Corrosion Corner W. Stephen tait, ph.D. Chief Science Officer & principal Consultant, pair O Docs professionals, LLC Why do spray package metals and coated metals corrode? Figure 1: three-dimensional cubic model of a solid 24 Spray November 2015 We will use the simpler model in Figure 1 for the discussion on why metals corrode. The atoms under the surface are surrounded by eight neighboring atoms and are consequently thermodynamically stable. However, the surface atoms have an incomplete number of neighbors and are thus thermodynamically unstable. Consequently, electrochemically active ions and molecules, such as water, can remove valence electrons from the surface atoms, changing their chemical state from a thermodynamically unstable surface atom to a thermodynamically stable ion. Figure 2 depicts a more complex cross section of the model in Figure 1. • Small blue circles represent water molecules in a formula • Grey circles represent metal atoms in an uncoated or coated package metal • Light gray circle with a positive sign in its center and the attached three blue circles represents a metal ion • Green negative signs represent metal valence electrons • Other colored circles represent various ions and molecules that are formula ingredients Notice in Figure 2 that the axis for atoms in the top group is 45 degrees to the axis for the bottom group under the parallel wavy lines. The two different groups represent Hello, everyone. Three ingredients are needed for metal or coated metal corrosion: 1. A metal or metal alloy—with or without a coating 2. Water—either as a contaminant or as a component of your formula 3. Ions and/or molecules that are electrochemically active Electrochemically active ions or molecules are chemicals with unsaturated valences or bonds, respectively. For example, water is electrochemically active. Hydrogen ions in water molecules can remove electrons from metal surface atoms. The positive metal ions resulting from corrosion balances the negative charge of the hydroxyl ions from the water molecules. The structure of a metal contributes to its susceptibility toward corrosion. Figure 1 is a simple cubic model for solids. Metal atoms are on the corner of each cube and the cubes are repeated under the surface atoms in all directions as indicated by the orange arrows. Iron has an additional atom inside each cube and aluminum has additional atoms on each cube face. The metals used for spray packaging are alloys, and the alloying elements are located either in one of the metal atom positions or wedged between the iron and aluminum atoms. Figure 2: Cross section of a metal with slight corrosion
Spray November 2015
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