A passive metal is one that is active in the EMF series but which corrodes nevertheless at a very low rate due to formation of very thin, oxidized, and protective films on its surface in corrosive solutions: e.g. Fe, Ni, Cr, Al, Ti, Zr, Nb and their alloys.

Definition 1 : A metal is passive if it substantially resists corrosion in a given environment resulting from marked anodic polarization. Definition 2 : A metal is passive if it substantially resists corrosion in a given environment despite of a marked thermodynamic tendency to react. Ex… Pb in acid, Mg in H2O.

Farady's Farady's Experiment Experiment (1840s)(1840s)

4. Passivity

Under certain conditions of potential and pH, some metals form protective films … i.e., they passivate.

We can examine the We can examine the kineticskinetics via an Evans diagram … via an Evans diagram …

Pourbaix diagram for the iron/water/dissolved oxygen system...

showing the effect of potential in moving the system from a corrosive (active) region (point 1) to a passive region (point 2)

NOBLE

APP

LIE

D P

OT

EN

TIA

L (V

)

active

passive

transpassive

log CURRENT DENSITYip ic

H2 M +

M

MxOy

M

O2

M

MxOy

ACTIVE

Epp

EF

Epp = passivating potentialIc = critical anodic current densityEF = Flade potentialip = passive current density

At low potential, corrosion rates increase with increasing anodic potential Above passivating potential Epp, the passive film becomes stable and corrosion rate falls to very low values At higher potential (ETransP), passive film breaks down and anodic rate increases again (trans-passive region)

Relation between Pourbaix diagram and polarization curve of active-passive metal

2H++2e-H2

Fe2O3

Fe3+

Fe2+

Fe

5 pH

E

Fe3O4

E

transpassive

passive

Active dissolution

2H2OO2+4H++4e-

MM++e-Epp

icorrioM+/M

ioH+/H2(Fe)

Er,H

Ecorr

Er,M

ip

log |i|

Theories of Passivation

1) Oxide film theory : The passive film is always a diffusion-barrier layer of reaction products, e.g, metal oxide or other compound which separates metal from its environment and which slows down the rate of reaction.

According to this model, the following processes are driven by the electric field across the film:

Transport of the metal cations or of oxygen anions through the oxide. Dissolution of metal cations from the film at the film/environment interface.

Properties required for protective passive film:

Stability over a wide potential range. Mechanical integrity. Low ionic conductivity. Low solubility and slow dissolution in the aqueous medium.

2) Adsorption theory : A chemisorbed layer of oxygen displaces the normally adsorbed H2O molecules and slows down the rate of anodic dissolution involving hydration of metal ions. Adsorbed oxygen decreases the exchange current density of metal:

M + (H2O)ads [M(O2-)]ads + 2 H+

Cathodic Reaction C2: (Ee)C2 > EppHowever, curve intersects Tafel line for anodic reaction below icriticalPassive film cannot form. … metal corrodes actively.Examples: Chromium in air-free H2SO4; Fe in dilute HNO3.

Cathodic Reaction C3: both passivating conditions are met:(Ee)C3 > Epp and i (intersecting Tafel line) > icrit) … metal passivates.Examples: Stainless steels & titanium in acid solutions containing oxidizers.

Impact of various cathodic reactions on the corrosion current and potential for a metal capable of undergoing an active-passive transitionCathodic Reaction C1: (Ee)C1 < Epp

(Ecorr)1 < Epp … … metal corrodes activelyExample: Titanium in HCl or H2SO4

Effect of oxidizer

To safely maintain passivity, oxidizer concentration should be greater than the minimum amount necessary to produce spontaneous passivation(that is, 5 or more).

Oxidizer undergoes the reduction reaction.Increasing the concentration of an oxidizer moves the cathodic curves from 1 to 7.

Effect of chloride ions• Chloride ions damage the protective

films and cause the metal surface to be pitted.

• Examples: – Stainless steel is subjected to

serious pitting by stagnant water containing a high concentration of chloride ions.

– Steel pipes are subjected to pitting in brackish water and seawater.

• The higher the concentration of chloride the greater is the tendency of pitting.

• The effect of chloride concentration on the passivation of steel is shown in Fig. 3.33.

• Chloride ions break down the passivity and increase the rate of anodic dissolution.

Effect of solution velocity

Effect of T and pHPassive region decreases with increasing temperature and higher acidity (H+)

Passivation (of Ti) by galvanically coupling (to Pt) in acid solution:

Titanium does not exhibit a transpassive region.

Corrosion rate isobtained by intersection of cathodic (H2 evolution) line with anodic passivation curve of titanium. This raises the possibility of anodic protection.

Alloying

Passivating potential too noble (highly +ve) for couple to passivate metal. If very large Pt cathode area is used, active anode potential can be increased to P.

Galvanic couple between an active-passive metal (Cr) and platinum (Pt) in air-free acid solution (no oxygen):

Cr – Pt coupling in not recommended in oxidizing conditions since the corrosion rate may be increased.

Alloying

Effect of adding Cr is to decrease corrosion rate at a given potential

(Corrosion Potential, Current and Rate)

Weight Loss Measurement Electrochemical Polarization Methods (ECPM)

Galvanostatic Measurement (current control) Potentiostatic Measurement (potential control)

1) Tafel Extrapolation Method (TEM)2) Linear Polarization Resistance Method (LPRM)

- ECPM are used for studying corrosion (e.g., evaluating the performance of a metal specimen in a test solution)

- Often involve the construction of potential vs. current curves … i.e., they involve the study of polarization characteristics.

5. Measurement of Corrosion

15

Polarization Measurements The simple potentiostat for applying a fixed potential (relative to a reference electrode) and measuring the current (flowing from the working electrode to the counter or auxiliary electrode) …

*ensure specimen potential (w.r.t. counter) constant … even though solution resistance might alter.

A typical arrangement of the classical potentiostat

A Real Potentiostat

Basic Cell Design / Components

• Electrodes– Working electrode (W.E.)– Counter electrode (C.W.) or Auxiliary electrode (A.E.)– Reference electrode (R.E.)

• Electrolyte solution• Cell container

2-Electrode cell: used for low conductivity solutions (e.g. cooling water)

3-Electrode cell: used for high conductivity solutions (e.g. chemical process industries

The PotentiostatVariable High Voltage Source 50-300 V

High Impedance Voltmeter 1012 Ω

Auxiliary ElectrodeWorking

Electrode Reference Electrode

Ammeter (current)

Working Electrode (Test Specimen)

• Requirements

– reproducible– representative– free of shape defects– Free of galvanic effects.

• Properties

– Often a small sphere, small disc or a short wire.– Has a useful working potential range– Usually < 0.25 cm2 surface area– Smooth with well defined geometry for even current and

potential distribution

Counter Electrodes Function to supply current required by the W.E. without limiting the

measured response. Current should flow readily without the need for a large over-

potential. Products of the C.E. reaction should not interfere with the reaction

being studied. Counter electrode should allow current to pass with tolerable

polarization Usually use platinum or graphite, although stainless steel can be

used in some situations (e.g. where only anodic polarization of specimen is used)

Reference Electrode• The role of the R.E. is to provide a fixed potential which does not vary during the experiment. • A good R.E. should be able to maintain a constant potential even if a few micro-amps are passed through its surface.

Reference electrodes: Examples• Calomel (Hg/Hg2Cl2)

the most popular R.E. in aq. solutions; usually made up in saturated KCl solution (SCE); may require separate compartment if chloride ions must be kept out of W.E.

• Silver – silver chloride gives very stable potential; easy to prepare; may be used in non aqueous solutions

Solution in SCE (or Ag/AgCl electrode) is saturated KCl

– beware of chloride contamination of test solution by Cl- leaking from reference electrode– make sure solution remains saturated

The electrolyte solution• Consists of solvent and a high concentration of an ionized salt and

electroactive species

• High conductivity of the solution and reduced resistance:

between W.E. and C.E. to help maintain a uniform current and potential distribution

between W.E. and R.E. to minimize the potential error due to solution resistance

add supporting electrolyte, such as sodium perchlorate

• Oxygen concentration often critical - aerate by bubbling air or O2 or deaerate with N2 or Ar

• Most reactions temperature sensitive. So, control or at least record temperature

• The lowest cost method of potential control is to connect the sample to a low resistance electrode with a stable potential.

• Potential is monitored… it is manually adjusted to the desired value.

The working Electrode

Tafel’s MethodPlot of the total current (iT = io + ic) versus potential showing the extrapolation of the Tafel regions to the corrosion potential, Ecorr, to yield the corrosion current, icorr.

Resolution, sensitivity and precision:

• Precision or accuracy: is the ability to measure the ‘true’ value.

• Resolution: the ability to detect small changes in a large value. for most corrosion measurements 1 mV is adequate for electrochemical noise and similar studies, 1mV

may be necessary

• Sensitivity: the ability to measure small values. it is relatively easy to obtain a sensitivity of 1 mV

when measuring 1 mV, but it is very difficult to obtain a resolution of 1 mV when measuring a 10 V signal

not usually a problem for corrosion measurements