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Forms of Corrosion (Intergranular Corrosion (Causes (Impurities at the…
Forms of Corrosion
Intergranular Corrosion
Causes
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Small amounts of iron in aluminum, wherein solubility of iron is low, have been shown to segregate in the grain boundaries and cause Intergranular corrosion
Depletion of chromium in the grain boundary regions results in
intergranular corrosion of stainless steels
Mechanisms
1) At 825°C, carbide precipitate along grain boundaries leading to the formation of chromium depleted zone adjacent grain boundaries
2) At 425°C, Diffusion rate of carbon atom is too low, carbide precipitation along grain boundaries cannot occur
In the intermediate temperature range carbide M23C6
is precipitated along grain boundaries.This precipitation consuming chromium atoms, results in Cr depleted zone
around precipitated carbide.
Chromium-depleted zone is much less corrosion resistant than the surrounding grains. An unfavorable area ratio consists of a large cathode and a small anode forms and results in microscopic localized galvanic attack leading to IGC.
Heating of austenitic stainless steels in this temperature
region will results in sensitized stainless steels.
Types
Knife line attack (KLA)
Stabilized austenitic stainless steel may become more susceptible to intergranular attack during welding
The zone immediately adjacent to the fusion line is heated to temperature high enough to dissolve the stabilizing carbide, but the rate of cooling is so rapid that carbide precipitation is prevented.
The narrow area is reheated during subsequent welding passes into the temperature range in which both stabilizing carbides (Nb and Ti carbide) and the chromium carbides (CrFe)23C6 co precipitate .
This narrow band near to the fusion line become susceptible to intergranular cracking due to the precipitated chromium rich carbides.
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Prevention of IGC
Use low carbon (e.g. 304L, 316L) grade of stainless steels
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Heating the alloy above 815°C to re-dissolve chromium carbide and followed by rapid cooling (often by water immersion)
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What is it?
Localized attack at and adjacent to grain boundaries, with relative little corrosion at the grains.
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Takes place due to higher rate of corrosion of grain boundary area of an alloy than that of grain interior
A metal corrode uniform attack usually results because grain boundaries are slightly more reactive than matrix
Pitting Corrosion
What is it?
Localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities or holes
One of the most damaging forms of corrosion because it is more difficult to detect, predict and design against
The shape of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the pit shape, the pit size, and the pit depth of penetration can be determined
Conditions for pitting
Breaks of the passivation film or other defects such as lack of homogeneity in the passive film on the metal surface
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Prevention/ Protection
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Control pH, chloride concentration and temperature
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Galvanic Corrosion
Factors
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The more active metal (anode) corrodes at an accelerated rate and the more noble metal (the cathode) corrodes at a slow rate
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the distance effect, with the greatest attack usually being near the junction
area effect, where it is better to have large anodes and small cathodes
Environmental affect (moisture, soil, humidity, temperature, pH)
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Galvanic Series
The further apart the metals in the galvanic series, the higher the rate of galvanic corrosion
Magnesium alloys have high susceptibility to galvanic corrosion due to excessive levels of heavy metal or flux contamination
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Crevice Corrosion
What is it?
Localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces
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The damage caused by crevice corrosion is normally confined to one metal at localized area within or close to the joining surfaces
This is a localized form of corrosion caused by the deposition of dirt, dust, mud and deposits on a metallic surface or by the existence voids, gaps and cavities between adjoining surfaces
Is usually associated with small volumes of stagnant solutions caused by holes, gasket surfaces, lap joints, surface deposit and crevice under bolt and rivet heads
Mechanisms
1) Anodic and cathodic reaction occur
uniformly over the entire surface of metals.
Anodic reaction: Fe→Fe+++2e
Cathodic reaction: O2 +2H2O+4e→4OH
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3) As dissolution of metal (Fe) inside crevice continues, excess positive charge is produced inside the crevice.
4) Migration of chloride ions into crevice, thus Increased
concentration of metal chloride.
5) Hydrolysis of ferrous ions is taking place
(except for alkali metals, Na & K):
Fe+2Cl- + 2H2O = Fe(OH)2 + 2H+Cl
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8) The rate of of oxygen reductions on adjacent surfaces increases as the corrosion within the crevice increases.
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