Corrosion Primer - What is Rust and Corrosion?
Rust is probably the most familiar kind of corrosion. It is generally classified as atmospheric corrosion, where the oxygen of the atmosphere reacts with the material in question. Most metals, with exception of the noble metals, like gold for example, can be oxidized by such atmospheric oxygen. In the most common case, water vapor must be present before any oxidation can take place. With iron for example, about 40% of relative humidity is needed at normal temperatures before rusting will occur.
In a situation sometimes called concentration cell corrosion, two solutions of different concentrations will set up an electrical potential between them similar to a battery. If oxygen is present in a liquid and if it is replenished continually by contact with air, then the oxygen concentration in this liquid will remain constant. Any liquid that is present in small holes or cracks on a metal surface will not be able to get oxygen from the main bulk of the solution, so when the supply in the holes and cracks are exhausted, no more oxygen can enter and replace it. Therefore, the oxygen concentration in the cracks is different from the main bulk and a concentration cell is set up. This minute electrical effect is sufficient enough to make corrosion proceed quite rapidly.
A similar type of corrosion is called two-metal corrosion. Two different metals in contact will set up an electrical potential between them. If the two metals are surrounded by electrolytes so a closed circuit is made, corrosion takes place. The speed and extent at which reaction takes place depends on the types of metals. Generally, metal pairs farther apart in nobility will corrode faster than those closer together.
It is evident that oxygen and hydrogen play an important part in metal corrosion. It can accelerate corrosion by participating in cathodic reactions, or it can retard corrosion by forming a protective film. The dual effect of oxygen is one of the factors that complicates corrosion processes. Some forms of corrosion are pitting resulting from local action currents, stress 'corrosion cracking from environmental and internal stresses. around the metal.
What is corrosion?
Rust is corrosion, but not all corrosion is rust. Corrosion is the deterioration of a material, usually a metal, that results from a reaction with its environment. For corrosion to take place, four components must be present:
- anode
- cathode
- electrical or mechanical connection between the anode and cathode
- electrolyte with allows the electrons to move between the anode and cathode.
The four most common methods used to control corrosion are:
1. Protective coatings and linings. Coatings and linings are principal tools for defending against corrosion. They are often used in conjunction with cathodic protection systems to provide the most cost effective protection for the structure. Coatings and linings help protect against corrosion in three ways:
a. They provide a barrier to prevent or limit contact between a structure's metal surface or components and its corrosive environment;
b. They release substances that inhibit the corrosion process and protect the structure from deteriorating; and
c. They serve as sacrificial materials, such as when galvanizing is used.
To be effective, protective coatings and linings must be properly selected and installed by personnel trained in surface preparation and application of the material selected.
2. Cathodic Protection
Cathodic Protection (CP) is a technology which uses direct electrical current to counteract the normal external corrosion of a structure that contains metal, such as a boat or a ship with steel reinforcing components. The term "cathodic" refers to the area of the metal where corrosion is controlled, as opposed to the anodic areas where corrosion occurs. The principle behind Cathodic Protection is to make the entire surface of a structure behave like a cathode with respect to an external anode. This behavior is induced by installing sacrificial materials to serve as anodes or by applying an external direct current power source in conjunction with anodes. On new structures, Cathodic Protection can help prevent corrosion from starting; on existing structures, Cathodic Protection can help stop existing corrosion from getting worse.
Effective CP system design will take into account variables such as:
- variations in the environment surrounding a structure;
- he presence of protective coatings and linings;
- the metal to protected; the expected useful life of the structure;
- the ability to maintain the Cathodic Protection system;
- the total electrical current required for protection;
The costs of installing and maintaining Cathodic Protection must be considered in context of the direct expenses associated with replacement of corroded structures and possible structural failure, as well as indirect costs such as environmental damage. Installing Cathodic Protection on any infrastructure can be very costly.
3. Materials Selection
Materials selection refers to the selection and use of corrosion-resistant materials such as stainless steels, plastics, and special alloys to enhance the life span of a product. Materials selection personnel consider the environment in which the product will exist and the desired life span. If more than one material is used, such as two metals joined together, controlling corrosion requires that the materials have compatible electrochemical properties. The two most common materials used in consumer products is steel and aluminum, which can be severely affected by corrosion.
4. Inhibitors, Vapor Corrosion
Vapor Phase Corrosion Inhibitors (VpCIs) are substances which, when added to a particular environment, decrease the rate of attack of that environment on material such as metal. VpCI's are commonly added in small amounts to liquids such as acids, cooling waters, and steam, either continuously or intermittently, to prevent serious corrosion. Inhibitors can stop or retard corrosion in many ways, such as "adsorption", forming films to coat materials at a molecular level and protect them from attack, and creating a "passive" layer on the surface of a material which inhibits further deterioration. VpCI's can: extend the life of equipment; prevent system shutdowns and failures; avoid product contamination; prevent loss of heat transfer; and preserve an attractive appearance of products.
Evaluating the environment in which a structure is or will be located is very important to corrosion prevention, no matter which control method is used. Modifying the environment immediately surrounding a structure, such as reducing moisture or improving drainage, can be a simple and effective way to reduce the potential for corrosion.
corrosion tour - prevent & protect >>>>