GRAIN BOUNDARIES CORROSION OF Ni-Mo
ALLOYS BY ELECTROCHEMICAL ETCHING
L. Beaunier, F. Salihi
To cite this version:
L. Beaunier, F. Salihi. GRAIN BOUNDARIES CORROSION OF Ni-Mo ALLOYS BY ELECTROCHEMICAL ETCHING. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-429-C1434. <10.1051/jphyscol:1990166>. <jpa-00230333>
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COLLOQUE DE PHYSIQUE
Colloque Cl, supplement au n0l, Tome 51, janvier 1990
GRAIN BOUNDARIES CORROSION OF Ni-MO ALLOYS BY ELECTROCHEMICAL ETCHING
L. BEAUNIER and F. SALIHI
Laboratoire de Physique des Liquides et Electrochimie, LP15 du CNRS,
Universite Paris VI, Tour 22, 4 Place Jussieu, F-75252 Paris Cedex 05,
France
I&m.mk - Le nickel et le molybdene sont des elements d'alliages qui jouent un rdle
important dans les aciers speciaux : resistance d l'usure, corrosion par piqores,
corrosion acide. Nous Btudions ici l'influence du molybdene sur la corrosion
intergranulaire d'alliages Ni-MO (0 d 18% pds MO) ainsi que l'action de
l'heptamolybdate d'ammonium et de l'acide molybdique ajoutes d la solution
H2SOq-2N. La corrosion intergranulaire est contr6lee rigoureusement par une
attaque electrochimique transpassive mise au point au laboratoire. Le molybdene
augmente les courants de passivite mais diminue ceux de la transpassivitb. Le
molybdene de l'alliage reduit la profondeur de lvattaque intergranulaire de 80%,
un resultat identique est observe si le molybdene provient de la solution et ceci
mgme sur le nickel pur. L'effet bbnefique du molybdene sur la structure de la
couche transpassive pendant la corrosion intergranulaire est un resultat majeur:
en R.B.S., nous avons mis en evidence que la couche d'oxyde est plus epaisse et
est enrichie en oxygene, en TEM-STEM et analyse par rayons X (EDS) nous observons
l'enrichissement de la couche en molybdene.
Abstract - Nickel and molybdenum are two alloying elements which influence some
properties of steels : wear resistance, pitting corrosion, acid corrosion. We
study the influence of molybdenum on the localized intergranular corrosion of NiMO alloys (0 to'18 wt% MO) and the effect of ammonium heptamolybdat and molybdic
acid dissolved in the aqueous H2S04-2N solution. The intergranular corrosion is
rigorously controlled by transpassive electrochemical attack performed in our
laboratory. Molybdenum increases passivity currents but decreases transpassivity
currents. Molybdenum in the alloys reduces the intergranular corrosion up to 80%.
The same effect is obtained with molybdenum compound dissolved in the solution
even on pure nickel. The beneficial effect of molybdenum on the transpassive layer
during the intergranular corrosion is a major result: R.B.S. analysis points out
the enrichmentof oxygen and X ray (EDS) analysis the enrichment of molybdenum in
the oxide layer.
Many studies have been made relating to the molybdenum effect on the pitting and general
corrosion of stainless steels and Ni-Cr-Fe-MO alloys. But they provide no satisfactory
mechanism for the modification of the corrosion process. In this study we work with a
more simple class of metals which are Ni-MO alloys. The number of publications on these
alloys is scarce. In the last years, different chemical solutions and,technics have been
used to study Ni-MO: sulphuric acid (AES, XPS) /1,2/, hydrochloric acid / 3 / , sulphate
solution (AES, RHEED, XPS) /4/ and TEM-STEM analysis / S / . No good correlation between the
presence of molybdenum in the anodic film and the modification of dissolution rate is
established.
The topic of this work is the effect of molybdenum on the intergranular corfosion of Ni0 , 2 , 10 and 18 wt% MO and especially the processes which result from the perturbation of
the passive-transpassive layer on the intergranular corrosion with molybdenum arising
firstly from the alloy and secondly from the solution.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990166
Cl-430
COLLOQUE DE PHYSIQUE
Ni-MO alloys are obtained by zone melting with refined Inco nickel (200 wt.ppm Fe, 50
wt.ppm C, 0 and S less than 15 wt.ppm) and pure molybdenum. We have tested only the solid
solution of Ni-MO alloys (limit at 20 wt% MO). Specimens Ni-0, 2, 10 and 1 8 wt% MO are
cold rolled up to 4 millimeters and heat treated at various temperature in quartz sealed
tubes with 1 atm
Argon-5% H>.
The electrochemical etching of grain boundaries is performed with a potentiostatic method
which consists of keeping the electrochemically polished surface at a given transpassive
potential 1500 mV/S.C.E. (saturated calomel electrode) equivalent to 1100 mV/S.S.E.
(saturated sulphate electrode) and of controlling the' same quantity of electricity
(20c/cm2) with an integrator for all specimens /6,7/.
The basic solution is an aqueous sulphuric acid solution (H2SOq-2N) used to prepare
ammonium heptamolybdat 0,OlM and saturated molybdic acid solutions. All solutions are
kept at 25°C.
Specimens are observed with a scanning electron microsoope S250 Cambridge Instrument.
They are inclined at 85' in order to get their profile. The intergranular corrosion is
developed at the emergence of grain boundaries as triangular grooves - Fig.1.
Fig.1 - a. Observation of a polycrystalline specimen after etching ; b. Groove attack at
grain boundary.
The width W, the depth D and the angle a of twenty five grooves are measured on each
polycrystalline specimen in order to have average values of the importance of the
corrosion which depends on the orientation of grains - Fig.2. The attack of grain
boundaries corresponds to a model which interprets the features of the grooves by an
equilibrium competition between the general corrosion current Js (on the general surface
and sides of grooves) and the grain boundary current of dissolution parallel to the plane
of the grain boundary Jg.b /6,7/- Fig.2.
The oxide layer on the surface, which has been created by the anodic attack, is observed
on thin foils of alloys, prepared by electrochemical polishing and etched in the three
solution studied. The transmission electron microscope (TEM-STEM JEOL 100CX-11) allows
the characterization of the layer structure by image, measurement, diffraction and
analysis. The X ray-EDS analysis (LINK) gives the concentration of Ni and MO elements in
the passive-transpassive layer and the amount of oxygen is quantified by Rutherford Back
Scattering analysis (R.B.S. ) .
groove
!
f i g r a i n boundary
grain boundary
Fig.2
3
-
-
Mechanism and morphology of the intergranular corrosion (see text).
RESULTS
Electrochemical transpassivation
Figure 3 shows the logarithmic current density potential curves obtained on Ni, Ni-10% MO
and Ni-18% MO in the three solutions studied. We note that the current density of pure
nickel is very low in the passive range of about 600mV/E.S.C. in H 2 S 0 4 - 2 N . The
introduction of 18 wt% molybdenum in the alloy increases this current by the factor two
hundred and the entire surface is covered by smal pits of about 20nm observed in TEM. If
we consider now HzSOq-2N with 0,OlM ammonium heptamolybdat, all the current densities of
the alloys are reduced in passivity, especially in the case of pure nickel, whereas in
the molybdic acid an increase of all current densities is observed.
m ,4jem2
- I t , SO, - 2 N
-- I t 2 SO* - 2 % - H E P I A
- iN.Mo
+ + H> so1
o1
Fig.3 - Current density potential curves of alloys in the passive range for different
solutions at 25OC.
COLLOQUE DE PHYSIQUE
Fig.4 - Current density potential curves in transpassivity. Effect of bulk MO amount and
solutions.
Figure 4 presents the current density potential curves in the transpassive range where
the intergranular attack of grain boundaries is developed. The corrosion rate for pure
nickel is highter than for the nickel alloys. With an increasing -amount of molybdenum
t h e current d e n s i t y d e c r e a s e s e x c e p t f o r Ni-18% M O . That i s t h e o p p o s i t e of t h e e f f e c t in
passivity. If heptamolybdat is added to the solution, the transpassive current densities
decrease. If molybdic acid is introduced then decrease even more. The process is very
dieferent than that in passivity.
Intergranular corrosion
The corrosion of grain boundaries is obtained in the transpassive range of potential at
1500 mV/S.C.E.. All the specimens are electrochemically attacked in order to have the
same quantity of metal alloy dissolved so we can directly compare their depth of
penetration at the emergence of grain boundaries. Specimens are incldned in the SEM in
order to measure the corrosion depth penetration (D pm) and width (W pm) of grooves at
the emergence of grain boundaries. In Figure 5a we note that with Ni-2% MO the
penetration of corrosion diminishes to 50% and with Ni-10% MO it diminishes to 70%.
1100mV/E.S.S
-- H z S 0 4
-MOLY BDAT .
Fig.5 - a. Evolution of width (W pm) and depth (D p) of intergranular corrosion versus
MO concentration in H2SOq and molybdat solution ; b. Evolution of the angle of the
grooves versus MO content.
The corrosion level of Ni-18% MO specimens is as high as in the case of pure nickel but
it has been observed that the molybdenum was not homogneously distributed in the bulk of
18% MO even after annealing for 380 hours at 1020°C.
In the molybdat solution, the corrosion of pure nickel is decreased by 80% and with
molybdenum content, the corrosion increases very slowly.
For the evolution of angle a of grooves - Fig.5 b, the value rises from pure nickel to
10% MO which confirms that the intergranular corrosion is reduced with increasing
molybdenum content. This is in agreement with our model : the larger the angle a, the
shallower the depth penetration /6,7/.
Observations and analysis
Transmission electron microscopy has showed that thin foils of pure nickel attacked by
H2SOq-2N are smooth but when molybdenum is incorporated in the alloys we observe very low
pits (2 to 10nm). In the same way, the solutions which contain a molybdenum compound give
rise to uniform pitting. In all cases, the oxide layer is epitaxied on the bulk. On the
same thin foils we can find some areas where the transpassive oxide film is alone for
example in etched grain boundaries grooves. The TEM-STEM apparatus with X ray-EDS
an
accessory allows analysis of elements and light elements. In Figure 6 we note
enrichment of oxygen and molybdenum in the oxide.
Fig.6 - X-ray analysis of the matrix and of the transpassive oxide layer.
Sulfuric a.
Molybdic a.
matrix
17,9%
18,9%
18,7%
OXYGEN ATOM
(105/cm2)
Ni
oxide
37,9%
24,9%
33,2%
Ni-18Mo
Molybdat
Table 1 : X-ray analysis on Ni-18% MO.
Sulfuric a.
Molybdat
16
30
24
196
Table 2 : R.B.S. analysis.
Table 1 gives the molybdenum analysis of the oxide layor with the different solutions
used. We note in all cases the enrichment of molybdenum. This analysis is completed by
the RBS analysis (Table 2) which points out the real increase of the amount of oxygen .
It proves that the oxide layer is thicker with molybdenum added in the alloy or in the
solution and more so if the two conditions are presents.
COLLOQUE DE PHYSIQUE
CONCLUSION
This work reports on a preliminary study about the modification of
corrosion process at Ni-MO alloys caused by molybdenum.
the intergranular
TEM diffraction has given the structure of the oxide layer as NiO but we have no evidence
for the presence of another compound as MoOg or NiM0O4 because these compounds are
hydrated and amorphous. Meanwhile, our results in TEM-STEM X-ray analysis prove that
passive and transpassive anodic dissolution introduce an important enrichment of
molybdenum in the oxide layer, at the surface of the metal, which comes from the alloy
itself or from the solution. From an other part RBS analysis indicate an increase of the
oxygen concentration and consequently indicates also an increase of the thickness of the
oxide. This improves the protection of the anodic surface and decreases the transpassive
current density. In passive range potential the increase of current is explained by the
pitting of the surface.
Correlatively, the intergranular corrosion which takes place in the transpassivity is
reduced to a very low level by the molybdenum effect. We have seen that molybdenum from
the alloy or from the solution decrease the rate of corrosion. The decrease of
intergranular corrosion with molybdenum amount seems to indicate a structural effect in
the atomic structure of the grain boundaries and also a modification of the process of
dissolution at grain boundaries due to composition and structure of the oxide layer.
BIBLIOGRAPHY
/l/ Tachibana, K. and Ives, M.B., Passivity of Metals, R. Frankenthal, J. Kruger (1978)
878.
/2/ Olefjord, I. and Marcus, P., Proceedings of EUROCORR 87, European Congress DEKEMA,
Karlsruhe (1987) 175.
/3/ Uhlig, H.H., Bond, P. and Feller, H., J. of Electrochem. SOC. l..l.Q,
no 6 (1963) 650.
/4/ Ives, M.B., Mitrovic-Scepanovic, V. and Moriya, M., Passivity of Metals and
Semiconductors, M. Froment Ed., Elsevier Science Publisher Amsterdam (1983) 175.
/5/ Moriya, M. and Ives, M.B., Corrosion Nace 411, no 2 (1984) 62.
/6/ Beaunier, L., Froment, M. and Vignaud, C., Electrochimica Acta 2.5 (1980) 1239.
/7/ Beaunier, L., Froment, M. and Vignaud, C., J. Electroanal. Chem. l.I.9 (1981) 125.