Ronak Macwan Ppt

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PRESENTATION

ON

METALLURGICAL & MATERIAL ENGINNERING DEPARTMENT

FACULTY OF TECHNOLOGY & ENGINEERING, KALABHAVAN

VADODARA

2011.

Prepared By-MACWAN RONAK R.

SEMINAR NO. 35ROLL NO. 912

B.E-IV(Metallurgy)

A SPECIAL TYPE OF CORROSION:

CREVICE CORROSION



CORROSION.

(A) Corrosion is the surface wastagethat occurs when metals are exposedto reactive environments.

(B) Corrosion is the result of interactionbetween a metal and environments

which results in its gradual destruction.



TYPES OF CORROSION.

There is not a unique classification ofthe types of corrosion, but thefollowing classification is adapted

hereafter.

1. GENERAL CORROSION

2. LOCALIZED CORROSION



1. GENERAL CORROSION

1. Atmospheric Corrosion

2. Galvanic Corrosion:

3. High-Temperature Corrosion4. Liquid-Metal Corrosion

5. Molten-Salt Corrosion

6. Biological Corrosion7. Stray-Current Corrosion



2. LOCALIZED CORROSION

1. Crevice Corrosion

2. Filiform Corrosion

3. Pitting Corrosion4. Oral Corrosion

5. Biological Corrosion

6. Selective Leaching Corrosion



CREVICE CORROSION

“crevice means thin and narrow crack”

1. OCCURRENCE, CONDITIONS:

- This is localized corrosion concentrated increvices in which the gap is sufficiently wide forliquid to penetrate into the crevice and sufficiently

narrow for the liquid in the crevice to be stagnant.



CONT…

-Crevice corrosion occurs on flange coupling,nail and screw heads and paint coating edges, inoverlap joints, between tubes and tube plates inheat exchangers etc.

-The most typical crevice corrosion occurs onmaterials that can easily be passivated (stainlesssteels, aluminium, unalloyed or low alloy steels inmore or less alkaline environments etc.), whenthese materials are exposed to aggressive species

(e.g. chlorides) that can lead to local breakdown ofthe surface oxide layer.



CONT…

- Materials like conventional stainless steels can be

heavily attacked by slowly flowing seawater. A criticalvelocity of about 2 m/s has often been assumed, but morerecent studies have indicated that crevice corrosion canoccur at higher velocities too.

-The corrosion form is most frequently observed inenvironments containing chlorides, but can also occur inother salt solutions.



2. MECHANISM:

-Figure shows two stages in the developmentof crevice corrosion. A flange joint of a metal M isexposed to neutral water containing oxygen andNaCl. For simplicity, we assume firstly that M is a

univalent metal. The development can be dividedinto the following four stages:

STAGE 1.

-In the beginning the metal corrodes at thesame rate inside and outside the crevice (Figure a).



CONT…

The anodic reaction:

M ----------> M+ + e-

The cathodic reaction:

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

-The oxygen is gradually being consumedwithin the crevice.

-Oxygen is more slowly transported into thecrevice than it is consumed inside it.



STAGE 1.



STAGE 2.

-When the oxygen has been completely

consumed inside the crevice, OH- can no longer beproduced there. Conversely, the dissolution ofmetal inside the crevice continues, by migration ofCl- into the crevice (Figure b).

-In this way an increasing amount ofdissociated metal chloride (M+Cl-) is produced inthe crevice.

-The metal chloride reacts with water

M+

Cl-

+ H2O ---------> MOH + H+

Cl-

-Metal hydroxide is deposited and hydrochloric

acid is formed in the crevice, which causes agradual reduction of the pH. This process leads to a

critical corrosion state.



STAGE 2.



STAGE 3.

-When the environment has become

sufficiently aggressive, the surface oxide film isattacked. The crevice surfaces are transferred to anactive state and the corrosion rate increases.

STAGE 4.- Because of increased corrosion rate, the

migration of Cl- increases also, which contributes toa further acceleration of the corrosion process. The

process promotes itself, i.e. it is “auto–catalytic”.-Since the pH has been strongly reduced, the

hydrogen reaction takes place.2H++2e- ----------> H2



CONT… This reaction act as second cathodic reaction.

-pH inside the crevice is 0 –4- pH outside the crevice is 9 –10

The corrosion current: Icorr = Io + IH

Wr, Io= oxygen reduction current outside thecrevice

IH= hydrogen reduction current inside thecrevice

-There is a potential drop E = Eo – Ei in theelectrolyte from the free surface outside to theinside of the crevice.



CONT…

-The area of the crevice surface is often smallcompared with that of the outer surface.

-This means that a large corrosion current isconcentrated on a small area, so that the corrosioncurrent density is very high in unfavourable cases.



ANODIC POLARIZATION CURVE FOR ASPECIMEN WITH AN ACTIVE CREVICE



CONT…

-Various metals, e.g. Al, Fe, Cr and Ni, may suffer

from crevice corrosion(various hydroxides deposited increvice.)

-The time until the crevice becomes active (stages 1-3) is called the incubation (or initiation or induction) period.

This may last, e.g. several months-After the incubation period, penetration of plates with

thickness of a few millimetres may in extreme cases occurwithin some days.

-The length of the incubation time is difficult to predict.-Crevice corrosion is affected by several factors, i.e.

metallurgical, environmental, electrochemical, surfacephysical, and geometrical nature.



PHOTOS OF CREVICE CORROSION



PREVENTION OF CREVICE CORROSION

-Crevice corrosion can be prevented or reduced by

appropriate:1. SELECTION OF MATERIAL:

-The conventional stainless steels, withmartensitic, ferritic, austenitic or ferritic –austenitic

(duplex) structure, are sensitive to crevicecorrosion, Newer high-alloy steels with high Mocontent show by far better crevice corrosionproperties in seawater and other Cl –-containingenvironments .

Table :Resistance of various materials to crevicecorrosion in stagnant seawater

1.



2. DESIGN & PRODUCTION

-Avoid as far as possible crevices and deposition. A

couple of examples are given in Figure: a and b.Comments:

a) Use butt welds instead of overlap joints. Alternatively,overlap joints can be sealed by continuous welds.

Be careful about the use of backing gas in weldingprocesses, and check that the penetration is complete.

After welding of stainless steels, carry out pickling,brushing, blasting or grindingfor removal of less

protective and less resistant surface layers.b) Design in a way that promotes complete drainageand prevents accumulation of deposits. Make it easy toinspect and clean.



EXAMPLE OF GOOD & BAD DESIGN



3. CATHODIC PROTECTION:

-The potential must

be kept below the protection potential. An efficientmethod for internal protection of pipe joints is the“resistancecontrolled cathodic protection” (RCP)

4. MEARURES FOR PREVENTING DEPOSITION:

a) Inspection and cleaning during and betweenservice periods.

b) Separation of solid material from flowing media

in process plants.c) Gravel filling around buried piping and structures.



REFERENCES

1. CORROSION & PROTECTIONBY EINAR BARDAL

2. CORROSION ENGINEERING

BY MARS G. FONTANA & NORBERT D. GREEN

3. INTERNET

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