Introduction
Corrosion is the destructive attack of a material by reaction with its environment. The serious consequences of the corrosion process have become a problem of worldwide significance. In addition to our everyday encounters with this form of degradation, corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, and expensive over design; it also jeopardizes safety and inhibits technological progress.
The multidisciplinary aspect of corrosion problems combined with the distributed responsibilities associated with such problems only increase the complexity of the subject. Corrosion control is achieved by recognizing and understanding corrosion mechanisms, by using corrosion- resistant materials and designs, and by using protective systems, devices, and treatments. Major corporations, industries, and government agencies have established groups and committees to look after corrosion-related issues, but in many cases the responsibilities are spread between the manufacturers or producers of systems and their users. Such a situation can easily breed negligence and be quite costly in terms of dollars and human lives.
1.1 importance of corrosion studies
Corrosion is both costly and dangerous. Billions of dollars are spent annually for the replacement of corroded structures, machinery, and components, including metal roofing, condenser tubes, pipelines, and many other items.
In addition to replacement costs are those associated with preventive maintenance to prevent corrosion, inspections, and the upkeep of cathodically protected structures and pipelines. Indirect costs of corrosion result from shutdown, loss of efficiency, and product contamination or loss.
Although the actual replacement cost of an item may not be high, the loss of production resulting from the need to shut down an operation to permit the replacement may amount to hundreds of dollars per hour. When a tank or pipeline develops a leak, product is lost. If the leak goes undetected for a period of time, the value of the lost product could be considerable. In addition, contamination can result from the leaking material, requiring cleanup, and this can be quite expensive. When corrosion takes place, corrosion products build up, resulting in reduced flow in pipelines and reduced efficiency of heat transfer in heat exchangers. Both conditions increase operating costs. Corrosion products may also be detrimental to the quality of the product being handled,
making it necessary to discard valuable materials.
Premature failure of bridges or structures because of corrosion can also result in human injury or even loss of life. Failures of operating equipment resulting from corrosion can have the same disastrous results.
When all of these factors are considered, it becomes obvious why the potential problem of corrosion should be considered during the early design stages of any project, and why it is necessary to constantly monitor the integrity of structures, bridges, machinery, and equipment to prevent premature failures.
Corrosion is not only limited to metallic materials but also to all materials of construction. costs of corrosion damage to bridges vary; however, because many states spend $20 million to $50 million annually, a total figure of $1 billion per year is not unreasonable. Although the costs attributed to corrosion damages of all kinds have been estimated to be of the order of 3 to 5 percent of industrialized countries’ gross national product
1.2 Examples of Catastrophic Corrosion Damage
1.2.1 Sewer explosion, Mexico
An example of corrosion damages with shared responsibilities was the sewer explosion that killed over 200 people in Guadalajara, Mexico, in April 1992.6 Besides the fatalities, the series of blasts damaged 1600 buildings and injured 1500 people. Damage costs were estimated at 75 million U.S. dollars. The sewer explosion was traced to the installation of a water pipe by a contractor several years before the explosion that leaked water on gasoline line laying underneath. The subsequent corrosion of the gasoline pipeline, in turn, caused leakage of gasoline into the sewers. The Mexican attorney general sought negligent homicide charges against four officials of Pemex, the government-owned oil company. Also cited were three representatives of the regional sewer system and the city’s mayor.
1.2.2 Loss of USAF F16 fighter aircraft
This example illustrates a case that has recently created problems in the fleet of USAF F16 fighter aircraft. Graphite-containing grease is a very common lubricant because graphite is readily available from steel industries. The alternative, a formulation containing molybdenum disulphide, is much more expensive. Unfortunately, graphite grease is well known to cause galvanically induced corrosion in bimetallic couples.
In a fleet of over 3000 F16 USAF single-engine fighter aircraft, graphite grease was used by a contractor despite a general order from the Air Force banning its use in aircraft.7 As the flaps were operated, lubricant was extruded into a part of the aircraft where control of the fuel line shutoff valve was by means of electrical connectors made from a of gold- and tin-plated steel pins. In many instances corrosion occurred between these metals and caused loss of control
of the valve, which shut off fuel to the engine in mid-flight. At least seven aircraft are believed to have been lost in this way, besides a multitude of other near accidents and enormous additional maintenance.
1.3 corrosion definition
Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
Or Destruction of a metal by chemical or electrochemical reaction with its’ environment.
Or A process in which a metal is destroyed by a chemical reaction
Or Degradation of the metal substrate by oxidation. That is the formation of an oxide layer on the metal surface. This process requires that the metal surface be exposed to oxygen, and is favored in the presence of water. In the case of iron and steel, corrosion is often referred to as rusting
The most widely used metal is iron (usually as steel) and the following discussion is mainly related to its corrosion.
1.3.1 THE CONSEQUENCES OF CORROSION
The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small.
Some of the major harmful effects of corrosion can be summarised as follows:
- Reduction of metal thickness leading to loss of mechanical strength and structural
- failure or breakdown. When the metal is lost in localised zones so as to give a crack like
- structure, very considerable weakening may result from quite a small amount of metal loss.
- Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars, aircraft)
- Loss of time in availability of profile-making industrial equipment.
- Reduced value of goods due to deterioration of appearance.
- Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities of heavy metals are released by corrosion)
- Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings.
- Loss of technically important surface properties of a metallic component.
- Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion
products.
- Added complexity and expense of equipment which needs to be designed to withstand
- a certain amount of corrosion, and to allow corroded components to be conveniently replaced.
1.4 corrosion principles
The cathode is that portion of the metal surface where reduction takes place and does not dissolve.
The anode is that portion of the metal surface that is corroded. It is the point at which metal dissolves, or goes into solution. When metal dissolves, the metal atom loses electrons and is oxidised.
Corrosion is defined as the deterioration of a material, usually a metal, because of a reaction with its environment and which requires the presence of an anode, a cathode, an electrolyte, and an electrical circuit. To understand the application of protective coatings or cathodic protection in corrosion control, the basic concepts of corrosion of metals in the presence of moisture needs to be reviewed.
a. Corrosion occurs by an electrochemical process.
The phenomenon is similar to that which takes place when a carbon-zinc “dry” cell generates a direct current. Basically, an anode (negative electrode), a cathode (positive electrode), an electrolyte (environment), and a circuit connecting the anode and the cathode are required for corrosion to occur (see Figure 1-1).
Dissolution of metal occurs at the anode where the corrosion current enters the electrolyte and flows to the cathode. The general reaction (reactions, if an alloy is involved) that occurs at the anode is the dissolution of
metal as ions:
M → M n+ + e n- ( eq 1-1 )
where
M = metal involved
n = valence of the corroding metal species
e = electrons
Examination of this basic reaction reveals that a loss of electrons, or oxidation, occurs at the anode. Electrons lost at the anode flow through the metallic circuit to the cathode and permit a cathodic reaction (or reactions) to occur. In alkaline and neutral aerated solutions, the predominant cathodic reaction is
O2 + H2O + 4e- → 4(OH) eq (1-2 )
The cathodic reaction that usually occurs in deaerated acids is
2H+ + 2e- → H2 eq (1-3)
In aerated acids, the cathodic reaction could be
O2 + 4H+ + 4e- → 2H2O eq (1- 4)
All of these reactions involve a gain of electrons and a reduction process.
b. The number of electrons lost at the anode must equal the number of
electrons gained at the cathode.
For example, if iron (Fe) was exposed to an aerated, corrosive water, the anodic reaction would be
Fe → Fe ++ + 2e- eq (1-5)
At the cathode, reduction of oxygen would occur
O2 + 2H2O + 4e- → 4(OH- ) eq (1-6)
Because there can be no net gain or loss of electrons, two atoms of iron must dissolve to provide the four electrons required at the cathode. Thus, the anodic and cathodic reactions would be
2Fe → 2Fe ++ + 4e- (anodic) eq (1-7)
O2 + 2H2O + 4e- → 4(OH- ) (cathodic) eq (1-8)
These can be summed to give the overall oxidation reduction reaction
2Fe + O2 + 2H2O → 2Fe ++ +4(OH- ) eq (1-9)
c. After dissolution,
ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++ ); these will combine with hydroxide ions (OH- ) formed at the cathode to give a corrosion product called rust (FeOOH or Fe2O3 x H2O).
Similarly, zinc corroding in an aerated, corrosive water (i.e., Zn → Zn++ + 2e-) will form the corrosion product Zn(OH)2 .
The important issue to remember is that anodic dissolution of metal occurs electrochemically; the insoluble corrosion
1.5 classification of corrosion
Table (1-1) , Classification of corrosion [5]
| General/Uniform Corrosion: Corrosive attack dominated by uniform thinning due to even regular loss of metal from the corrosion surface | corrosion or degradation of material exposed to the air and its pollutants rather than immersed in a liquid | ||
| corrosion that occurs when a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte | |||
| Stray-current | caused by an externally induced electrical current | ||
| General biological | corrosion of metals generally over the entire exposed surface in aqueous environments | ||
| Molten salt | corrosion of metals due to molten or fused salts | ||
| types of corrosion found in liquid metal / containment / component combinations | |||
| Oxidation | corrosion by direct reaction of exposed metals to oxidizing agents at elevated temperatures | ||
| Sulfidation | |||
| Carburization | |||
| Other forms | |||
| Localized Corrosion: all or most of the metal loss occurs at discrete areas | Filiform | occurs on metallic surfaces coated with thin organic film, typically .1 mm thick, characterized by the appearance of fine filaments in semi-random directions from one or more sources | |
| corrosion in narrow openings or spaces in metal to metal or non-metal to metal component sites | |||
| extremely localized corrosion marked by the development of pits | |||
| cases where biological organisms are the sole cause or an accelerating factor in the localized corrosion | |||
| Metallurgically influenced corrosion: form of attack where metallury plays a significant role | Intergranular | occurs when the corrosion rate of the grain boundary areas of an alloy exceeds that of the grain interiors | |
| Dealloying | a form of corrosion characterized by the preferential removal of one constituent of an alloy leaving behind an altered residual structure | ||
| Mechanically assisted degradation: form of attack where velocity, abrasion, hydrodynamics etc. play a major role | Erosion | removal of surface material by the action of numerous individual impacts of solid or liquid particles | |
| combined wear and corrosion between contacting surfaces when motion between the surfaces is restricted to very small amplitude oscillations | |||
| Cavitation & Water drop impingement | occurs on a metal surface in contact with a liquid, pressure differentials generate gas or vapor bubbles which upon encountering high-pressure zones, collapse and cause explosive shocks to the surface | ||
| occurs in metals as a result of the combined action of a cyclic stress and a corrosive environment | |||
| Environmentally induced cracking: forms of cracking that are produced in the presence of stress | Stress cracking | service failures in engineering materials that occur by slow environmentally induced crack propagation | |
| results from the combined action of hydrogen and residual or tensile stress | |||
| Liquid metal embrittlement | brittle failure of a normally ductile metal when coated with a thin film of a liquid metal and subsequently stressed in tension | ||
| Solid metal embrittlement | occurs below the melting point of the solid in certain liquid metal embrittlement couples | ||
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