FORMS OF CORROSION
 Corrosion may be
classified in different ways
 Wet / Aqueous corrosion &
Dry Corrosion
CORROSION
WET
CORROSION
DRY
CORROSION
CORROSION
 Room Temperature/ High
Temperature Corrosion
ROOM
TEMPERATURE
CORROSION
HIGH
TEMPERATURE
CORROSION
WET & DRY CORROSION
 Wet / aqueous corrosion is the major form of
corrosion which occurs at or near room temperature
and in the presence of water
 Dry / gaseous corrosion is significant mainly at high
temperatures
WET / AQUEOUS CORROSION
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
Uniform Corrosion
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.
Uniform Corrosion
 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.
Uniform Corrosion
EXAMPLES:
1.rusting of iron 2.tarnishing
of silver 3.Fogging of nickel
4.high - temperature
oxidation of metals
Corrosion Rate and Classification of
Metals
 mm/y – millimeters penetration per year
 gmd – grams per square meter per day
 ipy – inches penetration per year
 mpy – mils penetration per year (1000 mil = 1 inch)
 mdd – milligrams per square decimeter per day
Classification of metallic materials
according to their rate of uniform attack
A.
B.
C.
<0.005 ipy (<0.15 mm/y) : Metals in this category have good
corrosion resistance and can be used for critical parts
0.005 to 0.05 ipy (0.15 mm/y to 1.5 mm/y) : Metals in this
group are satisfactory if a higher rate of corrosion can be
tolerated
>0.05 ipy (>1.5 mm/y) : Usually not satisfactory
Galvanic Corrosion:
 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.
Galvanic Corrosion
 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
Galvanic Series:
Questions:
1. Worst combination?
2. Aluminum and steel?
3. Titanium and Zinc?
4. Stainless Steel and
Copper?
GALVANIC SERIES
Mercury
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
Liquid Cell Battery:
dry cell is a galvanic electrochemical cell with a pasty low-moisture
electrolyte. A wet cell, on the other hand, is a cell with a liquid electrolyte,
such as the lead-acid batteries in most cars.
Dry Cell - Zinc-carbon battery
- oxidation reaction that happens at zinc = anode :
Zn(s) → Zn2+(aq) + 2 e- reduction reaction at carbon rod = cathode
Galvanic Corrosion
Galvanic Corosion
 Dissimilar metals are physically
joined in the presence of an
electrolyte.
 The more anodic metal corrodes.
Bilge pump - Magnesium
shell cast around a steel core.
Galvanic Corrosion
Steel screws and brass
Steel screw in Mg
Dissimilar metals, the damage occurs
at the anode.
Design for Galvanic Corrosion?
 Material Selection: Do not connect dissimilar metals! Or if you
can’t avoid it:
 Try to electrically isolate one from the other (rubber gasket).
 Make the anode large and the cathode small
 Bad situation: Steel siding with aluminum fasteners
 Better: Aluminum siding with steel fasteners
 Eliminate electrolyte
 Galvanic of anodic protection
Design for Galvanic Corrosion?
 Galvanic severity depends on:
 NOT
 Not amount of contact
 Not volume
 Not mass
 Amount of separation in the galvanic series
 Relative surface areas of the two. Severe corrosion if anode
area (area eaten away) is smaller than the cathode area.
Example: dry cell battery
Steel bolt (less noble) is
isolated from copper plates.
Pitting
 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.
Pitting
 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.
304 stainless steel /
acid chloride solution
5th Century sword
Boiler tube
Pitting
Intergranular Corrosion
 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.
Intergranular Corrosion
 Corrosion which occurs preferentially at grain
boundries.
 Why at grain boundries?
 Higher energy areas which may be more anodic
than the grains.
Intergranular Corrosion
 The grain boundaries in
metals are more active than
the grains because of
segregation of impurities
and depletion of protective
elements. So preferential
attack along grain
boundaries occurs. e.g.
weld decay in stainless
steels
Intergranular Corrosion
How to recognize it?
 Near surface
 Corrosion only at grain boundries
 Corrosion normally at uniform depth for all grains.
Example 1: Intergranular Corrosion
 Sensitization of stainless steels:
 Heating up of austenitic stainless steel causes chromuim
carbide to form in the grains. Chromuim is therefore
depleted near the grain boundries causing the material
in this area to essentially act like a low-alloy steel which
is anodic to the chromium rich grains.
 Preferential Intergranular Corrosion will occur parallel
to the grain boundary – eventually grain boundary will
simply fall out!!
Intergranular corrosion
Corrosion along
grain boundaries,
often where precipitate
particles form.
Intergranular Corrosion
Example2:Intergranular Corrosion
 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 lead in to
increased corrosion propagation through the metal.
Intergranular Corrosion
Design for Intergranular corrosion
 Watch welding of stainless steels (causes
sensitization). Always needs proper annealling
after welding to redistribute Cr.
 Use low carbon grade stainless to eliminate
sensitization (304L or 316L).
 Add alloy stabilizers like titanium which ties up the
carbon atoms and prevents chromium depletion.
CREVICE CORROSION
Intensive localized
corrosion within
crevices & shielded
areas on metal surfaces
Small volumes of
stagnant corrosive
caused by holes,
gaskets, surface
deposits, lap joints
Crevice Corrosion
Narrow and confined spaces.
Crevice Corrosion
Crevice Corrosion
This is a concentration cell in action. Notice how the damage
occurs in out of sight places.
VELOCITY ASSISTED CORROSION
 Fast moving corrosives cause:
 a) Erosion-Corrosion
 b) Cavitation damage
 c) Impingement attack
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.
Erosion-corrosion
Combined chemical attack and
mechanical wear
(e.g., pipe elbows)
Brass water pump
Cavitation Damage
 Cavitation is a special case of
Erosion-corrosion.
 In high velocity systems, local
pressure reductions create
water vapour bubbles which
get attached to the metal
surface and burst at increased
pressure, causing metal
damage.
Cavitation Erosion
Impingement Attack
Dealloying:
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).
Dealloying
Two common types:
 Dezincification: preferential removal of zinc in brass
 Try to limit Zinc to 15% or less and add 1% tin.
 Cathodic protection
 Graphitization : preferential removal of Fe in Cast Iron
leaving graphite (C).
Dealloying
Dealloying
Preferred corrosion of
one element/constituent
[e.g., Zn from brass (Cu-Zn)].
Dezincification.
Dealloying
Alloys exposed to
corrosives experience
selective leaching out of
the more active
constituent. e.g.
Dezincification of
brass.
Loss of structural
stability and
mechanical strength
Dealloying:
Danger!
 The alloy may not appear damaged
 May be no dimensional variations
 Material generally becomes weak – hidden to inspection!
Fretting Corrosion