Final Report - Electrochemistry Lab, JU
Short Description
Jadavpur University Metallurgy Dept Electrochemistry Lab report 2014...
Description
Experiment-1 Aim of the Experiment –
Galvanic series To calculate the electrode potential of dierent materials and to study dierent aspects of the galvanic series
Theory: – An electrochemical cell is based on an oxidation-reduction (redox) reaction and consists of two half-cells: an anode half-cell and a cathode half-cell. Oxidation occurs at the anode; reduction occurs at the cathode. An electrochemical cell can produce an electric current, which is drien b! an electrical potential di"erence between the two halfcells. #n this experiment we will use a oltmeter to measure and compare the electrical potential di"erences of seeral electrochemical cells, some of which will hae di"erent concentrations of metal ions. Electrochemical Cell
#f two half-cells are connected b! placin$ a wire between the pieces of metal and b! addin$ a salt bridge between the two solutions, a direct direct electr electric ic curre current nt can %ow %ow throu$ throu$h h the circui circuit. t. &he electr electric ic current is $enerated because metal atoms in the more reducing metal conert to ions and leae one electrode electrode to enter the solution and and ions ions of the the less reducing metal accept electrons and plate out on the other electrode. &he electrons left behind when positie ions are formed at one electrode pass throu$h the external circuit and into the other electrode. electrode. &here the electrons combine with ions from the solution to form metal atoms. '! measurin$ the direction of cur current ent %ow, %ow, and and the the olt olta$ a$e e $ene $enera rate ted d in the the cell cell,, we can can deter determin mine e which which is the more more re reduc ducin$ in$ metal metal (str (stron$ on$er er reducing agent ), ), and b! how much. Salt Bridge
1
#n order for current to %ow, there must be a complete electric circuit. &he wire is part of the circuit and the salt brid$e completes the circuit. #n this experiment, the salt brid$e is a porous c!linder soaed soaed with aueous aueous potassi potassium um chloride chloride and a$ar a$ar-a$ar -a$ar $el. $el. &he solutions of salts, such as potassium chloride, are electrol!tes* the! the! cond conduc uctt elec electr tric ical al curr curren entt b! moe moeme ment nt of posi positi tie e and and ne$atie ions in the solution. &hus the porous c!linder proides a path for conduction of electricit!, +ust as the wire does, completin$ the electrical circuit. 'ecause di"usion of the solutions throu$h the porous c!linder is slow, there will be no mixin$ of the solution of one half-cell with the solution of another on the time scale of the experiment. &hus the half-cells are connected electricall!, but not chemicall!, b! the salt brid$e. ithout a salt brid$e a cell will not produce an electric current and we will not be able to measure the electrical potential di"erence between the two electrodes. Anode and Cathode
&he half-cell half-cell in which oxidation oxidation occurs is called the anode. anode. &his is the half-cell in which metal atoms lose electrons (are oxidied) to form positiel! char$ed ions (which $o into solution). &he electrons %ow into the external circuit from the anode . &he half-cell half-cell in which which reduction reduction occurs occurs is called called the cathode. cathode. &his is the half-cell in which metal ions from the solution $ain electrons (are reduced) and plate out onto the electrode as unchar$ed atoms. &he electrons electrons %ow out of the external external circuit circuit into the cathode. cathode.
Apparatus :1. eference Electrode-/aturated 0alomel Electrode(/0E) . /alt brid$e 2. 3otentiostat 4. 5ead strip 6. 7inc strip 8. 0opper strip 9. Aluminium strip . /tainless steel strip .ultimeter
Procedure:1. &he sample is immersed in a beaer containin$ tap water. . &he sample is connected to the positie end of the multimeter and the reference electrode is connected to the ne$atie end of the multimeter. 2. &he potential of the sample is measured. 4. Another sample strip is placed and the potential of it is measured followin$ the aboe procedure.
Observations:-
emar!s:1. 0are is taen to ensure that the electrodes do not touch the $lass-walls as it mi$ht lead to erroneous results. . 0are is taen to aoid an! parallax error. ???? 2
Experiment- Aim of the Experiment –
"yclic #oltammetry To study the Cyclic Voltagram of Pt in 1 mole KOH and 0.! glucose solution
Theory: – 0!clic 0!clic @oltam oltamme metr! tr! (0@) (0@) is an elect electro roche chemic mical al techni techniue ue which which measu measure res s the curr current ent that that deel deelops ops in an elect electro roche chemic mical al cell cell under conditions where olta$e is in excess of that predicted b! the ern e rnst st eua euati tion on.. 0@ is perf perfor orme med d b! c!cl c!clin in$ $ the the pote potent ntia iall of a worin worin$ $ elect electro rode de,, and and meas measuri urin$ n$ the re resul sultin tin$ $ curr curren ent. t. #t is a potentiod!namic electrochemical measurement. 0!clic olt oltam amme metr tr! ! is $ene $enera rall ll! ! used used to stud stud! ! the the elec electr troc oche hemi mica call properties of an anal!te in solution. &he potential of the worin$ electrode electrode is measured measured a$ainst a reference electrode which maintains a constant potential, and the result re sultin$ in$ applied applied potentia potentiall produc produces es an excita excitation tion si$nal si$nal such such as that of B$ure 1. #n the forward scan of B$ure 1, the potential Brst scans ne$atiel!, startin$ from a $reater potential (a) and endin$ at a lower potential (d). &he potential extrema (d) is call the switchin$ potential, and is the point where the olta$e is suCcient enou$h to hae caused an oxidation or reduction of an anal!te. &he reerse scan occurs from (d) to ($), ($), and and is wher where e the poten potentia tiall scan scans s positi positiel el! !. Di$ure i$ure 1 shows a t!pical reduction occurrin$ from (a) to (d) and an oxidation occurrin$ from (d) to ($). #t is important to note that some anal!tes under$o oxidation Brst, in which case the potential would Brst scan positiel!. &his c!cle can be repeated, and the scan rate can be aried. &he slope of the excitation si$nal $ies the scan rate used.
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Figure 1: CV Excitation Signal
A c!clic oltammo$ram is obtained b! measurin$ the current at the worin$ electrode durin$ the potential scans. Di$ure shows a c!clic oltammo$ram resultin$ from a sin$le electron reduction and oxidation.
Figure 2: Voltammogram of a Single electron oxidation-reduction
#n Di$ure , the reduction process occurs from (a) the initial potential to (d) the switchin$ potential. #n this re$ion the potential is scanned ne$atiel! to cause a reduction. &he resultin$ current is 6
called cathodic current (ipc). &he correspondin$ pea potential occurs at (c), and is called the cathodic pea potential (E pc). &he Epc is reached when all of the substrate at the surface of the electrode has been reduced. After the switchin$ potential has been reached (d), the potential scans positiel! from (d) to ($). &his results in anodic current (# pa) and oxidation to occur. &he pea potential at (f) is called the anodic pea potential (E pa), and is reached when all of the substrate at the surface of the electrode has been oxidied.
Experimental setup:A 0@ s!stem consists of an electrol!sis cell, a potentiostat, a current-to-olta$e conerter, and a data acuisition s!stem. A standard 0@ experiment uses a reference electrode (RE), a working electrode (WE), and a counter electrode (CE). &his combination is sometimes referred to as a three-electrode setu. An electrol!te is usuall! added to the sample solution to ensure suCcient conductiit!. &he solent, electrol!te, and material composition of the worin$ electrode will determine the potential ran$e that can be accessed durin$ the experiment. &he counter electrode $also !no%n as auxiliary electrode& , is an electrode which is used to close the current circuit in the electrochemical cell it does not participate in the electrochemical reaction. 'ecause the current is %owin$ between the E and the 0E, the total surface area of the 0E (sourcesin of electrons) must be hi$her than the area of the E so that it will not be a limitin$ factor in the inetics of the electrochemical process under inesti$ation. &he reference electrode is an electrode which has a stable and well-nown electrode potential and it is used as a point of reference in the electrochemical cell for the potential control and measurement. Fere saturated calomel electrode is used. &he hi$h stabilit! of the reference electrode potential is usuall! reached b! emplo!in$ a redox s!stem with constant (bu"ered or saturated) concentrations of each participants of the redox reaction. >oreoer, the current %ow throu$h the reference electrode is ept close to ero (ideall!, ero) which is achieed b! usin$ the 0E to close the current circuit in the cell to$ether with a er! hi$h input impedance on the electrometer (G 1== .D. as stated is positie, the reaction here is (n (n// / 0ehile at the ri$ht-hand electrode the electrons are remoed b! the process "u// / 0e- "u &he complete reaction is thus (n / "u// (n// / "u $ree Energ" and %eat Changes in Re#ersible Cells
/ince the uantitatie conseuences of the second law of thermod!namics are mainl! applicable to reersible processes, the stud! of reersible cells is of particular importance because it is possible to appl! thermod!namic methods to the results. #f the E.>.D. of a oltaic cell is E olts, and the process tain$ place in it is accompanied b! the passa$e of n farada!s, i.e., nD coulombs, where D represents 8,6== coulombs, the wor done b! the s!stem in the cell is nDE olt-coulombs or +oules. #f the cell is a reersible one, this wor represents maximum wor and since electrical wor does not inole mechanical wor resultin$ from a olume chan$e, it ma! be taen as eual to the chan$e of free ener$! accompan!in$ the cell reaction. &he increase of free ener$! of a process is eual to the reersible net wor, i.e., excludin$ mechanical wor, done on the s!stem, and hence it follows that 1G - n2E where 1G is the increase of free ener$! for the process tain$ place in the cell under consideration.
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Procedure:1. An electrochemical cell i.e. a Ianiel cell is prepared with 7n electrodes and 0u-electrodes with their respectie solutions 7n/O 4 and 0u/O4 is made. . 7n electrodes are dipped in 7n/O 4 solution and 0u-electrodes are dipped in 0u/O 4 solution, the two half-cells are connected b! a salt brid$e, which consists of L0l ions and a$ar-a$ar $el. &he reference electrode used here is saturated calomel electrode. 2. &he electrodes are connected b! wires to a oltmeter and an ammeter which measures the external olta$e and the current throu$h the circuit respectiel!. &he external olta$e is supplied b! the potentiostat. 4. At Brst when current is ero at the ammeter, we note the E ext b! the oltmeter readin$. 6. &hen we calculate the E = of the oerall cell, and b! the ernst euation we calculate the ratio of actiit! co-eCcients of 7n and 0u (a7n and a0u). 8. '! the formula of M< N nDE ext we calculate the thermod!namic propert! M< where DN 86== coulombs. 9. &hen the current is increased b! an interal of =.=1 m@ which is a er! small alue results in increase in E ext and chemical chan$e taes place. . e increase the current 6 times b! an amount of =.=1 m@ and b! each increase we note the alues of E ext, EN=, (a7na0u) and Mared increases or decrease in conductance are associated with the chan$in$ concentrations of the two most hi$hl! conductin$ ions *the h!dro$en and h!drox!l ions. As the titration pro$resses, the protons are neutralied to form water b! the addition of aOF. Dor each amount of aOF added euialent amount of h!dro$en ions is remoed. E"ectiel!, the mobile F cation is replaced b! the less-mobile a ion, and the conductiit! of the titrated solution as well as the measured conductance of the cell fall. &his continues until the euialence point is reached, at which one obtains a solution of sodium chloride, a0l. #f more base is added, an increase in conductiit! or conductance is obsered, since more ions a and OF- are bein$ added and the neutraliation reaction no lon$er remoes an appreciable amount of F. 0onseuentl!, in the titration of a stron$ acid with a stron$ base, the conductance has a minimum at the euialence point. &he conductometric titration cure is a plot of the measured conductance or conductiit! alues as a function of the olume of the aOF solution added. &he titration cure can be used to $raphicall! determine the euialence point. &he euation for the reaction in this experiment is: aO; $a& / ;"l $a& N a"l $l& / ;0O $l& 1
&he net ionic euation is: a/ $a& / O;- $a& / ;/ $a& / "l- $a& a"l $l& / ;0O $l&
Procedure: 1. &he conductiit! meter is calibrated. . 6 ml of $ien F0l is pipetted out in a clean beaer and then it is mae up to 1== ml b! addin$ 6= ml distilled water. 2. ow, the conductiit! cell is immersed in the beaer and and the initial conductance of the solution is taen b! stirrin$ the solution and eepin$ it constant. 4. &hen, ml portions of aOF is added from the burette and stirred well. &he conductance of the solution for each addition is to be noted. 6. &he conductance of the solution decreases till the euialence point of stron$ acid is obsered. 8. After the euialence point, on continuin$ the addition of aOF there will be a small raise in conductance alues till the end point of the acid is reached. 9. After that, the conductiit! alues increases suddenl! due to the conductance of OF ions. . A $raph is plotted with respect to the olume of aOF consumed ersus corrected conductance. Drom the intersection point on the $raph we obtain the euialence point of the acid-base combination. . Drom nown solution, we can calculate the stren$th of F0l.
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Observations:/tren$th of aOF*1= @olume of F0lN6ml #olume of aO; added$ml& + 0 , < = >+ >0 >, >< >? >= >@ 0+ 0> 00 07 0, 0 0< 0? 0= 0@ 7+ 7> 70 77 7, 7 7< 7= ,+ ,0 ,,
"onductance $*&
7+ 0 00 >@ >? >, >7 >0 >+ @ @ = = ? < , , 7 7 , < < ? ? = = @ @ >+ =
,< ,= +
>0 >7 >
"alculations:6 At euialence point, @1/1 N @/ B /1 N @/@1 where, #>N @olume of F0l N6 ml #0N@olume of aOF at euialence point *0N /tren$th of aOF N =.1 *>N/tren$th of F0l 1
esults:6 1. /tren$th of aOF solution N =.1 . /tren$th of acid F0l N +.>>
Precautions:6 1. 0are is taen to ensure that the electrodes do not touch the $lass-walls as it mi$ht lead to erroneous results. . 0are is taen to aoid an! parallax error whilst recordin$ the readin$s from the conductance meter. ????
Experiment-8 Aim of the Experiment:-
Pourbaix Cia'ram To plot the e/pH diagram of stainless steel
Theory: – Pourbaix dia'ram, also nown as a potential4p; dia'ram, E;p; dia'ram or a pE4p; dia'ram, maps out possible stable (euilibrium) phases of an aueous electrochemical s!stem. 3redominant ion boundaries are represented b! lines. As such a 3ourbaix dia$ram can be read much lie a standard phase dia$ram with a di"erent set of axes. /imilarl! to phase dia$rams, the! do not allow for reaction rate or inetic e"ects. Snder certain conditions, when a metal or allo! is exposed to an aueous solution with a concentration of inor$anicor$anic mixture, corrosion phenomena occur at a correspondin$ de$ree. Iurin$ corrosion, some metallic phases dissole, the metal or allo! surface $ets dama$ed and some secondar! solid phases form at the solidliuid interfaces (such as oxides, h!droxides, silicates, sulphides, sulphates, carbonates, nitrates, phosphates, borates, or halides). /uch corrosie chemical or electrochemical reactions can be studied b! means of the so-called 3ourbaix dia$rams if the reactions reach their eullibrium states . &he speciation and partition in the aueous solution and the interactin$ phases depend not onl! on pF and Eh, but also on other factors such as the bul composition, temperature and pressure in the s!stem. &he interactin$ phases ma! be $as mixtures, stoichiometric solids or solid solutions. A 3ourbaix dia$ram is diided in re$ions of Timmunit!U, TcorrosionU and Tpassiit!U. &hese re$ions proide information about the stabilit! of a particular metal or allo! in a speciBc aueous electrochemical enironment under certain pF, E F, pressure and temperature conditions .
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Di$. 1 .E-pF dia$ram of stainless steel
Apparatus pF meter,beaer,saturated calomel electrode,multimeter .
"hemicals Dsed aOF solution,F0l solution
Procedure:1.&he metal surface in contact with the worin$ solution of sodium h!droxide is polished. .&he pF is noted for each step wise addition of sodium h!droxide b! usin$ ph meter.Fere $raphite is used as counter electrode and standard calomel electrode as reference electrode. 2.At each step the correspondin$ olta$e is noted from the multimeter.
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Observations:p; 1.6 1.8 1.9 1. .1 . .9 2.6 2. 4. 4.9 6.9 8 8 8 8.2 8.2 8. 9 9.2 9.2 9. .1 .6 .9 . .1 .6 . 1=. 1=.2 1=.4 1=.9 1=. 1=.
E -8=8 -8=4 -8=2 -814 -8 -8 -82 -8= -81 -816 -81 -8=6 -8=8 -81= -8= -8=6 -8=2 -6 -68 -64 -666 -699 -8= -8= -816 -8=9 -8=6 -8=4 -8=4 -8=4 -8= -8== -64 -62 -62 -61
esults:6
'ased on the readin$s obtained from the multimeter,we plot the EpF dia$ram.
Precautions:6 1.>utlimeter readin$s should be taen usin$ an approximate mean of the readin$s since %uctuations occur often. . hen testin$ for acidic re$ion of the pF scale,fresh solution should be used and acid added. ????
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Experiment-9 Aim of the Experiment:–
Pit Censity To nd pit density along the given area of stainless steel sample.
Theory:– Pittin' corrosion, or pittin', is a form of localied corrosion that leads to the creation of small holes in the metal. &he driin$ power for pittin$ corrosion is the depassiation of a small area of the specimen, which becomes anodic while an unnown but potentiall! ast area becomes cathodic, leadin$ to er! localied $alanic corrosion. &he corrosion penetrates the mass of the metal, with limited di"usion of ions. &he mechanism of pittin$ corrosion is probabl! the same as creice corrosion. +echanism
3ittin$ can be separated into two di"erent re$ions, namel! pit initiation and pit $rowth. &he $rowth mechanism is reasonabl! well understood, while initiation mechanism is not er! clear. ,it nitiation
3it initiation is not well understood. 3it initiation time can ar! from er! short, da!s, to er! lon$ times, man! !ears. /mall chan$es in conditions can mae the di"erence in whether pits occur or not. &here are man! mechanisms of pit initiation. &he initiation mechanism could be metal speciBc and histor! dependent in some cases. #n other situations a $eneral t!pe of pit initiation mechanism ma! be inoed. >ost mechanisms inole a breadown of the passie la!er on a metal. &he passie la!er is thou$ht to be a complicated la!er on the surface of a metal. #t is a la!er which is 2= to 1== An$stroms thic. As an atom is onl! about An$stroms in diameter, then a passie la!er is onl! about 16 to 6= atoms thic. Experimentall! this is er! diCcult to examine, especiall! in pittin$ 9
inesti$ations when the experimentalist does not now which site is $oin$ to pit. &he passie la!er is thou$ht to be a two phase t!pe of structure with the side nearest the metal a cr!stalline phase while the la!er nearest the solution side is thou$ht to be an amorphous mixture of metal ions and h!drox!l ions. nitiation +echanisms
>. Cefect Theory Earl! inesti$ators suspected that defects in the Blm broe down. &he Blm defects were related to metal defects such as $rain boundaries or slip steps due to dislocations emer$in$ form the surfaces. &hese sites would be local anodes and initiate breadown as the Blm probabl! was not full! formed oer these local anomalies. Snfortunatel!, althou$h some materials show a relationship between pits and defects, it is not a $eneral rule. 0. "hloride 3on Cissolution >an! metal chlorides are soluble in water. One theor! used this fact to su$$est that at the solutionpassie la!er interface the chloride ion replaced the h!drox!l ion to form a metal chloride that dissoled. Another chloride ion at the same location then dissoled some more of the passie la!er until the bare metal was exposed. Durther chloride ion dissolution would then form a pit into the metal. &his mechanism predicts that once a pit forms it will continue to $row. Snfortunatel!, some pits cease to $row. ,it *rowth
Drom a mechanistic point of iew, the $rowth of a pit can be re$arded as similar to the corrosion process in a creice, coered in the preious section. &he exposed surface outside the $rowin$ pit is cathodicall! protected b! supportin$ the reduction of ox!$en to h!drox!l ion reaction:O0 / 0;0O / ,e- ,$O;-& As this cathodicall! protects the re$ion outside the pit, the metal dissolution re$ion cannot spread laterall! across the surface. #n
addition the lar$e cathodic surface can maintain this reaction and form a lar$e cathode to small anode ratio which will accelerate the anodic reaction. ithin the pit, which is re$arded as a small hemisphere at this sta$e, the metal dissolution reaction is tain$ place. &his is the $eneral anodic reaction of:9 9/ / eFoweer, it is the onl! reaction within the pit and results in an electrical imbalance a$ain which attracts ne$atiel! char$e ions, usuall! chloride ions. &he autocatal!tic reaction to form h!drochloric acid in the pit is initiated and continues:9n/ / "l- / ;0O 9$O;& / $;/"l-& 3ittin$, lie creice corrosion, is an autocatal!tic reaction once it is started and the pF decreases while chloride ion concentration increases inside the pit.
Procedure:1. A stainless steel sample is taen and the surface of the sample is thorou$hl! polished.
. #t is dipped in a solution containin$ 1 F/O4 and =.2 a0l for a desi$nated time.
2. &he surface is obsered under the optical microscope and the total number of pits on the surface are calculated.
4. &he procedure is repeated with a similar sample for a di"erent time interal
Observations:5en$th of the sample- .4 cm.
idth of the sample- =.2 cm. AreaN =.96 cm. &ime interalN seconds &otal number of pitsN1 3it densit! N > seconds &otal number of pits- 19. 3it Iensit!N 0+ pits4cm0. &ime interalN 0+ seconds &otal number of pitsN 6 3it densit!N7++ pits4cm0 Time interval 7+ seconds &otal number of pits N 4= 3it densit!N70+ pits4cm
Precautions:6 1. 0are is taen to ensure that the electrodes do not touch the $lass-walls as it mi$ht lead to erroneous results. . 0are is taen to aoid an! parallax error. 2. &he pit densit! doesnVt increase after a speciBc time interal as new pits stop formin$ and the initiated pits $row in depth. ????
2=
Experiment- Aim of the Experiment:-
Pittin' corrosion !easurement of the pitting potential% passivity 'rea#do"n potential through potentiodynamic polariation techni,ue
Theory: – ,itting Corrosion:- 3ittin$ corrosion is a localied form of corrosion b! which caities or WholesW are produced in the material. 3ittin$ is considered to be more dan$erous than uniform corrosion dama$e because it is more diCcult to detect, predict and desi$n a$ainst. 0orrosion products often coer the pits. A small, narrow pit with minimal oerall metal loss can lead to the failure of an entire en$ineerin$ s!stem. 3ittin$ corrosion, which, for example, is almost a common denominator of all t!pes of localied corrosion attac, ma! assume di"erent shapes. 3ittin$ corrosion can produce pits with their mouth open (uncoered) or coered with a semipermeable membrane of corrosion products. 3its can be either hemispherical or cup-shaped.
3ittin$ is initiated b!: a. 5ocalied chemical or mechanical dama$e to the protectie oxide Blm; water chemistr! factors which can cause breadown of a passie Blm are acidit!, low dissoled ox!$en concentrations (which tend to render a protectie oxide Blm less stable) and hi$h concentrations of chloride (as in seawater).
b. 5ocalied dama$e to, or poor application of, a protectie coatin$. c. &he presence of non-uniformities in the metal structure of the component, e.$. nonmetallic inclusions. 21
&heoreticall!, a local cell that leads to the initiation of a pit can be caused b! an abnormal anodic site surrounded b! normal surface which acts as a cathode, or b! the presence of an abnormal cathodic site surrounded b! a normal surface in which a pit will hae disappeared due to corrosion. #n the second case, post-examination should reeal the local cathode, since it will remain imperious to the corrosion attac as in the picture of an aluminium specimen shown on the ri$ht. >ost cases of pittin$ are belieed to be caused b! local cathodic sites in an otherwise normal surface. Apart from the localied loss of thicness, corrosion pits can also be harmful b! actin$ as stress risers. Dati$ue and stress corrosion cracin$ ma! initiate at the base of corrosion pits. One pit in a lar$e s!stem can be enou$h to produce the catastrophic failure of that s!stem
Procedure:1. &he specimen is $round and polished to remoe an! surface indulations and to obtain a smooth surface. . A part of the specimen surface is wrapped in &e%on and the sample is then immersed in the solution. 2. &he corrosion process is initiated from the a$nesium has the 4=
most ne$atie electron potential of the three and is suitable for onshore pipelines where the electrol!te resistiit! is hi$her. >a$nesium anodes are not suitable in sea-water, because low solution resistiities allow rapid consumption of the anodes. 7inc and aluminium are $enerall! used in sea-water where the resistiit! 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 tans.
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 hain$ composition of sea-water(2.6Q a0l) is taen.
2. 'oth steel and inc plate are partiall! dipped in the electrol!te. A 0u0u/O 4 reference 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 interals until the! reach a stead! alue. 41
Observations:3mpressed current$3&
Esteel
Einc
= -1.1 -1= -. -.6 -. -9. -9.9 -9.8 -9.4 -9.2 -9. -9 -8. -8. -8.9 -8.9 -8.6 -8.4 -8.2 -8.1 -8 -6. -6. -6. -6.9 -6.9 -6.9 -6.8 -6.8 -6.8
-89 -= -9 -1== -1==6 -1== -1=1= -1=1 -1=12 -1=16 -1=18 -1=19 -1=1 -1=1 -1=1 -1=1 -1= -1=4 -1=6 -1=8 -1= -1= -1=2= -1=21 -1=2 -1=2 -1=22 -1=22 -1=24 -1=24 -1=26
11=8 92 2 = 6 1==1 1==2 1==6 1==9 1== 1=1= 1=11 1=1 1=14 1=16 1=19 1=1 1== 1== 1=4 1=6 1=8 1=9 1= 1= 1=2= 1=22 1=2= 1=21 1=2 4
-6.6 -6.6 -6.4 -6.4 -6.4 -6.4
-1=26 -1=28 -1=28 -1=28 -1=28 -1=28
1=22 1=22 1=24 1=26 1=28 1=28
7inc(7n)
/teel
"alculations :esistiit! of sea-water([)N 46 ohm-cm /ubmer$ed portions : 7inc : 5en$th(5)N.1 idth()N.1cm &hicness(&)N=.2cm /teel : 5en$thN=.2cm idthN1.9cm &hicnessN=.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 - Oerall current demand N (9=.9?=.=6) N 2.628 mA
cm
ow, 42
esistance of the sacrificial anode N 16[(5=.=.&) N 189. \ #ohmN)( Esteel ] Einc)) 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 1.22 cc Iensit! N 91 $mcc Fence, >ass of sacriBcial anode necessar! N (91?1.22) N 2.2 $m /#0E &FE I#>E/#O/ OD &FE /A03 AE D#XEI &FE EDD#0#E0Y 'E >S5##EI &O &F#/ AI O& I#@#IEI Efficienc! of inc(^) N=Q Amount of inc present within that 2.2 N 2.2?.N4.629 $m 5et the serice life of the steel specimen be _xV hours ow, 1=== $m of inc can produce 1= A-hr &hus, 4.629 $m of inc can produce 8.496 A-hr 'ut, 0urrent supplied oer the entire serice life N (2.628?1= -2?x) Ahr &hus euatin$, we $et, xN .1 !ears /o, the probable serice life of the steel specimen would be 0.0> years
esults: &hus,the purpose of the experiment is sered and the serice life of the sample is about .1 !ears. 0onductiit! of the used sample solutionN46 moh-cm
Precautions:1.&he multimeter terminals should be connected properl! so as to obsere stead! alues of current and olta$es. 44
.3roper circuit should be made in accordance with the experiment. ????
46
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: – 0reice corrosion is a form of localied attac that occurs freuentl! on metals exposed to sta$nant solutions within shielded areas such as holes, $asets, lap +oints and creices under bolts. &his form of corrosion is usuall! er! diCcult to detect, predict and desi$n a$ainst due to the sie and locations of the corrodin$ creice. #t can also be thou$ht of as a $alanic process that occurs between di"erent areas of an identical metal $alanic couple immersed in an electrol!te. &his form of corrosion starts close to the creice mouth and becomes more widespread, pro$ressiel! moin$ to the interior of the material throu$hout the period of exposure to the a$$ressie solution. >aterials with hi$h corrosion resistance are usuall! the most ulnerable to this form of corrosion. ell nown examples of susceptible metals are stainless steel allo!s, nicel, titanium and aluminium. 0reice corrosion is encountered particularl! in metals and allo!s which owe their resistance to the stabilit! of a passie Blm, since these Blms are unstable in the presence of hi$h concentrations of 0l-and F ions. +echanism
&he $eneral conditions for creice corrosion include a sta$nant solution and a $ap between two surfaces, one of which is metal, of the order of 11==th of an inch. #nitiall!, the usual anodic and cathodic reactions occur oer the surface of the metal. &he $eneral anodic reaction is:> N > e48
&he $eneral cathodic reaction is :O- FO 4e- N 4(OF-) &hese initiall! occur oer the whole surface. Foweer a restriction occurs in the creice re$ion such that the dissoled ox!$en in the creice cannot easil! be replaced. &he re$ion inside the creice cannot then support a cathodic reaction. #t can still support an anodic reaction of the t!pe shown aboe. Outside the creice re$ion the cathodic reaction proceeds but anodic reaction ceases as it is concentrated in the creice. An electrical char$e imbalance exists between the hi$h positie char$e within the creice from metal ions and the ne$atie char$e outside the creice. As a result, ne$atie ions are attracted into the creice. &he limit is the small sie of the creice. 0hloride ions are the faored ions to be attracted into the creice. Associated with the ne$atie chloride ion is the er! small positie h!dro$en ion. 'oth the chloride ion concentration and the h!dro$en ion concentration increase within the creice. &hat is the pF in the creice decreases from alues of 8 to - 2. &he e"ect of this acidiBcation is that the corrosion rate inside the creice 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 creice increases, further protectin$ the re$ion outside the creice. eactions inside the creice include:9/ / "l- 9/ "l9// "l- / ;0O 9$O;& / $;/"l-& ;/"l- ;/ / "l &his results in acidiBcation within the creice. ote that onl! the re$ion inside the creice will be corroded. &his is also important as the anodic area is localied 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 seeral factors are inoled 49
in creice corrosion, electrical char$e imbalance, leadin$ to a lower pF and chloride ion concentration increase, and a er! unfaorable anode to cathode area ration all if which lead to an enhanced anodic rate.
Procedure:1. A stainless steel sample is taen and the surface of the sample is thorou$hl! polished.
. #t is dipped in a beaer containin$ tap water.
2. &he corrosion process is initiated b! the instrument and the software plots the polarisation cure (5inear /weep @oltammetr!)
Observations:-
4
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