Hello everyone. In previous editions of Corrosion Corner, I’ve discussed the cost of in-market corrosion failures. These are package corrosion failures that occur with an existing or derivative commercial product.

In this issue’s column, I’m going to refine the prior discussions on in-market failures and add in-development failures from package corrosion observed during new/derivative product development.

The parameters for calculating failure-costs are summarized in Table 1.

These types of failures have costs that include internal budget costs, out-of-pocket costs, and loss of revenue when a fraction of a firm’s consumers/customers switch to a competitor’s brand when a failure occurs.

The numbers in Table 1 are from discussions with multiple attorneys, our current research on package-failure cost and experience. The actual numbers for each company could be different from those used here.

Table 2 lists the typical number of full-time employees (FTE) on a team investigating both in-development and in-market package failures. The last row of Table 2 summarizes all the yearly FTE costs for in-development and in-market failures.

These examples of calculations for both types of failure costs assume:

• Annual expected sales of $15 million
• A one-year manufacturing delay
• A 20% brand-loyalty revenue loss for five years for an in-market failure with personal injury litigation
• A 50% brand-loyalty revenue loss for five years for in-market failure with wrongful death litigation

Let’s begin with in-development failures caused by package corrosion that occurs during development of a new or a derivative product.

Solving In-development failures for new and derivative products typically involve the FTE personnel listed in the first three rows of Table 2. In-development failure costs are typically resolved within 1–2 years; manufacturing begins shortly thereafter. The costs for 1–2 years are:

• $15,573,948 for a 1-year resolution
• $31,147,896 for a 2-year resolution

In-market failures of commercial products are significantly more costly than in-development failures. Indeed, during my more than five decades of corrosion research and experience, I’ve witnessed examples where a single in-market failure negated decades of profits for a given brand.

In-market failures are more complex, involve more people and have escalating costs as the consequences of the failure become more severe, such as litigation, product recall and revenue loss from reduced brand loyalty.

Table 3 summarizes several scenarios for in-market failure costs that are resolved after one year, using the same assumptions as those for the in-development cost examples. Each row in Table 3 provides a different failure-cost scenario with the estimated cost for each scenario in the last column of each row.

Table 3 illustrates that, as the consequences of a failure become more severe, the costs increase significantly from only paused manufacturing to scenarios involving an in-pantry recall, lost brand-loyalty revenue and litigation. Indeed, a failure resulting in only lost revenue from a manufacturing pause ($15,940,637) is significantly less expensive than a failure where revenue is lost from all consequences of the most expensive litigation ($183,440,637).

How do you minimize the probability of financial losses from in-development and in-market failures? A comprehensive, corrosion control program is needed to minimize the occurrence of both in-development and in-market failures.

A corrosion control program includes corrosion testing and is typically inside a company’s R&D program. The costs of an internal corrosion control program with 2–4 FTE would range from approximately $573,948 to $1,147,896 per year.

Thus, the cost for a corrosion control program is significantly less than the costs for both in-development failures and in-market failures, particularly when an in-market failure involves product recall, litigation and loss of brand loyalty!

Therefore, why doesn’t/wouldn’t your company have a comprehensive internal corrosion control program?

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: aristartec.com and pairodocspro.com. SPRAY

Hello, everyone. Which chemical most often contributes to or causes spray package material corrosion? Water! Water can be in a formula, either as an ingredient or as a contaminant. There are four reasons why I am so quick to implicate water as the most frequent contributor to or cause of spray package corrosion.

First: Water and metals are thermodynamically unstable when in contact with each other. The simple chemical equations for corrosion of the steel and aluminum used in spray packages by water are:

Fe0 + 2H2O → Fe(OH)2 + H2
2Al0 + 6H2O → 2Al(OH)3 + 3H2

These two equations show that iron (steel) and aluminum corrosion by water is possible, but do not tell us the corrosion reaction rates of aluminum and iron with water. Corrosion rates are important because they are directly proportional to the spray package service lifetime. Thus, corrosion reaction rates should be measured.

Second: A variety of corrosive ions and molecules dissolve in water. Consequently, water is also a carrier that delivers corrosive ions and molecules to spray package metals and metal foils.

Third: Water easily diffuses through materials, such as polymer coatings and laminate polymer films. Water diffusion degrades and disables coatings and film barrier properties, causing them to lose the ability to protect the underlying metal from corrosion. Water diffusion through polymer coatings/films brings corrosive water, molecules and ions to the metal under the polymer coating/film.

Fourth: Metals and metal-polymer interfaces have a molecular cloud of negative electrons on the surface and at the interface, respectively. Thus, water molecules are drawn to uncoated metal surfaces, and through polymer coatings and films, to these negative clouds. Water molecules cause corrosion when they adsorb onto metals and subsequently remove electrons from the water atoms:

2H2O + 2e- → H2 + 2OH-

What about anhydrous formulas?
It is extremely difficult to keep anhydrous formulas from being contaminated by small amounts of water. Thus, anhydrous formulas are not immune to corrosion by water.

Water is corrosive as a liquid and a cluster of only 90 water molecules is needed to form liquid water that initiates corrosion (based on thermodynamic calculations). Of course, additional water is needed to sustain corrosion after it initiates. The amount of additional water needed depends on the treatment of the metal or metal-foil surface (such as uncoated or coated/type of coating/etc.) plus the chemical composition of the formula inside the spray package.

Consequently, corrosion testing is needed to determine if an anhydrous formula is corrosive and what is the safe concentration of contaminant water. Corrosion testing can be either an electrochemical corrosion test with the appropriate measurement parameters or a minimum one-year constant temperature storage-stability test.

Please note that raising the storage temperature does not accelerate corrosion of spray package materials, such as aluminum, steel, polymer coatings and films. In other words, raising storage temperature does not shorten the length of the test.

Controlling & preventing corrosion
Spray package corrosion by both formula water and contaminant water can be controlled or prevented. For anhydrous formulas, there often is a non-corrosive concentration range for contaminant water. In other words, corrosion of anhydrous formulas might be controlled or prevented by keeping the concentration of contaminant water inside the non-corrosive range.

Corrosion by formula water can often be prevented with corrosion inhibitors. Indeed, in my almost five decades of corrosion research, testing and consulting, I’ve not found a corrosive formula that could not be inhibited! In some instances, slowing down the corrosion is required; in other instances, corrosion prevention is needed.

A corrosion inhibitor is often only effective for a very specific environment or formula. There are literally thousands of chemicals that might inhibit corrosion, hence finding the most effective corrosion inhibitor often takes time.

Ironically, corrosion inhibitors can actually cause corrosion when their concentration is above or below the effective concentration range. Hence, the effective corrosion inhibitor concentration range should also be determined when developing an inhibitor for a corrosive formula.

Storage testing can be used to screen potential corrosion inhibitors; however, a storage test requires at least one year to complete. Electrochemical corrosion testing is more precise and the quickest way to screen a large number of potential corrosion inhibitors—assuming the appropriate measurement parameters and data analysis protocols are used.

Changing package materials also can, in some situations, control and prevent corrosion. However, the options for changing package materials are more limited than the options for corrosion inhibitors.

In summary, water contributes to, or causes, spray package corrosive because water is:

• Thermodynamically unstable when in contact with metals
• Electrochemically active and causes corrosion
• Moves (diffuses) easily in solutions towards metal surfaces
• Attracted to charged surfaces—indeed the electrical field on charged surfaces often pull water towards the surface, as well as through polymer coatings and polymer films
• A carrier of corrosive ions and molecules
• Degrades polymers so they are not barriers between a formula and the package metal under the polymer coatings and films

Anhydrous formulas are not immune to corrosion. However, in some instances, there is a non-corrosive concentration range for contaminant water.

Corrosion inhibitors are an effective way to control or prevent spray package corrosion by formula water and anhydrous formulas. However, finding an inhibitor that works for individual formula-package systems takes time and the effective concentration for the inhibitor must also be determined.

Corrosion testing is needed to determine:

• If corrosion will occur
• Where in the package corrosion will occur
• The spay package service life with a specific formula
• The non-corrosive concentration range for contaminant water—if one exists
• An effective corrosion inhibitor and its effective concentration range

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

Hello, everyone. In March, we covered defects in polymer and tinplate coatings and their relationship with corrosion. The April column covered defects in laminated film bags, along with those found in traditional steel aerosol containers.

Most of the material defects in spray packages are very small, but can still be seen with the unaided eye. There are also microscopic defects that can lead to package corrosion and subsequent failure (leaking, clogged valves, etc.).

The metals used for aerosol spray containers are not pure metals. Instead, they are alloys that are mixtures of metals, plus non-metals, such as carbon and oxygen.

Metal/non-metal compounds, such as metal oxides or metal carbides, are insoluble in the host metal (i.e. aluminum and steel) and form microscopic precipitate particles. These particles are called inclusions and are dispersed throughout the host metals. Inclusions can be flattened and elongated when the metal is rolled from ingots into the sheets for package fabrication (typically for steel containers) and when the metal is extruded from slugs into a container (typically for aluminum containers).

Figure 1 provides a photomicrograph with examples of inclusions in steel. The arrows show the locations for only a few of the many different defects. The dark spots are inclusions and the thin, dark line is a row of inclusions, referred to as a stringer.

Please keep in mind that inclusions are not impurities. Inclusions are material defects resulting from the alloying of a metal with different elements to obtain physical properties, such as the strength and formability needed to form metals into containers.

Inclusions are sites for pitting corrosion and stress cracking. Stress cracking rarely occurs in spray packages. The chemical composition of a formula determines whether or not pitting corrosion occurs at inclusions.

Metals form regular arrangements of atoms, and these atomic arrangements form bulk formations referred to as crystal planes. Crystal planes are often not perfect, and, in some instances, include fragments of planes imbedded among complete planes. The fragments are referred to as dislocations.

Figure 2 provides an example of multiple dislocations that pile-up when a metal is rolled into a sheet. Figure 2 also shows that dislocations could be sites for the initiation of pitting corrosion.

Steel and aluminum metals and alloys all have dislocations. Indeed, there are approximately one million dislocations per square centimeter of metal surface. The chemical composition of a formula determines whether or not pitting corrosion occurs at dislocations.

Figure 3 provides a scanning electron micrograph of tinplated steel, after the tin coating was removed by mechanical polishing. Several different types of defects are noted in Figure 3:

• Different crystal planes form structures referred to as grains, and different types of grains corrode more rapidly than others
• The chemical composition of the boundary between two different types of grains is typically different from the chemical composition of the grains
• The boundary between different grains is not always metallic—the boundary could be either, metallic (dark shadows between grains) or non-metallic (white lines between grains)
• Non-metallic inclusions are also noted in Figure 3 (white particles inside grains)
• Iron carbide compounds are also present in the steel grains—(darker spots)

All material defects in Figure 3 could cause or contribute to spray package corrosion. Aluminum and steel both typically have more than one of the material defect types shown in Figure 3.

The chemical composition of a formula determines whether or not the material defects in Figure 3 contribute to or cause spray package metal corrosion. Hence, corrosion testing is essential for reducing the risk of costly surprise 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. 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

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. 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