Download - Corrosion of Steel Part A
-
Part ACorrosion of metals
Part ACorrosion 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.
-
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.
Why theory?Why theory?
-
What is Corrosion?What is Corrosion?
The deterioration of a substance, usually a metal, or its properties, because of a
reaction with its environment.
NACE definition:
-
Corrosion of IronCorrosion 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:
OxygenElectrolyte (moisture and ions)
AnodeCathode
Metallic pathway
Five things must be present for normal atmospheric corrosion
to occur:
OxygenElectrolyte (moisture and ions)
AnodeCathode
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.
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.
-
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.
ElectrolyteElectrolyte
-
ElectrolyteElectrolyte
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.
-
IonsIons 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 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 AnodeThe Anode
-
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.
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.
The CathodeThe Cathode
-
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-LA-L-U-M-I-N-U-M
Z-I-N-CThe 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.
Metallic Pathway
S-T-E-E-LA-L-U-M-I-N-U-M
Z-I-N-CThe 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
A Really Simple Corrosion Cell
-
Corrosion can be greatly influenced by:
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?
Why do some metals corrode faster than other metals?
-
Refining / Corrosion ProcessRefining / Corrosion Process
-
Forms of CorrosionForms of Corrosion
Uniform AttackGalvanic Attack
PittingErosionStressMIC
-
Uniform CorrosionUniform Corrosion
The corrosion process appears to be evenly distributed across the metal
that is corroding.
-
Galvanic AttackGalvanic AttackOccurs when 2 dissimilar metals are in electrical contact with each other & exposed to a suitable electrolyte.
-
Severe Galvanic
Corrosion
Severe Galvanic
Corrosion
Big Cathode Little Anode
Mild Galvanic
Corrosion
Mild Galvanic
Corrosion
Big Anode Little Cathode
-
Galvanic CorrosionGalvanic Corrosion
-
Galvanic CorrosionGalvanic Corrosion
Dissimilar Metals
-
Galvanic CorrosionGalvanic Corrosion
Stainless steel cabinet
Painted carbon steel
pad
Galvanized grating
Blisters on painted pad
-
The Galvanic SetupThe Galvanic Setup
Blisters forming at interface of
stainless band and vessel
Discoloration at strap buckle
-
The Big Galvanic SetupThe Big Galvanic Setup
Carbon Steel
Stainless Steel
Floor of a raw
sewage holding
tank
-
Pitting CorrosionPitting Corrosion
A form of extremely localized attack that results in either excessive metal
loss or holes in the metal.
/
-
Pitting CorrosionPitting Corrosion
-
Pitting CorrosionPitting Corrosion
Sludge on pipe wall
-
Pitting CorrosionPitting Corrosion
-
Pitting CorrosionPitting Corrosion
Part ACorrosion of metalsFive things must be present for normal atmospheric corrosion to occur:OxygenElectrolyte (moisture and ions) Anode CathodOxygenOxygen must be present for normal atmospheric corrosion to occur. Without oxygen, iron can not be oxidized to iron oxThe electrolyte is a solution of salts, minerals, acids, alkalis, industrial soot and fumes, or other chemical compounds in wElectrolyteIonsThe anode is a microscopic area of the corroding metal where the metal actually goes into solution and where the actual metalThe cathode is a microscopic area of the corroding metal adjacent to the anode. The cathode area accepts current flow from Metallic Pathway S-T-E-E-LA-L-U-M-I-N-U-MZ-I-N-CThe metallic pathway is provided by the metal actually undergoing cCorrosion can be greatly influenced by:Why do some metals corrode faster than other metals?Refining / Corrosion ProcessForms of CorrosionGalvanic AttackGalvanic CorrosionGalvanic CorrosionGalvanic CorrosionThe Galvanic SetupThe Big Galvanic SetupPitting CorrosionPitting CorrosionPitting CorrosionPitting CorrosionPitting Corrosion