Form of Corrosion (Part 1)
Affected By
Stress
Movement of Material
Material Compatibilty
Surface Condition
Shape
Temperature
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Uniform Corrosion
Mechanism of Uniform Corrosion
Aqueous Corrosion
Atmospheric Corrosion
Factor Affecting Uninform Corrosion
Velocity
-Higher velocity increases
mass transfer rate
-Accelerates erosion
Dissolve Gas
-Dissolved oxygen
pH
-Low pH accelerated the
cathodic reaction
-pH affects the stability
of passive films
Temperature
-Increased temperature,
will increase reaction rate
Corrosion Product Produce:
-Protective layer on the metal
-May be readily dissolved in the environment
Uniform corrosion is usually the result of breakdown in
protective coatings
Uniform corrosion is relatively easy to measure, predict
and design against.
Prevention of Uniform Corrosion
Change the environment
Cathodic protection
Proper material selection including coatings
Corrosion occurs evenly over the entire surface of the corroding metal.
Localised Corrosion
Attack or corrosion is limited to specific areas or
parts of a structure
7 Forms
Intergranular corrosion
Selective leaching
Pitting corrosion
Erosion-corrosion
Crevice corrosion
Environmentally induced cracking
Galvanic or two-metal corrosion
Factors
Material
-Inclusions
-Different phases
-Grain boundaries
Mechanical
-Static stress
-Cyclic stress
Environment
-Oxygen concentration
-Chloride ion concentration
-pH
-flow rate
Occurs when 2 different metals are electrically
connected in the same electrolyte
The more active metal (anodic) corrodes faster and the less active metal (cathodic) corrodes slower and will be protected
The galvanic series will predict which metal will corrode
Factors affecting
Size of exposed areas
Difference in potential between metals
Effect of anodic polarisation on anode
Prevention
Install a third metal which is anodic to both metals in the galvanic
contact, i.e. cathodic protection
Design for the use of readily replaceable anodic parts or make them thicker for longer life
Add inhibitors, if possible, to decrease the aggressiveness of the
environment.
Apply coatings with caution
If dissimilar metal must be joined, Insulate them electrically wherever practicable
Avoid the unfavorable area effect of a small anode and large cathode
Select combinations of metals as close together as possible in the galvanic series
Crevice corrosion occurs at shielded areas that contain
small volume of aqueous solution
Mechanism
Liquid entry but stagnant
Corrosion rate of crevice is higher than that on bulk
Crevice corrosion occurs mainly on alloys which
passivate
Crevice corrosion is initiated by changes in local
chemistry within the crevice
Preventions
the elimination of the crevice itself
controlled by good design
geometry (shape) to remove the crevice itself
Pitting corrosion is an intensive localised attack which results in the development of cavities or "pits" in the metal surface
Initiated by
Local breakdown of protective passive film
Localised damage to, or poor application of a protective coating
Compositional heterogeneity
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
Prevention
Decrease the aggressiveness of the environment
Increase the resistance of materials
The best protection against pitting corrosion is to select a
material with adequate pitting resistance