Crevice Corrosion  

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.

Mechanism of Crevice Corrosion

 

Forms of Corrosion

   

Uniform       Galvanic     Crevice      Pitting         Intergranular

    Erosion      Cavitation    S C C     Hydrogen Induced Cracking

    Galvanic Series   Back to Material Science                  

 

 

 

 

 

 

 

 

 

 Uniform Corrosion

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.

           

 

 

 

 

 

 

 

 

 

 

 

Galvanic 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.

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pitting Corrosion  

 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.                                     

Mechanism of Pitting Corrosion

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Intergranular Corrosion 

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 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cavitation Damage

 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. 

                                                               Cavitation Damage

 

Erosion Corrosion 

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

                Erosion Corrosion

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stress Corrosion Cracking

  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.

 

          

 

 

 

 

 

 

 

 

 

 

 

 

 

Hydrogen Induced Cracking

  The permeation of  adsorbed hydrogen into the interior of the metal under  stress, by diffusion is an important necessary step for hydrogen induced cracking. The formation of hydrogen takes place as cathodic reaction against the anodic metal dissolution which releases electron to be taken up by H+ ions to form adsorbed H atoms which diffuse into the metal under the conc. gradient of lower conc. into the metal from the higher      conc. outside it. Hydrogen diffuses through the lattice -- interstitial diffusion, displacing a little some atoms around it. The larger the lattice strain or distortion, larger conc. of H and all imperfections in crystals are regions of distortion or strain. Imperfections include voids. On reaching these regions adsorbed hydrogen atoms combine to form H .Thus pressure of gas builds up inside the void. The pressure of  H2 in equilibrium with atomic H is several thousands atmosphere which is sufficient to rapture any engineering material                                                                                                                                                                                                      

                                                                          Pipe cracked by HIC

   

 

 

                                                                                                                               

 

 

 

 

 

 

 

 

Go To the Top                                Prepared by S Paul