Written on: January 2, 2015 by W. Stephen Tait
Happy new year, everyone. Corrosion science and engineering, like all technical fields, has its own jargon. I thought we’d start out this year by defining those corrosion terms and a few statistical terms that are used with spray packages and spray package components.
Service life (SL)
Spray package service lifetimes are defined as the length of time before packages leak product or propellant, valves leak propellant or partially full packages no longer spray. In other words, service lifetime is the amount of time during which spray packages and aerosol valves function properly.
Corrosion rate
A corrosion rate is the rate at which metal corrosion penetrates through spray package metals and coating delamination, such as blisters, spreads laterally along the spray package surface. Corrosion rates are typically expressed as mils per year (1 mil = 0.001 inches) or millimeters per year.
Corrosion rates can be estimated from storage test data by measuring the corrosion depth or the blister diameter and dividing this depth or diameter by the spray package age when it was evacuated and opened.
Estimated service life
Spray package and valve service lives can be estimated when the corrosion rate is known or estimated. In reality, corrosion rates are always estimates of the actual rates. Rates can be estimated from storage test container examinations and electrochemical measurements—when the proper instrument parameters, measurement parameters and models are used for the estimation. Estimated service lifetimes are statistical distributions of numerous individual lifetimes.
Estimated cumulative percent failure
The number of failures in a given container population at a given time is expressed as the cumulative percent failure level. Each percent failure has an associated estimated life. For example, if 12 containers perforated after one year of storage testing with 96 containers, the cumulative percent failure level after one year is 12.5%.
Please note that the percentage for a storage test with a limited number of containers (96 in this example) significantly overestimates the actual percent failure at one year for a production of several million containers. In other words, in this example, the 12.5% estimated failure level from the storage test is significantly higher than the actual rate for commercial containers.
Each failure time will have a different associated percent failure level and the percentage of failures increases with time. A theoretical failure level versus time graph would have an S-shape with 0% when containers are filled (day one) and 100% at some later time.
Commercial life
Commercial life is the period from when sprays are filled to the time when they are exhausted and put in the recycling stream. Commercial lifetimes are also a statistical distribution of times.
The lowest service life of a spray package and its associated percent cumulative failure should exceed the target commercial life and be less than the acceptable associated cumulative failure, respectively. The lowest acceptable service life and its associated acceptable percent failure are both determined by each individual company and the length of the commercial life for each family of products.
General corrosion
General corrosion is the penetration of metal corrosion over a large area of the spray package surface. General corrosion is also the delamination of a polymer or laminate film over a large area of the spray package surface.
Localized corrosion
Localized corrosion is metal corrosion or polymer/laminate corrosion in small areas of the spray package. There are three types of localized corrosion: pitting corrosion, crevicing corrosion and blistering.
Pitting
Pitting corrosion penetrates spray package metals through a very small area. Thus, pitting corrosion is often referred to as pin-hole corrosion. Pitting corrosion causes spray packages to leak product or propellant.
Crevice corrosion
Crevicing also attacks spray package metals and occurs in the areas where two pieces of spray package material are joined together through seaming or welding. Both seaming and welding create a narrow crevice between the two separate pieces that allows formation of corrosive microenvironments inside the crevice.
Crevices are found in the a) top and bottom double seams for tinplated steel and tin-free-steel aerosol containers, b) the crimp area between the aerosol valve and aerosol container, c) the weld area between internal laminated-foil bags and the aerosol valve, and d) the various body welds for internal laminated-foil bags. Welds in tinplated steel and tin-free-steel aerosol containers are typically too shallow to be crevices.
Crevice corrosion could be general corrosion inside the crevice area, pitting corrosion inside the crevice area or both general and pitting corrosion.
Blistering
Blistering is a localized form of polymer coating and laminated film corrosion. Blisters grow laterally along the surface of the spray package. Blisters reduce spray package service life when pieces of coating or laminate film break free from the package surface and plug the holes in valve orifice(s).
Accelerated testing
Market time constraints often make it desirable to accelerate spray package corrosion testing. There are two traditional methods for accelerating corrosion: exposing packages to higher storage test temperatures and electrochemical corrosion testing.
Higher storage test temperatures
Raising test temperatures typically does not accelerate metal and polymer corrosion because the Arrhenius law does not apply to the corrosion process. It’s been observed by myself and numerous others that higher storage test temperatures stop the natural corrosion process that occurs at room temperature and often produce corrosion that is not naturally observed at room temperature—hence corrosion appears to be more intense, leading to the conclusion that a higher test temperature is accelerating corrosion.
Higher temperatures are only accelerating a process when there is an approximately doubling of the corrosion rate with each 20° increase in temperature and the process is controlled by the chemical activation energy of the process (corrosion is an electrochemical process and not a chemical process). There are numerous instances in corrosion literature where it was concluded that corrosion is not accelerated by increasing temperature.
Electrochemical corrosion testing
Electrochemical corrosion testing does not actually accelerate the natural spray package corrosion. The instruments used for electrochemical corrosion testing are very sensitive and thus detect and measure corrosion when it begins and measure corrosion long before it can be seen with the unaided eye or a light microscope. Thus, electrochemical corrosion tests actually provide accelerated results instead of accelerating corrosion. Attempts to accelerate spray package corrosion with applied electrical voltages have always failed.
We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Contact rustdr@pairodocspro.com or visit
www.pairodocspro.com. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestions/questions/comments. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in February.
