Studies on Localized Corrosion and Desalination Treatment of Simulated Cast Iron Artifacts
Ouyang Weizhen and Xu Chunchun
A simulated occluded cell was utilized to study the localized corrosion in seawatcr and desalination
treatment in alkaline solution of simulated cast iron artifacts. After the simulated occluded cell
galvanostatic test, high acidity and enrichment in chloride ions in the occluded cell were
demonstrated as a result of an autocatalytic process. In addition, scanning electron microscopy
(SEM) and energy dispersive X-ray analysis (EDX) indicated the degree of the localized corrosion
and the enrichment in chloride ions. An investigation was also made of the chemical changes within
the occluded cell during the desalination treatment; the results showed that the pH values and the
amount of chloride ion removed from the occluded cell increased with the treatment time. The
decrease in chloride of the corroded surface of the specimen was confirmed by EDX. The research
appeared to prove that the simulated occluded cell is capable of providing useful information on the
localized corrosion and desalination treatment of simulated cast iron artifacts.
INTRODUCTION
Nowadays the storage of cast iron artifacts recovered from marine sites has received extensive
attention. One of the most serious problems facing conservators and conservation scientists is how
to stabilize iron artifacts against further corrosion under favourable environmental conditions. The
ingress of chloride ions is of particular concern because they are aggressive and can promote
corrosion [1, 2J. Since they have deleterious effects on corrosion performance, chloride ions must
be removed [3]. Therefore, it is worthwhile investigating the mechanism of localized corrosion and
desalination treatment of cast iron artifacts.
The localized corrosion of cast iron artifacts is a complex electrochemical process, with the anodic
and cath-odic reactions occurring at spatially separate points [4]. During corrosion, the ironcontaining phases are corroded but the graphite remains as an interlinked three-dimensional
network which retains the original shape of the artifact [1]. It has been agreed that the solution
within the graphitized region of corroded marine cast iron is essentially a ferrous chloride solution,
with a concentration of 1 M or greater and pH 4.8 [5]. Local acidification and chloride ion
enrichment could be explained by the model developed by Turgoose [6]. which was in some ways
similar to crevice or pitting corrosion on iron exposed to a solution containing chloride ions. Within
these occluded cavities, the presence of chloride ions indeed contributes to a corrosion cycle (7).
The autocatalytic process is a feature common to the localized corrosion of cast iron artifacts in
seawater. Detailed studies of the autocatalytic process will certainly help in understanding the
localized corrosion process. To date, there have been no direct techniques to measure either the pH
values or the chloride concentration in the pore solution.
On the other hand, the removal of chloride ions is absolutely essential for the conservation of iron
artifacts. The principal aim of any treatment of archaeological iron of marine origin is to eliminate
chloride from the corrosion products. It this is done successfully, the development of the corrosion
process due to chloride ceases and the rate of corrosion is reduced to a more manageable level,
where conventional methods to counter corrosion can operate efficiently. When artifacts are soaked
in a treatment solution, the hydroxide ions diffuse into the corrosion product reacting with chloridecontaining compounds to produce free chlonde ions, and then chloride ions diffuse out from the
pores of high chloride ion concentration into the treatment solution [8J. The chloride ion diffusion
tends to be relevant to the processes occurring in solution-filled pores. At present, there is no
consensus as to the optimum treatment and one reason for this appears to be that there is no reliable
method of assessing the ongoing process during treatment. Consequently, a thorough knowledge of
the chloride ion migration in occluded pores during the treatment is essential if desalination
methods are to be carried out successfully.
It would be of great practical interest to have a physical model which enabled the study of the
microscopic mechanism of the localized corrosion of cast iron artifacts and chlonde ion migration
during corrosion and desalination. The method using a simulated occluded cell is a convenient way
to reveal the chemical changes within an occluded cell, and has been confirmed by many
researchers [9], including Mars G. Fontana and Marcel Pourbaix. Experimental data, combined with
the physical description of the localized corrosion process, should prove useful for developing more
versatile, phenomenological corrosion prediction and conservation for cast iron artifacts.
In this paper, it is intended to investigate the changes of chemical states within the occluded cells of
simulated cast iron artifacts in 3.5 wt% NaCl solution and 2.5 wt% NaOH solution by use of a
simulated occluded cell. In addition, scanning electron microscopy (SEM) and energy dispersive Xray analysis (EDX) were adopted to analyse corrosion morphology and components.
EXPERIMENTAL DESIGN
The specimens were made of grey cast iron to simulate cast iron artifacts. A simulated occluded cell
apparatus was used in the study of localized corrosion and desalination treatment. (A full
description of the simulated occluded cell is provided in the Appendix.)
A simulated occluded cell galvanostatic test was carried out in 3.5 wt% NaCl solution, which was
the medium to simulate seawater corrosion. The glass container was filled with the solution and its
pH was adjusted to 7, though seawater is normally in the range 8.2-7.8. At room temperature, the
solution was injected into the occluded cell. An anodic current density of
1 mA-cm 2 was applied to the occluded specimen to simulate the couple current in and out of the
occluded cell. The chloride ion concentration and pH in the simulated occluded cell were monitored
at various time intervals. During desalination treatment the simulated occluded cell, after 48 hours
of galvanostatic tests, was immersed in a glass container containing 2.5 wt% sodium hydroxide.
The pH values and the chloride contents in the occluded cell at different time intervals were
monitored in order to determine the progress of chloride ion release. Details of the experimental
procedures are described in the Appendix. Specimens after tests were photographed by SEM and
analysed by EDX.
RESULTS AND DISCUSSION
After the simulated occluded cell galvanostatic tests, the pH values in the occluded solution over
time under 1 mA-cm-2 anodic current density are shown in Figure 1. It can be seen that the pH
dropped sharply within the first hour from 7 to 5.68, then tended to decrease gradually. After 72
hours of accelerated corrosion, the pH value dropped to 3.86.
The reason for this was that localized corrosion took place inside the occluded cell. The localized
corrosion inside the occluded cell resulted from the establishment of differential oxygen cells and
was continued by the stimulation of the autocatalytic process which promoted local acidification
and chloride ion enrichment. Local acidification that occurred inside the occluded cell involved
dissolution of the anode and hydrolysis of the dissolved metal ions. Cations dissolved from the
anode
Figure 1
Fall in pH in the occluded cell with time with a 1 mA-cm-2 current density passing
through the specimen.
found difficulty in diffusing outwards because of the greater immobility of the solution in the
occluded cell compared to the bulk solution. In consequence, their concentrations increased. The
accumulation and hydro-lysis of Fe2+ resulted in local acidification in the occluded cell. The
hydrolysis reactions are given below [6]:
The generation of free acid lowered the pH value and, inversely, the increase in acidity accelerated
localized corrosion. The effect in the occluded cell of acidification causing accelerated localized
corrosion activity is called the 'autocatalytic effect' [10]. As anodic current passes through the
occluded specimens, the autocatalytic process continues at a steady rate in the occluded cell, and the
pH value gradually decreases.
On the other hand, chloride ions migrated inward from the bulk solution simultaneously with local
acidification. Figure 2 is representative of the quantity of chloride ion migration in the occluded cell
at various time intervals under 1 mA-cm-2 anodic current density. It was found that the chloride ion
migration increased in a linear relationship with the increase of time.
As noted previously, dissolution of the anode resulted in an increase in Fe2+ ion concentration. In
order to balance the excess positive charge produced in the occluded cell, negative chloride ions
have to migrate inwards from the bulk solution to maintain electrical neutrality [7], which causes
chloride ion enrichment in the occluded cell. Moreover, the autocatalytic process
keeps chloride ions migrating inwards, and therefore chloride ion concentration increases with time.
As long as the cast iron specimen is corroding and producing Fe2+ ions, chloride ions will diffuse in
and concentrate in the occluded cell.
As a result of the autocatalytic process inside the occluded cell, the acidity- (via hydrolysis) and
chloride concentration (via migration) increased with time. The relationship between pH and
chloride ion concentration m the occluded cell at 1 mA.cm-2 current density is presented in Figure 3.
It is evident that the pH value in the occluded cell dropped rapidly from 7 to below 5, and thereafter
decreased linearly, indicating that the decrease in the pH value was accompanied by an increase in
chloride ion concentration.
Figure 3 Relationship between pH value and the chloride ion concentration in the occluded cell.
Based on the results above, the pH value below 5 was proportional to the chloride ion
concentration. The following empirical equation was derived:
pH = 6.82-2.71 [Cl-]
R = 0.9910
where [Cl-] was the chloride ion concentration in the simulated occluded cell and R. was the
confidence in the fitting equation.
SEM micrographs of specimens after galvanic testing tor 24 and 48 hours at 1 mA-cm-2 anodic
current density-are presented in Figure 4. As can be observed, the surface patterns differed from one
another. Figure 4a exhibited a rust laver with a uniform, adherent and con-
Figure 4 SEM micrographs of cast iron under an anodic current density of 1 mA.Cm-2 for (a) 24
hours and (b) 48 nours:;(c) and (d) show the detailed surface morphology of sample 4b
tinuous structure. In contrast, Figure 4b demonstrated a more severe attack on the rust layer. The
insoluble structure, and micro-cavities and cracks, can be clearly observed in Figures 4c and 4d.
The more severe attack on the rust layer in Figure 4b was attributed to the autocatalytic effect,
which promoted acidity and chloride ion concentration, thus accelerating the dissolution of cast iron
with longer exposure time. The insoluble structure m Figure 4c, which was flake-like, indicated that
the residual graphite formed a three-dimensional network. In addition, micro-cavities and cracks
could be seen in Figure 4d. The formation of these micro-cavities and cracks was evaluated in
relation to the acid produced and the formation of the solid products, which could give rise to local
stresses and cause dissolution of other corrosion products, thereby chemically assisting the
propagation of cracks and aiding the disintegration of the rust layers [11]. The general conclusion
was that the autocatalytic effect exerts a greater influence on the sample after the 48-hour test than
on the sample after the 24-hour test.
In order to obtain a better understanding ot outward diffusion of chloride ions, the simulated
occluded cell is also used in the desalination treatment. Monitoring the pH in the occluded cell over
time provides more details during chloride removal, which is shown in Figure 5. The pH value in
the occluded cell rises gradually with the square root of the treatment time.
The reason tor the increase in the pH value is the increase m hydroxide ion concentration in the
occluded cell. The inward flow of the hydroxide ions is due to the concentration gradient between
inside and outside oí the occluded cell. In general, the high mobility of hydroxide ions and their
concentration in the treatment solution maintain the inward diffusion of hydroxide ions, thus the pH
value increases gradually over time.
As a result of the ingress of hydroxide ions, chloride ions diffuse outwards from the occluded cell
into the washing solution. After immersion, the amount of chloride ion removed from the occluded
cell increased with the square root of time as shown in Figure 6. It can also be seen that the results
are divided into two stages.
It is generally accepted that two steps might occur when a marine iron artifact is soaked in the
treatment solution [8]. One is the reaction of dissolution of the 'FeOCl' to produce free chloride
ions; the other is the movement of the chloride ions from their point of production inside the
corrosion product to the bulk wash solution. After galvanic testing for 48 hours at 1 mA.cm-2 anodic
current density, the specimen released Fe2+ ions into the occluded solution. It has been reported that
chloride ions present in the iron corrosion products are trapped within the lattice structure of the
various iron oxyhydroxides; among them, ferric oxychloride (FeOCl) is the dominant chloridecontaining corrosion product on cast iron [12—14|. In the case of the galvanic test, FeOCl, which is
not stable m air, may be present as an intermediate phase during the corrosion process due to lack of
oxygen in the occluded cell and be converted to ß-FeOOH once exposed to air. North has proposed
Figure 5 Relationship between pH in the occluded cell and the square root of the treatment time.
Figure 6 The amount of chloride ion removed as a function of the square roof of the treatment
time.
the mechanism by which chlorides in the corrosion products are converted to free chloride ions
during chloride removal. The reaction is given by [15]:
The hydroxide ions diffuse into the occluded cell and react with chloride-containing compounds to
produce free chloride ions. Then the chloride ions migrate outwards because of the concentration
gradients between inside and outside of the occluded cell, and the removal of chloride should be
achieved.
The data in Figure 6 indicate that there may be two stages involved in the washing. The initial stage
is controlled by the reaction ot dissolution of the FeOCl and in turn this is controlled by the outward
diffusion of chloride ions. In stage I. it is due to the rapid ingress ot hydroxide ions in the occluded
cell, which allows the dissolution of the FeOCl, and the more chloride ions present in the occluded
solution the more chloride ions can be transported. Thus, the amount of chloride ion removed
rapidly increases. In stage II, the amount of chloride ions removed increases gradually because of
the decrease in the concentration gradients ot hydroxide ions between inside and outside of the
occluded cell. The reaction of dissolution of the FeOCl may not be a significant factor in
determining chloride release rate. The outward diffusion of chloride ions may then play an
important role in the washing. As the rate-controlling process is generally outward diffusion ot
chloride ions in the washing method, the rate of chloride ion release from the occluded cell into the
wash solution follows the diffusion law [8]. According to this law, the diffusion equation predicts
that the amount of chloride ion removed varies linearly with the square root of the treatment time.
Such a line is shown in Figure 6, and the agreement between theory and experiment indicates that
diffusion is the rate-controlling process in stage II.
The release rate of chloride ions can be obtained by analysis of chloride ion removal data measured
in the occluded solution, which has seldom been used in the literature. The chloride ion extraction
rate at a specific time is defined by the instantaneous slope at that time, which is the derivative of
the amount of chloride ions removed from the occluded cell with respect to time [3]. Figure 7 is a
plot of the chloride ion extraction rate as a function of treatment time.
Figure 7 The chloride ion extraction rate from the occluded cell with treatment time.
It was tound that the chloride ion extraction rate decreased rapidly at the beginning and then became
almost stable, with a minor decrease with the increase of washing time. By monitoring the amount
of chloride ion extracted and constructing a graph from this information, the chloride ion extraction
rate can be used to evaluate the effectiveness of different treatment solutions by using the same
occluded cell. Moreover, the chloride ion extraction rate as a guide to the progress of the treatment
can provide a signal to change the treatment solution when the curve levels out into a plateau
region. Having achieved an understanding of the process of chloride removal, it can be applied to
increase the desalination efficiency.
X-ray energy dispersive spectrometry (EDX) of samples before and atter the 72-hour desalination
treatment is shown in Figure 8. Figure 8a shows the chemical composition of the specimen after the
48-hour galvanic test at 1 mA-cm-2 anodic current density and Figure 8b is the EDX analysis of the
corroded specimen atter the 72-hour desalination treatment which followed the application of a 1
mA-cm-2 anodic current density for 48 hours. The presence of Fe. Mn, Si, P, S and Cl could be
observed. Comparing these two pictures. Figure 8a was rich in chloride and Figure 8b was low in
chloride.
Figure 8 Chemical composition of specimens before and after the treatment analysed by EDX. (a)
EDX analysis of the cast iron with 1 mA.cm-2 applied anodic current density for 48 hours, (b) EDX
analysis of the corroded specimen after 72-hour desalination treatment.
The EDX analysis in Figure 8a provided evidence that the chloride ion enrichment was the result of
the autocatalytic process inside the occluded cell. Figure 8b provided intormation that the decrease
in chloride was due to the diffusion of the chloride ions from the occluded cell to the washing
solution during the treatment.
CONCLUSIONS
A simulated occluded cell was used to study the localized corrosion and the desalination treatment
of simulated cast iron artifacts. It is a convenient method to reveal the chemical changes within an
occluded cell, which can not only study the mechanism ot the localized corrosion of cast iron, but
can also investigate chloride ion migration in and out of the occluded cell during localized corrosion
and desalination treatment.
In the present work, the changes of chemical state for localized corrosion of simulated cast iron
artifacts in 3.5 wt% NaCl solution were studied. It was found that as an anodic current was passed
through the cell, the pH value inside the cell initially fell quickly and then decreased gradually.
Meanwhile, the chloride ions migrated into the occluded cell. The quantity of chloride ion migration
increased with the time that the current was flowing. The results of the SEM analysis showed
acceleration of dissolution of cast iron with the time of application of the anodic current, and EDX
analysis indicated the enrichment in chloride ion.
A study was made of the chemical changes within the simulated occluded cell after the 72-hour
desalination treatment which was applied to samples that had first been corroded at 1 mA-cm-2
anodic current density for 48 hours. During chloride removal, the pH value in the occluded cell
gradually increased over time. In addition, the amount ot chloride ions removed from the occluded
cell increased with the treatment time. The chloride ion extraction rate initially decreased rapidly
and then became almost stable with the washing time. Furthermore, the EDX analysis disclosed the
change of composition ot chlonde after the treatment.
It is evident that the simulated occluded cell could help in understanding the degree of localized
corrosion and the progress of the treatment of cast iron artifacts, thereby enabling their safe and
effective preservation.
ACKNOWLEDGEMENTS
The authors would like to thank the National Key Technologies R&D Program of the 10th FiveYear Plan Period for financial support (Contract No. 2001BA805B01). This work was also
supported by the State Key Laboratory for Metallic Corrosion and Protection. Thanks are also
extended to Dr L.S. Selwyn for her kind help.
APPENDIX: EXPERIMENTAL
A simulated galvanostatic occluded cell apparatus [16], as shown in Figure 9, was adopted to study
the localized corrosion.
Figure 9
Schematic diagram of the simulated occluded cell: 1 bulk specimen, 2 occluded
specimen. 3 simulated occluded cell. 4 magnetic stirrer.
As can be seen, the simulated occluded cell made of hard glass was in the centre of a glass
container, the effective volume of which was 1.5 mL. The cell was separated from the bulk solution
by a glass tube (1.5 x 15 mm), which was filled with filter-paper scraps to retard diffusion and
convection between the occluded and the bulk solutions. The specimen was inserted into the cell
through a rubber stopper. The outer end of the specimen was connected to the positive terminal of a
set of batteries, the negative terminal of which was connected to the external bulk specimen of
graphite.
The specimens used were grey cast iron to simulate cast iron artifacts, so the corrosion process
would be similar in soine ways to that observed in the field. The chemical composition of
specimens (wt%) was: C 3.00, Si 1.84, Mn 0.82, P 0.098, S 0.089. The surfaces of the specimens
were wet-polished with silicon carbide paper to grade 1000. The samples were then rinsed with
deionized water, degreased with CP-grade acetone (propanone) and stored in a desiccator filled with
nitrogen until they were ready for testing. The exposed area of each specimen was 25 mm2, and the
remaining surface was shielded with silicone paste. The exposed area ratio between the occluded
and bulk specimen was about 1:100. NaCl (AR grade) and deionized water were used to make up a
3.5 wt% solution, which was the medium to simulate seawater corrosion. The glass container was
tilled with 2 L of the solution and its pH was adjusted to 7. At room temperature, about 1.5 mL of
the bulk solution was injected into the occluded cell. An anodic current density of 1 mA.cm-2 was
applied to the occluded specimens to simulate the couple currents in and out of the occluded cell via
resistances. After various time intervals the tests were stopped, and the occluded solution was
removed for analysis. The pH value of the occluded solution was recorded with a pH meter (pHs-25
type) at ambient temperature. The chloride ion concentration was obtained with a Metrohm (model
751 Titrino) potentiometric titrator.
The desalination treatment was conducted in 2.5 wt% NaOH solution. After a 48-hour galvanic test
at 1 mA.cm-2 anodic current density, the simulated occluded cell was immersed m a glass container
containing the sodium hydroxide. The outer end of the specimen was directly connected to the
external bulk specimen of graphite. The pH values and the chloride ion contents in the occluded cell
at different time intervals were monitored.
After exposure in the simulated occluded cell galvanostatic tests at 1 mA.cm-2 anodic current
density for 48 hours and the subsequent desalination treatment for 72 hours, specimens were rinsed
in deionized water and then dried in hot air and stored in a container filled with nitrogen. The
microstructural characterizations of the samples were performed with a Cambridge (model S250MK3) scanning electron microscope, fitted with a Link (model AN-10000) energy dispersive
spectrometer.
REFERENCES
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AUTHORS
OUYANG WEIZHEN completed his master's degree in corrosion and protection from Beijing
University of Chemical Technolog)1 in 1996. He studied for a doctorate in 2(102. He is an associate
professor and his research focuses on the corrosion and conservation problems related to cast iron.
Address: Beijing University of Chemical Technology. 15 Bcisanhuau East Road, Beijing 100029,
PR. China. Email: ouyangwz@grad.buct.edu.cn
Xu CHUNCHUN has an engineering degree (1964) from Beijing University ot Chemical Technology.
She is a professor at the University and Vice-Secretary General and Director, Chinese Society of
Corrosion and Protection (CSCP). She has conducted research in corrosion science. Address as for
Ouyang. Email: chunchunxu@263.net
Résumé — Une cellule fermée simulée a été utilisée pour étudier la corrosion localisée dans l'eau
de mer et le traitement de dessalement en solution alcaline pour des objets eu foute simulés. Après
un test galvauostatique de ¡a cellule, on a pu observer, comme résultat d'un processus
autocatalytique, une forte acidité et un enrichissement en ions chlorure dans la cellule. Par
ailleurs, les analyses au MEB/EDS indiquaient le niveau de corrosion localisée et l'enrichissement
eu ions chlorure. Une investigation a également été menée sur les changements chimiques à
l'intérieur de la cellule fermée pendant le traitement de dessalement; les résidtats montrent que les
valeurs du pH et la quantité d'ions chlorure éliminés de la cellule augmentaient avec la durée du
traitement. La diminution de la quantité d'ions chlorure à la surface corrodée de l'échantillon a été
confirmée par EDS. La recherche semble prouver que la cellule fermée simulée est eu mesure de
fournir des informations utiles sur ¡a corrosion localisée et sur le traitement de dessalement
d'objets en fonte.
Zusammenfassung — Eine Simulation einer Einschlußzclle wurde verwendet, um die lokale
Korrosion in Meerwasser und die Entsalzung durch die Behandlung mit alkalischeu Lösungen au
Schmiedeeisen zu untersuchen. Gemäß einem galvanostatischen Test konnte ein holier pH-Wert und
eine Anreicherung von Chloridionen in der Eiuschlusszelle beobachtet werden, welche auf
autokatalytisclie Prozesse zurückgeführt werden. Das Ausmaß der Korrosion und der
Chloridanreicherung wurden durch Untersuchungen mit Hilfe der Rastcrclektronenmikroskopie I
energiedispersiveu Röutgenmikroaualyse (REM/EDX) bestätigt. Weitere Untersuchungen betrafen
die chemischen I eranderungen in der Einschlusszelle während der Entsalzung. Dabei stiegen der
pH-Wert und der Grad der Chloridentjernung mit der Behandlungsdauer. Letzteres wurde mit EDX
bestätigt. Die Untersuchungen legen nahe, dass anhand der simulierten Eiuschlusszelle nützliche
Informationen über lokale Korrosionen und Entsalzungseffektc an Schmiedeeisen gewonnen werden
können.
Resumen — Se empleó una simulación de célula cerrada con el fin de estudiar, tanto la corrosión
localizada por efecto del agua de mar, como los tratamientos de desalinizacióu en disoluciones
alcalinas para artefactos simulados de hierro fundido. Según mostró el test, en la célula
galvanoestática cerrada se manifestó una elevada acidez y un enriquecimiento de iones cloruro,
todo ello resultado del proceso autocatalítico. Adicioualmente, micrografías obtenidas por
microscopía electrónica de barrido (SEM) y energía dispersiva de rayos X (EDS) mostraron el
grado de corrosión localizada y el aumento de iones cloruro. Se realizó además una investigación
sobre los cambios químicos ocurridos en el interior de la célula cerrada durante el tratamiento de
desalinización; los resultados mostraron que los valores del pH y la cantidad de iones cloruro
eliminados aumentaban cu el transcurso del tratamiento. La disminución de los cloruros en la
superficie corroída de las muestras se confirmó mediante EDS. La investigación parece probar que
la célula cerrada simulada es capaz de suministrar información muy útil sobre corrosión
localizada y tratamientos de desalinización en objetos simulados de hierro fundido.