Written on: October 1, 2024 by W. Stephen Tait
Hello everyone. In the last issue, we discussed the complexity of corrosion and how one could determine if spray package corrosion will or will not occur. I listed the nine known factors that cause or contribute to spray package corrosion.
The empirical equation for these nine factors help determine what type(s) of corrosion occurs and how fast corrosion occurs.
Equation 3 provides an empirical equation for all nine factors that could be used to estimate corrosion rates and help determine what type(s) of corrosion are occurring.
The symbols in Equation 3:
• Ψ and Γ have the same meanings as in Equation 2
• Subscript and superscript items have the same meanings as in Equation 2
• ß is proportionality constant that converts the probabilities into corrosion rates
• f (letter or phrase) is the exponent for each group of factors—each is a complex function
Obviously, Equation 3 is more complex than Equation 2. The increased complexity occurs because it answers both the What type(s)? and How fast? questions. Equation 2 only answers the question, Will it corrode?
There are ~362,880 possible combinations of the nine factors that could cause corrosion or contribute to spray package corrosion—making corrosion probabilistic instead of deterministic. Groups that have probabilities (ψ1-9) >1 affect corrosion and those that are (ψ1-9) = 1 have no effect on corrosion. Consequently, Equation 3 provides information about the type(s) of corrosion that occur.
Let’s look at each group in Equation 3, proceeding from left to right for the first two (pH and metal type), and then from the second line (surface tension) to the bottom.
The first group estimates how pH affects the corrosion rate magnitude. ψ1 is the probability that the pH of formula water or contaminant water will decrease or increase the corrosion rate, and the exponent “a” determines how much pH affects the rate, and is typically a single number.
The second group (metal type) estimates how the type of package interacts with the product. Spray package metals could be tinplated steel, tin-free steel, aluminum and aluminum foils. Coated metals, laminated metals and uncoated metals corrode at different rates when exposed to the same formula. Consequently, the second factor accounts for how different types of metals/coated-metals influence the rate of spray package corrosion. The exponent f(h) is a complex function that generates a single number for each metal type exposed to a specific formula.
The third group estimates how surface tension affects package corrosion. Surface tension determines how easy or difficult it is for formula ingredients to:
1. Absorb onto uncoated metal surfaces;
2. Diffuse through polymer coatings and laminate films; and
3. Adsorb onto substrate metals under laminated films and coatings.
The exponent f(i) is a complex function that generates a single number for each component surface tension.
The fourth group estimates how electrochemically active (ECA) ions and molecules affect corrosion rates. The ECAϒj symbol represents the electrochemical activity for individual ions and molecules in a formula that are electrochemically active. The exponent f(j) is a complex function that generates a single number for each specific electrochemically active ion or molecule in a formula.
The fifth group estimates how the metal surface treatments affect corrosion rates. Surface treatments include polymer coatings or laminate films, tin coatings on steel and chromium/chromium oxide coatings on steel (tin-free steel). The exponent is a complex function that generates a single number for each type of surface treatment.
The sixth group estimates the cathode/anode area ratios on the metal surface that determines if pitting corrosion will occur and how fast pitting corrosion will penetrate through a package. Metal surfaces—both coated and uncoated—are composed of cathodic areas where valence electrons are transferred from surface atoms to ECA formula ingredients and anodic areas where the atoms are ejected from the bulk metal as ions.
This factor also helps determine if corrosion will be either general or localized. Pitting corrosion occurs when the exponent f(m) is >0. The exponent is a complex function whose magnitude is determined by the specific chemical composition of a formula and the type of package materials.
The seventh group accounts for emulsion stability. Emulsions break after a certain age and when exposed to either high or low temperatures. Water and cream phases are typically generated when an emulsion breaks—and one or more of these phases could be very corrosive. Consequently, the exponent for this factor is an equation that is a function of both temperature and emulsion age. This particular factor is zero for non-emulsion products and greater than one for emulsions.
The eighth group is the age of a product in its spray package. General corrosion typically occurs shortly after a package is filled; pitting corrosion follows afterward. The exponent “n” for this factor is typically a single number that is determined by the specific chemical composition of a formula and the type of package materials.
The ninth factor accounts for formulas incorporating corrosion inhibitors. There is no such thing as a one-size-fits-all corrosion inhibitor.
There are also many types of formula ingredients, such as fragrances, that in some instances act as corrosion inhibitors. Consequently, the exponent of this factor is a complex function that accounts for specific formula chemical compositions, pH, synergy between all formula ingredients that could inhibit corrosion and the effective concentration range for each ingredient that inhibits corrosion.
To summarize both Parts 1 and 2 of this series:
• Predicting corrosion with Equation 2—the theoretical equation for whether corrosion will or will not occur—does not provide operational data for making decisions on product-package longevity.
• There is no available public domain knowledge for the parameters needed to use empirical Equation 2 and Equation 3 for predicting corrosion and corrosion rates.
Consequently, corrosion testing with either a storage stability test and/or an electrochemical corrosion test are the only ways to reliably measure and predict if corrosion will occur and how fast corrosion will penetrate spray packaging.
Both types of tests must be conducted with the appropriate testing procedures (e.g., a minimum of one year for a storage test), data analysis procedures and models for predicting both service lifetimes and their associated percent failures.
Thanks for your interest and I’ll see you in an upcoming issue. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
