Lesson 5-1 CORROSION
Since the corrosion of metal results in such tremendous loss of materials, time, and money, just what is corrosion? Let's consider three different definitions:
Though the definitions vary, they all boil down to the fact that corrosion is a natural act of a metal trying to return to its lowest level of energy. In the case of a metal structure, iron and steel try to return to their natural state of iron oxide (iron ore). The so-called noble metals, such as gold and platinum, do not corrode since they are chemically uncombined in their natural state. Your concern is with the chemical changes that occur when metal is exposed to the elements.
a. Electrochemical Theory. The most common theory of corrosion is called electrochemical. An electrochemical-corrosion theory is best explained by the action that takes place in a battery cell (Figure 5-1). A galvanic battery cell is produced by placing two dissimilar metals in a suitable electrolyte that is a conducting medium in which the flow of current is accomplished by the movement of matter in the form of ions. The resulting electrochemical reaction develops a potential difference between these two metals, causing one metal to be anodic and the other metal to be cathodic. In a dry cell, the zinc case is the anode (positive terminal) and the carbon rod is the cathode (negative terminal). When an external electrical circuit is completed, current flows from the zinc case into the electrolyte, taking with it particles of zinc. This is an example of galvanic corrosion of the zinc case.
Figure 5-1. Electrochemical-corrosion theory of a battery cell
b. Electrochemical Conditions.
(1) Four conditions must exist before electrochemical corrosion can take place; namely, the presence of a metal anode, a cathode, an electrolyte, and a conductor.
The four conditions required for the corrosion process are shown in Figure 5-2. Elimination of any of the four conditions will automatically stop the corrosion process. For example, an organic paint film on the surface of metal will prevent an electrolyte (corrosion path) from connecting the cathodic and anodic areas and a current cannot flow; therefore, no corrosion occurs (Figure 5-3).
Figure 5-2. Electrochemical-corrosion condition
Figure 5-3. Paint film preventing corrosion
(2) The possibility of corrosion problems and the necessity for control measures will vary accordingly because some metals are more subject to corrosive action than others. A corrosive attack begins on a metal surface that is exposed to a corrosive environment. If allowed to progress, corrosion works down into the core of the metal's material (Figure 5-4). Since corrosion never originates in the core, there will always be evidence on the surface when an attack is in progress (Figure 5-5).
Figure 5-4. Core corrosion
Figure 5-5. Corrosion point of entry
Before you can identify corrosion, you must know what it looks like on a metal's surface. Corrosion takes place in various forms, depending on the metal type, the environment, and the mechanical conditions. Among the many types of corrosion are the following:
a. Uniform (General). This corrosion is the most common form, which is a general attack on a metallic surface. Uniform metal etching is the surface effect produced by most direct chemical attacks. On a polished surface, this corrosion type is first seen as a dulled surface. If such corrosion is allowed to continue, the surface becomes rough and possibly frosted in appearance.
(1) This corrosion type is a complete corrosion class that involves an electrochemical action between two metals or between different areas of the same metal having different heat treatments or other metallurgical differences. Galvanic corrosion occurs when dissimilar metals are in contact and the presence of moisture provides an external circuit (electrolyte). One recognizable feature of galvanic corrosion is the presence of corrosion buildup at a joint between metals. For example, aluminum and magnesium skins that are riveted together in an aircraft wing form a galvanic couple if moisture and contamination are present.
(2) When aluminum pieces are attached with steel fasteners or screws, galvanic corrosion (Figure 5-6) can occur between the aluminum and steel. Table 5-1, is a list of metal and alloy classifications. The metals listed within each classification group have no strong tendency to produce galvanic corrosion and are relatively safe to use in contact with each other. The coupling of metals from a different group and the increased distance the groups are from each other will generally result in galvanic or accelerated corrosion of the metal higher on the list. The farther apart the metals are listed in the table, the greater a galvanic tendency will be. A galvanic tendency is determined by measuring the potential electrochemical difference between any two metals.
Figure 5-6. Galvanic corrosion
Table 5-1. Metal and alloy classification
(3) A metal's tendency to corrode in a galvanic cell is determined by its metal compatability position in the metal and alloy galvanic series shown in Table 5-2. The metal compatibility order listed is only appropriate for seawater at 77°F. Both temperature and the makeup of an electrolyte (water or soil) may vary the order in which the metal is listed. For example, in fresh water at a temperature above 150°F, iron may become anodic (more corrosive) with respect to zinc. The less noble (anode) the metal is, the more it suffers from an accelerated corrosion attack. The more noble (cathode) the metal is, the greater it is cathodically protected by a galvanic current. The more noble the metal or alloy, is the more chemically inert or inactive it is, especially toward oxygen.
Table 5-2. Metal and alloy galvonic series
c. Pitting. This corrosion occurs in most alloys, but it is most common in aluminum and magnesium. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny pits or holes are seen in the surface. Pitting corrosion is a localized form that begins at a break in the passive (protective) film. A cell is formed between the exposed metal and the passive metal where the film is broken. Such breakdowns in the protective coating can occur at a rough spot, machining mark, scratch, or other surface flaw. Pitting corrosion can also occur under a small deposit (weld spot or dirt particle) that prevents the access of oxygen to the metal. Pitting corrosion proceeds at a rapid rate if the products of corrosion are conductive.
d. Stress Cracking.
(1) Stress cracking is caused by the distortion of a metal's grain structure. Distortions are induced when metals are punched, cold-riveted, shrink-fitted, or otherwise distorted after hot-finishing. Distortion is also caused by the metal bending or twisting. This bending or twisting may be a continuous stressed condition or it may be an alternating stressed and unstressed condition. Stress corrosion is due primarily to the fracture of a metal's surface, film, or coating. The corrosion process is accelerated by fractures or tiny cracks that permit moisture to enter.
(2) After surface cleaning, stress cracks are usually visible at the bottom of corrosion pits. Corrosion pits may form before or simultaneously with any stress condition. The pitting rate is more rapid with simultaneous stress. In the second stage, a stress crack at the pit base develops rapidly and gradually penetrates the section until a fracture occurs.
(3) A good example of stress corrosion is the use of cold-worked rivets to join steel sheets used in storage tank construction. The rivets carry an internal strain—the deformation of the rivet shank—that makes them anodic (dissimilar metals) to the steel. The stressed area forms the anode to the adjacent unstressed parts. Since the rivets are very small in area compared with the steel, they become badly corroded in comparison with the action on the steel.
e. Intergranular. This corrosion is the result of the metallic grain boundaries and the grain particles creating a cell in an electrolyte, such as a corrosive solution or atmosphere. This type of corrosion is an attack on the metal's basic grain structure. A highly magnified metal cross section shows its composition is made up of a number of tiny crystals or grains. Each of these tiny grains has a clearly defined boundary, and each grain differs chemically from the one in the center of the metal. The adjacent grains of different elements react with each other as anodes and cathodes when they are in contact with an electrolyte. The early stages of this corrosion cannot be detected by normal visual inspections. Some of the stainless steels are prone to intergranular corrosion if they are heated. This is because corrosion may begin when heat from welding causes chromium carbides to collect at the grain boundaries.
f. Exfoliation. This corrosion is the visible evidence of intergranular corrosion. It shows itself by metal surface grains lifting up and lifting is caused by the force of expanding corrosion occurring at the grain boundaries just below the metal's surface. This corrosion is most often seen on rough-finished metal surfaces. The rougher, more strained, and less uniform a metal's surface, the sooner corrosion starts and the more localized the corrosion develops.
g. Concentration Cell (Differential Environmental). This corrosion occurs when several areas of a metal's surface are in contact with different concentrations of the same electrolyte. Corrosion results from a difference in the composition of the electrolyte and from the difference in the concentrations. Both conditions cause metal corrosion. An example of differential-environmental corrosion is shown in Figure 5-7.
Figure 5-7. Differential-environmental corrosion
There are three types of concentration cell corrosion; they are metal-ion, oxygen, and active-passive.
(1) Metal-ion concentration cell corrosion consists of different concentrations of metallic ions in various water parts. High metal-ion concentrations will exist under the surfaces where the water is stagnant, whereas low metal-ion concentrations will exist adjacent to the crevice that is created by the raised surfaces. An electrical-potential will exist between the high- and low-concentration points. The area that has the high metal-ion concentration will be anodic and will corrode. For example, when a large object (such as a pipeline) passes through different soil environments, major corrosion cells are established and will extend over several miles. This condition results in several amperes of current flowing in the pipeline metal. Corrosion of the pipeline will occur wherever the current leaves the pipe's surface.
(2) Oxygen concentration cell corrosion occurs when a solution contains varying amounts of dissolved oxygen cells. Oxygen develops—
Figure 5-8. Low oxygen-concentration areas
(3) Active-passive concentration cell corrosion is commonly found on metals that depend on a tightly adhering passive film, usually an oxide, for corrosion protection. The corrosive action starts as an oxygen-concentration cell. For example, salt deposits forming on metal surfaces in the presence of water containing oxygen can create an oxygen cell. The corrosive action of the low-oxygen cells will break the passive film beneath the dirt particles, which in time will expose the active metal beneath the film to corrosion action (Figure 5-9). An electrical potential develops between the large area of the cathode (passive film) and the small area of the anode (active metal), which can produce rapid pitting of the active metal.
Figure 5-9. Low-oxygen cell corrosion action
The corrosive action on pipelines, structures, and equipment conveying water, petroleum, and gases is a problem of vast importance to the Army. Instead of maintaining a few feet of pipe as we do in our homes, the Army maintains thousands of feet. A substantial saving is made if the effect of corrosion on equipment is decreased. Corrosion may develop under a number of conditions, among them are mill scale, cinder, dissimilarity of pipe surface, different soil condition, stray current, bacteria, dezincification, graphitization, and hydrogen embrittlement.
a. Mill Scale. One cause of pipe corrosion is mill scale which is embedded in the walls of iron pipe during its manufacture. The component parts of this corrosion condition are the—
The current leaves the iron-pipe wall, passes through the electrolyte soil to the mill scale, and returns to the iron pipe. This electrochemical action causes severe metal pitting at the anodic areas. Continued action of this type will eventually weaken the pipe and cause it to fail.
b. Cinder. Another type of corrosion occurs when iron pipe is laid in a cinder fill and is in direct contact with the cinders. The component parts of cinder corrosion are the—
The current leaves the pipe through the soil to the cinders and returns to the pipe. Severe corrosion occurs at the points where the current leaves the pipe. Galvanic corrosion wears away the pipe at an accelerated rate because of the nonpolarizing effect of the cinders and the highly ionized soil contamination of the cinders.
c. Dissimilarity of Pipe Surface. This galvanic corrosion occurs when there are bright or polished surfaces on some areas of iron-pipe walls, and the dissimilar pipe surfaces are in contact with suitable electrolytic soil. A pipe wrench can produce bright surfaces, such as scars and scratches, on the pipe when assembling it. The threads on both ends of a coupling may expose polished surfaces that corrode easily. Corrosion in the threads will eventually cause the perforation of the iron-pipe wall.
d. Different Soil Condition. This is a general corrosion problem that is especially prevalent in high-alkaline areas. Corrosion currents enter or pass through an iron-pipe wall from compact soils. Corrosion currents also enter or pass through the iron-pipe wall from light sandy soils. The intensity of the corrosion currents and the resulting corrosion rate at the pipe's anodic areas are directly proportional to the soil's conductivity. Earth current meters are used to determine the location of the anodic and cathodic areas and the extent to which a corrosion current exists. This meter determines if the pipe requires protection.
e. Stray Current. Stray currents, many of which are direct causes of corrosion, are usually direct-current circuits that pass in and out of an electrolyte. This condition poses the greatest problem in the vicinity of electrical-transportation systems, electrified coal mines, or manufacturing plants where the direct-current distribution system requires a ground as a complete or partial circuit return. If a metallic structure, such as a tank or pipeline, is laid in such an area, a large galvanic cell is created, making a perfect setup for corrosion. Corrosion does not occur at the point where the current enters the structure because it is cathodically protected. However, at the section where the current leaves the structure, severe, stray-current corrosion occurs. Over a period of a year, this type of corrosion is known to displace as much as 20 pounds of iron-pipe wall for every ampere of current.
(1) Another distinct corrosion that results from the electrolytic or galvanic cell action of minute organisms is bacteria (microbiological corrosion). Microbiological corrosive action is the deterioration of metal by a corrosion process that occurs as either a direct or indirect result of the activity of certain bacteria, particularly in water or soil environments. Organisms that cause microbiological corrosion are bacteria, slime, and fungi.
(2) The microbiological corrosive action that occurs in the soil is due to the physical and chemical changes in soil by the action of bacterial-type organisms. Some bacterial-type organisms are responsible for the production of active galvanic cells. These cells are produced by variations of the oxygen content in the soil (differential aeration) or by the reduction of the hydrogen film over the cathodic areas (depolarization). Bacterial-type organisms are mostly found in highly water-logged, sulfate-bearing, blue-clay soils. The bacterial concentration as well as the corrosion rate varies considerably with the different seasons of the year. Cast iron and steel pipe are corroded mostly by the sulfides produced by the bacteria.
g. Dezincification. Dezincification is a selective corrosion that occurs in copper and zinc alloys. When alloys of this kind (brasses) are exposed to dezincification corrosion, the zinc will dissolve out of the alloy, leaving only copper. Since most pipe fittings are made of brass, dezincification attacks and weakens these brass fittings to the point of failure. In this case, the zinc ions go into solution, leaving the copper. The solution may be impure water or oil that acts as an electrolyte.
h. Graphitization. Graphitization, or graphite softening, is a peculiar disintegration form that attacks gray cast iron. Cast iron is an alloy made of iron and carbon, the carbon being in the form of graphite. When cast iron with such a composition is subjected to graphitization, the graphite pipe may last for many years if it is not subjected to any mechanical forces or sudden pressures.
i. Hydrogen Embrittlement. Hydrogen embrittlement is a term applied to metal that becomes brittle due to hydrogen action on its surface. When hydrogen forms on the surface of steel, the hydrogen action may form blisters or actually embrittle the metal. It has been demonstrated that hydrogen, which is liberated near the surface of steel in an electrolyte environment, will diffuse into the metal quite rapidly. This hydrogen, picked up by the steel in an atomic state, causes the steel to become brittle. When atomic hydrogen production on the metal's surface stops, the hydrogen leaves the metal in a few days and the metal again regains its original ductility. Carbon steels have shown that they are affected by hydrogen embrittlement according to the hardness in the steel. The harder the metal, the greater the susceptibility to hydrogen embrittlement. Hydrogen embrittlement in carbon steel is also increased by the presence of stress.
Now that you know what corrosion is and what causes it, you must know how to control it. Basically, the same principle that causes corrosion is used to counteract it. This is done in several ways, but the most common are known as passivation and cathodic protection.
(1) A passivator is an inhibitor that changes the potential of a metal to a more cathodic value. An inhibitor is a chemical substance or mixture which, when added to an environment, usually in small concentration, effectively decreases corrosion. Table 5-3 lists several mixture types for passivating metal surfaces. The term passivity may be defined as the property by which certain metals become inactive in a specific environment. Metals that do not form protective films under service conditions are protected by the method of immersion in a chemical bath containing inhibitors. The passivity of a metal, such as stainless steel, is gained by the method of a protective-film formation (electroplate) on the metal's surface through its absorption of atoms or ions.
Table 5-3. Mixtures for passivating metal surfaces
(2) By either of the methods (immersion or protective film), the passivation of the metal's surface serves to make the surface more resistant to corrosion by either physical or chemical treatment. One is based on the corrosive behavior of the metal or alloy, and the other is based on the electrochemical behavior of the metal or alloy.
(3) A metal's passivity is attributed, directly or indirectly, to a protective film that changes the metal's or alloy's galvanic potential in a more noble direction. For example, the inhibiting action of the chromate ion in zinc chromate priming paint provides protection by passivation or mechanical means. The zinc chromate diffused in the paint provides a concentration of chromate ions on the metal's surface and tends to passivate the base metal. The chromate film shares electrons from surface iron atoms to make the metal surface less reactive and more noble in the galvanic series (Table 5-2). Sodium nitrate is also used as a passivator and will render iron several tenths of a volt more noble than iron in distilled water. The nitrate ion is oxidizing in nature and, like chromates and other oxidizing passivators, reduces corrosion.
(4) The use of metallic coatings and claddings is another way of making a metal passive to its environment. This means of metal protecting plates a metal's surface with another corrosion-resistant metal. In some cases, plating gives the base metal a hard, wear-resistant surface in addition to providing protection against corrosion.
(5) The application of anodic coating to magnesium and zinc protects them against corrosion. The protective oxide film that is provided by an anodic treatment is of the same general type as that afforded by a natural oxide film. The natural film is very thin; thus, anodic coating, because of its greater thickness, uniformity, and abrasion-resistance, offers better protection against corrosion. Look again at Table 5-3, and study the passivation types that are applicable to different metals as well as the chemicals used for passivation.
(6) Passivation by the electron theory deals specifically with metals and alloys that become more noble because of their electrochemical behavior. In application, advantage is taken of the corrosion-inhibiting action of ions in protective coatings that passivate by both electrochemical and mechanical means. Coatings and claddings are also used and frequently provide the most economical solution to corrosion problems.
b. Cathodic Protection. Cathodic protection is a method used to protect metal structures from corrosive action. As explained before, galvanic cell corrosion is the major contributing factor to the deterioration of metal by an electrochemical reaction. The area of a structure that corrodes is the anode or positive electrical current that leaves the metal and enters the electrolyte. Galvanic cathodic protection is designed to stop this positive current flow. When the current is stopped, the corrosive action stops and the anodes disappear. This type of protection depends upon the neutralization of the corroding current and the polarization of the cathodic metal areas. Galvanic cathodic protection is a procedure for reducing or preventing metal surface corrosion by using sacrificial anodes or impressed current methods. The sacrificial anode method is known as the galvanic anode method. The impressed current method is the galvanic cathodic method; however, it is referred to as the impressed current method. Depending on the corrosive characteristics of the electrolyte surrounding the structure, galvanic anode and impressed current methods are used separately or in conjunction with each other.
(1) Galvanic anode method. This cathodic protection method uses an electrode that is referred to as a sacrificial anode which corrodes to protect a structure. This sacrificial anode is electrically connected to, and placed in, the same electrolytic area of the structure. To protect iron or steel structures, use a sacrificial anode made of magnesium or zinc so that it will produce a sufficient potential difference to cause the structure to become a cathode. The action of this galvanic protection causes the electrical current to flow from the sacrificial anode through the electrolyte to the structure to be protected. The electrical connection between the two metals completes the circuit and allows current to return to the corroding metal. The sacrificial anode becomes the anode of the established, dissimilar metal galvanic cell; the structure to be protected becomes the cathode. The current from a sacrificial anode is intense enough to oppose or prevent all positive current flows from leaving the anodes in the structure to be protected. The prevention of the positive current flow from the anodic areas in the structure reduces the corrosion rate to almost zero. Galvanic cathodic protection is used in areas where the corrosion rate is low and electric power is not readily available. The gas-fired, hot-water tank and the buried oil tank, shown in Figure 5-10, are typical examples of galvanic cathodic protection.
Figure 5-10. Galvanic cathodic protection
(2) Impressed current method. The impressed current method of cathodic protection is designed to protect large metal structures that are located in corrosive areas. An alternating current source is required with this protection method. In addition, a rectifier is necessary to obtain the required direct-current potential. The basic principal of the impressed current method is merely the application of the galvanic-celled reaction. The component parts of this method are the—
The operation of this method depends on a rectifier that forces direct electrical current from the anode through the electrolyte to the metal structure that needs protection. This method causes the metal structure to be the cathode, suppresses all anodic currents from it, and prevents corrosion of the structure. Figure 5-11 shows a setup of an impressed current method of cathodic protection.
Figure 5-11. Impressed current method of cathodic protection