April 2013

How does corrosion start with uncoated and coated metals?

 

Hello everyone. I’m often asked how corrosion starts in spray packaging. The corrosion of spray packages begins in so many different ways that a comprehensive discussion is beyond the scope of Corrosion Corner. However, I’d like to provide an overview on what have been, in my experience, the more common beginnings of spray package corrosion.

Last month’s Corrosion Corner provides background information for this month. Let’s begin with a brief discussion of the most common types of spray package metals: aluminum and steel.

 Corrosion Basics: the surfaces of spray package metals

Aluminum spray packages with a few exceptions are coated with a polymer or laminate film. The surface morphology of aluminum is a complex mixture of grains, grain boundaries, inclusions, crystal defects and precipitates.

Polymer coatings and laminate films (also a polymer) must bond to this complex mixture. Consequently, there are a variety of different bonds with different bond-strengths distributed over the surface of the aluminum. There are also areas on aluminum containers where the polymer coating did not wet the aluminum leaving a small hole in the coating.

Some areas of coated and laminated aluminum will be more susceptible to corrosion than others. The chemical composition of your formula—and not polymer-metal bond strength or holes—determines which areas of the coating are more susceptible to becoming corrosion sites.

The steel used for spray packaging normally has either a tin metal coating, or a chromium/chromium oxide layer on top. Tin coated steel is commonly called tin plate, and chromium/chromium oxide coated steel is called tin-free-steel (TFS).

The metal coatings do not completely cover the steel. Indeed, image analysis measurements of tinplate indicated that 0.2 tinplate is approximately 20% exposed steel and iron-tin alloy (the ratio of the exposed steel to the alloy has not been determined/published). Thus polymer coatings and laminate films on tinplate will have a) a variety of polymer-metal coating bonds, b) polymer-alloy bonds—tinplate only, c) a variety of steel-polymer bonds and d) small areas where the coating is not bonded to a surface.

irst stages of coated/laminated aluminum corrosion

For the sake of simplicity, I will hereafter refer to both polymer coatings and laminates as polymers.

Formula water and ingredients absorb into polymers and diffuse through them to the underlying aluminum metal. The liquid reaching the aluminum is not pure water and it could cause polymer disbonding from the aluminum (delamination), and potentially initiate aluminum corrosion under the polymer.

Diffusion of water and formula ingredients through the polymer is a necessary first step for delamination (a form of polymer corrosion) and aluminum corrosion. Metal corrosion under a polymer occurs when a) the diffusing liquid is corrosive, b) enough liquid accumulates to initiate corrosion and c) liquid continuously diffuses through the coating to sustain corrosion.

The chemical composition of your formula determines a) which areas of the coating are more susceptible to diffusion, b) the chemical composition of the liquid reaching the aluminum metal substrate, c) if the liquid reaching the aluminum is corrosive and d) what type of corrosion will occur.

 First stages of uncoated and coated steel corrosion

Steel containers without a polymer coating are referred to as uncoated containers—even though the “uncoated” steel is actually coated with a metal. Steel containers with a polymer coating are referred to as coated containers—even though technically they are double coated containers (a polymer coating over the metal coating). I will again refer to both laminate films and polymer coatings as polymers.

The first stages for uncoated steel corrosion will be different from those for coated steel. Please remember that metal coatings have holes, some of which expose steel.

Uncoated steel corrosion could start as a) tin corrosion (commonly referred to as detinning), b) galvanic corrosion between the steel and metal coating and c) non-galvanic steel corrosion at holes in the metal coating. Notice that galvanic corrosion is not needed for steel corrosion.

Steel pitting corrosion could also start with the dissolution of inclusions (see March ’13 Corrosion Corner). Pitting corrosion could also initiate after general corrosion covers the steel with a thin porous layer of iron oxide/hydroxide. The amount of exposed steel can be very small in both cases.

Coated steel corrosion also begins with absorption and diffusion of water and formula ingredients into and through the polymer. However, there are additional ways that metal corrosion could start under polymers. For example, diffusion of water and formula ingredients through the polymer could cause detinning, leaving polymer that is not attached to a surface.

Polymer coatings could either fill holes in a metal coating; form a cap over the holes or both. Holes with polymer caps are cavities where liquid could accumulate, and liquid-filled cavities could be pitting corrosion sites.

The chemical composition of your formula determines a) which areas of the coating allow more or less diffusion, b) the chemical composition of the liquid reaching the metal coating and the steel substrate, c) if the liquid reaching the metal coating and the steel is corrosive and d) what type of corrosion will occur.

Please send your questions/comments/suggestions to [email protected] Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in May. SPRAY