Research on corrosion has shown us that there are two main mechanisms of corrosion, electrochemical and chemical oxidation. Both of these types have many forms and ramifications, some more accentuated than others but the mechanism with which they degrade a substance or material can always be traced back to one of these two types.
Corrosion is all around us, in our everyday life, silently working and taking its course. Corrosion is the degradation of a material’s properties because of a reaction with its surroundings. NACE defines corrosion as “The destruction of a substance (usually a metal) or its properties because of a reaction with its environment”1 While this statement is true it also points at the fact that not just metals corrode, concrete, plastics, and other materials also corrode.
The effects of corrosion can be seen everywhere, on our houses, on children’s playgrounds, on our cars, buildings and bridges. If left unchecked it can cause severe accidents like the I-35W Minneapolis Bridge that collapsed on August 1, 2007 killing 13 people and injuring 145 others2 or the Aloha 243 flight which experienced an explosive decompression in mid-flight, tearing off part of the fuselage, killing a crew member and injuring 65 passengers and crew3.
Corrosion also has financial consequences. In the USA corrosion causes yearly losses of $47.9 billion in the utility industry, $29.7 billion in the transportation industry, $22.6 billion on the Infrastructure, $20.1 billion on Government structures and $17.6 billion in the production and manufacturing industry4. This totals $275.7 billion yearly because of corrosion with the biggest cost being in the drinking water and sewer systems at $36 billion and increasing5.
Research on corrosion has shown us that there are two main mechanisms of corrosion, electrochemical and chemical oxidation. Both of these types have many forms and ramifications, some more accentuated than others but the mechanism with which they degrade a substance or material can always be traced back to one of these two types. In this portion of the article we will be covering the electrochemical corrosion mechanism and its most commonly forms.
Electrochemical corrosion is a process through which a metal returns to its lowest energy oxidation state. To properly understand electrochemical corrosion one has to understand how ores are found and turned into metal.
Raw ores are picked up from nature where some of them exist in a chemical compound with other elements in a thermodynamically stable state. Iron, for example, is found along with oxides, hydrates and sulfides amongst others. In order to extract or purify the iron, energy (heat) is forced into the ore to turn it into a metal. This causes the iron ore to reduce and become thermodynamically unstable. The fundamental laws of thermodynamics dictate that an unstable system eventually returns to its stable form, equilibrium.
Some metals, however, are found in nature in a metallic form already, such as gold, and therefore are already stable in metallic form, which is why gold doesn’t corrode. This is not to say that only those metals that are found in pure metallic form in nature are the only ones that won’t corrode, there are ores that when oxidized produce a metal that won’t corrode, the difference is that the amount of energy (heat) needed to oxidize the ore in this case is minimal.
Unstable metals become stable throughout the years by electrochemical corrosion if left unprotected. In order for electrochemical corrosion to take place a corrosion cell must be present. A corrosion cell is a combination of four elements, anode, cathode, electrolyte and a metallic pathway connecting the anode and cathode. Without any of these four elements corrosion will not take place, all elements must be present. Modern studies have shown that oxygen can greatly affect corrosion and experts often consider it a very important element stating that in some forms of corrosion oxygen is the final completing step to reach the true lowest energy state.
- The anode is where the actual metal loss takes place. By ionizing into the electrolyte the ions release electrons through an elaborate series of chemical reactions producing ferric oxide (Fe2O3) and oxygen. Ferric oxide is what we commonly refer to as Rust.
- The cathode is an extremely important element of the corrosion cell as it determines the rate of corrosion. Electrons from the anode travel to the cathode and are accommodated there. The more electrons a cathode can accommodate the faster the anode will corrode. This is the primary relationship between the anode and the cathode in a corrosion cell.
- The Electrolyte is also an important element of the corrosion cell as it is the solution that surrounds the anode and the cathode. The electrolyte also has an effect over corrosion rate since the conductivity of the electrolyte will either allow electrons to freely move from the anode to the cathode or restrict their flow, thus reducing corrosion rate.
- Metallic Pathway is what some might refer to as the internal circuit as it is provided by the metal where the anode and cathode reside. The metallic pathway closes the corrosion circuit.
Oxygen is considered to be an important factor to corrosion since without oxygen corrosion slows down and eventually stops. Oxygen is responsible for reacting with the hydrogen ions that are released at the cathode due to the electron accommodation. In the lack of oxygen, hydrogen ions accumulate on the cathode and prevent electron accommodation, thus stopping the corrosion process. This phenomenon is called polarization6.
Although electrochemical corrosion refers to a corrosion mechanism it can be manifested in many forms. Galvanic, pitting and crevice corrosion are the most common ones.
Galvanic corrosion occurs when two different metals are submerged in a solution and joined together by a metallic pathway. An example is an iron ship with aluminum propellers. The electrolyte is the sea and the metallic pathway is the actual ship’s structure. The anode and the cathode are determined by looking into the galvanic series. Table 1 shows a simplified galvanic Series.
From Table 1 it can be seen that on an iron ship the aluminum propellers would be the most active metal or the anode and therefore corrode. It should be noted that the further apart the two metals are on the galvanic series the more accelerated the rate of corrosion will be.
On this specific case it’s also worth noting the size of the anode vs. the size of the cathode. The bigger the cathode is, the more electrons it can accommodate from the anode, causing it to ionize at a faster rate, thus corroding much faster. This relationship between the two is linear, the bigger the size difference between the cathode and the anode, the faster the anode will corrode. This only applies if the cathode is bigger than the anode. Should the anode be bigger than the cathode, the corrosion rate slows down. Therefore, the relationship is now inverse.
Pitting is a form of corrosion that often relates back to the size relationship between anode and cathode explained above. The most common cause of pitting is inhomogeneity in metals. Sometimes metals are not consistent in their content, pot metals or other grains of easily corroded metals can be included in them. If there are inclusion of this nature in the metal, that are more anodic than the rest of the metal, these impurities corrode at a faster rate and cause pits. The inverse is also possible, the impurities can be cathodic to the rest of the metal, this will induce pits in the surrounding area of the impurity.
Inhomogeneity is not the only cause of pitting. Protective coatings can break and expose a portion of the substrate that it was meant to protect. This causes the rest of the coated metal to act as a cathode while the small discontinuity becomes the anode. As per the size relationship between anode and cathode, the rate of corrosion on this small anode will be high, thus causing pit.
This is a particular form of corrosion that happens in very small crevices on metals. Crevice corrosion is a very interesting form because at some point it involves one of the two main mechanisms, Electrochemical and Chemical. Crevice corrosion is often located in places where the electrolyte becomes stagnant. These places often include lap joints, under gaskets and under insulation. In these places crevice corrosion usually starts out as electrochemical corrosion with the ingression of an electrolyte into a crevice. Due to the small size of the crevice, oxygen that is needed to maintain electrochemical corrosion is soon depleted and the electrolyte becomes acidic. This happens by the hydrolyzation of the metal ions produced by the electrochemical corrosion7. At this point the type or mechanism of corrosion is through chemical attack.
Although it may seem a difficult process for a corrosion cell to be established it happens with more ease than thought. Electrochemical corrosion can happen everywhere, it can interact with more than one metal and also make its way inside concrete and corrode the reinforcement bars. Our whole infrastructure is subject to corrosion. Many other forms of corrosion can be observed in industrial environments, however, as explained, all these forms can be traced back to electrochemical. These other forms might include filiform, atmospheric and mill scale corrosion.
Electrochemical corrosion is the most common mechanism in today’s industry and its effects can be measured from an economical, health, safety and even cultural perspective. This mechanism of corrosion can exist inside our bodies, in pace makers, it can cause bridges to fall, gas pipelines to burst and even cultural structures such as the Liberty statue to rust if not properly taken care of. Electrochemical corrosion is a force to be reckoned with in today’s world and it needs to be addressed as such. In the next issue we will cover the chemical oxidation mechanism and its most common forms.
Also see Corrosion: Chemical oxidation
1VanDelinder, L.S. ed. Corrosion Basics – An Introduction, Chapter 1, The Scope and Language of Corrosion. National Association of Corrosion Engineers, p.14, 1984
2Frommer, Frederic And Lowy, Joan. "Bridge design flaws may not be as rare as thought" USA Today. December 25, 2010.
3Kristoff, Susan. “The Fatigue Failure of Aloha Flight 243: How Cyclic Loading and Corrosion Caused a Mid-Flight Failure” Suite 101. December 27, 2010.
4Thompson, Neil. “Cost of Corrosion: In the USA $276 Billion/Yr.” CorrosionCost. December 28, 2010.
5Thompson, Neil. “Cost of Corrosion: In the USA $276 Billion/Yr.” CorrosionCost. December 28, 2010.
6Munger, Charles. Corrosion Prevention by Protective Coatings, Chapter 1, The Corrosion Cell. National Association of Corrosion Engineers, p.21, 1984
7Thompson, Neil. “Chronology of a Crevice.” Corrosion-Doctors. December 28, 2010.