Patrimonio asociado

experiment:-"athodic protection usin' sacriFcial anode

To cathodically protect a sample of steel using inc as sacricial anode and appro&imate the average service life of the steel sample

Theory:

0athodic protection preents corrosion b! conertin$ all of the anodic sites on the metal surface to cathodic sites of an electrochemical cell b! suppl!in$ electrical current from an alternate source. &his practise is also referred to as sacriBcial s!stem, since the $alanic anodes sacriBce themseles to protect the structural steel from corrosion. 0athodic protection addresses protection in two wa!s. &he Brst is passiel! b! connectin$ a sacriBcial metal to the metal to be protected. &his sacriBcial anode taes the corrosion on behalf of the structure (cathode), and is therefore replaced as it oxidises and is depleted. &he structure does not corrode, onl! the desired sacriBcial anode. 3olariation of the tar$et structure is caused b! the electron %ow from the anode to the cathode, so the two metals must hae a $ood electricall!

conductie contact. &he driin$ force for the cathodic protection current is the di"erence in electrode potential between the anode and the cathode. &here must be two other conditions existin$

besides the anode and the cathode for the sacriBcial anode method to wor. &here must be a return current path for the electrons to

%ow from the anode to the material it is protectin$ and an electrol!te to cone! the electrons. &he second method of cathodic protection utilies impressed current to actiel! alter the electrical properties of the metal to be protected from corrosion.

/acriBcial anodes are hi$hl! actie metals that are created from a metal allo! with a more ne$atie electrochemical potential than the metal it will be used to protect. /acriBcial anodes $enerall! come in three metals: ma$nesium, aluminium, and inc. >a$nesium has the

most ne$atie electron potential of the three and is suitable for on-shore pipelines where the electrol!te resistiit! is hi$her.

>a$nesium anodes are not suitable in sea-water, because low solution resistiities allow rapid consumption of the anodes. 7inc and aluminium are $enerall! used in sea-water where the resistiit!

is $enerall! lower. &!pical uses are for the hulls of ships and boats, o"shore pipelines and production platforms, in salt-water-cooled marine en$ines, on small boat propellers and rudders, and for the internal surface of stora$e tans.

Experimental *et-up:

1. &he experiment of sacriBcial cathodic protection was performed on a steel sample usin$ inc as sacriBcial anode.

 &he steel sample can be considered as a hull of a ship.

. #n a small $lass tumbler containin$ an electrol!te hain$

composition of sea-water(2.6Q a0l) is taen.

2. 'oth steel and inc plate are partiall! dipped in the electrol!te. A 0u0u/O4reference electrode is used to measure the potential across half-cell formed b! steel.

4. 4. A saturated calomel electrode is used in a similar wa!

across inc.

6. &wo multimeters are connected across the reference electrodes and half-cells to record the potential across them.

Another multimeter is connected across the electrochemical cell formed b! inc and steel which measures the amount of  current %ow from cathode to anode.

8. &he current %ow and the potential across anode and cathode are recorded at interals until the! reach a stead! alue.

-6.6 -1=26 1=22

-6.6 -1=28 1=22

-6.4 -1=28 1=24

-6.4 -1=28 1=26

-6.4 -1=28 1=28

-6.4 -1=28 1=28

"alculations

:-esistiit! of sea-water([)N 46 ohm-cm /ubmer$ed portions :

7inc :

5en$th(5)N.1 cm

idth()N.1cm  &hicness(&)N=.2cm /teel :

5en$thN=.2cm

idthN1.9cm

 &hicnessN=.6cm

Area of steel submer$ed N 9=.9 cm^ Area of inc submer$ed N 1=.= cm^

0urrent densit! of steel in sea waterN =.=6 mA cm^-

Oerall current demand N (9=.9?=.=6) N 2.628 mA

ow,

7inc(7n)

/teel

esistance of the sacrificial anode N 16[(5=.=.&) N 189. \

#ohmN)( Esteel ] Einc)) N 1=.64 mA

/ince #ohm is bi$$er than the current demand, hence the inc plate can act as the sacriBcial anode.

Assumin$ that the submer$ed inc is completel! used to protect the steel,

@olume of sacriBcial anode N (.1?.1?=.2) cc N 1.22 cc Iensit! N 91 $mcc

Fence, >ass of sacriBcial anode necessar! N (91?1.22) N 2.2

$m

/#0E &FE I#>E/#O/ OD &FE /A03 AE D#XEI &FE EDD#0#E0Y 'E >S5&#35#EI &O &F#/ AI O& I#@#IEI

Efficienc! of inc(^) N=Q

Amount of inc present within that 2.2 N 2.2?.N4.629 $m 5et the serice life of the steel specimen be _xV hours

ow,

1=== $m of inc can produce 1= A-hr

 &hus, 4.629 $m of inc can produce 8.496 A-hr

'ut, 0urrent supplied oer the entire serice life N (2.628?1=^-2?x) A-hr

 &hus euatin$, we $et, xN .1 !ears

/o, the probable serice life of the steel specimen would be 0.0>

years

esults:- &hus,the purpose of the experiment is sered and the serice life of  the sample is about .1 !ears.

0onductiit! of the used sample solutionN46 moh-cm

Precautions:-1.&he multimeter terminals should be connected properl! so as to obsere stead! alues of current and olta$es.

.3roper circuit should be made in accordance with the experiment.

????

Experiment-1

Aim of the

Experiment:-"revice "orrosion

To nd the crevice corrosion rate 3 generate the polarisation curves sho"ing the nature of crevice corrosion.

Theory: –

0reice corrosion is a form of localied attac that occurs freuentl!

on metals exposed to sta$nant solutions within shielded areas such as holes, $asets, lap +oints and creices under bolts. &his form of  corrosion is usuall! er! diCcult to detect, predict and desi$n a$ainst due to the sie and locations of the corrodin$ creice. #t can also be thou$ht of as a $alanic process that occurs between di"erent areas of an identical metal $alanic couple immersed in an electrol!te. &his form of corrosion starts close to the creice mouth and becomes more widespread, pro$ressiel! moin$ to the interior of the material throu$hout the period of exposure to the a$$ressie solution. >aterials with hi$h corrosion resistance are usuall! the most ulnerable to this form of corrosion. ell nown examples of susceptible metals are stainless steel allo!s, nicel, titanium and aluminium.

0reice corrosion is encountered particularl! in metals and allo!s which owe their resistance to the stabilit! of a passie Blm, since these Blms are unstable in the presence of hi$h concentrations of  0l-and F ions.

+echanism

 &he $eneral conditions for creice corrosion include a sta$nant solution and a $ap between two surfaces, one of which is metal, of  the order of 11==th of an inch. #nitiall!, the usual anodic and cathodic reactions occur oer the surface of the metal.

 &he $eneral anodic reaction

is:-> N is:->^  e

- &he $eneral cathodic reaction is :-O^-  FO  4e^- N 4(OF^-)

 &hese initiall! occur oer the whole surface. Foweer a restriction occurs in the creice re$ion such that the dissoled ox!$en in the creice cannot easil! be replaced. &he re$ion inside the creice cannot then support a cathodic reaction. #t can still support an anodic reaction of the t!pe shown aboe. Outside the creice re$ion the cathodic reaction proceeds but anodic reaction ceases as it is concentrated in the creice.

An electrical char$e imbalance exists between the hi$h positie char$e within the creice from metal ions and the ne$atie char$e outside the creice. As a result, ne$atie ions are attracted into the creice. &he limit is the small sie of the creice. 0hloride ions are the faored ions to be attracted into the creice. Associated with the ne$atie chloride ion is the er! small positie h!dro$en ion.

'oth the chloride ion concentration and the h!dro$en ion concentration increase within the creice. &hat is the pF in the creice decreases from alues of 8 to  - 2. &he e"ect of this acidiBcation is that the corrosion rate inside the creice increases.

 &he chloride ion repeatedl! reacts as shown below where the chloride ion associates with the metal ion and the metal chloride reacts with water to form metal h!droxide and h!drochloric acid.

 &he dissociated chloride ion can react a$ain with the metal ion and the series of reactions repeat. &his is termed 4autocatalytic 'ehaviour5 . ith the increase in anodic rate the cathodic reaction of ox!$en outside the creice increases, further protectin$ the re$ion outside the creice.

 eactions inside the creice include:-9^/ / "l^-  9^/"l

-9^// "l^- / ;⁰O 9$O;& / $;^/"l^-&

;^/"l^-  ;^// "l

- &his results in acidiBcation within the creice. ote that onl! the re$ion inside the creice will be corroded. &his is also important as the anodic area is localied and small in comparison to the cathodic area. &he area e"ect then also comes into pla! with a small anode carr!in$ the same current as the cathode, leadin$ to an increased current detnsit! and corrosion rate. /o seeral factors are inoled

in creice corrosion, electrical char$e imbalance, leadin$ to a lower pF and chloride ion concentration increase, and a er! unfaorable anode to cathode area ration all if which lead to an enhanced anodic rate.

Procedure:-1. A stainless steel sample is taen and the surface of the sample is thorou$hl! polished.

. #t is dipped in a beaer containin$ tap water.

2. &he corrosion process is initiated b! the instrument and the software plots the polarisation cure (5inear /weep

@oltammetr!)