure 1. An example of how chloride ions do not affect spray package service lifetime
Spray February 2016
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