Transcript DAY 36: INTRODUCTION TO CORROSION Importance of Corrosion  What is Corrosion? Some theory.  The four things that are required for corrosion  Types.

DAY 36: INTRODUCTION TO
CORROSION
Importance of Corrosion
 What is Corrosion? Some theory.
 The four things that are required for corrosion
 Types of Corrosion

ENVIRONMENTAL DEGRADATION OF
MATERIALS
Materials are “attacked” by their operating
environment.
 We will focus on the degradation of metals. This
is called corrosion.
 Some attention will be paid to polymers, but none
to ceramics.

In metals, corrosion is produced by the loss of
actual material, which leaves the piece as an ion
in solution, and is carried away by an electrolyte.
 Rust is a symptom of this problem in steel, but
there can be corrosion without it.

IMPORTANCE OF CORROSION

The impact of corrosion on society is very
significant.
From NACE:
Direct Costs of
Corrosion:
Nearly 300 G$ in 1998.
Clearly, they rise
proportionally till today.
Between 3-5% of the
Gross Domestic Product.
WHEN PROTECTIVE COATINGS BREAK
DOWN, THINGS CAN GET UGLY.
From Corrosion
Doctors web
site
This obvious, up front relatively uniform corrosion is relatively benign.
We see it for a long time before it hurts us. Not all corrosion is so
nice.
CORROSION DISASTER
Atlantic Southeast 529, 8-21-1995
Lead wool added for
balancing
Cork stopper
A CORROSION DISASTER
The Safety Board concludes that one of the four blades from the left
engine propeller separated in flight because a fatigue crack that
originated from multiple corrosion pits in the taper bore surface of the
blade spar propagated toward the outside of the blade, around both
sides of the taper bore, then reached critical size. (See Section 1.16.1.)
Results of investigations conducted in two previous propeller blade
failures in 1994, one in Brazil with this model blade and the other in
Canada with a similar model blade, indicated that corrosion was
produced when entrapped moisture reacted with residual chlorine in a
bleached cork used to retain the lead wool in the taper bore hole of the
propeller.
Point: Corrosion is subtle and very hard to detect
SO, WHAT EXACTLY IS CORROSION?
Corrosion is an irreversible interfacial
reaction of a material (metal, ceramic,
polymer) with its environment which
results in consumption of the material or in
dissolution into the material of a
component of the environment.
 Chemistry is at work. We are talking about a
certain class of chemical reactions between a
metal and the environment.

EXAMPLE – THE DANIELL CELL

This example illustrates some of the basics of
corrosion.

On the surface of the Zn
bar we have the following
2
Zn  Zn  2 e


On the surface of the Cu
bar we have the following
2

Cu  2 e  Cu
Note the current path. The salt bridge provides for ion exchange.
DISSIMILAR METALS HAVE GALVANIC
POTENTIAL
Anodic
Cathodic
Any voltage, even
if small will
produce corrosion
damage over
time.
Clearly dissimilar
metals will create
a corrosion cell.
The anodic metal
will be damaged.
PLEASE NOTE THE PRESENCE OF
STAINLESS STEEL
Yes, under certain circumstances, stainless
becomes active.
 Factors: (These are bad for any metal!)
1. Low aeration in water
2. Low velocity water
3. Presence of Cl-. Chlorine is one of the worst
offenders in promoting corrosion.

REDOX REACTIONS

Here is an oxidation reaction. Fe is the symbol
for iron. Note that metal looses electrons.
Fe  Fe2  2 e

Here is a typical reduction reaction involving
hydrogen ions in solution. Note that the H gains
electrons.


2 H  2 e  H2
THESE REACTIONS WANT TO OCCUR IN
PAIRS
We are assuming that the Fe
is surrounded by a weak acid
in which H+ ions are
abundant.
This acid is called an
electrolyte. It provides a
home for the dissolve Fe+2
ion.
Note that there has to be an
internal movement of
electrons through the Fe.
WHERE IS THE CATHODE?


At the location at which hydrogen is being
liberated, we have a local cathode, associated
with what is called a hydrogen overvoltage.
Summary: What’s needed for Corrosion
1.
2.
3.
4.
An anode. This is where the damage occurs.
Oxidation takes place.
A cathode. Here’s where the reduction reaction
takes place.
An electrolyte. (Almost any moisture will do.)
A current path between the cathode and anode.
GENERAL REACTIONS

Anode: (Metal basically dissolves in the
electrolyte.)
M M

n
 ne

Cathode: (This is a very common reaction!)
2 H 2O  O2  4 e  4 OH 


Surfaces near high O2 concentration are cathodic!
CONCENTRATION CELL
M  M  n  n e
2 H2O  O2  4 e  4 OH 

TYPES OF CORROSION
Uniform - common surface effect.
 Galvanic - dissimilar metals.
 Crevice corrosion.
 Pitting.
 Intragranular.
 Errosion corrosion.
 selective leaching. De-zincification of brass
 Stress corrosion.
 Hydrogen embrittlement

UNIFORM CORROSION
This one is common in steel that is unprotected by
any surface coating. Most noticeable. Surface
effect, leaving rust on the surface.
 The good thing about this, if there is one, is that
the corrosion is widely spread around.

The more dangerous
forms of corrosion are:
1. Highly localized,
concentrated.
2. Hidden.
Electrolyte?
GALVANIC CORROSION
Steel screws and brass
Steel screw in Mg
Dissimilar metals, the damage
occurs at the anode.
CREVICE CORROSION
This is a concentration cell in action. Notice how the
damage occurs in out of sight places.
PITTING
This is similar to crevice corrosion. It is based on
low oxygen concentration at the bottom of the pit.
 This is very common in materials that protect
themselves with a passive layer, i.e. stainless.
Also, aluminum.

Highly localized. Goes
deep into the metal.
Chloride ions find their way
into the pits, exacerbating the
situation.
STRESS CORROSION
Sometimes called stress corrosion cracking.
 Ingredients: (1) tensile stress in the metal (2)
corrosive (electrolyte) environment.
 Accelerators: presence of Chloride ion and high
temp.
 Victims: Stainless steel is unsafe in water above
50C and over a few ppm of chloride, if any
tension exists. Others: mild steel in alkaline
environment, copper alloys in ammonia env.
 The anode is the stresses region.

SCC IN STAINLESS STEEL
Failure is along grain
boundaries.
INTERGRANULAR CORROSION
This is a segue from the previous. It is closely
related.
 Again, stainless steel is the ideal victim here.
The problem is triggered by improper heating,
and often this comes with welding. Carbides of
chromium form in the grain boundary regions.
 The chromium is tied up in the carbides. It can’t
protect by forming the passive layer.
 PLUS, there is a dissimilarity in metals
producing a small but definite galvanic corrosion.

MORE INTERGRANULAR
Exfoliation corrosion in Aluminum that has been
heavily worked, such as in extrusion.
 Corrosion products start to build up in between
the long elongated grains, separating them and
leadin to increased corrosion propagation through
the metal.

SELECTIVE LEACHING
Another example of microstructural corrosion.
 In an alloy system, one phase may be anodic
with respect to another phase.
 Example: dezincification of brass.
 Example: graphitization of cast iron.

EROSION CORROSION
This is caused by the impingement of a high
velocity turbulent flow on a surface.
 The flow is often multi-phase. This means there
can be entrained solid particles, or even gas
bubbles, as in cavitation of a propeller.
 The flow will carry away any protective layer that
was intended to protect the material, and even
abrade the flow surface.

HYDROGEN EMBRITTLEMENT
This is not exactly galvanic corrosion, but it
definitely is a form of environmental attack.
 Hydrogen atoms diffuse into the metal from
outside. Deep in the metal, they combine to form
H2 gas or combine with C, if present to form CH4.
 The pressure in this internal pockets of gas is
enough to initiate cracking.
 The metal is already seeing a lot of tensile stress.
 Normally ductile high strength metals,
particularly steels, are not so ductile anymore
because of these internal cracks.

WHERE DOES THE HYDROGEN COME
FROM?
Arc welding can a source.
Hydrogen might be
released from the
electrode.
 Galvanic corrosion can
produce hydrogen in a
reduction reaction.
 Sour gas wells
 Hydrogen storage (You just
don’t use high strength
steel!)

CORROSION PROTECTION
Protection of the Anode. (Passivation)
 Reduce the activity of the cathode and or
electrolyte. (Polarization)
 Sacrificial Anodes
 Impressed Voltages

PASSIVATION OF THE ANODE
We have two examples already. Stainless and
aluminum.
 A thin oxide layer forms on the surface and
isolates the metal from the environment.
 Zn, Mg, Cu and Ti are also capable of passivation
under normal conditions of operation.
 Steel will also passivate in the presence of an
alkaline environment, such as rebar in concrete.
 Corrosion inhibitors. Some of these, such as the
chromates, are capable of coating a steel and
passivating it.
 Coatings, paints, etc.

POLARIZATION
This is an effect which reduces the actual
chemical potential driving the cell. If the
thermodynamic force driving the ion into solution
is reduced, this is polarization.
 Easy example. By lowering the electrolyte
temperature, we find that it is usually less
corrosive. Diffusion of ions is slowed.
 Inhibitors are chemicals which slow corrosion.
Some of them do this by promoting the
polarization of the cathode.

SACRIFICIAL ANODES
Galvanization of Steel
 Dip steel sheet in molten zinc. Get a pretty thin
coating.
 Zinc will be anode. Steel exposed by crack is the
cathode. Since we have a huge anode having to
be served by a small cathode, corrosion rate will
be slow.

Tiny cathode (steel)
Large area
anode (zinc)
An example of a favorable area ratio. Bad deal: huge cathode, tiny anode
ANOTHER EXAMPLE

Zinc is attached to the steel hull of the vessel.
Attachment points
SACRIFICIAL ANODE FOR A PIPELINE
IMPRESSED VOLTAGE
By imposing a voltage which causes electrons to flow towards
the object to be protected, we make it less anodic and protect it
from corrosion damage.
POLYMER DEGRADATION
Swelling and Dissolution (Solvents)
 Bond Rupture

Radiation (UV and higher)
 Chemical Reaction Effects (Oxygen and Ozone)
 Thermal Effects

http://inside.mines.edu/~dwu/classes/CH351/links/images/Foxtrot%20comic%20UVbull.gif
UV DEGRADATION
Exposure to UV can result in deterioration of
appearance and mechanical properties.
 UV photons have sufficient energy to break
carbon-carbon bonds
 UV + Oxygen is photooxidation
 The property degradation is due to

Chain scission (reduction in molecular weight)
 Crosslinking (loss of ductility)
 Induced Crystallization

http://en.wikipedia.org/wiki/UV_degradation
Free Radical oxidation of UHMWPE
tibial implant. Could happen in vivo
or in vitro. Vitamin E has been tried
to deal with the free radicals.
http://www.informaworld.com/smpp/260129
48671745926/ftinterface~content=a906724758
~fulltext=713240928
http://media.iupac.org/publications/pac/1972/pdf/3001x0135.pdf
http://media.iupac.org/publications/pac/1972/pdf/3001x0135.pdf