Corrosion is a naturally occurring phenomenon commonly defined as the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment. Corrosion can cause dangerous and expensive damage to water and wastewater systems, pipelines, bridges and public buildings. According to a current corrosion study, it costs the United States approximately $276 billion on an annual basis. Production and manufacturing industries are widely affected by corrosion, with pulp-and-paper corrosion costing approximately $6 billion per year, followed by oil-and-gas exploration at $4.1 billion, food processing at $2.1 billion and mining at $0.1 billion. Fortunately, there are time-proven methods to prevent and control corrosion, reducing or eliminating its impact.
The science of corrosion prevention and control is highly complex, exacerbated by the fact that corrosion takes many different forms and is affected by numerous outside factors. Corrosion professionals must understand the effects of:
- Environmental conditions.
- Type of product to be processed or handled.
- Required lifetime of the structure or component.
- Proximity to corrosion-causing phenomena.
- Appropriate mitigation methods.
Corrosion professionals must take into account the above-mentioned affects as well as other considerations before determining the specific corrosion problem and specifying an effective solution.
There are 10 primary forms of corrosion, but it is rare that a corroding structure or component will suffer from only one. The combinations of metals used in a system — and the wide range of environments encountered — often cause more than one type of attack. Even a single-cell alloy can suffer corrosion from more than one form, depending on its exposure to different environments at different points within the system. Except for some types of high temperature corrosion, all forms of corrosion occur through the action of the electrochemical cell.
The elements that are common to all corrosion cells are:
- An anode, where oxidation metal loss occurs.
- A cathode, where reduction and protective effects occur.
- Metallic and electrolytic paths between the anode and cathode through which electronic and ionic current flows.
- A potential difference that drives the cell.
The driving potential may be the environment, including chemical concentrations. There are specific mechanisms that cause each type of attack; different ways of measuring and predicting them; and various methods that can be used to control corrosion in each of its forms.
Uniform corrosion is characterized by a corrosive attack occurring evenly over the entire surface area of a large fraction of the total area. General thinning takes place until failure. On the basis of tonnage wasted, this is the most important form of corrosion.
However, uniform corrosion is fairly easily measured and predicted, making disastrous failures relatively rare. The breakdown of protective coating systems on structures often leads to this form of corrosion. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of surface corrosion. If surface corrosion is permitted to continue, the surface may become rough, and surface corrosion can lead to more serious types of corrosion.
Pitting corrosion is a localized form of corrosion by which cavities or holes are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Corrosion products often cover the pit. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Pitting corrosion may assume different shapes, producing pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits either can be hemispherical or cup shaped.
Oftentimes, pitting is initiated by localized chemical or mechanical damage to the protective oxide film such as water chemistry factors, which can cause breakdown of a passive film like acidity, low-dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater). Poor application of a protective coating or the presence of non-uniformities in the metal structure of the component also has been known to cause pitting corrosion.
Apart from the localized loss of thickness, corrosion pits can be harmful by acting as stress risers. Fatigue and stress corrosion cracking may initiate at the base of corrosion pits. One pit in a large system can be enough to produce the catastrophic failure of that system.
Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environment level, which tends to occur in crevices. This type of corrosion is initiated by changes in local chemistry within the crevice, including:
- Depletion of inhibitor in the crevice.
- Depletion of oxygen in the crevice.
- A shift to acid conditions in the crevice.
- Buildup of aggressive ion species in the crevice.
As oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between the crevice (a micro environment) and the external surface (a bulk environment). The cathodic oxygen-reduction reaction cannot be sustained on the crevice area, giving it an anodic character in the concentration cell. This anodic imbalance can lead to the creation of highly corrosive micro-environmental conditions in the crevice, conducive to further metal dissolution.
Galvanic corrosion refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two or more dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would on its own. The other becomes the cathode and corrodes slower than it would alone. Either metal in the couple may or may not corrode by itself. When contact is made, the self-corrosion rates will change: corrosion of the anode will accelerate while corrosion of the cathode will decelerate or stop.
Erosion corrosion is an acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surface of a tube can result in rapidly increasing erosion rates and eventually a leak. A combination of erosion and corrosion can lead to extremely high pitting rates.
Rough surfaces and those designs that create turbulence, flow restrictions and obstructions are generally undesirable. Abrupt changes in flow direction should be avoided.
Stress Corrosion Cracking
Stress corrosion cracking, characterized by the multi-branched lightning bolt crack pattern, is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of stress corrosion cracking on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses.
Cold deformation and forming, welding, heat treatment machining and grinding can introduce residual stresses. The magnitude and importance of such stresses often is underestimated. The buildup of corrosion products in confined spaces also can generate significant stresses and should not be overlooked.
Usually most of the surface does not suffer attack, but fine cracks penetrate into the material. Macroscopically, stress corrosion cracking fractures have a brittle appearance. Stress corrosion cracking is classified as a catastrophic form of corrosion because the detection of such fine cracks can be very difficult, and the damage not easily predicted.
Corrosion fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time.
In a corrosive environment, the stress level at which it could be assumed a material has infinite life is lowered or removed completely. Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue. Much lower failure stresses and much shorter failure times can occur in a corrosive environment compared to the situation where the alternating stress is in a non-corrosive environment.
Protection possibilities include:
- Minimizing or eliminating cyclic stresses.
- Reducing stress concentration or redistributing stress.
- Selecting the correct shape of critical sections.
- Preventing rapid changes of loading, temperature or pressure.
- Avoiding internal stress.
- Avoiding fluttering and vibration producing or transmitting design.
- Increasing natural frequency for reduction of resonance corrosion fatigue.
- Limiting corrosion factor in the corrosion fatigue process.
The total annual U.S. cost for organic and metallic protective coatings is $108.6 billion. Fully 50 percent of all corrosion costs are preventable, and approximately 85 percent of these are in the area of coatings. Companies depend upon technological expertise as well as a wide variety of applications, including coatings for corrosion resistance.
No material is resistant to all corrosive situations, but materials selection is critical to preventing many types of failures. Factors that influence materials selection are:
- Corrosion resistance in the environment.
- Availability of design and test data.
- Mechanical properties.
- Compatibility with other system components.
- Life expectancy.
Appropriate system design also is important for effective corrosion control and includes the consideration of many factors. These include:
- Materials selection.
- Process and construction parameters.
- Geometry for drainage.
- Avoidance or electrical separation of dissimilar metals.
- Avoiding or sealing of crevices.
- Corrosion allowance.
- Operating lifetime.
- Maintenance and inspection requirements.
Finally, a corrosion inhibitor reduces the corrosion rate of a metal exposed to that environment. Inhibition is used internally with carbon steel pipes and vessels as an economic corrosion control alternative to stainless steels and alloys, coatings or nonmetallic composites. It can often be implemented without disrupting a process.
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