WFF GT HB year

1.  Corrosion
• Corrosion is the deterioration of materials
through various electrochemical, chemical, and
environmental factors
 Almost all metals corroded to greater or lesser
degree.

2.  Economic impact
• The study showed that for 1998 the total
annual estimated direct cost of corrosion in
the U.S. was approximately $276 billion
(approximately 3.2% of the US gross
domestic product).
 Due to corrosion everywhere, bridges and
buildings collapse, failure of machinery,
catastrophe and los of life.

3.  Factors govern corrosion:
1. The metal from which the component is
made.
2. The protective treatment the component
surface received.
3. The environment in which the component is
kept.

4.  Types of corrosion
 Corrosion can be categorized in some
common types:
• uniform corrosion
• pitting corrosion
• galvanic corrosion
• crevice corrosion
• concentration cell corrosion
• graphitic corrosion

5. Uniform or General Corrosion
• The metal loss is uniform from the surface.
• The reaction starts at the surface and proceeds
uniformly.
• Often combined with high-velocity fluid erosion, with
or without abrasives.
•
• Fig. Uniform corrosion
or

6.  Pitting Corrosion
• Pitting Corrosion is the localized corrosion of a metal surface
confined to a point or small area, that takes the form of cavities.
• Pitting is one of the most damaging forms of corrosion.
• Pitting factor is the ratio of the depth of the deepest pit resulting
from corrosion divided by the average penetration as calculated
from weight loss.
• Fig. Localized corrosion (pitting corrosion)
 The basis metal is eaten away and perforated in places in the manner of
holes, the rest of the surface being affected only slightly or not at all.

7. chart for rating of pitting corrosion.

8.  Galvanic Corrosion
• Occurs when two metals with different electrode
potential is connected in a corrosive electrolytic
environment.
• The anodic metal develops deep pits and groves
in the surface.
 Increased corrosion in crevices or cracks or at contact surfaces between
two metal articles.
or

9. • Crevice Corrosion
• Occurs at places with gaskets, bolts and lap joints
where crevice exists.
• Crevice corrosion creates pits similar to pitting
corrosion.

10.  Concentration Cell Corrosion
• Occurs where the surface is exposed to an electrolytic
environment where the concentration of the corrosive
fluid or the dissolved oxygen varies.
• Often combined with stagnant fluid or in areas with
low fluid velocity.

11. • Graphitic Corrosion
• Cast iron loosing iron in salt water or acids.
• Leaves the graphite in place, resulting in a soft
weak metal.

12.  Corrosion Type Immunity
 Immunity is the lack of measurable
attack on a metal when exposed to
operational environments.
A metal that is immune to corrosion in a
given environment will not show any
change due to corrosion after exposure.
Their strength, weight, size, shape, and
surface finish are unaffected by exposure.

13.  Mechanisms
 Immunity can result from two basic
mechanisms:
1. In the first case, the energy content of the
metal is lower (more stable) than any of
the corrosion products that could possibly
form.
2. In the second case, there is an energy
tendency for the metal to suffer corrosion,
but corrosion activity is prevented by the
presence of a highly stable passive film.

14.  Examples
 Gold and platinum are examples of
metals that are normally immune in
a wide variety of environments due
to the low energy content of their
metallic state.
The woman in the picture is
wearing anodized titanium
earrings

15. Methods of protection from corrosion
Surface treatments
Surface treatments
Fig. Galvanized surface

16.  Applied coatings
 Plating, painting, and the
application of enamel are the most
common anti-corrosion treatments.
 They work by providing a barrier of
corrosion-resistant material between the
damaging environment and the (often
cheaper, tougher, and/or easier-to-process)
structural material.

17.  Protective coatings
1. Hot dipping-immersing into molten metal
(Zinc).
2. Electroplating-immersing into solution
called electrolyte to allow flow of elect.
Current.
3. Spraying
4. Cementation
5. Plastic coating
6. Painting
7. Reactive coatings
8. Etc.

18. Type of environment Typical rate of rusting for low carbon steel*
Rural
Urban
Industrial
Chemical
Marine
0.025-0.05
0.05-0.1
0.1-0.2
0.2-0.375
0.025-0.15
* The steel is unprotected.

19.  LEARNING MAKES YOU
PERFECT; LEARN MORE!!!
 REFER DIFFERENT BOOKS
AND INTERNET SITES.
 PREPARE SEMINAR ON:
CORROSION OF NON-METALLS
AND DEGRADIATION OF
POLYMERIC MATERIALS