CORROSION RATES MEASUREMENTS BY LINEAR POLARISATION AND AC IMPEDANCE TECHNIQUES USING DIFFERENT STEEL BARS AND ACIDIC SOLUTION
SIPIL MESIN ARSITEKTUR ELEKTRO
e kC O RRO SIO N RA TES MEA SUREMENTS BY LINEA R PO LA RISA TIO N A ND A C
IMPEDANC E TEC HNIQ UES USING DIFFERENT STEEL BA RS AND A C IDIC SO LUTIO N
- * G id io n Turu’ a llo
A b stra k
La ju ko ro si b a ta ng tula ng a n d a la m b e to n d ip e ng a ruhi o le h ting ka t ke a sa ma n a ta u ting ka t p H
d a ri ling kung a n b e to n ya ng me nye limuti b a ta ng tula ng a n. Pe ne litia n ini d ila kuka n d e ng a n
me ng g una ka n la ruta n a sa m d e ng a n d e ra ja t ke a sa m a n (p H) d a n je nis a sa m ya ng b e rb e d a se rta
je nis b e si tula ng a n ya ng b e rb e d a . Ha sil p e ne litia n ini m e nunjukka n b a hwa la ju ko ro si ya ng
d ip e ro le h d e ng a n m e ng g una ka n ke d ua me to d e b a ik Line a r Po la risa tio n Re sista nc e (LPR)
ma up un AC Imp e d a nc e ha m p ir sa ma . Ha l ini d ise b a b ka n ling kung a n a sa m me nd ukung p ro se s
ko ro si. Ha sil p e ne litia n ini jug a m e nunjukka n b a hwa je nis d a n d ia m e te r b a ta ng tula ng a n se rta
ko nse ntra si la ruta n a sa m a d a la h fa kto r ya ng me mp e ng a ruhi la ju ko ro si d a ri b a ta ng tula ng a n.
Ba hka n la m a p e ng e te sa n jug a m e m p e ng a ruhi la ju ko ro si ka re na ta ha na n p o la risa si b e rkura ng
me nurut wa ktu.Ke ywords: la ju ko ro si, ko nse nta si, la ruta n a sa m, ta ha na n p o la risa si Abstrac t
The c o rro sio n ra te o f ste e l b a rs in c o nc re te wa s a ffe c te d b y c o nc e ntra tio n o f a c id o r p H le ve l o f
c o nc re te e nviro nme nt whic h c o ve re d the ste e l b a rs. This re se a rc h wa s c o nd uc te d b y using
d iffe re nt a c id a nd c o nc e ntra tio n with d iffe re nt d ia me te r a nd kind o f ste e l b a rs. The re sults
o b ta ine d fro m the te st using b o th the p o la risa tio n re sista nc e (LPR) a nd the AC imp e d a nc e
te c hniq ue s a re simila r. This is b e c a use the a c id ic e nviro nme nt sup p o rts the c o rro sio n p ro c e ss. It is
a lso fo und tha t the typ e a nd d ia me te r o f b a rs imme rse d in a c id so lutio n a nd the c o nc e ntra tio n o f
a c id a re the d e te rmining p a ra me te rs o f the c o rro sio n ra te s o f the b a rs. Eve n the le ng th o f te st
p e rio d a lso a ffe c ts the c o rro sio n ra te s a s the p o la risa tio n re sista nc e d e c re a se s b y time .Ka ta kunc i: C o rro sio n Ra te , C o nc e ntra tio n, Ac id So lutio n a nd Po la riza tio n Re sista nc e
the hyd ra tio n o f the c e m e nt in the
1. Intro d uc tio n
c o nc re te . Und e r the se c irc um sta nc e s no C o rro sio n is the d e te rio ra tio n o f c o rro sio n o f the re info rc e m e nt c a n m a te ria ls b y c he m ic a l inte ra c tio n with o c c ur. the ir e nviro nm e nt. The te rm c o rro sio n is
Ho we ve r, if the p ro te c tive o xid e so me tim e s a lso a p p lie d to the film is lo c a lly d e stro ye d , fo r e xa m p le b y d e g ra d a tio n o f p la stic s, c o nc re te a nd the ing re ss o f c hlo rid e io ns, a re a s o f wo o d , b ut g e ne ra lly re fe rs to m e ta ls. The d iffe re nt p o te ntia l c a n b e se t up o n the m o st wid e ly use d m e ta l is iro n (usua lly a s surfa c e . The p re se nc e o f a c id a ffe c ts ste e l) a nd the fo llo wing d isc ussio n is the c o rro sio n ra te s o f ste e l b a rs in ma inly re la te d to its c o rro sio n. c o nc re te . The ste e l b a r is p a ssive in a
Whe n ste e l re info rc e m e nt is hig h p H e nviro nm e nt (b e twe e n 12 – 14) e nc a se d in so und d e nse c o nc re te , the b ut the e xiste nc e o f a c id in the e ntire surfa c e o f the ste e l is c o ve re d b y c o nc re te b re a k d o wn the p H o f the a sta b le p ro te c tive o xid e film tha t fo rm s c o nc re te fro m the hig h le ve l (a lka line in the a lka line e nviro nm e nt c re a te d b y
- Sta f Pe ng a ja r Jurusa n Te knik Sip il Fa kulta s Te knik Unive rsita s Ta d ula ko , Pa lu
e nviro nm e nt) to the lo w le ve l (a c id ic e nviro nm e nt). The re fo re , it is ne c e ssa ry to und e rsta nd the b e ha vio r o f the ste e l in a c id ic e nviro nm e nt with d iffe re nt a c id s a nd va rio us c o nc e ntra tio ns.
2. Lite ra ture Re vie w
C o nc re te is a ve ry d ura b le m a te ria l, whic h c a n b e use d fo r m o st typ e s o f c o nstruc tio n. Its p ro p e rtie s a nd p e rfo rm a nc e a re influe nc e d b y the se le c tio n o f mix ing re d ie nts, mix d e sig n, p la c ing , c o m p a c tio n, c uring c o nd itio ns, d e sig n a nd d e ta iling , a nd inte ra c tio n with se rvic e e nviro nm e nt. The p ro c e ss o f d e g ra d a tio n, suc h a s c o rro sio n o f ste e l re info rc e m e nt, is the re fo re d e p e nd e nt o n c o nc re te q ua lity a s we ll a s e xp o sure c o nd itio ns. The initia tio n a nd p ro p a g a tio n o f c o rro sio n in c o nc re te struc ture s c a n b e influe nc e d b y b o th inte rna l a nd e xte rna l fa c to rs. The se so urc e s o f d e te rio ra tio n d e p e nd o n c o nc re te p ro p e rtie s a nd e xp o sure c o nd itio ns a nd , to a la rg e e xte nt, g o ve rn struc tura l p e rfo rma nc e a nd re m e d ia tio n p ra c tic e s.
2.2 C o rro sio n Ba sic s
In this re g a rd , ste e l a nd c o nc re te a re in m o st a sp e c ts m utua lly c o m p a tib le , a s e xe m p lifie d b y the fa c t tha t the c o e ffic ie nt o f the rm a l e xp a nsio n fo r e a c h is a p p ro xim a te ly the sa m e . Also , while b o ld ly e xp o se d ste e l c o rro d e s a c tive ly in m o st na tura l e nviro nm e nts a t a ra te tha t re q uire s use o f e xtrinsic c o rro sio n c o ntro l me a sure s (fo r e xa mp le , p ro te c tive c o a ting s fo r a tm o sp he ric e xp o sure s a nd c a tho d ic p ro te c tio n in sub m e rg e d a nd b urie d situa tio ns), the re la tive ly hig h p H o f c o nc re te p o re wa te r (p H > 13.0-13.8) p ro mo te s fo rma tio n o f a p ro te c tive p a ssive film suc h tha t c o rro sio n ra te is ne g lig ib le a nd d e c a d e s o f re la tive ly lo w m a inte na nc e re sult.
re d uc tio n a nd H
2.1 O ve rvie w o f C o nc re te De te rio ra tio n Pro c e sse s
………..(1)
I
I I
∑ ∑ − = red ox net
o n the sa me me ta l (Bro o m fie ld , 1997) .
2 - e vo lutio n) o c c urring
2 -
The surfa c e o f the c o rro d ing m e ta l a c ts a s a m ixe d e le c tro d e , up o n whic h c o up le d a no d ic a nd c a tho d ic re a c tio ns ta ke p la c e . At a no d ic site s, me ta l a to ms p a ss into so lutio n a s p o sitive ly c ha rg e d io ns (a no d ic o xid a tio n) a nd the e xc e ss o f e le c tro ns flo w thro ug h the me ta l to c a tho d ic site s whe re a n e le c tro n a c c e p to r like d isso lve d o xyg e n is a va ila b le to c o nsum e the m (c a tho d ic re d uc tio n). This re p re se nts the e le c tro c he mic a l the o ry o f me ta l c o rro sio n; d e sc rib ing the me ta l c o rro sio n p ro c e ss, a s a c o m b ina tio n o f a n a no d ic o xid a tio n, suc h a s m e ta l d isso lutio n, a nd a c a tho d ic re d uc tio n, suc h a s o xyg e n re d uc tio n o r hyd ro g e n e vo lutio n.
simulta ne o us re d uc tio n re a c tio ns (O
While c o nc re te ha s e vo lve d to b e c o me the mo st wid e ly use d struc tura l m a te ria l in the wo rld , the fa c t tha t its c a p a c ity fo r p la stic d e fo rma tio n is e sse ntia lly nil im p o se s ma jo r p ra c tic a l d e sig n limita tio ns; this sho rtc o ming is mo st c o m m o nly o ve rc o me b y inc o rp o ra tio n o f ste e l re info rc e m e nt into tho se lo c a tio ns in the c o nc re te whe re te nsile stre sse s a re a ntic ip a te d . C o nse q ue ntly, c o nc e rns re g a rd ing p e rfo rma nc e must no t o nly fo c us up o n p ro p e rtie s o f the c o nc re te p e r se b ut a lso o f the e mb e d d e d ste e l a nd , in a d d itio n, the m a nne r in whic h the se two c o mp o ne nts inte ra c t.
is g e ne ra lly ze ro , i.e . fo r the situa tio n whe re a m e ta l c o rro d e s d ue to a n o xid a tio n re a c tio n o f the m e ta l a nd o ne (O
ne t
wo uld re p re se nt a ne t e ffe c t o f the p a rtia l c urre nts re sulting fro m o xid a tio n a nd re d uc tio n. I
ne t
If the c urre nt c a use d b y the e le c tro n flo w c o uld b e m e a sure d a t a ll, the m e a sure d q ua ntity, I
The e le c tro ns c re a te d in the a no d ic re a c tio n m ust b e c o nsum e d e lse whe re o n the ste e l surfa c e e sta b lishing the c o rro sio n re a c tio n. The p ro c e ss is c o m p le te d b y the tra nsp o rt o f io ns thro ug h the a q ue o us p ha se , le a d ing to the fo rma tio n o f c o rro sio n p ro d uc ts a t the a no d ic site s e ithe r so lub le (e .g . fe rro us c hlo rid e ) o r inso lub le (e .g . rust, hyd ra te d fe rric o xid e ).
2 -re d uc tio n) o r two
- 2
ρ = d e nsity o f iro n (= 7.86 g / c m
1 ) ( 3 .
c o rr
) a nd c o rro sio n p e ne tra tio n (p ) to ra te s o f c o rro sio n is g ive n in Ta b le 1
This g ive s a c o nve rsio n o f 1 μ A = 11.6 μ m ste e l se c tio n lo ss p e r ye a r to o b ta in the ra te o f c o rro sio n. The c o rro sio n c urre nt tha t is inve rse ly re la te d to the p o la riza tio n re sista nc e c a n b e c a lc ula te d b y the e q ua tio n (Bro o m fie ld , 1993):
F = Fa ra d a y’ s c o nsta nt (= 96500 C / m o l)
−
) e Fe 2 Fe for 2 (
) z = num b e r o f e le c tro ns tra nsfe rre d p e r a to m
3
)) I c o rr = c o rro sio n c urre nt A = m e a sure m e nt a re a t = time
= ……(3) whe re β
(c o rro sio n c urre nt d e nsity (A c m
A Icorr
whe re , M = a to m ic we ig ht (= 55.85 g / m o l fo r iro n) i c o rr =
ρ = …………………..(2)
F z M t i p corr
A c o rro sio n c urre nt d e nsity o f 1 mA/ m 2 iro n surfa c e is the re fo re e q ua l to a c o rro sio n ra te o f 1.16 µm/ ye a r. If a re b a r with a d ia m e te r o f 16 m m is c o rro d ing with 100 mA/ m2 surfa c e fo r 20 ye a rs - whic h c a n lo c a lly b e the c a se - the c ro ss se c tio n wo uld ha ve re d uc e d to 11.4 m m . This c a n c a use a lre a d y sta tic p ro b le ms fo r the struc ture . In fa c t, the c o lla p se o f the Be rlin C o ng re ss Ha ll a nd o f a p a rking g a ra g e in Minne so ta is two e xa m p le s o f sp e c ta c ula r fa ilure s b e c a use the sta tic lo a d c a p a c ity wa s re d uc e d e xc e ssive ly d ue to c o rro sio n (Bo rg a rd , B., 1990).
C o rro sio n Ra te s Me a sure me nts b y Line a r Po la risa tio n a nd AC Imp e d a nc e Te c hniq ue s Using Diffe re nt Ste e l Ba rs a nd Ac id ic So lutio n (G id io n Turu’ a llo )
The e le c tro c he mic a l syste m "ste e l c o rro d ing in c o nc re te " c a n b e d e sc rib e d b y a p p lying the mixe d me ta l the o ry. The c urre nt d e nsity-p o te ntia l c urve c a n b e a c hie ve d the o re tic a lly b y so lving the Butle r Vo lm e r e q ua tio ns in c o m b ina tio n fo r the re a c tio ns tha t ha p p e ne d in a no d ic a nd c a tho d ic .
- →
In a lka line a nd o xyg e n ric h e le c tro lyte s suc h a s a tm o sp he ric a lly e xp o se d re info rc e d c o nc re te struc ture s, the se c o nd a nd o r the third e le c tro c he m ic a l re a c tio ns a re invo lve d in the o ve ra ll c o rro sio n re a c tio n. If the Iro n we re just to d isso lve in the p o re wa te r o f the c o nc re te , c ra c king a nd sp a lling o f the c o nc re te a re no t visib le . Se ve ra l m o re ste p s m ust o c c ur fo r fo rming “ rust” . O ne c o m b ina tio n is sho wn b e lo w whe re fe rro us hyd ro xid e a nd the n hyd ra te d Fe rric o xid e o r rust (Bro o mfie ld , 1993):
2 I =
a
- 2 Fe →
2
) OH ( Fe H 4 Ohyd ra te d fe rric o xid e (rust) ………….(6)
3
2
3
2
2
2
O H H 2 O . O Fe ) OH ( Fe
→ + + fe rric hyd ro xid e …(5)
4
) 2 O OH ( Fe
2
2
, β
3
fe rro us hyd ro xid e …(4)
− +
2 ) OH ( Fe OH
2
- →
137 The c o rro sio n ra te p is the n p ro p o rtio na l to the sum o f the p a rtia l a no d ic c urre nts (c o rro sio n c urre nt) c a using me ta l d isso lutio n. p is d e fine d a s the lo ss o f the c o rro d ing m e ta l in mic ro m e te rs p e r ye a r [µm / y] a nd c a n b e c a lc ula te d b y (And ra d e , 1996):
p p c a c a corr
R B R
a re the a no d ic a nd c a tho d ic Ta fe l c o nsta nt re sp e c tive ly, whic h is kno wn a s the Ste rn-G e a ry c o nsta nt, B. B is ta ke n a s a p p ro xim a te ly 25 mV fo r a c tive ly c o rro d ing ste e l a nd a ro und 50 m V fo r p a ssive ste e l in c o nc re te (And ra d e , 1993 ) . Ho we ve r, so me so urc e s to o k 26 mV a nd 52 m V (Milla rd , 1994) fo r a c tive ly a nd p a ssive c o rro d ing re sp e c tive ly, with the e rro r fa c to r is 2. Furthe rmo re , g uid a nc e re la ting p o la riza tio n re sista nc e (Rp ), c o rro sio n c urre nt d e nsity (i
β + β β β
c So urc e : G o we rs a nd Milla rd , 1999
Unhyd ra te d d e nse fe rric o xid e
C o rro sio n p e ne tra tio n: p
< 1 1 < p < 10 Pa ssive Rp > 250 i c o rr < 0.1 P < 1
c o rr
Lo w 250 > Rp > 25 0.1 < i
< 100 100 < p < 1000 Me d ium 25 > Rp > 2.5 1 < i c o rr < 10 10 < p < 100
c o rr
Hig h 2.5 > Rp > 0.25 10 < i
( μm/year)
2 )
(Fe
( μA/cm
) C o rro sio n c urre nt d e nsity: i c o rr
2
( k Ω.cm
C o rro sio n Po la riza tio n re sista nc e : Rp
Ta b le 1. Typ ic a l c o rro sio n ra te s fo r ste e l in c o nc re te Ra te o f
Jurna l SMARTe k, Vo l. 4, No . 3, Ag ustus 2006: 135 - 145
3 ) ha s a vo lume o f a b o ut twic e
2 O
2.3. The e ffe c t o f p H The c o rro sio n ra te o f a c tive m e ta ls is stro ng ly d e te rm ine d b y the p H va lue a nd in ne utra l me d ia b y the o xyg e n c o nte nt. Alka line c o nc re te ha s a p H va lue o f a b o ut 12.5. In this e nviro nm e nt c a rb o n ste e l is p a ssive a nd suffe rs the re fo re no no tic e a b le c o rro sio n in a b se nc e o f c hlo rid e s. In ne utra l wa te r the re la tive ly slo w d iffusio n o f the o xyg e n to the m e ta l surfa c e is the limiting ste p in the c o rro sio n p ro c e ss. The ra te o f c o rro sio n o f a c tive me ta ls in wa te r c a use d b y the O
Also in the c a se o f lo c a lise d c o rro sio n o f p a ssive ma te ria ls the c o rro sio n ra te s a re usua lly ve ry hig h. Fo r p itting a nd c re vic e c o rro sio n m a te ria l lo sse s up to 3 mm/ ye a r a re no t unusua l. In the lite ra ture e ve n c o rro sio n ra te s o f 20 m m / ye a r a re re p o rte d . Stre ss c o rro sio n c ra c king a nd inte rg ra nula r c o rro sio n c a n, a t le a st in unfa vo ura b le c a se s, le a d to fa ilure s p ra c tic a lly witho ut p re limina ry wa rning .
tha t o f ste e l re p la c e d . Whe n it b e c o m e s hyd ra te d it swe lls e ve n m o re a nd b e c o m e s p o ro us, inc re a sing the vo lum e a t the ste e l/ c o nc re te inte rfa c e two to te n tim e s. This le a d s to the a lre a d y m e ntio ne d c ra c king a nd sp a lling o f the c o nc re te o b se rve d a s a usua l c o nse q ue nc e o f ste e l c o rro sio n in c o nc re te . The e le c tro c he m ic a l b e ha vio r o f ste e l in a q ue o us so lutio n ha s to b e c o nsid e re d a s the b a se fo r und e rsta nd ing the c o m p le x c o rro sio n p ro c e ss in the ve ry inho m o g e ne o us c o nc re te with lo c a l g ra d ie nts o f p H a nd c o nc e ntra tio n o f a g g re ssive io ns.
It c a n a lre a d y' b e se e n tha t the re d uc tio n o f H
- to H2 is a the rm o d yna mic a lly fe a sib le wa y to a llo w the o xid a tio n o f Fe to ta ke p la c e . This a lso imp lie s tha t (e sp e c ia lly in d e o xyg e na te d so lutio ns) the c o nc e ntra tio n o f H
- is ve ry im p o rta nt.
- ], the n: using the Ne rnst e q ua tio n o ne c a n write :
In ma c ro c e lls the re is a c urre nt flo w whic h c a use s a n a d d itio na l m e ta l d isso lutio n a t the a no d e . The c urre nt a nd thus the a mo unt o f ma te ria l lo ss d e p e nd s m a inly o n the d iffe re nc e o f the c o rro sio n p o te ntia ls, the e le c tric a l re sista nc e b e twe e n a no d e a nd c a tho d e , the ra tio o f the a no d ic a nd c a tho d ic a re a s a nd the p o la risa tio n b e ha vio r o f the two me ta ls. In p ra c tic e typ ic a l c o rro sio n ra te s a re in the ra ng e b e twe e n 0.5 a nd 2 mm/ ye a r [Bind sc he d le r, D., 2001].
g e ne ra lly lo w a nd ha rd ly e xc e e d s 0.1 mm/ ye a r.
2 - c o rro sio n typ e is
E pH a nF RT E E H
059 , log 3 ,
2 − = + = +
……(7) So , E fo r the hyd ro g e n e vo lutio n
(H+ re d uc tio n) c ha ng e s b y 59 m V fo r e ve ry c ha ng e in p H unit. C o rro sio n e xp e rim e nts inte nd e d to sim ula te ste e l in c o nc re te ha ve histo ric a lly e mp lo ye d a sa tura te d C a (O H)
2 so lutio n, the p H o f whic h is
a p p ro xim a te ly 12.4. Ho we ve r, with the a d ve nt o f the p o re wa te r e xp re ssio n m e tho d (12-14) a nd the o re tic a l c o nsid e ra tio ns, it wa s re c o g nize d tha t K+ a nd Na + a re the p re d o mina nt c a tio ns; a nd the so lub ility a nd c o nc e ntra tio n o f the se is suc h tha t a p H in e xc e ss o f 13 typ ic a lly o c c urs.
Sinc e p H = -lo g [H
Simila rly, to the Ana lo g o us Re sisto r-C a p a c ito r C irc uits te st, the te st is c o nd uc te d b y c o nne c ting the re fe re nc e e le c tro d e , wo rking e le c tro d e a nd a uxilia ry e le c tro d e fro m the te st sp e c im e n to the re fe re nc e e le c tro d e , wo rking e le c tro d e a nd a uxilia ry e le c tro d e fro m the AC M Fie ld Ma c hine a s sho wn in Fig ure 18 b e lo w. The me a sure me nts o f c o rro sio n ra te s we re p e rfo rm e d a fte r the ste e l b a rs we re imm e rse d fo r a d a y in the a c id so lutio n in a g la ss c o nta ine r.
2
Furthe rm o re , the e xp e rim e nt use s two kind s o f a c id i.e ., a c e tic a c id a nd sulp huric a c id . The p urp o se o f this is to c o mp a re the c o rro sio n ra te s in the d iffe re nt a c id s. Bo th the a c id s use d c o nsist o f thre e d iffe re nt p H's i.e ., the va lue o f 3, 4 a nd 5.
The re fe re nc e e le c tro d e use d wa s a n Ag / Ag C l o f 3 % so lutio n a nd the a uxilia ry e le c tro d e a s we ll a s the wo rking e le c tro d e s c o mp rise d ste e l b a rs with 10 c m lo ng . The wo rking e le c tro d e we re fo ur d iffe re nt b a rs i.e . mild ste e l b a r with d ia m e te r o f 6 mm, 10mm, 25 mm a nd a no the r o ne whic h is a sta inle ss b a r with a d ia m e te r o f 10 m m . A 10 c m le ng th o f sta inle ss ste e l with a d ia me te r o f 10 m m is use d a s the a uxilia ry e le c tro d e .
The p urp o se is to e va lua te the ra te o f c o rro sio n o f the ste e l b a r a nd ho w this is a ffe c te d b y a c id with va rio us p H, b a r with d iffe re nt d ia me te rs, typ e o f ste e l b a r, a nd tim e o f le ng th o f e xp o sure . The te sts a re p e rfo rm e d b y b o th the line a r p o la risa tio n re sista nc e (LPR) a nd AC im p e d a nc e (EIS) te c hniq ue s using a c id so lutio n a s a m e d ium a nd ste e l b a rs a s the te st sp e c im e n. The te st re sults o f the two te c hniq ue s the n a re c o m p a re d .
Exp e rim e nta l Pro g ra m
3 .
3
The re g io ns whe re the so lub le sp e c ie s Fe
2 , Fe 3O 4 a nd Fe 2O 3 a re sta b le .
Inc re a sing so lub ility o f the o xid e la ye r will o fte n im p ly a re d uc e d p a ssivity a nd a n inc re a se in the c o rro sio n ra te . Iro n in a q ue o us e le c tro lyte s is p a ssive whe n the hyd ro xid e a nd o xid e sp e c ie s Fe (O H)
- a re sta b le a re the zo ne s in whic h a c tive c o rro sio n is e xp e c te d .
2.4. Bre a kd o wn o f the p a ssivity d ue to p H-d e c re a se Pa ssive hyd ra te d o xid e s inte ra c t with the so lutio n d ue to the ir c e rta in so lub ility. If the so lub ility o f the hyd ro xid e (hyd ra te d o xid e ) in a g ive n a q ue o us e nviro nm e nt is sm a ll the n it is p ro b a b le tha t it will fo rm a sta b le p ro te c tive film o n the m e ta l surfa c e . Ho we ve r, the p a ssiva ting (hyd ra te d ) o xid e o r hyd ro xid e films o n ma ny me ta l surfa c e s e xhib it inc re a sing so lub ility with d e c re a sing p H o f the surro und ing so lutio n.
The se limita tio ns a re m inimize d b y the in- situ m e tho d b e c a use o nly a b o ut 0.4 m l o f d istille d wa te r is e mp lo ye d ; ho we ve r, wa te r sa tura tio n o f the sp e c im e n is re q uire d he re a lso . Re c e ntly, a mo d ific a tio n o f the e xsitu me tho d wa s p ro p o se d whe re b y a c o rre c tio n is m a d e fo r the [O H-] re sulting fro m C a (O H)2 d isso lutio n (Sa g üé s, A.A., e t a ll, 1997); ho we ve r, so lub le s in unre a c te d c e me nt p a rtic le s ma y a lso b e c o me d isso lve d , the re b y e le va ting the c a lc ula te d p H c o m p a re d to wha t a c tua lly e xiste d in the p o re wa te r. C o nse q ue ntly, this p ro c e d ure ma y b e mo re a me a sure o f inhe re nt a lka linity tha n o f p o re wa te r p H.
2 , if sa tura te d , b uffe rs the le a c ha te a t a p H o f a b o ut 12.4.
139 Limita tio ns a sso c ia te d with p o re wa te r e xp re ssio n inc lud e , first, p rio r wa te r sa tura tio n o f sa mp le s is re q uire d a nd , se c o nd , the m e tho d is mo re use ful fo r p a ste s a nd mo rta rs sinc e e xp re ssio n yie ld s fo r c o nc re te , p a rtic ula rly hig h p e rfo rm a nc e o ne s, is lo w. C o nse q ue ntly, b o th e x-situ a nd in-situ le a c hing me tho d s (Sa g üé s, A.A. e t a ll, 1997) ha ve a lso b e e n d e ve lo p e d , whe re the fo rm e r invo lve s e xp o sure o f a p o wd e r sa mp le to d istille d wa te r a nd the la tte r p la c e m e nt o f a sm a ll q ua ntity o f wa te r into a d rille d c a vity in ha rd e ne d c o nc re te . A limita tio n in the c a se o f e x- situ le a c hing is tha t so lid C a (O H)2 fro m the c o nc re te b e c o me s d isso lve d a nd e le va te s [O H-] c o m p a re d to wha t o the rwise wo uld o c c ur. Also , the d isso lve d C a (O H)
C o rro sio n Ra te s Me a sure me nts b y Line a r Po la risa tio n a nd AC Imp e d a nc e Te c hniq ue s Using Diffe re nt Ste e l Ba rs a nd Ac id ic So lutio n (G id io n Turu’ a llo )
- a nd HFe O
Fig ure 1. C o nne c tio n b e twe e n the e le c tro d e s fro m the AC M Ma c hine a nd the e le c tro d e fro m Sp e c im e n Ho we ve r, unlike the wo rking e le c tro d e b a rs, o nc e the te sts we re finishe d the a uxilia ry e le c tro d e wa s ta ke n o ut fro m the sp e c im e n (a q ue o us so lutio n) to ke e p its surfa c e a re a a s we ll a s the re fe re nc e e le c tro d e .
The a c id so lutio n wa s a e ra te d c o ntinuo usly a s lo ng a s the te sts we re p e rfo rm e d b y a n e le c tric d rive n a ir p ump . This a im wa s to p ro vid e o xyg e n, whic h is ne e d e d , fo r the c o rro sio n p ro c e ss. The e le c tro d e a re a wa s o b ta ine d fro m the surfa c e a re a o f the wo rking e le c tro d e . The 'se q ue nc e ' p ro g ra m wa s use d to se t up the te sts o f b o th the line a r p o la risa tio n re sista nc e a nd the AC im p e d a nc e te c hniq ue s p ro vid ing a d e la y tim e b e twe e n the two te sts b y me a ns the 'p a use ' te c hniq ue .
The e xp e rim e nt o f e a c h c o nc e ntra tio n o f b o th the a c e tic a c id a nd sulp huric a c id wa s p e rfo rme d fo r 7 d a ys. In g e ne ra l, a ll o f te st re sults sho w tha t the re is a ra p id e xp o ne ntia l d e c re a se in the p o la risa tio n re sista nc e until a c e rta in tim e . The re is the n a slo w d e c re a se in the ne xt tim e p e rio d a s sho wn in Fig ure 2 b e lo w. The g ra p h p re se nts the te st re sult o f the p o la risa tio n re sista nc e te st using a mild ste e l b a r o f 6 m m d ia m e te r a s a c o rro sio n sp e c im e n in a a c e tic a c id o f p H 3.
Fro m the re sults se e n in Fig ure 2, it is se e n tha t the p o la risa tio n re sista nc e d e c re a se s with tim e fo r the m ild ste e l sp e c im e n. As the c o rro sio n c urre nt is inve rse ly re la te d to the p o la risa tio n re sista nc e the n it c a n b e se e n in Fig ure 3 tha t the c o rro sio n ra te inc re a se s in m a g nitud e o ve r 5 tim e s d uring the 6 d a ys te st p e rio d . Ho we ve r, b o th the g ra p hs sho w tha t a fte r 5 to 6 d a ys (o n the e nd o f te st p e rio d ) the ra te o f c o rro sio n a p p e a rs to sta b ilise .
The Fig ure 2 sho ws tha t the p o la risa tio n re sista nc e o f the 6 mm mild ste e l b a r d e c re a se s fo r the first thre e d a ys. It re d uc e s to mo re tha n ha lf o f its va lue a t the sta rt o f the te st i.e ., fro m the va lue o f 418 Ω o n the first d a y to 165 Ω o n the third d a y (a ve ra g e re sults o f the line a r p o la risa tio n re sista nc e a nd the AC im p e d a nc e ). The p o la risa tio n re sista nc e the n re d uc e s m o re slo wly b e twe e n the third d a y a nd the sixth d a y o f the te st o f the va lue o f 165
Ω
a nd 113
Ω
re sp e c tive ly. Fina lly, the c o rro sio n re a c tio n lo o ks to b e c o nsta nt a t the la st two d a ys o f the te st i.e ., fro m the va lue o f 113
Ω
o n the sixth d a y to 111
Ω
o n the la st d a y. In c o ntra st, the Fig ure 3 b e lo w sho w tha t the re is a ra p id ly inc re a se in
ACM Field Machine Computer to record data Acidic Solution Electrodes From the ACM Field
Machine
4. Re sults a nd d isc ussio n
C o rro sio n Ra te s Me a sure me nts b y Line a r Po la risa tio n a nd AC Imp e d a nc e Te c hniq ue s Using Diffe re nt Ste e l Ba rs a nd Ac id ic So lutio n (G id io n Turu’ a llo )
4
2
Fig ure 3. Plo tting the C o rro sio n c urre nt ( μ A/ c m
2 ) L P R E IS A v e r a g e Acetic acid solution pH 3 Mild steel bar 6 mm diameter
^
C o rro s io n c u rre n t ( μA/cm
C o r r o s i o n c u r r e n t v s T i m e M i l d s t e e l b a r 6 m m d i a m e t e r w i t h A c e t i c a c i d ( p H - 3 ) 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 1 2 3 4 5 6 7 8 T im e s ( d a y s )
Acetic acid solution pH 3 Mild steel bar 6 mm diameter
.cm^2 ) L P R E I S a v e r a g e
8 T i m e ( d a y s ) Po la ri sat io n resi s tan ce ( Ω
7
6
5
3
141 the c o rro sio n c urre nt fo r the first thre e d a ys fro m a va lue o f 63 μ A/ c m 2 o n the first d a y o f the te sts to a va lue o f 125 μ A/ c m
2
1
P o l a r i s a t i o n r e s i s t a n c e v s T i m e M i l d s t e e l b a r 6 m m d i a m e t e r w i t h A c e t i c a c i d ( p H - 3 ) 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0
) vs. Tim e (d a ys)
2
Fig ure 2. Plo tting the p o la risa tio n re sista nc e ( Ω .c m
. The re fo re , the ra te o f c o rro sio n in the sp e c im e n will re m a in c o nsta nt if the e nviro nm e nt o f the te sts is no t c ha ng e d .
2
to a va lue o f 239 μ A/ c m
2
o n the third d a y o f the te sts. This is a lm o st twic e the va lue o f the re sult o n the first d a y. The c o rro sio n re a c tio n the n b e c o m e s m o re c o nsta nt o n the la st two d a ys o f te st i.e ., fro m a c o rro sio n c urre nt va lue o f 235 μ A/ c m
2
) vs. Tim e (d a ys) Jurna l SMARTe k, Vo l. 4, No . 3, Ag ustus 2006: 135 - 145
Fig ure 4 b e lo w p re se nts the re sult o f the te sts, whic h we re p e rfo rm e d using the sa m e o f a c id a nd c o nc e ntra tio n with d iffe re nt d ia m e te r o f the b a rs. It c a n b e se e n fro m the fig ure tha t the p o la risa tio n re sista nc e o f a sta inle ss ste e l b a r is hig he r tha n the p o la risa tio n re sista nc e o f a mild ste e l b a r. A c o mp a riso n the te st re sults o f p o la risa tio n re sista nc e sho ws tha t the p o la risa tio n re sista nc e o f the sta inle ss ste e l b a r is a lmo st twe nty time s hig he r tha n the p o la risa tio n re sista nc e o f the mild ste e l b a r i.e ., 4062 Ω. c m
2
. c m ^
7 T im e (d a ys ) P o la ri sati o n r e si st a n c e (
6
5
4
3
2
1
P o l a r i s a t i o n r e s i s t a n c e v s T i m e ( a v e r a g e r e s u l t s o f L P R a n d E IS M e t h o d s ) A c e t i c a c i d ( p H - 3 ) w i t h d i f f e r e n t b a r s 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 0 0
Furthe rmo re , Fig ure 4. Me a sure me nt o f p o la risa tio n re sista nc e fo r d iffe re nt b a r d ia m e te rs with the sa me a c id ic so lutio n
2 re c e p tive ly.
, a nd 111 Ω. c m
, 150 Ω. c m
2
2
And a t the e nd o f te st the p o la risa tio n re sista nc e s o f the b a rs a re 206 Ω. c m
2 .
, a nd 419 Ω. c m
2
, 746 Ω. c m
2
The fig ure a lso sho ws tha t using a b ig g e r d ia m e te r o f the sa m e typ e o f b a r i.e ., mild ste e l g ive s a hig he r p o la risa tio n re sista nc e in the b e g inning o f te st. Ho we ve r, a t the e nd o f the te sts it is fo und tha t the re sults a re just a little d iffe re nt b ut still sho w tha t the b ig g e r d ia m e te rs o f the ste e l b a r g ive the hig he r p o la risa tio n re sista nc e . The re sults fro m first d a y o f te st using mild ste e l b a rs o f 6 mm, 10 mm a nd 25 mm d ia m e te r g ive s the p o la risa tio n re sista nc e re sults o f 3246 Ω. c m
re sp e c tive ly. The re sults sho wn a re fo r the sa me 10 mm d ia m e te r o f b a r a nd fo r im m e rsio n in the sa m e c o nc e ntra tio n o f a c id . This is b e c a use sta inle ss ste e l b a r ha s a p a ssive o xid e la ye r tha t a c ts a s a c o rro sio n inhib ito r tha t p ro te c ts the b a r surfa c e fro m c o rro sio n.
2
Ω. c m
a nd 206
2 ) M i l d s te e l b a r 6 m m d i a m e te r M i l d s te e l b a r 1 0 m m d i a m e te r S ta i n l e s s s te e l 1 0 m m d i a m e te r M i l d s te e l b a r 2 5 m m d i a m e te r H igh corrosi on Medi um corrosi on
C o rro sio n Ra te s Me a sure me nts b y Line a r Po la risa tio n a nd AC Imp e d a nc e Te c hniq ue s Using Diffe re nt Ste e l Ba rs a nd Ac id ic So lutio n (G id io n Turu’ a llo )
Polarisation resistance vs Time (average results of LPR and EIS methods) Mild steel bar 10 mm with different concentration (pH) of Acetic acid 100 200 300 400 500 600 700 800
High cor ro sion
2 ) Acetic acid pH 3 Acetic acid pH 4 Acetic acid pH 5
(⎠ .c m^
8 Time (days) P o lar isat io n r esi st an ce
7
6
5
4
3
2
1
Mils steel bar 6 mm diameter Mild steel bar 10 mm diameter Stainless steel bar 10 mm diameter Mild steel bar 25 mm diameter Hig h co rr o sio n M edium corr osion co rro sio n
143 Fig ure 5. Me a sure m e nt o f c o rro sio n c urre nt fo r d iffe re nt d ia me te r o f b a rs with the sa me a c id ic so lutio n
( ÷ A/ c m ^2 )
8 Time (days) C or ros ion c ur rent
7
6
5
4
3
2
1
50 100 150 200 250 300
Corrosion current vs Time (average results of LPR and EIS Methods) Acetic acid (pH-3) with different bars
Fig ure 6. Po la risa tio n re sista nc e o f the m ild ste e l b a r 10 m m d ia m e te r im m e rse d in d iffe re nt c o nc e ntra tio n o f a c id Jurna l SMARTe k, Vo l. 4, No . 3, Ag ustus 2006: 135 - 145
Ano the r p re se nta tio n o f the te st re sults c a n b e se e n in Fig ure 5 b e lo w. By p lo tting the c urre nt d e nsity a g a inst tim e , the fig ure sho ws tha t the c o rro sio n c urre nt o f sta inle ss ste e l b a r is q uite c o nsta nt. It is va rie s fro m a va lue o f 4 μ A/ c m
2
While the re sults o f the e xp e rim e nt, whic h use d the sta inle ss b a r, sho w tha t the c o rro sio n ra te o f a sta inle ss b a r, whic h wa s im m e rse d in the sa m e a c id a nd c o nc e ntra tio n with the mild ste e l b a rs, is lo we r tha n c o rro sio n c urre nt o f the mild ste e l b a rs. The c o rro sio n ra te o f the sta inle ss b a r is e xp e c te d to b e a p a ssive c o rro sio n, ho we ve r, a s it wa s im me rse d in a stro ng a c id so lutio n with a va lue o f p H 3, whic h b ro ke d o wn the p a ssive la ye r o f the sta inle ss b a r. The sta inle ss
2 .
1. The re sults o f the e xp e rime nt whic h use d the m ild ste e l b a rs sho w tha t the c o rro sio n ra te s o f the mild ste e l b a rs whic h we re imm e rse d in a c id a re ve ry hig h c o rro sio n in whic h e a c h the mild ste e l b a rs ha ve a va lue o f the c o rro sio n c urre nt o ve r o f 100 µA/ c m
C o m p a ring the to ta l inc re a sing o f the c o rro sio n c urre nt fo r e a c h c o nc e ntra tio n o f the a c e tic a c id so lutio n sho ws tha t the b ig g e st inc re a se o f the c o rro sio n c urre nt is in the a c e tic a c id so lutio n o f p H 3, whic h is the stro ng e st o f the a c id s.
2 .
to 175.09µA/ c m
2
re sp e c tive ly. The c o rro sio n c urre nt fo r the e xp e rim e nt use d the a c e tic a c id so lutio n o f p H 3 inc re a se d fro m 35.07µA/ c m
2
to 115 µA/ c m
2
a nd fro m 64.38 µA/ c m
to 155.82µA/ c m
2
2
The c o rro sio n c urre nt fo r the te sts, whic h use d a c e tic a c id so lutio n o f p H 4 a nd p H 5, a re fro m 103.67µA/ c m
The re fo re , the lo we r p H o f a n a c id so lutio n use d in the e xp e rim e nt the hig he r ra te s o f c o rro sio n c a n b e o b ta ine d . The c o rro sio n p ro c e ss is fa ste r with the lo we r p H o f a c id so lutio n ra the r tha n with the hig he r p H o f a c id a s p re se nte d in G ra p h 10 a b o ve . It a lso se e ms tha t the c o rro sio n c urre nt fo r b o th o f the two d iffe re nt p H i.e ., p H 4 a nd p H 5 inc re a se s mo re slo wly tha n the c o rro sio n c urre nt with the a c e tic a c id so lutio n o f p H 3.
The p o la risa tio n re sista nc e o f b a r in a c e tic a c id o f p H 3 d e c re a se s m o re d ra m a tic a lly tha n the o the r c o nc e ntra tio ns o f a c e tic a c id . This me a ns tha t stro ng a c id s a re mo re c o rro sive tha n we a k a c id s. As the re sult o f the stro ng a c id mo re c o rro sio n c urre nt c a n b e p a sse d . In o the r wo rd s, the use o f stro ng a c id s will a c c e le ra te the c o rro sio n re a c tio n.
The g ra p h o n fig ure 6 is d isp la c e d b e c a use the e xp e rim e nts use the sa m e a c tua l b a rs fo r the te st o f e a c h c o nc e ntra tio n o f a c id . The first te st use s the stro ng e st a c id i.e ., a c e tic a c id o f p H 3, the re fo re , whe n the o the r te sts using a c e tic a c id o f p H 4 a nd p H 5 the b a r use d a lre a d y ha s a c o rro d e d surfa c e . Ho we ve r, a t the e nd o f e xp e rim e nt it sho ws tha t the stro ng e st a c id i.e ., a c e tic a c id o f p H 3 g ive s the lo we st o f the p o la risa tio n re sista nc e re sults. It c a n b e se e n tha t the re sults o f b o th the a c e tic a c id o f p H 4 a nd o f p H 5 g ive a hig he r p o la risa tio n re sista nc e re sult tha n using a c e tic a c id o f p H 3.
Fina lly, Fig ure 6 b e lo w sho ws the e ffe c ts o f d iffe re nt c o nc e ntra tio ns o f a c id to the c o rro sio n p ro c e ss o f the ste e l b a r. The fig ure sho ws the p o la risa tio n re sista nc e re sults o f a mild ste e l b a r o f 10 m m d ia m e te r.
The g ra p h a lso sho ws tha t the c o rro sio n re a c tio n in the mild ste e l b a rs is slo w o n the fifth d a y a nd the n lo o ks q uite c o nsta nt o n the la st two d a ys o f the te st. This sug g e sts the e q uilib rium ra te o f the c o rro sio n in e a c h b a r ha ve b e e n re a c he d . The re fo re , the c o rro sio n ra te re a c he s e q uilib rium if the e nviro nm e nt is no t c ha ng e d .
μ A/ c m2 a nd 140 μ A/ c m 2 re sp e c tive ly.
re sp e c tive ly. And the re sults o n the e nd o f the e xp e rim e nt a re 271 μ A/ c m2, 190
μ A/ c m2, 37 μ A/ c m 2 a nd 10 μ A/ c m2
a t the e nd o f the te st, whe n the te sts a re p e rfo rm e d fo r 7 d a ys. Ho we ve r, the c o rro sio n c urre nt o f a ll the m ild ste e l b a rs inc re a se d ra p id ly a nd e xp o ne ntia lly o ve r the sa m e p e rio d . The c o rro sio n c urre nt o f the mild ste e l b a rs o f the d ia m e te r 6 m m , 10 m m a nd 25 mm o n the first d a y o f te sting we re 64
2
a t the b e g inning o f the te st to a va lue o f 6.86 μ A/ c m
5. C o nc lusio n
C o rro sio n Ra te s Me a sure me nts b y Line a r Po la risa tio n a nd AC Imp e d a nc e Te c hniq ue s Using Diffe re nt Ste e l Ba rs a nd Ac id ic So lutio n (G id io n Turu’ a llo )
Bro o mfie ld J.P. 1993, C o rro sio n ra te m e a sure m e nt in re info rc e d c o nc re te struc ture s b y a Line a r Po la risa tio n d e vic e , AC I fa ll c o nve ntio n, Minne a p o lis.
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3. The o b ta ine d re sults a re simila r to the e xp e c ting re sult, b e fo re p e rfo rm e d the te sts with va rio us va ria b le s suc h a s using d iffe re nt c o nc e ntra tio n o f the a c id , d iffe re nt d ia m e te r a nd typ e o f the b a rs p a rtic ula rly fo r the m ild ste e l b a rs. The re is little b it d iffe re nt fro m the e xp e c ting re sults fo r the sta inle ss b a r, whic h is e xp e c te d to b e a p a ssive c o rro sio n le ve l, ho we ve r, the re sults sho w tha t the b a r is in the me d ium c o rro sio n le ve l. This is b e c a use the a c id use d wa s stro ng . Ho we ve r, the re sults a re still re a so na b le to b e g o o d re sults b e c a use the surfa c e o f the sta inle ss b a r wa s lo o ke d muc h d a ma g e d a fte r p e rfo rm e d the e xp e rim e nt.