Hello, everyone. In the last issue, I began a series on material defects in spray packaging and the relationship between these defects and corrosion.

This month, I’ll discuss the more common material defects found in laminated foil bag packages—bag-on-valve (BOV) packaging—as well as both coated and uncoated steel aerosol containers. The final, Part Three discussion will focus on microscopic material defects.

Laminated aluminum foil bag (BOV) packaging
Aluminum foils are laminated on both sides with one or more polymer films and fabricated into bags with an aerosol valve (referred to as bag-on-valve [BOV] packaging) to be inserted into traditional aerosol containers.

Figure 1 shows an example of a micro-bulge on the internal laminate film for a BOV package. This type of material defect is common with this type of packaging. Multiple attempts to obtain a clear cross-sectioning of this defect type have not been successful. Consequently, the properties of micro-bulges and their most likely causes are unknown.

Micro-bulges typically do not contribute to, or cause, laminated aluminum foil bag corrosion. In other words, the probability is not zero, so corrosion tests should be conducted.

Figure 2 provides an example of a polymer film that delaminated at the bag weld. It has been our experience that this type of material defect is rare and can be avoided with optimized welding.

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 metal foil corrosion and bag rupture at the delaminated area.

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 random.

We have not yet observed instances where this type of material defect contributes to or causes laminate foil bag corrosion. However, product that diffuses through the inner laminate layer/layers would also be able to diffuse at the crack through the outer film and cause container perforation and leaking. Consequently, corrosion testing should be conducted.

Tinplated steel aerosol containers
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 either the substrate steel or a very thin, iron-tin alloy layer. Numerous holes in tin coatings, such as that in Figure 4, are present in all 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 three decades.

Notice when comparing Figure 4 and Figure 5 that the newer holes are more symmetrical than the traditional holes. Most likely, the newer hole morphology comes from either a new tinplating bath chemistry composition or a new tinplating process.

Holes in tin coatings are potential sites for pitting corrosion and a formula’s chemical composition determines if pitting corrosion will or will not occur in tin coating holes.

It is unknown if either the traditional hole morphology or the newer hole morphology 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 a tin coating.

Three-piece steel aerosol container bodies are welded by a diffusion-weld process, using heat and pressure. The heat is produced by an electrical current flowing between the overlapping ends of the tinplated steel sheet used to form the cylindrical container body. In some instances, the combination of pressure and heat is not optimum and a small amount of metal is ejected from under the overlapping ends.

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 typically not common. However, pitting corrosion could occur if weld spatter produces a small cavern in the weld and the chemical composition of a formula determines if pitting corrosion will occur in a weld splatter cavern.

 

This column, and the previous one, both contain examples of macro-defects that can be seen with either the unaided eye or a light microscope. Next month, I’ll complete this series with a discussion of microscopic defects that cannot be seen with the unaided eye and their relationship to spray package corrosion.

Thanks for your interest and I’ll see you in May for Part Three. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY

Hello, everyone. There is no such thing as defect-free spray package materials. There are always potential concerns as to whether or not defects will contribute to, or cause, spray package corrosion and if corrosion at the point of defect will cause spray packages to fail (leak).

This issue starts a three-part discussion on material defects and their relationship to package corrosion. Material defects in traditional aluminum aerosol containers will be discussed here, and material defects in laminated foil bags in aerosol containers and tinplated steel aerosol containers will be discussed in the next two issues.

Figures 1–6 provide examples of material defects in coated aluminum aerosol containers.

All metal alloys have inclusions in the metal matrix. Inclusions are typically microscopic spherical particles of non-metal components of the aluminum alloy and aluminum/non-metal compounds. Spherical inclusions become distorted and flattened when the metal is formed into a container. Figure 1 has an example of an alloy material inclusion in aluminum aerosol container metal.

I’ve only observed rare instances when inclusions like the one in Figure 1 cause container pitting corrosion. However, corrosion could rapidly perforate containers when corrosion occurs around this type of defect.

Small pieces of metal (divots) are removed from aluminum during the container-forming process. Figure 2 has an example of a divot found in an aluminum aerosol container.

Notice in Figure 2 that the coating backfilled the divot. I have only observed rare instances where this type of metal defect contributes to or causes container corrosion. However, corrosion tests should be used to qualify all new and derivative container-formula systems.

 

Aluminum aerosol containers are formed with multiple extrusion stages. The containers resemble long tubes open at the top with a bottom during one of the later stages.

The coating is sprayed inside the open tubes with a nozzle that moves from the bottom of the tube to the top during the coating application. In some instances, coating drips from a nozzle after spraying. Figure 3 has an example of a coating drip in an aluminum aerosol container (referred to as a drool). Corrosion caused by drools is rare, but corrosion testing is still needed.

Entrained air in the bulk coating material sometimes causes a coating nozzle to instantaneously eject the air with a small amount of excess coating. The excess coating on the container surface is referred to as a spit, examples of which are shown in Figure 4. Notice that there are two spits at two different locations inside this example.

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.

 

Spits are very common in aluminum aerosol containers. Coating color variations also noted in Figure 4 are caused by coating thickness variations. These variations are also common and I have observed instances where they caused or contributed to random container failures (leaking).

High temperatures are used to cure aerosol container coatings. Coatings and coating components are dissolved in solvents that evaporate during the curing process and small bubbles can form during solvent evaporation. Sometimes these bubbles harden, producing solvent pops like the one in Figure 5.

Solvent pops rarely contribute to, or cause, corrosion. However, pitting corrosion inside solvent pops could occur when there is also extensive coating corrosion surrounding a solvent pop.

Holes in coatings are very common. Figure 6 has an example of a small area where a coating did not wet (cover) the container metal, resulting in a hole that exposes metal.

This type of defect typically causes pitting corrosion when there is also coating corrosion around the hole.

One or several of the defects shown in Figures 1–6 are present in virtually all aluminum aerosol containers. Consequently, corrosion testing is needed to determine when these defects will contribute to, or cause, container corrosion that leads to failure.

In the next issue, we’ll continue this discussion on material defects in bag-on-valve (BOV) packaging and traditional steel aerosol containers.

Thanks for your interest and I’ll see you in April for Part Two. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY

Hello, everyone. Static storage testing—also known as storage stability testing (both SST)—is the oldest form of corrosion testing for spray packaging. The goal of an SST is to ensure that products are safe to use, provide the expected efficacy and do not leak during the time consumers use the product.
Package failures in the market, and unexpected corrosion, both occur from a number of faulty SST practices. These include:
• Using high temperatures to accelerate package material corrosion
• Storage times that are too short
• Insufficient numbers of samples examined at each interval (sample pull)
• Unrealistic test parameters
• Not evaluating the effect of variability on corrosion
• Not determining the effective concentration range for corrosion inhibitors
• Not examining packages from first product runs
Let’s examine these SST practices.
Using high temperatures to accelerate corrosion
SST package samples are typically stored at room temperature and at least one higher temperature. The higher temperature(s) is (are) used to accelerate the packaging material’s corrosion rate and reduce the length of an SST so that products can be commercialized more quickly. On numerous occasions, at the beginning of a package failure investigation, I’ve been told that, “We put a few cans in the oven for a week; there was no corrosion and we marketed the product.” However, if a low corrosion risk is desired, there’s no such thing as a short-cut when conducting corrosion tests!
Package material corrosion rates are not accelerated by higher temperatures. Although in some instances, higher temperatures can actually cause formula ingredient degradation.
Storing packages at high temperatures for multiple months also does not simulate actual package storage. The storage time at higher temperatures should be based on the number of hours per year that packages are stored in warmer areas or during the warmer seasons; the test-temperature should be realistic for the various areas where products are warehoused and sold.
Short storage times
We’ve seen numerous instances where pitting corrosion that perforates aerosol containers in one year was not detected until around 6–9 months of storage testing.
It takes time for corrosion to initiate, increase to a critical size/mass that sustains corrosion growth through the materials and generate enough corrosion to be seen either with the unaided eye or with a light microscope.
Accelerated corrosion test results can be obtained from measurements with sensitive electronic instruments (i.e., electrochemical corrosion tests) that detect corrosion and measure its rates before there is enough to be seen. Electrochemical tests conducted with the correct parameters provide accurate and precise predictions for actual corrosion with less risk than a traditional SST.
Insufficient numbers of samples
Limited storage room space often restricts the number of spray packages tested. However, a small number of samples could result in low statistical confidence for a given examination time (sample pull). In addition, a small number of samples per examination probably won’t be large enough to find the corrosion that will eventually cause package failure, such as leaking. For example, if corrosion will actually cause 10% of packages to leak, it is highly unlikely one will know one has a corrosion issue when examining only 1–3 packages.
Unrealistic test parameters
Test packages are sometimes stored in several different orientations, such as upright, inverted and on their sides. The upright and inverted orientations for aerosol containers typically provide the same results and don’t evaluate potential leaking of aerosol valve components. Evaluating corrosion of aerosol containers on their side is only appropriate when the commercial packages are routinely stored on their sides by consumers.
Not evaluating the effect of variability 
Variability is a fact of life that could contribute to or cause spray package corrosion. Storage room space limitations also often prevent evaluating how variability affects spray package corrosion.
Variabilities that could contribute to or cause spray package corrosion include:
• Formula ingredient concentration range variability
• Variability of contaminant concentrations, such as water
• Corrosion inhibitor concentration variability
• Package material composition variability
• Variability of package component physical attributes, such as valve crimp (clench) dimensions
Variability causes random package failures for commercial products.
Not determining the effective concentration range for corrosion inhibitors
Small concentrations of corrosion inhibitors are often very effective in preventing and controlling package corrosion and thus enable marketing of corrosive formulas. However, corrosion inhibitors often have an effective concentration range, above which and below which package corrosion occurs. Hence, the effective corrosion inhibitor concentration range should be determined, and that range should be part of manufacturing specifications.
Not examining packages from first product runs
A corrosion SST has an approximately 7% risk of unexpected corrosion in commercial products when the SST is completed after one year. Consequently, it is advisable to also run confirmatory storage tests on packages from one or more early production batches to ensure the effect of variability on commercial package corrosion is known and to help lower the risk to below 7%.
I realize that the best SST practices mentioned in this article might sometimes strain both limited personnel and storage test facility resources, hence, my preference and recommendation for conducting quicker and more accurate electrochemical corrosion tests.
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

Happy New Year, everyone! In the August 2023 edition of Corrosion Corner, I discussed how corrosion failures are very expensive. These occur when packages leak, do not completely exhaust when spraying or when product is discolored or has a malodor because of package corrosion.
The cost of spray package corrosion could include:

• Corrosion testing
• Development of corrosion inhibitors
• Product recall when corrosion unexpectedly causes package or product failures
• Time and resources for investigating package failures
• Commercial or personal injury litigation
• Loss of sales
• Loss of both customer and consumer confidence in the products that failed from corrosion

Precise information on the corrosion costs to our industry is difficult to estimate. The global corrosion cost in 2022 was estimated to be $2.5 trillion, which is approximately 3.4% of the global gross domestic product (GDP) for that year. Clearly, corrosion is a major global concern to all industries, including the aerosol industry.

There are still many gaps in our knowledge about what causes spray package corrosion, as well as how to control and prevent it. These knowledge gaps in spray package corrosion lead to multiple questions that should be considered while developing a new spray product or derivative products for existing product lines.

My original perspective was published in the February 2014 edition of Corrosion Corner. That list still applies to spray packaging corrosion today and has since expanded, so an update is useful. The list of corrosion questions to consider during development of all spray products, changes to a formula’s ingredients, as well as changes in packaging and package materials includes:

1. How does the type of formula water cause or contribute to spray package corrosion?
2. Will an anhydrous formula be contaminated with water at some point in the manufacturing and filling process?
3. Is there a critical contaminant water concentration range for an anhydrous formula, above or below which the formula becomes corrosive?
4. How do the different types of fragrances (e.g., citrus, vanilla, etc.) cause or contribute to the corrosivity of various different types of spray formulas (e.g., aqueous, ethanol-water, emulsions, etc.)?
5. How and when do fragrances behave like corrosion inhibitors?
6. Which fragrance types exhibit the ability to inhibit spray package corrosion?


7. What initiates the various types of spray package corrosion, such as vapor phase, crevice and pitting corrosion?
8. What are the safe formulas—ones that will not cause or contribute to spray package corrosion?
9. Are there specific chemicals to avoid using in specific types of formulas?
10. What causes package corrosion to be random in its type—and its location—inside a package?
11. What are the best practices for spray package corrosion testing?
12. How should corrosion tests be designed to address:

a. Formula composition variability during manufacturing
b. Raw material variability
c. Variability of package materials?

This is quite a list! Reviewing it gives an appreciation for the breadth and depth of factors that influence whether or not spray package corrosion will occur with a given type of package/formula combination.

Hopefully this list also provides guidance on what areas of corrosion research need further development. Obviously, research produces more expedient and effective spray package corrosion control and prevention programs for companies using spray packaging for their 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

Part 3

Hello, everyone. This month, we’ll complete the three-part series begun in (part 1, October), (part 2, November) with a discussion on how coating glass transition temperature (Tg) changes when formula ingredients permeate coatings, as well as how higher storage test temperatures can produce misleading corrosion test results.

Coatings have physical properties, such as tensile strength and barrier strength that disappear when the coating temperature is at or above the coating’s Tg. In other words, a coating loses its physical properties when the temperature is above its Tg.
The internal coating for a spray package is dry before it is filled. Examples of dry coating Tg magnitudes are:

• Epoxies and polybutadiene are around 100°C (212°F)
• Nylon 6 is around 47°C (116.6°F)
• Nylon 6,6 is around 79°C (174.2°F)
• Polypropylene is around -10°C (14°F)
• PET has a Tg between 69°C (156.2°F) and 85°C (185°F), depending on the PET grade

Internal package coatings typically become wet shortly after the package is filled with product, because formula ingredients absorb into and diffuse through the coatings. Consequently, the internal dry coating Tg for a new, unfilled package is higher than the Tg for the same coating after the package is filled with product. For example, the Tg for a dry epoxy coating is 100°C (212°F), but the corresponding Tg for the same wet coating is around 50°C (122°F).

It is often assumed that a higher storage temperature will accelerate both the coating’s and the metal’s corrosion rates, thus allowing the time needed for storage stability corrosion tests to be reduced. This assumption is based on the Arrhenius equation that states chemical reaction rates double for every 10°C (50°F) increase in temperature.

However, metals and coatings do not follow the Arrhenius equation because the corrosion rates for both do not satisfy the two conditions for valid application of the Arrhenius equation:

1. The chemical reaction is first order
2. The chemical reaction rate is controlled by its energy of activation

In addition, metal corrosion is not a pure chemical reaction, but a combination of a chemical reaction (metal atoms change state to metal ions) with an electrical charge transfer between the metal and its environment. An environment can be either a formula or the permeate diffusing through a coating to the metal underneath.

Figure 1 illustrates how the Tg for a wet coating changes with increasing temperature. The metal corrosion rate under an epoxy coating is plotted on the Y-axis (log scale) and the corresponding temperature is plotted on the X-axis. The liquid-permeate in this case is water, and the coating was completely saturated.

The inflection point for the graph is around 50°C (122°F), which is the most likely estimation for the wet epoxy coating Tg.

The corrosion rate in Figure 1 decreases from approximately 0.016mm per year at 20°C (68°F), to approximately 0.004mm per year at 60°C (140°F) (-1.8 and -2.3, respectively on the Y-axis). In this case, increasing the temperature actually decreases corrosion instead of increasing it, contrary to expectations from the Arrhenius equation.

In addition, the corrosion rate at 20°C (68°F) (0.0159 mm/year) is more than three times larger than the rate at 40°C (104°F) (0.005 mm/year), indicating that corrosion is worse at room temperature (20°C/68°F). This trend also contradicts the Arrhenius equation. I have actually seen numerous storage tests where the room temperature corrosion was worse than the higher temperature corrosion, and vice versa.

It’s tempting to say from Figure 1 that temperature accelerates corrosion as the temperature is increased above 50°C (122°F). However, the corrosion rate at 80°C (176°F) should actually be greater than 0.016mm per year instead of 0.008mm per year–also not consistent with the Arrhenius equation. Consequently, it’s more accurate to conclude that the wet coating is no longer a barrier when temperatures are above the 50°C (122°F) wet epoxy Tg.

Therefore, I typically do not recommend shortening storage test length with higher storage temperatures. Corrosion test length can be reduced with electrochemical measurements when the appropriate instruments, measurement parameters, exposure times, analysis protocols, sample size and data analysis and interpretation protocols are used.

Summary of Parts 1–3
Part 1: Coatings do not always prevent corrosion and are not always needed to prevent corrosion. Whether or not a coating is needed to protect a metal from corrosion is determined by a complex interaction between a formula’s chemical composition, the type of metal and the type of coating, as well as a variety of metal surface attributes that produce various coating defects.

Part 2: Defects in coatings are always present and typically not visible with either the unaided eye or a light microscope. Consequently, electrochemical measurements with sensitive instruments are needed to detect and measure if these defects will or will not cause package corrosion.

Part 3: The Tg of a coating decreases when it becomes wet. Absorption of formula ingredients into coatings cause them to become wet and subsequently degrades coating properties, such as its Tg. Raising storage temperatures to evaluate the corrosion resistance of coated metal packages does not reliably predict package corrosion in a smaller amount of time.
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. In the last issue, we began a three-part series (Part 1) with a discussion on how the complex composition of metal surfaces causes coating pores and voids and how they allow formula ingredients to absorb into and through coatings.
Epoxy, Micoflex, polyethylene terephthalate (PET) and bi-layer nylon-polypropylene laminate films are typically used as internal coatings for spray packages. I will continue to refer to both coatings and laminate films as “coatings” because the corrosion science is the same for both.
The intention is for the coating to act as a barrier between a formula and the underlying metal or metal foil. Consequently, it often comes as a surprise when a c oating is corroded by a formula.
Internal coatings on metal are dry when packages are removed from their pallets for product filling. Liquid dispersed throughout a coating transforms it from a dry coating to a wet coating, and a dry coating has significantly different properties than those of a wet coating, such as the ability to be a barrier between the package substrate metal and a formula.
A “skin over metals”
Think of coating as a skin over metals. Human skin is a natural polymer that provides a good analogy for the corrosion of coatings used for spray package coatings and laminate films.
Formula ingredients, such as water and emollients, can absorb into skin and subsequently modify its properties, such as causing it to wrinkle or make it feel soft, respectively. Skin properties could also be modified by formula pH. For example, a liquid will cause a burning sensation on skin when the liquid pH is higher or lower than around 5.5 and the burning sensation typically intensifies as the pH moves further from 5.5.
Like skin, the properties of coatings are also modified when materials absorb into a coating. The intensity of the modification depends on the type of coating, its morphology, how it was deposited inside the package, the chemical composition of the formula and the amount of liquid dispersed throughout the coating.
Diffusion of a liquid-permeate into and through a coating could cause:
1. The coating to become a semi-permeable membrane that allows an either a non-corrosive or corrosive liquid to diffuse into and through the coating
2. Loss of the coating’s barrier property
3. Lowering of the coating’s glass transition temperature (Tg)
As mentioned in the above list, formula ingredients absorbing into coatings could cause the coatings to become a semi-permeable membrane instead of a barrier layer. A semi-permeable membrane—originally a dry coating—allows select formula ingredients to diffuse into and through the coating. Hence, the chemical composition of the liquid-permeate is typically different from the chemical composition of the formula inside the package.
In this situation, the liquid-permeate typically causes corrosion of the coating (with delamination from the metal), corrosion of the underlying metal or both coating and metal corrosion. Both the formula’s chemical composition and the pH of the liquid-permeate determine if coating or metal corrosion will occur, or if both will occur together.
Some of the more common formula ingredients that typically absorb into coatings, and potentially cause both coating and metal corrosion, are:
• Water
• Emollients
• Surfactants
• Fragrances
• Formula pH
• The metal cations from organic and inorganic  salts
• Organic acids
This list is by no means an exhaustive one.
Formula-ingredient absorption into a coating could also cause it to lose its barrier properties—either partially or completely. Loss of barrier properties often leads to coating delamination from the substrate metal and/or corrosion of the metal under the coating.
Coating corrosion could be either localized—such as blisters (Figure 1) or general, widespread delamination (Figure 2). Package metal corrosion could also either be localized under blisters or under wide areas of a coating. Metal corrosion also typically causes and/or accelerates the delamination of a coating.
Absorption of formula ingredients also lowers a coating’s glass Tg. Discussion of how Tg is changed by absorption, plus how increasing temperature affects a coating’s Tg.
We will complete this three-part series in the next issue. We’ll also discuss how raising temperature affects coating performance as a barrier and sometimes produces metal corrosion that does not occur at room temperature.
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.comSPRAY

Hello, everyone. It’s commonly believed that a barrier is needed between a spray package’s base metal/metal foil and a formula to prevent corrosion of the metal/metal foil packaging. It’s also commonly believed that coatings and laminate films are these barriers. I’ll refer to both coatings and laminate films as “coatings.”

Coatings do not always prevent corrosion and are not always needed to prevent corrosion—Why? This month starts a three-part series discussing why coatings are not always corrosion barriers for spray packages.

Why coatings are not perfect barriers
Metal surfaces are a complex mixture of:

• A variety of metal/metal alloy crystal groups, called grains
• Thin, non-metallic intergranular boundaries between different grains—the glue keeping them together
• A variety of metal-alloy/non-metallic precipitate particles inside grains
• Metal structural defects inside grains

Consequently, complete coating coverage over a package’s metal surface requires that a coating bonds to all the metal surface attributes listed above. Not surprisingly, the complexity of a metal surface prevents complete coating coverage, resulting in microscopic and macroscopic coating defects wherever coating/metal bonding is incomplete or missing.

Figure 1 provides an example of a macroscopic solvent pop in a coating. This solvent pop has an approximately 100-micron diameter.

 

Solvent pops are very common in both coated steel and coated aluminum packages. Indeed, in one instance, approximately 25% of the packages from the same manufacturing lot had coating solvent pops.

A thin cap over the solvent pop is the transparent area inside the dark halo ring. Caps are typically removed/dissolved shortly after a package is filled. However, metal pitting corrosion in solvent pops only initiate and are sustained when a large area of the coating surrounding the solvent pop is also corroding.

Figure 2 provides an example of a microscopic hole in a coating where it did not wet the substrate metal. The hole diameter is approximately 20-microns—similar to that of human hair—and it exposes the metal to a formula. Coating holes are commonly found in both coated steel and coated aluminum packages.

 

It’s tempting to conclude that a hole in a coating causes metal pitting corrosion. However, in a coated metal, pitting corrosion initiates, and is sustained, only when a large area of the coating surrounding the hole is also corroding, much like that for the solvent pop in Figure 1.

Coatings also have other microscopic defects that allow liquids to absorb into, and accumulate in, a coating. Absorbed liquid often saturates a coating, breaks down coating barrier properties and causes metal corrosion under the coating (Absorbed liquid diffuses into and through a coating; Adsorbed liquid sits on a coating surface).

The two most common microscopic coating defects are pores and voids, which occur in all coatings, are very small and difficult to observe. Thus, coating pores and voids are typically indirectly detected with analytical methods, such as positron annihilation spectroscopy and electrochemical measurements.

Liquid (from a formula) moving into and through pores often enlarge pores and create microscopic rivers. These rivers often cause extensive coating corrosion (e.g., blistering), loss of the coating’s barrier properties and metal corrosion under the coating.

In some instances, swelling of a coating can slow or stop corrosion by pinching off rivers. Corrosion products can also slow or stop metal corrosion when metal corrosion products backflow into and plug a river.

Figure 3 has a diagram of a model for how liquids move into and through coating pores and voids. The blue lines depict pores full of liquid and the blue ovals depict voids full of liquid.

 

Liquid-filled pores form microscopic rivers in the coating that disperse liquid throughout the coating and deliver it to voids at the coating-metal interface. The chemical composition of the microscopic rivers could be either the same as, or significantly different from, a formula’s chemical composition. The numbers in Figure 3 indicate:

1. Pores end in voids: #1 depicts pores that end at a coating void. Capillary action fills both the pores and void with liquid.

2. Meandering pores: #2 depicts pores where a liquid meanders throughout the coating without ending at a void. Both pores ending at coating voids and meandering pores typically cause coating corrosion—such as swelling and blistering—that can also delaminate a coating from its substrate metal in addition to breaking down coating barrier properties.

3. Pores with Voids at the Metal-Coating Interface: #3 depicts pores that end at a void at the coating-metal interface. The liquid accumulating at the interface voids often leads to metal corrosion, as well as coating delamination, such as blisters.

Metal corrosion occurs under a coating when:

• The liquid moving through the coating is corrosive toward the metal
• Sufficient liquid accumulates at the metal-coating interface
• Liquid continuously flows through the microscopic rivers and the flow is large enough to sustain metal corrosion
• Coating swelling and corrosion products do not pinch or plug the microscopic rivers

In other words, when/if a coating will either protect, or not protect, the substrate metal from corrosion is determined by a very complex interaction between a formula’s chemical composition and the type of metal, type of coating, and metal/coating morphologies (grains/grain boundaries/precipitates/defects, coating pores and voids, respectively).

Pores and voids are always present in coatings and typically not visible to the unaided eye or a light microscope. Consequently, electrochemical measurements with sensitive instruments are needed to determine if these defects will or will not cause package corrosion with a given formula.

We’ll continue this three-part discussion in the next issue. 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

They’re processes, not one-time events…

Hello, everyone. The risk of corrosion is always greater than zero and could be as high as 62%, particularly when there is no corrosion data for new formulas or derivatives of current formulas (line extensions).

Two types of corrosion tests are available for spray packaging: the traditional long-term storage stability test and the electrochemical corrosion test. Figure 1 depicts the risk associated with each of these corrosion tests as a function of test length.

 

The storage test curve in Figure 1 is from one-year-old package examinations for pitting corrosion, from approximately 7,500 aluminum, tin-plated steel and tin-free steel aerosol containers (approximately 750 to 1,000+ storage tests). The electrochemical curve in Figure 1 was generated from approximately 1,500 direct comparisons between predictions from test results and actual package corrosion.

Notice in Figure 1 that storage test risk decreases as the test length increases. In other words, results from a short storage test typically have a higher risk than results from a long storage test. Hence, this is why I recommend conducting storage tests for at least one year before making a go/no-go decision to commercialize a new product or a derivative product (line extension).

Figure 1 also illustrates that the risk for a properly-designed:

• Storage test—typically decreases from 62% to around 7% after one year of testing
• Electrochemical test—typically decreases from 62% to < 1% within 90 days of testing

Therefore, an electrochemical test can be completed in a significantly shorter time than a storage test with a corresponding lower risk.

Corrosion risk is always a concern
Changes to a formula’s chemical composition can cause very large changes in corrosion behavior. For example, a new fragrance or a new surfactant in a non-corrosive base formula sometimes results in a new, corrosive derivative product.

Corrosion can unexpectedly appear with changes in the concentrations of formula ingredients either by design or from manufacturing batch-to-batch variations. For example, an inhibitor can lose its effectiveness when its concentration is either above or below the effective concentration range. This means an inhibitor could actually cause corrosion when its concentration is outside of its effective range.

Substituting formula ingredients from alternate suppliers could also cause unexpected corrosion in spray packaging. Additionally, contamination, such as water in anhydrous formulas, is a common cause for spray package corrosion.

Variability in formula ingredients, formula ingredient concentrations, package components and materials of construction also causes random corrosion in some instances.

Comprehensive corrosion control & prevention
A corrosion control and prevention program has multiple components, as illustrated in Figure 2.

Corrosion testing is the linchpin of a comprehensive corrosion control and prevention program. Testing is used to determine formula compatibility with package components, metals and coatings.

A corrosive formula doesn’t necessarily have to be abandoned. In most instances, a corrosion inhibitor can be developed to transform a corrosive formula to a non-corrosive formula.

Also, maintained corporate knowledge, as well as an experience database, can be used to help avoid formula/package combinations and formula ingredients that have a high probability of causing spray package corrosion. Such a database can also reduce the number and length of corrosion tests in some situations.

“Knowledge” and “Experience” are shown in Figure 2 with a plus sign between them to emphasize their dependence on each other. Knowledge without practical experience, and experience without theoretical frameworks, are incomplete and will not result in useful, predictive professional practice; experience and knowledge work together to complete a comprehensive corrosion control and prevention program.

Understandably, corrosion control and prevention are typically not part of most corporate business plans and mission statements. However, product efficacy, quality and safety are!

Ignoring corrosion risk does not make it disappear; disregarding risk could lead to a major disruption of a business plan. Corresponding to that may be loss of income, re-deployment of resources to determine the cause and find a solution, as well as the delay of other projects. Ignoring risk can also result in either an expensive product recall or litigation—with subsequent potential loss of future sales when customers/consumers lose confidence in your products.

In most instances, corrosion testing is significantly cheaper than an in-market corrosion failure, as discussed in last month’s Corrosion Corner. A comprehensive corrosion control and prevention program also helps prevent the loss of productivity and expense that accompany an in-market product failure (leaking packages).

Consequently, corrosion prevention and control should be a continuous process and not a one-time event.

Thanks for your interest and I’ll see you in October. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY

Hello, everyone. Your spray products have probably both changed and evolved over time. Changes/evolutions are driven by the desire to have:

1. Sustainable raw materials;
2. Environmentally friendly (Green) raw materials;
3. Water used as a solvent instead of petrochemicals;
4. New ingredient technologies; and
5. New package material technologies, to name a few.

All of these changes/evolutions provide the unwanted possibility of transforming a formula that was typically non-corrosive to one that is very corrosive.

Indeed, a change/evolution that transforms a formula into a severely corrosive one often leads to unexpected package leaking. In other words, changes to a product’s formula ingredients and package materials can result in corrosion that was normally not experienced prior to the changes.

Corrosion risk is always present
Package corrosion is always present, as illustrated in Figure 1. The base corrosion risk is approximately 62% when either no-corrosion tests are conducted or relevant corrosion data is not available.

The graph for storage testing in Figure 1 is an empirical corrosion-risk graph that was estimated from spray package corrosion observed in over 7,500 spray packages. The risk magnitude is similar for other types of consumer goods packaging, such as food containers, pumps and tubes.

Notice in Figure 1 that the corrosion risk is significantly reduced to approximately 2%–7% after one year of storage testing (using appropriate test and sampling parameters), and less than 1% after 30–90 days with appropriate electrochemical corrosion test parameters. I refer to these lower risks as information-based corrosion risks, because they are based on corrosion test data. The risk decreases faster with the electrochemical testing because it uses sensitive instruments that can measure corrosion long before it can be seen.

Will consumers observe leaking packages?
Whether or not consumers observe leaking packages is determined by two factors: the package corrosion rate and the rate of sales. Fast corrosion appears more quickly and results in consumer-observed package leaking. Very slow corrosion typically does not cause consumer-observed package leaking when sales are timely (on-target), and when all packages are exhausted and in the recycle stream before they leak.

The rate of sales is essentially the time it takes to sell all units from a given manufacturing batch of product. Figure 2 illustrates how a given failure-time profile (green graph) interacts with both on-target and slow sales. On-target sales (blue graph) produce a very low percentage of leaking packages that are typically not seen by consumers. Slow sales (red graph)—caused by a pandemic or a recession—produce a situation where a significant number of leaking packages are seen by consumers.

Corrosion failure disrupts company operations
Package failure investigations are typically not a regular budget item. Thus, a package failure disrupts production and typically triggers initiation of a non-budgeted internal investigation into the root cause of the failure. An internal investigation typically lasts from 6 to > 12 months, depending on the number of manufacturing batches and units sold.

The investigation team typically includes personnel from all R&D disciplines (including management)—Manufacturing, Quality Assurance, Legal, Marketing, Sales and Customer Service. A failure could also include one or more members of senior management, such as the Chief Science Officer, CEO, CFO, Legal Counsel, etc.

Consequently, an internal failure investigation team could include total of 10–30 personnel (usually not all involved at the same time). An investigation also often includes vendor personnel and external consultants.

Corrosion failure costs
Figure 1 illustrated that the risk associated with no-corrosion-data (testing) is high (~62%). I’ve been involved in multiple failure investigations, and my experience has been that most of them
take approximately one year to complete. Table 1 summarizes an example of corrosion costs for a failure.

The costs in Table 1 are based on the assumptions of:

• 3,000 hours of personnel time in one year
• $250 per hour internal personnel costs
• The failed product has an annual sales of $50,000,000
• $30,000 to > $100,000 for external consultant(s)
• $10,000,000 to > $30,000,000 for product recalls
• $100,000,000 for personal injury litigation (based on estimates from several personal-injury attorneys)

 

Are corrosion tests cost effective?
The cost summary in Table 1 indicates that the minimum cost for a corrosion failure could be at least $135,780,000 when a product recall and personal injury litigation are involved.

Consequently, the cost of corrosion testing is significantly less than the cost of a corrosion failure. Indeed, the minimum cost for a failure would easily fund corrosion tests for multiple decades.

Corrosion testing also helps prevent leaking packages for new products, products with new ingredient technologies, alternate raw material vendors, packages with new material technologies and changes to both ingredient and package vendors.

Thanks for your interest and I’ll see you in September. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY

Hello, everyone. “Natural, sustainable ingredients” and “Green ingredients” are the current guiding phrases when formulating new products and derivatives of existing product bases. We are all used to “natural” ingredients in spray products; indeed, the quintessential natural formula ingredient is a fragrance. More recently, probiotics (microbiomes) and enzymes, to name a few, have been discussed as new sources for natural, sustainable and Green ingredients.
I’m not a biologist or a biochemist, so I can only provide a very basic overview of these ingredients. There are also many different types of natural ingredients currently being researched for skin and personal care formulas, so I’ve narrowed down my discussion to probiotics (microbiomes) and enzymes.
Probiotics (Microbiomes)
Probiotics are various types of benign bacteria, algae and yeast/fungi. Older medical research on probiotics indicates they might improve gastrointestinal health; more recent research on both cosmetic and personal care products indicates that probiotics might also contribute to skin and oral health.
Enzymes
Both UV light and oxygen can damage skin, resulting in visible aging. UV light damages skin DNA. Skin oxidation is similar to metal package corrosion, but is also produces free radical molecules that can cause visible skin aging. Photolyase enzymes repair skin DNA damage by UV while antioxidant enzymes neutralize the free radicals produced during skin oxidation.
MIC
What do these natural and sustainable ingredients have to do with spray package corrosion? Microbe colonies on surfaces can induce corrosion referred to as MIC (microbial induced corrosion). MIC is a complex type of metal corrosion that occurs under microbial colonies when formed on both uncoated metal and coated metal surfaces.
MIC is typically very rare in spray packaging. Indeed, I’ve only seen a few instances of MIC that have caused spray package failures (perforations). The main attributes of MIC are:
• MIC-causing microbes are attracted by the electrical fields around metal grain boundaries.
• MIC microbes fit into the cavities between the metal grain boundaries, making them prime sites for microbes to attach on surfaces and subsequently grow into colonies.
• Microbial colonies produce organic acid metabolic wastes, such as lactic acid and hyaluronic acid. Organic acids are typically electrochemically active and thus could contribute to or cause package metal corrosion.
Consequently, the organic acid metabolic waste from colony-forming microbes could also contribute to or cause package metal and polymer corrosion via the MIC corrosion mechanism.
Skin is a polymer, much like the internal coatings in spray packages or the laminate used on aluminum foils to fabricate bag-on-valve packaging, ointment tubes and squeeze tubes. As a result, natural ingredients, such as enzymes that modify skin chemistry, might also modify the chemistry of internal coatings and laminated foil packages. In some instances, polymer/laminated film modification by natural ingredients degrades the barrier-protection properties of a polymer coating or laminated film, thus causing or contributing corrosion of both the coating/laminate film and/or the substrate metal/metal foil.
Research of probiotics and enzymes for personal care and skin care products is still relatively new. The technical issues with these new ingredients in spray packages include:
1. What are their effective concentrations?
2. How to keep the live natural ingredients, such as probiotics, algae and microbiomes alive in spray packages, particularly with formulas incorporating biocides that prevent growth of harmful microbes;
3. How to keep natural ingredients, such as enzymes, from reacting with other formula ingredients and degrading with age inside the spray package; and
4. When do these new ingredients cause or contribute to spray package corrosion?
Much like fragrances, other natural ingredients will not always cause corrosion. However, corrosion by natural ingredients is unpredictable, and corrosion testing with formulas using any new ingredient is essential to avoid unexpected spray package corrosion with subsequent package failure.
In other words, corrosion testing is recommended to determine long term product-spray package corrosion compatibility for all new formula ingredients and new formula ingredient chemistries.
The following shortlist provides a few, select papers and articles on natural biologicals for personal care and skin care products. The list is not intended to be comprehensive.
1. Mei-Chiung Jo Huang & Jane Tang, Microbiology Discovery, Volume 3, Article 5, 2015 (probiotics)
2. Herbert Open Access Journals (probiotics with extensive references)
3. COSSMA Issues 04-2023 and 05-2023 (articles on probiotics and enzymes)
Thanks for your interest and I’ll see you in August. Contact me at 608-831-2076; rustdr@pairodocspro.com or from our two websites: pairodocspro.com and aristartec.com. SPRAY