Corrosion and spray package failures

Written on: October 2, 2018 by W. Stephen Tait

Hello, everyone. All spray package materials are susceptible to corrosion. There are a variety of materials used to fabricate spray packages:

Metals, such as aluminum, steel and tinplated steel (ETP)

• Aluminum spray package components are fabricated from sheet metal or foil
• Steel and ETP are fabricated from sheet metal

Polymer coatings, such as epoxy, Micoflex, PAM, PET, etc. 

Polymer films

• polypropylene
• double film layers, such as nylon on polypropylene
• (laminates)

Nylon used for valve bodies

Stainless steel used for valve springs

Metal corrosion is caused when valence electrons are transferred from a packaging metal to ingredients in a formula. The metal atoms become metal ions during their electron transfer to formula ingredients; this changes the chemical states of the metal atoms from atoms to ions as a result of the electrical transfer. Thus, metal corrosion is referred to as electrochemical corrosion because it is a hybrid reaction that has both electricity (electron transfer) and a chemical change of state (atom to ion).

Polymer corrosion is a chemical degradation of a polymer structure when formula ingredients absorb into the bulk polymer. There can also be an electrical charge transfer with polymer corrosion when ions and polar molecules absorb into and diffuse through the polymer.

Consequently, corrosion is broadly defined as the degradation of materials by an environment.

Aluminum aerosol cans

Steel aerosol can

The degradation of materials includes loss of:

• Package metal
• Polymer coating or film properties, such as the ability of a coating or film to be a barrier between its metal substrate and the environment (i.e., a formula)
• Adhesion between a polymer and its substrate metal (delamination)

There are several types of possible spray package failures modes:

• A package leaks product, propellant or both
• A package no longer sprays but is still partially full
• The product has a malodor
• The product is discolored
• The product’s active ingredients are below their specified concentrations

Different types of corrosion lead to various failures modes. The different types of corrosion that contribute to or cause different types of spray package failures are:

General metal corrosion

• Malodor
• Discoloration
• Valve orifice plugging
• Decreased concentrations of active ingredients
  

  General metal corrosion can lead to valve orifice plugging
  (photo courtesy of Innovative Group)

Localized metal corrosion

• Leaking product
• Leaking propellant

General polymer corrosion

• Delamination of film or coating from substrate metal; could cause valve orifice plugging when there are free-floating coating or film particles

Localized coating or laminate film corrosion

• Blisters—typically do not cause package failure unless blisters become free-floating polymer particles and plug valve orifices

Hybrid corrosion consisting of polymer delamination and metal corrosion under the delaminated polymer coating or film

• Malodor
• Discoloration
• Product leaking
• Propellant leaking
• Valve plugging

Material corrosion with subsequent package failure cannot be predicted using first-principles. Fortunately, corrosion tests are available to determine if material corrosion is possible, how fast materials are corroding and the length of service life when package materials corrode. Corrosion testing could be either a conventional storage test (conducted for at least one year) or a shorter length electrochemical corrosion test.

Integrating a company corrosion database with results from corrosion testing will help prevent surprise corrosion that leads to a disrupted market or a delayed new product introduction.

Thanks for reading Corrosion Corner and I’ll see you in November. SPRAY