Single-step synthesis of SrMoO4 particles from SrSO4 and their anti-corrosive activity

Article abstract of DOI:10.1016/j.jallcom.2014.03.151

The transformation of three different sizes (<38-165 μm) of SrSO ₄ mineral powder into SrMoO₄ scheelite-type tetragonal particles was investigated via conventional hydrothermal treatments under both static and stirring conditions (20 rpm) at 100-250 °C and varying reaction intervals (0.08-48 h) in alkaline solutions saturated with MoO4^2-. A partial SrSO₄ transformation was found to proceed in mild to concentrated alkaline NaOH solutions (0.1-2.5 M), the complete transformation of SrSO₄ to SrMoO₄ was observed to proceed at temperatures lower than 200 °C over 48 h in a static 5 M NaOH solution, but this process was accelerated when the autoclave was stirred at 20 rpm during the treatment, which led to the production of uncontaminated SrMoO₄ particles at 200 °C within 6 h. The resultant SrMoO₄ particles had octahedral bipyramidal morphology and particle sizes between 1 and 5 μm; these particles exhibited a noticeable agglomeration at the intermediate and final stages of the reaction due to the formation of agglomerates with an average size of 60 μm. However, stirring of the mixture in the autoclave markedly reduced the growth of the bulky agglomerations of SrMoO₄ particles. A coupled process involving the bulk dissolution of the SrSO₄ powder and, subsequently, the massive precipitation of SrMoO₄ in an alkaline solution (5 M NaOH) saturated with MoO₄^2- ions was utilised to crystallise the SrMoO₄ particles. Kinetic studies demonstrated that the activation energy required for the formation of the octahedral shaped SrMoO₄ particles was 25.8 kJ mol^-1 in the system without agitation; in contrast, the activation energy decreased to 12.8 kJ mol^-1 when the transformation was stirred. In a subsequent investigation, the ionic permeability resistance of a commercial paint applied to a AISI 1020 steel substrate was improved by adding a 2.0 wt.% of the transformed SrMoO₄ particles to the paint.

Full text of DOI:10.1016/j.jallcom.2014.03.151

Journal of Alloys and Compounds 607 (2014) 73–84
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Single-step synthesis of SrMoO4 particles from SrSO4 and their
anti-corrosive activity
d
e
b
J. Diaz-Algara a, J.C. Rendón-Angeles b,c, , Z. Matamoros-Veloza , K. Yanagisawa , J.L. Rodriguez-Galicia ,
⇑
J.M. Rivera-Cobo a
a Institute of Engineering, Autonomous University of Baja California, Mexicali, Baja California 21280, Mexico
b Research Institute of Advanced Studies of the National Polytechnic Institute, Saltillo Campus, Ramos Arizpe 25900, Coahuila, Mexico
c Department of Nanoscience and Nanotechnology of the Research Institute of Advanced Studies of the NPI, Campus Mexico City, U. Professional Adolfo López Mateos, Mexico
d Technological Institute of Saltillo, Faculty of Metal-Mechanics, Saltillo 25280, Mexico
e Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi 780-8073, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2013
Received in revised form 12 March 2014
Accepted 23 March 2014
Available online 18 April 2014
The transformation of three different sizes (<38–165 lm) of SrSO4 mineral powder into SrMoO4
scheelite-type tetragonal particles was investigated via conventional hydrothermal treatments under
both static and stirring conditions (20 rpm) at 100–250 °C and varying reaction intervals (0.08–48 h) in
alkaline solutions saturated with MoO42ꢀ. A partial SrSO4 transformation was found to proceed in mild
to concentrated alkaline NaOH solutions (0.1–2.5 M), the complete transformation of SrSO4 to SrMoO4
was observed to proceed at temperatures lower than 200 °C over 48 h in a static 5 M NaOH solution,
but this process was accelerated when the autoclave was stirred at 20 rpm during the treatment, which
led to the production of uncontaminated SrMoO4 particles at 200 °C within 6 h. The resultant SrMoO4
Keywords:
SrMoO4 Scheelite-type
Solid–liquid reaction
Bulk dissolution-precipitation
Kinetics
Anticorrosive activity
Hydrothermal
particles had octahedral bipyramidal morphology and particle sizes between 1 and 5
exhibited a noticeable agglomeration at the intermediate and final stages of the reaction due to the
formation of agglomerates with an average size of 60 m. However, stirring of the mixture in the auto-
lm; these particles
l
clave markedly reduced the growth of the bulky agglomerations of SrMoO4 particles. A coupled process
involving the bulk dissolution of the SrSO4 powder and, subsequently, the massive precipitation of
SrMoO4 in an alkaline solution (5 M NaOH) saturated with MoO24ꢀ ions was utilised to crystallise the
SrMoO4 particles. Kinetic studies demonstrated that the activation energy required for the formation
of the octahedral shaped SrMoO4 particles was 25.8 kJ molꢀ1 in the system without agitation; in contrast,
the activation energy decreased to 12.8 kJ molꢀ1 when the transformation was stirred. In a subsequent
investigation, the ionic permeability resistance of a commercial paint applied to a AISI 1020 steel
substrate was improved by adding a 2.0 wt.% of the transformed SrMoO4 particles to the paint.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
compound exhibits green or blue luminescence emissions at room
temperature [9]. Hitherto, conventional ceramic and low tempera-
Recently, alkaline earth molybdates (MMoO4, M = Ba, Sr, and Ca)
with scheelite-type structures have been intensively investigated
due to their luminescence and structural properties, as well as their
great potential for technological applications and others [1–3].
These compounds can be employed in a wide array of applications,
such as scintillator detectors, laser host materials, efficient cata-
lysts, optical fibres and microwave materials [1–8]. One of the most
important compounds within the molybdate group is SrMoO4,
which is important due to its opto-electrical properties: this
ture chemical preparative processes have been widely applied to
prepare scheelite-type molybdates. Under solid-state conditions,
the formation of these compounds is carried out at temperatures
from 700 to 1100 °C using stoichiometric mixtures of carbonate
species as raw materials [10–15], but this process is plagued by
problems, such as poor control of the chemical composition homo-
geneity, size and morphology of the powders. In contrast, low tem-
perature chemical routes have been shown to overcome these
problems: alkaline earth molybdates have been synthesised using
such techniques as the formation of sol–gels [16], as well as copre-
cipitation [3] and electrochemical [1], sonochemical [17,18] and
microwave-assisted methods [19–21]. In contrast, chemical
processes carried out in solution, such as solvothermal [22,23]
⇑
Corresponding author at: Research Institute of Advanced Studies of the National
Polytechnic Institute, Saltillo Campus, Ramos Arizpe, 25900, Coahuila, Mexico.
E-mail address: jcarlos.rendon@cinvestav.edu.mx (J.C. Rendón-Angeles).
http://dx.doi.org/10.1016/j.jallcom.2014.03.151
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

74
J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
(large), 69
was conducted only for the <38
l
m (medium) and <38
l
m (small), respectively. X-ray powder analysis
and hydrothermal methods [24–27], have demonstrated their
efficiency in synthesising different molybdates particles with
interesting morphologies and size control (micron or submicron).
The particle size and morphology are mainly controlled by adding
organic templates, surfactants or substrates to the mother liquor,
but certain of these compounds promote the formation of second-
ary phases (as impurities) in the reaction products [19].
l
m powder (Fig. 1); the results indicate that the
mineral powder contains an orthorhombic system with a Pnma space group (JCPDS
card 05-0593) and unit cell parameters as follows: a = 8.3624(5) Å, b = 5.3513(5) Å,
and c = 6.8670(5) Å. However, these ‘‘a’’ and ‘‘c’’ cell parameters are different from
the parameters determined for a pure SrSO4 sample (a = 8.359 Å, b = 5.352 Å and
c = 6.866 Å, JCPD card 05-0593). The pure sample has a very low content of impu-
rities:<0.11 wt.%; with Ba < 0.1 wt.%, Na < 0.01 wt.%, and Al < 0.001 wt.%. Therefore,
the enlargement of the unit cell parameters ‘‘a’’ and ‘‘c’’ of our collected SrSO4 sam-
ple is attributed to the incorporation of a large amount of impurities. Wet chemical
analyses of the SrSO4 powders (1 g) indicated that the major constituents of the
While soft solution processing, which includes hydrothermal
synthesis, is environmentally friendly it depends on the solubility
of the precursor salts in water at the temperature (>100 °C) and
pressure (>1 atm) utilised by the process. This parameter affects
the optimal chemical reaction equilibrium necessary to saturate
the hydrothermal media with solute, leading to the crystallisation
of particles [27–30]. Previously, overcoming the challenges posed
by the use of low-grade reactants during the preparation of pure
inorganic compounds has been investigated by some of the present
authors [31]. Perovskite type SrTiO3 submicron cubic particles
without by-products were prepared under alkaline hydrothermal
conditions with mineral strontium sulphate (celestite) crystals
and powders as the Sr2+ sources. The crystallisation of the oxide
phase proceeded in a single step from a highly concentrated alka-
line solution (KOH 5 M) and was found to be affected by the rate of
dissolution of the precursor SrSO4. The single-step reaction was
accelerated by increasing the temperature and concentration of
the alkaline solution, leading to better control over the morphology
and size of the particles. The single-step transformation of the
SrTiO3 particles occurs through a coupled mechanism: the bulk
dissolution-precipitation of ionic species. In addition, kinetic
analyses have indicated that the chemical reaction involving the
solid–liquid interaction, which generates the particle crystallisa-
tion, displays first-order kinetic behaviour. The activation energy
for this reaction is 27.9 kJ/mol. Importantly, the impurities con-
tained in the mineral precursor did not contaminate the product
during the crystallisation process and instead remained as ionic
species in the hydrothermal fluid media after the transformation
process [28,31]. Therefore, this process, when used in conjunction
with a suitable mineral species, might have potential applications
for the chemical preparation of different oxide materials, such as
scheelite-type alkaline earth-derived materials, with efficiency, as
well as economic and environmental advantages.
experimental sample are as follows: 46.61 wt.% Sr, 1.33 wt.% Ba, 52.05 wt.% SO42ꢀ
which corresponds to 96.8 wt.% SrSO4, 2.25 wt.% BaSO4 and the remainder as minor
elements (0.73 wt.% CaO, 0.19 wt.% Fe2O3, 0.02 wt.% MnO, and 0.007 wt.% Al2O3).
,
A literature search suggests that none of the numerous research
studies conducted toward the synthesis of SrMoO4 scheelite type
particles has used celestite SrSO4 ore as a reactant in the context
of the conventional hydrothermal method. Therefore, in this study
we have focused on evaluating the feasibility of employing SrSO4
powder as the Sr2+ source during the synthesis of SrMoO4 powder
under alkaline hydrothermal conditions. Our preliminary attempts
determined the chemical stability of the SrSO4 and Na2MoO4
precursors and set out to optimise the experimental conditions
(reaction temperature and time, and particle size of SrSO4) neces-
sary for successfully synthesising SrMoO4 scheelite-type powders
in a single step from the SrSO4 ore. Additionally, the reaction path-
way and the morphology of the synthesised products are discussed
in detail relative to the reactivity of SrSO4 in a concentrated hydro-
thermal NaOH fluid. In addition, the powders were applied in the
context of an anticorrosive paint coating and evaluated.
2. Experimental procedure
2.1. Materials preparation
Fig. 1. XRD patterns of the reaction products obtained from the hydrothermal
Polycrystalline aggregate celestite (SrSO4) ore was collected from the north-
eastern portion of Coahuila State (Mexico). The granulated aggregate was pulver-
ised for 1 h with zirconia ball milling media. The pulverised powder was sieved
treatments of celestite powders with (a) large (165 lm) and (b) medium (69 lm)
average particle sizes. The samples were obtained at 250 °C in a 5 M NaOH solution
with an autoclave filling ratio of 45% without stirring for several reaction intervals.
The continuous line corresponds to the peaks of the single phase SrMoO4 with
scheelite-type structures (JCPDS card No. 08-0482). (.) SrSO4 mineral celestite
(JCPDS card 05-0593), (w) SrCO3 strontium carbonate (JCPDS card No. 05-0418).
using stainless steel meshes for the following particle sizes: 80 (180
(150 m), 200 (75 m)–230 (63 m) and 400 (<38 m). To simplify the identifica-
tion of the three different particle sizes, these intervals are referred to as 165
lm)–100
l
l
l
l
lm

J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
75
Na2MoO4ꢁ2H2O is one of the most popular raw material MoO24ꢀ sources used to
prepare SrMoO4 powders, despite the fact that this compound has a very low water
solubility. However, in highly concentrated alkaline solutions, i.e., 5 M NaOH, even
the stoichiometric amount of Na2MoO4ꢁ2H2O required by the equilibrium of the
chemical reaction (1) was completely dissolved in the alkaline media before the
hydrothermal treatments.
acrylic paint when different amounts of SrMoO4 powders (0–2.5 wt.%) were added.
The powders were well dispersed in the paint with stirring, and the film coatings
(2.1 0.3 mm thickness) were applied via spray gun to the mirror-like polished sur-
faces of low carbon steel (SAE 1020) substrate plates (20 ꢂ 20 ꢂ 6 mm). The imped-
ance measurements of the acrylic paint coatings were made in an electrochemical
cell composed of carbon and Ag/AgCl working and standard electrodes respectively,
and used a 0.8 M NaCl solution as the electrolyte. The impedance spectra of four
samples placed on epoxy resin bases were collected for 12 h using a perturbation
voltage from ꢀ10 to 10 V with a CH Instrument 600 potensiostat and the electro-
chemical CMS 100 Gamry impedance software.
SrSO4ðsÞ þ Na2MoO4ðaqÞ þ NaOHðaqÞ ! SrMoO4ðsÞ þ Na2SO4ðaqÞ þ NaOHðaqÞ
ð1Þ
2.2. Hydrothermal treatments
The amount of the reagent grade chemical Na2MoO4ꢁ2H2O (99.9% purity, Wako
Chemical Japan) for the transformation was calculated based on the MoO4/Sr molar
ratio in accordance with the proposed chemical reaction (Eq. (1)) and the preliminary
dissolution experiments; to ensure the complete transformation of SrSO4, an alkaline
hydrothermal media saturated with MoO24ꢀ was selected. Therefore, the experiments
were conducted with a MoO4/Sr molar ratio of 1.5/1. The Na2MoO4ꢁ2H2O was
dissolved in the NaOH solutions using different concentrations from 0.1 to 5 M prior
to the addition of 0.2 0.01 g of SrSO4 powder with a specified particle size (<38, 69
3. Results and discussion
3.1. Structural aspects of the synthesis of SrMoO4 particles via the
transformation of SrSO4 powders
The hydrothermal treatments of differently sized SrSO4
or 165
l
m) to the Teflon-lined stainless steel vessel. The hydrothermal treatments
particles (large 165 lm, medium 69 lm and small < 38 lm) were
were conducted at 45% of the inner vessel volume (70 ml), which remained constant.
The autoclaves were sealed and placed in a forced-air convection oven heated to a
predetermined temperature (150–250 °C). The vessels were held at each of selected
temperatures with varied reaction times (1–48 h). In another set of experiments, the
treatments were conducted with stirring at 20 rpm under the same reaction condi-
carried out to examine variation in the following principal reaction
factors: temperature, reaction interval and stirring of the system
during the treatment. The relevant experimental conditions at
which the complete transformation of SrSO4 into SrMoO4 pro-
ceeded under hydrothermal alkaline conditions are summarised
in Table 1. For each case, the lattice parameters of the SrMoO4
are included. Additionally, the lattice parameters were also calcu-
lated for the partially transformed reaction products and for the
sample that produced reaction by-products; these are given also
in Table 1 for comparison.
tions as the preliminary static treatments above mentioned using a <38 lm Celestite
powder. After each hydrothermal treatment, the reaction products were carefully
separated from the remaining solution and ultrasonically washed three times with
deionised water at 80 °C.
2.3. Characterisation
X-ray powder diffraction analysis was employed to determine the crystalline
phases of the reaction products. The X-ray diffractometer (Rigaku Ultima IV) uses
The typical crystalline structural aspects of the reaction prod-
ucts obtained without stirring at 200 °C in a 5 M NaOH solution
at several reaction times are listed in Fig. 1. In particular, the for-
mation of a secondary crystalline phase occurred with medium
and large sized SrSO4 particles reacted for 6 h at mild temperatures
(200 and 250 °C). The diffraction peaks for the newly formed
phases corresponded to a SrMoO4 compound with a scheelite-type
tetragonal structure and I41/a (88) space group (JCPDS card No. 08-
0482), which was the major phase; additionally, a minor amount of
SrCO3 (JCPDS card No. 05-0418) was produced as the main reaction
by-product, but this phase was preferentially produced during the
transformation of SrMoO4 carried out with medium (Fig. 1a) and
large sized (Fig. 1b) SrSO4 particles. The amount of SrCO3 slightly
increased with longer reaction times, as observed in Fig. 1. The
same result was found when the reaction temperature was varied
from 150 to 250 °C. However, the formation of this secondary
phase was limited when powder containing the smallest particle
graphite monochromatised Cu K
a radiation (a = 1.54056 Å) at 40 kV and 20 mA.
The diffraction patterns were collected in the 10–80° 2h range at a scanning speed
of 4°/min in 2h/h scanning mode with a 0.02° step. The lattice parameters ‘‘a’’ and
‘‘c’’ were calculated with the least squares method using Si as an internal standard.
The morphology of the particles was observed via scanning electron microscopy
(JEOL FE-SEM), and the particle size was estimated from SEM images of 50 particles.
The kinetics of the proposed chemical reaction (2) was determined from powder
samples (0.15 0.01 g) placed over a non-diffracting silicon plate. The content
(wt.%) of the formed SrMoO4 and residual SrSO4 were calculated from the
refinement of the X-ray diffraction patterns using the reference intensity ratio
(RIR) quantitative routine of the PDXL Rigaku Integrated software. Additionally,
the kinetics relating to the consumption of the SrSO4 powder together with the for-
mation of SrMoO4 powder were evaluated using the shrinking core model for solid–
liquid systems reported previously [32,33]. Generally, the chosen model involved
bulk solid dissolution in the liquid phase, which is similar to the chemical equilib-
rium for the transformation of SrSO4 into SrMoO4 in the current study, Eq. (1).
The model considers that the reaction rate ‘‘r’’ of the SrSO4 powder might be
controlled by either the surface chemical reaction or diffusion through the solution
boundary layer. As a function of the consumption ratio ‘‘
follows:
a’’, r can be written as
size, <38 lm, was employed. In this particular case, the X-ray pow-
der diffraction results indicated that the transformation proceeded
in a single step via the proposed reaction (Eq. (1)) under alkaline
hydrothermal conditions and produced only the SrMoO4 phase.
r ¼ d
a=dt ¼ kfðaÞ
ð2Þ
where the solid (SrSO4) consumption,
a
, can be determined when the chemical reac-
tion reaches equilibrium by using the expression a ¼ 1 ꢀ CiSrMoO or, alternatively,
4
In contrast, the transformation of the small (<38 lm) SrSO4 parti-
a ¼ CiSrMoO . Within infinitesimal reaction intervals, CiSrMoO and CiSrMoO represent
4
4
4
cles into SrMoO4 was markedly improved by increasing the reac-
tion temperature from 150 to above 200 °C. In this case, the
powders were rapidly transformed into SrMoO4 at 200 °C even
within 24 h (Fig. 2a). Additionally, the results also demonstrated
that the SrSO4 transformation process was more dependent on
the particle size than the reaction time because the large raw SrSO4
the amount of the raw SrSO4 remaining and the amount of SrMoO4 formed after each
hydrothermal treatment conducted in a 5 M KOH solution, respectively. The con-
sumption ratio (a) for each experimental run was obtained from the reciprocal of
the SrMoO4 content (wt.%) measured using the X-ray diffraction analyses carried
out after the hydrothermal treatment. The rate constant ‘‘k’’ was then determined
from the linear regression of the experimental data in ln(1 ꢀ
a
a
) vs. t plots. This par-
ticular model makes use of empirical kinetic laws in which f(
) = (1 ꢀ
a
)n, where ‘‘n’’
particles (165
media, even after intermediate reaction times. The transformation
process was faster when the medium (69 m) or small (<38 m)
particle sizes were used. The <38 m SrSO4 powders were almost
lm) were partially dissolved in the hydrothermal
is the reaction order. This expression allows the calculation of different empirical
functions to predict the kinetics and mechanism of the reaction occurring at the
solid–liquid interface [32,33]. Finally, the activation energy ‘‘Ea’’ was calculated with
the Arrhenius equation using linear regression. This activation energy is related to
the chemical reaction (1) and associated only with the dissolution of bulk SrSO4 in
an alkaline solution containing a high concentration of MoO24ꢀ ions, which might
lead to the synthesis of SrMoO4 powders.
Molybdate inorganic compounds have well known for their anticorrosive prop-
erties [34]. Therefore, additional measurements were carried out to determine the
capability for ionic transfer resistance of an acrylic paint film containing the hydro-
thermally prepared SrMoO4 anticorrosive powders, and this parameter can be
determined by impedance measurements using an electrochemical cell. The imped-
ance electrochemical analyses were carried out using a non-corrosive commercial
l
l
l
completely converted into SrMoO4 after 12 h of reaction using
temperatures as low as 150 °C; under these conditions trace SrSO4
was observed (Fig. 2b). Likewise, at 200 °C the concentration of the
alkaline solution was found to accelerate the synthesis of SrMoO4
particles above 2.5 M NaOH, because in mild concentrated
solutions (<2.5 M) trace SrSO4 was observed (Fig. 2c). Hence, in
accordance with our results, the particle size and the concentration

76
J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
Table 1
Summary of the hydrothermal treatments conducted to study the transformation of SrSO4 particles into SrMoO4 particles.
Sample
SrSO4 particle
Temperature
(°C)
Alkaline solution
concentration (M)
Time (h)
Stirring
speed
(rpm)
Crystalline
phases
Lattice
Lattice
Cell volume
(Å3)
size (l
m)
parameter
parameter
a = ba (Å)
ca (Å)
Mo2
Mo4
Mo7
Mo8
165
69
250
250
250
200
200
150
225
225
225
225
200
150
100
200
200
200
200
5
5
5
5
5
5
5
5
5
5
5
5
5
0.1
0.5
1
48
48
48
48
24
48
0.5
2
4
6
6
6
0
0
0
0
0
SrMoO4, SrCO3
SrMoO4, SrCO3
SrMoO4
SrMoO4
SrMoO4
5.4006(1)
5.4006(1)
5.3981(1)
5.3994(1)
5.3983(1)
5.3997(1)
5.4025(1)
5.3992(1)
5.3992(1)
5.3966(1)
5.3989(1)
5.4019(1)
5.4082(1)
–
12.0455(3)
12.0420(2)
12.0385(2)
12.0411(2)
12.0388(3)
12.0383(3)
12.0471(3)
12.0396(2)
12.0387(2)
12.0318(3)
12.0382(2)
12.0448(2)
12.0593(4)
–
351.33(2)
351.22(1)
350.80(1)
351.04(1)
350.83(2)
351.00(2)
351.62(2)
350.97(1)
350.94(1)
350.41(2)
350.89(1)
351.47(1)
352.72(2)
–
<38
<38
<38
<38
<38
<38
<38
<38
<38
<38
<38
<38
<38
<38
<38
Mo10
Mo13
Mo15
Mo18
Mo19
Mo21
Mo24
Mo26
Mo28
Mo44
Mo45
Mo46
Mo47
0
SrMoO4
20
20
20
20
20
20
20
0
SrMoO4, SrSO4
SrMoO4, SrSO4
SrMoO4
SrMoO4
SrMoO4
SrMoO4
SrMoO4
SrMoO4
SrMoO4
6
12
12
12
12
0
0
0
–
–
–
–
–
–
–
–
–
SrMoO4
SrMoO4, SrCO3
2.5
Strontium Molybdate (SrMoO4)b
5.3944
12.02
349.78
a
Lattice parameter calculated from the Rietveld refinement analyses, which were conducted considering anisotropic lattice strain, particle size and preferential orientation
refinement parameters.
b
Strontium molybdate (scheelite), JCPDS card No. 08-0482.
of alkaline solution concentration appear to be the principal factors
that affect the rate of the desired transformation under hydrother-
mal unstirred conditions.
the reaction time was increased to 24 h, crystal growth occurred to
form bipyramidal octahedral-shaped particles with an average size
of 8.5 lm; these particles exhibited significant coalescence in their
Stirring the mixture at 20 rpm brought about a substantial
improvement to the SrSO4 transformation; these experiments
were conducted employing the smallest particle size at which
the SrSO4 reaction by-products were produced under static condi-
tions. Stirring within the reaction vessels reduced both the temper-
atures and reaction times required to produce the same result
when unstirred. Although a similar trend was exhibited by all of
the stirred experiments, complete transformation of the SrSO4
powder into a crystalline SrMoO4 phase was obtained after 6 h
without the formation of any by-products (SrCO3), even at temper-
atures as low as 150 °C. In addition, the ‘‘a’’ and ‘‘c’’ lattice param-
eters of the SrMoO4 particles that were obtained after different
reaction times and temperatures using the three different SrSO4
particle sizes, are summarised in Table 1. The lattice parameters
and unit cell volumes of hydrothermally synthesised SrMoO4
powders approach closely the values reported in JCPDS card No.
08-0482 (a = 5.3944 Å and c = 12.02 Å) for the single SrMoO4
phase. Furthermore, the minute deviations exhibited by the lattice
parameters and cell volume values might be attributed to minor
strain, which causes distortion of the unit cell. A coupled dissolu-
tion-recrystallisation process occurring at the early stage of the
reaction might promote this phenomenon, which was induced
and enhanced by the hydrothermal processing conditions
employed [24,27].
pyramidal faces to produce large aggregates (Fig. 3b). However, the
bipyramidal particles were dissolved by the hydrothermal media
after 48 h, leading to the formation of a large number amount of
truncated octahedral (average size of 1.5
lm) and a few bipyrami-
dal octahedral particles (average size of 5
l
m, Fig. 3c). Further-
more, some morphological differences were observed between
the SrMoO4 particles prepared at different temperatures over
48 h (Fig. 4); when the small SrSO4 particles (<38
at 150 °C, the hydrothermally transformed SrMoO4 particles exhib-
ited an irregular nodular shape in two average sizes (1 and 6 m)
lm) were treated
l
(Fig. 4a). In contrast, increasing the temperature to 250 °C pro-
duced a noticeable change in the SrMoO4 particle morphology; at
this temperature, the micrographs revealed two particle shapes
resembling faceted quasi-octahedral and bipyramidal octahedral
morphologies. A large amount of 9 lm (average) bipyramidal octa-
hedral particles were formed preferentially at this temperature,
and these particles did not exhibit faceted (001) or (101) surfaces.
These particles agglomerated massively due to the particle coales-
cence on the pyramidal (111) faces, producing large aggregates
(approximately 60 lm) (Fig. 4b). Likewise, no marked morpholog-
ical differences were seen with hydrothermally transformed
SrMoO4 particles prepared using the small SrSO4 particles at
200 °C for 12 h under unstirred treatments in NaOH solutions with
different concentrations (0.1–5 M NaOH, Fig. 5). The bipyramidal
octahedral particles were even produced using mild concentration
NaOH solutions (<0.5 M, Fig. 5b), these particles have a particle
3.2. Morphological aspects of the transformed SrMoO4 particles
average size of 4 lm and only slight particle tendencies. Increasing
The morphology of the reaction products obtained under static
hydrothermal conditions is shown in Fig. 3. When small particles
of SrSO4 were treated with a 5 M NaOH solution at 200 °C for dif-
ferent reaction intervals, FE-SEM revealed that the resultanting
SrMoO4 particles had a bimodal size distribution, even with the
complete consumption of the SrSO4. In general, the SrMoO4 pow-
ders obtained at reaction intervals shorter than 6 h were composed
of large amounts of irregular octahedron-shaped particles with
the NaOH solution concentration up to 5 M (Fig. 5d) produced par-
ticle growth and coalescence, which resulted in the agglomeration
of these particles preferentially on the bipyramidal (111) phases
similar to that trend revealed by the treatments conducted at dif-
ferent temperatures.
The differences of morphology and the extent of crystal growth
observed in the transformed SrMoO4 particles are proposed to
occur in accordance with the scheme proposed in Fig. 6. The general
hydrothermal transformation reaction trend proposed suggest that
single SrMoO4 quasi octahedral particles are obtained at short and
small uniform sizes (0.9–2
lm) and a small number of octahe-
dron-shaped particles with an average size of 5
l
m (Fig. 3a). When

J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
77
Fig. 2. XRD patterns of the reaction products obtained during the hydrothermal treatment of celestite powders with small particle sizes (<38
lm) in a 5 M NaOH solution
without stirring (a) at 250 °C for several reaction intervals, (b) for 48 h at different reaction temperatures and (c) in NaOH solutions with different concentration at 200 °C for
12 h, respectively. The continuous line corresponds to the peaks of the single phase SrMoO4 with scheelite-type structures (JCPDS card No. 08-0482), (.) SrSO4 mineral
celestite (JCPDS card 05-0593), (w) SrCO3 strontium carbonate (JCPDS card No. 05-0418).
intermediate reaction intervals, caused initially by the simulta-
neous formation of single bipyramidal SrMoO4 particles; these par-
ticles then formed very large aggregations (averaging 60 lm) and a
concentration and pH of the NaOH solution are important variables
affecting outcomes in the preparation of single-phase scheelite oxi-
des, including SrMoO4, under hydrothermal conditions [24,27,31].
Indeed, the present inference is in accordance with experimental
results published recently, that demonstrate the low chemical sta-
bility of the SrMoO4 particles, which were partially dissolved in
deionized water even at a lower temperature of 70 °C [35]. In con-
sequence, these phenomena promote the dissolution of the aniso-
tropic quasi octahedral fine SrMoO4 particles produced during the
early stages of the reaction (>12 h, Fig. 6a); subsequently, the re-
crystallisation of the well-defined bipyramidal octahedral SrMoO4
particles proceeded as an intermediate reaction (24 h). The coupled
dissolution-recrystallisation mechanism was further extended,
small amount of fine anisotropic polyhedral particles due to the fur-
ther dissolution of the quasi octahedral SrMoO4 shaped particles
(Fig. 6a). Therefore, we might surmise that the differences in the
SrMoO4 particle morphology and size caused by varying reaction
time and temperature occur because the chemical stability of trans-
formed SrMoO4 at early and intermediate reaction stages is very
low in the highly concentrated hydrothermal media (NaOH 5 M).
Consequently differences in the solute (Sr2+ and MoO42ꢀ) saturation
steady state are likely to affect the crystallisation and particle
growth. The stability of these species is low because the metal

78
J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
Fig. 4. FE-SEM micrographs of the SrMoO4 produced after the hydrothermal
transformation of <38 m SrSO4 powders subjected to an unstirred 5 M NaOH
l
solution for 48 h at two reaction temperatures of (a) 150 °C and (b) 250 °C.
Furthermore, slight differences in the SrMoO4 particle
morphology were produced by hydrothermally treating small
SrSO4 powders (<38 lm) in a 200 °C 5 M NaOH solution for varying
reaction times with stirring at 20 rpm. Fig. 7 shows the morphology
typical of the SrMoO4 particles obtained under these conditions. In
general, stirring the mixture during the reaction promoted the crys-
tallisation of a large amount of well-defined bipyramidal octahedral
particles (average size 5
lm) alongside a few flower-like aggregates
of 1 m SrMoO4 particles, even after very short intervals (0.25 h,
l
Fig. 7a). A bimodal particle size distribution was observed for the
Fig. 3. FE-SEM micrographs of the SrMoO4 obtained after the hydrothermal
transformation of the SrSO4 powders that were <38 lm in size subjected to an
unstirred 200 °C 5 M NaOH solution for (a) 6, (b) 24 and (c) 48 h.
SrMoO4 powder and consisting of similar amounts of flower-shaped
crystallites (approximately 10
particles, which were produced after 2 h of stirring (Fig. 7b). In par-
ticular, a few platelets (50 m long and 20 m width) composed of
extremely tiny SrMoO4 particles (average size of 0.5 m) were dis-
lm in size) and large shuttle-like
l
l
l
covered during the cooling stage after the crystallisation of the
other major particles; the remaining saturated hydrothermal media
precipitated minute nuclei that coalesced along the longitudinal
and lateral axes of the platelet particles. During stirring, the growth
of all the particles was slower when the temperature in the hydro-
thermal system fell below 100 °C and when the solute concentra-
tion in the fluid media was reduced. Over the longest reaction
interval studied (>4 h), the crystal growth of both large bipyramidal
even up to the complete transformation of SrMoO4 at the longest
reaction interval (48 h, Fig. 6b), resulting in the dissolution of the
smaller anisotropic quasi-octahedrons to form the well-defined
bipyramidal particles [24,27]. Likewise, the kinetic aspects of the
dissolution-recrystallisation mechanism might generate the
changes in morphology and size that correlate with variation in
the temperature; this thermal energy controlled parameter is capa-
ble of promoting massive crystal growth, even under severe static
alkaline hydrothermal conditions. This inference follows the trend
revealed by increasing the temperature, because increase in the
rate of the dissolution-recrystallisation mechanism caused rapid
growth of large bipyramidal octahedral SrMoO4 particles; the
formation of the SrMoO4 aggregates was promoted by an aligning
processes involving crystallographic cohesion between some pref-
erential faces (pyramidal (111) faces) with high energy surfaces
[24,31] as is illustrated in Fig. 6c.
octahedral and polyhedral-shaped particles generated 10–20 lm
particles. The observed variations in particle morphology and size
occur because, under stirring conditions, the artificial fluid convec-
tion enhances the chemical reactivity of SrSO4 further. Likewise, for
the SrMoO4 particles, the increased fluid convection caused by
stirring and the particle sedimentation during the dissolution-
recrystallisation process must influence the relationship between
the particle growth and reaction time.

J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
79
hydrothermal and microwave assisted processes [5,21]. The syn-
thesis of these powders was achieved through a dissolution-
precipitation mechanism enhanced by increasing and controlling
the pH with NH4OH to reach values between 9 and 12, and the
reaction pathway was irrespective of the synthesis method; the
hydrolysis of the coprecipitated Sr-Mo precursor was further accel-
erated at milder alkaline pH values under hydrothermal conditions
[21,24]. In contrast, the transformation of the strontium source
(micron sized SrSO4 particles) might proceed by the process
described in chemical reaction (1), shown in Section 2.1 when a
strong alkaline hydrothermal fluid is used instead of mild concen-
trated alkaline fluids proposed recently [24,27]. In general, in the
present case of study, our system is analogous to those systems
proposed on previous studies above mentioned because the trans-
formation of SrSO4 particles is achieved via the dissolution-crystal-
lisation mechanism. However, we surmise that the synthesis of
SrMoO4 carried out in this work differs from the previously pub-
lished systems because the ionic molybdenum species were dis-
solved in the hydrothermal media before treatment. In the
present work, the single-step synthesis of the SrMoO4 particles
was promoted by a massive dissolution of the SrSO4 particles;
however, saturating the alkaline NaOH solvent with MoO24ꢀ species
might have further accelerated this bulk dissolution. The high con-
centration of MoO24ꢀ ions might decrease the chemical stability of
SrSO4; consequently, the massive dissolution of Sr2+ and SO42ꢀ ionic
species might be achievable at lower temperatures (125–150 °C)
and proceed because the dissolution of SrSO4 is further accelerated
under hydrothermal conditions when the temperature and alkaline
(NaOH) concentration are increased [29,35]. This inference is sup-
ported by the fact that in a recent study has shown that the kinet-
ics of the SrSO4 dissolution is markedly slowed when other
sparingly soluble alkaline phases are mixed with the alkaline
media under hydrothermal conditions, such as was observed when
Ti(OH)04 was employed as a Ti4+ source during the transformation
of SrSO4 into SrTiO3 [31].
The macroscopic aspects of the reaction pathway that promote
the transformation process were determined by experiments con-
ducted using the 165 lm SrSO4 powder, without stirring over 6 h,
in a 5 M NaOH solution at 150 or 200 °C, the resulting precipitate
morphology being displayed in Fig. 8. In general, the surface obser-
vations of the partially transformed SrSO4 particles indicate that
the transformation to SrMoO4 is pseudomorphic because the pre-
cipitation of the SrMoO4 phase occurred only on the surfaces
exposed to the alkaline fluid under static conditions without fluid
convection (Fig. 8a). The SrMoO4 covers the surface of the unre-
acted SrSO4 particles homogeneously, generating a face boundary
crevasse between the crystallised SrMoO4 and the remaining SrSO4
core. The consistent homogenous coating, which composes of large
Fig. 5. FE-SEM micrographs of the SrMoO4 produced after the hydrothermal
transformation of <38 m SrSO4 powders subjected to unstirred treatments at
quasi-octahedral shaped particles (approximately 10 lm in size),
l
reduces the transference of fresh hydrothermal media to and from
the reaction interface. This coating phenomenon is aided by the
difference in the molar volumes when small SO24ꢀ ions are replaced
by larger MoO24ꢀ ions. Therefore, molar volume increase associated
with the formation of the SrMoO4 layer initially disfavours that
pathway, but eventually creates a porous and cracked structure
so as to allow the completion of the transformation. The dissolu-
tion of the SrSO4 and the precipitation of the SrMoO4 might be
controlled by local solution chemistry and the homoepitaxial
precipitation of fine SrMoO4 particles on the surface of large,
previously-crystallised SrMoO4 particles (Fig. 8a) during the
cooling stage. Meanwhile, the blockage of the mass transfer was
overcome by the generation of anisotropic stresses in the SrMoO4
coating. When the large SrSO4 particles were hydrothermally trea-
ted at 250 °C, the steady stresses induced in the SrMoO4 layer
within even 6 h of reaction fracture the large coated regions and
allow the fluid to react with the remaining Sr2+ (Fig. 8b). These
200 °C for 12 h in NaOH solutions with different concentrations of (a) 0.1 (b) 0.5 (c)
1 and (d) 5 M, respectively.
3.3. Single step hydrothermal transformation of SrSO4 into SrMoO4
powders
In earlier studies, a series of single step chemical reactions have
been investigated as routes to synthesise various strontium inor-
ganic compounds employing celestite ore, crystals or powders, as
the main strontium source in highly concentrated alkaline hydro-
thermal medias [28–31]. However, the feasibility of obtaining
SrMoO4 by the transformation of SrSO4 ore, and the related reac-
tion mechanism, had not yet been studied under hydrothermal
conditions at a pH above 12. At pH values below 12, these milder
fluids allowed the preparation of various alkaline earth metal
MMoO4 or MWO4 scheelite compounds with both conventional

80
J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
Fig. 6. Schematic illustrations that are associated with the SrMoO4 particle crystallisation process occurring after the dissolution of the SrSO4 particles, and the course of the
dissolution-recrystallisation mechanism that produces the formation of bipyramidal octahedral SrMoO4 particles coupled with the marked particle agglomeration process.
phenomena agree with physical and chemical factors affecting the
coupled dissolution-reprecipitation process models proposed for
the various pseudomorphic reactions during the syntheses of var-
ious functional inorganic compounds from mineral raw materials
under hydrothermal conditions [35]. In contrast, the dynamic
transformation conditions achieved by mechanically stirring the
autoclave caused dramatic convection to occur in the hydrother-
mal media, promoting a homogeneous distribution of the solvent
and controlling the saturation state of the ionic species, for exam-
ple Sr(OH)+ and WO24ꢀ reported in earlier studies [30]. Controlling
the saturation state by stirring enhanced the bulk nucleation and
growth processes for the SrMoO4 particles and avoided the
localised precipitation of any hydrothermally transformed SrMoO4,
precluding the formation of layers coating the surface of the parti-
cles during the early and intermediate stages of the reaction, as
reported for other reactions [31].
over the same time when this solution was used (Fig. 9b). Stirring
caused the SrSO4 consumption to proceed even faster with stirring:
a yield above 50% (a = 0.5) was obtained at a temperature of 100 °C
over 2 h period. Although the rate of the SrSO4 consumption was
slow at lower temperatures (100–150 °C), the transformation of
SrSO4 into SrMoO4 occurred with yields near 98% (
a
= 0.98); the
complete consumption (
more than 3 h (Fig. 9c).
a = 0.98) was promoted at 225 °C over
Previously, several kinetic models have been proposed for
solid–liquid reactions occurring at low temperature under atmo-
spheric pressure [32,33], in addition to the standard models with
closed form analytic solutions, such as the Johnson–Mehl–Avrami
model [36,37] and newer models based on the population balance
approaches developed for reactions that proceed even under
hydrothermal conditions [38]. Here, a number of those models that
are not excluded by our observations are tested against the exper-
imental data.
3.4. Kinetics of the hydrothermal transformation of SrSO4 into SrMoO4
Naturally, the experimental conditions under which any trans-
formation proceeds constitute the critical factors determining the
selection of any descriptive quantitative kinetic model. In the pres-
ent reaction system, the optimal fastest reaction rate for forming
regular SrMoO4 particles is likely to be that promoting homoge-
neous nuclei formation and steady particle growth [35]. This state
is favoured when a homogeneous saturation of the precursor reac-
tants (Sr2+ and MoO24ꢀ) for a given reaction temperature and con-
centration of the alkaline solution exists [31,35]. Solid–liquid
reaction models that match the reaction state above mentioned,
and which were to fit the experimental kinetic data obtained under
both static and dynamic hydrothermal conditions include the
The global kinetic process summarised by Eq. (1) is only valid
where is no production of secondary phases, which was the case
for the transformation of the <38
lm SrSO4 particles under static
and stirring conditions. The consumed fraction (
a) of the small
SrSO4 particles treated under static and stirring (20 rpm)
hydrothermal conditions was determined within the range of
100–250 °C for reaction times up to 48 h with 5 M NaOH solutions.
The variation in ‘‘
presenting in Fig. 9a. In particular, the small SrSO4 particles
(<38 m) were consumed very rapidly during the static treat-
a’’ under static hydrothermal conditions is
l
ments, even at the lowest temperature (150 °C) within 1 h; under
these conditions, the reaction reached 65% consumption of SrSO4
Jander diffusion controlled function (D3, (1–(1 ꢀ
first-order function (F1, –ln(1 ꢀ ) = kt), and the Johnson–
Mehl–Avrami function (JMA, ln[ln(1/1 ꢀ )] = kt). These models
a)
1/3)2 = kt), the
a
(a
= 0.65) increasing to 75% (
a
= 0.75) consumption over the same
a
time at 250 °C. Over 95% of the small of SrSO4 particles were con-
sumed at temperatures above 200 °C over a 48 h period. Addition-
ally, a remarkable consumption yield of nearly 75% (a = 0.75) was
found at 200 °C after 12 h in mild concentrated NaOH solutions
below 0.5 M, the rate of consumption of SrSO4 was slightly acceler-
ated by increasing the concentration of the hydrothermal media up
to 5 M, with complete transformation of the fine SrSO4 powder
were selected because the kinetics associated with the proposed
solid–liquid reaction upon which each model is based might be
good analogues of the formation process of SrMoO4 from SrSO4
particles. High correlation coefficients R2 were determined for
the D3 (99.7%) and F1 (98.0%) models compared to a lower value
of 80.3% when the experimental data were fitted to the JMA func-
tion. The F1 model fits the kinetic data well because of the high R2

J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
81
Fig. 8. FE-SEM micrographs of the SrSO4 particles surfaces partially converted to
after the hydrothermal transformation of <165 m SrSO4 powders in an unstirred
l
5 M NaOH solution for 6 h at (a) 150 °C and (b) 200 °C.
because the stirring causes a strong convection in the reaction
media and consequently might affect a localized liquid boundary
layer diffusion rate, which could be associated with the D3 model.
We might infer that formation of a solid product layer controlling
the surface reactions did not proceed during the hydrothermal
transformation. However, the bulk particle dissolution that pro-
duced the rapid, homogeneous saturation of the alkaline solution
in solute triggered the controlled homogeneous formation of nuclei
and particle growth; these phenomena are associated with the dif-
fusion-controlled processes depicted by the D3 model.
In accordance with the above mentioned results, the Arrhenius
activation energy ‘‘Ea’’ for the temperature dependence of the
single-step transformation of SrSO4 into SrMoO4 conducted under
static conditions was calculated from a linear regression using
the rate constant ‘‘k’’ determined at different temperatures. Hence,
for the D3 model in this case, the value for ‘‘Ea’’ was of
25.8 kJ molꢀ1, while the ‘‘Ea’’ calculated for the F1 model was
26.8 kJ molꢀ1. Both values are of the same order as the activation
energies determined for chemical reactions occurring in liquid
state, which have been proposed for the synthesis of functional
ceramic oxides, such as BaTiO3 and SrTiO3 [31,39]. Nevertheless,
a noticeable reduction in the calculated activation energy of
hydrothermal transformation process was observed for the exper-
iments conducted with stirring at 20 rpm. For the D3 model fitting
the experimental data in this particular case, the ‘‘Ea’’ was
12.8 kJ molꢀ1. Although the kinetics for a solid–liquid reaction con-
ducted under hydrothermal conditions are strongly affected by the
reactivity of the solid reactant phases, we surmise that this phe-
nomenon was avoided because the alkaline solution 5 M NaOH
was highly saturated (molar ratio of MoO24ꢀ/Sr2+ = 1.5) and the
molybdenum precursor was highly soluble in the alkaline solution.
Therefore, the chemical stability of the SrSO4 phase was reduced
Fig. 7. FE-SEM micrographs of the SrMoO4 produced in a stirred (20 rpm) autoclave
from <38 m SrSO4 powder treated with a 200 °C 5 M NaOH solution for (a) 0.25,
(b) 2 and (c) 4 h, respectively.
l
coefficient value; however, this was discarded because the reaction
mechanism associated with the model is not likely to achieve the
SrMoO4 formation investigated. The D3 function assumes that
SrMoO4 formation is controlled by bulk ionic diffusion at the reac-
tion interface; this might be established between the shrinking
SrSO4 core through the SrMoO4 product layer including through
the liquid present in any cracks or crevices in the product layer.
Our reaction path observations suggest that ionic species diffusion
in the liquid was likely a relevant mechanism in those areas such
as SrMoO4 film cracks and the crevasse produced at the surface
of the shrinking SrSO4 particles, this reaction limiting pathway
was inferred to be significant when the SrSO4 particles were trea-
ted without agitation (Fig. 8). Moreover, the reaction kinetics
results obtained for the treatments conducted with stirring gave
a lower correlation coefficient for the D3 model the R2 value was
93.5%, which is lower than that obtained for the F1 function
(97.7%) under the same conditions. This difference is likely to occur

82
J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
Fig. 9. Consumption ratio (
a
) curves for the <38
lm SrSO4 particles partially and completely transformed into SrMoO4 under hydrothermal conditions (a) without stirring and
(b) stirred at 20 rpm in a 5 M NaOH solution at various temperatures: (N) 100 °C, (d) 150 °C, (j) 200 °C, (.) 225 °C and (ꢀ) 250 °C, and (c) treatments unstirred using different
concentration of the NaOH solution at 200 °C for (s) 6 and (h) 12 h. All of the experiments were conducted with an autoclave-filling ratio of 45% for various reaction intervals.
further, resulting in a massive SrSO4 particle bulk dissolution accel-
erated further by the enhanced convection of the fluid, in the par-
ticular case when the system was stirred.
incorporated different amounts of the hydrothermally transformed
SrMoO4 powders were evaluated. This present analysis was based
on the crystalline structure of chromate, which is similar to the
tetragonal structure of the alkaline earth metal molybdate com-
pounds, as well as on the inherent features of molybdate pigments
[41]. The impedance of the commercial coatings when mixed with
different contents of SrMoO4 particles (the product of hydrother-
mally transformation at 200 °C in a 5 M NaOH solution for 6 h sub-
ject to a stirring speed of 20 rpm), and the impedance curves of a
commercial paint coating on AISI 1020 substrates immersed in a
0.8 M NaCl solution were measured for 12 h; the results are pre-
sented in Fig. 10. The AISI 1020 substrates were not primed in
any way, but even a low content of SrMoO4 particles (1.0 wt.%)
caused an obvious increase in the impedance of the coating. The
3.5. Commercial paint mixed with hydrothermally transformed
SrMoO4 particles for impedance evaluation
Currently, chromate compounds (SrCrO4, ZnCrO4) are one of the
most commonly used chemicals for preventing the corrosion of the
coatings applied to metallic surfaces because they boast an excel-
lent cost to benefit ratio. However, these compounds are currently
being gradually eliminated from metal finishing processes because
they are quite toxic [40]. Therefore, in a search for potential alter-
natives, the corrosion behaviours of commercial coatings that

J. Diaz-Algara et al. / Journal of Alloys and Compounds 607 (2014) 73–84
83
sized (0.9–10 lm) SrMoO4 particles with two different morpholo-
gies, quasi-octahedral and bipyramidal octahedral, were produced
as a result of the direct conversion.
The experimental kinetic data were fitted to analytic functions
based on theoretical models of bulk diffusion control of ionic spe-
cies into the hydrothermal media. The activation energy required
for the latter transformation associated with the diffusion con-
trolled function to occur under static hydrothermal conditions
was 25.8 kJ molꢀ1; this is the minimum energy required for the
transformation of SrMoO4 powders from small SrSO4 particles
(<38 lm) over a wide range of temperatures in a single step. The
activation energy was further decreased to 12.8 kJ molꢀ1 when
the vessel was stirred at 20 rpm during the treatment; under these
conditions; neither the dissolution nor reaction surface controlling
functions fitted the experimental kinetic data. In this particular
case, the fluid convection produced by the stirring resulted in a
massive dissolution of the SrSO4 particles, which enhanced bulk
diffusion within the solvent media of the dissolved ionic species.
A potential application of the micron-sized SrMoO4 powders gen-
erated hydrothermally with stirring was demonstrated. Electro-
chemical impedance experiments demonstrated that the SrMoO4
powders are effective anticorrosion agents for acrylic paint coat-
ings with the addition of 2 wt.% of SrMoO4 powder triggering a
marked increase in the impedance of a commercial coating.
Fig. 10. Nyquist plots of the commercial paint coatings deposited without primer
on the AISI 1020 substrates immersed in a 0.8 M NaCl solution for 12 h. The
commercial paint was mixed with different contents of SrMoO4 powders (s) 0, (h)
1.0, (ꢀ) 1.5, (N) 2.0 and (.) 2.5 wt.% before application.
Acknowledgments
The present work was supported by the CONACYT project
CB-2008-01-0107052. Three of the authors, J.C.R.A, Z.M.V. and
J.L.R.G., are indebted to the CONACYT-SNI. J.D.A, which is indebted
to CONACYT in Mexico, for financial support, for Ph. D scholarship.
Many thanks are also given to Prof. T. Yamamoto from Centre of
Advanced Marine Core Research, Kochi University, Japan, for his
assistance with FE-Scanning Electron Microscopy. Special acknowl-
edge is offer to Dr. Jonathan Willis-Richards Ph.D. at Loeb Aron Co.
& Ltd. for his particular technical suggestions regarding the content
and English correction of the present research work.
anticorrosive behaviour of the commercial coating was found to
increase 10-fold when 2.0 wt.% of SrMoO4 particles were added.
The increased content of homogeneously distributed SrMoO4 par-
ticles in the commercial acrylic paint might decrease the anionic
exchange of Clꢀ ions because they interact with the sites in the
polymer structure with enhanced anionic exchange. This should
increase the impedance of the coating [40]. However, the anticor-
rosive behaviour was slightly reduced from values seen at 2.0 wt.%
when 2.5 wt.% of SrMoO4 was added to the acrylic paint. This
decrease might be caused a minor particle agglomeration, but fur-
ther experimental work is needed to repeat and clarify this occur-
rence. This anticorrosive behaviour is similar to that observed for
alkyd paints formulated with ion-exchange pigments and ZnCrO4
particles, and which has been reported elsewhere [40]. Hence,
we conclude that micron-sized SrMoO4 particles produced by
hydrothermally transformation at a low temperature (200 °C) in
2 h through a single-step process using SrSO4 ore, might have
industrial application as an anticorrosive agent within polymer-
based coatings.
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