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Forms of corrosion (Macroscopic (When dissimilar metallic materials are…

- - - - Galvanic Corrosion
        
        an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.
        
        Factors affecting
        
        Ratio of the cathodic to anodic areas
        
        Current density
        
        Environment
        
        Moisture, soils or salts will induce corrosion
        
        Electrode potential
        
        Ecathode–Eanode=small
        
        corrosion potentials difference between two metals/alloys hsve to below 250mV
        
        Control
        
        Small potential difference
        
        Low current density (ratio of cathodic to anodic areas)
        
        Insulate dissimilar metals
        
        Applied coating
        
        Add inhibitors
        
        Avoid threaded joints
        
        Design for the use of readily replaceable anodic parts
        
        Install a third metal that is anodicto both metals
        
        Mechanism
        
        When two metals come in contact in the presence of an electrolyte, the more reactive metal acts as an anode whereas the less reactive metal acts as a cathode. The electrolyte offers a channel for the movement of particles, which in turn leads to the rapid eroding of the anodic metal.
  - - - Crevice Corrosion
        
        a localized attack on the material, which is usually associated with a stagnant solution on the microenvironment level
        
        Control
        
        Avoid riveted or bolted joint
        
        Close crevices
        
        Design vessels for complete drainage
        
        remove deposit frequently
        
        Remove solids in suspension early in the process
        
        Use solid nonabsorbent gaskets
        
        Factors affecting
        
        crevice type: metal-to-metal, metal-to-non-metal
        
        crevice geometry: gap size, depth, surface roughness
        
        material: alloy composition (e.g. Cr, Mo), structure
        
        environment: pH, temperature, halide ions, oxygen
        
        Mechanism
        
        Anodic & cathodic reactions occur over the entire surface
        
        O2 is consumed inside the crevice by passive corrosion (depletion of O2)
        
        Attraction of Cl ions inside the crevice (charge balance)
        
        pH decreases inside the crevice (more acid): Passive film breaks down in acid and rapid active corrosion starts
        
        Crevice corrosion is an “autocatalytic process”
  - - - Passive film breakdown
        
        Pitting Corrosion
        
        a localized form of corrosion by which cavities or "holes" are produced in the material
        
        Control
        
        Cathodic protection
        
        Increase resistance of material
        
        Adding inhibitor
        
        Decrease aggressiveness of environment
        
        Type of Pitting
        
        Sideway Pits
        
        Elliptical
        
        Vertical
        
        Wide, Shallow
        
        Narrow, Deep
        
        Through Pits
        
        Horizontal
        
        Undercutting
        
        Mechanism
        
        Local breakdown of passive film (initiation) – act as anode
        
        The unbroken film (protective film) acts as cathode
        
        Pits develop at the anodic region.
        
        Presence of Clreduces the pH inside the electrolyte of the growing pit to about 1 (acidic) – increase corrosion (autocatalytic process)
- - - - Mechanism
        
        Precipitates rich in alloying elements which are essential for corrosion resistance result in the regions adjacent to grain boundaries are depleted of these elements. The metal is said to be sensitized and it suffers from Intergranular attack in a corrosion environment.
        
        Segregation of impurities at grain boundaries may give rise to galvanic corrosion. For example, chromium increase the corrosion resistance of the steel. This steel is sensitized to corrosion when heated approximately from 950-1450 ̊F. In this range chromium carbide is virtually insoluble.So Cr, come out of solid solution results in metal with low chromium contents in the area adjacent to the grain boundaries. The chromium depleted zone near the grain boundary is corroded because it does not contain sufficient corrosion resistance to resist attack in many corrosive environment. Carbon diffuse out more readily towards the grain boundaries at sensitizing temperature but chromium is much less mobile. So chromium carbide is formed on the grain boundaries.
      - Control
        
        Use low carbon grade of stainless steels
        
        Use stabilized grades alloyed with titanium or niobium
        
        Use post-weld heat treatment
      - Types
        
        Knife line attack
        
        occur in narrow band in the parent metal immediately adjacent to the weld
        
        occurs in stabilized steel
        
        Weld Decay
        
        develops at the appreciable distance from the weld
        
        occurs in non- stabilized steels.