Of all materials, metals are the most susceptible to corrosion, which is defined as deterioration due to a reaction with the surrounding environment. We go into more detail in our first post on corrosion.
Corrosion is the deterioration of a material, and its properties, due to its reaction with the surrounding environment. It is a particularly dangerous phenomenon because it leads to the pre-mature, and often undetected, thus unexpected, failure of components. In addition to being dangerous, corrosion is also tremendously costly; the National Association of Corrosion Engineers (NACE) estimates that approximately 3 percent of the United States’ gross domestic product (GDP) is directly lost to corrosion each year. Almost every economic sector is hurt by corrosion.
Thermodynamics provide the groundwork for corrosion. Energy is added to metals throughout the extraction and fabrication process, which leaves metals at a high energy, thermodynamically unstable state. Metals release energy while attempting to return to their natural ore state and corrosion is the visual indication of this process.
Of all materials, metals are the most susceptible to corrosion. There are five types of atomic bonds: ionic, covalent, metallic, Van der Waals, and hydrogen. Metals are composed of both metallic and covalent bonds, but it is the metallic bonds which prove to be the Achilles heel of metals. In metallic bonding, the electrons in the highest energy state (valence electrons) have a tendency to leave their parent atoms and combine to form a sea of delocalized valence electrons. This easy movement of electrons feeds the electrochemical cell, allowing corrosion to occur.
Although, buyers beware: not all metals are created equally. Some are more likely to corrode than others. Metals are organized into a list called the galvanic series which orders metals based on their electrode potentials, and consequently their tendency to corrode. From this list, metals can be categorized into three general corrosion behaviors: immune, active, and passive.
Immune metals are those that do not react with the environment. For example, noble metals, such as gold and platinum, often exhibit immune behavior. Contrarily, active behavior metals are quite finicky and react with the environment and exhibit corrosion. Differently, passive behavior metals corrode in their environment, and this corrosion product forms a protective film which protects from further corrosion. Many alloys, such as stainless steel, exhibit passive behavior.
|Corrosion Behavior||Metals in this Group|
|Immune||Gold, Silver, Platinum|
|Active||Magnesium, Tin, Carbon Steel|
|Passive||Iron, Stainless Steel, Titanium, Cobalt-Chrome, Nitinol|