Fundamentals of Corrosion and Corrosion Control for Civil and Structural Engineers - Thomas Sputo.pdf

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Fundamentals of Corrosion and Corrosion Control for
Civil and Structural Engineers
Thomas Sputo, Ph.D., P.E., S.E., SECB
Corrosion of a Coated Handrail
Corroded Rain Gutter
Corrosion on the Nose of the Statue of
Liberty
Introduction
Corrosion can be defined as the degradation of a material due to a reaction with its
environment.
Degradation implies deterioration of physical properties of the material. This can be a
weakening of the material due to a loss of cross-sectional area, it can be the shattering of
a metal due to hydrogen embrittlement, or it can be the cracking of a polymer due to
sunlight exposure.
Materials can be metals, polymers (plastics, rubbers, etc.), ceramics (concrete, brick,
etc.) or composites-mechanical mixtures of two or more materials with different
properties. Because metals are the most used type of structural materials most of this
paper will be devoted to the corrosion of metals.
Most corrosion of metals is electrochemical in nature.
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Electrochemistry Fundamentals
Atoms:
All matter is made of atoms composed of protons, neutrons, and electrons. The center,
or nucleus, of the atom is composed of positively charge protons and neutral neutrons.
The outside of the atom has negatively charged electrons in various orbits. This is shown
schematically in the picture to the right where the electrons are shown orbiting the center,
or nucleus, of the atom in much the same way that the planets orbit the sun in our solar
system.
All atoms have the same number of protons (positively charged) and electrons
(negatively charged). Therefore all atoms have a neutral charge (the positive and negative
charges cancel each other). Most atoms have approximately the same number of neutrons
as they do protons or electrons, although this is not necessary, and the number of
neutrons does not affect the identity of the element.
The number of protons (atomic number) in an atom determines which kind of atom we
have, and the atomic mass (weight) of the atom is determined by the number of protons
and neutrons in the nucleus (the electrons are so small as to be almost weightless).
There are over 100 different elements that have been discovered. These are shown in
the Periodic Table of the Elements below. The letter symbols for the elements come from
their Latin names, so for example, H stands for hydrogen, C for Carbon, O for oxygen,
while Fe stands for iron and Cu stands for copper.
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Only a few of the elements are common, and most corrosion occurs due to only a
dozen or so metallic elements (iron, aluminum, copper, zinc, etc.) reacting with common
nonmetallic elements (oxygen, chlorine, sulfur, etc.).
Ions:
Ions are formed when atoms, or groups of atoms, lose or gain electrons.
Metals lose some of their electrons to form positively charged ions, e.g.
Fe
+2
, Al
+3
, Cu
+2
, etc.
Nonmetals gain electrons and form negatively charged ions, e.g.
Cl
-
, O
-2
, S
-2
, etc.
Molecules:
Compounds are groups of metals and nonmetals that form distinct chemicals. Most of
us are familiar with the formula H
2
O, which indicates that each water molecule is made
of two hydrogen atoms and one oxygen atom. Many molecules are formed by sharing
electrons between adjacent atoms. A water molecule has adjacent hydrogen and oxygen
atoms sharing some of their electrons.
Acids and bases:
Water is the most common chemical on the face of the earth. It is made of three
different constituents, hydrogen ions, hydroxide ions, and covalently bonded (shared
electron) water molecules. Most of water is composed of water molecules, but it also has
low concentrations of H
+
ions and OH
-
ions.
Neutral water has an equal number of H
+
ions and OH
-
ions. When water has an excess
of H
+
ions, we call the resultant liquid an acid. If water has more OH
-
ions, then we call it
a base.
The strength of an acid or a base is measured on the pH scale. pH is defined by the
following equation:
pH = -log [H
+
]
A detailed explanation of pH and acids and bases is beyond the scope of this paper. It
is sufficient to note that some metals (e.g. zinc and aluminum) will corrode at faster rates
in acids or bases than in neutral environments. Other metals, e.g. steel, will corrode at
relatively high rates in acids but have lower corrosion rates in most neutral and basic
environments.
Even a strong acid, with a pH of 0, will be less than 1/1000
th
by weight hydrogen ions.
Neutral water, at a pH of 7, is less than 1 part H
+
in 10 million parts covalently bonded
water molecules.
Mnemonic device:
Many people have a hard time remembering whether an acid or a
base has a high pH number. Just remember that acid comes before base in the alphabet
and that low numbers come before high numbers. Acids have low numbers (less than 7),
bases have high numbers (greater than 7). Neutral waters have pH's near 7 and tend to be
relatively noncorrosive to many materials.
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Electrochemical Cells
Oxidation and Reduction:
Metals are elements that tend to lose electrons when they are involved in chemical
reactions, and nonmetals are those elements that tend to gain electrons.
Sometimes these elements form ions, charged elements or groups of elements. Metallic
ions, because they are formed from atoms that have lost electrons, are positively charged
(the nucleus is unchanged). When an atom or ion loses electrons it is said to have been
oxidized.
A common oxidation reaction in corrosion is the oxidation of neutral iron atoms to
positively charged iron ions:
Fe » Fe
+2
+ 2e
-
The electrons lost from a metal must go somewhere, and they usually end up on a
nonmetallic atom forming a negatively charged nonmetallic ion. Because the charge of
these ions has become smaller (more negative charges) the ion or atom which has gained
the electron(s) is said to have been
reduced.
4H
+
+O
2
+ 4e
-
» 2H
2
O
or
2H+ +2e
-
» H
2
While other reduction reactions are possible, the reduction of oxygen is involved in well
over 90% of all corrosion reactions. Thus the amount of oxygen present in an
environment, and its ability to absorb electrons, is an important factor in determining the
amount of oxidation, or corrosion, of metal that occurs.
Mnemonic device:
Many people have a hard time remembering what oxidation and
reduction mean in terms of chemical reactions. If you just remember that reduction means
“get smaller,” then you can remember that the electrical charge on a reduced chemical
has gotten smaller (has more negative charges). The opposite reaction, oxidation, means
that the charge has gotten larger (not so easy to remember).
Electrochemical Reactions:
The two metal strips shown below are exposed to the same acid.
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Both metals undergo similar oxidation reactions:
Cu » Cu
+2
+ 2e
-
and
Zn » Zn
+2
+ 2e
-
The electrons freed by the oxidation reactions are consumed by reduction reactions.
On the copper the reduction reaction is:
4H
+
+O
2
+4e
-
» 2H
2
O
The corrosion rate of the copper is limited by the amount of dissolved oxygen in acid.
On the zinc the reduction reaction is:
2H
+
+2e
-
» H
2
The hydrogen ions are converted to hydrogen gas molecules and can actually be seen
bubbling off from the acid.
If we now connect the two metal samples with a wire and measure the electricity through
the connecting wire, we find that one of the electrodes becomes different in potential than
the other and that the corrosion rate of the copper decreases while the corrosion rate of
the zinc increases. By connecting the two metals, we have made the copper a cathode in
an electrochemical cell, and the zinc has become an anode. The accelerated corrosion of
the zinc may be so much that all of the oxidation of the copper stops and it becomes
protected from corrosion. This method of corrosion control is called cathodic protection.
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