3.2 corrosion 3.2.1 electrolytic corrosion 3.2.2 applied voltages 3.2.3 connecting to different...

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

The Cost of Corrosion

Concrete International December 2004

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Electrolytic corrosion. When a metal is placed in water there is a tendency for it to dissolve (ionise) in the solution. Fe Fe++ + 2e-

 where e- is the electron which remains in the metal. Positive metal ions are released into the solution and the process continues until sufficient negative charge has built up on the metal to stop the net flow.

-ve

Fe++

Electrode PotentialsMetal Electrode Potential

Magnesium -2.4

Aluminium -1.7

Zinc -0.76Chromium -0.65

Iron (ferrous) -0.44

Nickel -0.23Tin -0.14Lead -0.12Hydrogen (reference) 0.00

Copper (cupric) +0.34

Silver +0.80Gold +1.4

Current and exchange current

The current will depend exponentially on the difference between the potential and the rest potential:

where V is the Voltage across the anode and B1' is a constant for all samples

Similarly for the exchange current:

(1) e]B)/V[(V

I = I10a

aoa

(2) e]B)/ [(V

I = I10a

ao-aV

Anode current and exchange current

Anode current and exchange current

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

-1 -0.5 0

Voltage V

Cu

rren

t A

Ia+

Ia-

Notation for logarithms

Log(x) = Log to base 10

Ln(x) = Log to base e (natural log)

Thus Ln(x) = Ln(10)

Log(x)

The anode current

It may be seen that at voltages well above Va0

the exchange current is negligible and the voltage may be expressed by rearranging equation 1:

(3) I

I LogB + V = V0a

a10a

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Applied Potential to cause corrosion (reversed to stop it)

+

Fe++

Power

Supply

-

Current (electrons go the other way)

Cathodic protection

Cathodic Protection. Preparing the steel

(the cathode)

Cathodic protectionConductive paint anode (left)Titanium mesh anode (right)

Connection to rebar (left)Main junction box (right)

Bonding steel beams together (left), Casting in connection to the rebar (right)

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Zinc and Copper

Zinc anode system for reinforcement

protection

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

pH

pH = log(1/H+)where H+ is the number of grammes of hydrogen ions per litre.In pure water the following equilibrium reaction takes place:

H2O H+ + OH-

and there are 10-7 grammes of hydrogen ions per litre. Thus the pH of water is 7 and is defined as neutral. Acids have pH below 7 and alkalis (bases) have pH above 7. Concrete has a pH of 12.5.

Corrosion in pure water

The small amount which does take place is caused but the pH of water being 7, not infinite. i.e. there are 10-7 grammes of hydrogen ions per litre in neutral water. They are the product of the equilibrium of the reaction:

H2O H+ + OH-

in which the OH- is a hydroxyl ion which may combine with the iron ions in solution:

Fe++ + 2(OH)- Fe(OH)2 The product is ferrous hydroxide which is a green precipitate.

Anode and Cathode (Could be caused by applied voltage, different metals etc.)

+

Fe+++ 2(OH)- Fe(OH)2

-

CathodeAnode

e-

H2

H+

Anode and Cathode reaction

The reaction of the hydrogen ions with the electrons in the metal:

2H+ + 2e- H2

is known as the cathodic reaction and the dissolution of the metal ions:

Fe Fe++ + 2e-

is the anodic reaction.

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Corrosion with OxygenIf oxygen is present in the water it will react at the cathode: 

2H2O + O2 + 4e- 4(OH)-

 this uses up electrons at the cathode (increasing its potential) and provides hydroxyl ions to react with the iron ions in solution and thus greatly accelerates the corrosion. If there is a good supply of oxygen the final product is ferric hydroxide Fe(OH)3, this is common "red rust". If the air supply is limited, however, the product is Fe3O4 which is "black rust".

Oxygen

+

Fe++

-

CathodeAnode

e-

O2+ water 4(OH)-

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Corrosion in acids

Acids contain free positive hydrogen ions. Provided the metal has a potential below that of hydrogen the hydrogen ions will combine with the electrons in the metal to release hydrogen gas. 

2H+ + 2e- H2 The metal ions will then combine with the acid in solution and the process will continue until either the metal or the acid is exhausted.

Acid corrosion

+

Fe++

-

CathodeAnode

e-

H2

H+

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Pitting

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Pourbaix diagram for steel

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Anode and cathode currents

Anode

CathodeCurrentIc

CurrentIa

Current Ix

Voltage V

STEEL +

CONCRETE -

The current will therefore depend on the resistance in the circuit.

This consists of :

• a. Surface resistance at the cathode

• b. Surface resistance at the anode

• c. Resistance in the solution

Solving for no applied voltageIf a cathodic process is initiated by any of the above processes (e.g. oxygen) its voltage may be expressed as:

If there is no external applied voltage the voltage is known as the rest potential Eo and the current flowing round the "loop"

is the corrosion current Icorr. Thus:

Thus subtracting from (3) and (4)

(4) LogB - V = V0c

c20c

I

I

(5) I

ILogB - V =

ILogB+ V=E

co

corr20c

0a

corr10a0

I

(6) I

I LogB- =

I

I LogB = EV

corr

c2

corr

a10

The linear approximation

but when x is close to 1: x-1 Ln(x)

Thus: (x-1) Log(x)

Ln(10)

Thus when Ia and Ic are close to Icorr

 (7) 1-

I

I (10)LnB- = 1-

I

I (10)Ln

= EVcorr

c2

corr

a10

B

The Tafel Constants B1 and B2

and the Stern-Geary equationsWith the following definitions: 

and 

 Equation (7) reduces to: 

(8) (10)Ln)B+B(

BB = BConstant 21

21

(9) I

B = R resistanceon Polarisati

corr

p

(10) R

= I - I = ICurrent Externalp

0

cax

EV

Combination of Anode and Cathode Currents

-100-80-60-40-20

020406080

100

-600 -400 -200

Voltage mV

Cu

rren

t m

icro

A

Anode Current

Cathode Current

External Current(= Anode-Cathode)

Linear V-E0/Rp

Equivalent circuit for corroding surface

Resistance Rp

Diffusion Potential E0

Combination of Anode and Cathode Currents

-100-80-60-40-20

020406080

100

-600 -400 -200

Voltage mV

Cu

rren

t m

icro

A

Anode Current

Cathode Current

External Current(= Anode-Cathode)

Linear V-E0/Rp

Increased Anode Current (Ia0 increased)

-100

-80

-60

-40

-20

0

20

40

60

80

100

-600 -500 -400 -300 -200

Voltage mV

Cu

rren

t m

icro

A

Effect of increasing the anode current – Increased gradient indicating higher corrosion

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

NCE October 06

The corrosion Circuit

2Fe(OH)2Current

4e-

Electrons

2Fe++

4(OH)-

O2 and 2H2O

STEEL

CONCRETE

Cathodic reactionAnodic reaction

Pinholes in coating on bar – cathode will be 10 times larger than anode

Anode

Cathode

Offshore oil retaining structureAnode and cathode may be several metres apart

Air

Water Oil supplies oxygen to cathode

Cathode

Anode in splash zone

Large reinforced structure

Air (provides oxygen to cathode)

Cathode Anode

Two main differences in the equivalent circuit when in

concrete: 1. The circuit must pass through the concrete

which has a resistance. 2. The steel/concrete interface has a

capacitance. This is known as the "double layer capacitance" and is caused by charge build up at the interface.

Equivalent circuit of steel in concrete

The characteristics of the circuit1 If a voltage different from E0 is applied to it there will be a

high initial current through the capacitor but this will decay to zero. Thus in order to make a linear polarisation resistance measurement it is necessary either to:

•Wait about 30 second after applying the voltage. This has the disadvantage of causing possible changes to the corrosion process. or

•Apply a very slowly changing voltage. or

•Apply a pulse of voltage and make a measurement when it is switched off.

2 When measuring the polarisation resistance the concrete resistance will also be measured. Fortunately the capacitance has a very low resistance to alternating current so this may be used (50 - 100Hz) to measure the concrete resistance and it may then be subtracted.

Current decay

Experimental results for

linear polarisation

(high corrosion)

Experimental results for

linear polarisation

(low corrosion)

Equivalent circuit shown

with potentiostat

Potentiostat in use

Causing corrosion with an anodic voltage

Black rust from samplesTurns red when exposed to air

Circuit for resistance measurement

Measuring Resistivity

Potential Survey

Looking again at equation (5)

It may be seen that when comparing systems with similar cathode conditions (i.e. the same Ic0 and Vc0) as the rest potential Eo increases the log of the corrosion current Icorr decreases. This is the basis of a method of detecting corrosion called potential survey.

I

ILogB - V = E

0c

corr20c0

Rest potential

vs. corrosion current

Linear polarisation apparatus with guard ring

Polarization Resistance

Eo

Voltmeter

Ammeter

Switch D.C. Reference cell

Counter electrode

Working electrode

• Step 1: Measure open circuit potential, Eo

Eo + E

Ip

• Step 2: Close switch and apply small current• Step 3: Measure current, Ip, to produce small change in

voltage, E - 4 mV• Step 4: Increase current, and repeat measurement until E

-12 mV

Polarization Resistance, Rp

E

ip

Current/Area of Bar, ip, (µA/cm2)

Voltage Rp =E

ip

Corrosion Rate:

icorr = B

Rp

(µA/cm2)

B = 25 to 50 mV

From Faraday's Law:1 µA/cm2 = 0.012 mm/y

Guard-Electrode Method

Voltmeter

Ip

Guard ElectrodeAmmeter

VoltageFollower

Confine current so that affected area of bar is well defined

3.2 CORROSION• 3.2.1 Electrolytic corrosion• 3.2.2 Applied voltages• 3.2.3 Connecting to different metals• 3.2.4 Slow corrosion in pure water• 3.2.5 Oxygen• 3.2.6 Acids• 3.2.7 Pitting• 3.2.8 The effect of pH and potential• 2.3.9 Corrosion rates• 3.2.10 Corrosion of steel in concrete• 3.2.11 Corrosion Prevention

Corrosion Prevention

Coatings: This is the standard method (e.g. paint).Weathering Steels: Carbon steel with a 0.2% copper content forms a very stable oxide layer (in the absence of chlorides). It is therefore very durable, but equally ugly.Stainless Steels: are alloys of steel with some chromium and some other elements. Most stainless steels corrode to some extent..Cathodic protection: This method makes the metal cathodic (negative) relative to the solution and thus stops the anodic reaction.

Corrosion of stainless

steel

Sample panel of stainless

steel cladding

Corrosion Prevention

Coatings: This is the standard method (e.g. paint).Weathering Steels: Carbon steel with a 0.2% copper content forms a very stable oxide layer (in the absence of chlorides). It is therefore very durable, but equally ugly.Stainless Steels: are alloys of steel with some chromium and some other elements. Most stainless steels corrode to some extent..Cathodic protection: This method makes the metal cathodic (negative) relative to the solution and thus stops the anodic reaction.