Eight forms of Corrosion: Wk2a

Eight forms of corrosion:

1. Uniform or general corrosion
2. Galvanic corrosion
Identifiable by
visual inspection 3. Pitting corrosion
4. Crevice corrosion
5. Intergranular corrosion
6. Selective leaching
Identifiable with special
inspection tools
7. Erosion corrosion
8. Stress corrosion
Other important forms of Corrosion:

Some other forms of corrosion:

1. Impingement Corrosion
2. Corrosion Fatigue
3. Fretting corrosion
4. Hydrogen Embrittlement
5. Biological Corrosion
 An example of findings in Chemical processing company

Summarization of the findings obtained from 363 corrosion failure

cases investigated in a major chemical processing company .
 1. Uniform Corrosion:

Uniform corrosion is one sort of electrochemical

corrosion
It occurs with equivalent intensity over the entire
exposed surface and often leaves behind a scale
or deposit.

Example: i) rusting of steel and iron

ii) tarnishing of silverware.

This is a general form of galvanic corrosion i.e.

anode and cathode are random and occurs in the
same material!
 1. Uniform Corrosion

Weathering Steel Cleat Carbon Steel Manhole METAL: Carbon Steel

House Drain and Drain Cap

1 year old cap

30 yrs old cap

METAL: Cast Iron; ENVIRONMENT:
Residential basement water exposure
 Uniform (or general) corrosion mechanism (Iron in water):

Uniform Corrosion
mechanism

In anodic reaction, electrons

and metallic radicals (Fe2+)
are released.

On the other hand, released

electrons are consumed in
cathodic reaction and OH- is
produced.

Corrosion compounds
FeO(OH) and/or Fe2O3 are
generated by the interaction
of OH- and Fe2+ radicals.

Iron is corroded by oxidation reaction and rusts are generated on reduction reaction.
Uniform (or general) corrosion mechanism (Iron in water):
Q1: Define rust. Explain the influence of water and
oxygen on the formation of rust.

Rust is a generic term used to describe different iron

hydroxides and oxides, Fe(OH)2, Fe(OH)3, FeO(OH),
Fe2O3.H2O that form when iron corrodes.

The common form of rust is a red product, Fe2O3 called

hematite.

The causes of rust formation through the reaction

between iron and water or iron exposed to the
atmosphere could be either water condensing from
air or rain.

The oxygen in the air dissolves in the water and

causes rust to form.
The associated reactions in the formation of rust are given below:

Anodic Dissolution of Metal (Iron) that goes into solution (water)

Fe -----> Fe2+ + 2e-

Cathodic Reduction of Oxygen dissolved in water

O2 + 2H2O + 4e- ----> 4OH-

The final reaction is: Fe2+ + 2OH- -----> Fe(OH)2

The formed iron hydroxide will then further react with oxygen to give the final
red product (Rust): Fe2O3.H2O
 1. Uniform Corrosion

Major characteristics of uniform corrosion:

i. Thinning might take place until failure.

ii. Uniform in nature leaves scale or deposit over the
entire exposed area this is called rust (e.g. iron-oxide
Fe(OH)3 or Fe2O3.)

iii. Corrosion rate will decrease once the oxide layer has
been established.
iv. Corrosion rates should not be used to estimate the extent
of localized form of corrosion.

v. Weight loss can indicate the thickness reduction.

vi. Fairly predictable and therefore the effects can be
minimized.
 1. Uniform Corrosion

After cleaning the rust:

weight loss
corrosion rate

Corrosion penetration rate (mils/yr):

Constant depending on
desired units

Weight loss after

KW exposure time t
CPR
At
Exposure time
density
Exposed area
 1. Uniform Corrosion
(a) Corrosion rate in mpy:

Kw 10 3 g
Corrosion rate
Dt A g 1
3
( ) y inch 2 where
cm 8760
w : weight loss (mg )
g
3
10 8760 cm 3 D : Density 3
cm
y inch 2 t : Exposure time (h)
10 3 8760 (0.3937 inch) 3 A : Area (inch 2 )

y inch 2 milli inch
Corrosion rate :
534 10 3 inch y

y
534 milli inch

y

K 534
Not in SI Unit!!
534 w milli inch
So, Corrosion Rate .........
Dt A y
 1. Uniform Corrosion
(b) Corrosion rate in m/y:

Kw 10 3 g
Corrosion rate where
Dt A g 1
3
( ) y cm 2 w : weight loss (mg )
cm 8760
g
D : Density 3
8760 10 3 cm cm

y t : Exposure time (h)
8760 10 5 m
A : Area (cm 2 )
y
87600 m m
Corrosion rate :
y y
K 87600

87600 w m
So, Corrosion Rate .........
Dt A y
Not in SI Unit!!
 1. Uniform Corrosion
(c) Corrosion rate in m/y:
Kw kg
Corrosion rate
Dt A kg 1 2
where
( ) y m
m 3 8760 w : weight loss (kg)
kg
8760 m D : Density 3
m
y t : Exposure time (h)

8.76 10 m
9 A : Area (m 2 )
y
m
Corrosion rate :
y
K 8.76 109

8.76 109 w m
So, Corrosion Rate .........
Dt A y SI Unit!!
 1. Uniform Corrosion

534 w milli inch 534 w 3 m 534 w m

Corrosion rate
D t A y y
10 ( 0.0254) 25.4
y D t A D t A

m mm
So, 1 mpy 25.4 0.0254
y y

1 mpy = 0.0254 mm/y = 25.4 m/y

 1. Uniform Corrosion

Uniform Corrosion:
Corrosion rate in terms of current:
r = rate in terms of mol/m2-s
i
r
i = current per unit surface area of
material corroding
nF n = # of electrons associated with
ionization of metal ion
F = constant = 96,500 C/mol
 1. Uniform Corrosion

Uniform corrosion prevention:

i. By removing electrolyte (i.e. lower relative humidity

below 30%).
ii. By choosing material that doesnt rust in that particular
environment. Potential-pH diagram can be used for
selection of materials.

iii. Add design allowance for rust

iv. Cathodic protection.
v. Use of coating/paints.
vi. Ensure adequate metal thickness for a specified design
life
 1. Uniform Corrosion

Test and Design Considerations (Uniform Corrosion)

Design Considerations
Mass loss
Reduction in load bearing capacity

Measurement
Thickness loss
Weight loss/Corrosion rate

Misapplication of Data
Corrosion rate is average value
Uniform corrosion rate cant be used to understand localized corroision
 Potential-pH diagram
Under certain conditions of potential and pH, some
metals form protective films i.e., they passivate:

Pourbaix diagram for the iron /water system showing the effect of potential in moving
the system from a corrosive (active) region (point 1) to a passive region (point 2).
20
 2. Galvanic Corrosion
Galvanic corrosion is an electrochemical oxidation-reduction
(redox) process, which occurs when two dissimilar metals or
alloys are brought into electrical contact and immersed into
an electrolyte solution.

Electrolytes are aqueous solutions of salts, acids and bases.

It is also known as bimetallic corrosion or dissimilar metal
corrosion.
 It occurs due to the difference in oxidation potentials of
metallic ions between two or more metals.

The less noble metal will corrode (i.e. will act as the anode)
and the more noble metal will not corrode (acts as cathode).

The greater the difference in oxidation potential, the greater

is the galvanic corrosion
 Example of galvanic corrosion:

i) Steel screws in brass marine hardware,

ii) steel pipe connected to copper plumbing
iii) steel propeller shaft in bronze bearing
iv) zinc coating on mild steel
v) leadtin solder around copper wires.

vi) Copper and steel tubing are joined in a

domestic water heater, the steel will corrode
in the vicinity of the junction.

vii) Low-cost household batteries typically contain

carbon-zinc cells. As a part of closed circuit,
the zinc within the cell will corrode
preferentially.
 Galvanic corrosion mechanism:
Iron (less noble) and copper (more noble) in a marine environment.

In anodic reaction,
electrons and
metallic radicals
(Fe2+) will be released
into the solution.

On the other hand,

released electrons
are consumed in
cathodic reaction and
OH- is produced.

Corrosion compound
Fe(OH)2 is generated
by the interaction of
OH- and Fe2+ radicals
Major characteristics of galvanic corrosion:

Galvanic corrosion always will be at the joint of two different metals

having different electrode potential for each. The greater the
difference, the higher the driving electric force is for corrosion.

Higher contact resistance at the boundary decreases the corrosion

rate.

It also depends on electric resistance of electrolyte solution.

Electrolyte solution properties (PH, oxygen content, temperature,
flow rate) can influence the rate of corrosion.

Corrosion rate also depends on anode-to-cathode areas ratio. Large

anode connected to a small cathode result in low corrosion rate.
 Prevention of galvanic corrosion:

Galvanic corrosion can be avoided by coupling metals

close to the electrochemical series.

By cathodic protection (Electrically connect a third metal

which is more anodic to the other two).

Fixing insulating material between two metals.

By using larger anodic metal and smaller cathodic metal.

Bad situation: Steel siding with aluminum fasteners
Better: Aluminum siding with steel fasteners
GALVANIC SERIES
Galvanic Series in Seawater (supplements Faraq Table 3.1 , page 65), EIT Review Manual, page 38-2
Tendency to be protected from corrosion, cathodic, more noble end

Mercury
Platinum
Gold
Zirconium Graphite
Titanium
Hastelloy C Monel
Stainless Steel (316-passive)
Stainless Steel (304-passive)
Stainless Steel (400-passive)
Nickel (passive oxide) Note, positions of
Silver
Hastelloy 62Ni, 17Cr ss and al**
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
 Galvanic corrosion: area effects

The rate of galvanic attack depends on the relative anode-to-cathode

surface areas that are exposed to the electrolyte.

A smaller anode will corrode more rapidly than a larger one. The reason
for this is corrosion rate depends on current density.

High current density results for the anode when its area is small
relative to that of the cathode.

The ratio of cathodic to anodic area will determine the rate of

corrosion:
AreaCathode /AreaAnode >> 1 Bad!
 Iron (Anode)

Aluminum (Cathode)

Big Cathode, Small Anode = Big Trouble

 Dry Cell - Zinc-carbon battery

Zn(s) Zn2+(aq) + 2 e- - oxidation reaction that happens at zinc = anode

2MnO2(s) + 2 H+(aq) + 2 e- Mn2O3(s) + H2O(l) - reduction reaction at carbon rod =

cathode

Dry cell is a galvanic electrochemical cell with a pasty low-moisture electrolyte.

Steel bolt (less noble) is isolated
from copper plates.
 2. Galvanic Corrosion

Galvanic corrosion around the inlet of a

single-cycle bilge pump.
 2. Galvanic Corrosion

copper

steel plug

A galvanized steel plug was fastened to a Galvanic corrosion due to differing anodic
copper fitting. index between the bolts and the plate
 2. Galvanic Corrosion

Test and Design Considerations (Galvanic Corrosion)

Design Considerations
Loss of strength
Perforation in applications that are required to be sealed (e.g. valves)
Electronic components
Fasteners must be cathodic

Measurement
Galvanic series (difference in corrosion potentials between metals)

Misapplication of Data
The order of metals on the galvanic series chart changes depending
on electrolyte (seawater versus salt water NaCl)
Identifiable by visual inspection

Uniform Corrosion Pitting Corrosion

Crevice Corrosion Galvanic Corrosion