Assignment 2

Types of Corrosion

Aviral Jain CE10B011

Two types of Corrosion1. Galvanic CorrosionGalvanic corrosion occurs when two different metals (alloys) are in electrical contact in an electrolyte. Nobler element is protected and more active element corrodes at higher rate. Galvanic corrosion, resulting from a metal contacting an-other conducting material in a corrosive medium, is one of the most common types of corrosion. It may be found at the junction of a water main, where a copper pipe meets a steel pipe, or in a microelectronic device, where different metals and semiconductors are placed together, or in a metal matrix composite material in which reinforcing materials, such as graphite, are dispersed in a metal, or on a ship, where the various components immersed in water are made of different metal alloys.

2. Crevice CorrosionLocalized corrosion resulting from a concentration difference b/w the electrolyte within the crevice and the electrolyte outside the crevice due to stagnation of electrolyte. Possible concentration difference in O2, H+ (acidity), Cl-.

3. Pitting CorrosionRapid corrosion penetration at small discrete areas where passive film breaks down and appears as small diameter holes (0.1-5 mm). The remaining surface is not attacked. Pitting corrosion is common in passive metals above pitting potential (Ep).

4. Intergranular CorrosionThis is preferential attack of the grain boundaries of the crystals that form the metal. It is caused by the physical and chemical differences between the centres and edges of the grain.

5. Selective leachingCorrosion in which one constituent of an alloy is preferentially removed, leaving behind an altered (weakened) residual structure.

6. Erosion CorrosionAn increase in corrosion brought about by a high relative velocity between the corrosive environment and the surface. Removal of the metal may be:

• As corrosion product which "spalls off" the surface because of the high fluid shear and bares the metal beneath;

• As metal ions, which are swept away by the fluid flow before they can deposit as corrosion product.

7. Stress CorrosionStress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses.

Crevice CorrosionCrevice corrosion is a type of localized corrosion that occurs in the small gap between two pieces of overlapping metal. Crevice corrosion could be general corrosion, pitting corrosion or both. This form of attack is generally associated with the presence of small volumes of stagnant solution in occluded interstices, beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. Deposits of sand, dust, scale and corrosion products can all create zones where the liquid can only be renewed with great difficulty. This is also the case for flexible, porous or fibrous seals (wood, plastic, rubber, cements, asbestos, cloth, etc.)

Crevice corrosion is encountered particularly in metals and alloys which owe their resistance to the stability of a passive film, since these films are unstable in the presence of high concentrations of Cl- and H+ ions.

The basic mechanism underlying crevice corrosion in passivatable alloys exposed to aerated chloride-rich media is gradual acidification of the solution inside the crevice, leading to the appearance of highly aggressive local conditions that destroy the passivity.  In an interstice, convection in the liquid is strongly impeded and the dissolved oxygen is locally rapidly exhausted. A few seconds are sufficient to create a "differential aeration cell" between the small de-aerated interstice and the aerated remainder of the surface. However, "galvanic" corrosion between these two zones remains inactive.As dissolution of the metal M continues, an excess of Mn+ ions is created in the crevice, which can only be compensated by electro-migration of the Cl- ions (more numerous in a chloride-rich medium and more mobile than OH- ions). Most metallic chlorides hydrolyze, and this is particularly true for the elements in stainless steels and aluminum alloys. The acidity in the crevice increases (pH 1-3) as well as the Cl- ion concentration (up to several times the mean value in the solution). The dissolution

reaction in the crevice is then promoted and the oxygen reduction reaction becomes localized on the external surfaces close to the crevice. This "autocatalytic" process accelerates rapidly, even if several days or weeks were necessary to get it under way.Means of preventing or limiting crevice corrosion : Use welds rather than bolted or riveted joints, design installations to enable complete draining (no corners or stagnant zones), hydrofuge any interstices that cannot be eliminated, and in particular, grease all seals and seal planes, use only solid, non-porous seals, etc.

Microbially induced Corrosion (MIC)Microbial corrosion, also called bacterial corrosion, bio-corrosion, microbiologically influenced corrosion, or microbially induced corrosion (MIC), is corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and non-metallic materials. Studies have implicated various organisms in microbial induced corrosion including, iron oxidizing bacteria such as Gallionella, Leptothrix (and related genera) and Thiobacillus (and relatives) as well as sulfate reducing bacteria such as Desulfobacter (and relatives). 

There are biological organisms (microbes) which influence corrosion. The primary, and to many, the only concern is that this "influence" often results in an extremely accelerated rate of corrosion. It affects most alloys, such as ductile iron, steel (including stainless and galvanized), and copper, but it doesn't seem to affect titanium. The affect does vary between the different alloys with ductile iron corroding slower than steel. There was also a case where MIC caused the water in copper pipe to turn blue.

MIC is not caused by a single microbe, but is attributed to many different microbes. These are often categorized by common

characteristics such as by-products (i.e., sludge producing) or compounds they effect (i.e. sulfur oxidizing). In a general sense, they all fall into one of two groups based upon their oxygen requirements; one being aerobic (requires oxygen) such as sulfur oxidizing bacteria, and the other being anaerobic, (requires little or no oxygen), such as sulfate reducing bacteria.

General corrosion affects the entire surface or at least the wetted surface. MIC, on the other hand, is very localized. It creates a nodule and a pit beneath the nodule. There can be only a few nodules or there can be many. Within these nodules microbes rarely work alone but operate as a mixed community of differing types and groups. The different microbes perform different functions within the community. This interaction allows a community to thrive in environments that are actually hostile to some of its members. For example, in an aerobic environment, anaerobic bacteria are generally inhibited or killed. But within a community the aerobic bacteria reside in the outer layer of the nodule consuming the oxygen in the water as it penetrates the nodule. Thus, the inner portion of the nodule experiences a reduced oxygen level allowing anaerobic bacteria to thrive.

How MIC WorksAs already identified, MIC operates as an individual nodule covering a pit. The development of this process occurs in three phases, which are:

Attachment of microbes. Growth of nodule and initial pit. Mature pit and nodule.

Phase One is depicted by Figure 1. Obviously, for MIC to occur the microbes must be introduced into the sprinkler system. Even though a nodule can contain many different bacteria, they do not necessarily arrive and/or thrive simultaneously. In order for microbes to attach themselves to the inner wall of the pipe, the bacteria must find a desirable site. Such sites are defined as containing absorbed nutrients and having a metallurgical feature that the microbes can attach to. These features seem to be critical for MIC to occur and consist of irregularities in the pipe surface such as from welded connection, pipe seams, pre-existing corrosion, inclusions, etc.

After successfully attaching to a location, Phase Two starts. As shown in Figure 2, a lot occurs at this stage. (Actually, only a fraction of the activity is shown, since there can be an immense amount of chemical interaction occurring). Among the microbe's by-products are sticky polymers which retain organic and inorganic materials aiding in the creation of the nodule. Once the nodule is formed, it allows the underlying conditions to become

chemically dissimilar to the surrounding surface. This is the start of accelerated corrosion, which initially leads to crevice corrosion. Some of the characteristics of the community at this phase are: microbes are located throughout the nodule and the pH level is lowered (acidic) within the crevice, but it is still above 4. This lower pH adds to the corrosiveness of the environment, as well as stimulating the growth of certain types of bacteria. The increased acidic level is commonly initiated by acid-producing bacteria which produce organic acids as a by-product. This acid provides a nutrient source for other bacteria whose by-product results in a buildup of hydrogen protons and an even lower pH level.

In the final phase, as depicted by Figure 3, there is continued formation of the nodule over a mature pit. Such a pit not only increases in depth but also produces a tunneling characteristic. A significant condition is that the pH is less than 4. One of the factors which can contribute to the high acidic level is the weak organic acids discussed in Phase Two. These can be converted to a stronger acid by combining with chloride from the water, thus producing hydrochloric acid. As a result of the high acidity in the pit area, live bacteria are present only in the outer portion of the nodule. At this point, the bacteria could be eliminated and the corrosion would continue as a traditional electrochemical corrosion process. Because of a better understanding of the final phase, whereby the presence of the bacteria is not required, the name was changed from microbiological "induced" corrosion to microbiological "influenced" corrosion.

Hydrogen EmbrittlementHydrogen embrittlement is a physical-chemical-metallurgical phenomenon. It is developed by chemical interaction and diffusion in the metal. Depending on the metallurgical nature of the substrate where it takes place may cause critical changes in the capacity of load transmission of the elements under stress. This can lead to catastrophic fracture at lower loads which supports the material under normal conditions.Even if this phenomenon takes place preferentially in carbon or low-alloy, other metals and alloys are susceptible to this kind of damage. The hydrogen embrittlement phenomena, in whatever their form of expression, severely restricts the applicability of certain materials. The interaction between hydrogen and metals, can lead to the formation in the metal of solid solution of hydrogen, molecular hydrogen and gaseous products formed by the reaction between hydrogen and the constitutive metals of the alloy.

The embrittlement of of metal or alloy by atomic hydrogen involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal.

Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions which inject hydrogen into the component. Presently this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys and aluminum alloys.

Sources of HydrogenSources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized below:

a. by some manufacturing operations such as welding, electroplating, phosphating and pickling; if a material subject to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any hydrogen is employed

b. as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”

c. The use of cathodic protection for corrosion protection if the process is not properly controlled.

Referenceshttp://corrosion-doctors.orghttp://en.wikipedia.org/wiki/Hydrogen_embrittlementhttp://microbedetectives.comhttp://www.firesprinkler.org/techserviceshttp://www.cdcorrosion.com/mode_corrosion