L12-Cathodic & Anodic Protection

8/12/2019 L12-Cathodic & Anodic Protection

1/41


2/41

2

The theoretical basis


3/41

3

Free corrosion potential E corr of iron inaerated water is in the range of -600 to -700 mV SSC at pH 7.

Different ways of affecting the condition ofthe system at point O are: Decreasing the pH Increasing the pH Apply a more negative potential

Cathodic Protection (CP) Make the potential more positive

Anodic Protection (AP)


4/41

4

CP Design Considerations


5/41

5

The corrosion rate, i p, at a given polarizationcathodic to E corr (at which the corrosion rate is i corr )

is expressed by

K T Z

T R

F Z ii

corr p

2832

5.0

)exp(


6/41

6

For a given polarization of -200 mV, we get

corr p

corr p

ii x

x x xii

00027.0283314.8

494,962)2.0(5.0exp

Therefore, the corrosion rate at -850 mV SSC will be reduced to 0.03%of the rate of unprotected steel.

At -750 mV SSC, corrosion rate is reduced to 2% of the rate at E corr .


7/41

Cathodic Protection The most important of all approaches to corrosion control.

Using an externally applied electric current, corrosion is reduced(approaching zero).

The mechanism of cathodic protection depends on external current thatpolarizes the entire surface to the thermodynamic potential of theanode.

The surface becomes equipotential (cathode and anode potentialsbecome equal), and corrosion currents no longer flow.

Or, looked at another way, at a high enough value of external currentdensity, a net positive current enters the metal at all regions of themetal surface (including anodic areas); hence, there is no tendency formetal ions to enter into solution.

Cathodic protection can be applied in practice to protect metals, suchas steel, copper, lead, and brass, against corrosion in all soils and inalmost all aqueous media.

7


8/41

Pitting corrosion can be prevented in passive metals, such as thestainless steels or aluminum.

Cathodic protection can be used effectively to eliminate

stress - corrosion cracking (e.g., of brass, mild steel, stainless steels,magnesium, aluminum),

corrosion fatigue of most metals (but not fatigue), intergranular corrosion (e.g., of Duralumin, 18 8 stainless steel), or

dezincification of brass.

It can be used to avoid SCC of high-strength steels, but nothydrogen cracking of such steels. Corrosion above the water line (e.g., of water tanks) is not affected,

because the impressed current cannot reach metal areas that areout of contact with the electrolyte.

Nor does the protective current extend into electrically screenedareas, such as the interior of water condenser tubes (unless theauxiliary anode enters the tubes), even though the water box maybe adequately protected.

8


9/41

9

Types of CP

Two types:1) Sacrificial anode CP (SACP)

2) Impressed current CP (ICCP)


10/41

10

Example of Sacrificial Anode CP over ground


11/41

11

Example of Sacrificial Anode CP over ground


12/41

12


13/41

Sacrificial anodes The sacrificial anodes are usually composed of magnesium or magnesium-

based alloys .

Occasionally, zinc or aluminum has been used. Approximately 10 million pounds of magnesium is annually used for this

purpose. The open-circuit potential difference between magnesium and steel is about

1 V.

This means that one anode can protect only a limited length of pipeline.However, this low voltage can have an advantage over higherimpressed voltages in that the danger of overprotection to some portions ofthe structure is less; and because the total current per anode is limited, thedanger of stray-current damage to adjoining metal structures is reduced.

Magnesium anode rods have also been placed in steel hot-water tanks toincrease the life of these tanks. The greatest degree of protection isafforded in hard waters where the conductivity of the water is greaterthan in soft waters.

13


14/41

Anode Requirements To provide cathodic protection, a current density

of a few milliamps (mA) is required. To determine the anodic requirements, it is

necessary to know the energy content of theanode and its efficiency.

From this data the necessary calculations canbe made to size the anode, determine itsexpected life, and determine the number of

anodes required. The three most common metals used are

magnesium, zinc, and Aluminum. 14


15/41

15

Principle of impressed current CP


16/41

16


17/41

17


18/41

18

Potentials more negative than -850 mV SSC show evenless metal loss. Why not use -1000 mV SSC, or lower?

There are two reasons:1) More hydrogen evolution at more negative

potentials. It may cause damage like

hydrogen embrittlement etc.2) More negative potentials produce high local

concentrations of hydroxyl ions which ,may

cause chalking or damage any barriercoatings such as paint.


19/41

19

Current should be controlled or potential?

Fixing current may cause decrease in currentdensity if the area is increased by damage incoating. This may change potential towards

E corr . Area may increase or decrease

Regulating potential can provide required

current density.


20/41

20

Sacrificial Anode Design


21/41

21

Protection Potential Ep

It is the least negative potential necessaryto achieve a satisfactory level ofprotection.

For steel in aerated seawater, this isconsidered to be 800 mV SSC

The anodes connected for protection are

termed as Sacrificial Anodes . The anodes are usually welded or

mechanically coupled with the structure at

predetermined points.


22/41

22

Anodes Material Usually Zinc, Magnesium & Aluminum Traditionally for steel in seawater; Zinc &

Cast Iron Using sacrificial anodes on Copper reduced

the consumption, however, fouling wasincreased.

C-Sentry __ Zn-alloy with 0.1 0.5% Al, 0.025 0.15% Cd. Extensively used by marine industry.


23/41

23

Sacrificial anodes are supposed to bedissolving at uniform rate.

Commercially pure zinc corrodes in seawaterforming an impermeable skin which severelylimits its current output.

Fe the most detrimental impurity Its solubility in Zn is so low (


24/41

24

Addition of Cd reduces the adverse effect of Pbimpurity

Aluminum undergoes pitting corrosion in seawater because of

cathodic oxide film Therefore, pure Al most unreliable for use as sacrificial

anodes Al-alloys containing Zn & Hg or Zn & In Have much higher electrical power/weight ratios than Zn-

alloys

Magnesium Too vigorous corrosion rate in seawater Used in soil or estuarine waters where resistivity is

relatively high to limit effectiveness of Al or Zn alloys


25/41

25


26/41

26

Efficiency of anodes Magnesium in seawater

E corr ~ -2.12 V SCE (theoretical) E corr ~ -1.7 V SCE (practical) Theoretically, 1 kg should yield ~ 2200 A h

Practically, 1200 A h Therefore, efficiency = 50 60%

Efficiency for Zn and Al alloys is >90%.


27/41

27

Factors that affect current requirements are:1. The nature of the electrolyte2. The soil resistivity3. The degree of aeration

The required current to provide cathodicprotection can vary from 0.5 to 20 mA/ft 2 of baresurface.


28/41

28


29/41

29

Benefit of coating

Less anode material is required for a coatedstructure A surface with a fresh coal tar paint will be well

protected with a current density of 20 30 mA/m 2

to accommodate holidays in the coating A bare steel structure requires >100 mA/m 2.


30/41

30

Few important terminologies Capacity of an anode

The no. of Ampere-hours supplied by each kg ofthe material

Wastage rate

Current output per unit of exposed surfacearea expresses the rate of loss of metal byvolume or mass

Throwing power The effective distance between the metal and

the anode


31/41


32/41


33/41


34/41

34


35/41


36/41

36


37/41

37


38/41

38


39/41

Test ing fo r Com pleteness of Protect ion

The preferred method is to take potentialmeasurements.

Position of reference electrode as close as possible to the protected structure avoids and minimizes internal resistance (IR)

drop through the soil. For buried pipelines, a compromise location is directly over

the buried pipe at the soil surface because cathodicprotection currents flow mostly to the lower surface and areminimum at the upper surface of the pipe buried a few feetbelow the surface.

39


40/41

Overpotential of steel structures, If to a moderate degree, does not cause any problems.

The primary disadvantages are waste of power and increased consumption of auxiliary anodes. hydrogen can be generated at the protected structure,

causes blistering of organic coatings, hydrogen embrittlement of the steel, or hydrogen cracking.

Overprotection of systems with amphotericmetals (e.g., tin, lead, aluminum, zinc)

will damage the metal by causing increased attackinstead of reduced corrosion.

40


41/41

Several ways to check effectiveness of protection.1. C o u p o n t es t . A metal coupon is shaped to conform to the contour of the pipe,

weighed, and attached by a braze-connected cable to the pipe. Both the cable

and the surface between the coupon and the pipe are coated with coal tar. Thecoupon is allowed to remain buried for weeks or months, uncovered, cleaned,and weighed. The weight loss, if any, is an indication as to whether or not thecathodic protection is complete.

2 . Color imet ri c t es t . A piece of absorbent paper soaked in potassium ferricyanidesolution is placed in contact with a cleaned section of the buried pipeline and

the soil replaced. After a relatively short time, the paper is retrieved. A blueferrous/ferricyanide reaction indicates incomplete cathodic protection, whereasan absence of blue on the paper indicates that cathodic protection is complete.

3 . Po ten t i a l measurements . By measuring the potential of the protectedstructure, the degree of protection, including overprotection, can be quantitavelydetermined. This measurement is the generally accepted criterion and is usedby corrosion engineers. The basis for this determination is the fundamentalconcept that cathodic protection is complete when the protected structure ispolarized to the open-circuit anodic potential of the local action cells.

Date post:	03-Jun-2018
Category:	Documents
Upload:	amir-khalid
View:	236 times
Download:	0 times

L12-Cathodic & Anodic Protection

Documents