Corrosion

Rust, the most familiar example of corrosion.

Corrosion is the deterioration of a material's essential properties as a result of reactions with its environment. The rusting of iron is a well-known example of corrosion. Other metals may be similarly damaged, typically producing their oxides, hydroxides, and salts. Corrosion also refers to the dissolution of ceramic materials or the discoloration and weakening of polymers affected by the Sun's ultraviolet light. A substance that causes corrosion is called a corrosive substance, or simply, a corrosive (as a noun). A corrosive can damage living tissue as well as inanimate materials.

• It is the loss of an electron of metals reacting with water or oxygen. Weakening of iron due to oxidation of the iron atoms is a well-known example of electrochemistry (a branch of chemistry that studies the reactions that take place when an ionic and electronic conductor interfere) corrosion.

Most structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances (see below). Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area to produce general deterioration. While some efforts to reduce corrosion merely redirect the damage into less visible, less predictable forms, controlled corrosion treatments such as passivation and chromate-conversion will increase a material's corrosion resistance.

Corrosive substances

Warning symbol for a corrosive.

Corrosive chemicals can be solids, liquids, or gases, and they can belong to any of several classes of materials. The following are some examples.

• Acids, such as sulfuric acid, nitric acid, and hydrochloric acid.
• Bases ("caustics" or "alkalis"), such as sodium hydroxide and potassium hydroxide.
• Dehydrating agents, such as phosphorus pentoxide or calcium oxide.
• Halogens and certain halogen salts, such as bromine, iodine, zinc chloride, and sodium hypochlorite.
• Organic halides and organic acid halides, such as acetyl chloride and benzyl chloroformate.
• Acid anhydrides, such as acetic anhydride.
• Some organic materials, such as phenol ("carbolic acid").

Corrosion of nonmetals

Most ceramic materials are nearly immune to corrosion. Their atoms are bound together by strong ionic or covalent bonds that resist disruption. When corrosion does occur, it is almost always a simple dissolution of the material or a chemical reaction, rather than an electrochemical process.

A common example of corrosion protection in ceramics is the lime (calcium oxide) added to soda-lime glass to reduce its solubility in water. Though it is not nearly as soluble as pure sodium silicate, normal glass forms submicroscopic flaws when exposed to moisture. Given the brittleness of glass, such flaws dramatically reduce the strength of a glass object during its first few hours at room temperature.

In the case of polymeric materials, their degradation can be caused by any of a wide array of complex and often poorly understood physicochemical processes. These are strikingly different from the other processes discussed below, so the term "corrosion" is applied to them in just a loose sense of the word.

Given their high molecular weight, polymers are generally quite difficult to dissolve. In those instances where dissolution is a problem, it is relatively simple to design against. A more common and related problem is swelling, in which small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible vinyl) are intentionally swelled with plasticizers, and when these get leached out, the structure becomes brittle or undergoes other undesirable changes.

The most common form of degradation of polymers is a decrease in chain length. In the case of DNA, agents that break their chains include ionizing radiation (most commonly ultraviolet light), free radicals, and oxidizers (such as oxygen, ozone, and chlorine. Additives can slow these processes effectively, and they can be as simple as a UV-absorbing pigment (such as titanium dioxide or carbon black). Plastic shopping bags often do not include these additives, so they can break down more easily as litter.

The remainder of this article is about electrochemical corrosion.

Electrochemical corrosion of metals

Consider a metal in contact with an electrolyte—that is, a medium that can conduct electricity by allowing the flow of ions. Corrosion of the metal involves electrochemical changes. In other words, the metal is chemically changed by a process that involves the flow of an electric current. In most cases, electrons are transferred from one substance to another.

One way to understand the metal's structure is to think of it as an array of positively charged metal ions sitting in a negatively charged "gas" of free electrons. Electrostatic attraction holds these oppositely charged particles together. Other negatively charged particles, such as anions in the electrolyte, are also attracted to the metal ions. [Consequently, there is ]

A particular metallic ion at the metal's surface may dissolve into the electrolyte or remain part of the metal, depending on the energy gained or lost by the process. This process reflects an atomic-scale tug-of-war between the electron "gas" and dissolved anions. The quantity of energy gained or lost depends on a host of variables, including the types of ions in solution, their concentrations, and the number of electrons present at the metal's surface. In turn, corrosion processes strongly affect all of these variables.

For a given anion at the surface of the metal, a certain amount of energy is gained or lost when the ion dissolves into the electrolyte or becomes attached to the metal. The quantity of energy gained or lost depends on a host of variables, including the types of ions in solution, their concentrations, and the number of electrons present at the metal's surface. The overall interactions between electrons and ions tend to produce a state of local thermodynamic equilibrium that can often be described using basic chemistry and a knowledge of initial conditions.

Galvanic corrosion

If two metals are placed in an electrolyte (such as aerated seawater) and electrically connected, one metal will be more "active" while the other will be more "noble" (less active). The more active metal will experience what is called "galvanic corrosion."

The activity of the metals is based on how strongly their ions are bound to their surfaces. When the two metals are in electrical contact, they share the same electron gas, so that the tug-of-war at each surface is translated into a competition for free electrons between the two materials. The noble metal tends to take electrons from the active one, and the electrolyte facilitates the flow of electricity by hosting a flow of ions.

Based on the relative activity of metals within a given environment, they can be arranged in a hierarchy known as a galvanic series. This series can be a useful guideline for choosing materials for electrochemical processes.

Protection from corrosion

Some metals are intrinsically more resistant to corrosion than others, based on the fundamental nature of the electrochemical processes involved or the manner in which the reaction products are formed. If a material is more susceptible to corrosion, it may be protected from damage by various techniques.

Intrinsic chemistry

Gold nuggets do not corrode, even on a geological time scale.

The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. For example, corrosion products of gold or platinum tend to decompose spontaneously into the corresponding pure metal. These elements, therefore, can be found in metallic form in nature, and their resistance to corrosion is a large part of their intrinsic value. By contrast, the more common "base" metals can be protected in ways that are more temporary.

For some metals, their corrosion may be thermodynamically favorable, but the rate of reaction is acceptably slow. Examples of such metals are zinc, magnesium, and cadmium. In the case of graphite, it releases large amounts of energy upon oxidation, but the reaction rate is so slow that it is effectively immune to electrochemical corrosion under normal conditions.

Passivation

For some metals under appropriate conditions, a thin film of corrosion products forms spontaneously on the metal's surface, acting as a barrier to further oxidation. If this layer stops growing after reaching a thickness of less than one micrometer (under the conditions in which the material will be used), the phenomenon is known as passivation.

This effect, which in some sense is a property of the material, serves as an indirect kinetic barrier: the reaction is often quite rapid until an impermeable layer is formed. Passivation can be observed with materials such as aluminum, stainless steel, titanium, and silicon, when they are exposed to air and water at moderate pH. By contrast, the rusting of iron, which involves the formation of mixed oxides, is not considered passivation because the layer is not protective and usually grows to be much thicker.

Conditions required for passivation are specific to each material. Some conditions that inhibit passivation include high pH for aluminum, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal), and fluoride ions for silicon. On the other hand, sometimes unusual conditions can bring on passivation in materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms.

Surface treatments

A galvanized surface.

Applied coatings

The most common anti-corrosion treatments are plating, painting, and the application of enamel. They function by providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper, tougher, or easier-to-process) structural material. Aside from cosmetic and manufacturing issues, there are tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only in small sections, and if the plating is more noble than the substrate (such as chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with a more active metal such as zinc or cadmium.

Reactive coatings

If the environment is controlled, corrosion inhibitors can often be added to it. These inhibitors form an electrically insulating or chemically impermeable coating on exposed metal surfaces, thereby suppressing electrochemical reactions. Such methods make the system less sensitive to scratches or defects in the coating. Chemicals that inhibit corrosion include some of the salts in hard water, chromates, phosphates, and a wide range of chemicals that are designed to resemble surfactants (long-chain organic molecules with ionic end groups).

This figure-8 descender, used in rock climbing, is anodized with a yellow finish. Climbing equipment is available in a wide range of anodized colors.

Anodization

Aluminum alloys are often given a surface treatment known as anodization in a chemical bath toward the end of their manufacture. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to close, forming a harder-than-usual (and therefore more protective) surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.

Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode (negative electrode) of an electrochemical cell. This technique is most commonly used to protect steel pipelines and tanks (for water and fuels), steel pier piles, ships, and offshore oil platforms.

For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With

In this technique, the steel surface is given a uniform, negative electrical potential relative to another material that acts as the anode. Current flows from the anode to the cathode, driven by the difference in electrochemical potential between the two. The corrosion of steel is halted, while the anode material corrodes and must be replaced eventually.

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a direct-current power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rods of various specialized materials, such as high-silicon cast iron, graphite, mixed metal oxide, or platinum-coated titanium.

Corrosion in passivated materials

Passivation is extremely useful in alleviating corrosion damage, but one must be careful not to trust it too thoroughly. Even a high-quality alloy will corrode if its ability to form a passivating film is compromised. The modes of corrosion may be more exotic and their immediate results less visible than rust and other bulk forms of corrosion. Consequently, they may escape detection and cause problems.

Pitting corrosion

Pitting is among the most common and damaging forms of corrosion in passivated alloys. In the worst case, almost the entire surface remains protected, but tiny local fluctuations degrade the oxide film at a few critical points. Corrosion at these points can be greatly amplified, causing corrosion pits of several types, depending on conditions in the environment. Some conditions—such as low availability of oxygen or high concentrations of anions (for example, chloride ions)—can interfere with a given alloy's ability to re-form a passivating film.

Although corrosion pits may begin to be formed under unusual circumstances, they can continue to grow even when conditions return to normal, because the interior of each pit is naturally deprived of oxygen. In extreme cases, the sharp tips of long, narrow pits can cause stress concentration to the point that otherwise tough alloys may shatter. Alternatively, a thin film pierced by an invisibly small hole may hide a thumb-sized pit from view. These problems are especially dangerous because they are difficult to detect before failure of the structure or its part.

Pitting can be prevented by controlling the alloy's environment. This approach often includes ensuring that the material is exposed to oxygen uniformly (i.e., eliminating crevices).

Fretting

Many useful passivating oxides are also effective abrasives, particularly titanium dioxide (TiO₂) and alumina (Al₂O₃). Fretting corrosion occurs when particles of corrosion product continuously abrade away the passivating film, as two metal surfaces are rubbed together. Although this process often damages the frets of musical instruments, they were named separately.

Weld decay and knifeline attack

Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a minor alloying component (Chromium, typically only 18%). Due to the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries near welds) in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name applies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable¹.

Microbial corrosion

Microbial corrosion, or bacterial corrosion, is a corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and non-metallic materials, in both the presence and lack of oxygen. Sulfate-reducing bacteria are common in lack of oxygen; they produce hydrogen sulfide, causing sulfide stress cracking. In presence of oxygen, some bacteria directly oxidize iron to iron oxides and hydroxides, other bacteria oxidize sulfur and produce sulfuric acid. Concentration cells can form in the deposits of corrosion products, causing and enhancing galvanic corrosion.

High temperature corrosion

High temperature corrosion is chemical deterioration of a material (typically a metal) under very high temperature conditions. This non-galvanic form of corrosion can occur when a metal is subject to a high temperature atmosphere containing oxygen, sulphur or other compounds capable of oxidising (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.

The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions. Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.

Economic impact

The US Federal Highway Administration released a study, entitled Corrosion Costs and Preventive Strategies in the United States, in 2002 on the direct costs associated with metallic corrosion in nearly every U.S. industry sector. The study showed that for 1998 the total annual estimated direct cost of corrosion in the U.S. was approximately $276 billion (approximately 3.1% of the US gross domestic product). FHWA Report Number:FHWA-RD-01-156. The NACE International website has a summary slideshow of the report findings. Jones¹ writes that electrochemical corrosion causes between $8 billion and $128 billion in economic damage per year in the United States alone, degrading structures, machines, and containers.

References

ISBN links support NWE through referral fees

• Jones, Denny (1996). Principles and Prevention of Corrosion, 2nd edition, Upper Saddle River, New Jersey: Prentice Hall. ISBN 0-13-359993-0. 
• Working Safely with Corrosive Chemicals

See also

• Chemical hazard label
• Copper band corrosion.
• Rusting
• Electronegativity
• Cathodic protection
• Galvanization
• Oxidation
• Periodic table
• Society of Tribologists and Lubrication Engineers

External links

• NACE International -Professional society for corrosion engineers ( NACE )
• corrosion-doctors.org -Site dedicated to corrosion of all forms
• [1] -European Federation of Corrosion
• [2]-Corrosion Properties of Stainless Steel

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