Written on: January 1, 2017 by W. Stephen Tait
Happy new year, everyone. The May 2016 through August 2016 Corrosion Corners have a series of discussions on corrosion risks with spray packaging, how the cost for corrosion failures is significantly higher than corrosion testing, as well as the difference between risk management and gambling and how corrosion prevention and control is a continuous process and not a one-time event. All these discussions centered on a figure for the risk of spray package corrosion as a function of test time for the traditional storage corrosion test and electrochemical corrosion tests.
I received many requests for a more detailed discussion on this figure and decided to start the detailed discussion this month. The discussion is long, so I divided it into two parts—Part I is this month and Part II will be in the February edition of Corrosion Corner.
Figure 1 contains the risk versus time curve for storage corrosion testing with traditional metal aerosol containers and correlations between electrochemical corrosion test predictions and their corresponding actual commercial spray package corrosion.
The data used to generate Figure 1 were obtained from corrosion tests on a wide range of household spray products, personal care spray products, health care spray products and commercial spray products. The curves and correlations in Figure 1 are aggregates for all data and are not for individual families of products or specific spray package-formula systems.
Figure 1: The estimated risk of localized corrosion as a function of test length.
The Y-axis for Figure 1 is the localized corrosion probability for spray packages and the X-axis is the length of a corrosion test in days. In other words, the curve provides the probability (risk) of localized corrosion in spray packaging as a function of test length. Localized corrosion was chosen as the metric for Figure 1 because it leads to package metal perforations with subsequent product and/or propellant leaking and delamination, such as blistering that could potentially produce loose pieces of coating or laminate that clog valve orifices.
The storage test curve in Figure 1 is for traditional metal aerosol containers. Risks for multiple times were generated because a traditional storage test typically takes one year to complete in order to obtain a risk below 10%.
Figure 1 depicts the probability of metal pitting corrosion. The data for Figure 1 were obtained from:
We do not have enough data for generating similar curves from storage tests on TFS containers with other types of coatings, such as polyethylene terephthalate (PET) and laminated aluminum foil packaging, such as bag-on-valve (BOV) packages. However, preliminary data suggests that the zero-time (or no corrosion data) risks for these types of packages are approximately 43% for PET-coated TFS and 20% for BOV-types of packages.
The two points below the storage test curve in Figure 1 are correlations for electrochemical corrosion tests on multiple types of uncoated spray packages, coated spray packages and laminated-foil spray packages. There are two correlation points instead of two curves because electrochemical corrosion test lengths are significantly shorter than a corresponding storage test. Correlations were calculated between predictions from electrochemical tests and their corresponding actual commercial package corrosion.
The correlations are empirical estimations for the probabilities of localized metal and coating/laminate film corrosion in spray packages. They were obtained from:
Approximately 7,500 traditional aerosol containers were used to generate the Storage test curve in Figure 1. Data from over 12,800 spray packages were used to generate the two electrochemical corrosion test correlations in Figure 1. The square is the electrochemical test correlation for uncoated spray packages and the diamond is the correlation for coated and laminated-foil coated spray packaging.
I will complete this discussion in the February edition of Corrosion Corner. In that issue, I’ll discuss how to read the curve and correlations in Figure 1, plus the limitations associated with storage corrosion tests and electrochemical corrosion tests.
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 February.
