Corrosion: Chemical oxidation
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 the previous issue it was discussed how a corrosion cell containing an anode, cathode, electrolyte and a metallic pathway is needed in order for electrochemical corrosion to occur. The most common forms and their effects were also discussed, such as galvanic, crevice and pitting. On this issue we will discuss chemical oxidation, which is often referred to in the industry as chemical corrosion or acid corrosion, and explain its most common forms.
Chemical oxidation, unlike electrochemical, can occur in the lack of oxygen and does not need a complex cell to be in place. Chemical oxidation is caused by a substance, this substance can be categorized as either an acid or an alkali. A general rule is that the more acidic a substance is, the more corrosive it will be and the more alkaline the less corrosive6.
The level of acidity or alkanity of a substance is usually measured by it’s pH. pH is defined as “the negative decimal logarithm of the hydrogen ion activity in a solution”7. According to this definition the higher the pH of a substance, the more alkaline, the lower the pH the more acidic it is. Table 2 displays the pH of common substances.
The difficulty of protecting against chemical attack is that acids can become vapors, travel to a metal’s surface, quickly react with moisture and corrode the metal. Acids such as Nitric can not only evaporate and move through the air but it can also penetrate organic materials, such as coatings, and attack the metal behind it. Alkalies can also become corrosive through the right environment. Sodium hypochlorite is a manufactured salt that is highly alkaline yet extremely corrosive. It is important to note that although an oxidizing salt might be alkaline, it can still be highly corrosive under the right circumstances. The most common forms of chemical oxidations are concrete corrosion, heat corrosion and microbial corrosion.
Corrosion in concrete can happen two ways, through the reinforcement bars and through chemical attack. Corrosion of the concrete though the reinforcement bars is a form of electrochemical corrosion because of the moisture that makes its way to the bars through the concrete since it’s very porous. Chemical, however, is a much more complex form, it attacks by either leaching the calcium hydroxide or by penetrating it. Concrete is a highly alkaline building material that is essentially composed of a hydrated calcium silicate, a cement paste matrix. The calcium is the element that is often “leached” out of the concrete, weakening it.
When an acid such as sulfuric acid attacks a concrete the ensuing effect is the deterioration or complete dissolution of the cement paste matrix. This deterioration weakens the concrete and produces that “edged” or “exposed concrete” look that is often reported on failure reports. Concrete can also be affected by carbon dioxide in the air through a process called carbonation. In this process the carbon dioxide reacts with the calcium hydroxide to form calcium carbonate. This process reduces the thickness of the concrete and also lowers the pH, allowing for electrochemical corrosion to be easily initiated should any reinforcement bars be present.
Microbial corrosion is caused by microorganism, bacteria, on not just metals but concrete, plastics and other materials. This form of chemical corrosion can happen when oxygen is present, through aerobic bacteria and in the lack of oxygen through anaerobic bacteria. The most common type of microbial corrosion is caused by Acidithiobacillus, where the bacteria acts as a sulfide-reducing agent that produces sulfuric acid, deteriorating the surface. Other bacteria, while in the presence of oxygen, can actually oxidize iron into iron oxide. Bacterial corrosion can also promote electrochemical corrosion by producing oxygen concentrations and causing pitting10. Microbial corrosion is a subject that is currently undergoing a large amount of research. Bacteria that can grow and corrode on both salt and fresh water as well as bacteria that can utilize the hydrogen formed during electrochemical corrosion processes thus eliminating the need for oxygen.
Heat corrosion, also known as high temperature corrosion, is the degradation of a metal through the scaling of salts and other compounds from hot gases. As the title suggests, this form of chemical corrosion occurs on environments where temperatures are high, such as the hot gas path in a gas turbine. A common sequence of heat corrosion is the carburization – dusting – green rot sequence. This sequence starts when the high temperatures are in the presence of carbon compounds that cause the carbon content of the metal (usually a Chromium-Nickel alloy) to increase on the surface. The resulting effect is the hardening of the surface of the metal which leads to embrittlement, cracking and eventual failure. Further exposure to this environment with high carbon content leads to carbides forming in the metal structure and decomposing into graphite. This graphite acts as a catalyst for the decomposition of local carbon monoxide in carbon and oxygen. This second stage is known as metal dusting. The third stage happens only if the metal is, as listed above, a chromium-nickel alloy. This rather catastrophic form of corrosion is when a rapid cyclic between carburization and the discharge of oxides on the metal surface takes place. This usually results in a greenish residue of chromium oxide and a significant loss of metal11. This last stage is known as green rot. All these stages can occur in a temperature range between 572oF and 1922oF11.
In conclusion, the terms electrochemical corrosion and chemical oxidation are just mechanisms for materials degrading because of a reaction with their environments. Technologies for preventing against corrosion have been developed and evolve everyday due to the fact that we still don’t fully understand why some materials corrode. Industrial environments are becoming more and more aggressive, requiring that these technologies to protect against corrosion evolve as well. Some of these methods include the development of corrosion resistant metals such as super-duplex stainless steel, environmental control technologies such as the use of chemical inhibitors, barriers such as coatings and epoxies, cathodic protection and fabrication modifications to avoid, say, crevices leading to crevice corrosion.
All these different forms of corrosion can always be traced back to one of the two main types of mechanisms, electrochemical and chemical oxidation. The forms discussed here are the ones most commonly seen in the industry. Corrosion is a very wide and serious problem in today’s industry, causing loss of productivity on plants and sometimes tragedies. Not just metals corrode, plastics, concrete and wide range of materials degrade because reactions with their environments. Corrosion is the ongoing life cycle of our equipment and materials, the ongoing life cycle of our industrial world.
Also see Corrosion: Electrochemical
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.
8Munger, Charles. Corrosion Prevention by Protective Coatings, Chapter 1, The Corrosion Cell. National Association of Corrosion Engineers, p.28, 1984
9Nick, Miloslav. IUPAC Compendium of Chemical Terminology. International Union of Pure and Applied Chemistry, 2005.
10Schwermer, C. U., G. Lavik, R. M. M. Abed, B. Dunsmore, T. G. Ferdelman, P. Stoodley, A. Gieseke, and D. de Beer. Impact of nitrate on the structure and function of bacterial biofilm communities in pipelines used for injection of seawater into oil fields. Applied and Environmental Microbiology, 2008.
11C.M. Chun, J.D. Mumford and T.A. On the Mechanism of Metal Dusting Corrosion, Ramanarayanan. (No Date)