C 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
* 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 1. Intro d uc tio n
C o rro sio n is the d e te rio ra tio n o f m a te ria ls b y c he m ic a l inte ra c tio n with the ir e nviro nm e nt. The te rm c o rro sio n is so me tim e s a lso a p p lie d to the d e g ra d a tio n o f p la stic s, c o nc re te a nd wo o d , b ut g e ne ra lly re fe rs to m e ta ls. The m o st wid e ly use d m e ta l is iro n (usua lly a s ste e l) a nd the fo llo wing d isc ussio n is ma inly re la te d to its c o rro sio n.
Whe n ste e l re info rc e m e nt is e nc a se d in so und d e nse c o nc re te , the e ntire surfa c e o f the ste e l is c o ve re d b y a sta b le p ro te c tive o xid e film tha t fo rm s in the a lka line e nviro nm e nt c re a te d b y
the hyd ra tio n o f the c e m e nt in the c o nc re te . Und e r the se c irc um sta nc e s no c o rro sio n o f the re info rc e m e nt c a n o c c ur.
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.
2.1 O ve rvie w o f C o nc re te De te rio ra tio n Pro c e sse s
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.
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.
2.2
C o rro sio n Ba sic sThe 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.
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 ).
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, Ine 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. Ine 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 (O2 -re d uc tio n) o r two
simulta ne o us re d uc tio n re a c tio ns (O2
-re d uc tio n a nd H2 - e vo lutio n) o c c urring
o n the sa me me ta l (Bro o m fie ld , 1997)
.
∑
−
∑
=
ox rednet
I
I
(G id io n Turu’ a llo )
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)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. 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.
2.4. 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.
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)2, Fe 3O 4 a nd Fe 2O 3 a re sta b le .
The re g io ns whe re the so lub le sp e c ie s
Fe2+ a nd HFe O3- 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 .
3. Exp e rim e nta l Pro g ra m
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 .
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 .
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.
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 .
4.Re sults a nd d isc ussio n
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
(G id io n Turu’ a llo )
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 μA/ c m 2 o n the
first d a y o f the te sts to a va lue o f
125 μA/ c m2 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 μA/ c m2 to
a va lue o f 239 μA/ c m2. 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 .
Fig ure 2. Plo tting the p o la risa tio n re sista nc e (Ω.c m2) vs. Tim e (d a ys)
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 )
0 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
0 1 2 3 4 5 6 7 8
T i m e ( d a y s )
Po
la
ri
sat
io
n
resi
s
tan
ce (
Ω
.
cm^2 )
L P R E I S a v e r a g e
Acetic acid solution pH 3
Mild steel bar 6 mm diameter
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 )
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0
0 1 2 3 4 5 6 7 8
T im e s ( d a y s )
C
o
rro
s
io
n
c
u
rre
n
t
( μ
A/cm
^
2
)
L P R
E IS
A v e r a g e
Acetic acid solution pH 3
Mild steel bar 6 mm diameter
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 m2 a nd 206
Ω.c m2 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.
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 m2, 746 Ω.c m2, a nd 419 Ω.c m2.
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 m2,
150 Ω.c m2, a nd 111 Ω.c m2 re c e p tive ly.
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
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
0 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
1 2 3 4 5 6 7
T im e (d a ys )
P
o
la
ri
sati
o
n
r
e
si
st
a
n
c
e
(
. c
m
^
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
(G id io n Turu’ a llo )
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
Corrosion current vs Time (average results of LPR and EIS Methods) Acetic acid (pH-3) with different bars
0 50 100 150 200 250 300
0 1 2 3 4 5 6 7 8
Time (days)
C
or
ros
ion c
ur
rent
(÷
A/
c
m
^2
)
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
Polarisation resistance vs Time (average results of LPR and EIS methods) Mild steel bar 10 mm with different concentration (pH) of Acetic acid
0 100 200 300 400 500 600 700 800
0 1 2 3 4 5 6 7 8
Time (days)
P
o
lar
isat
io
n
r
esi
st
an
ce
(⎠
.c
m^
2
)
Acetic acid pH 3
Acetic acid pH 4
Acetic acid pH 5
High cor
ro
sion
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
4μA/ c m2 a t the b e g inning o f the te st to
a va lue o f 6.86 μA/ 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
μA/ c m2, 37μA/ c m 2 a nd 10 μA/ c m2
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 μA/ c m2, 190
μA/ c m2 a nd 140 μA/ c m 2 re sp e c tive ly.
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 .
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 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.
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 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 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 m2 to
155.82µA/ c m2 a nd fro m 64.38 µA/ c m2 to
115 µA/ c m2 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 m2 to 175.09µA/ c m2.
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.
5. C o nc lusio n
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 m2.
(G id io n Turu’ a llo )
b a r the n c o rro d e d whic h is c a te g o rise d in a me d ium c o rro sio n with the c o rro sio n c urre nt b e lo w o f 10
µA/ c m2.
2. The re sults o f the c o rro sio n ra te
m e a sure m e nt, whic h use d a n a c id so lutio n, sho w tha t b o th the line a r p o la risa tio n re sista nc e (LPR) a nd the AC im p e d a nc e te c hniq ue s g ive simila r re sults. The a na lo g o us re sisto r-c a p a r-c ito r r-c irr-c uit te sts ha ve b e e n p e rfo rm e d to me a sure the
p o la risa tio n re sista nc e Rp using b o th
the line a r p o la risa tio n re sista nc e (LPR) a nd the AC im p e d a nc e te c hniq ue s
b y me a ns the AC M Fie ld ma c hine .
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.
6. Re fe re nc e s
Akino v G .V., 1993, The o ry a nd Te sting Me tho d s o f C o rro sio n o f Me ta ls, STNL Pra ha .
And ra d e C . a nd Ala nso C ., 1996, C o rro sio n ra te mo nito ring in the la b o ra to ry a nd o n site , C o nstruc tio n a nd Build ing Ma te ria ls, vo l. 10, No . 5, p p . 315-322.
And ra d e C ., C a ste lo V., a nd G o nza le s J.A., 1993, the d e te rmina tio n o f the c o rro sio n ra te o f ste e l e mb e d d e d in c o nc re te b y the p o la risa tio n re sista nc e a nd A.C im p e d a nc e m e tho d s, ASTM STP 906.
Bind sc he d le r, D. 2001: G a lva nic c o rro sio n in wa ste wa te r insta lla tio ns - C o rro sio ns g a lva niq ue s d a ns le s ré se a ux d ’ e a ux usé e s, Pro c e e d ing s o f C EO C O R p le na ry d a ys Bia rritz 2001 (C EO C O R)
Bo rg ra d B., 1990, Me c ha nisms o f C o rro sio n o f Ste e l in C o nc re te in: C o rro sio n Ra te s o f Ste e l in C o nc re te , ASTM STP 1065.
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.
Bro o mfie ld J.P. 1997, C o rro sio n o f ste e l in c o nc re te . E & F.N. Sp o n, 1997
Ha rtt, W.H., C ha rvin, S., a nd Le e , S.K., 1999, Influe nc e o f Pe rma e a b ility Re d uc ing a nd C o rro sio n Inhib iting Ad mixture s in C o nc re te up o n Initia tio n o f Sa lt Ind uc e d Emb e d d e d Me ta l C o rro sio n, fina l re p o rt sub mitte d to FDO T b y FAU o n Pro je c t WPI 0510716,
Lo p e z W. 1993, Influe nc e o f d e g re e o f p o re sa tura tio n o n the re sistivity o f c o nc re te a nd the c o rro sio n ra te o f
ste e l re info rc e m e nt. C e m e nt a nd
C o nc re te Re se a rc h, vo l. 23, No . 23, p p . 368-376.
Milla rd S.G . 1999, C o rro sio n ra te s m e a sure m e nt o f in-situ re info rc e d c o nc re te struc ture s, Pro c . Instn. C iv. Eng rs. Struc ture s a nd Build ing s, vo l. 99, p p .84-88.
Milla rd S.G ., G o we rs K.G ., G ill J.S., a nd G ill R.P. 1994, Pro g ra mma b le line a r p o la risa tio n m e a sure m e nt fo r d e te rmina tio n o f c o rro sio n ra te o f re info rc e m e nt in c o nc re te struc ture s, British C o rro sio n Jo urna l, vo l. 28, No .4, p p 25-31.
