- 226Solidilication ofMetals and Alloys, No.28, 1996
Krzepmęcie Me rali i Sropów. Nr 28. 1996
PAN- OddziDI Katowice: PL. ISSN 0208-9386
MECHANISTIC MODEL OF CORROSION PITTING
ON 2024 T3 ALLOY
OLIVE Jean Marc
Laboratoire de Mecanique Physique, Universite Bordeaux I, 351 Cours de la Liberation,
33405 Talence cedex, France.
ABSTRACT
Under free corrosion potential in a 0.5 M NaCI solution, 2024 T3 alloy is sensitive to
localized corrosion in the form of pitting. This pitting has been shown to develop at
constituent particles which are either anodic or cathodic relative to the matrix. The driving
force considered in the modelling is then the galvanic coupling between constituent particles
and the matrix. The modelling of the growth kinetic of severe pits that are potential sites for
corrosion fatigue crack initiation is presented. It is explained how the kinetic of a severe pit
that involve several particles belonging to a eluster can be reached from the measurement and
modelling of a pi t associated with a single particie.
l. INTRODUCTION
The role of rnicrostructural heterogencity at the metal surface in the localized corrosion
was reported for various alloys [1,3] . The mechanism involved in the developpement of
localized corrosion on 2024 T3 alloy is the galvanic coupling between constituent particles
aand the matrix [4]. Two types of particles were identified ; one containing Al, Cu and Mg
acting asanode versus the matrix and the other, Al, Cu, Mn and Fe acting as cathode relative
to the matrix. 25 o/o of the overall particles (300,000 cm-2) are of type cathodic and banding
and clustering along the rolling direction is evident.
In situ monitoring of pitting process on 2024 T3 in 0.5 M NaCI was performed using
long focal video rnicroscopy [5]. Two modes of pitting corrosion were identified : namely, (i)
pitting associated with single particles, and (ii) severe pitting resulting from growth in depth
of corrosion cavities at selected sites. Because severe pits are potential sites for corrosion
fatigue crack initiation, emphasis is placed herein on the mechanistic understanding and
modelling of this process. Probabilistic modelling for the growth of corrosion pi ts taking into
account the role of clustered particles was proposed [6] . Three different models with
increasing complexity relative to geometrical assumptions on the shape of corrosion cavities
were detailed. A Jack in mechanistic understanding was apparent. A mechanistic model fully
based on galvanic corrosion is presented here.
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2. DEVELOPMENT OF THE MODEL
The corrosion process considered in this model is the galvanic coupling between
constituent particles and the matrix. The first step consist on the measure and the modelling
of the growth of pitting associated with single particie. This is the base of the severe pit
modelling since a galvanic current as a function of the ratio anodic surface area - cathodic
surface area is obtained. The second step is the assessment of the anodic and cathodic surface
areas during the propagation of the severe pi t through the particie ciuster. The growth kinetic
is then evaluated from the knowledge of the galvanic current inferred from the first step
result.
2.1. Pitting associated with single particles.
This is a typical problem of galvanic corrosion where two media of different
electrochemical features are in contact. Several investigations on accelerated corrosion of
dissimilar metais which are electrically coupled and exposed to a corrosive environment were
reported [7-12]. On a
ge nerał
theory of galvanic corrosion [10, 11] , the dependence of the
anodic and cathodic galvanic current densities and the galvanic potential were derived .
However, the knowledge of electrochemical features of each uncoupled metal is necessary
and only coplanar electrodes were considered. For galvanic corrosion between constituent
particles and the matrix , the pit morphology Jead to geometrical changes of anodic and
cathodic surfaces with the time. Furthermo re, electrochemical behavior o f uncoupled particles
and matrix are not known. The evaluation of galvanic current, cathodic and anodic surface
areas must bethen approach experimentally making some assumptions on the pit geometry.
This is illustrated in a fl ow diagram presented in figure l .
Particie morphology and its electrochemical behavior were obtained after a corrosion
test and removal of corrosion products. The scanning electron microscopy picture presented
in figure 2 shows typical pitting produced by galvanic coupling between cathodic particie and
matrix . From this observation, a model of pitting associated with single particie is proposed
making the following assumptions :
- Cathodic particie is a rectangular prism with the dimension along the rolling direction
being higher than those in the transverse and short directions.
- Pit initiation occurs at the interface particle-matrix
- Anodic dissolution of the matrix is isotropic.
- the cathodic surface where reduction reaction occurs is the particie surface in contact
with the electrolyte. The anodic surface is the surface of the pi t.
- 228- .
Particie morphology and
electrochernical behavior
~
~
l
FARADAYLAW
Y
(~: =f(tJ~~g=f( ~~J~8
Fig. l. Quantitative assessment of the galvanic current between constituent
particie and matrix. V : pit volume, r : pit radius, Sc : cathodic surface area, Sa :
anodic surface area, Ig : galvanic current.
-
lO )lm
Fi g. 2. Corrosion pitting resulting from galvanic coupling between cathodic
particie and the matrix. 2024 T3 alloy in 0.5 M NaCI at free corrosion potential.
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The pit model is presented in figure 3. The dimensions of the particie along the rolling,
transverse and short directions are respectively a, b and c. The pit size is given by r. For r ::; c,
the cathodic and anodic surface areas as well as the pit volume are easily obtained :
For r ::; c, Sc = a b+ 2 r ( a+ b )
S3 =nr ( a+ b+ 2 r )
vpit
=1tr
( l )
z(a+b 2r)
-2- + 3
Pit
particie
._1'-------------- - - - - matrix
a
Fig. 3. Pit morphology associated to a cathodic constituent particie.
The pit kinetic r =f (t) need to be measured experimentally. In situ monitoring of the
pitting proces s was perfo rmed during corrosion test by using lon g focal video ruieroscapy . A
particie with a rectangular shape was chosen before immersion of the specimen and r was
measured. Pictures presented in figure 4 show the shape of the cathodic constituent particie
and the pit growth at 41 min and 16 h after immersion. The pit size r was measured
continuously from immersion to 17 h and could be expressed according to time by :
r = 6.10 - 8 t 0.45
R = 0.93
(2)
From ( l ) and ( 2 ), Sc ( t), Sa (t) and Vpi t (t) are deduced, the particie dimensions a
and b bei ng respectively 17.10- 6 m and 6.10- 6 m:
Sc = 3.3 .10-11 t 0.23 R = 0.97
Sa= 2.7.10- 12 t 0.53 R = 0.94
Ypit=9.10-20 t0.95 R=0.94
( 3)
-230-
Before immersion
10 llffi
10 llffi
10 llffi
Fig. 4. In situ monitoring of corrosion pitting resulting from galvanic coupling
between cathodic particie and the matrix. 2024 T3 alloy in 0.5 M NaCI at free
corrosion potential.
From V( t ), the galvanic current associated to the anodic dissolution of the matrix
surrounding the cathodic particie is obtained by the Faraday law
I = n p F (Jy
g
M
( 4)
at
where M is the molecular weight of the alloy ( M = 27 g/mol ), n the valence of
oxidized species ( n= 3 ), F the Faraday constant (F= 96,514 C/mol) and p the density
( p = 2.7 !O 6 gfm3 ).
Consequently, from ( 3) and ( 4) the expression for Ig can be approximated versus the
ratio cathodic surface area- anodic surface area as :
Ig=l.7 . 10
-9
+5 .10
-!O
Sa
log-
sc
R=0.94
(5)
This quantitative assessment of the galvanic current as a function of surface areas is
used in the severe pitting model described in the following section.
- 231 -
2. 2. Severe pitting model.
Severe pitting is clearly associated with clustering of type cathodic particie. lt can be
noted that the severe pitting surface density is very low according to the one of pitting on
single particles. After 20 h of a corrosion test, the severe pit density is about 1.5 pit/mm·2.
The approach aim to modelize the growth of this severe pi t in a particie cluster, the postulated
driving force being the galvanic coupling between particles and matrix. Only the
generał
features of the model are presented in this section, numerical computations are currently
performed and the results will be presented later.
Morphological observations of severe pits after corrosion test showed that their shape
could be approach by a rectangular prism. The scheme in figure 5 defines this shape. The
dimension A along the rolling direction is in most of the case higher than B and C, and the
side 6 is the mouth o f the pi t.
ej ~
~
Particie j of the eluster
Fig. 5. Severe pit and particie shape considered in the model.
Severe pi t propagates in a particie cluster. Therefore, at eacb time t of the propagation ,
the cavity sides intersect cathodic particles of the cluster. It is then possible to compute the
cathodic and anodic surface areas on each side of the pi t at time t. From these informations
and equation ( 5 ), the propagation rate of each side is given by :
The different steps of the numerical simulation are :
- Generation of the cathodic particie eluster coherent with statistical analysis of particie
distribution .
- Definition o f the initial cavity for severe pi t initiation.
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- Computation of the cathodic and anodic surface areas on each side of the pi t.
- Computation o f propagation rate o f each side.
3. CONCLUSION
A mechanistic model for severe pitting on 2024 T3 alloy is proposed. The mechanism
of pit growth considered is the galvanic coupling between cathodic constituent particles and
the matrix. From in situ monitoring of pitting process by video microscopy and some
assumptions it has been possible to give the galvanic current as a function of the cathodic and
anodic surface areas. This galvanic current can be used in a model to compute the growth
rate of a severe pit. This is of a great interest in the prediction of corrosion fatigue crack
initiation time.
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