Corrosion is often thought to be a very simple process. However, most types of corrosion are very complex, multiple step processes. For example, the equation for corrosion of aluminum in hydrochloric acid is:
The equation is straight forward—aluminum metal comes into contact with hydrogen ions (from the hydrochloric acid); aluminum corrodes and hydrogen gas is formed.
However, the actual process for corrosion of an uncoated metal by hydrochloric acid (i.e., hydrogen ions) is a complex, multi-step process:
- Hydrogen ions diffuse from the bulk solution to the metal surface
- Hydrogen ions displace adsorbed water molecules on the metal surface—water molecules must de-adsorb to give hydrogen ions access to the metal
- Hydrogen ions adsorb on the metal surface
- Hydrogen ions remove electrons from the metal to form hydrogen atoms
- Hydrogen atoms move over the metal surface (diffuse) to find other hydrogen atoms
- Two hydrogen atoms combine to form a hydrogen molecule
- Hydrogen molecules combine to form gas bubbles (also a multi-step process)
- Hydrogen gas bubbles de-adsorb from the metal surface
The process continues until either all of the hydrogen ions or the metal is consumed. Most steps must be completed before the other could occur, and many of the steps have very rapid rates.
Some steps interact with others making the corrosion reaction much more than a first order reaction controlled by the chemical activation energy for the chemical portion of the reaction (metal atoms change to metal ions). Thus, increasing temperature does not speed-up the corrosion reaction rate as predicted by the Arrhenius law (increasing temperature by 20 degrees approximately doubles the reaction rate).
Corrosion could be sped-up or arrested at step. For example, stirring would increase the corrosion rate by a) removing hydrogen gas bubbles, b) increasing the amount of hydrogen ions that contact the metal surface and c) making sites for the exchange of electrons with hydrogen ions more readily available to other hydrogen ions. Adding a corrosion inhibitor could also block access of the hydrogen ions to the metal surface and slow or stop corrosion or inhibit or prevent electron transfer from the metal to the hydrogen ions.
The corrosion of coated metals is also often more complex than believed. For example, the corrosion of a coated aerosol container is not the result of holes in the coating.
Instead, water and formula ingredients create microscopic rivers through the container coating, and these rivers establish the fluid dynamics necessary for general and pitting corrosion under the coating. Thus, the steps involved with general and pitting corrosion under aerosol container coatings are:
- Water and other formula ingredients diffuse from the bulk solution to the coated metal surface
- Water and other formula ingredients absorb onto the coating surface
- Water and other formula ingredients diffuse into and through the coating
- Continued diffusion of water and other formula ingredients forms microscopic rivers through the coating
- Water and formula ingredients break coating-metal bonds
- Water and other formula ingredients adsorb on the metal surface under the coating, wherever coating-metal bonds are broken or there are voids at the coating-metal interface
- Accumulation of water and other formula ingredients continues causing:
- Increasing amounts of coating disbonding from the metal (breaking coating-metal bonds)
- Initiation of general corrosion
- Initiation of pitting corrosion
- The area of degraded coating continues to spread
- Pitting corrosion initiates when the area of degraded coating is large enough to support a pitting corrosion rate
- Metal ions from corrosion diffuse out of the coating
- Note: the diffusion of metal ions creates a counter current flow of
- Metal ions out of the coating
- Water; formula ingredients and ions into the coating.
- Pits continue to grow under the coating until they perforate the container metal
Corrosion inhibitors could also arrest or prevent corrosion under coatings and laminate films. Increasing coating thickness is typically only effective when the coating thickness is increased by a very large amount, such as increases in coating thickness from four microns to 152 microns.
Higher temperatures could be above the glass transition temperature for the wet polymer or laminate film and produce both polymer corrosion and metal corrosion under the polymer that would not normally occur at room temperature.
- Metal and polymer corrosion is a complex process and corrosion problems are often not solved with simple solutions.
- Increasing temperature does not typically increase metal and polymer corrosion because the complexity of the corrosion process invalidates the applicability of the Arrhenius law for metal and polymer corrosion—in other words, increasing temperature does not increase metal and polymer corrosion rates.