Written on: December 1, 2016 by W. Stephen Tait
Hello, everyone. In the May–July 2016 Corrosion Corners, I discussed corrosion risk and how the cost for corrosion failures is significantly higher than corrosion testing. Corrosion prevention and control is a continuous process and not a one-time event.
The risk of corrosion is always greater than zero and the risk could be as high as approximately 62% when there is no corrosion data for new formulas and derivatives of current formulas (line extensions). Figure 1 is a reproduction of the risk curve provided in the July 2016 Corrosion Corner.
Figure 1: The estimated risk of pitting corrosion as a function of test length.
The graphs in Figure 1 illustrate that the risk of corrosion is significant when there is no corrosion data on a new formula or a derivative formula, risk decreases as the corrosion test length increases (i.e., the amount of data collected) and risk decreases more rapidly with electrochemical corrosion tests to a lower magnitude than the risk for a corresponding storage test.
Why is corrosion risk always a concern?
Very small changes in formula chemistry can cause very large changes in corrosion behavior. For example, the number of SKUs for an existing successful, non-corrosive formula is extended by adding more fragrances to a base formula. However, one or more of the derivative formulas turn out to be corrosive (base formula with new fragrances).
In this type of situation it’s typically not that the fragrance is corrosive. Instead, one or more of the new fragrances are not as effective at inhibiting corrosion as the original fragrance. In other words, the unfragranced formula base is actually corrosive.
Corrosion can unexpectedly appear with changes in the concentrations of formula ingredients either by design or by manufacturing batch-to-batch variations. Corrosion inhibitors have an effective concentration range above which the inhibitor no longer effectively controls or prevents corrosion and below which the inhibitor no longer effectively controls or prevents corrosion. Thus, corrosion inhibitor concentration variations actually cause corrosion, particularly when the effective concentration range is small.
Substituting formula ingredients from alternate suppliers could also cause unexpected corrosion in spray packaging. Also, contamination, such as water in anhydrous formulas, is a common cause for spray package corrosion.
What are the elements of an effective and comprehensive corrosion prevention and control program?
A comprehensive corrosion control and prevention program consists of several elements as illustrated in Figure 2.
Figure 2: Elements of a comprehensive corrosion control and prevention program.
A brief overview for each of the elements in Figure 2 is:
Two types of corrosion testing are available for a comprehensive corrosion prevention and control program. The traditional test is a storage stability test. I strongly recommend at least one year of storage testing before making commercializing new spray formulas and derivative spray formulas. A properly designed, one-year-long storage test typically has a 2% to 7% risk that corrosion will occur in commercial containers.
Electrochemical corrosion testing is the other corrosion test. A properly designed electrochemical test can be completed in 1–3 months with an associated risk of less than 1%. In other words, a properly designed electrochemical test can provide lower risk results than a one-year storage test.
Understandably, corrosion is not a main element of most business plans. However, ignoring corrosion risks does not make them disappear and could lead to a major disruption in a business plan with a corresponding loss of income and loss of customer/consumer confidence in your products. In most instances corrosion testing is significantly cheaper than a corrosion failure. Consequently, corrosion prevention and control is a continuous process and not a one-time event.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course, which provides a more comprehensive discussion on spray package corrosion science and technology, corrosion prevention and control, at your R&D facility. We have also introduced the Corrosion Partnership Program, which makes our electrochemical corrosion laboratory, corrosion consulting and anti-corrosion technology conveniently available for your R&D program. Contact: rustdr@pairodocspro.com; 608-831-2076; www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in January.
