CORROSION:- TYPES EFFECTS & PREVENTION PRESENTED BY: GAUTAM AHUJA 1207535

What is Corrosion? Corrosion is the deterioration of materials by chemical interaction with their environment.  The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.

TYPES OF CORROSION:- Uniform corrosion. Galvanic corrosion. Pitting & Crevice corrosion. Stress corrosion cracking. Erosion corrosion. Microbiologically induced corrosion.

UNIFORM CORROSION:- Uniform (or general) corrosion refers to the relatively uniform reduction of thickness over the surface of a corroding material. It is relatively easy to measure, predict and design against this type of corrosion damage. While uniform corrosion may represent only a small fraction of industrial corrosion failures, the total tonnage wasted is generally regarded as the highest of all forms.

GALVANIC CORROSION:- Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte. When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.

PITTING & CREVICE CORROSION:- Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting is one of the most damaging forms of corrosion. Pitting corrosion forms on passive metals and alloys like stainless steel.The resulting pits can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal.

STRESS CORROSION CRACKING:- It 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. Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material.

EROSION CORROSION:- Erosion 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 surfaces of a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also be aggravated by faulty workmanship.

MICROBIAL CORROSION:- Microbial corrosion or biological corrosion, is the deterioration of metals as a result of the metabolic activity of microorganisms.There are about a dozen of bacteria known to cause microbial corrosion of carbon steels, stainless steels, aluminum alloys and copper alloys in waters and soils with pH 4~9 and temperature 10oC~50oC. These bacteria can be broadly classified as aerobic & anaerobic. Sulphate reducing bacteria (SRB) is anaerobic and is responsible for most instances of accelerated corrosion damages to ships and offshore steel structures. Iron and manganese oxidizing bacteria are aerobic and are frequently associated with accelerated pitting attacks on stainless steels at welds.

EFFECTS OF CORROSION:- Reduced Strength Downtime of equipment Escape of fluids Lost surface properties Reduced value of goods Economic losses The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal.  Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. 

CORROSION PREVENTION:- Electrically insulate the two metals from each other. Unless they are in electrical contact, there can be no galvanic couple set up. Coating the two materials or if it is not possible to coat both, the coating shall be applied to the more noble, the material with higher potential. Another way is to keep the metals dry and/or shielded from ionic compounds (salts, acids, bases), for example by painting or encasing the protected metal in plastic or epoxy, and allowing them to dry. Proper selection of materials with known resistance to the service environment . Control pH, chloride concentration and temperature Cathodic protection and/or Anodic Protection. Regular mechanical cleaning if possible. Chemical treatment with biocides to control the population of bacteria. Complete drainage and dry-storage.

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