1
Sensing elec ochemical ac i i y in polyme coa ed me als du ing he ea ly
s ages o coa ing deg ada ion by means o he scanning ib a ing elec ode
echnique
J.J. San ana+, J. González-Guzmán, J. Izquie do, S. González, R.M. Sou o
Depa men o Physical Chemis y, Uni e si y o La Laguna, E-38205 La Laguna (Tene i e,
Cana y Islands), Spain
+ On lea e om he Depa men o P ocess Enginee ing, Uni e si y o Las Palmas de G an
Cana ia, Campus Uni e si a io de Ta i a, E-35017 Las Palmas de G an Cana ia, Cana y
Islands, Spain.
Abs ac
Applica ion o scanning ib a ing elec ode echnique (SVET) o co osion s udies o o ganic
coa ings on eac i e me als is p esen ed. SVET was used o moni o he elec ochemical
p ocesses a pain ed s eel imme sed in ei he 10 mM Na2SO4 o 10 mM NaCl aqueous
solu ions. The coa ed samples we e in es iga ed a e a sc a ch was ope a ed h ough he
polyme ma ix down o he me al-subs a e su ace in o de o simula e a de ec ac oss he
coa ing. SVET imaging p obes ha he elec ochemical beha iou o he sys em is di e en
depending on he elec oly e employed. Enhanced coa ing delamina ion o igina ing om he
de ec is obse ed when chlo ide ions a e p esen in he en i onmen .
Keywo ds: Co osion; O ganic coa ings; Scanning ib a ing elec ode echnique; I on
dissolu ion; Oxygen educ ion.
2
1. In oduc ion
The es ing o me al-coa ing sys ems o e icacy and sc eening o ial p oduc s a e
gene ally ca ied ou by a mosphe ic exposu e and accele a ed sal sp ay labo a o y es s bu
hese me hods p o ide e y li le in o ma ion abou he complex in e ac ions wi hin he
sys em comp ising me al, polyme ilm and co osion medium. The e o e, elec ochemical
me hods, pa icula ly elec ochemical impedance spec oscopy (EIS) ha e come o be widely
employed in moni o ing he physicochemical changes occu ing du ing he exposu e o a
me al-coa ing sys em o a gi en co osi e en i onmen , and ecen ly a s anda d es ing
p ocedu e based on EIS has been implemen ed [1]. A ecen e iew on uses o EIS in he ield
o co osion can be ound in e . [2]. In his way, he EIS echnique ha e p obed o be e y
use ul o anking he co osion esis ance o di e en me al-coa ing sys ems, and hough
di icul ies a e o en me du ing a emp s o co ela e accele a ed labo a o y es s wi h he
esul s o con en ional a mosphe ic exposu es, his echnique has shown o be a e y e ec i e
ool o he de ec ion o po es o de ec s in he coa ing [2,3] a signi ican ly ea lie imes han
non-elec ochemical es s. Indeed, ele an in o ma ion ega ding bo h he elec ochemical
beha iou o he me al/coa ing sys em and he ex en o he unp o ec ed me allic a eas can be
ex ac ed om he analysis o he impedance spec a [3].
Fo co osion p o ec ion o be op imized, he localized deg ada ion mechanisms
ope a ing in coa ed me als should be ully unde s ood. Only hen he mos e ec i e me al-
coa ing combina ion can be chosen o a pa icula applica ion and he exac causes o damage
be asce ained. Though con en ional elec ochemical echniques, including elec ochemical
impedance spec oscopy, p o ide aluable in o ma ion abou hese p ocesses, hey a e
in eg al me hods ha a e age he beha iou o he sample and hey lack spa ial esolu ion. As
a consequence, he cu en unde s anding o how coa ings p o ec and he mechanisms o
co osion b eakdown a e no ye comple ely known.
To cha ac e ize localized p ocesses occu ed in he deg ada ion o coa ed me als,
du ing he pas wo decades, se e al local elec ochemical measu emen echniques ha e been
in oduced o he in es iga ion o hese p ocesses because hey p o ide spa ially esol ed
in o ma ion ypically a mic oscopic and submic oscopic anges. Among hem, he use o he
Scanning Kel in P obe (SKP) has p oduced he bigges numbe o publica ions in he ield
un il now, since his echnique is sui ed o moni o bu ied in e aces, such as hose o igina ing
om de ec s and cu edges a coa ed me als [3-6]. Bu his echnique do no ac ually ope a e
in si u, ha is, in he elec oly ic en i onmen esponsible o he co osi e a ack, wi h he
ou come ha he eal elec ical s a e o he unde lying me al subs a e which de elops in he
3
agg essi e en i onmen canno be es ablished. Thus, he e is need o o he
mic oelec ochemical echniques o be employed, including scanning e e ence elec ode
echnique (SRET) [7-9], scanning ib a ing elec ode echnique (SVET) [6,8,9], local
elec ochemical impedance spec oscopy (LEIS) [10,11], and scanning elec ochemical
mic oscopy (SECM) [12-14]. Scanning elec ochemical mic oscopy consis s o measu ing a
a adaic cu en a he ul amic oelec ode ip, which esul s om he edox ans o ma ion o
an oxidizable/ educible species used as media o in he solu ion o o igina ing om he
co osion p ocess a he su ace [15]. The echnique hus supplies bo h opog aphic and
elec ochemical eac i i y in o ma ion whe eas he ip scans a subs a e, and has been
success ully employed o in es iga e in ac as well as de ec i e coa ings. In he i s case,
local swelling could be de ec ed o he i s ime a ea ly exposu es [16-20], whe eas in he
second, he elease o me al ions a he mic oanodes and he consump ion o oxygen a he
mic oca hodes could be moni o ed [21-23]. Localized elec ochemical impedance
spec oscopy also u ilizes a pola izable mic oelec ode, bu i can exclusi ely be employed o
cha ac e ize de ec i e coa ings by measu ing he local impedance a he exposed me al-
elec oly e in e ace [10,11], and i has no chemical selec i i y.
An al e na i e app oach is employed in he emaining wo mic oelec ochemical
echniques, which consis basically o scanning a e e ence mic oelec ode o e he imme sed
specimen, ei he in s a ic mode (scanning e e ence elec ode) o in a ib a ing mode
(scanning ib a ing elec ode) and hus allow o po en ial dis ibu ions in he elec oly e
a ound he de ec o be de ec ed. Among he wo, he highes sensi i i y is exhibi ed by SVET
because i de ec s local g adien s ins ead o absolu e po en ial dis ibu ions in he elec oly ic
phase [9]. Fu he mo e, SVET can moni o ionic luxes a ising om he de ec p o ided he
conduc i i y o he elec oly e is known [6], which is especially use ul o moni o he
homogeneous in e ac ion o he chemical species gene a ed a he locally sepa a ed anodic
and ca hodic a eas on he su ace in co osion eac ions. Cu en ly, he SECM has become a
e y powe ul echnique o p obing a a ie y p ocesses in me al-coa ing sys ems, including
delamina ion p ocesses om cu edges and sc a ches, as well as sel -healing p ocesses a
de ec s [24-35].
He ein we epo on he elec ochemical pe o mance o a comme cial pain sys em o
he co osion p o ec ion o mild s eel panels as de i ed om expe imen s employing he
scanning ib a ing elec ode echnique. The choice o his echnique has been de e mined by
he p e ious inding ha a highly po ous laye is o med when a epoxy-polyamine esin
con aining glass lakes is applied on he s eel as deduced om elec ochemical impedance
4
spec oscopy measu emen s [36], which may a ec he local dis ibu ion o he co osion
eac ions in and a ound a sc a ch p oduced h ough he polyme ilm un il he unde lying
me al, which should be moni o ed in he mic ome ic ange. Speci ically, we epo SVET
images o complex elec ochemical beha iou ha occu s on me allic subs a es p o ec ed by
o ganic coa ings in he icini y o a de ec when exposed o di e en elec oly e solu ions.
.
2. Expe imen al
2.1. Ma e ials and subs a e p epa a ion
A wo-componen epoxy-polyamine p ime con aining glass lakes as pigmen was
examined. Me al samples we e cu om mild s eel pla es ha ha e been g inded wi h silicon
ca bide pape down o 1200 g i o emo e any oxides and ilms om he su ace, washed
ho oughly wi h Millipo e deionised (Milli-Q) wa e , subsequen ly insed in ace one and
e hanol, and d ied p io o he coa ing p ocess. The samples we e pain ed in he labo a o y
wi h a wi hd awing echnique using an accu a e speed mo o and a sys em o gea s. Coa ed
specimens we e allowed o cu e a oom empe a u e and humidi y o 10 days p io o
es ing. The coa ing ma e ial used was comme cial, supplied by Sigma Coa ings (Ams e dam,
The Ne he lands), wi h a d y ilm hickness o 60 µm. The coa ing hickness was measu ed
wi h a Mega-Check FN Coa ing-Thickness-me e (Lis -Magne ik GmbH, Ge many). A
sc a ch o ca. 1 cm leng h and 1 mm wid h was p oduced in he coa ing o he me al subs a e
wi h a scalpel. A pho og aph o he esul ing su ace pe o med wi h an op ical mic oscope is
shown in Figu e 1. The di ec exposu e o he unde lying me al inside he sc a ch is obse ed
h ough i s cha ac e is ic me allic b illiance, a clea indica ion ha he polyme ic ilm has
been comple ely emo ed inside he a i icial de ec ope a ed h ough he o ganic coa ing.
2.2. SVET measu emen and da a analysis
The scanning ib a ing elec ode ins umen a ion used was manu ac u ed by
Applicable Elec onics Inc. (Fo es dale, MA, USA) and con olled by dedica ed so wa e. The
p obe mic oelec ode consis ed o P /I (80%/20%) wi es insula ed wi h pa alene C® and
a ced a he ip o expose he me al, and hey we e pla inized in o de o p oduce a sphe ical
pla inum black deposi o 10-20 μm diame e . A ideo came a connec ed o an op ical
mic oscope was in oduced in he sys em bo h o es ablish he p obe o sample dis ance, and
o ollow he mo emen o he ib a ing elec ode o e he sample du ing ope a ion. The
measu emen s we e made wi h he elec ode ip ib a ing in a plane pe pendicula o he
5
sample a an ampli ude o 40 μm. The mean dis ance be ween he mic oelec ode and he
sample su ace was 150 μm.
The coa ed samples we e embedded in a esin exposing he coa ed side upwa ds o he
es solu ion. The moun s wi h he samples we e hen su ounded la e ally by sello ape, hus
c ea ing a small con aine o he elec oly e solu ion, and he elec oly e co e ed he
specimen unde in es iga ion by ca. 4 mm. The elec ochemical cell o SVET was comple ed
wi h he inclusion o he co esponding mic oelec odes, namely he sphe ical pla inized
p obe and a P wi e as e e ence elec ode. Figu e 2 shows he dis ibu ion o he elemen s in
he cell. A e e ence measu emen wi h he mic oelec ode away om he ac i e a ea was
sub ac ed om he alues measu ed du ing he scan. Tes ing was ca ied ou in ei he 10 mM
Na2SO4 o 10 mM NaCl aqueous solu ions made om analy ical g ade eagen s and Milli-Q
wa e . The solu ion was na u ally ae a ed and expe imen s we e conduc ed a ambien
empe a u e. No pola iza ion was applied o he samples, which we e a hei co esponding
open ci cui po en ial in he es elec oly e.
The ionic cu en maps a e composed by 900 da a poin s, esul ing om a 30 x 30
poin s ma ix in X and Y di ec ions. The sample a ea imaged in each expe imen was 6 cm2
app oxima ely, whe eas he ime needed o comple e each map amoun ed o ca. 9 min. The
expe imen s we e ollowed du ing 80 minu es, hus allowing o 8 ionic cu en maps o be
de e mined consecu i ely. A calib a ion ou ine con e ed he measu ed po en ials in o
cu en densi y a he co oding su ace by using he expe imen al conduc i i ies measu ed in
each es elec oly e (namely, 877.96 Ω·cm in 10 mM NaCl, and 483.09 Ω·cm in 10 mM
Na2SO4 solu ions).
3. Resul s and discussion
The SVET is a echnique ha allows he de ec ion o ionic cu en s in a solu ion
caused by a concen a ion g adien . I is based in he de ec ion o elec ic ields gene a ed in a
solu ion due o homogeneous dis ibu ion o elec ic cha ges such as ions. The elec ic ield is
ze o when he solu ion is a es , bu i he e is a g adien o concen a ion caused by a sou ce
o ions, a a ia ion o po en ial in he solu ion occu s. Ionic lows can a ise om co osion
p ocesses on a me al as schema ically shown in Figu e 3. The oxida ion eac ions occu ing a
anodic si es on a me al su ace in con ac wi h an elec oly e cause elec ons o low h ough
he me al subs a e o adjacen ca hodic a eas. This low o elec ons h ough he me al is
suppo ed by a low o ionic cu en in he elec oly e, which in u n causes po en ial g adien s
o exis in he solu ion close o si es o localised co osion.

6
3.1. Imaging a sc a ch in epoxy-polyamine coa ed ca bon s eel du ing imme sion in 10 mM
Na2SO4.
Figu e 4 shows SVET maps o ionic luxes associa ed wi h he onse o deg ada ion
eac ions om he sc a ch ope a ed in he polyme -coa ed ca bon s eel o di e en exposu e
imes in 10 mM Na2SO4. The solu ions we e open o ai , hus na u ally ae a ed, and he
samples we e le unbiased o a ain hei co esponding open ci cui po en ials in each
elec oly e. The same a ea has been scanned o each sample du ing he expe imen .
Anodic ac i i y s a s inside he sc a ch immedia ely a e imme sion, and he
magni ude o he associa ed ca ionic lux is obse ed o inc ease wi h he elapse o ime.
Indeed, ca ions a e obse ed o di use om all o e he sc a ch. Con e sely, SVET
measu emen s show ha he ca hodic p ocess mainly occu s below he coa ing, since he
anionic luxes a e obse ed om he su ounding coa ed a eas. The di using chemical species
in ol ed in he anodic hal -cell eac ion a e Fe(II) ions which di use om inside he sc a ch
in o he bulk elec oly e. Dissol ed molecula oxygen mus di use om he elec oly e
owa ds he su ace o he coa ed me al, and subsequen ly pe mea e h ough he polyme
ma ix owa ds he me al-coa ing in e ace, o be combined wi h he elec ons eleased by he
me al in he anodes wi h he p oduc ion o hyd oxyl ions as p oduc s in he ca hodic hal -cell
eac ion. The sligh change in colou ing ha may be obse ed in he sequence o mic og aphs
aken in si u wi h he ideo mic oscope o he in es iga ed sample wi h he elapse o ime
migh be an indica ion ha some p ecipi a ion o co osion p oduc s (b ownish i on oxy-
hyd oxides) inside he a i icial de ec may al eady occu a he longes exposu es unde
conside a ion (c . Figu e 5).
In summa y, when he de ec i e sample was imaged wi h he SVET du ing i s
imme sion in 0.1 M Na2SO4 ai -sa u a ed aqueous solu ion, all he in o ma ion om he maps
is exclusi ely om ionic cu en s due o he di usion o cha ged soluble species. Collec ion
e iciency is hus hinde ed when p oduc p ecipi a ion occu s [37], hough his e ec is
ega ded o be a he small in his case as suppo ed by he op ical images shown in Figu e 5.
On he o he hand, SVET measu emen s suppo ha he anodic p ocess is dis ibu ed o e he
exposed me al inside he sc a ch, whe eas he co esponding ca hodic eac ions a e obse ed
o occu below he coa ing in zones su ounding he de ec .
3.2. Imaging a sc a ch in epoxy-polyamine coa ed ca bon s eel du ing imme sion in 10 mM
NaCl.
7
SVET images we e also eco ded du ing he exposu e o a coa ed sample wi h a
sc a ch o 10 mM NaCl solu ion. Selec ed maps a e shown in Figu e 6, and hey allow he
collec ion o su ace in o ma ion in he cou se o he co osion p ocess inside he sc a ch.
Signi ican changes in he dis ibu ion o he ca ionic and anionic luxes in he scanned a ea
a e obse ed wi h ime. Wi hin he i s 10 min o exposu e, a na ow anodic peak exhibi ing
a he high ionic cu en s due o he di usion o he dissol ing Fe2+ ions om he sample is
clea ly displayed. This is an indica ion ha he anodic eac ion is g ea ly localized compa ed
o he o al me al exposed inside he sc a ch, in an a ea ound a he uppe pa o he sc a ch
as depic ed in he sequence o ideo mic og aphs gi en in Figu e 7. The dissolu ion o i on
may ha e s a ed h ough he nuclea ion o a co osion pi . On he o he hand, he ionic luxes
ela ed o he ca hodic eac ion a e ound o be dis ibu ed o e a signi ican ly g ea e su ace,
mainly abo e he coa ed su ace a he igh side o he sc a ch. The p ecipi a ion o co osion
p oduc s a his ea ly exposu e is also no iceable om he co esponding op ical mic og aph
in igu e 7, and i is obse ed o become mo e gene alized o e he sc a ch as ime elapses.
SVET images aken a longe exposu es show ha he anodic ac i i y u he ex ends
bo h o e he me al exposed in he sc a ch and below he coa ing a he le o he de ec (c .
Figu es 6 B-D). This is an e idence o he onse o an anodic delamina ion p ocess below he
coa ing has occu ed, and his ini ially akes place om he coa ed edge in he p oximi y o
he si e a which he co osion o i on was ini ia ed. Addi ionally, wi h he inc ease o he
o al a ea om which posi i e cu en s (i.e., anodic ac i i y) a e de ec ed, he ca hodic egions
a e obse ed o occu below he coa ing a bigge dis ances om he sc a ch.
3.3. Gene al obse a ions.
Rega dless he na u e o he es elec oly e employed, he ca hodic p ocess has been
ound o ake place unde he coa ing since he onse o co osion, whe eas he anodic ac i i y
was mainly obse ed inside he sc a ch, hough i s occu ence below he coa ing was also
obse ed o longe exposu es when he sample was exposed o 10 mM NaCl aqueous
solu ion. Fo ionic cu en s o be de ec ed o e he coa ed a eas, mic opo es mus be p esen
h ough he coa ing which may ei he exis om he manu ac u ing p ocess, o be de eloped
he co osion p ocess p og esses. On he basis o a p e ious in es iga ion on he co osion
p o ec ion cha ac e is ics o his me al-coa ing sys em conduc ed by elec ochemical
impedance spec oscopy [36], EIS spec a could be modelled conside ing he coa ing ac ed as
a de ec i e ba ie ilm, which is consis en wi h a a he po ous s uc u e esul ing om i s
manu ac u ing s age. A possible speci ic e ec owa ds ionic pe mea ion h ough he coa ing
8
due o chlo ide may be emp a i ely p oposed om he epo ed da a as compa ed o he
measu emen s in he sulpha e-con aining solu ion, and his will co ela e well wi h
obse a ions conduc ed wi h scanning elec ochemical mic oscopy o his sys em [38]. Ye ,
such p oposed speci ic e ec o chlo ide ions o p omo e coa ing delamina ion canno be
es ablished unambiguously a his s age om he only conside a ion o ou cu en da a.
Ano he impo an e ec in he sys em is ela ed o he onse o p oduc p ecipi a ion
in he in es iga ed sys em. I on oxy-hyd oxide species a e o med when Fe(II) and hyd oxyl
ions libe a ed in he dis ibu ed anodic and ca hodic egions a e anspo ed in o he same
elec oly e olume. The p ecipi a ion o hese p oduc s may pa ially block he eac ing me al
su ace, and may jus i y he p og ess o delamina ion below he coa ing wi h he elapse o
ime.
4. Conclusions
SVET imaging o de ec i e polyme coa ed s eel samples exposed o an aqueous
elec oly ic solu ion p o es ha he elec ochemical beha iou o he sys em is qui e di e en
depending on he elec oly e employed.
The anodic ac i i y, ela ed o he elec odissolu ion o he unde lying me al, occu s
almos exclusi ely wi hin he sc a ch in he sulpha e-con aining solu ion. Con e sely, he
ca hodic ac i i y is obse ed always below he coa ing a ound he de ec , as ollowed by he
di usion o hyd oxyl ions owa ds he elec oly ic phase. The p ecipi a ion o i on oxy-
hyd oxide may e en ually ake place om he elec oly e wi h he elapse o ime, due o he
local alkaliniza ion o he aqueous phase in he p oximi y o he co oding si es. Fu he mo e,
in 10 mM Na2SO4 solu ion, he anionic luxes measu ed a e smalle han hose due o he
ca ions, indica ing ha he anodic ac i i y is mo e localized and he echnique exhibi s highe
collec ion e iciency owa ds he me al ions in his case. This is a u he con i ma ion ha he
ca hodic si es a e dis ibu ed unde he coa ing. No signi ican changes in he magni ude o
he ionic luxes moni o ed by SVET wi h ime is obse ed, which suppo s he obse a ion
ha he p ecipi a ion o co osion p oduc s only occu s in a small ex en in his en i onmen .
G ea e anionic and ca ionic luxes a e measu ed by SVET o he same sys em when
imme sed in 10 mM NaCl aqueous solu ion, which is e idence ha he co osion p ocess
occu s as e in his en i onmen . This e ec may be emp a i ely a ibu ed o a speci ic
e ec o chlo ide ions owa ds acili a ing an inc ease anspo o ionic species h ough he
coa ing owa ds he me al-polyme in e ace. Delamina ion e ec s a e also obse ed o occu
in his case, when e en anodic ac i i y occu s below he coa ed a eas, hough s ill he main
9
con ibu ions owa ds me al elec odissolu ion a e obse ed om he sc a ch. Inc eased
p ecipi a ion o co osion p oduc s is also obse ed.
To ou knowledge, he de ec ion o ionic luxes ela ed o he ca hodic eac ion om
below he coa ed me al has been obse ed o he i s ime using SVET, which can be
asc ibed o he pa icula mic opo ous cha ac e is ics o his coa ing al eady desc ibed on he
basis o elec ochemical impedance spec oscopy (EIS) da a [36].
Acknowledgemen s:
This wo k was suppo ed by he Minis e io de Ciencia y Tecnología (Mad id, Spain) unde
P ojec No. CTQ2009-14322. A g an awa ded o JJS by he Gobie no de Cana ias (Spain) o
conduc a esea ch s ay a he Uni e si y o La Laguna is g a e ully acknowledged. Thanks
a e due o Sigma Coa ings (Ams e dam, The Ne he lands) o p o iding he coa ings.
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