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What is corrosion Corrosion can be defined as the reaction of a material with its environment. The problem of corrosion arises in various environments ranging from urban and marine atmospheres to industrial chemical plant installations. It is a major factor governing the design and operation of plant and equipment as it reduces their useful life and can often result in unscheduled shutdowns or, in some cases, it cause failure. The control of corrosion presents a considerable challenge to engineers and, in spite of our best efforts, the annual costs of corrosion damage and corrosion related service failures run into many millions of pounds. However, there is scope to reduce this cost burden by making improvements in materials selection, methods of protection, design and in- service monitoring. In aqueous environments, corrosion may occur as uniform (general) or non- uniform (local) attack. Uniform corrosion results in general wastage, is reasonably easy to inspect and to predict from weight loss experiments or electrochemical data. Local corrosion can take a number of various forms and is much less predictable. It can result in more serious damage to structures.

Corrosion Mechanisms  Modern corrosion science has its roots in electrochemistry and metallurgy. Electrochemistry contributes an understanding of the mechanism that is basic to the corrosion of all metallic objects. Metallurgy provides knowledge of the characteristics of metals and their alloys as well as the methods of combining the various metals and working them into the desired shapes. Corrosion can proceed by several different mechanisms, including:  · Rusting  · Pitting  · Galvanic attack  · Intergranular attack  · Leaching (selective corrosion)  · Corrosion and erosion  · Stress corrosion cracking (SCC)  · Corrosion fatigue  · Hydrogen damage

Corrosion Types - Corrosion Forms  Uniform Corrosion  Uniform Corrosion  Concentration Cell Corrosion  Intergranular Corrosion  Atmospheric Corrosion  Fretting Corrosion  Dealloiyng  Microbiological Corrosion - MIC  Galvanic Corrosion or Bimetallic Corrosion  Pitting Corrosion - Crevice Corrosion  Stress Corrosion Cracking  Cavitation Corrosion  Hydrogen Embitterment  Iimmunity  Corrosion Fatigue

UNIFORM ATTACK  This is the most common form of corrosion.  A chemical reaction (or electrochemical reaction) occurs over entire exposed surface (or large areas) more or less uniformly.  Not usually serious and is typically predictable from simple tests.  Can be designed “around” by specifying an adequate CORROSION ALLOWANCE for the expected lifetime of the component.

Concentration Cell Corrosion  Concentration cell corrosion is corrosion that is accelerated by differences in environment between separated areas on a single metal  A difference in environment between sites on a single metal can also result in increased electrochemical activity.

Intergranular Corrosion  Intergranular corrosion is a selective attack of a metal at or adjacent to grain boundaries.  Intergranular attack caused by high grain boundary energies or impurities at the grain boundaries results in attack with a grainy residue and rough surface. Under high magnification, the individual grains are often visible. Intergranular attack of aluminum alloys is associated with pitting or other localized attack.

Fretting Corrosion  Fretting corrosion is an attack that is accelerated by the relative motion of contacting surfaces. 1. Fretting was common in riveted joints on ships and other riveted structures where cyclic loads were experienced, but this has largely been eliminated through welded construction. Fretting is, however, still encountered in bolted joints and flanges where there is not enough bolt tension to eliminate movement in the joint. Thermal expansion with frequent cycling can also result in fretting attack. Any combination of corrosion and wear will almost always be worse than the action of either one separately.

Dealloying Corrosion  Dealloying is the selective corrosion of one or more components of a solid solution alloy. It is also called parting, selective leaching or selective attack. Common dealloying examples are decarburization, decobaltification, denickelification, dezincification, and graphitic corrosion or graphitization. Graphitic corrosion of a gray cast iron valve

Bimetallic Corrosion  Bimetallic corrosion is a localised mechanism by which metals can be preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another.

Pitting & Crevice Corrosion  What is Pitting Corrosion? Under certain specific conditions, particularly involving chlorides (such as sodium chloride in sea water) and exacerbated by elevated temperatures, small pits can form in the surface of the metal. Dependent upon both the environment and the metal itself these small pits may continue to grow, and if they do can lead to perforation, while the majority of the metal surface may still be totally unaffected. Under certain specific conditions, particularly involving chlorides (such as sodium chloride in sea water) and exacerbated by elevated temperatures, small pits can form in the surface of the metal. Dependent upon both the environment and the metal itself these small pits may continue to grow, and if they do can lead to perforation, while the majority of the metal surface may still be totally unaffected. What is Crevice Corrosion? What is Crevice Corrosion? Crevice Corrosion can be thought of as a special case of pitting corrosion, but one where the initial "pit" is provided by an external feature; examples of these features are sharp at corners, overlapping metal surfaces, non-metallic gaskets or incomplete weld penetration. To function as a corrosion site a crevice has to be of sufficient width to permit entry of the corrodent, but sufficiently narrow to ensure that the corrodent remains stagnant. Accordingly crevice corrosion usually occurs in gaps a few micrometers wide, and is not found in grooves or slots in which circulation of the corrodent is possible

Stress Corrosion Cracking  Stress corrosion cracking is the intergranular or transgranular cracking of a material due to the combined action of tensile stress and a specific environment. 

Corrosion rate vs. Pressure

Cavitation Corrosion  Cavitation corrosion is corrosion that is enhanced through the formation and collapse of gas or vapor bubbles at or near the metal surface.  Under high velocity flow conditions, particularly when the flow is turbulent, areas is high and low pressure will be induced. In areas of low pressure, gas and vapor bubbles will be produced. When these bubbles move to an area of higher pressure, they collapse and their implosion creates a pressure wave that can remove protective films and cause increased corrosion.

Hydrogen Embrittlement  Hydrogen embrittlement is the severe loss of ductility of a metal when hydrogen has been introduced into the metal structure. Hydrogen can enter most metals. Due to the small size of the hydrogen atom, it can migrate through the metal structure and cause a loss of ductility similar to that experienced in stress corrosion cracking. Hydrogen atoms can enter a metal either from hydrogen gas, usually at elevated temperatures, or from atomic hydrogen that is electrolytically formed on its surface. This hydrogen can either reduce the energy required for forming cracks under stress or can accumulate at areas of high stress, such as crack tips, and cause pressure

Immunity  Immunity is the lack of measurable attack on a metal when exposed to operational environments. The first form of corrosion described is the lack of attack, or immunity. This can result from the action of two basic mechanism. Corrosion test measurements that are used to measure very low corrosion rates must be used to validate that corrosion activity is completely absent. Immunity can result from two basic mechanisms. In the first case, the energy content of the metal is lower (more stable) than any of the corrosion products that could possibly form. Such metals are commonly found in nature as metals that indicates the stability of the metallic state for these elements.

Corrosion rates vs. temperature

Corrosion Fatigue  Corrosion fatigue is the reduced ability of a metal to withstand repeated stress when exposed to the combined action of stress and a corrosive environment as compared to the effects of stress alone. Many materials will exhibit a substantial reduction in fatigue life when exposed to a corrosive environment. In some cases, the reduction is severe, in other cases it is less dramatic, but only a very few materials show a fatigue resistance in a corrosive environments as great as that in dry air.

Corrosion rate vs. velocity

Corrosion Control & Corrosion Protection There are four basic methods for Corrosion Control & Corrosion Protection. 1. Materials resistant to Corrosion 2. Protective coatings 3. Cathodic protection 4. Corrosion Inhibitors- Modify the operating environment. Ways to protect metal from corrosion: There are four basic methods for Corrosion Control & Corrosion Protection. 1. Materials resistant to Corrosion 2. Protective coatings 3. Cathodic protection 4. Corrosion Inhibitors- Modify the operating environment. Ways to protect metal from corrosion: 1. Galvanization is method of protecting steel & iron from rusting by coating them with a thin layer of zinc. The galvanised is protected against rusting even if the zinc coating is broken. 2. Painting, greasing & oiling are some commonly used methods to prevent rusting. 3. Chrome plating, anodising & making alloys are some other ways to prevent corrosion of metals.

Methods of preventing the atmospheric corrosion of metals fall into two broad categories: categories: i) The choice of a suitably resistant metal or alloy or combination of materials, or the provision of a protective coating which supplements or enhances the protection given by the air-formed oxide film on the metal surface. ii) The control of the environment by the exclusion of water or aggressive contaminants, or by the introduction of a corrosion inhibitor. Tin plating, Galvanization, Coating, Rubber paints Copper plating

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