Corrosion, which can be simply defined as rust, is more appropriately the tendency for metals to revert to their natural, lower energy state of ore. Metallic corrosion is an electrochemical process meaning it involves both chemical reactions and the flow of electrons. A basic electrochemical process that drives the corrosion of metals is galvanic action, where current is generated internally by physical and chemical reactions occurring among the components of the cell.
There are two primary types of galvanic cells that cause corrosion: the bimetallic couple and the concentration cell. A bimetallic couple is like a battery, consisting of two dissimilar metals immersed in an electrolyte solution. An electric current (flow of electrons) is generated when the two electrodes are connected by an external continuous metallic path. A concentration cell consists of an anode and cathode of the same metal or alloy and a return current path. The electromotive force is provided by a difference in concentration of the solutions contacting the metal(s). In a galvanic cell, there are four elements necessary for corrosion to occur:
- Anode – An anode is an electrode at which negative ions are discharged and positive ions are formed, or other oxidizing reactions occur. Corrosion occurs at the anode.
- Cathode – A cathode is an electrode at which positive ions are discharged, negative ions are formed, or other reducing reactions occur. The cathode is protected from corrosion.
- Electrolyte – An electrolyte is a conducting medium in which the flow of current is accompanied by movement of matter. Electrolytes include water solutions of acids, bases, and salts.
- Return Current Path – The return current path is the metallic pathway connecting the anode to the cathode. It is often the underlying substrate.
Removing any one of these elements will stop the current flow and corrosion will not occur.
Substituting a different metal for the anode or cathode may cause the direction of the current to reverse, resulting in a change of the electrode experiencing corrosion.
The galvanic series lists metals and alloys in decreasing order of electrical activity. Metals toward the top of the list are “less noble” metals and have a greater tendency to lose electrons than metals found lower on the list. Utilizing hot-dip galvanized steel basically exploits this phenomenon by offering up zinc (anode) to protect the underlying steel (cathode).
Corrosion of Steel
The corrosion process that takes place on a piece of bare steel is very complex due to variations in the composition/structure of the steel, presence of impurities due to the higher instance of recycled steel, uneven internal stress, or exposure to a non-uniform environment.
It is easy for microscopic areas of the exposed metal to become relatively anodic or cathodic, and many of these areas can develop in a small section of the exposed metal. Therefore, it is highly possible several different galvanic corrosion cells are present in the same small area of the actively corroding piece of steel.
As the corrosion process progresses, the electrolyte may change due to materials dissolving in or precipitating from the solution. Additionally, corrosion products might tend to build up on certain areas of the metal. As time goes by, there may be a change in the location of cathodic and anodic areas and previously uncorroded areas of the metal are attacked and corrode.
The corrosion rate of metals is controlled by factors such as temperature, humidity, pH of the
electrolyte, and the electrical potential and resistance of anodic and cathodic areas.