one example of corrosion that we have already met is y.iven by.!!! electrode in the Daniell cell. When this cell discharp,es

Corrosion mechanisms.

,

. · ! . The products of metallic corrosion are ionic in nature, and· corrosion.mechanisms involve electron transfer. They may thereto~~· ·b;··-dl ;~~-;sed in term~ of ~!~£!!!~£!!~!!!!£~!_£2!!£~1?~~. An understanding of corrosion mechanisms is very necessary if corrosion is to be effectively controlled~

.....

t~nt metals, e.g., ~e}/ coz-r-oe Lon represents a major problem, nd vast sums of money are being spent annually in attempts to a . . . - , . . . . .I • • ! . • . ~ . •

pre~.~n~. or. minimi~e 1.ts e.ffect •

of .this stabili-ty .Ls that once a pure m_etal has been extracted it often exhibi~s.a tendency, when exposed to an oxi~izing

,.' ·atmo~pher, to rev:~r~ to. the combined state. The l,atter process is .referred_. to as; metallic corrosion. In case of some impor-

modynamically with respect to the pure metals. A consequence - . . - ............

Although some1 metals (e.g., the platinum, mercury) are found in the earth's crust in more or less pure state, others,

~·such as.Fe, Al. and Zn'are normally found in combination with -~--....c:·-~ - ... ~~-

02 or other elements.· These compounds are highly stable ther- ·-· - -- .. .. .. ' ' -

r

• ·1. · , attack. accompanies physical deterioration.

'? . . • • ~ • '( ~· I • . f: . -

Co~.x;:~~.Q!l 'is the destructive attack on a metal through chemical and electrochemical-reactions by the environment. -- ,,_.---. '

~ '<, .... _Deteriora tio~)by physical causes such as errosion, p;alling or wear .. ie also included i~ corrosion. In many cases, chemical

NOTE ON CORlli)SION

- 112 -

r ..

2 e __ ,._ 2 u+ ,. '-~·- . ,_. .. :: ,.-</

takes p1ace. On a ca.thocli.e area a corresponding reduction reaction ... ta_k~_s

place. 1 .. In d~lute acids the aoet li~elJ reduction reaction is

-- . I •

Zn2+ + 2 e Zn

on an anodic area of Zn the oxidation reaction (i.e •• anodic disso1ation vhi.ch leads to the loss of •etal)

forms vb.at is known as local galvanic cell of corrosion. Numer­

ous~1oca1 cells thus occur' on the surface of a small piece of meta1. This shows that corrosion is electrochemical.

areas. __ The coabi.nation-·of' one anodic and one cathodic reaction

, . . It is now considered that the surface of a corroding metal is divided up into, sometimes numerous, anodic areas and cathodic

acid.

~~er,, etallic corrosion can occur when a single metal ~~ is placed in a corrosive environment: e.g., pure 'zinc in dilute

wou1d not take place. I// In aosence of th1.a reduction process, the corrosion oC zinc~--

Cu

the reduction is the deposition of copper according to

Cu2+ + 2 e

are zn >· zn2+·; + 2 e

taken up bv the reduction ~ reaction {in the Daniell cell,

Rive Znso4. The rate of thi~ corrosion i a liai ted by the rate at ·w- hich - -- - ., . ...,.

·-·-~··· electrons liberated by the oxidation reaction --·- ------~· · --· - -~·- - ~--· --~ ..... --

the zinc dissolves: we can eay that in the cell.en~ironment

the zinc corrodes to

113 -

~) One_important: reason being that metal aurraces are not uniform, when considered at the microscopic level. There

be iess .rapid. Th~s.can be for a variety of reasons:

2- However, !!~~-!~-~h~-~~~~~£~-~!-~~!~-~£~!~-~~~~!_!~E~!!~!~~· metalscan still corrode, though the rate o~ corrosion may ----------------------- ··-

' ! •. in HCl acid if a little CuS04 solution is added.

!

whole aerie~ _o~ .!!!.£!:~!!£~E!£_~~~!~!!_£!:!!~ (local gal vnnic ~-ells) .. . -

envisaged on'the surface of _the metal. In confirmation of this mechanism we might note that zn will dis~olve much more readily

copper, and the dilute acid also contained copper ions then a . . ( ; ~

areas and these could be quite sufficient to permit a measurable -- _,. --~..:~ • ;···· 'i· ' ·~--..-: . ra~e of corrosion. If, e.g., impurities were in the rorm or

• ~ • . ! 1 f ,, • .• • ' . .

of -~:;~~g~~-~~~~t~~.£~~il1 ~~~!._E~.s~i-}~~atoms of this m~t_a~ _at--~~-~-• s.~r~~ce would const1_:.:i..!_e ~!~i~~~~E!~-~!!!!!~~~£

--....--::':-:.. - _7At"'~..J.'"""'.-.-lw.· ..., , •'-.C • ! ~---·- -~--~----~-- ·--.....-- .... - •

If very small amounts 1- Metals are seldom compl~tely pure. r· t ';

In case dr a single piece of zinc, it might be expected to be quite uniform in its properties. How, therefore could the sepa­ rate areas arise.J

being anodic.

In the baniell cell the tw~~different metals are anodic and cathodic because of their different stnnclnrd electrode potJm_j;-

• ials,\the metal witb th~_more +ve potential (more noble) being_)

. --~ . - -

~athodic i8nd the one with the more -ve potential (more active) ---·---·· - ~

'But why should these different anodic and cathodic areas exist on the surface of the zinc·?

leading to the evolution of H2 gas.~

- 2 e ~ 2 .. -+

The activit7 or (A) is due to the fact that the valency· of the

ato•s in th:l.a part is not eai"isfied. The electrons released in the above reaction are transported in the metal f'r-om (A.) to

the Clat part o'C the surface (B). On._(B) the two electrons are taken up b7 211+ :ions from the solution.and a reduction (or cathodic) rea.ct:l.on occurs leading to the evolution of H2

Zn >s Zn2 + + 2 e

. ~ the anode {act~Ye) and an anodic reaction occurs according to

proceeds in an e1ectr.oc}temical manner. The given d La g r-ara shows that on the heterogeneous Zn surface the part (A) will act as

For this reason the chemical reaction: .: sn .,_ 2 HCl - ZnCl2 +(Ii~)

actiTe with respect to hydrogen (which is more noble than 7,n).

is above H2 (e0 =zero) in the electromotive series, Zn is more

local galvRnic cell of corrosion

~A r ~ d. a no a c area

B ~ cathodic area I

r - 115 -

. .• . r -.·, .... ;: ._.,.,. ..

. ::.:..:....::.. .......... -~-----.

tion of' zn in pure ;:cids. In thi.s caee the •etal

Thus consider the diEsolu-

po-tentials. ..... - .... ,_,,. _ ....... - .. ~ -- ~ ..... ~ .... ""':"'-. ~ ....

tly rli~ferent electrodr • .:"';'_ .... ,"'!.~ ..... .,,~)or" ..

will · ' e.g., be different crystal faces exposed at different points at the surface and these diffe~. ent races will have slirn- .. - .. '':-.

-~-~!:' .. f~~.!:-1- .:'I. ~-~9. ~·--·~°-~~ g~.f_!~~_:>, '-~~~~~~~~~~~~~~~--~~-I since Zn (e0 = -0.1~ V)

,. liquids used in industry.

as an acid solution. Among the 0th.er environments which cause corrosion one may cite air: (o2,' tt2o, co2), see water and va r-f ou s

In the above.examples, the envtronment has been considered . I

hydrogen evolution type of corrosion. -- --~---~ ~

' more noble metal) ihe type of corrosion in this case is known as In the above mentioned examples (in which H2 evolves on the

liberates. -----

Unstrained pa r t s (cathodic areas\ unstrained parts. $,trained area acts

. as .~n<:?.c!.e (active or less noble) and in ------~ acid media they will ~issolve, while the unstrained areas act as cathode (more noble) at which hydrogen

.r: _a~ the potential of the strained area ! I l:·-~,"' ... I I \.\-. differs slightly from the surrounding

.. - ~......_~--;=:;·y--~, ..• ,- -. • ·:-·· - . ... ~!

Strained part (corroding) (anodic area) · to_ take_· 'p Lace in the area under strain, ~ ., ··.::!.-.

corrosion in these areas may represent a serious hazard. Even stra~n in the metal cause corrosion

tT_~--~-~?_ond reason i_!.,~e presence of strain in the metal') fo~.~xample, when rivets pass through a metal and extensive

b)

The two reactions are exactly balanced because the electron released in the·fi~st are consumed in the second.

anodic dissolu~ion (or oxi~ation) which leads to the loss of met~~,~ cathodic as the reduction of H+ ions.

two reactions:

Conclusion: From above it is clear that corrosion is composed of

- 116 -

·-- ........... --.,_

• te(OH) 2 whl;h~le·~~~i~iaed- .. by .. o;J~~?;~~fcn_{)3. f-·:Perric hydro- rld.e reacts with the enrtronm·e.nt in two waya:· with 02 to form the oxide Pe203 With co2 to for11 ferric carbonate.

(Z ~ 4)

The H'ern8t reaction is • equation ·for the cathodic

Eo _ RT ln [OH-) 4

ZiJ [ o2 J E =

. r

oC a aore complex eube tanee.. of variable composition known -~s_

{"~t~ (contains Pe(OH)2 part· of which remains (unoxidized), '-· .~e(Oll)3, Pe203 and ferric carbonate (in acid medium)

If'. a.a is of'ten the caee, the anodic and o a t hod Le areas of" the

corroding iron are in c1ose __ pr~ximity0e~) ions produced at

the anod~_,,r.ea~t-~1 th theee0~~~one, prod~ced at ~b~-· c~~~ode " . . ' : -.· i . ":"',

resa.1ting i;n the precipi 'tation of Fe(OH)2• J If suf'1"icient o2 is

present further reactions are possible leading to the formation

giv~ng rise to the formation of hydroxide ions

; ., .. i

The· ·cathodic renction is: 2 H2o + o2 .... 4 e --)+ 4 OJC

~!?!:!:~~~-!!!_!~!~!:_!f_Q2_!!!_~~!"!:!:~!!l_!:!~!~~~~. }

since iron will not Thus thie reaction involves 02 invo'lvied.

Th" .. cnthorlic proce~A could be the same RR described for zinc ·if the iron were corroding in an acidic environment · nu. t 1 t

is vel.l known that 1;·on will corrode unde r _neutral or alkaline conditions and some other, cathodic reactions must therefore be

--\~ Fe2+ + 2 e Anodic proceee: Fe

The rusting of iron iR a well known example of corrosion

whteb w~ eee in our every-day life.

Corrosion Of Iron (Rusting of Iron)

- 117 -

will.not occur.

The prevention of Corrosion. This can be achieved in several ways:

1. The commonest way of preventing corrosion is to insulate the metal from the corroding at•oaphere by a suitable coat­ ing such as paint. So long as the paint completely covers the surface is.not porous and remains undamaged, corrosion

....

anodic area ·Fe ·~Fe2.+ +.2e

Fe rod

surface. -===-- iron corroding in water. Corrosion takes place below the The figure below shows anodic and cathodic areas on a piece or

corrosio~~

showing that the potential of a cathodic area in water is de­

pendent on the concentration of dissolved o2. If, therefore, different areas are exposed to different concentration of 02, ,.

l the_Q2_=-E!£~_!!!!~ will be ~ore cathodic (more noble) than . ~~e 22=~~f!£!~!!~_!!~!! ( l~·ss noble)~ If a pt ece of Fe 1 a

partially immersed' in water the o2=.concentration will be great­ est at the surface of the water: this area of the metal will therefore act as a·cathode to the submerged area (anode) and

·corrosion of the submerged area will then take plnce. This type of corrosion is known as Differential oxygenation

connected by an e1ectrical condttQ tor,

(see figure). This metal forms the , . .,...... ~ .,.. • .te ,...,.., rat)•n

steel structures a more (1--,,.}t -.; t_J{,( ~ protection or /'!!''l-' , . .., ~~-~·-·:. ,... __ a~ti_V:e ttet~.l, e.g. .!.!!-• or~ ie buried ~· af..\)· ~ »; close to the structure to. which it ie

e.

may be :

1- Galvanic protection. In the galvanic

very interesting application or electrochemical princjples is te be found in cathodic protec~ion of metals: this is used often for buried etructuree nuch as ~pelinee. Cathodic protection -

Protection against corrosion is done in several ways. a

.. ; ' ~ '

Cathodic Protection.

----- -~--·--=- ---=- •nd cathodic inhibitors.

------- ------

. . ~ - . ' ' .. using substances known· ae ' iilhi bi tore. Hal-ting one of thesre .. .reac- tions preve.iiis ·the eiectron transfer processes which are essential for the start oC the corrosion. There are anodic

4. Other methods of corrosion control depend on·preventing __ either the anodi~ or cathodic reaction .f'rom taking place by

. . .-_ -

ing the metal wi_lli-.6_sui-ta-bl-e-~r-e.agent. Oxide coatings, often -·--------- • . -- -------~--~ ~~~~t'ff!i?r --·-

resulting naturally from expoeure of. the· metal. to air, are sometimes, sufficiciently hard and coherent to protect from further atmospheric effect

Alternatively a metal coating can be applied such as chrom­ ium or zinc~· Zinc, e.g., provides a very effective corrosion

·-reeiatant coating for ·iron, the process of application being known as galvanizing.

3. A third type of protective coating can be produced bl _reac~-=---

2. - 119 -

Corrosion will be studied in detail in a separate course.

,. _:.=-.--~ -~·- ___, - ~..-~..:..u:~.::__:-~·~~ corrosion. · 7

-·~~ - cture at a sufficiently negative potential to prevent p'lati_num, . or, scrap metal, the. battery maintaining the stru-

- r-,,_..: -·,. . -- _...,. . ....,~-----·- -- _.__ - • - -

e1ectrode:can-be any conducting.material such Hence, th~ positive f'igure).

lntrL· t leeh- • .le ,.,. ... - . :... 6 f~) :r h<..

as graphite -

' ' ·cell ia a1so·establiehed but In polarization protection a

in this case a' battery is included in the circuit (see

2. Polarization·Protection.

an9de of a galvanic cell and is·eacrificially corroded, the· structure1.being ~hereby protected.

- - 120 -

4. ~nod~c Oxidation or Organic Compounds: also on inert electrodes.

~- Anodic Oxidation Of Inorganic Compounds: also on inert .electrodes.

2• Gas .EW'olu tion : Such as 02 and the halogens. This occurs

on inert electrodes (e.g., noble metals like Pt) which are not active, and do not undergo anodic dissolut]on.

b) Concentration polarization when the ~low step is the diffu­

si~n or the metal cation fro• the solution near the anode to the bulk oC solution.

. I

charge transfer rencti.on. In thi!l cane , the ove r-po t.e n t t a I

is ·represented by a Tafel line and a Tafel equation chara­ cteristic of acti.vation-control •

a) ActiYalion overpotential when the slov 3tep is either the detochaent of a metal atom from the Metal lattice, or a

ciated vi th an overpotential which may be: ThP. rroeefHJ of Rn()d i c dissol n t f.on of a r:t(' t-.n l Et node 1 ~ :tr.~o-

( 1 ) + 2 e ~ ·­ Cu· Cu

nn nno~~ mny hr.hnv~ nct1vP.ly nn~ di~~olv~ in the elPctroJyte. e , fY.·. vhen Cu is mnrle an an nnode in acidified cuso,. ~olu-

tj onn

Dependent on the type of metRl and the nature of solution,

One •ay summRri~~ the Bnorlic prnc~R~~~ previou~ly rtln­

~usoed in this cour~e. These nre:

,~·~cntrq~<rxtt'tl'F.ir~rf,ft~~!!HAT;[QI ~· '. ~1._,· ... C:'..)''.tt'"'•c )-.',(.,.,-, .. # .. 1--;P. .... 'l°'/.z LI z.ea I c I

- ] ?] -

Kuch thicker oxide films can be produced as a result of corro­ sion,~~~· the rusting of iron. It is believed that the

and oxides oC various thickness are produced on the surface dependent on the nature of the metal and the composition of a~r (apart Crom B2, 02, H20 vapour, air may contain other ga~es resulting rrom'various industrial plants). When thin oxide rtims or other thin films (e.g. silver sulphide) are formed on the metal in air, the reaction ia called a tarnishing reaction . . .

When exposed to air, metals undergo atmo~pheric oxidation I

oxidation oC metals to form oxide films on the anode surface.

Howaver. another important anodic.~eaction, which ha~ not been mentioned before, still exists, and this is the anodic·

H2 gas as in ·the case of the corrosion of Zn jn acids.

b) a pa~tial cathodic reaction which exactly balances the anod­ ic ~eaction. e.g., the cathodic reduction of H+ ions to form

-- ...

a) a partial anodic metal dissolution reaction (corrosion), like the dissolution of Zn in acid solutions~ ~~d

is the basis of co r r-o a Lon which involves.

ion of a salt of a more nohJ.c metol. The actJ.ve metRl din~ solves spontaneously (anodic reaction) and the cations of the more noble metal are reduced and deposited (cathodic reaction). This is a displacement reaction. Such a displacement reaction

This occurs when an active metal is immersed in a solut-

6. Spontaneous Anodic Dissolution

Anodic Oxidation Of Fuel Gae i~ a fuel cell, such as the oxidation of H2 to H+ ions in the 112/02 fuel cell.

5.

- 122 -

---· ------ --- -·------~- and the aebll disso1Tes giving rise •o ~as evolution. occurs. ~--=----

Wh<'n a piece or iron iR 1-ereed in dilate HN03• corrosion

are ~oosldered:

Tvo types of pasgivity not und~r~o d1.ssolutton or corrosion. A pa~sive metal does

·~ ... :;-}-._.;.~--~-~ .... - ,., .. sJcJ~;,._th1-ck oxide f"l.l.lft~

' ,. 'v ,

-:- •. ·.- .. "\ ~; • \ :· ,. - - .'. \ 1 t·.!:•. valve' metals ''.such as

----------·- . are ~~rm~d on cer~tn mPtRl~~ nfo.bi Wll ~!~.b) and tantalum (Ta)_:___.-/

... . . --::-.------:-----·--- -- - .. ~----------=.-:::::: =:: !____ ..!~- =: ~.

= 1010 .A . • .. ·.· R 1 r~ =·· 108 A: (Rn~trom), l m~ter = 100 c• = lOOxlo-

. 1 ·~· ·(-~ic~on ~- io-6 met.er = 10-6 I 1010 = 104 l since .-

i 1 ••.••

Note th.Rt: or more. 1 Jl (micron)

.. b) · Pormn t ion of' thick oxide :fi l•~ whi.ch mr..y reA.~h R thl clrn~m1 of

\:

- . -·~. P.tttdi~ct under the euh.1ect cf rassivity.

tt) Formntion of very· thin oxide films (50 1). and the rroc~!l~ is

f,• -----Here we \ha,;.e · ·tvo po e e f. bill t Le s ;

,.,,, ~ .,,,,. .... _,_. , , .. ..,. ,...

' I -r: .,

t : ,.

. of eolut1nn one may have: sui.tRblc el-ectrolvte. Dependent on the type of metal and nature ox\dc fll•e rormed hy polarizJn~ the met~l nR nn anode in a

In Auch an introductory couree, one may concentrate on the

nion 1 n 111A.ny 111onogrnphn nnd t;e.x t bo ok a on corrosion.

atmoApheric oxidation of metals (thin and thick oxides) proceeds

by electrochemical re~ctlonA. The subject of atmospheric oxide

film formation io usually dealt with under the subject or r.orro-

- 123 -

·~

- .

I sudden decrease in c.d. is called the paasivating potential (EP). Thie potential is sometimes known as the Flade potential.

Thie oxide causes a resistance to the passage of current, with the-result that the current (and consequently the c.d.) decreas­ ws abruptly along (be). The potential corresponding to this

be passivated ae an anode in a cell using the proper electrolyte. For exe.mple, when we passi va t e iron anodically in H2so4 solut­ ions. the metal starts by dissolving actively along the Tafel line (ab) as shown in the follow:i.ng figure. At (b) an oxide fil• is formed (similar to that formed in chemical passivity)

*'~~~.l$.~~'ft~}1l --- ~.:.,;.-Thie type of pasei vi ty is produced by polarizing the metal to

-~---· -=- ·- _..._ -- --------- chemical passivity.

Metal / I oxide

passivity of Fe. This pansjvity arises as a result of formation bf a thin (1 b t SO') a ou A, coherent, transparent oxide film which covers the entire metal surface and pre-

vents .the metal from contacting the solution. Therefore, this oxide fi~m protects the metal against corrosion. Since this type of passivity is caueed by a chemical (cone HN03) it ie cnlled

Corrosion 10/ HN03

passivity

dil HN03

Fe I J Fe / '

However, .. when iron ie put in

concentrated HN03 (70 r o~ more), no reaction occurs· (no gas is evolved), and the - metal surface remaine bi ht r g •

Therefore, it is said that

concentrated HNOT causes the ~

- 124 -

oxide surrace.

'- ........... ~rod"4:,00. .... l'W'~fl..__~"'e:"" -~·

ca1 to ~orm o2 gae and n2o. The ~xy~en gae ta evolved on the to the ortde. Thie is followed by .~he combination of' OH radi-

02 evolution starts and becomes visible to the naked eye at the poi~t (d). o2 eYolution r~leee the c.d. along the line (de) vh~ch represents what is known as transpasaive region. o2

• ; f evolution reeulte fro• the paeeage of electrons from OH- ions

...t:.7:'- .. ·· r .... "'.;.;.';::.'.1"1.-- '·"""'r~ .•. ~•- .. ,,_ .. -· ·-~ .. --. ----- -~--- .. -.. "' 1.-""

Therefore. when the anode po~ential is increased or electrons • . ~;,.,,....,;;;.._-.,-,.x•r-· .. - .. ; ,j_

until the eYolution of 02 becomes thermodynamically possible,

(cd) is called the passivity region wher~ a stable oxide film -- ----- - ~ ( Pe.e~) e_es-ts on the surface. (Thi~- o~~~~---~a a good conductor -.=::..-:.:::::-...::::::::___ _,,____ - ~ ------ - ... - -- -- - --- .::-.

. -1 - . . . .. ~ The region represented by the line es '"'"to more poei ti ve values.

Thermodymudc caleulation<have shown t~ the most likely oxide . . ""'. ·tic .: . )

to· he f'orwed on iron at the pnnf!ivntinP,-potentinl is Pe o • ~~·,.._,....-..;;:, .. ·--- - .=-: ~-~ -.: _:__ - ~-"~,~.--. ~i --.:.:- .. ~~:-~~ - 2 3

The meta1 remaina JlR.fHtive alonP. the line ( cd) which is chara- cterized by a constant low c.d. although the potential increas-

c

b t ~ ~ ~ '>-t .

~ a

- 125 -

h

. .

in the direct.i.D.ILQ_f the metal. - - . :::::::::=: 3) Cations and anions (e.g. 0--) have their characteristic

·1

) · -- ii n (resulting from the decomposition 2 Th~- _tr~~~_po~~=-<?:-~ -~ ~ ~

of water at the oxide/so~ution boundary) through the oxide ~----

the~ide in the direction of the solution.

betwee~ metal and oxide, and the second between oxide and solution (see figure). The passage o.f an anodic cur­

/ rent in thie system is brought about by:

t t 1 atom info a cation which is transported 1) The oxidation o a me ·a ..,. ____

th metal/oxide boundary (or interface), then through across e ·~ _, ~-

phnao houndnry (2)

phaoc boundary (1)

boundaries the Ciret

system (metal/oxide/solu- ao l u t t on· ox i de metal tion) with two phaee

Conside~ a three-phase metal by tarnishing Teactions grows to a considerable thickness.

is made the anode.in an electrolyte which does not dissolve the_ metal or oxide, the thin oxide film already .formed on the -- --­ '

,,

The name valve metals is given for a number of metals which allow the free passage of current in one direction (cathodic direction) and hinders the current passing in the other direc­ tion (anodic direction). Thie is because the paeaage of current in the anodic direction requires a· ·higher free energy of acti­ vation than that required for~the cathodic direction. Typical valve ~etalo are Nb (niobium) and Ta (tantalum). Lesa typJcnl valve metals include Ti (titanium), Zr (zirconium), Al (alumin­ ium) and W (tungsten). When a typical valve metal (Nb or Ta)

... - 126 -

- . oC increase oC potential with time at constant c.•. (i) in region

-..-- - ------'7.. ... its thickn~se increasefl when the potential inan?lJ.M•~. The rate

ti-"e {,i)

cqnstant c.d. A o

. "·J.

200 v

E

formed on the surface Rnd that .:;:;•: ··- indicate that an oxide film is

surface of a Yariery of inter­ ference coLeu.re {golden yellow, blue, pink. etc). These colours

•

with the appearance on the Th

high potentials (e.g. 200 V vit.h

the f'ol lovlng curve from whi.ch it a ppen r-s that the po t e n t La l rises linearl.7 with time along the region (A) reaching very

- '

•easured ap;ainst a suitable ref"erence 'iR f'ollowed es e. function of" time. we ohta.1 n

oxi 1fr p:rowth (new oxide

=cation potential. (E) as etnticnl1y. nnd itn c.d., i.e. fralvano- growth) at constant which favours oxide

solution metal· is nnodically polari?.­ e d (in a solution

When a valve metaJt-~~~~~~~~~~....,,,_,.--~~~~~~~~ r oxide phase. This ia illustrated in the follo..,1.ng. figure.

. ·-----------------=~- Therefore, the growth of oxide occurs in the middle of the

... '' .. ,J

- 127 -

(4) or

. id th1cknesa (6) is directly proportional to The increase of ox e~~"---~-~ ) .--:-:.i-herefore, the oxide formation (or growth) increase in {Er· -------- rate m-;;; be expressed by

( 3) +constant = E

Hence:

~---6 ·>

~~f~~---

1 I

f!f 2

cathode reference -}-~---------t-:-----~~--- Solution oxide metal

,,

In the electrical circuit, the. reference electrode remains the

same, and if we assume that the po~~ntials (~1 and ¢2) at the two phase boundaries remain constant during the passage of current, the increase of the measured potential (E) with time

(~ '<f-J ... <'c>-?te • j -~ (1'.Y"' is due to the inc reaee in the .!'oten tial difference Q E f") -"'_':'.'"."

the growing oxide. phase with time due to th~_ consu~ption ~f the ~current in oxide growth (see figure).

. ~hie can be explained in the ~ollowing manner:

( 2) (dE/dt)i =oxide formation rate

(A) is known as the oxide formatiori'rate •

- 128 -

( 7)

the oum of the pre-polartz•tion thickness (60) due to the film formed in nir (very thin, nbout 20 t. o; Lo n s ) plua the increase in thickness (66) cauAed by the p~esnge of current (coulometric increeae of oxide thickness)

( time(_!) during oxide growth the t h i ck ne s n ([•) of the ox f d e is

Let un consider the coulometric (electrochemical) method. At any

3. Coulomctry which uses the amount of electricity passed.

2. Ellipsometry 1. TntP-rferometry

can be determined by several me tb od s among which one may cite:

The thickness (l)) of the ox Id e .film formed in region (>.)

hrcnun~ wear~ dealing vith an Rnodtc pronees:

the ~olution. Note that the expon~nt1nl hR~ n positiv~ sign

(6) A e:xp ( RH ) • i

1'he relation bPtveen the c , d. ( i) and the electric f"ield l rPP;ion (A) i~ rxpresf'ec't by e e v e r-a I e:x-~onei.~ie1 lawe. th/

~-- ~implP.ot of which j~

--- conAtant field process. .//.------·-------j - .. -------.........__ ------- <, ------- '\

~~-~-ns constant in r~_g1on (A) R.R Long aa the c.d .. (i·) remains

e orrs t nn t , The:refore, r,RlTnnoetatic anodic film growth ie a

( 5)

.. The proportionality betv~en the 1ncreAB~ of (&) end the increase

or·(Er) suggests thnt the electric field Atrengtb (ff) (Potent1Rl

divided by dietnnce) Rcroes the oxide phaRe:

- 1?~ -

•·.;.' ,,. ·- '. ~; \,.:,

tial, and sometimes region (8) shows a bend-down of the potential/ tion. Furthermore,

in the slope of the potential/time i a decrease time relation, .e.

( ) of' the galvanoeta.tic curve showa that breakdown Region B t · t occur This Ls known as dielectric of the oxide film st~j"J o . .

1 martlfe~ted by oacilla tiona :in the valve poten­ brea.kdown and s

by sparking between the electrode and solu-

region (B) of the galvanoatatic curve.

The situation is different in electronic conductance occurs.

i.e. the conductance is ionic, and no

that only cationa and anions are trans- It muet. pe emphaaized ported in region (A),

The value of (r) is given by:

= cm3 /coulomb (10) r

The units can be introduced in equation (8) thus (note that i

is in A/cm2).

( 9) = (gf!im)( cm3)( :1le )(coulomb)= ~ gram co omb cm2 cm

ft = molecular weight of oxide l!I = density.of oxide n = number of faradays required for the formation of one mole

oxi.de, e.g.' n = 10 for Ta205. 'F = J"araday, 96500 coulomb. r = Tolume of oxide formed per coulomb Q = amount of' electricity

(8) M /

= ( 8 n F) 1. t - . ri t = rQ

gets f'or (bb) :

When all the anodic ·current le used ~·for oxide formation, one

- 130