p77 ethos for determination of corrosion Rate

|- Micro gravimetric or weight loss method

We have two methods for Metal or surface polishing
I- Mechanical polishing
The surface can be polished by using emery paper with
different grades or polishing machine.

II- Chemical polishing

The surface can be polished by using chemical bath with
certain composition. This bath should not attack the surface
itself. Itis used only for polishing the surface. The chemical
bath was prepared according to (ASTM) American Standers
for Testing Materials .

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Penetrating rate (mpy) = corrosion rate = 534 W/ADT

W: weight loss (gm)

A: area (inch)

t > time (year)

D > density (gm \ cm?)

mpy > mils per year

Mils > = 0.001 inch

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Advantage = it’s,easy to applied for any metal

Disadvantage : if the loss in weight is very small it

—

can’t detected, so it isn’t very sensitive to any small mass

change .

Quartz Crystal Microbalance (QCM)

Itis avery sensitive technique to determine a very small

mass changes. It is a sensitive to detect the mass changes in

10°? gm ( nano-gram)
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© aide
;

Ifisa restricted only for a reaction accompanied by a gas

evolution.

Example

The amount of hydrogen evolved can be determined, which is

proportional to the corrosion rate.
Disadvantageous: It is not be applied generally for
determination the mass change, but it 1s a restricted only for a

reaction accompanied by a gas evolution. 9

7
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 KH,
—

~. Hydrogen Evolution Measurements

Figure 1 is a simple set-up designed for hydrogen
evolution measurements. It consists of an inverted
funnel and _ burette assembly above’ the
experimental sample.

a) b)

6
2
H gas I, ow]
61 wit ‘e 5
as ° ~

5 E4
c
_ A)
jo) S 3
3.5% Nact {ie | So
a
e. pa 2
= 1 wi

: ess ol «
Mg exposed 0 24 48 72 96
‘che Time/hours

Figure . (a) Hydrogen evolution measurement set-up and (b) hydrogen evolution as a function of
time -

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—

The hydrogen evolution rates were calculated from

_, the following equation:

CRy io Vv, = V;

t{ - ¢

where V, and V; are the volumes of hydrogen

evolved at time t, and t; ,respectively.

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— method

Mostof corrosion process depends on temperature where

—

corrosion rate changes by change of temperature . Corrodant

solutions are put in isolated beaker to prevent transfer

temperature from the surrounding then sample and

thermometer are put where corrosion rate changes with

temperature , so we measure the change in temperature at

different time interval.

a, i
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 YY)
r (RN) a corrosion rate

r ; RN = Corrosion rate = T,,- T;/t

mperature

i)
—
fe
OS
ar
a
=o)
EH

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wo method (Rp)
——<

. = Geary equation
—

|e i B / Rp

B= bab. 2303 (b. FB)

Rp: Polarization Resistance

b, slope of anodic curve

b, slope of cathodic curve

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—
qe)
ea

&
t region
Non-linea
cD)
—

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an
—
b, slope of cathodic curve

Current Density (log I)

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This mé¢thod isn’t accurate when applied for very diluted

solution due its high polarization resistance (Rp).

We can determine Rp accurately using EIS technique in dilute

solution

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 FORMS OF CORROSION
Stress corrosion cracking
Stress & corrosion
Uniform Attack Erosion-corrosion
Oxidation & reduction Break down of passivating
occur uniformly over layer by erosion
surface.
Forms Pitting
Intergranular Downward propagation
Corrosion along of of small pits & holes.
grain boundaries,
often where special corrosion
phases exist.

Galvanic
Atmospheric corrosion
Dissimilar metals are
Degradation of the Crevice Between two
physically joined. The
material with external more anodic one pieces of the same metal.
environment corrodes.(Zn & Mg Rivet holes
very anodic.
0 Pe
* ae "7

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 TYPES OF CORROSION
1 Uniform: Uniform attack of all surfaces exposed to corrodent:
Electrochemically or Direct attack, (where the material has poor
conductivity properties).

2 Pitting: It is a localized Corrosion and its attack produces a pit —

formed at the passive metal surface via access of some aggressive
anion such as Cl-
3. Crevice: Local attack in a crevice between metalto metal, and/or
Non-metal surfaces. Corrodent must be 1n the crevice
4 Galvanic: 2 or more dissimilar metals electrically immerseded in an
electrolyte. One metal become anodic and corroded by galvanic cell
action.

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 5- STRESS CORROSION CRACKING:

RESULTING FROM THE APPLICATION OF

STRESS ON THE MATERIAL PRESENT IN

AGGRESSIVE ENVIRONMENT LEADING

TO A FAILURE OF THE MATERIAL .

6 Intergranular attack: Corrosion occur preferentially at grain boundaries

in metals/alloys.

7 Erosion Corrosion: It is a form of corrosion resulting from the

relative motion of the corrosive fluid and metal surface .

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 SH,
w/ @ Pitting corrosion

It is a localized Corrosion and its attack produces a pits

formed at the passive metal surface via access of some
aggressive anion such as Cl

wa ae —_— Tea J77T7F

Figure Different shapes of corrosion pits. (ASTM [7.21].)

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 MECHANISM OF PITTING
I. Initiation: Local breakdown of passivity (breakdown of the passive
film) where anion such as Cl: attack the film and lead to the dissolution
of the metal. One pit is formed then continue to grow

II. Propagation : rapid dissolution of metal (anodic reaction) according

to:

Anodic reaction: M= Mt +e

Cathodic reaction : O,+,H,O+4e = 40OH-

Hydrolysis
Mirch vir MOH + HCl

Hydrochloric acid is formed inside the pit, which causes a gradual

reduction of the pH.

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I~ THE PIT MAY BE DEEP AND

HORIZONTAL OR DEEP AND NARROW (PIT GROWTH).
THE PIT MAY BE POSSIBLY REPASSIVATED
(TERMINATION). SOME OF CORROSION PRODUCTS
MAY BE DEPOSIT OVER THE TIP MOUTH AND
PREVENT CONTINUOUS PROCESS OF PITTING
CORROSION.

an i
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eT ~ Factor affecting Pitting Corrosion

1 Effect of alloying composition:

The presence of some alloying element such as ( Cr , Ni, Mo ) increase
the Pitting Corrosion resistance

2 Environmental effect :
Pitting corrosion increases .with increasing the concentration of the
aggressive ion

3. Metallurgical properties:
Impurities contribute to the localization of pits on surface and enhances
the pitting process.

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2 surface roughness:
Rough surfaces get numerous smaller pits and enhances the pitting
process.

4 The insulating ability of the oxide: If the oxide insulates efficiently,

the surface is inactive as a cathode and causes a relative slow growth of
pits

5 Temperature
Increasing temperature is accompanied by increasing liability to pitting
corrosion

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 PROTECTION AGAINST PITTING CORROSION

1 Metal must be polished well and contains no defects .

2 Addition of some alloying element such (Cr , Ni , Mo ) increase
corrosion resistance .

3. Coating the metal.

4 Selection of an appropriate materials with high resistance to pitting
corrosion.

5 The surface should be free from defects

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 GALVANIC CORROSION
The potential difference resulting from the presence of two dissimilar

metals in corrosiove solution 1s called gavanic corrosion.

Factor affecting galvanic corrosion

Potential difference between anode and cathode: galvanic corrosion
increases as the potential difference increase.

Ratio of anode to cathode surface area: Small surface area of anode

to

cathode increases the galvanic corrosion.

3 Distance between cathode and anode. Galvanic corrosion decrease as

the distance increases.

4 Temperature and pH value of surrounding environment

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 PROTECTION AGAINST GALVANIC CORROSION

1 Decrease the potential difference between the anode and the cathode.
2 The area the anode should be larger than the area of the cathode.
3. The distance between the anode and cathode should be adjusted to
decrease the Galvanic Corrosion.

4 Control the temperature and pH of the Solution to decrease the

galvanic Corrosion.

5 Selecting adequate materials.

6 Insulating the parts of the couple from each other

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 we
7- Using a metallic coating on one of the materials
8. Avoiding access of humidity to the contact region between the
materials

9. Adding inhibitors.

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