Corrosion process and control - PowerPoint Presentation

Slide1

Corrosion process and control(TKK-2289)

15/16 Semester genap

Instructor: Rama Oktavian;

Vivi

Nurhadianty

.

Email: rama.oktavian86@gmail.com

Office Hr.:

T. 11-12

, Th.

08

-10; 13-15, F. 08-10; 13-15Slide2

Corrosion types

Uniform corrosion

The

uniform thinning of a metal

without any

localized attack

Corrosion does not penetrate very deep inside. Familiar example is the rusting of steel in airFactors that can cause corrosionSlide3

Corrosion types

Uniform corrosion

Corrosion can proceed in a dry

environment without

any moisture

if traces

of sulfur compounds or H2S or other pollutants are present in the air.Slide4

Corrosion types

Uniform corrosionSlide5

Corrosion types

Uniform corrosion

Corrosion can be caused in the

atmosphere when

about 70% of the humidity is

present

. In the presence of such humidity, an invisible thin film of moisture is formed on the surface of a metal.

Effect of humiditySlide6

Corrosion types

Uniform corrosion

If visible water layers are formed on the

metal surface

, corrosion initiates. Splashing of

seawater, rain

and drops of dew provide the wet environment. Water layerDew Formation

If the dew becomes acidic, due to the

presence of

SO

2

, it increases the rate of

corrosion.Slide7

Corrosion types

Uniform corrosion

In atmospheric corrosion, a very thin layer of electrolyte is

present

. Corrosion

products are

formed close to the metal surface. Mechanism of uniform corrosionSlide8

Corrosion types

Uniform corrosion

At the anodic area

Mechanism of uniform corrosion

At the cathodic area

The OH ions react with the Fe++ ions

produced at

the anodeSlide9

Corrosion types

Uniform corrosion

With more access to oxygen in the air,

Fe(OH)2 oxidizes

to Fe(OH)3 and later it loses its water

Mechanism of uniform corrosion

Ferrous hydroxide is converted to hydrated ferric oxide or rust by oxygenSlide10

Corrosion types

Uniform corrosion

Tarnishing

of silver

ware

Tarnishing of electrical

contactsRusting of steels in open airCorrosion of offshore drilling platformsFailure of distillation

columns

Corrosion of underground pipes

Corrosion of heat exchanger tubes

Example of uniform corrosionSlide11

Corrosion types

Galvanic corrosion

Occurs

when two metals

with different

electrochemical potentials or with different tendencies to corrode are in

metal-to-metal contact in a corrosive electrolyte.When two metals with different potentials are joined, such as copper (+0.334 V) and iron (−0.440 V), a galvanic cell is

formed.

The driving force for corrosion is a potential difference between different

materials

DefinitionSlide12

Corrosion types

Galvanic corrosion

MechanismSlide13

Corrosion types

Galvanic corrosion

The positive ions (Fe++) flow from

the anode

(iron) to cathode (copper) through the electrolyte, which is water

.

The hydrogen ions (H+) are discharged at the copper cathode, and ultimately hydrogen is released.The Fe++ ions travel towards the cathode and OH− towards the anode. They combine to

form insoluble iron hydroxide,

Fe(OH)2.

MechanismSlide14

Corrosion types

Galvanic corrosion

Position

of metals in the galvanic series

.

The magnitude of galvanic corrosion primarily depends on how

much potential difference exists between two metals.the metals selected should be close to each other in the galvanic series to minimize galvanic corrosion.

Example: aluminum should not be joined to

steel, as

aluminum being more active would tend

to corrode

Factors affecting galvanic corrosionSlide15

Corrosion types

Galvanic corrosion

The nature of environment.

Consideration

must be given to the environment that surrounds the

metal.

Water containing copper ions, like seawater, is likely to form galvanic cells on a steel surface of the tank

Factors affecting galvanic corrosionSlide16

Corrosion types

Galvanic corrosion

Area, Distance and

Geometric Effects

.

The anode to cathode area ratio is

extremely important as the magnitude of galvanic corrosion is seriously affected by itFactors affecting galvanic corrosionSlide17

Corrosion types

Galvanic corrosion

Area, Distance and

Geometric Effects

.

The

solution conductivity varies inversely with the length of the conduction path.If two different metals are far away from each other, there would be no risk of galvanic corrosion, because of very little

current flow.

As current does

not flow

around the corners, the geometry of the

circuit affects the degree of galvanic corrosion

Factors affecting galvanic corrosionSlide18

Corrosion types

Galvanic corrosion

Galvanic corrosion of steel pipe with

brass fittings.

Galvanic corrosion of the body of the ship

in contact

with brass or bronze propellers.Galvanic corrosion between the tubes and the tube sheet in heat exchangers.Galvanic corrosion of steel coated

with copper

due to the defects in copper coating

Example of galvanic corrosionSlide19

Corrosion types

Crevice corrosion

Caused by the

deposition of dirt, dust, mud and deposits

on a

metallic surface or by the existence of

voids, gaps and cavities between adjoining surfaces.Presence of narrow spaces between metal-to-metal or non-metal to metal components

Presence of cracks, cavities and other

defects on metals

Deposition of

biofouling

organisms and similar

deposits

Deposition of dirt, mud or other deposits

on a

metal surfaceSlide20

Corrosion types

Crevice corrosion

The conventional steels, like SS 304 and SS

316, can

be subject

to crevice

corrosion in chloride containing environmentsFactors affecting crevice corrosionSlide21

Corrosion types

Crevice corrosion

Effect of bulk concentrationSlide22

Corrosion types

Crevice corrosion

Mechanism

Most

of the

mechanism is

based on certain type of concentration cells. a difference in metal ions exists between the crevice and outside, hence, a corrosion cell is formed.

A high concentration of oxygen on the surface outside the crevice and a low

oxygen concentration

inside a crevice creates a differential aeration cell, which initiates

crevice corrosionSlide23

Corrosion types

Crevice corrosion

MechanismSlide24

Corrosion types

Crevice corrosion

Mathematical model for crevice corrosion

Developed by Oldfield and

Sutton suggested four stages of crevice corrosionSlide25

Corrosion types

Crevice corrosion

Mathematical model for crevice corrosion

Stage 1- the initial reaction when

stainless steel

is placed in

a oxygenated neutral chloride solution

The solution in the crevice

becomes slowly

depleted in oxygenSlide26

Corrosion types

Crevice corrosion

Mathematical model for crevice corrosion

Stage

1

Oxygen

is depleted and the solution inside becomes deoxygenatedThe metal ion concentration is increasedSlide27

Corrosion types

Crevice corrosion

Mathematical model for crevice corrosion

Stage 2- the cathodic reduction of

oxygen proceeds

outside the crevice and slow dissolution of the metal takes place inside the creviceSlide28

Corrosion types

Crevice corrosion

Mathematical model for crevice corrosion

Stage 3- accelerated

corrosion takes

place due to the breakdown of the

passive filmSlide29

Corrosion types

Crevice corrosion

Mathematical model for crevice corrosion

Stage 4- the crevice corrosion continues to

propagate.

Example of crevice corrosion

- Development of leaks at the rolled joint of the pipe and pipe bottom in a heat exchanger.

- Crevice corrosion of a tubing in a hydraulic

oil cooler.Slide30

Thank You !