Forms of Corossion

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TRAINING MANUAL – PIPING

FORMS OF CORROSION –

RECOGNITION AND PREVENTION

Uhde India Limited

DOC No. : 29040-PI-UFR-0026

Rev. : R0

Page : 1

CONTENTS

Page

0.0 Cover Sheet 1

1.0 Introduction 2

2.0 Thermodynamics 2

3.0 Electrochemical Criteria 2

4.0 Forms of Corrosion 3 – 6

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Uhde India Limited

DOC No. : 29040-PI-UFR-0026

Rev. : R0

Page : 2

1.0 INTRODUCTION:

Corrosion can be defined in a more simplistic way as deterioration of materials under the

influence of an environment. Without exception, the corrosion of metals and alloys (majority of materials used in industry) in aqueous environments (the most often encounteredenvironment) is an electrochemical reaction. A brief description of the electrochemicalprinciples for the beginners of corrosion are made before attempting on the forms of corrosion. For a more detailed study the participants can refer the bibliography given at theend.

2.0 THERMODYNAMICS:

At the outset it is necessary to pose the question WHY DO METALS CORRODE? And set acriteria for the process. Corrosion of metals and alloys involves oxidation from its metallicstate and therefore must obey the thermodynamic criteria as any other processes. Take for

example, the corrosion of Mg in water, which can be represented by the following reaction.

Mg + H2O(l) + 1/2 O2 = Mg(OH)2 G° = -142.6 K Cals

The corrosion of Mg is spontaneous as it leads to a -ve change in free energy. On thecontrary the reaction of Au with H2O and O2 results in a +ve change in free energy and so Aucannot be corroded.

3.0 ELECTROCHEMICAL CRITERIA:

It must be remembered that corrosion of metals and alloys by aqueous media mustnecessarily proceed through electrochemical process, hence it is imperative to setelectrochemical criteria for corrosion of metals. As indicated earlier, corrosion of metalsinvolves oxidation of metals and alloys from their metallic state in ionic form. It is representedas

M = Mn+

+ ne (Oxidation)

One can notice, the oxidation of metal simultaneously leads to release of electrons. Shouldthe reaction to continue there must be a sink for electrons. This is accompanied by areduction reaction. WHAT ARE THESE CATHODIC REACTIONS?. One or more of thefollowing reactions are possible. They are

2H+

+ 2e = H2 (Hydrogen Reduction reaction)

O2 + 4H+ + 4e = 2H2O (Reduction of dissolved O2 in acidic media)

O2 + 2H2O + 4e = 2OH-

(Reduction of dissolved O2 in basic media)

Mn+

+ ne = M (Metal Deposition)

Mn+

+ e = M(n-1)+

(Metal ion reduction)

Interestingly oxidation (also called anodic) and reduction (also called cathodic) reaction occur on the same surface of the metal. In uniform corrosion these anodic and cathodic sites keepsinterchanging with respect to time.






Uhde India Limited

DOC No. : 29040-PI-UFR-0026

Rev. : R0

Page : 3

4.0 FORMS OF CORROSION:

4.1 UNIFORM CORROSION:

Though uniform corrosion is an idealized form of corrosion and cause less damage than theother forms of corrosion, it is more appropriate to understand this form of corrosion.

This leads to uniform thinning of the structures. The attack is measured in terms of penetration. They have the units mpy(mills per year)or mm per year. This can be determinedby any gravimetry method. Depending on the applications the tolerance of the alloy towardscorrosion is chosen as given in the Table 1.

Table 1 : Range of corrosion rates

Corr osionRate

Application

<5 valves, pumps, shafts andimpellers

5-50 tanks, piping, valve bodies,bolt heads

>50 not satisfactory

<.5 pharmaceuticals, implants for bodies.

4.1.1 Prevention

One or more of the following methods can be adopted to prevent uniform corrosion.

• Cathodic Protection

• Inhibitors

• Protective Coatings

• Selecting Proper Materials

4.2 GALVANIC CORROSION:

When dissimilar metals or alloys differing in their galvanic or corrosion potential are employedand if they are electrically shorted they induce this type of corrosion. The corrosion rate of thealloy with lower corrosion potential will be accelerated by that of higher corrosion potential.

4.2.1 Identification

• The active metal is corroded

• Grooving of the interface

• Noble metal deposits from the stream

• Graphite lining or bricks






Uhde India Limited

DOC No. : 29040-PI-UFR-0026

Rev. : R0

Page : 4

4.2.2 Prevention

• Provide electrical insulation between the metal

• Choose alloys closer in the galvanic series

• Provide design in structure so as to make anodic to cathodic ratio extremely large.• Coat both anode and cathodic areas. Otherwise coat only the cathode.

• Protect the corroding metal with a sacrificial anode, which is anodic to the corrodingmetal.

4.3 CREVICE CORROSION:

Accelerated corrosion occurs if differential aeration exists due crevice, metal joining (lap joints, flanges etc.)or any deposits. Interestingly the location starving for oxygen is forced tobecome anodic and the region having free access to oxygen becomes cathode.


• Rivets, flanges, lap joints are attacked internally.

• Deposits such as corrosion products, organic deposits, growth of organisms etc. causecorrosion.

• Improper drainage of vessels, pipelines cause accelerated attack.

4.3.2 Prevention

• Avoid riveting, go in for welding

• Design for proper drainage

• For stainless steels high Mo content (316,317 and Haste alloys) reduces crevice

corrosion• Remove the deposits

• Use solid non absorbent gaskets

4.4 PITTING CORROSION:

Alloys in presence of certain ions (such as halides) are prone to pitting. The rate of penetration within the pit can be as high as one million times as compared to thesurroundings.


• Pinholes

• Normally grow in the direction of gravity

• The alloy environment combination is likely to promote pitting

• Pitting has taken place along inclusion

4.4.2 Prevention

• Eliminate the specific ions responsible for pitting (say halides in the case of SS)

• Choose alloy resistant to pitting. In stainless steels high Mo promotes resistance (hastealloys, duplex stainless steels)

• Mild steels serve better in chloride environment than SS if certain amount of uniformcorrosion is tolerated. Monel has more resistance in this environment.






Uhde India Limited

DOC No. : 29040-PI-UFR-0026

Rev. : R0

Page : 5

4.5 SELECTIVE LEACHING (DEZINCIFICATION):

When noble and active elements form an alloy it result in selective removal of the latter. As a

consequence the alloy looses its strength and fails prematurely. Cu-Zn alloys are well knownwhere in dezincification occurs if Zn content exceeds beyond 15 wl. Similarly we havedenickelification, desiliconation, decobaltification.


• They give rise to plug and layered types of attack.

• Change in color (from yellow to brown in the cases of brasses)

• X-ray diffraction can some times reveal selective removal of one element

• There can be a change in density in some cases.

4.5.2 Prevention

• Addition of any one of the elements namely Sn, As, Sb and P

• Al addition reduces overall corrosion and to some extent dezincification.

4.6 INTERGRANULAR CORROSION:

This type of corrosion occurs as a result of selective attack of the grain boundaries wheneither grain boundary becomes highly active or phases prone to selective attack are formed.Stainless steels, which are normally resistant to intergranular attack, when subjected to anheat treatment between 400-900 C become sensitive to intergranular corrosion (IGC). Thisrange can vary depending on the composition of the alloy. This treatment is called

sensitization treatment and alloy is said to be sensitized. This is mainly due to the formation of Cr 23C6 and the consequent grain boundary depletion. Welding, a common practice infabrication causes such an IGC attack.


• Attack of the alloy away from the weldment called heat affected zone.

• Clear ditch type of attack along the grain boundary and consequent weakening seen athigher magnification.

4.6.2 Prevention

• Choose low carbon and extra low carbon stainless steels (such ss are 3041,3161,3171)

• Choose Ti or Ta and Nb containing alloys (321,347)

• Provide a solutions treatment to redissolve the carbides (1050 °C, 30 m)

4.7 EROSION CORROSION:

When there is a relative movement of the corrosive environment with respect to the alloy itcan lead to erosion corrosion. Pipelines and heat exchangers are subjected to such a kind of failure.






Uhde India Limited

DOC No. : 29040-PI-UFR-0026

Rev. : R0

Page : 6


• Attack at the bends in pipelines

• Grooves in the direction of liquid flow.

4.7.2 Prevention

• Reduce the velocity of the medium

• Choose hard materials

• Avoid sharp turns

• Provide hard coatings.

4.8 CAVITATION DAMAGE:

Some variation in erosion corrosion is cavitation damage. Here there is damage due to bubbleformation and collapse when there is hydrodynamic variation in pressure difference along the

line. At low pressure water/liquid vaporizes. When the same is subjected to higher pressurebubble forms and subsequently implodes. This leads to plastic deformation and formation of cavities as brought out in.

4.9 FRETTING DAMAGE:

Moving/vibrating interfaces under load causes a damage akin to wear called fretting damage.Here the relative movement is relatively small in angstroms. Typical failed surface under thisprocess is brought out in.

4.10 STRESS CORROSION CRACKING:

When there is a conjoint action of stress and environment. Stress corrosion cracking occurs(SCC). However SCC is specific to environment. The alloys are susceptible to SCC only whenspecific ions are present akin to pitting corrosion. In addition the alloys fail only if the stressexceeds a threshold level below which they are safe.


• SCC in austenitic stainless steels are predominantly transgranular in nature .

• Failure occurs by brittle mode.

• Ions promoting SCC of that particular alloy must be present. Say Cl and O2 for austeniticSS and ammoniacal solution for Cu base alloys.

• If the alloy is sensitized it can promote integranular mode of cracking.

4.10.2 Prevention

• Select the alloy that is not susceptible to the environment.

• In the case of SS control either Cr or O2. As seen from the we can keep either one of them low.

• Apply load lower than the threshold stress.

• Provide compressive stresses by sand blasting. or shot blasting.

• Avoid stress concentration.

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Forms of Corossion