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Corrosion can be defined as the deterioration of material by reaction to
its environment.
Corrosion occurs because of the natural tendency for most metals to
return to their natural state; e.g., iron in the presence of moist air will
revert to its natural state, iron oxide.
4 required components in an electrochemical corrosion cell:
1) An anode; 2) A cathode; 3) A conducting environment for ionic
movement (electrolyte); 4) An electrical connection between the anode and
cathode for the flow of electron current.
If any of the above components is missing or disabled, the
electrochemical corrosion process will be stopped.
This example illustrates some of the basics of corrosion.
On the surface of the Zn
bar we have the following
eZnZn 22
On the surface of the Cu bar
we have the following
CueCu 22
Note the current path. The salt bridge provides for ion exchange.
• Ranking the reactivity of metals/alloys in seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel (passive)
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel (active)
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
more
anodic
(act
ive)
more
cat
hodic
(iner
t)
Based on the appearance of the corroded metal, wet
corrosion may be classified as:
Uniform or General
Galvanic or Two-metal
Pitting
Environment-assisted cracking
Intergranular
Crevice
Velocity-assisted
Dealloying
Fretting
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.
Corrosion over the entire
exposed surface at a
uniform rate. e.g..
Atmospheric corrosion.
Maximum metal loss by
this form.
Not dangerous, rate can
be measured in the
laboratory.
EXAMPLES:
1.rusting of iron 2.tarnishing
of silver 3.Fogging of nickel
4.high - temperature
oxidation of metals
Possibility when two dissimilar metals are electrically connected in an electrolyte
Results from a difference in oxidation potentials of metallic ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion.
Refer to Galvanic Series
The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode).
Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials.
When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction
Fig. Al sheets covering underground Cu cables
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 steel and aluminum.
Highly localized. Goes
deep into the metal.
Chloride ions find their way
into the pits, intensifies the
situation.
A form of extremely
localized attack causing
holes in the metal
Most destructive form
Autocatalytic nature
Difficult to detect and
measure
Mechanism
Pitting
Pitting is a localized form of corrosive
attack. Pitting corrosion is typified by the
formation of holes or pits on the metal
surface. Pitting can cause failure, yet the
total corrosion, as measured by weight loss,
may be minimal.
5th Century sword
Boiler tube
304 stainless steel /
acid chloride solution
Questions:
1. Worst combination?
2. Aluminum and steel?
3. Titanium and Zinc?
4. Stainless Steel and
Copper?
5. Mild steel and cast iron?
Galvanic Series:
GALVANIC SERIES
Platinum
Gold
Zirconium Graphite
Titanium
Hastelloy C Monel
Stainless Steel (316-passive)
Stainless Steel (304-passive)
Stainless Steel (400-passive)
Nickel (passive oxide)
Silver
Hastelloy 62Ni, 17Cr
Silver solder
Inconel 61Ni, 17Cr
Aluminum (passive AI203)
70/30 copper-nickel
90/10 copper-nickel
Bronze (copper/tin)
Copper
Brass (copper/zinc)
Alum Bronze Admiralty Brass
Nickel
Naval Brass Tin
Lead-tin
Lead
Hastelloy A
Stainless Steel (active)
316 404 430 410
Lead Tin Solder
Cast iron
Low-carbon steel (mild steel)
Manganese Uranium
Aluminum Alloys
Cadmium
Aluminum Zinc
Beryllium
Magnesium
Note, positions of
ss and al
Big Cathode, Small Anode = Big Trouble
When a metal is subjected to a tensile stress
and a corrosive medium, it may experience
Environment Assisted Cracking. Four types:
Stress Corrosion Cracking(SCC)
Hydrogen Embrittlement
Liquid Metal Embrittlement
Corrosion Fatigue
Static tensile stress and
specific environments
produce cracking
Examples:
1) Stainless steels in hot
chloride
2) Ti alloys in nitrogen
tetroxide
3) Brass in ammonia
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.
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.
High strength materials
stressed in presence of
hydrogen crack at
reduced stress levels.
Hydrogen may be
dissolved in the metal or
present as a gas outside.
Only ppm levels of H
needed
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.
Intensive localized
corrosion within
crevices & shielded
areas on metal
surfaces
Small volumes of
stagnant corrosive
caused by holes,
gaskets, surface
deposits, lap joints
This is a concentration cell in action. Notice how the damage
occurs in out of sight places.
Fast moving corrosives cause:
a) Erosion-Corrosion
b) Impingement attack
c) Cavitation damage in
metals
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.
Forms of Erosion:
› Liquid Impingement
› Liquid erosion
› Slurry Erosion
› Cavitation
Combined chemical attack and
mechanical wear
(e.g., pipe elbows)
Erosion-corrosion
Brass water pump
When one element in an alloy is anodic to the
other element.
Example: Removal of zinc from brass (called
dezincification) leaves spongy, weak brass.
Brass alloy of zinc and copper, and zinc is anodic
to copper (see galvanic series).
Cathodic protection: Make the structure more cathodic by
› Use of sacrificial anodes
› Impressed currents
› Galvanized steel
Anodic protection: Make passivating metal structures
more anodic by impressed potential. e.g. 316 s.s. pipe
in sulfuric acid plants.
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
Zinc is attached to the steel hull of the vessel.
Attachment points
Aluminium anodes mounted on a steel jacket
structure – using galvanic corrosion for
corrosion control! Called cathodic protection
(aka sacrificial anode)
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.
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