st27

SprayMarch13

Corrosion Corner W. Stephen Tait, Ph.D. Chief Science Officer and Principal Consultant, Pair O Docs Professionals, LLC Why isn’t spray package corrosion more consistent? H ello everyone. One of the most difficult questions to answer about corrosion is why it’s so random. For example, you have a static storage test on a new formula or line extension, and you open twelve sample containers after six months and you find: • Five clean containers • One container with rusting on the bottom • Three containers with rusting in the vapor area • One container with a pit on the body in the product phase • One container with rusting and pitting in the bottom crevice (bottom double seam) • One container with rusting on the weld in the product phase The formula in the containers was from the same batch, so the chemical composition is the same; all the containers came from the same pallet (i.e., manufactured at the same time); and the containers were all filled at the same time. Frustrating! It would be even more frustrating if you found eleven corroded containers and one pristine, corrosion-free container. Why isn’t the corrosion more consistent? The answer is complex because a) the surface metallurgy of spray package metals is very complex; b) laminate film, polymer and tin coatings do not bond uniformly with the complex metal surface; c) there are many different types of spray package corrosion; and d) the initiation of corrosion is often a complex chain of events resulting from interactions between your formula and the container. Let’s explore the metallurgical reasons for spray package corrosion inconsistency. 26 Spray March 2013 Corrosion Basics: Metal surfaces & coated metal surfaces are not homogeneous The Figure on the facing page contains a scanning electron micrograph that shows the metallurgical structure details of the steel used to fabricate tinplated steel aerosol containers. The aluminum used to fabricate aerosol containers and laminated film bags have a similar structure. The metal in the Figure was prepared by removing the tin coating, mechanical polishing and then etching the steel with an acidic solution to show the metallurgical structure. The larger shapes are referred to as grains. There are also inclusions, precipitates, grain boundaries and crystal defects identified in the Figure. Notice in the Figure that there are several different grain sizes and shapes. Notice also that there are different grain heights, plus different thicknesses and heights for grain boundaries. The variety of heights and thicknesses resulted from different grains and grain boundaries having different corrosion rates in the etching solution. Thus the Figure demonstrates that non-uniform corrosion results in part from the complex mixture of the different types of materials and metallurgy on the metal surface. Certain formula compositions will also corrode the different steel and aluminum grains at different rates (e.g., a disinfectant formula verses an aqueous air freshener formula). Theoretically, polymer coatings and laminate films will bond with all the different materials on the package metal surface identified in the Figure. However, polymer coatings and laminate films actually bond differently with the different types of grains, inclusions, precipitates, grain boundaries and crystal defects. Some of the polymer-surface-material bonds will be very


SprayMarch13
To see the actual publication please follow the link above