Part A
Corrosion of metals
This section on the corrosion of metals is
greatly simplified for purposes of clarity. It
is not meant to train the student with the
intention of making him/her an expert in this
area but rather to give a sense of the depth
and scope of the problem.
Why theory?
If we are to be effective in controlling corrosion,
and in making an intelligent selection and
recommendation of protective coating systems, we
need to understand clearly the actual processes by
which a clean, useful iron or steel structure can
eventually be reduced to a collection of rusty
scrap.
What is Corrosion?
NACE definition:
The deterioration of a substance, usually a
metal, or its properties, because of a
reaction with its environment.
Corrosion of Iron
• Electrochemical process, involving:
• a chemical change of iron to iron oxide,
and
•an electrical process involving current
flow.
Five things must be present for
normal atmospheric corrosion
to occur:
Oxygen
Electrolyte (moisture and ions)
Anode
Cathode
Metallic pathway
Oxygen
Oxygen must be present for normal
atmospheric corrosion to occur. Without
oxygen, iron can not be oxidized to
iron oxide (rust), and corrosion will not
occur. There is more than enough oxygen
available in the surrounding atmosphere,
and dissolved in fresh or salt water, to
support corrosion.
Electrolyte
The electrolyte is a solution of salts,
minerals, acids, alkalis, industrial soot and
fumes, or other chemical compounds in
water or atmospheric moisture, which is
capable of conducting electrical current.
Electrolyte
The current carrying capacity of the
electrolyte is due to the presence of tiny,
electrically charged particles, called IONS,
derived from salt or other chemicals
dissolved in the water.
Ions
 Formed by dissociation (upon mixing with
water) of salts, acids, and caustic
chemicals into electrically charged
fragments of the original chemical.
 The greater the concentration of ions in
the electrolyte, the greater the current
carrying capability and resulting corrosion
rate. This is why salt-laden marine
environments are more corrosive than dry,
desert environments.
 Purified water, containing very few ions, is
a poor current conductor.
The Anode
The anode is a microscopic area of the
corroding metal where the metal actually
goes into solution and where the actual
metal loss takes place. A piece of corroding
steel contains millions of microscopic
anodic areas.
The Cathode
The cathode is a microscopic area of the
corroding metal adjacent to the anode.
The cathode area accepts current flow
from the anode, completing the corrosion
circuit. The cathodic area is not attacked
and does not corrode.
In the corrosion of a metal such as a
steel pipe exposed to atmospheric
elements, millions of microscopic
anodic and cathodic areas are located on
the same metal. The overall effect
is eventually complete corrosion
of the metal.
Metallic Pathway
S-T-E-E-L
A-L-U-M-I-N-U-M
Z-I-N-C
The metallic pathway is provided by the
metal actually undergoing corrosion.
Adjacent anodic and cathodic areas are
located on the same metal surface. This
metal surface, a good conductor of electric
current, provides the internal current path
necessary to complete the corrosion circuit.
A Really Simple
Corrosion Cell
Corrosion can be greatly
influenced by:
 Oxygen aka air
 Gases such as carbon dioxide, hydrogen
sulfide, sulfur dioxide
 Ingredients in a particular metal i.e.
carbon steel vs. stainless steel
 Ion concentration in an electrolyte
Why do some metals corrode
faster than other metals?
Refining / Corrosion Process
Forms of Corrosion
ÂUniform Attack
ÂGalvanic Attack
ÂPitting
ÂErosion
ÂStress
ÂMIC
Uniform Corrosion
The corrosion process appears to be
evenly distributed across the metal
that is corroding.
Galvanic Attack
Occurs when 2 dissimilar metals are
in electrical contact with each other
& exposed to a suitable electrolyte.
Severe Mild
Galvanic Galvanic
Corrosion Corrosion
Big Cathode Big Anode
Little Anode Little Cathode
Galvanic Corrosion
Galvanic Corrosion
Dissimilar Metals
Galvanic Corrosion
Stainless
steel cabinet
Painted
carbon steel
pad
Galvanized
grating
Blisters on
painted pad
The Galvanic Setup
Ó Discoloration at
strap buckle
Blisters forming at Ð
interface of
Ï
stainless band and
vessel
The Big Galvanic Setup
Carbon Floor of
Steel a raw
Stainless sewage
Steel holding
tank
Pitting Corrosion
A form of extremely localized attack
that results in either excessive metal
loss or holes in the metal.
/
Pitting Corrosion
Pitting Corrosion
Sludge on
pipe wall
Ï
Pitting Corrosion
Pitting Corrosion