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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

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Part A Corrosion of metals - Pacific Southwest Coatings

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

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


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