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Forms of Corrosion -convenient to classify corrosion by the form…
Forms of Corrosion
-convenient to classify corrosion by the form appearance of the corrode metal
Pitting Corrosion
Mechanism
Localized adsorption of aggressive ions.
Passive film breakdown due to corrosion potential increases
Pitt growth if passivation cannot occur. The rate of corrosion is significantly high due to autocatalytic process
Autocatalytic Process in a corrosion of pit
:
Dissolved oxygen is difficult to diffuse into the pit leading to formation of very small anodic area inside the pit and very large cathodic area outside the pit which is still covered by a passive film.
Excess positive charge due to metal ions accumulation inside the pit is necessarily balanced by the migration of aggressive ions into the pit.
Because of metal ions are diffusing outward and being hydrolyzed and deposited just in front of the pit, hydrogen ions are diffusing inward to replace the metal ions that will result in gradual increase in acidity
Control
Materials that show pitting, or tendency to pit should not be used to built the plant or equipment.
Adding inhibitor including increase of pH
Cathodic protection
Use materials which are more resistance to pitting such as stainless steel 304, stainless steel 316, titanum
Properties
It is a form of extremely localized attack in environment containing
aggressive ions that results in holes in the metal.
These holes may be small or large in diameter but in most cases they are
relative small.
The surface diameter of pits are about the same as or less than the depth.
Conditions for pitting:
-Breaks of the passivation film or other defects such as lack of
homogeneity in the passive film on the metal surface
-Presence of halogen ions
-Stagnancy of electrolyte
Crevice Corrosion
Control
Use welded butt joints instead of riveted or bolted joints in new equipment.
Close crevices in existing lap joints by continuous welding or soldering.
Design vessels for complete drainage to avoid sharp corners and stagnant areas.
Inspect equipment and remove deposit frequently.
Remove solids in suspension early in the process.
Use solid non absorbent gaskets such as Teflon.
Properties
This is a localized form of corrosion caused by deposition of dirt, dust, mud and deposits on a metallic surface or by the existence of voids, gaps and cavities between adjoining surfaces.
It 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.
A crevice must be wide enough to permit liquid entry and also sufficiently narrow to maintain a stagnant zone.
Mechanism
Stage 1
At time zero, the oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere.
Stage 2
Due to the difficult access caused by the crevice geometry (convection restriciton), oxygen consumed by normal uniform corrosion is very soon depleted in the crevice.
Stage 3
The metal ions produced bt the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products.
The acidification of the local environment can produce a serious increase in the corrosion rate of most metals.
The corrosion products seal even further the crevice environment.
The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in the environment.
As the corrosion within the crevice increases, the rate of oxygen reductions on adjacent surfaces also increases. The cathodically protects the external surface. Thus, the attack is localized within the shielded areas..
Intergranular Corrosion
Control
Alteration of environment
Lower acidity and less oxidizing conditions will reduce the susceptibility of IGC
Solution annealing
Decrease Carbon content to < 0.03%
Chromium carbide will form material which is not sensitive to IGC
USe stabilized stainless steel.
7 Heating the alloy to redissolve chromium carbide and followed by rapid cooling.
Properties
Localized attack at and adjacent to grain boundaries, with relative little
corrosion at the grains
It caused by impurities at the grain boundaries, enrichment of one of the alloying elements or depletion of one of these elements in the grain boundary areas.
Small amounts of iron in aluminium wherein the solubility of iron is low has been shown to be segregate in the grain boundaries .
Depletion of chromium in the grain boundary regions result in intergranular corrosion of stainless steel. Chromium content in stainless steel alloys generally prevent corrosion as it helps to procastinate to combine pure iron with oxygen and water to form rust
Mechanism
Intergranular corrosion is caused by Microsegregation of impurities and alloying elements on the grains boundaries.
The driving force of intergranular corrosion is the difference between the Electrode potentials of the grain boundary and the grain itself, which form a galvanic cell in presence of an electrolyte.
Dissolution of anodic grain boundaries starts from the surface and advances along the grains interfaces. The process results in deterioration of the bonding between the grains and drop of mechanical properties.If the precipitates at the grain boundaries have higher electrode potential the grains will dissolve (anodic reaction). In this case the grain boundaries will not be attacked.
Knife Line Attacl (KLA)
is a highly localized form of IGC that occurs only a few grain diameters immediately adjacent to weld bead in stabilized austenitic steels
Intergranular corrosion (IGC)
of austenitic stainless steel by precipitate along grain boundaries leading to formation of chromium depleted zone adjacent to grain boundaries and dissolution of carbide.
IGC of ferritic iron-chromium stainless steel
as Ferritic stainless steels sensitize only after heating above 925 C where solubility of carbon and nitrogen become significant in ferrite. Because of low solubility of interstitials in ferrite, it sensitize much more rapidly at lower temperature.
IGS of other alloys
High strength of aluminum alloys depend on precipitated phases for strengthening and susceptible to IGS.When these alloys are solutionquenched to keep the copper in the solution, their susceptibility to intergranular
corrosion is very small but they posses low strength.
Galvanic Corrosion
Properties
It is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another in the presence of electrolyte
It will be significant if the corrosion potential difference between two metals/alloys is >250mV
If two different metals/alloys have to be used in contact, is is suggested to select alloys with *close corrosion potentials.
Factors Affecting a Galvanic Corrosion
Area Effect
-Ratio of the cathodic to anodic areas
An unfavorable area ratio consists of a large cathode and a small anode, this is because the corrosion of metal will be taking palace with high current density.
Current Density
(I/A)
Electrode potential
based on EMF/Galvanic Series position
The further apart the position of metals in the series, the higher the corrosion potential difference,
Environmental Affect
(moisture, soil etc)
-Corrosion is greater near seashore than in dry rural atmosphere.
-Condensate near seashore contains salts – more corrosive.
-Not occur when metals are completely dry since no electrolyte to produce current between 2 electrode.
Control
Select combinations of metals as close as possible in galvanic series..
Avoid unfavourable area effect of a small anode and large cathode
Insulate dissimilar metals
Applied coating with caution
Add inhibitors to decrease the aggressiveness of the environment
Avoid threaded joints for materials far apart in galvanic series
Design for the use of readily replaceable anodic parts
Install a third metal that is anodic to both metals in the galvanic contact
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Pee Kai Xuan 138724
EBB316 CORROSIONS & DEGRADATION
