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 can either be hemispherical or cup shaped.
Often times pitting is initiated by localized chemical or mechanical damage to the protective oxide film; 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) causing localized damage. Poor application of a protective coating or the presence of non-uniformities in the metal structure of the component have also 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 tend to occur in crevices. This type of corrosion is initiated by changes in local chemistry within the crevice:
- Depletion of inhibitor in the crevice
- Depletion of oxygen in the crevice
- A shift to acid conditions in the crevice
- Build up 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 crevice (micro environment) and the external surface (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 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 are generally undesirable as are designs that create turbulence, flow restrictions and obstructions. Abrupt changes in flow direction should be avoided.
STRESS CORROSION CRACKING (SCC), 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 SCC 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 is often underestimated. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked.
Usually most of the surface remains unattacked, but with fine cracks penetrating into the material. Macriscopically, SCC fractures have a brittle appearance. SCC is classified as a catastrophic form of corrosion, as 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 a 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
- Providing against 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
METHODS OF CONTROL
Materials Selection and Design. 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, cost, availability, maintainability, compatibility with other system components, life expectancy, reliability and appearance.
Appropriate system design is also important for effective corrosion control, and includes the consideration of many factors, such as 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, and maintenance and inspection requirements.
Protective Coatings. The total annual U.S. cost for organic and metallic protective coatings is $108.6 billion. 50% of all corrosion costs are preventable, and approximately 85% of these are in the area of coatings. Companies depend on technological expertise as well as a wide variety of applications, including coatings for corrosion resistance.
Cathodic & Anodic Protection & Corrosion Inhibitors. A corrosion inhibitor reduces the corrosion rate of metal exposed to that environment. Inhibition is used internally with carbon steel pipes and vessels as an economic corrosion control alternative to stainless steel and alloys, coatings, or non-metallic composites, and can often be implemented without disrupting a process.
Contact your U.S. Water Representative for a plant audit to identify potential corrosion areas and the proper treatment plan for your plant.