Written on: July 1, 2020 by W. Stephen Tait

Hello, Everyone. By the time this edition of Corrosion Corner reaches your mailbox I hope the pandemic is over and everyone is healthy.

The June edition of Corrosion Corner began a series on why spray package corrosion measurements are necessary. I started with a discussion on the various materials used to fabricate spray packaging; how metals, coated metals and laminated metals corrode and how polymer coatings and films corrode.

There are always market and resource pressures to either reduce the time for corrosion testing or measurements and, in some instances, to skip them altogether. Indeed, I’ve often been asked if corrosion could be mathematically modeled as a substitute for storage corrosion tests/measurements. The short answers are Yes and No.

Two questions should be asked for every new formula (as well as any formula derived from an existing formula-chassis), particularly whenever the formula chemistry is altered and package materials or vendors are changed:

1. Will the formula cause spray package corrosion?

2. How fast will corrosion by the formula degrade the package materials?

Many years ago, I derived empirical, probability equations to address these questions (and have been periodically improving them ever since). An overview for each equation follows.

**Will the formula cause spray package corrosion?**

The Gibbs free energy determines whether or not a chemical reaction is possible1. Corrosion is a chemical reaction associated with an electrical current, so the Gibbs free energy can also be used to estimate the probability for an electrochemical corrosion reaction with a specific formula/package system.

**Figure 1** provides my empirical equation for the probability of spray package corrosion. This equation estimates the probability that package corrosion will occur with a specific formula.

There are approximately 15 parameters in the **Figure 1** equation. Consequently, there are approximately 15 possible combinations of these parameters (i.e., 1,307,674,368,000)! I say “approximately” because some of the exponents are also equations with their own set of parameters.

Each formula-family and package material have unique parameters for this equation. Thus, each estimation from this equation is typically only for a specific formula/package system. The **Figure 1** equation does not provide information as to how fast the corrosion will degrade the packaging materials, thus reducing the package service lifetime. Service lifetime is defined as the filled package age when:

• Packages leak product or propellant

• Valves leak propellant

• Partially full packages cease to spray, or

• Corrosion degrades product performance and/or efficacy.

In other words, service lifetime is the amount of time during which spray packages function properly and do not degrade the product.

**How fast will corrosion by the formula degrade the package materials?**

**Figure 2** provides the empirical probability equation to estimate how fast corrosion by a specific formula degrades spray package materials.

There are approximately 25 parameters in the **Figure 2** equation. In other words, there are 25 possible parameter combinations to estimate package corrosion rates (i.e., 15,511,210,043,3 30,985,984,000,000)! I again say “approximately” because some of the exponents are actually equations with their own set of parameters.

Each formula-family and package material also has unique parameters for this equation. Thus, each estimation from this equation is typically also only for a specific formula/package system.

Extensive knowledge of how each parameter affects corrosion enables complex equations to be simplified—e.g. use fewer groups in an equation—for estimations. However, most of the parameters in the **Figure 1** and **Figure 2** equations are either unknown or not available in the public domain at this time. In addition, a simplified version of one or both of the two equations might only apply to a very specified formula-package combination.

Hence, the current, state-of-the-art corrosion science is insufficient to allow substituting mathematical models for corrosion tests/measurements, which are the only reliable way to determine:

• Will the formula cause spray package corrosion?

• How fast will corrosion by the formula degrade the package materials?

The empirical equations in **Figure 1** and **Figure 2** are complex because there are many factors that determine if corrosion will occur and how fast it will degrade package materials. In the next issue, I’ll continue the discussion with the most common types of spray package corrosion causing/contributing factors. Please visit www.pairodocspro.com for more information.

Thanks for reading and I’ll see you in August. *SPRAY*

**1Editor’s note:** *Gibbs free energy is a measure of the **potential for reversible or maximum work that may be done **by a system at constant temperature and pressure. It is a **thermodynamic property that was defined in 1876 by Josiah Willard **Gibbs to predict whether a process will occur spontaneously at constant **temperature and pressure. Source:* ThoughtCo.com