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 1 22 Spray January 2016 ion concentration in water. Therefore a pH of two is 10-2 moles per liter of hydrogen ions (acidic) and a pH of 12 is 10-12 hydrogen ions (basic). The shape of a pH-corrosion rate curve for individual formulapackage systems is dependent on the chemical composition of your formula and the type of package metal. Figure 1 is an example of how water pH affects the corrosion rate of iron. The Y-axis is the corrosion rate in mils per year (1 mil = 0.001 inches) and the X-axis is the pH. Notice that the corrosion rate is very high at a pH of three (approximately 580 mils per year), and increasing the pH to 13 decreases the corrosion rate to approximately 50 mils per year. A normal rated tinplated steel container has a nominal thickness of six mils. Therefore, if Figure 1 is valid for your container and formula, the container service life is approximately 44 days when the pH is 13. Water (either as a formula ingredient or as a contaminant) Water is electrochemically active and therefore could cause spray package metal corrosion. Water also absorbs into polymer coatings and laminate films, causing their physical properties to degrade, such as with the polymer glass transition temperature. Polymers lose their physical properties when the temperature is above the glass transition temperature. Liquid water is necessary for metal and polymer corrosion. The amount of water molecules needed to form liquid water depends on the chemical composition of your formula. However, approximately 90 water molecules thermodynamically constitute pure liquid water. Water is consumed during metal and polymer corrosion and must be replenished to maintain the progress of the corrosion. For example, it would take approximately 0.042 ppm of water in 100 grams of formula to sustain pit growth until it perforated a 6-mil thick container. There have been many instances where spray package corrosion does not occur when the water concentration range is above and below a critical concentration. Therefore, the critical concentration for package corrosion by water is determined by the chemical composition of the liquid (formula + propellant + water) in the commercially filled spray package. There are no public domain lists of critical water concentrations for spray formulas, so the non-corrosive concentration of water in specific formulas should be determined on a case-by-case basis. Corrosion inhibitors Corrosion inhibitors typically have an effective concentration range that prevents or controls corrosion. Corrosion typically is higher when inhibitor concentrations are both below and above the effective concentration range. Happy New Year, everyone. The chemical composition of a formula determines if corrosion will occur, where it will occur and how fast corrosion proceeds through the package metal to cause leaking or how fast the package polymer degrades. The concentrations of formula ingredients may also affect spray package corrosion. A list of the most likely formula ingredients that could contribute to or cause spray package corrosion is: • pH • Water • Corrosion inhibitors • Fragrance • Surfactants • Electrochemically active formula ingredients, copper ions, chloride ions, nitrate and sulfate ions • Propellants I will discuss the first four items this month, and the remainder next month. pH: (hydrogen ion concentration) The concentration of hydrogen ions in the formula or contaminant water is quantified with pH—the negative logarithm of the hydrogen Figure 1. Example of how pH could affect steel corrosion rates
Spray January 2016
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