Basics of Corrosion 2

WELCOME TO ME 575(Advanced Corrosion Engineering)

(6.30~7.45 PM / MW)

Instructor: Dr. Ihsan-Ul-Haq Toor

Office: 63-358/Phone:7493

Office Hours: 12.30~1.30 (UTR)

or appointment via E-mail:

[email protected]

Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING

Corrosion process requires a complete corrosion cell/circuit, which

includes;

How Corrosion Takes Place?

Electrochemical Cell

1. Anode

2. Cathode

3. Electrolyte

4. Electrical path


Anodic reactions/Oxidation reaction

General Reaction (metal oxidation)

MMn+ + ne-(gives off electrons))

Zn Zn2+ + 2e- (Zn corrosion)

Fe Fe2+ + 2e- (Fe corrosion)

Al Al3+ + 3e- (Al corrosion)

Fe2+ Fe3+ + e- (Ferrous ion oxidation)



Cathodic reactions/reduction reaction:


O2 + 2H2O + 4e- → 4OH- (oxygen reduction in water/bases)

O2 + 4H+ + 4e- → 2H2O (oxygen reduction in acids)

2H2O + 2e- → H2 + 2OH- (hydrogen evolution in

water/bases)

2H+ + 2e- → H2 (hydrogen evolution in acids)

Cu2+ + 2e- → Cu (metal deposition=>copper plating)

Fe3+ + e- → Fe2+ (metal/ferric ion reduction)



Fig. 2.4 Coupled electrochemical reactions

occurring at different sites on the same metal

surface for iron in an acid solution. The

electrons lost by the oxidation of Fe atoms

are consumed in the reduction of two H+

ions to form hydrogen gas (H2)



Fig. 2.5 The heterogeneous nature of a metal surface

showing various types of imperfections



Fig. 2.6 Coupled electrochemical reactions occurring at different sites

on the same metal surface for iron in a neutral or a basic solution


Consider what happens when Zn is placed in aerated dilute HCl solution (acidic solution):

Electrochemical nature of Corrosion

CR↑ or ↓?



Atmospheric Corrosion & Relative Humidity

Fig. 2.8 Corrosion of iron in air containing 0.01% SO2 after 55

days of exposure showing the effect of a critical relative

humidity (approximately 60%). Redrawn from Vernon [3] by

permission of the Royal Society of Chemistry



Atmospheric Corrosion & Relative Humidity

Fig. 2.9 Adsorption isotherms for water vapor on α-Fe2O3

[4] showing that multi molecular layers of adsorbed water

are formed at a relative humidity of 60% and higher


Secondary Effects of Cathodic Reactions


During SCC=> hydrolysis of Fe ions decrease in pH=>adsorption of

hydrogen atoms and Hydrogen embrittlement

Fig. 2.10 Crack tip reactions can produce hydrogen atoms

available for migration into the metal at stressed regions ahead

of the crack tip


Secondary Effects of Cathodic Reactions


CR in basic solutions=> O2 reduction OH- formation=>accumulation will

increase pH more alkaline solution more corrosion of some metals

Fig. 2.11 Aluminum has high corrosion rates for both acidic (low pH) and basic

(high pH) solutions [5]. Reproduced by permission of ECS – The

Electrochemical Society


Types of Cells

1: Dissimilar Electrode Cells/Galvanic cell

A potential difference exists when two dissimilar metals, electrically connected, are

immersed in a corrosive solution. A cell that produces electricity as a result of the

spontaneous cell reaction.


Types of Cells

2: Concentration Cells

-Difference in the composition/salt content of the soil or solution

-Change in oxygen concentration


Types of Cells

3: Electrochemical Cells

A cell in which non spontaneous reaction is

driven by an external power source.

For example: Electrolysis

M+ M

m+


ActivePassive

Behavior of Active and Passive metals

Corrosion of Metals


Corrosion Types


Corrosion Types-Uniform/General

Rates of uniform attack are reported in various units;

millimeters penetration per year (mm/y); grams per

square meter per day (gmd), inches penetration per

year (ipy), mils (1 mil = 0.001 inch) per year (mpy),

and milligrams per square decimeter per day (mdd).


Metals are classified into three groups according to their corrosion rates and

intended application.

A. < 0.15 mm/y ( < 0.005 ipy) — Metals in this category have good corrosion

resistance to the extent that they are suitable for critical parts, for example,

valve seats, pump shafts and impellors, springs.

B. 0.15 to 1.5 mm/y (0.005 to 0.05 ipy) — Metals in this group are satisfactory

if a higher rate of corrosion can be tolerated, for example, for tanks, piping,

valve bodies, and bolt heads.

C. > 1.5 mm/y ( > 0.05 ipy) — Usually not satisfactory.



This is a localized type of attack, with the rate of corrosion being greater at

some areas than at others.

If appreciable attack is confined to a relatively small, fixed area of metal, acting

as anode, the resultant pits are described as deep.

If the area of attack is relatively larger and not so deep, the pits are called

shallow.



FC is the result of “slight relative motion (as in

vibration) of two substances in contact, one or

both being metals.

It usually leads to a series of pits at the metal

interface.

Metal - oxide debris usually fills the pits so that

only after the corrosion products are removed do

the pits become visible.

Corrosion Types- Fretting


Cavitation – erosion is the loss of material

caused by exposure to cavitation, which is the

“formation and collapse of vapor bubbles” at a

dynamic metal – liquid interface.

Example, in rotors of pumps or on trailing faces

of propellers.

Corrosion Types- Cavitation


Dealloying is the “selective removal of

an element from an alloy by corrosion”.

One form of dealloying, dezincification,

is a type of attack occurring with zinc

alloys (e.g., yellow brass) in which zinc

corrodes preferentially, leaving a porous

residue of copper and corrosion

products.

The tensile strength and ductility are

seriously reduced.

Corrosion Types- Dealloying


This is a localized type of attack at the grain boundaries of a metal, resulting in

loss of strength and ductility.

Improperly heat - treated 18 - 8 stainless steels or Duralumin - type alloys (4%

Cu – Al) are among the alloys subject to intergranular corrosion.

Corrosion Types- Intergranular

Basics of Corrosion 2

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