Lesson 7. Corrosion and ais classification

7.1 INTRODCTION

Except noble metals (Au and Pt) most other metals exist in nature in combined forms as their sulphide, oxides, silicates, carbonates etc. These are reduced to their metallic states from their ores, during their extraction processes. During extracting of metals, considerable amount of energy is required. Consequently, isolated pure metals can be regarded in exited state then their corresponding ores, and they have a natural tendency to revert back to combined state. Hence, when metals are put in to use in various forms, they are exposed to environment, the exposed metal surfaces begin to decay in to more stable compounds more or less rapidly. When they come in contact with gaseous and/or liquid environment. In other words, destruction or deterioration of metal starts at the surface. This type of the metallic destruction may be due to direct chemical attack or by electrochemical attack.

            Any process of destruction and consequent loss of solid metallic materials, through an unwonted chemical or electro chemical attack by its environment, starting at its surface, is called corrosions.  Thus corrosion is a process “reverse of extraction of metals”.

            The mos familiar examples of corrosion are rusting of iron, when expose to the atmospheric conditions. During this, a layer of reddish scale and powder of oxide (Fe3O4) is formed, the iron becomes weak. Another common example is formation of green film of basic carbonate (CuCO3 + Cu(OH)2)    on the surface of copper, when expose to moist air containing carbon dioxide.

                Gravidity of corrosion problem; the process of corrosion is slow and occurs only at metal surfaces, but the lows incurred due to corrosion, are enormous weast or destruction of machines, equipments and different type of metallic products. Losses occurring, due to corrosion cannot be measured in turns of the cost of metals alone, but the high cost of fabrication into equipments- machine tools   structure should be considered.           

7.2  CLASSIFICATION OF CORROSION

It is convenient to classify the corrosion by the forms in which it manifests itself, the bases of this classification being the appearance of the corroded metal. Each form can be identified by mere visual observation. In most cases the naked eye is sufficient, but sometimes magnification is helpful or required. Valuable information for the solution of a corrosion problem can often be obtained through careful observation of the corroded test specimens or failed equipment. Examination before cleaning is particularly desirable.

            Some of the eight forms of corrosion are unique, but all of them are more or less inter -related. The eight forms are: 1. Uniform or general attack, 2. Galvanic or two metal corrosion, 3. Crevice corrosion, 4. Pitting, 5. Intergranular corrosion,  6. Selective leaching or parting, 7. Erosion corrosion,  8. Stress corrosion . This listing is arbitrary but covered practically all corrosion failure and problems. The forms are not listed in any particular order of importance.

            Below, the eight forms of the corrosion are discussed in terms of their characteristics, mechanism and preventive measures. Hydrogen damage, though not a form of corrosion, often occurs indirectly as a result of corrosive attack.

7.2.1UNIFORM ATTACK

            Uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or over a large area. The metal becomes thinner and eventually fails. For example, a piece of steel or zinc immersed in dilute sulphuric acid will normally dissolve at a uniform rate over its entire surface. A sheet iron roof will show essentially the same degree of rusting over its entire outside surface.

Uniform attack, or general overall corrosion, represents the greatest destruction of metal on a tonnage basis. This form of corrosion, however, is not of too great concern from the technical stand point, because the life of equipment can be accurately estimated on the basis of comparatively simple tests. Merely immersing specimens in the fluid involved is often sufficient.   Uniform attack can be prevented or reduced by (1) proper materials, including coatings, (2) inhibitors, or (3) cathodic protection. These expedients, which can be used singly or in combination. Most of the other forms of corrosion are insidious in nature and are considerably more difficult to predict. They are also localized; attack is limited to specific areas or parts of a structure. As a result, they tend to cause unexpected or premature failures of plants, machines or tools.

7.2.2 GALVANIC CORROSION

A potential difference usually exists between two dissimilar metals when they are immersed in a corrosive or conductive solution. If these metals are placed in contact (electrically connected), this potential difference produced electron flow between them. Corrosion of the less corrosion resistant metal is usually increased and attack of the more resistant material is decreased, as compared with the behaviour of these metals when they are not in contact. The less resistant metal becomes anodic and the more resistant metal cathodic. Usually the cathode or cathodic metal corrode very little or not at all in this type of couple. Because of the electric currents and dissimilar metals involved, this form of corrosion is called galvanic or two metals corrosion. It is electrochemical corrosion, but we shall restrict the term galvanic to dissimilar metal effects for purposes of clarity.

The driving force for current and corrosion is the potential developed between the two metals. The so called dry cell battery is a good example of this point. The carbon electrode acts as a noble or corrosion resistant metal – the cathode- and the Zinc as the anode, which corrode. The moist paste between the electrodes is the conductive (and corrosive) environment that carries the current. Magnesium may also be used as the anodic material or outer case.    

7.2.3 CREVICE CORROSION

Intensive localized corrosion frequently occurs within crevices and to other shielded areas on metal surfaces exposed to corrosives. This type of attack is sally associated with small volumes of stagnant solution cased by holes, gasket surfaces, lap joints, surface deposits, and crevices under bolt and rivet heads. As a result, this form of corrosion is called crevice corrosion or sometimes, deposit or gasket corrosion.

7.2.4 PITTING:

Pitting is a form of extremely localized attack that results in holes in the metal. These holes may be small or large in diameter, but in most cases they aree relatively small. Pits are sometimes isolated or so close together that they look like a rough surface. Generally a pit may be described as a cavity or hole with the surface diameter about the same as or less than the depth. Pitting is one of the most destructive and insidious forms of corrosion it causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. It is often difficult to detect pits because of their small size and because the pits are often covered with corrosion products. In addition, it is difficult to measure quantitatively and compare the extent of pitting because of the varying depths and numbers of pits that may occur under identical conditions. Pitting is also difficult to predict by laboratory test. Sometime the pits require a long time to show up in actual service. Pitting is particularly vicious because it is a localized and intense form of corrosion, and failures often occur with extreme suddenness.

7.2.5 INTERGRANULAR CORROSION

The more reactive nature of grain boundaries effect is of little or no consequence in most applications of uses of metals. If a metal corrodes, uniform attack results since grain boundaries are usually only slightly more reactive then the matrix. However under certain conditions, grain interfaces are very reactive and intergranular corrosion results. Localized attack at an adjacent to grain boundaries, with relatively little corrosion of the grains is intergranular corrosion. The alloy disintegrates and / or loses its strength. Intergranular corrosion can be impurity at the grain boundaries, enrichment of one of the alloying elements or depletion of one of these elements in the grain boundary areas. Small amount of iron in aluminium, where in the solubility of iron is low, have been shown to segregate in the grain boundaries and cause intergranular corrosion. It has been shown that based on surface tension considerations the zinc content of a brass is higher at the grain boundaries. Depletion of chromium in the grain boundary region results in intergranular corrosion of stainless steel.

7.2.6 SELECTIVE LEACHING

Selective leaching is the removal of one element from a solid alloy by corrosion processes the most common examples is the selective removal of zinc in brass alloy (Dezincification). Similar process occur in other alloy systems in which aluminium, iron, cobalt, chromium and other elements are removed. Selective leaching is the general term that describes the processes, and its use precludes the creation of terms such as dealluminiumification, decobaltification  etc. Parting is a metallurgical term that is some time applied but selectively leaching is proffered.

 7.2.7 EROSION CORROSION

Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface. This movement is quiet rapid, and mechanical wear effects or abrasion are involved. Metal is removed from the surface as dissolved iron, or it forms solid corrosion products that are mechanically swept from the metal surfaces. Sometimes movement of the environment decreases corrosion, practically when localized at attack occurs under stagnant conditions, but this is not erosion corrosion because deterioration is not increased. Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys and usually exhibits a directional pattern. For example: heat exchanger tube handling water, failure because of erosion corrosion occurs in relatively short time and they are unexpected largely because evaluation corrosion tests were run under static conditions or because the erosion effects were not considered. Most metals and alloy are susceptible to erosion corrosion damage. Many depend upon the development of surface film of some sort (passivity) for resistance to corrosion. Examples are aluminium, lead, and stainless steels. Erosion corrosion results when these protective surfaces are damaged or worn and the metal and alloy are attacked at a rapid rate. Metals that are soft and readily damaged or worn mechanically, such as copper and lead are quiet susceptible to erosion corrosion.

 7.2.8 STRESS CORROSION

Stress corrosion cracking (SCC) refers to cracking caused by the simultaneous presence of tensile stress and specific corrosive medium. Many investigators have classified all cracking failures occurring in corrosive medium as stress corrosion cracking, including failures due to hydrogen embrittlement. These two types of cracking failures respond differently to environmental variables. To illustrate cathodic protection is an effective method of preventing stress corrosion cracking where as it rapidly accelerates hydrogen – emrittlement effects. Hence the importance of considering stress corrosion cracking and hydrogen embrittlement as separate phenomena is obvious. For this reason two cracking phenomena are considered. During stress corrosion cracking the metal or alloy is virtually attacked over most of its surface, while fine cracks progress through it. This cracking phenomenon has serious consequences since it can occur at stresses within the range of typical design stresses. The stresses required for stress corrosion cracking are compared with the total range of strength capability for type 304 stainless steel.

Last modified: Monday, 3 February 2014, 6:36 AM