Crevices are very small openings formed between a metallic surface and rivets or washer or gaskets. Stagnant volume of liquid , say water, is entrapped in the crevice where cathodic reduction reaction 2H2O +O2 +4e = 4OH- ceases to occur after initial consumption of O2 due to restricted convection of from the bulk and only anodic reaction Fe= Fe2+ + 2e occur within the crevice supporting total electron charge balance of the cathodic reaction taking place over the vast areas other than crevices which corrode locally at fast rate with high current density.
It is the uniform degradation of materials from the metallic surface exposed to corrosive environment which has same access to all parts. Atmospheric corrosion is prevalent example of Uniform Corrosion. The other cited example is that of steel in acid solution. It is the greatest destruction of material on tonnage basis , but it is visible and predictable unlike other forms of localized corrosion.
Electrochemical corrosion is caused due to flow of electrochemical current between cathode and anode under a potential difference of Cathode (Noble) potential Ec and anode (active) potential Ea. When two metals or alloys having far apart Ec and Ea are in contact, a large amount of current flows, leading to high corrosion rate preferentially at the junction, giving rise to galvanic or Bimetallic corrosion. The corrosion potential of different metals and alloys in a particular environment are tabulated in a series known as Galvanic series which guides to avoid contacts of materials far apart in the series ,while designing structures.
It is a localized corrosion, producing perforations over the passive metallic surface in contact with aqueous environment containing some aggressive ions such as Cl- ions. Stainless steel which is highly corrosion resistant , because of formation of thin passive film is perforated due to penetration of Cl- ions into the passive film with formation of pits. It is quite unpredictable and may occur at the bottom of ship hull, opening liquid entry into it, over the outer surface of underground pipeline , producing leakage of liquid petroleum products, leading to accidents. Pits may have different shapes depending on metallurgy of the alloy and chemistry of the environment.
Grain boundary is normally slightly more reactive than grain body, but sometimes reactive elements segregate at the grain boundary, giving rise to Intergranular corrosion. When stainless is heated in the temperature range of 425 to 815 C ,Cr ,the passivating element in the steel, migrate from the grain body to the grain boundary with precipitation of chromium carbide Cr23C6 . As a result grain boundary or adjacent regions become less corrosion resistant, causing preferential corrosion at grain boundary, leading to detachment of grains out of the surface. When two stainless steels plates are welded, regions away from welding zone may get heated up in the above mentioned temperature range, leading to Intergranular Corrosion
According to Bernoulli's Principle, if the velocity of the fluid flowing horizontally is very high, the pressure becomes very low. When a liquid such water flows over the turbine, impeller or through the pipe at very high velocity, the pressure of the liquid may decrease to such a level that it may start boiling at room temperature and form large number of very small bubbles which are short lived and grow in size and burst , producing hammering action over surface of passive metal oxide.
It is a conjoint action of mechanical force and corrosive environment and arises due to relative movement between a fluid such as water containing suspended solids like sand and a metallic component such as pipe or turbine blades. The impact of high velocity fluid breaks the oxide layer already formed over the metal surface. Repassivation takes place at exposed metal sites and again the protective oxide layer are eroded out by impact of liquid with suspended solids. Breaking and reformation of oxide layer continues with appearance of groovy surface of erosion corrosion having a directional pattern in accordance with the direction of fluid flow
A structure under static tensile stress, much below the yield stress, in contact with corrosive environment may fail due to SCC. Three conditions must be present simultaneously to produce SCC: a critical corrosive environment, a susceptible alloy and some component of tensile stress. Environmental species are often specific to the alloy system and may not have the same effect on other alloys. Hot aqueous chloride solutions readily cracks stainless steel but not carbon steels, aluminum or other non ferrous alloys. Level of the stress may be as low as 10% of Y.S. and may arise in fabricated structures such as hull of a ship from bolting or fastening, uneven differential cooling after welding , giving rise to residual stresses or corrosion products may even act as stress raiser. Crack morphology is brittle transgranula or intergranular branched sharp tip. Electrochemical effects play important role in initiation of cracking. Cracks may initiate at pits due to stress concentration.
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