Hello, everyone. This month, I’m starting a two-part series on the roles that water plays in causing or contributing to spray package corrosion. Water is in a formula as either an ingredient or a contaminant.
Steel alloys and aluminum alloys are both used to fabricate spray packages with and without coatings or laminate films. Steel alloys are in sheets and aluminum alloys can be either thin solid plugs (about the size of an ice hockey puck) or thin foils.
Water and metals are both thermodynamically unstable when in contact with each other. Thus, water can cause or contribute to package metal corrosion.
Water is a useful ingredient in a formula because it dissolves a wide range of other formula ingredients and also dissolves into a wide range of formula chemicals, solvents and propellants. For example, inorganic and organic salts, such as sodium chloride or sodium benzoate, dissolve in water.
Most gases dissolve in water, and some of these gases—such as carbon dioxide—react with water to form corrosive carbonic acid. Dimethyl ether (DME) propellant is water-soluble and small amounts of water will dissolve in HFC152a propellant, liquefied petroleum gas (LPG) propellants and most organic solvents.
Water and surfactants are used to form emulsions with water-insoluble ingredients. Emulsions can be either water in oil (oil-out or W/O emulsions), or oil in water (water-out or O/W emulsions).
Consequently, water is both a solvent and, in some instances, a corrosive species.
Water chemistry & properties
You probably remember from your first chemistry course that water molecules have one oxygen atom bonded to two hydrogen atoms (H2O). The valence electrons in water molecules are not shared equally between the hydrogen and oxygen atoms. Thus, one area of the water molecule is positively charged (the hydrogen end) and the other area is negatively charged (the oxygen end). The positive end is attracted to the negative charge associated with surface metal atom valence electrons.
Water’s small size also allows it to absorb into and migrate through interior package coatings and laminate films. Polymers swell when water absorbs into and diffuses through a polymer, thereby creating microscopic rivers through both polymer coatings and laminate films. Microscopic rivers that terminate at the package metal can cause both metal corrosion and polymer delamination, such as blisters.
Water & spray package corrosion
There is an approximate 62% probability of package-metal corrosion whenever corrosion testing is not conducted. Water molecules are electrochemically active, but water also dissociates into electrochemically active hydrogen ions and hydroxyl ions. Electrochemically active water and dissociated hydrogen ions can both remove electrons from spray package metals, thereby causing corrosion. In other words, in addition to being a solvent, water is also potentially a corrosive ingredient or contaminant. 
Liquid-water plays multiple roles in a formula and in package material corrosion (more on liquid-water in the next issue). Water’s various corrosion roles are it:
• Could be a corrosive formula ingredient
• Transports corrosive formula ingredients to the package metal
• Moves ions to/from corrosion sites to maintain the electrical balance between the corrosion sites and their surrounding areas
• Hydrates metal ions formed by corrosion
• Provides hydroxyl ions that bond with metal ions, forming visible corrosion, such as the various colors of rust observed on steel or the white and black corrosion on aluminum
• Migrates through package polymer coatings and laminate films to the package metal to cause metal corrosion and/or coating/laminate film corrosion, such as blistering
• Transforms coatings/laminate films into semi-permeable membranes that allow only specific formula ingredients to migrate through the coating/film to the package-metal
• Forms microscopic rivers of formula ingredients in a container coating or laminate film that could subsequently cause coating delamination and/or metal corrosion
• Is a solvent for the various corrosion inhibitors used to prevent and control spray package corrosion
Next time, we’ll discuss the mechanism for water corrosion and how water supports pitting corrosion, as well as how much water is needed to initiate spray package corrosion.
Thanks for your interest and I’ll see you in June. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. One of the most difficult questions to answer about corrosion is why it is so random. For example, you may open and inspect 12 traditional aerosol container spray packages after six months of storage stability testing to find:
• Five corrosion-free packages
• One package with corrosion on the liquid product area
• Three packages with corrosion in the vapor area
• One package with a pit in the product phase
• One package with corrosion and pitting in the bottom crevice (bottom double seam)
• One leaking at a pit that perforated the container body
The formula in this example was from the same batch, so the chemical composition is the same; all the packages came from the same pallet (i.e., manufactured at the same time); and all the packages were all filled at the same time. Should you:
a) Ignore the corroded and leaking packages;
b) Put the program on hold; or
c) Something else?
It would be even more frustrating if you found 11 corroded packages and one pristine, corrosion-free package. Why isn’t corrosion more consistent?
Variability is the culprit that causes the inconsistency of the observed corrosion in the same product-package system. Variability can also cause the unexpected appearance of corrosion in a previously corrosion-free commercial product, particularly if the corrosion tests were conducted with the wrong parameters.
There are three generic types of variability:
1. Within each individual package—package variability
2. Within individual product and package production batches—within-lot variability
3. Between different product and package production batches—lot-to-lot variability
The main corrosion causing/contributing factors associated with the generic types of variability are:
1. Product chemical composition
2. Package coating, valve coating and laminate film morphology (e.g., thickness and adhesion)
3. Laminate film and coating chemical compositions
4. Package geometry
a) Aerosol valve crimps (crevices)
b) Package crimps (crevices)
c) Aerosol container double seams (crevices)
d) Aerosol container and laminated foil bag welds
It has been my experience that variability of the package and laminated foil metal chemistries typically do not have a significant effect on spray package corrosion. Consequently, variations in foil and package-metal chemistries are not included in the above list.
The four associated factors could cause corrosion either by themselves or in combination with one or more of the others. Consequently, the number of combined factors could be very great. For example, there are 5,040 possible combinations for all of the above associated factors.
Let’s briefly discuss the relationship between the associated factors and the inconsistency of the observed spray package corrosion.
1) Product chemical composition
Small changes in chemical composition, such as pH, fragrance concentration or amount of water, could transform a benign formula into a package eater. Variability is exacerbated by lot-to-lot variations in raw materials. There are also chemistry differences and variations in raw materials from different sources, even when the specification for the different sources is nominally the same.
2) Package & valve laminate/coating morphology
This pertains to thickness, adhesion and metal wettability. Laminate films and coatings have variable thickness on individual packages, among packages within the same lot and among packages from different lots. Variability in the metal cleaning process and the coating application process could also produce small areas on individual package surfaces that lead to delamination and metal corrosion under the film or coating. Coatings from different sources could also produce a narrower or wider thickness range, even when the specifications for the two sources are nominally identical.
3) Laminate film & coating chemical compositions
Variations in the chemistry of laminate films and coatings could cause variability in how the coating adheres to the substrate metal. Variability in adherence could cause laminate/coating delamination with subsequent metal corrosion under the coating.
4) Package geometry
Spray package crevices are formed when:
• Aerosol valves are crimped to the container curls
• Rolling the bottom of a laminated foil bag or tube to form a seal
• Seaming aerosol container tops and bottoms to the container body
Not all crevices are created equal. Some have a large opening with a short length, some have a small opening with a long length and so on. The ratio of the crevice opening to its length is referred to as its aspect ratio.
Crevice aspect ratios interact with a formula’s chemical composition and physical properties to determine if and when crevice corrosion will occur, if the crevice corrosion will be general corrosion with or without pitting corrosion and how fast pitting crevice corrosion will penetrate the package metal or laminated foil.
Spray packages typically have a range of crevice aspect ratios within individual production lots and between different production lots. Consequently, crevice corrosion—with or without pitting—does not occur in every spray package unless the product is extremely corrosive.
Spray package welds might also be vulnerable to coating corrosion, laminated film corrosion and foil and metal corrosion. Corrosion vulnerability is determined by the interaction between a formula’s chemical composition and the weld.
There are two ways to account for variability when conducting corrosion tests:
1. Develop a database on how variability affects package corrosion for each product family in your line of spray products
2. Design your corrosion tests to include:
a) A large number of replicate samples for each variable
b) Variables with package components from different production lots
c) Variables with different concentrations of potential corrosion-causing formula ingredients, such as water
The corrosion test could be a storage test, an electrochemical test or a combination of both. Skipping corrosion stability tests and conducting tests with the wrong parameters might result in surprise corrosion in the marketplace.
Thanks for your interest and I’ll see you in May. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. In the previous issue, I began a series on material defects in spray packaging to discuss their relationship with spray package corrosion. We began by focusing on the more common defects in aluminum aerosol containers. This month, we’ll continue with the more common material defects found in laminated foil bags and tinplated steel aerosol containers.
Laminated aluminum foil bags
Flat sheets of laminated foils are folded into a bag, the seams welded together and an aerosol valve is attached to the top of the bag. The aluminum-foil is sandwiched between layers of polymer film—typically one layer on the foil’s exterior and two layers on the interior. The laminated bags are inserted into traditional aerosol containers and the product is filled into the bag.
Figure 1 has a typical example of a micro-bulge in a laminated bag. Bulges are typically small, thin and transparent, as seen in Figure 1. The micro-bulge in Figure 1 is in the interior bi-layer film (nylon on top of polypropylene). Hence, the bulge might be the result of one layer delaminating from the other. However, the actual cause for micro-bulges is unknown at this time.
It has been our experience that micro-bulges do not contribute to or cause laminated aluminum foil bag corrosion.
Figure 2 provides an example of a laminate polymer film that is delaminated at the bag welds. It has been our experience that this type of material defect is rare and can be avoided with good welding practices.
We have not observed instances where this type of material defect contributes to or causes spray package bag corrosion. However, this type of defect could cause bag-rupture around the delaminated area if the weld is weak.
Figure 3 provides an example of a crack in the aluminum foil under the laminate film. In our experience, this type of material defect is typically associated with a single manufacturing batch. We have not observed instances where this type of material defect contributes to or causes laminate-foil bag corrosion.
It is possible that a bag with this type of defect might leak product. However, the product would have to diffuse through both the inner and outer film layers to cause a bag to leak. We have not yet observed instances where this type of defect causes bag leaking.
Tinplated steel aerosol containers
Tinplated steel (tinplate or electrolytic tinplate [ETP]) aerosol containers are available both with and without an internal polymer coating. The same coating defects discussed in last issue’s Corrosion Corner for aluminum aerosol containers are also found in coated tinplate aerosol containers. Hence, I will not repeat last issue’s discussion and will focus on unlined tinplate defects.
Figure 4 provides an example of an area where the tin coating did not cover (wet) the steel substrate. Non-wetting produces holes in the tin coating that expose the substrate steel, and/or a very thin iron-tin alloy layer—sometimes referred to as K-plate. Multiple holes in tin coatings—such as that in Figure 4—are frequently present in tinplated steel aerosol containers.
There are two different morphologies for the holes in tin coatings. Figure 4 shows the traditional type of hole morphology and Figure 5 shows a newer type of hole morphology that has appeared within the last two decades.
Notice that, when comparing Figure 4 and Figure 5, the newer holes are more symmetrical than the traditional holes. Most likely, the symmetrical newer holes arise from either a new plating bath chemical composition, a new tinplating process or variations in plating bath chemical composition.
Holes in tin coatings are potential sites for pitting corrosion. However, the chemical composition of your formula determines if holes in tin coatings will or will not result in pitting corrosion.
It is unknown at this time if the traditional hole morphology (Figure 4) or the newer hole morphology (Figure 5) result in different susceptibilities to and/or magnitudes of pitting corrosion. Corrosion testing is the only way to determine if a given formula will cause pitting corrosion at holes in the tin coating.
Steel aerosol container welds are diffusion welds formed with heat and pressure. The heat is produced from an electrical current flowing between the overlapping edges of the tinplated steel sheet used to form the cylindrical container body.
A small amount of steel metal could be ejected from under the overlapping edges when the combination of pressure and heat is not optimum. Figure 6 has an example of a small particle of metal ejected from a weld. This phenomenon is referred to as weld spatter.
Weld spatter is not common. However, pitting corrosion could occur if weld spatter produces a small cavern in the weld. The chemical composition of a formula determines if pitting corrosion will occur in a cavern formed by weld splatter.
The material defect examples in this and last issue’s Corrosion Corner both provided examples of macro-defects that can be seen either with the unaided eye or a light microscope. These types of defects typically do not cause corrosion. However, there are formulas that will cause or contribute to spray package corrosion at these defects. Thus, corrosion testing is necessary for both new products and existing-product derivatives to prevent unexpected corrosion.
Thanks for your interest and I’ll see you in April. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. Defect-free spray package materials do not exist and these defects are often misidentified as corrosion.
Even though defects don’t often contribute to or cause spray package corrosion, some formulas will interact with defects to produce package corrosion. Thus, there are always potential concerns as to whether or not defects will contribute to or cause spray package corrosion and if this corrosion will cause spray packages to fail (leak).
This issue begins a two-part discussion on material defects and their relationship to package corrosion, beginning with traditional aluminum aerosol containers. Discussion on material defects in laminated foils and tinplated steel aerosol containers will follow in the next issue.
Photographs of material defects are used to discuss the relationships between defects and corrosion.
Aluminum aerosol container material defects
Figures 1–6 provide examples of material defects in coated aluminum aerosol containers. Figure 1 shows an example of an inclusion in the container metal.
Inclusions are microscopic materials that are insoluble in the aluminum metal. The two major types of inclusions include:
1. Particles of the metals and non-metals added to aluminum to give it the desired strength and formability; and
2. Crystals of aluminum complexed with other metallic and non-metallic elements, for example, aluminum silicate.
Inclusions are often spherical and become distorted and flattened when the aluminum alloy is formed into a container.
I’ve only observed one rare instance when an inclusion such as the one in Figure 1 caused container pitting corrosion with subsequent leaking.
Aluminum tubes and aerosol containers are formed by extruding a small aluminum disc into the desired container shape. Small pieces of metal (divots) are often ripped-from the aluminum during extrusion, an example of which is shown in Figure 2.
Notice in Figure 2 that the internal package coating backfilled the divot. I have only observed rare instances where this type of defect contributes to or causes package corrosion.
Aluminum aerosol containers are open at the top and closed at the bottom. The tubes are cleaned with a spray nozzle to remove the lubrication applied during extrusion and the internal coating is subsequently sprayed inside the tube prior to forming the container top dome. The nozzle moves from the bottom of the tube to the top during the coating application.
In some instances, coating drips from the nozzle when spraying is stopped near the top of the tube. Figure 3 shows an example of a coating drip in an aluminum aerosol container, commonly referred to as a drool. I’ve not observed an instance where a coating-drool contributed to or caused aerosol container corrosion.
Entrained air in the bulk coating material sometimes causes a coating nozzle to eject a small amount of excess coating. The excess coating that sticks to the surface is referred to as a spit, examples of which are shown in Figure 4. Notice that the spits are at two different locations inside the two containers.
Spits are very common in aluminum aerosol containers. However, I have not observed an instance where a spit contributed to or caused container corrosion.
Figure 4 also shows variations in coating color. Coating color variations could be caused by variations in the thickness of the coating—a well-known phenomenon in the coatings industry. I have observed a few rare instances where variations in color appeared to cause random container failures (leaking).
Aerosol container coatings are cured with high temperatures. Coatings are dissolved in solvents that evaporate during the curing process and small bubbles can form during evaporation. Sometimes these bubbles harden, producing solvent pops like the one in Figure 5.
Solvent pops rarely contribute to or cause corrosion. Pitting corrosion inside solvent pops can only occur when there is also extensive coating corrosion in a large area surrounding a solvent pop.
Small holes form in coatings wherever the coating does not wet (cover) the package metal. Figure 6 shows an example of a hole in a coating. The hole is inside the meandering dashed line in Figure 6. Holes like that shown in Figure 6 are very common and often expose the container metal to your formula.
This type of defect causes pitting corrosion when there is also extensive coating corrosion in a large area around the hole.
One or several of the defects shown in Figures 1–6 are present in most aluminum aerosol containers and tubes. Consequently, corrosion testing with your specific formulas is needed to determine when material defects will contribute to or cause container corrosion that leads to package failure (leaking).
In the next issue, I will complete this discussion on material defects for laminated foil packages, as well as for both coated and uncoated tinplated steel aerosol containers.
Thanks for your interest and I’ll see you in March. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. New products and derivative products typically have several timetables. One of these is an ideal timetable for sales (Target Sales) and the other is the actual sales timetable that is determined by the strength or weakness of the economy.
Package corrosion also has a timetable, referred to as the package service life (PSL). A PSL for each product interacts with its product sales timetables to determine if customers and consumers will observe package corrosion.
Observed corrosion manifests itself as leaking packages, partially full packages that no longer spray, packages that dispense discolored product and packages that dispense product with active ingredient concentrations below the specified concentrations.
Let’s review the definitions for Target Sales (TS), Slow Economy Sales (SES) and PSL, as well as how they interact to determine if or when corrosion is observed by customers and consumers.
TS & SES
Target Sales (TS) are typically developed from market research and profit budgets. Typically, TS is the shortest time after production when packages are emptied and recycled. TS is often thought of as a single number, such as “100% of the units produced are purchased, used and recycled within X-days.”
However, TS is typically a curve composed of the unsold inventory at different times. Figure 1 graphically illustrates generic curves for both TS and slow economy sales (SES). Notice that both curves have an S-shape.
The X-axis in Figure 1 is the time after production when empty packages are recycled. The numbers are left off the X-axis to make it generic. The Y-axis is the percentage of unsold inventory at a given time.
The curve on the left side of Figure 1 is the TS curve and the one on the right is a curve for SES. A fast economy normally decreases production-to-recycle times (desirable) and a slow or stagnant economy increases production-to-recycle times (undesirable). These types of curves could be generated either individually for single production batches or for an entire year’s production.
TS and SES curves do not consider package corrosion. Slow corrosion rates can be hidden by fast economies and a short TS, particularly when all the empty inventory is recycled before package corrosion is observed. Very fast and severe corrosion is typically observed even with a short TS. However, intermediate corrosion rates might not cause detectable corrosion for a TS, but often produce detectable corrosion with a long SES.
Let’s define PSL and then how it interacts with both TS and sales during a slow/stagnant economy, such as occurred during the COVID-19 pandemic.
PSL with corrosion
Package service lifetimes (PSL) are also typically a range of times with associated estimated failure levels (EFL). Figure 2 is an example of PSL-EFL curves with lower-range and upper-range curves. Both are generated with simulated data, but have the same shapes as actual PSL-EFL curves.
The X-axis in Figure 2 has a range of PSL magnitudes and the Y-axis has their corresponding, cumulative EFL magnitudes. The numbers are left off the X-axis to make it generic. The estimated percent package failure levels (EFL) increase with increasing time—in contrast to the TS and SES curves in Figure 1 that both decrease with increasing time.
The magnitude of a PSL-EFL curve is determined by:
• The type of package materials;
• Package metal surface treatment (e.g., coated, or uncoated);
• The physical form of a formula (e.g., emulsion or single phase); and
• The chemical composition of your formula and the package corrosion rate
PSL data and its corresponding EFL data are both generated from corrosion data collected from either a long-term storage test or electrochemical corrosion testing.
Figure 3 illustrates the relationship between TS, SES and PSL by overlaying Figure 1 with a single PSL-EFL curve from Figure 2 (done for clarity).
The small dashed box on the left of Figure 3 illustrates corrosion overlapping TS and the large one on the right illustrates corrosion overlapping SES.
Figure 3 illustrates why it is less likely that customers and consumers will observe corrosion with a TS than with a SES, particularly when corrosion rates are low. If you shift the PSL-EFL curve to the left you will see why severe corrosion is observed even when the TS is short.
I recommend generating a PSL-EFL curve for all spray package products and derivative products and comparing the curves with the product’s TS and several scenarios with different SES curves. These comparisons can enhance production planning and avoid surprise corrosion when sales become slower than originally planned.
This same recommendation applies to other types of metal packages or laminated metal foil packages that are used for various consumer packaged goods, foods, beverages and pharmaceutical products.
Thanks for your interest and I’ll see you in February. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. I have often been asked if corrosion could be mathematically modeled so that long-term corrosion testing could be either shortened or skipped. The short answers are Yes-and-No.
There are many potential formula and package material factors that cause or contribute to the corrosion of spray package materials.
The seven major categories of corrosivity factors are:
1. Electrochemically active (ECA) formula ingredients are often those whose molecules are unsaturated and thus can contribute to or cause corrosion of package metal components. Ions such as hydrogen ions and molecules such as water are also ECA formula ingredients, as are many insecticides.
2. Liquid Water is an EAC molecule. Water can be either part of the formula or present as a contaminant.
• Only 90 water molecules are needed to form liquid water and thus initiate corrosion
• Water opens up polymer coatings and laminate films by forming microscopic rivers through the coatings and the films
3. Formula pH is the hydrogen ion concentration in your formula-water or contaminant-water. pH is typically expressed as the negative log of the hydrogen ion concentration. For example, water with a pH of seven has a 10-7 moles/liter hydrogen ion concentration.
Different metals have different ranges where they are either more—or less—corrosive by formula pH. For example, steel alloys usually corrode rapidly at low pH, but slowly at high pH. Aluminum does not corrode in pure water when the pH is from around 3–7, but corrodes when the pH is lower than three and higher than seven in pure water.
4. Fragrances have been shown to function as corrosion inhibitors in many instances—and a few types of fragrances contribute to or cause spray package corrosion. The concentration of the fragrance often determines either the level of corrosion inhibition or the corrosivity of a given fragrance.
5. Surfactants make it easier for ECA ions and molecules to contribute to or cause corrosion by enhancing water adsorption to metal surfaces and the absorption that creates water molecular-rivers through polymer coatings and films.
6. Propellants in some instances might contain EAC molecules. There are multiple types of propellants:
• Compressed gases such as carbon dioxide (EAC is carbonic acid), air (EAC is oxygen) and nitrogen
• LPG propellants that are blends of butane isomers, pentane isomers and propane (no EAC molecules)
• Hydrofluorocarbons, such as HFC152a and Solar (typically no EAC molecules unless contaminated with water)
• Dimethyl ether (DME)—this is a good solvent for a wide range of polymers and can, in some instances, contribute to metal corrosion by degrading internal package coatings and films
7. Spray package materials can be metals, polymers or a combination of both.
The metals for spray packages can be:
• A thin, chromium-coated steel (tin-free-steel or TFs)
• Tin coated steel (tinplate)
• Aluminum
The polymer coatings for spray package metals can be:
• Epoxy coatings
• PVC particles suspended in a epoxy matrix
• PAM coatings
• Laminate films composed of a layer of nylon (e.g., OPA) on top of polypropylene
• Vinyl coatings
The chemical composition of your formula and the type of package materials determine if a formula will contribute to or cause spray package materials corrosion.
In summary, spray package corrosion is a complex interaction of multiple, potential corrosion-causing factors in at least seven categories. Consequently, the mathematical equations to predict spray package corrosion must have parameters for all of these factors.
Three corrosion questions should be answered before marketing a new spray product or a derivative spray product:
1) Is corrosion expected?
2) How fast is corrosion expected to degrade and/or penetrate package materials?
3) What type or types of corrosion are expected?
Question #2 is about package service lifetime. Service lifetime is defined as the filled package age when:
a) The package leaks product or propellant;
b) Valves leak propellant;
c) Partially full packages cease to spray; or
d) Package corrosion degrades product performance and/or efficacy.
In other words, service lifetime is the length of time during which spray packages and their products function properly.
Empirical equations for the first two corrosion questions need to incorporate all the formula and package. Let’s discuss the equations for Questions #1 and #2. Discussion on the equation for the third corrosion-question is deferred for a later edition of Corrosion Corner.
1. Is corrosion expected?
The Gibbs-free-energy equation can be used to estimate the probability of package corrosion. An empirical version of the Gibbs-free-energy equation for the probability of spray package corrosion is:
The various symbols in the Gibbs-equation mean or indicate:
• ∆G = Gibbs free energy
• n = 2 or 3 for steel /n = 3 for aluminum
• F is the Faraday’s constant
• K is a proportionality constant
• Ψ is probability
• Single, lower-case, superscripted letters are exponents for the equations inside parentheses. These can range from 0–infinity
• G indicates that all the factors between square-brackets are multiplied
• A combination of superscripted lower-case letters with sub-scripted lower-case letters represents unknown equations. Some of these unknown equations are exponents for equations in both parentheses and square-brackets
• Single, lower-case letters represent numbers ranging from 1–infinity
There are approximately 15 parameters in the Gibbs-equation. Consequently, there are 15 [!] (15-factoral) possible combinations. In other words, there are 1,307,674,368,000 possible combinations of the corrosivity-factors that determine if a formula will contribute to or cause spray package corrosion.
A negative ∆G indicates that package corrosion is expected by a formula. However, the Gibbs-equation does not tell us how fast the corrosion by a formula degrades and/or penetrates a package, thus reducing the package service lifetime. Hence, let’s move on to discuss the empirical equation for how fast corrosion is expected to degrade and/or penetrate package materials.
2. How fast is corrosion expected to degrade and/or penetrate package materials?
The empirical equation to estimate the most probable corrosion rate can be seen in Figure 1.
The new symbol, b, is a conversion factor that converts calculations from the Equation into a corrosion rate. The probability equation for package material corrosion rates has approximately 25 parameters in it. In other words, there are approximately 15,511,210,043,330,985,984,000,000 possible combinations for the corrosivity factors needed to estimate a package corrosion rate with a specific formula.
Clearly, the two empirical equations are very complex and require knowledge of many, many corrosion factors and equation parameters before either can be solved for a specific package-formula system. To further complicate matters, no-to-very-little research is available for the specific corrosion factors and equation parameters in both equations. Hence, the corrosion factors for consumer packaged goods spray products are virtually unknown at this time.
Consequently—while theoretically possible—substituting mathematical models for corrosion tests is not practical at this time. Thus, corrosion testing is currently the only practical and reliable way to determine:
1) Will a formula or line extension corrode the chosen spray package materials?
2) How fast will corrosion proceed through the materials—in other words, what is the package service lifetime with a specific formula?
3) What type or types of corrosion are expected?
Thanks for your interest and I’ll see you next year. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. I started a two-part series about the science for using higher temperatures to accelerate the rate of spray package corrosion as a possible way to reduce storage test time in the last issue. The first part centered on the science for the Arrhenius equation, often used to justify using higher temperatures to accelerate corrosion. It was demonstrated that the Arrhenius equation is usually invalid for spray package materials; hence higher temperature does not accelerate metal and polymer corrosion rates.
I’ll pick up where we left off with a continuation of how higher temperatures actually affect polymer and metal corrosion.
3. Do higher temperatures actually accelerate corrosion? (Continued from last issue)
Sometimes higher temperatures appear to produce more corrosion than lower temperatures, leading to the impression that a higher temperature is accelerating the corrosion. Polymer-coated metals provide a good example of why this is not the case.
Figure 1 summarizes data from corrosion measurements on coated tinplate at temperatures from 20°C to 80°C (68°F to 176°F). Notice in Figure 1 that instead of the corrosion rate increasing linearly with increasing temperature, the corrosion rate actually decreases between 20°C (68°F—room temperature) and 60°C (140°F). Thus, increasing temperature in this range does not accelerate coated tinplate corrosion. Notice also that the corrosion rate doubles as the temperature is increased from 60°C to 80°C (140°F to 176°F). Figure 1 demonstrates that the Arrhenius equation is not valid for coated metal corrosion.
The reason why the Arrhenius equation does not predict corrosion for coated metals is because polymer corrosion is a complex interaction between absorption of water, molecules and ions into and through the polymer, plus breaking metal-polymer bonds. In other words, polymer-corrosion is not a first order reaction.
Absorption also changes a polymer’s physical properties—such as lowering the glass transition temperature (Tg) and often breaking metal-coating bonds.
Notice in Figure 1 that the wet-epoxy Tg is approximately 40°C (104°F) lower than the dry-epoxy Tg. The wet coating corrosion rate increases as temperature is increased because the wet-epoxy is no longer a protective-barrier between the formula and the container metal. Both polymer and metal corrosion occurred in this situation. In other words, exceeding the coating glass transition temperature causes coated tinplate corrosion. Consequently, Figure 1 also helps to explain why coated metals sometimes appear to have significantly more corrosion at higher storage temperatures.
To summarize, polymers lose their properties—including corrosion barrier-protection—when the temperature is at and above their Tg. Thus, higher storage test temperatures subject package polymer materials (coatings) to temperatures above the wet-polymer Tg, with the result that the polymers no longer have their original corrosion-barrier protection property. In this situation, the corrosion at a higher temperature is not indicative of actual corrosion.
Both metal corrosion and polymer coating corrosion are not pure chemical reactions and both types of corrosion rarely meet the two requirements for valid use of the Arrhenius equation. Consequently, higher temperatures rarely accelerate the corrosion of the metals, internal coatings and laminated films used for spray packaging.
4. Should storage tests be conducted −and for how long?
Higher temperatures should be part of a storage test. It might not seem likely and I’m not contradicting myself. Higher temperature corrosion data provides a way to determine if the formula is temperature-sensitive, but not as a way to accelerate corrosion.
Products may be exposed to higher temperatures while being transported or stored during the Summer months. However, actual exposure times at higher temperatures are often short, because peak daily temperatures typically only last for a few hours every 24 hours. Thus, the majority of filled spray packages spend most of their service lifetime at or near room temperature (assuming no long exposures to direct sunlight).
Higher temperatures could degrade components of a formula, making it corrosive. Higher temperatures could also destabilize the physical properties of a formula. For example, higher temperatures could break water-in-oil emulsions, producing a corrosive, free-water phase. Consequently, higher temperature storage testing is recommended to determine if:
• Your formula is stable at higher temperatures
• Formula instability at higher temperature produces conditions that cause spray package corrosion
5. How long should test samples be exposed to higher temperatures?
The amount of time that packages are stored at higher temperature depends on the local climate where your products are purchased and used.
Figure 2 shows a graph of the hourly temperatures for Middleton, WI for 24 hours during Summer (Source: National Oceanic & Atmospheric Administration). Indoor temperatures are typically 5–10 degrees lower than the outside temperatures, so Figure 2 includes a simulated graph for indoor temperatures.
Notice in Figure 2 that the outside air temperature is only at or above 30°C (86°F) for approximately seven hours during this particular day. If there are 40 days each year where the temperature is at or above 30°C, then there are cumulatively about 10 days per year when the temperature is at or above 30°C.
In other words, 10 days of continuous storage at or above 30°C (86°F) simulates approximately one year of exposure. Storing packages at 30°C for 30 days would simulate three years and continuously storing packages at 30°C for one year would simulate 36 years.
Please keep in mind that these times and temperatures do not simulate one, three and 36 years of package corrosion.
Higher temperature testing is needed, particularly when products are marketed in regions having high temperatures. Consequently, I recommend the following guidelines for higher temperature storage testing:
• Don’t use a higher storage temperature to accelerate package material corrosion rates in order to reduce test times.
• Use higher temperature storage to determine if there are possible formula thermal instability issues that could cause package material corrosion.
• Conduct tests at higher temperatures that reflect the actual high temperatures for the regions where your products are marketed.
• Use test lengths that reflect the actual number of hours per year when your products are exposed to higher temperatures.
Summary & conclusions…
There is no reliable way to accelerate spray package corrosion rates. However, electrochemical corrosion testing can provide accelerated results because sensitive electronic instruments detect corrosion much sooner than it can be observed with either the unaided eye or a microscope. In some instances, a comprehensive company knowledge database also allows reduction of the number of variables in a corrosion test without increasing the risk of unexpected package failure. Higher temperatures for storage tests should be used to determine product stability at higher temperatures but not to accelerate package material corrosion.
Thanks for your interest and I’ll see you in December. Contact me at 608-831-2076, rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. Limited budgets for testing, as well as pressure to quickly commercialize new products and derivative products (line extensions), often force the taking of short-cuts in corrosion tests. Indeed, higher temperatures are often used to accelerate spray package corrosion to shorten the time needed for corrosion testing. However, there are many pitfalls to this approach.
Parts 1 & 2 of this column will present an updated compilation of previous Corrosion Corners concerning the use of high temperatures to accelerate spray package corrosion. The questions I’ll address will be:
1. Why consider higher temperature storage testing?
2. The science often used to justify increasing temperature to accelerate package corrosion.
3. Do higher temperatures actually accelerate corrosion?
4. Should higher temperatures be part of storage tests?
5. How long should test samples be exposed to higher temperatures?
1. Why consider higher storage temperatures?
Storage testing is probably the most common form of spray package corrosion and product stability testing. A basic protocol for these corrosion and stability tests is:
• Multiple, replicate packages are stored in one or more constant temperature rooms
• A small number of the packages are periodically removed from each room, inspected for corrosion and evaluated for product stability
• The corrosion data are used to predict long-term package corrosion (e.g., 3–5 years)
One of the most often used short-cuts for a storage test is to increase the storage temperature and (hopefully) decrease the test time. This short-cut is often justified on the basis that corrosion is always worse at a higher temperature, particularly with coated metal package components. However, several others and I have witnessed numerous instances where this type of assumption (and its associated short-cut) results in unexpected corrosion with package leakage.
Using higher temperatures to accelerate spray package corrosion typically does not work because metal and polymer corrosion typically do not follow the Arrhenius Equation. A review of the science for the Arrhenius Equation helps to understand why higher temperature does not increase corrosion rates and thus does not predict actual long-term corrosion.
2. The science often used to justify increasing temperature to accelerate package corrosion
The Arrhenius Equation
The rates of a gaseous decomposition and nuclear decay were found to be a linear function of temperature by Svante August Arrhenius in 1889. The mathematical expression for his equation in corrosion rate terms is:
CR2 = CR1(exp [-Ea/R ∆T]) (1)
Equation (1) states that the corrosion rate at room temperature (CR1) exponentially increases as temperature increases (∆T). Equation (1) also indicates that a reaction (corrosion) rate doubles for every 10 degrees Kelvin (10°K) increase in temperature. The term symbols in Equation (1) represent the following:
• Ea is the activation energy of a corrosion reaction in calories per mole per degrees Kelvin (°K)
• R is the universal gas constant in calories per mole °K (°C +273)
• T1 is the lower storage temperature in °K
• T2 is the higher storage temperature in °K
• ∆T = T2 – T1
• CR1 is the corrosion rate (in moles/second) at the lower temperature
• CR2 is the corrosion rate (in moles/second) at the higher temperature
Equation (1) is also believed to apply to polymer film and polymer coating corrosion, such as delamination and blistering. In practice, using Equation (1) for polymer corrosion rates is difficult because the rate has to be the number of moles of broken polymer-metal bonds per time.
Being able to conduct a corrosion storage stability test in half the amount of time would be very useful indeed. Thus, the Arrhenius Equation is often used as the technical justification to increase storage temperature to accelerate corrosion and subsequently shorten storage testing times.
However, Arrhenius established two conditions that must apply for his equation to be valid for a given chemical reaction:
1. The kinetics of the reaction is first order, or can be assumed to be quasi-first order
2. The rate of the chemical reaction is controlled by its activation energy: Ea
In addition, a plot of the logarithms of corrosion rates as a function of temperature should produce a linear graph like that in Figure 1 when higher temperatures are indeed accelerating corrosion rates.
A linear graph like that in Figure 1 is rarely observed with room temperature and high temperature corrosion data.
A useful form to test the validity of the Arrhenius Equation for specific formula-packages systems can be derived by taking the natural log (Ln) of Equation (1) and rearranging it as follows:
Ea = R (T2T1)(Ln[CR2/CR1]) Equation (2) (T2-T1)
Equation (2) enables calculation of Ea for any chemical reaction. Corrosion is accelerated by increasing temperature when the calculated Ea ranges between approximately 18,000 calories/mole °K to approximately 70,000 calories/mole°K.
However, my experience has been that Ea calculated for pitting corrosion are typically millions of calories/mole °K, indicating that pitting won’t occur when it actually does occur. I’ve seen similar situations for both tinplate detinning corrosion and the blistering of coatings.
Consequently, increasing temperature rarely predicts the actual corrosion in commercial spray packages. Indeed, other spray package specialists and I have observed higher temperatures producing corrosion that does not occur at room temperature, as well as no package corrosion of samples stored at a higher temperature with corrosion and package perforation of the corresponding samples stored at room temperature. Both of these situations indicate that the higher temperatures are either causing, or inhibiting, corrosion—but not accelerating it.
A key thing to remember is that I’m talking about the rate of corrosion and not the amount of corrosion. In other words, there might appear to be more corrosion at a higher storage temperature, but the high temperature did not actually increase the corrosion rate.
3. Do higher temperatures actually accelerate corrosion?
Why doesn’t corrosion follow the Arrhenius Equation? One answer is that the two assumptions that underlie Equation (1) are not applicable to corrosion kinetics. In addition, corrosion kinetics are typically not first order; plus, corrosion is often controlled by a variety of other phenomena instead of activation energy.
Metal corrosion is also not a chemical reaction and the Arrhenius Equation was developed for chemical reactions. Like a chemical reaction, corrosion involves a change in a metal’s chemical state (metal atoms become metal ions). However, the change in the chemical state is brought about by the transfer of electrons from the metal to ingredients in a formula. In other words, metal corrosion is a hybrid electrical-chemical (i.e., electrochemical) reaction.
In addition, most corrosion reactions involve a complex interaction of:
• Formula chemical composition
• Surface chemistry
• Material transport between the formula and the package; and
• Package material type and properties
The interaction of these different processes produces an overall process that does not typically fulfill all the conditions needed for valid application of the Arrhenius Equation to spray package corrosion. Hence, increasing temperature by 10° does not usually double the rate of corrosion for spray package materials.
Indeed, it has been my experience that metal container pitting corrosion, steel aerosol container detinning, plus both delamination and blistering of internal polymer coatings, do not usually follow the Arrhenius Equation. In other words, one cannot reliably accelerate pitting corrosion, detinning or coating delamination and blistering by raising the storage temperature. Consequently, using temperature to accelerate these types of corrosion often leads to erroneous results and unexpected spray package corrosion.
We’ll complete this series in the next issue with more discussion of the science of why high temperatures do not accelerate metal or polymer corrosion, when high temperature storage conditions should be used and how long to evaluate packages at higher temperatures.
Thanks for your interest and I’ll see you in November. Contact me at 608-831-2076, rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
Hello, everyone. Corrosion prevention and control for all types of spray packaging is a continuous process and not a one-time event.
The risk of corrosion is always greater than zero and the risk could be as high as approximately 62% when there is no corrosion data for new formulas and derivatives of current formulas (line extensions). Figure 1 is a general risk curve for pitting corrosion in spray packages.
Figure 1: The estimated risk of pitting corrosion as a function of test length.
The graphs in Figure 1 illustrate that the risk of corrosion is significant when:
• There is no corrosion data on a new formula or a derivative formula;
• Risk decreases as the corrosion test length increases (i.e., the amount of data collected increases); and
• Risk decreases more rapidly with electrochemical corrosion tests to a lower magnitude than the risk for a corresponding storage test.
Why is corrosion risk always a concern?
Very small changes in formula chemistry can cause very large changes in corrosion behavior. Corrosion can unexpectedly appear with changes in the concentrations of formula ingredients either by design or by manufacturing batch-to-batch variations. Corrosion inhibitors have an effective concentration range above which the inhibitor no longer effectively controls or prevents corrosion and below which the inhibitor no longer effectively controls or prevents corrosion. Thus, corrosion inhibitor concentration variations actually cause corrosion, particularly when the effective concentration range is small.
Substituting formula ingredients from alternate suppliers could also cause unexpected corrosion in spray packaging. As well, contamination, such as water in anhydrous formulas, is a common cause for spray package corrosion.
Elements of an effective & comprehensive prevention program
A comprehensive corrosion control and prevention program consists of several elements, as illustrated in Figure 2.
Below is a brief description for each of the elements in Figure 2 (going clockwise from the top):
Figure 2: Elements of a comprehensive corrosion control and prevention program.
• Testing: corrosion testing is used to determine package-formula compatibility
• Container: the most corrosion-resistant forms of packaging components are selected with corrosion tests
• Coatings: coatings for spray package components are selected with corrosion tests
• Inhibitors: the most effective corrosion inhibitor and its effective concentration range are selected and determined with corrosion tests
• Experience & Knowledge: a corporate knowledge and experience database is developed and maintained to avoid formula-package combinations and formula ingredients that have a high probability of causing spray package corrosion.
Two types of corrosion testing are available for a comprehensive corrosion prevention and control program. The first is the traditional corrosion test via a storage stability test.
I strongly recommend at least one year of storage testing before making new spray formulas and derivative spray formulas for commercial use. A properly designed, one-year-long storage test typically has a 2% to 7% empirical risk that corrosion will occur in commercial containers.
Electrochemical corrosion testing is the other type of spray package corrosion test. A properly designed electrochemical test can be completed in 1–3 months with an associated statistical risk that is less than 1%. In other words, a properly designed electrochemical test can provide lower risk results than one-year storage in significantly less time with a lower statistical risk.
Understandably, corrosion is not a main focus of most consumer packaged goods companies. However, ignoring corrosion risks does not make them disappear and ignoring risk could disrupt a product’s marketing with a corresponding loss of income and loss of customer/consumer confidence in the product. In most instances, corrosion testing is significantly cheaper than corrosion failure.
Consequently, corrosion prevention and control for spray packaging is a continuous process and not a one-time event.
Thanks for your interest and I’ll see you October. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY
