GENERAL CORROSION DUE TO ENVIRONMENTAL EXPOSURE.
Uniform Corrosion.
During uniform corrosion the complete exposed surface of the metal corrodes. A
good example would be the formation of rust on unprotected steel such as unpainted
automobile bodies. This type of corrosion is very general and resulted in a great loss of
metal and money.
Uniform corrosion was occurring on this tank when the paint system did not provide
complete protection. A considerable amount of rust was found on the surface where the
paint had broken down and corrosion occurred.
Uniform corrosion is basically thermodynamically controlled, with the Redox
potentials and the Nernst equation dictating the process until concentration polarization
takes place when the transport of oxygen is limited. As noted previously for the case of iron,
the anodic reaction is:Fe -> Fe2+ + 2ewhile the cathodic reaction is oxygen reduction to form hydroxyl ions:O2 + 2H2O + 4e- -> 4(OH-)
As the whole surface of steel corrodes the anodic sites and cathodic sites are the
same surface.
A further example is shown for a steam return pipe. In this case, water condensed in
the pipe and uniform corrosion occurred on the inside of the pipe. As the pipe thinned from
the inside, enough material was removed until the internal diameter of the threads on the
bottom of the pipe was reached. Eventually a drip was found at the joint when the threads
were corroded away and a tight seal was no longer possible. This is a long term process.
Another case of uniform corrosion is zinc dissolving in hydrochloric acid. The zinc
dissolves with the formation of hydrogen gas. The oxidation of zinc to zinc ions is the
anodic reaction and the cathodic reaction is the reduction of hydrogen ions to hydrogen gas.
Zn + 2HCl = ZnCl2 + H2
Note that this reaction is both chemically and electrochemically balanced.
Anode:-
Zn -> Zn2+ + 2e-
Cathode:-
2H+ +2e- -> H2
The rate of reaction is controlled by the slowest anodic or cathodic process. For
example, concentration polarization controls the cathodic reduction of oxygen to hydroxyl
ions. This rate is a function of velocity and so faster flowing solutions will corrode iron at a
faster rate. Temperature is also a factor in activation controlled corrosion. Raising the
temperature will also increase the corrosion rate as the activation energy decreases with
temperature. Environment is important. Chloride containing solutions will increase the rate
of corrosion over non chloride containing solutions. Instead of Fe2+ reacting with only
hydroxyl ions it can now react with chloride ions. Most common metals are very soluble in
chloride form and so the reaction is increased.
Materials.
Most metals which do not passivate and which have potentials lower than the Redox
potential for reduction of oxygen in water will corrode in water. Examples are steels, zinc,
and magnesium. Passivators such as stainless steels and nickel containing alloys such as
monels and superalloys do not corrode in water.
Environment.
Data on corrosion rates for a variety of metals is readily available. In general a large
range of corrosion rates is obtained depending on the specific exposure conditions. For
example, corrosion rates from 0.03 mpy in a polar environment to 17.4 mpy at a location at
ground level, 60 yards from the ocean at Cape Kennedy for carbon steel were measured. On
the beach at Cape Kennedy, the corrosion rate was 42 mpy. In flowing seawater, rates of 15
mpy in the splash zone and 5 to 10 mils just below the surface were reported. From a design
aspect, the local conditions of the material must be known to determine thickness to be used
for the lifetime of the component. For metals and alloys such as nickel aluminum bronzes,
rates of 1 to 2 mpy in quiet seawater were reported.
In designing a piling in a river from unprotected steel, if both sides are exposed to
slow flowing water, a 15 mpy rate can be used. For both sides exposed, this becomes 30
mpy of steel lost. For a configuration such as an “I” beam, for a twenty year life expectancy,
600 mils or thousands of an inch of steel will be lost. This does not include the amount of
material required for the loading from the decks on the piling which must still be present
after twenty years to provide structural stability.
Protection techniques.
1
Exclude the environment.
One of the required components is an electrolyte, which for water also
contains the cathodic reactant of oxygen. If this component can be excluded from the
surface, then corrosion will not occur. The simplest method is to paint the material.
However, this is not always a long lasting solution if the paint is not applied well. The
introduction shows a possible problem where the paint will not provide a long term solution.
2.
Sacrificial Anodes.
Apply a more active metal. A good example would be zinc on steels. It must
be noted that there is a concept called "throwing power" in that the kinetics of the reactions
are such that a small piece of zinc cannot protect a large area. Zincs have to be placed at
frequent distances with a higher density at increased velocity areas. Cadmium plating on
steel was a good choice but it along with chromates are deemed to be environmentally
“unfriendly”. These materials are being replaced.
3.
Impressed Current techniques
These make the component to be protected a cathode by supplying electrons
from an external source.
4.
Material selection.
Weathering steels are a steel with 0.5 - 1.0% Cu additions. The copper
stabilizes the oxide product on the surface such that after the initial oxidation, the rust
formed becomes protective and no further metal penetration occurs. Examples include many
light poles throughout the USA. These are form a corrosion perspective, maintenance free
structures. The drawback is the only color available is rust!. Other materials which will not
corrode can be selected.
Weathering steel light pole, no maintenance of the pole is required.
5.
Inhibitors.
Examples of inhibitors are the waxy brown paper surrounding tools. In the
paper is a vapor phase inhibitor which coats the metal and displaces the moisture necessary
for corrosion.
Testing Technology.
Salt Spray Testing.
Salt spray testing in a chamber is a recognized test for uniform corrosion. It follows
the ASTM B117 standard. A 5% by weight salt solution at 100% humidiy and 95F is passed
into a chamber in which the samples are contained. The equipment is shown below.
An example of results of a test is shown for two steels, one of which is suitable and one
which exhibited extensive rust over the same time period.
On the left of the figure is a typical untested component, the center one corroded
while the one on the right was covered on a white deposit from the salt. There are many
variations on the salt spray test with each particular industry setting up exposure limits based
on their experience. Some groups use 1000 hours with no corrosion, others 24 hours with no
“white rust” for galvanized parts. For painted parts an “X” scratch is often placed through
the paint into the substrate and in addition to damage in the scratch, damage spreading away
from the scratch monitored. This is known as “undercutting”. Other cycles and combinations
are used, such as some time in salt spray, followed by a period at elevated temperature and
high ultra violet light exposure, followed by a time below freezing. These conditions were
defined on the basis that they reflect accurately an acceleration of the failure mechanisms
that lead to failure of the particular component.
Another process to measure uniform corrosion is the Tafel extrapolation shown
previously. From this test corrosion rates can be established to put into design calculations
for the corrosion life of a component. For example, if a corrosion rate of 15mpy is
established, or 15 thousands of an inch of metal thickness will be removed each year, the
thickness of metal required for a 30 year corrosion life as a piling in a river would be 0.6
inch as water is on both sides of the component.
A case of uniform corrosion is the corrosion of reinforcing steel in concrete. Steel is
placed in concrete to reinforce it as concrete is a brittle material in tension, so the steel
reinforces it. If the Pourbaix diagram for iron is examined, Chapter 2, then the conditions of
concrete a high pH, are such that a passive film should be present for steel in concrete.
However, actions such as salting the roads to prevent ice forming, or the local areas
proximity to a marine environment, results in salt building up in the concrete cover
overlaying the steel reinforcing bar. It is this concrete cover that protects the steel from
environmental exposure and uniform corrosion. The chloride ion from salt gradually
permeates down to the level of the steel and breaks down the passivation. Once the
passivation is destroyed then uniform corrosion of the steel then occurs. The corrosion
products require a volume of space considerably greater than the original steel, from 4 to 10
times the volume. This produces tensile stresses in the concrete cover separating the rebar
form the external environment. Once the stresses reach the strength of the concrete, it
“spalls” and the concrete separates from the structure leaving the reinforcing steel exposed,
which then continues to corrode. A good example of the process is shown below.
The possible solutions to this include cathodic protection of the steel, adding
inhibitors to the concrete, change the composition of the reinforcing steel, coatings on the
reinforcing steel, and non metallic reinforcement materials. This problem is worldwide and
very costly.
In the figure below, a steel wheel nut on a boat trailer has corroded uniformly.
On the left hand side of the nut a large piece of rust is still attached. The rust
contains many cracks and is about to fall from the surface. The exansion of corrosion
products can be seen here. The steel bearing is on the left hand side and rust containing
cracks can be seen as well. The area of concern with an article like a boat trailer is that this
rust will build up in the leaf springs rather like a wedge of material and separate them over
time.
Galvanic Corrosion.
Galvanic corrosion results from two different metals being in contact in the
environment. Examples would be brass plumbing fittings on a cast iron pipe. In this case
several reactions are possible, but in general the corrosion rate of the most anodic or active
metal is increased and the corrosion rate of the more cathodic metal is decreased. This is
shown in the following diagrams. The first figure shows the corrosion rate for a single metal
corroding in an environment. A good example would be for zinc in hydrochloric acid. The
anode reaction favored is:Zn -> Zn2+ + 2eCathode:2H+ +2e- -> H2
Potential
(V)
Cathodic Reaction 1
Eoc(2)
Anodic reaction 2
icorr(2)
log Current Density
2
A/cm
Corrosion rate determination for a two Redox reaction system
Using Faradays Law, substitution of the value of icorr will calculate the corrosion
rate in mils per year. The next figures shows how to calculate the oveall corrosion rate as
well as the individual reactions rates for each half cell reaction when a third Redox reaction
is added at a potential between the first two Redox reactions. The rule that must be applied
is that the TOTAL OXIDATION RATE MUST EQUAL THE TOTAL REDUCTION
RATE. In figures below, the dashed lines represent the TOTAL rates.
E
(V)
Cathodic Reaction 1
Anode Reaction 3
EOC(3)
Eoc(2)
Er(3)
Total Cathode Rate
Cathode Reaction 3
Anodic Reaction 2
icorr(2)
icorr (3)
log Current Density
2
A/cm
Corrosion rate determination for a three Redox reaction system. The corrosion rate for
electrode 2 has increased from icorr to icorr 1+2 as it the only anodic reaction.
Two cases are shown, the first in the figure above, when the corrosion potential for
three electrodes, Eoc(3), is above the two electrode potential Eoc(2), and in the figure
below, when the three electrode corrosion potential is below the two electrode potential.
In the figure above, the open circuit potential for the three Redox system, Eoc (3) is
more negative than the individual third electrode Reox potential, Er(3). As such the third
Redox reaction can only contribute to the cathodic reaction rate. The third Redox reaction is
therefore protected from corrosion. The total reduction rate is provided by the addition of
the cathodic rates from Redox reactions 1 and 3, which is shown by the dashed line. The
second Redox dissolution rate increased significantly from icorrr(2) to icorr(3) by the
introduction of the third Redox cathodic reaction increasing the total cathode reaction which
must be matched by the anode reaction from Redox reaction 1.
In the figure below, a similar process can be followed for determination of corrosion
rates. In this case the open circuit potential from the three Redox reactions, Eoc(3) is more
positive than the potential for two Redox reactions. In this case both the second and third
anodic Redox half cell reactions are proceeding and both will be corroding, but the third
electrode is at a lower rate than if the second electrode was not present.
E
(V)
Cathodic Reaction 1
Anode Reaction 3
Eoc (3)
Cathode Reaction 3
Total Anode
Anodic Reaction 2
icorr (3)
log Current Density
2
A/cm
The introduction of a less noble metal will decrease the corrosion rate of the more noble
metal.
Both these figures show that introducing a more anodic metal will decrease the
corrosion rate for a more noble metal. This is the process behind galvanic corrosion. It can
also be used for protection by galvanizing.
Factors Affecting Galvanic Corrosion.
1. Area Effect.
When current flows between the anode and cathode, the CURRENT will be the same
in the anode and cathode independent of the surface area of each electrode. It is the
CURRENT rather than the CURRENT DENSITY which is equal for the anodic and
cathodic reactions. Therefore, if the current flowing between the anode and the cathode is
one amp and the surface areas are one cm2, then the current density in each electrode is one
A/cm2. However, if the area of the anode is only 0.1 cm2, then the current density in the
anode with the same one amp flowing is 10 A/cm2. From Faradays Law, the corrosion rate
depends on the CURRENT DENSITY in the anode. In this case decreasing the surface area
of the anode increases the corrosion rate by a factor of 10.
As a general rule to minimize galvanic corrosion, the anode area should be large and
the cathode area should be small. This leads to a result that on first appearance would seem
incorrect. For protection from galvanic corrosion, the CATHODE of the system should be
painted if a coating is applied. This arises from the area effect, in that if the paint is damaged
by a scratch for example, then a small cathode to large anode area ratio is formed which
results in minimizing corrosion rates. If the anode is painted, then damage to the paint
results in a large cathode to small anode ratio which results in large corrosion rates in the
anode and rapid penetration into the metal.
The anode to cathode area effect is an important characteristic. It is important in
several other forms of corrosion including pitting corrosion, crevice corrosion, stress
corrosion cracking and corrosion fatigue. Remember the important effects of the anode to
cathode area ratios.
2. Physical Distance Effect.
Another important factor for galvanic corrosion is the distance effect. Galvanic
corrosion rates are the largest at the interface between the anode and cathode and decrease
with distance away from the contact region. As the anodes and cathodes are both good
conductors - remember carbon can cause galvanic corrosion by acting as a cathode - electron
transport is very good. What is more difficult is the ionic transport in the electrolyte. As the
distance between the anodic reaction site and the cathode reaction site increases the
transport of the ions becomes more difficult and the corrosion rate decreases. Essentially the
resistance between the anode and cathode increases with distance.
This is an important factor in determination of the form of corrosion. If a suspicion
of galvanic corrosion exists then it should follow the rule of the corrosion rate being higher
adjacent to the galvanic contact region. If the corrosion rate is greater away from this area
then another type of corrosion may be involved. For example if corrosion appears at a
constriction some distance from a galvanic contact, then erosion corrosion was the cause not
galvanic contact.
3. Distance Apart in the Galvanic Series.
A listing of the corrosion resistance in seawater for many metallic systems was
produced and it is called the galvanic series. It is an empirical listing of the corrosion
resistance of metals. Its advantage over the Redox series is that it refers to alloys in a real
environment. Metal at the top of the system are highly cathodic while metals at the bottom
highly anodic.
For selection purposes metals close together on the list are desirable as there is little
driving force for corrosion to be accelerated.
Clearly, it is undesirable to connect metals widely spaced on the galvanic series.
4.Temperature Effects.
With increasing temperature above 180oF, zinc will form a protective layer and
become cathodic to iron. The zinc becomes nonprotective and aggressive to the iron.
It is best to conduct a test program to determine the relative corrosion rates for the
metals under the conditions they will be exposed to in order to determine any unusual
effects which may take place due to unpredicted reactions.
Prevention of Galvanic Corrosion.
1. Electrically Insulate the Metals.
Electrical insulation of the dissimilar metals will avoid galvanic corrosion. One
method demonstrated in the class is to use non metallic spacers.
2. Materials Selection.
Try to avoid selecting dissimilar metals. If that cannot be avoided refer to some of
the following protection techniques.
3. Design.
Several features can be designed in to minimize the effects of
galvanic corrosion:a) Design for the cheapest replacement.
Allow corrosion to occur in a component that can be replaced without loss of
production capability. This may mean some redundant systems such as multiple valves and
pipes in one area. However, this would allow one pipe system to be used as a bypass while
the corroded pipe from the first system is replaced without loss of production. After the
second pipe exhibits corrosion then use the first pipe system and replace the second pipe.
This is corrosion control rather than prevention. However, there is no plant downtime and
the corroded component is a piece of pipe rather than an expensive and difficult to replace
valve.
b) In sealed systems use the area effect.
In a sealed system where the supply of dissolved oxygen for the cathodic reaction is
limited and easily expended, if a large anode to small cathode area is designed then only a
very small degree of corrosion over the large anode will occur before the oxygen consumed.
Then no more corrosion will occur. If a brass fitting is placed on a water tank fro a closed
system where the steel surface area of the tank is much larger the brass area, then the steel
will corrode slightly until all the oxygen is used up and then it will cease.
4 Coatings.
Paint the cathode to prevent the cathodic reaction.
5. Sacrificial Protection
The addition of a sacrificial element will avoid corrosion .
6. Impressed Current Protection.
If electrons are supplied to the anode then it will not corrode despite the galvanic
contact with a more nobler metal.
7. Inhibitors.
Cars use inhibitors in antifreeze as there is often galvanic contact in the water
cooling system between the aluminum alloy cylinder head and the cast iron engine block
and water pump.
Testing Technology.