Ci a ion: Fe nández, A.G.;
González-Fe nández, L.; G osu, Y.;
Labidi, J. Physicochemical
Cha ac e iza ion o Phase Change
Ma e ials o Indus ial Was e Hea
Reco e y Applica ions. Ene gies 2022,
15, 3640. h ps://doi.o g/10.3390/
en15103640
Academic Edi o s: Wenjin Ding,
Ziye Ling, Xianglei Liu and
Wenji Song
Recei ed: 6 Ap il 2022
Accep ed: 13 May 2022
Published: 16 May 2022
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ene gies
A icle
Physicochemical Cha ac e iza ion o Phase Change Ma e ials
o Indus ial Was e Hea Reco e y Applica ions
Angel G. Fe nández 1,* , Luis González-Fe nández 2, Ya osla G osu 2,3 and Jalel Labidi 1
1Depa men o Chemical and En i onmen al Enginee ing, Uni e si y o he Basque Coun y, UPV/EHU,
20018 San Sebas ián, Spain; [email p o ec ed]
2Cen e o Coope a i e Resea ch on Al e na i e Ene gies (CIC ene giGUNE), Basque Resea ch and
Technology Alliance (BRTA), Ala a Technology Pa k, Albe Eins ein 48, 01510 Vi o ia-Gas eiz, Spain;
[email p o ec ed] (L.G.-F.); yg osu@cicene gigune.com (Y.G.)
3Ins i u e o Chemis y, Uni e si y o Silesia in Ka owice, Szkolna 9 S ee , 40-006 Ka owice, Poland
*Co espondence: [email p o ec ed]
Abs ac :
The eco e y and s o age o p ocess hea in indus ial applica ions a e some o he key
ac o s o imp o e he sus ainabili y and eliabili y o high empe a u e applica ions. In his sense, one
o he main d awbacks is ocused on he selec ion o p ope he mal ene gy s o age (TES) ma e ials.
This pape pe o ms a ull cha ac e iza ion o ou phase change s o age ma e ials (PCM), KOH,
LiOH, NaNO
3
and KNO
3
, which a e p oposed o s o age applica ions be ween 270 and 500
◦
C,
acco ding o he esul s ob ained h ough di e en ial scanning calo ime e and he mog a ime ic
analysis. One o he main inno a ions includes he co osi e e alua ion o hese ma e ials in a
p omising alumina o ming alloy (OC4), close o hei co esponding phase change empe a u e
du ing 500 h. The physicochemical p ope ies ob ained con i m he op imal use o NaNO
3
and
KNO
3
and ecommend he use, wi h cau ion, o KOH, due o i s highe co osi e po en ial. FeC
2
O
4
,
NiC
2
O
4
and FeAl
2
O
4
we e he main p o ec i e spinels o med in he alloy su ace, howe e , he
c oss-sec ion s udy in he alloy imme sed in KOH, e ealed a non-uni o m beha io , p esen ing some
c acks and spalla ion in he su ace. On he o he hand, he p oposal o LiOH was dis ega ded since
i p esen s a na ow ope a ion empe a u e ange be ween mel ing and solidi ica ion poin .
Keywo ds: indus ial was e hea ; phase change ma e ials; he mal cha ac e iza ion; co osion
1. In oduc ion
Nowadays, ene gy is an essen ial ac o o e alua e he economic g ow h o any
coun y and egion as an indica o o i s le el o de elopmen , indus ializa ion and ech-
ni ica ion, ne e heless, despi e he impo ance o his esou ce, he e a e s ill ine icien
beha io s ela ing o i s use, ans o ma ion and managemen , especially a high ope a-
ion empe a u es.
The Indus ial sec o uses abou 37% o he wo ld’s o al ene gy and is he la ges
gene a o and consume o p ocess hea om non- enewable sou ces o ene gy [
1
]. Among
he objec i es se by he Eu opean Union o he yea 2030, hey highligh an impo an
educ ion in g eenhouse gas emissions (40%), as well as p omo ing an ene gy ma ix in
which 27% o ene gy mus come om enewable sou ces [1].
In his di ec ion, hea ing p ocesses a e esponsible o 17% o ene gy consump ion
o indus ial pu poses and i is w ongly assumed ha he hea is inco po a ed in o he
p oduc s. In ac , a signi ican pa o he hea used in indus ial p ocesses (20–50%), as well
as ha gene a ed in he successi e ans o ma ions, is dissipa ed in he o m o esidual
hea , discha ged in o he en i onmen h ough he e apo a ion o wa e , gases exhaus ,
coolan s, chemical was e, among o he s [2].
Due o he impo ance and olumes o hese losses, was e hea eco e y was ecen ly
iden i ied by he In e na ional Ene gy Agency (IEA) as one o he lines o be de eloped o
Ene gies 2022,15, 3640. h ps://doi.o g/10.3390/en15103640 h ps://www.mdpi.com/jou nal/ene gies
Ene gies 2022,15, 3640 2 o 12
he yea 2030 [
3
]. I s eco e y is especially in e es ing, due o i s impac on he dec ease in
economic cos s and he g owing en i onmen al es ic ions imposed on indus ies, whe e i
is p oposed o educe CO
2
emissions a a es o up o 60% by he yea 2040, h ough be e
esou ce managemen and he implemen a ion o ene gy e iciency p og ams [4].
The p oposal and in eg a ion o TES sys ems a e some o he main impo an opics
o esea ch o indus ial p ocess hea eco e y. In his di ec ion, he p oposal o di e en
he mal ene gy s o age (TES) sys ems con aining PCMs we e s udied in he li e a u e [
5
–
8
].
The main impo an equi emen s ha an op imal PCM mus mee [
9
] a e shown in Table 1:
Table 1. Requi emen s o phase change ma e ials o he mal ene gy s o age.
The mophysical Kine ic Chemical O he s
-Mel ing/solidi ica ion empe a u es in
he applica ion ange -Supe cooling educed -The mal
s abili y/ eliabili y -Low cos
-High hea capaci y -Low oxici y
-High he mal conduc i i y -High c ys alliza ion a e -Low co osion wi h he
con aine ma e ials -Sus ainabili y
As can be seen in Table 1, he e a e many desi able equi emen s ha phase change
ma e ials mus mee , bu hei main selec ion is based on he mophysical p ope ies, ha
is, he empe a u e ange whe e hei usion/solidi ica ion occu s, which will co espond
o he hea lux o be s o ed acco ding o he applica ion. H. Mehling and L.F. Cabeza [
10
]
es ablished a classi ica ion o PCMs based on hei mel ing poin and mel ing en halpy
(Figu e 1).
Ene gies 2022, 15, 3640 2 o 13
Due o he impo ance and olumes o hese losses, was e hea eco e y was ecen ly
iden i ied by he In e na ional Ene gy Agency (IEA) as one o he lines o be de eloped
o he yea 2030 [3]. I s eco e y is especially in e es ing, due o i s impac on he dec ease
in economic cos s and he g owing en i onmen al es ic ions imposed on indus ies,
whe e i is p oposed o educe CO2 emissions a a es o up o 60% by he yea 2040,
h ough be e esou ce managemen and he implemen a ion o ene gy e iciency p o-
g ams [4].
The p oposal and in eg a ion o TES sys ems a e some o he main impo an opics
o esea ch o indus ial p ocess hea eco e y. In his di ec ion, he p oposal o di e en
he mal ene gy s o age (TES) sys ems con aining PCMs we e s udied in he li e a u e [5–
8]. The main impo an equi emen s ha an op imal PCM mus mee [9] a e shown in
Table 1:
Table 1. Requi emen s o phase change ma e ials o he mal ene gy s o age.
The mophysical Kine ic Chemical O he s
-Mel ing/solidi ica ion empe a u es in
he applica ion ange
-Supe cooling e-
duced
-The mal s abili y/ e-
liabili y -Low cos
-High hea capaci y -Low oxici y
-High he mal conduc i i y -High c ys alliza ion
a e
-Low co osion wi h
he con aine ma e i-
als
-Sus ainabili y
As can be seen in Table 1, he e a e many desi able equi emen s ha phase change
ma e ials mus mee , bu hei main selec ion is based on he mophysical p ope ies, ha
is, he empe a u e ange whe e hei usion/solidi ica ion occu s, which will co espond
o he hea lux o be s o ed acco ding o he applica ion. H. Mehling and L.F. Cabeza [10]
es ablished a classi ica ion o PCMs based on hei mel ing poin and mel ing en halpy
(Figu e 1).
Figu e 1. PCMs acco ding o hei mel ing empe a u es and en halpy. Adap ed om [10].
Figu e 1. PCMs acco ding o hei mel ing empe a u es and en halpy. Adap ed om [10].
La inaga e al. [
11
] es ablished ha a highe hea eco e y po en ial could be ob ained
be ween 200 and 500
◦
C, especially in he i on and s eel sec o and he pe ochemical
indus y. This empe a u e ange will be he aim o his pape , ne e heless, ew ma e ials’
amilies can ac as TES ma e ial in his empe a u e ange, whe e ni a es and hyd oxides
a e he main candida es o be used. Ni a es we e widely s udied as sensible hea s o age
ma e ials [
12
–
14
] and he main sal p oposed is he so-called sola sal , composed o
60%NaNO
3
+ 40%KNO
3
. Mo eo e , hey we e widely s udied as sensible and la en
Ene gies 2022,15, 3640 3 o 12
hea s o age solu ions. In his las case, Maldonado e al. [
15
] p oposed his mix u e as
a PCM ma e ial o high empe a u e powe plan s, along wi h myo-inosi ol (C
6
H
12
O
6
),
de eloping co osion es s in di e en ma e ials a 250
◦
C du ing 2000 h and es ablishing
an accep able co osion esis ance in aluminum and ca bon s eel. In his di ec ion, he
selec ion o ma e ials o he cons uc ion o he sal con aine p esen s one o he majo
challenges o de elop easible TES sys ems o indus ial was e hea eco e y applica ions,
and he e is a lack o in o ma ion e alua ing he co osion po en ial in TES sys ems able o
wo k be ween 200 and 500 ◦C.
The LiOH-KOH mix u e was p oposed by Cabañas e al. [
16
] as la en s o age media,
due o i s high ene gy densi y and p ope mel ing poin o di ec s eam gene a ion (DSG)
echnology. The au ho s [
16
] ca ied ou a co osion es o his s o age mix u e in A516,
A316L and in 625, concluding ha he lowes co osion a e (6.3
µ
m/yea ) was ob ained o
A316L. Ku a i e al. [
17
] p o ided a lis o PCM ma e ials o be used in a high empe a u e
ange, bu he p o ided co osion da a e e only o PCMs a a low empe a u e [
18
–
20
],
hence mo e in o ma ion is equi ed, especially o he co osi e po en ial, when PCMs o
high empe a u e applica ion a e p oposed.
The aim o his pape is o ca y ou a ull cha ac e iza ion o he physicochemical
p ope ies, including he co osi e po en ial, o ou ino ganic phase change ma e ial
(NaNO
3
, KNO
3
, LiOH and KOH) ha could be applied in hea eco e y sys ems be ween
270 and 470 ◦C.
2. Ma e ials and Me hods
PCMs employed in his esea ch we e: KNO
3
(mp: 334
◦
C, 99.5% pu i y), LiOH (mp:
462
◦
C, 99% pu i y), KOH (mp: 360
◦
C, 99% pu i y) and NaNO
3
(mp: 308
◦
C, 99.5% pu i y),
all pu chased om Me ck (Me ck KGaA, Da ms ad , HD, Ge many). These ma e ials
p esen an analy ical deg ee; ne e heless, du ing he expe imen al wo k, each sal was
handled ca e ully o a oid wa e abso p ion.
The mal p ope ies as solidi ica ion/mel ing poin s and usion en halpy we e ana-
lyzed by di e en ial scanning calo ime y (DSC) and he he mal s abili y was s udied by
he mog a ime y (TGA). This cha ac e iza ion was ca ied ou in a Simul aneous The mal
Analysis (STA) 449 F3 de ice om Ne zsch, which allows he analysis o all he he mal
p ope ies a he same ime. A p e-d ied sample o each sal was es ed in ine a mosphe e
(ni ogen) o h ee consecu i e hea ing and cooling cycles be ween 100
◦
C and 400
◦
C o
he ni a es, 550
◦
C o LiOH and 500
◦
C o KOH. Masses o app ox. 70 mg we e employed
o he measu emen s and hea ing/cooling a es o 10 ◦C/min.
On he o he hand, he co osion s udies we e ca ied ou using he alumina- o ming
aus eni ic (AFA) alloy OC4, an expe imen al alloy p o ided by Oak Ridge Na ional Labo a-
o y. The chemical composi ion o his ma e ial is shown in Table 2.
Table 2. Chemical composi ion o he AFA alloy.
Alloy Weigh %
Fe C Mn Si P S C Ni Cu Ti Al Mo Co V Nb N
OC4
47.43
0.89 1.80 0.22 0.06 0.01
13.88 26.16
0.57 0.01 4.28 2.24 0.08 0.06 2.27 0.04
Di e en au ho s [
21
–
23
] ha e s udied he use o alumina- o ming alloys, since hey
can gene a e alumina scales (gene a ed on he su ace o s eel) ha a e mo e p o ec i e han
ch omia scales, usually o med on con en ional s ainless s eel. OC4 alloy was p oposed as
a ma e ial con aine in di e en TES sys ems using mol en sal s as ni a es [
24
] a 550
◦
C,
ca bona es [
25
] a 650
◦
C and chlo ides [
26
] a 720
◦
C. The alloy gene a ed a p o ec i e
alumina scale wi h high co osion esis ance, especially o ni a e sal s, o his eason i
was selec ed o con ain he PCM es ed in his pape .
The co osion es s we e ca ied ou h ough an iso he mal, s a ic and open o ai
se -up a 320
◦
C (NaNO
3
), 350
◦
C (KNO
3
), 380
◦
C (KOH) and 470
◦
C (LiOH), du ing 500 h.
Ene gies 2022,15, 3640 4 o 12
A scheme o he expe imen al se up is shown in Figu e 2, including he da a used o he
weigh change g a ime ic cu e.
Ene gies 2022, 15, 3640 4 o 13
han ch omia scales, usually o med on con en ional s ainless s eel. OC4 alloy was p o-
posed as a ma e ial con aine in di e en TES sys ems using mol en sal s as ni a es [24]
a 550 °C, ca bona es [25] a 650 °C and chlo ides [26] a 720 °C. The alloy gene a ed a
p o ec i e alumina scale wi h high co osion esis ance, especially o ni a e sal s, o his
eason i was selec ed o con ain he PCM es ed in his pape .
The co osion es s we e ca ied ou h ough an iso he mal, s a ic and open o ai se -
up a 320 °C (NaNO
3
), 350 °C (KNO
3
), 380 °C (KOH) and 470 °C (LiOH), du ing 500 h. A
scheme o he expe imen al se up is shown in Figu e 2, including he da a used o he
weigh change g a ime ic cu e.
Figu e 2. Expe imen al se up o co osion es s (le ) including he da a used o he weigh change
g a ime ic cu e ( igh ).
The coupons used o he iso he mal expe imen p esen dimensions o 12 mm × 12
mm × 2 mm, and be o e he co osion es hey we e g ounded wi h #600 SiC ab asi e
pape , washed wi h dis illed wa e , deg eased wi h e hanol and d ied unde a wa m ai
s eam, acco ding o he s anda d p ocedu e o ASTM G1-03 [27]. A digi al calipe was
used o dimension each coupon be o e he co osion es and an analy ical balance (wi h
a 0.1 mg esponsi eness) was used o weigh he coupons in oduced in he u nace. The
coupons we e main ained inside a labo a o y desicca o while he mol en sal mix u e o
he co osion es was p epa ed. The expe imen al se up (Figu e 2) in ol ed he use o
alumina c ucibles wi h he amoun o sal mix u e ecommended by he s anda d p oce-
du e ISO 17245 [28]. The c ucibles we e in oduced inside a esis ance u nace and hea ed
h ough a s epwise hea ing p ocedu e (main aining 1 h each hea ing empe a u e e e y
100 °C un il he co osion es empe a u e). Once he co esponding es empe a u e was
eached, i was main ained o 24 h o homogenize and s abilized he mol en sal . Then,
one coupon pe c ucible was in oduced o he u nace o s a he co osion es , main-
ained du ing 500 h, e ie ing he samples a e 100, 170, 250, 415 and 500 h. A he end
o he exposu e ime, each coupon was cooled slowly in wa m ai and cleaned wi h dis-
illed wa e o emo e he emaining sal . The g a ime ic weigh change was de e mined
h ough he Equa ion (1):
∆𝑚
𝑆
𝑚𝑚
𝑆
(1)
whe e m
i
is he o iginal mass o coupons, m
is he mass o coupons a e exposu e, and S
0
is he ini ial a ea o coupons.
The mo phology and elemen al composi ion o he co osion p oduc s we e analyzed
by Scanning Elec on Mic oscopy (SEM, Quan a 200 FEG model, FEI Company, Hillsbo o,
OR, USA) coupled o an ene gy dispe si e X- ay spec ome e (EDX). Supe icial and
c oss-sec ion iews we e examined. Fo he c oss-sec ion, he coupon was imbedded in a
esin composed o Aka-Resin and Aka-Cu e ca alys (100 g o 12.5 g, espec i ely). A e
Figu e 2.
Expe imen al se up o co osion es s (
le
) including he da a used o he weigh change
g a ime ic cu e ( igh ).
The coupons used o he iso he mal expe imen p esen dimensions o 12 mm
×
12 mm
×
2 mm, and be o e he co osion es hey we e g ounded wi h #600 SiC ab asi e
pape , washed wi h dis illed wa e , deg eased wi h e hanol and d ied unde a wa m ai
s eam, acco ding o he s anda d p ocedu e o ASTM G1-03 [
27
]. A digi al calipe was
used o dimension each coupon be o e he co osion es and an analy ical balance (wi h
a 0.1 mg esponsi eness) was used o weigh he coupons in oduced in he u nace. The
coupons we e main ained inside a labo a o y desicca o while he mol en sal mix u e
o he co osion es was p epa ed. The expe imen al se up (Figu e 2) in ol ed he
use o alumina c ucibles wi h he amoun o sal mix u e ecommended by he s anda d
p ocedu e ISO 17245 [
28
]. The c ucibles we e in oduced inside a esis ance u nace and
hea ed h ough a s epwise hea ing p ocedu e (main aining 1 h each hea ing empe a u e
e e y 100
◦
C un il he co osion es empe a u e). Once he co esponding es empe a u e
was eached, i was main ained o 24 h o homogenize and s abilized he mol en sal . Then,
one coupon pe c ucible was in oduced o he u nace o s a he co osion es , main ained
du ing 500 h, e ie ing he samples a e 100, 170, 250, 415 and 500 h. A he end o he
exposu e ime, each coupon was cooled slowly in wa m ai and cleaned wi h dis illed wa e
o emo e he emaining sal . The g a ime ic weigh change was de e mined h ough he
Equa ion (1):
∆m
S0
=
m −mi
S0
(1)
whe e m
i
is he o iginal mass o coupons, m
is he mass o coupons a e exposu e, and S
0
is he ini ial a ea o coupons.
The mo phology and elemen al composi ion o he co osion p oduc s we e analyzed
by Scanning Elec on Mic oscopy (SEM, Quan a 200 FEG model, FEI Company, Hillsbo o,
OR, USA) coupled o an ene gy dispe si e X- ay spec ome e (EDX). Supe icial and
c oss-sec ion iews we e examined. Fo he c oss-sec ion, he coupon was imbedded in a
esin composed o Aka-Resin and Aka-Cu e ca alys (100 g o 12.5 g, espec i ely). A e
he comple e solidi ica ion o he esin, he samples we e cu using a p ecision cu ing
machine (S ue s Mini on) equipped wi h a diamond blade. Then, he samples we e
g ound by successi e SiC sandpape s o inc easing g i size up 2500 by means o ATM
Saphi s a iona y polishing machine.
Coupons we e also examined by X- ay di ac ion (XRD) wi h a B uke D8 Disco e
di ac ome e equipped wi h a LYNXEYE de ec o using CuK
α
1 adia ion in B agg–
B en ano θ:2θgeome y, o de e mine he c ys allog aphic phases.
Ene gies 2022,15, 3640 5 o 12
3. Resul s and Discussion
The s udy o he he mal p ope ies o he PCM ma e ials selec ed (NaNO
3
, KNO
3
,
LiOH and KOH) we e ca ied ou by STA in o de o ob ain he mel ing and solidi ica ion
poin s, as well as he usion en halpy (∆H us). The esul s ob ained a e shown in Table 3.
Table 3. The mal p ope ies ob ained by STA in NaNO3, KNO3, KOH and LiOH.
Phase Change Ma e ial Mel ing Poin (◦C) Solidi ica ion Poin (◦C) Fusion En halpy (kJ/kg) Hea Capaci y (J/g K) [29]
NaNO3299.0 283.5 201.6 1.83
KNO3317.5 292.3 92.2 1.48
LiOH 413.1 391.5 No applicable 3.90
KOH 345.6 336.4 81.6 2.09
The ma e ials p oposed as PCMs we e also selec ed based on hei hea capaci y, in
his case, hyd oxides p esen ed highe hea capaci ies compa ed o ni a e sal s [29].
The he mal s abili y o he mol en sal s p oposed was also es ed in Figu es 3–5. In
his case, a simila beha io was ob ained o NaNO
3
and KNO
3
(Figu e 3), showing a
sligh ini ial weigh loss due o wa e con en elease (0.35%). Fo he es o he he mal
cycles, he sal weigh emained cons an du ing he ime be ween 100 and 400 ◦C.
Ene gies 2022, 15, 3640 5 o 13
he comple e solidi ica ion o he esin, he samples we e cu using a p ecision cu ing
machine (S ue s Mini on) equipped wi h a diamond blade. Then, he samples we e
g ound by successi e SiC sandpape s o inc easing g i size up 2500 by means o ATM
Saphi s a iona y polishing machine.
Coupons we e also examined by X- ay di ac ion (XRD) wi h a B uke D8 Disco e
di ac ome e equipped wi h a LYNXEYE de ec o using CuKα1 adia ion in B agg–
B en ano θ:2θ geome y, o de e mine he c ys allog aphic phases.
3. Resul s and Discussion
The s udy o he he mal p ope ies o he PCM ma e ials selec ed (NaNO3, KNO3,
LiOH and KOH) we e ca ied ou by STA in o de o ob ain he mel ing and solidi ica ion
poin s, as well as he usion en halpy (ΔH us). The esul s ob ained a e shown in Table 3.
Table 3. The mal p ope ies ob ained by STA in NaNO3, KNO3, KOH and LiOH.
Phase Change
Ma e ial
Mel ing Poin
(°C)
Solidi ica ion
Poin (°C)
Fusion En halpy
(kJ/kg)
Hea Capaci y
(J/g K) [29]
NaNO3 299.0 283.5 201.6 1.83
KNO3 317.5 292.3 92.2 1.48
LiOH 413.1 391.5 No applicable 3.90
KOH 345.6 336.4 81.6 2.09
The ma e ials p oposed as PCMs we e also selec ed based on hei hea capaci y, in
his case, hyd oxides p esen ed highe hea capaci ies compa ed o ni a e sal s [29].
The he mal s abili y o he mol en sal s p oposed was also es ed in Figu es 3–5. In
his case, a simila beha io was ob ained o NaNO3 and KNO3 (Figu e 3), showing a
sligh ini ial weigh loss due o wa e con en elease (0.35%). Fo he es o he he mal
cycles, he sal weigh emained cons an du ing he ime be ween 100 and 400 °C.
Figu e 3. TGA cu es o KNO3 and NaNO3 be ween 100 and 400 °C.
Figu e 3. TGA cu es o KNO3and NaNO3be ween 100 and 400 ◦C.
In he case o KOH, he TGA cu e (Figu e 4) showed a highe i s s ep compa ed
wi h he sodium and po assium ni a e. The weigh los due o he wa e con en was 23%
in he i s cycle o he he mal es , a e ha , he sal weigh emained cons an be ween
100 and 500 ◦C.
Finally, he TGA cu e o LiOH (Figu e 5) was ca ied ou om 100 o 550
◦
C, due
o he highe empe a u e ange p oposed o he indus ial applica ion. In his case, he
he mal decomposi ion has s a ed and i p esen s a highe inc ease beyond 500
◦
C. This is
con i med in he ollowing he mal cycles, whe e a cons an weigh loss, a ound 2% a each
he mal segmen analyzed, was ob ained.
Rega ding he co osion es s pe o med, ou independen s udies we e ca ied ou
o NaNO
3
(T = 320
◦
C), KNO
3
(T = 350
◦
C), KOH (T = 380
◦
C) and LiOH (T = 470
◦
C),
du ing 500 h. The g a ime ic cu es ob ained a e shown in Figu e 6.
Ene gies 2022,15, 3640 6 o 12
Ene gies 2022, 15, 3640 6 o 13
Figu e 4. TGA cu e o KOH be ween 100 and 500 °C.
Figu e 5. TGA cu e o LiOH be ween 100 and 550 °C.
In he case o KOH, he TGA cu e (Figu e 4) showed a highe i s s ep compa ed
wi h he sodium and po assium ni a e. The weigh los due o he wa e con en was 23%
in he i s cycle o he he mal es , a e ha , he sal weigh emained cons an be ween
100 and 500 °C.
Finally, he TGA cu e o LiOH (Figu e 5) was ca ied ou om 100 o 550 °C, due
o he highe empe a u e ange p oposed o he indus ial applica ion. In his case, he
he mal decomposi ion has s a ed and i p esen s a highe inc ease beyond 500 °C. This
is con i med in he ollowing he mal cycles, whe e a cons an weigh loss, a ound 2% a
each he mal segmen analyzed, was ob ained.
Rega ding he co osion es s pe o med, ou independen s udies we e ca ied ou
o NaNO3 (T = 320 °C), KNO3 (T = 350 °C), KOH (T = 380 °C) and LiOH (T = 470 °C),
du ing 500 h. The g a ime ic cu es ob ained a e shown in Figu e 6.
Figu e 4. TGA cu e o KOH be ween 100 and 500 ◦C.
Ene gies 2022, 15, 3640 6 o 13
Figu e 4. TGA cu e o KOH be ween 100 and 500 °C.
Figu e 5. TGA cu e o LiOH be ween 100 and 550 °C.
In he case o KOH, he TGA cu e (Figu e 4) showed a highe i s s ep compa ed
wi h he sodium and po assium ni a e. The weigh los due o he wa e con en was 23%
in he i s cycle o he he mal es , a e ha , he sal weigh emained cons an be ween
100 and 500 °C.
Finally, he TGA cu e o LiOH (Figu e 5) was ca ied ou om 100 o 550 °C, due
o he highe empe a u e ange p oposed o he indus ial applica ion. In his case, he
he mal decomposi ion has s a ed and i p esen s a highe inc ease beyond 500 °C. This
is con i med in he ollowing he mal cycles, whe e a cons an weigh loss, a ound 2% a
each he mal segmen analyzed, was ob ained.
Rega ding he co osion es s pe o med, ou independen s udies we e ca ied ou
o NaNO3 (T = 320 °C), KNO3 (T = 350 °C), KOH (T = 380 °C) and LiOH (T = 470 °C),
du ing 500 h. The g a ime ic cu es ob ained a e shown in Figu e 6.
Figu e 5. TGA cu e o LiOH be ween 100 and 550 ◦C.
Ene gies 2022, 15, 3640 7 o 13
Figu e 6. G a ime ic co osion cu es o mol en sal s es ed du ing 500 h o imme sion.
The highes co osi e po en ial was obse ed o KOH sal wi h a mass gain o 8.38
mg/cm2, which co esponds o a co osion a e o 0.186 mm/yea , ollow by LiOH, NaNO3
and KNO3. I is no ewo hy ha he lowes co osi e a e was ob ained in KNO3, despi e
ha ing ca ied ou he es a 350 °C, 30 °C highe han he NaNO3 es .
Scanning elec on mic oscopy s udies we e ca ied ou o he alloys imme sed in
li hium and po assium hyd oxides, since hey p esen ed a highe g a ime ic weigh
change. The supe icial image and EDX analysis (a ed ma k) o OC4 alloy, imme sed in
LiOH a 470 °C du ing 500 h, is shown in Figu e 7.
Elemen O Al Si C Mn Fe Ni Cu
Weigh (%) 7.06 0.44 0.27 7.51 1.86 51.67 23.69 3.08
Figu e 7. Supe icial image o OC4 alloy a e 500 h in LiOH a 470 °C.
The s udy e eals he p esen o Fe-C -O-Ni as main componen s in he co osion
scale. Some black spo s we e obse ed in he alloy and a de ailed cha ac e iza ion o his
sec ion was analyzed in Figu e 8, ob aining a highe con en in Al.
Figu e 6. G a ime ic co osion cu es o mol en sal s es ed du ing 500 h o imme sion.
Ene gies 2022,15, 3640 7 o 12
The highes co osi e po en ial was obse ed o KOH sal wi h a mass gain o
8.38 mg/cm
2
, which co esponds o a co osion a e o 0.186 mm/yea , ollow by LiOH,
NaNO
3
and KNO
3
. I is no ewo hy ha he lowes co osi e a e was ob ained in KNO
3
,
despi e ha ing ca ied ou he es a 350 ◦C, 30 ◦C highe han he NaNO3 es .
Scanning elec on mic oscopy s udies we e ca ied ou o he alloys imme sed in
li hium and po assium hyd oxides, since hey p esen ed a highe g a ime ic weigh
change. The supe icial image and EDX analysis (a ed ma k) o OC4 alloy, imme sed in
LiOH a 470 ◦C du ing 500 h, is shown in Figu e 7.
Ene gies 2022, 15, 3640 7 o 13
Figu e 6. G a ime ic co osion cu es o mol en sal s es ed du ing 500 h o imme sion.
The highes co osi e po en ial was obse ed o KOH sal wi h a mass gain o 8.38
mg/cm2, which co esponds o a co osion a e o 0.186 mm/yea , ollow by LiOH, NaNO3
and KNO3. I is no ewo hy ha he lowes co osi e a e was ob ained in KNO3, despi e
ha ing ca ied ou he es a 350 °C, 30 °C highe han he NaNO3 es .
Scanning elec on mic oscopy s udies we e ca ied ou o he alloys imme sed in
li hium and po assium hyd oxides, since hey p esen ed a highe g a ime ic weigh
change. The supe icial image and EDX analysis (a ed ma k) o OC4 alloy, imme sed in
LiOH a 470 °C du ing 500 h, is shown in Figu e 7.
Elemen O Al Si C Mn Fe Ni Cu
Weigh (%) 7.06 0.44 0.27 7.51 1.86 51.67 23.69 3.08
Figu e 7. Supe icial image o OC4 alloy a e 500 h in LiOH a 470 °C.
The s udy e eals he p esen o Fe-C -O-Ni as main componen s in he co osion
scale. Some black spo s we e obse ed in he alloy and a de ailed cha ac e iza ion o his
sec ion was analyzed in Figu e 8, ob aining a highe con en in Al.
Figu e 7. Supe icial image o OC4 alloy a e 500 h in LiOH a 470 ◦C.
The s udy e eals he p esen o Fe-C -O-Ni as main componen s in he co osion scale.
Some black spo s we e obse ed in he alloy and a de ailed cha ac e iza ion o his sec ion
was analyzed in Figu e 8, ob aining a highe con en in Al.
Ene gies 2022, 15, 3640 8 o 13
Elemen O Al Si Ca C Mn Fe Ni Cu
Weigh (%) 27.49 34.64 0.41 2.19 1.92 1.65 19.43 7.99 2.29
Figu e 8. Supe icial image magni ica ion o OC4 alloy a e 500 h in LiOH a 470 °C and mapping
analysis (Al).
This high aluminum con en in he su ace laye was demons a ed o be a mo e
e ec i e ba ie o he a ack and pene a ion o co osi e ions [24–26]. In his case he
aluminum con en was only de ec ed in hese speci ic black spo s, and he p o ec ion laye
is no uni o m. Du ing he co osion es s, some d awbacks we e de ec ed in he p oposal
o LiOH as PCM, especially ela ed o hei high iscosi y and he limi ed ope a ional
empe a u e ange, since LiOH s a s o decompose as soon as i mel s, ende ing i nec-
essa y o e ill he c ucible wo imes du ing he expe imen al es . Indeed, some mass loss
can be obse ed a a empe a u e as low as 420 °C unde ni ogen a mosphe e (Figu e 4),
less han 10 °C abo e he mel ing poin .
On he o he hand, he highes co osion a e was ob ained o he OC4 alloy in con-
ac wi h KOH (Figu e 5), despi e o he co osion expe imen was ca ied ou a a lowe
empe a u e (380 °C) compa ed he li hium hyd oxide sal .
The supe icial SEM image (Figu e 9) shows se e al co osion pa icles in he alloy
su ace. A magni ica ion o he co osion p oduc s ob ained in he alloy su ace is shown
in Figu e 10.
Figu e 8.
Supe icial image magni ica ion o OC4 alloy a e 500 h in LiOH a 470
◦
C and mapping
analysis (Al).
Ene gies 2022,15, 3640 8 o 12
This high aluminum con en in he su ace laye was demons a ed o be a mo e
e ec i e ba ie o he a ack and pene a ion o co osi e ions [
24
–
26
]. In his case he
aluminum con en was only de ec ed in hese speci ic black spo s, and he p o ec ion laye
is no uni o m. Du ing he co osion es s, some d awbacks we e de ec ed in he p oposal
o LiOH as PCM, especially ela ed o hei high iscosi y and he limi ed ope a ional em-
pe a u e ange, since LiOH s a s o decompose as soon as i mel s, ende ing i necessa y
o e ill he c ucible wo imes du ing he expe imen al es . Indeed, some mass loss can be
obse ed a a empe a u e as low as 420
◦
C unde ni ogen a mosphe e (Figu e 4), less han
10 ◦C abo e he mel ing poin .
On he o he hand, he highes co osion a e was ob ained o he OC4 alloy in
con ac wi h KOH (Figu e 5), despi e o he co osion expe imen was ca ied ou a a lowe
empe a u e (380 ◦C) compa ed he li hium hyd oxide sal .
The supe icial SEM image (Figu e 9) shows se e al co osion pa icles in he alloy
su ace. A magni ica ion o he co osion p oduc s ob ained in he alloy su ace is shown in
Figu e 10.
Ene gies 2022, 15, 3640 9 o 13
Elemen O Al Si K C Mn Fe Ni
Weigh (%) 5.02 1.36 0.48 3.67 8.40 1.60 50.30 28.63
Figu e 9. Supe icial image o OC4 alloy a e 500 h in KOH a 380 °C.
Elemen O Al Si K C Mn Fe
N
i
Weigh (%) 4.35 0.24 0.26 5.68 1.11 0.68 57.49 29.98
Figu e 10. Supe icial image magni ica ion o OC4 alloy a e 500 h in KOH a 380 °C, including
EDX analysis ( ed squa e).
The main con en o his co osion p oduc was iden i ied as Fe-Ni-O, including also
some po assium con en as an impu i y om he mol en sal . I is wo h men ioning ha
he C con en is signi ican ly lowe in hese pa icles han he a e age o he co osion
laye (Figu e 9).
The o ma ion o his co osion scale was esponsible o he highe weigh gain in
his sal , showing a non-uni o m scale g owing. The c oss-sec ion s udy (Figu e 11) con-
i med his beha io , showing some c acks and spalla ion in he scale o med.
Figu e 9. Supe icial image o OC4 alloy a e 500 h in KOH a 380 ◦C.
Ene gies 2022, 15, 3640 9 o 13
Elemen O Al Si K C Mn Fe Ni
Weigh (%) 5.02 1.36 0.48 3.67 8.40 1.60 50.30 28.63
Figu e 9. Supe icial image o OC4 alloy a e 500 h in KOH a 380 °C.
Elemen O Al Si K C Mn Fe
N
i
Weigh (%) 4.35 0.24 0.26 5.68 1.11 0.68 57.49 29.98
Figu e 10. Supe icial image magni ica ion o OC4 alloy a e 500 h in KOH a 380 °C, including
EDX analysis ( ed squa e).
The main con en o his co osion p oduc was iden i ied as Fe-Ni-O, including also
some po assium con en as an impu i y om he mol en sal . I is wo h men ioning ha
he C con en is signi ican ly lowe in hese pa icles han he a e age o he co osion
laye (Figu e 9).
The o ma ion o his co osion scale was esponsible o he highe weigh gain in
his sal , showing a non-uni o m scale g owing. The c oss-sec ion s udy (Figu e 11) con-
i med his beha io , showing some c acks and spalla ion in he scale o med.
Figu e 10.
Supe icial image magni ica ion o OC4 alloy a e 500 h in KOH a 380
◦
C, including EDX
analysis ( ed squa e).
Ene gies 2022,15, 3640 9 o 12
The main con en o his co osion p oduc was iden i ied as Fe-Ni-O, including also
some po assium con en as an impu i y om he mol en sal . I is wo h men ioning ha
he C con en is signi ican ly lowe in hese pa icles han he a e age o he co osion
laye (Figu e 9).
The o ma ion o his co osion scale was esponsible o he highe weigh gain in his
sal , showing a non-uni o m scale g owing. The c oss-sec ion s udy (Figu e 11) con i med
his beha io , showing some c acks and spalla ion in he scale o med.
Ene gies 2022, 15, 3640 10 o 13
Elemen O Al Si C Mn Fe Ni
Weigh (%) A 8.78 1.18 0.76 1.15 0.56 55.9 31.28
Weigh (%) B - 2.35 0.52 14.82 1.8 52.41 27.27
Figu e 11. C oss-sec ion image o OC4 alloy a e 500 h in KOH a 380 °C, including EDX analysis
a A-B poin .
The co osion p oduc s o med we e iden i ied h ough XRD, ob ained as he main
co osion p oduc s LiFeO
2
, Fe
2
O
3
( o LiOH) and NiO and Fe
3
O
4
( o KOH). Addi ionally,
some p o ec i e spinels wi h con en in aluminum and ch omium we e de ec ed.
The di ac og ams ob ained o KOH and LiOH a e shown in Figu e 12.
Figu e 12. XRD ob ained o OC4 alloy imme sed in KOH (le ) and LiOH ( igh ) mol en sal s a e
500 h.
The alloy imme sed in he LiOH sal p esen ed he o ma ion o FeC
2
O
4
and NiC
2
O
4
as he main p o ec i e componen s and we e esponsible o he highe co osi e e-
sis ance o his alloy, in compa ison wi h he KOH sal . Fo KOH, no p o ec i e spinels
we e de ec ed and he OC4 alloy gene a es NiO and Fe
3
O
4
as main oxida ion componen s.
Figu e 11.
C oss-sec ion image o OC4 alloy a e 500 h in KOH a 380
◦
C, including EDX analysis a
A-B poin .
The co osion p oduc s o med we e iden i ied h ough XRD, ob ained as he main
co osion p oduc s LiFeO
2
, Fe
2
O
3
( o LiOH) and NiO and Fe
3
O
4
( o KOH). Addi ionally,
some p o ec i e spinels wi h con en in aluminum and ch omium we e de ec ed.
The di ac og ams ob ained o KOH and LiOH a e shown in Figu e 12.
Ene gies 2022, 15, 3640 10 o 13
Elemen O Al Si C Mn Fe Ni
Weigh (%) A 8.78 1.18 0.76 1.15 0.56 55.9 31.28
Weigh (%) B - 2.35 0.52 14.82 1.8 52.41 27.27
Figu e 11. C oss-sec ion image o OC4 alloy a e 500 h in KOH a 380 °C, including EDX analysis
a A-B poin .
The co osion p oduc s o med we e iden i ied h ough XRD, ob ained as he main
co osion p oduc s LiFeO
2
, Fe
2
O
3
( o LiOH) and NiO and Fe
3
O
4
( o KOH). Addi ionally,
some p o ec i e spinels wi h con en in aluminum and ch omium we e de ec ed.
The di ac og ams ob ained o KOH and LiOH a e shown in Figu e 12.
Figu e 12. XRD ob ained o OC4 alloy imme sed in KOH (le ) and LiOH ( igh ) mol en sal s a e
500 h.
The alloy imme sed in he LiOH sal p esen ed he o ma ion o FeC
2
O
4
and NiC
2
O
4
as he main p o ec i e componen s and we e esponsible o he highe co osi e e-
sis ance o his alloy, in compa ison wi h he KOH sal . Fo KOH, no p o ec i e spinels
we e de ec ed and he OC4 alloy gene a es NiO and Fe
3
O
4
as main oxida ion componen s.
Figu e 12.
XRD ob ained o OC4 alloy imme sed in KOH (
le
) and LiOH (
igh
) mol en sal s a e 500 h.
