Dinkar Kokje Ppt

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PassivityMetal is passive when its corrosion in course of

chemical or electrochemical reaction is lower athigher affinity of reaction than at lower affinityof chemical or electrochemical process


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Concept of Passivity It is known that certain metals and alloys can exist in 2

states, namely ACTIVE & PASSIVE.

In Active state metals and alloys exhibit greater activityw.r.t. certain types of environment than others.

In Passive state metals and alloys behave as if they arenoble.

Passivity is the characteristic of a metal exhibited whenthat metal does not become active in Corrosionatmosphere.

Passivity is caused by build-up of a stable, tenacious layerof metal oxide on the surface of the metal.


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Passivity can be defined in 2 ways Thermodynamically

the phenomenon by which the

metal remains unattacked by a

medium in which,thermodynamically a netdecrease of free energy isassociated with corrosion

reaction

Electrochemically

the phenomenon by which the

metal, when acting as an anode

does not pass into the solutioneven though the value ofequilibrium potential seems to

indicate that it should


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Metals such as Zr, Cr, Al and S.S. form thin,tenacious oxide films when exposed to theatmosphere or to pure water at room temperature.

In some cases, the film is extremely thin and is

almost invisible to eye, but is still effective ingiving these metals a marked PASSIVITY.

The metals like Fe, Co, Ni can be rendered passiveby heating in air.

S.S. and Cr plate provide well known examples ofpassivity resulting from exposure to air.


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Characteristic of Passive State In general, oxidizing condition is required for

passive state while reducing condition for active

state. In passive state, corrosion rate reduces to a very

small value. Generally reduction in corrosion ratefrom active to passive state will be of the order of

104 to 106 In passive state, a metal may fail to displace other

metal lower in electrochemical series from its salts.


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ELECTROCHEMICAL PASSIVITY Most metals as anode dissolve when the potential is just

slightly positive than the reversible value in the particularsolution used as an electrolyte.

If the current density is increased, a point is, howeverreached at which the potential increases suddenly or veryrapidly, and there is a corresponding decrease in the

current, at the same time the anode practically ceases todissolve, although its appearance is unchanged. The metalis said to be in Passive State and the phenomenon iscalled ELECTROCHEMICAL PASSIVITY


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Active Passive

Region1) Active Region :- when electrodepotential is made nobler the metalsfollow a typical tafel behaviour &dissolution rate increasesexponentially.

2) Passive Region :- thedissolution rate decreases to a verysmall value and remainsindependent of potential over aconsiderable range

3) Transpassive Region :- aspotential is further increasedfurther in noble direction, thedissolution rate again increases.


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Mechanism of Chemical PassivityA) Oxide film theory

In the film theory the passivation of a metal is caused bythe formation of a thin oxide film on the s/f of the metal,

which ceases to interact with the surrounding medium andis thus protected from surrounding.

1) Formation of Salt Layer

2) Removal of Salt Layer

3) Formation of Oxide Layer


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B) Adsorption Theory

It can be achieved through the retardation of themetal dissolution reaction caused by adsorbed oxygenatoms. The adsorbed oxygen atoms passivate the metaleither by forming a continuous monomolecular layeron its s/f or by blocking the most active centers or elseby changing the effective potential difference acrossthe metal-solution interface.

Also here arises the possibility of passivation byblocking active centers on the s/f of a dissolving metalor by retarding the dissolution p/c electrochemically.


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Increase in Passivity

There are two ways of increasing the passivation and thecorrosion resistance of metal systems by alloying, i.e.introducing components which directly facilitate passivity

(for instance Cr, Si, Mo when added to iron) or introducingcomponents which lower the cathodic polarization (Pt, Pdand other noble metals for stainless steels and titanium).

Experimental data on increasing the passivity and

corrosion resistance of metal by means of alloying withcathodic metals and by other methods, in particular by theuse of anodic protection, are considered


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