The corrosion when generally confined to a metal surface as a whole is known as general corrosion. This corrosion occurs uniformly over the entire exposed surface area, for example, swelling, cracking, and softening of plastic materials. Whereas localized fluid corrosion includes intergranular, pitting, stress, fretting corrosion, and corrosion fatigue. The metabolic action of micro-organisms either directly or indirectly causes the deterioration of a metal called biological fluid corrosion, Fig.1.
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Fig.1: Macroscopic and Microscopic Forms of Corrosion
We know the basics of corrosion, fundamental chemical reactions, and environments in which corrosion can occur. The schematic of various types of corrosion is given in Fig.2.
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| Fig.2: Schematic of Common Types of Corrosion |
As corrosion most often
occurs in aqueous environments, the different types of corrosion a metal can
experience in such conditions are described below.
Types of Corrosion
1. Uniform Corrosion
2. Galvanic Corrosion
3. Flow-Assisted or
Erosion Corrosion
4. Fretting Corrosion
5. Crevice Corrosion
6. Pitting Corrosion
1. Uniform Corrosion
Uniform corrosion is the
most common type of corrosion and is due to the uniform attack across the
surface of a metal. The driving force for this type of corrosion is the
electrochemical activity of the metal in the environment to which the metal is
exposed. It is most simple to identify as the extent of the attack is judged
easily, and the resulting impact on material performance is fairly evaluated
due to an ability to consistently reproduce and test the phenomenon. This type
of corrosion typically occurs over relatively large areas of an exposed
material’s surface. Rust on a steel structure or the green thin layer (patina)
on a copper roof are examples of uniform corrosion.
2. Galvanic Corrosion
Galvanic corrosion, or
dissimilar metal corrosion, occurs when two different metals are located
together in a corrosive electrolyte. Galvanic corrosion is the degradation of
one metal near a joint or juncture that occurs when two electrochemically
dissimilar metals are in electrical contact in an electrolytic environment; for
example when copper is in contact with steel in a saltwater environment.
However, even when these three conditions are satisfied, many other factors
affect the potential for, and the amount of, corrosion, such as temperature and
surface finish of the metals. The driving force for the corrosion reaction is
the difference in electrode potentials between the two metals. A galvanic
couple forms between the two metals, where one metal becomes the anode and the
other the cathode. The anode, or sacrificial metal, corrodes and deteriorates
faster than it would alone, while the cathode deteriorates more slowly than it
would otherwise.
Large engineered systems
employing many types of metal in their construction, including various fastener
types and materials, are susceptible to galvanic corrosion if care is not
exercised during the design phase. Choosing metals that are as close together
as practicable on the galvanic series help to reduce the risk of galvanic
corrosion. In areas where corrosion is a concern, stainless steel products
offer value and protection against these threats. Stainless’ favorable chemical
composition makes it resistant to many common corrosives while remaining
significantly more affordable than specialty alloys such as titanium and
Inconel® alloys.
3. Flow-Assisted or Erosion Corrosion
Flow-assisted corrosion
(FAC) or erosion-corrosion results when a protective layer of oxide on a metal
surface is dissolved or removed by wind or water, exposing the underlying metal
to further corrosion and deterioration. It leads to erosion-assisted corrosion,
impingement, and cavitations.
4. Fretting Corrosion
Fretting corrosion occurs
as a result of repeated wearing, weight, and/or vibration on an uneven, rough
surface. Corrosion, resulting in pits and grooves, occurs on the surface.
Fretting corrosion is often found in rotation and impact machinery, bolted
assemblies, and bearings, as well as on surfaces exposed to vibration during
transportation.
5. Crevice Corrosion
Crevice corrosion is also
a localized form of corrosion and usually results from a stagnant
micro-environment in which there is a difference in the concentration of ions
between two areas of metal. Crevice corrosion occurs in shielded areas such as
those under washers, bolt heads, gaskets, etc. where oxygen is restricted. It
also occurs in crevices between components and also under polymer coatings and
adhesives. These smaller areas allow for a corrosive agent to enter but do not
allow enough circulation within, depleting the oxygen content, which prevents
re-passivation. As a stagnant solution builds, pH shifts away from neutral.
This growing imbalance between the crevice (microenvironment) and the external
surface (bulk environment) contributes to higher rates of corrosion. The
driving force for the corrosion is the difference between the oxygen
concentration inside the crevice and outside the crevice. Crevice corrosion can
often occur at lower temperatures than pitting. Proper joint design helps to
minimize crevice corrosion.
6. Pitting Corrosion
Pitting is one of the
most destructive types of corrosion as it can be hard to predict, detect and
characterize. Pitting is a localized form of corrosion, in which either a local
anodic point or more commonly a cathodic point, forms a small corrosion cell
with the surrounding normal surface. Pitting occurs in metals that are normally
passive when the passive layer breaks down. Once a pit has initiated, it grows
into a “hole” or “cavity” that takes on one of a variety of different shapes,
Fig.3. Pits typically penetrate from the surface downward in a vertical
direction. Pitting corrosion can be caused by a local break or damage to the
protective oxide film or a protective coating; it can also be caused by
non-uniformities in the metal structure itself. Pitting is dangerous because it
can lead to the failure of the structure with a relatively low overall loss of
metal. Examples of passive metals are aluminum and stainless steel. Pitting is
a problem if it leads to the weakening or perforation of the metal. In
applications where appearance is important pitting is a problem.
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| Fig.3: Types of Pitting Corrosion |
Make sure you also check our other amazing article on : Methods of Prevention of Corrosion?