May 17 - 19, 2023, B no, Czech Republic, EU
EFFECT OF CITRIC ACID AS A CHELATING AGENT ON THE MICROSTRUCTURE AND
COMPOSITION OF HYDROTHERMALLY-PREPARED CAP COATINGS ON AZ31 MAGNESIUM
ALLOY
Lenka HORÁKOVÁ, Ja omí WASSERBAUER, Leoš DOSKOČIL, Ma ěj BŘEZINA,
Ma in BUCHTÍK
Ma e ials Resea ch Cen e, Facul y o Chemis y, B no Uni e si y o Technology, B no, Czech Republic, EU,
xckno licko a@ u b .cz
h ps://doi.o g/10.37904/me al.2023.4653
Abs ac
In his s udy, he e ec o ci ic acid as a chela ing agen on he mic os uc u e, chemical, and phase
composi ion o hyd o he mally p epa ed CaP coa ings on AZ31 magnesium alloy was obse ed. The coa ings
we e deposi ed in a eac ion mix u e a pH 5, 7, and 9, whe e he concen a ion o he chela ing agen was se
a 5, 50, and 500 mM, espec i ely. The mic os uc u e and chemical composi ion o all deposi ed coa ings
we e cha ac e ized by scanning elec on mic oscopy wi h an ene gy dispe si e spec ome e (SEM-EDS). The
measu ed da a we e compa ed wi h he modynamic calcula ions using Hyd a-Medusa chemical equilib ium
so wa e.
Keywo ds: AZ31 magnesium alloy, CaP coa ings, ci ic acid, chela ing agen , hyd o he mal syn hesis
1. INTRODUCTION
Magnesium-based ma e ials and magnesium alloys ha e gained much a en ion as biodeg adable me als due
o hei simila mechanical p ope ies o human bone, biocompa ibili y, and biodeg adabili y [1,2]. Calcium
phospha e (CaP)-based bioma e ials a e cu en ly e y p omising ma e ials. In addi ion, hei p ope ies can
be posi i ely in luenced by he use o chela ing agen s such as e hylenediamine e ace ic acid (EDTA),
ni ilo iace ic acid (NTA), ci ic acid, gluconic acid, e c [3]. These agen s p e en he p ecipi a ion o di alen
and i alen me al ca ions in alkaline media and o m a s able complex wi h Ca ions (Figu e 1) in which he
me al ions a e bound igh ly enough o p e en hem om eac ing u he [3-5]. Fu he mo e, hese eagen s
p e en he complex om u he eac ions ha may occu du ing p ocessing. The s uc u e o CaP coa ings
and he size o he c ys als a e u he in luenced by he eac ion empe a u e, eac ion ime, and pH alue
[4,6]. The aim o his s udy is o assess he e ec o ci ic acid as a chela ing agen on he s uc u e o CaP
coa ings p epa ed a a ious pH alues.
Figu e 1 Coo dina ion o me al ions wi h ci ic acid [3]
May 17 - 19, 2023, B no, Czech Republic, EU
2. EXPERIMENTAL
The samples o AZ31 magnesium alloy we e cu om o igin shee s o samples wi h dimensions o 20×20×5
mm. The elemen al composi ion o AZ31 magnesium alloy was he same as in ou p e ious s udies [6-7]. The
su ace o magnesium samples was g ound using #1200 SiC pape . Then, he samples we e insed in dis illed
wa e and e hanol and ai -d ied.
To uni y he su ace, and o p e en he excessi e dissolu ion o he Mg subs a es du ing he hyd o he mal
syn hesis, a s able and compa ible Mg(OH)2 laye was deposi ed on he su ace o g ound samples. Mg(OH)2
laye was p epa ed unde hyd o he mal condi ions in 2 mol/l NaOH solu ion a 120 °C o 12 hou s. A e his,
he samples we e insed in dis illed wa e and e hanol and ai -d ied [8].
The p epa a ion o he eac ion mix u e was ca ied ou a labo a o y empe a u e. The eac ion mix u e was
p epa ed by mixing 175 ml o solu ion A con aining 0.15 mol/l Ca(NO3)2·4H2O and ci ic acid wi h di e en
concen a ions (0, 5, 50, o 500 mM). Then, 125 ml o solu ion B con aining 0.14 mol/l o NH4H2PO4 was
d opwise added in o igo ously s i ed solu ion A. The pH alue was hen adjus ed o he pH alues 5, 7, o 9
using 1 mol/l NaOH solu ion.
Magnesium samples we e hung on a special holde co e ed wi h Te lon and we e ans e ed in o a 250 ml
Te lon-lined s ainless s eel p essu e essel con aining 150 ml o he p epa ed eac ion mix u e. The p essu e
essel was placed in o he Memme UF55 (Memme GmbH + Co.KG, Büchenbach, Ge many) elec ic o en
hea ed up o 120 °C o 150 minu es. A e he deposi ion, coa ed magnesium samples we e insed wi h
dis illed wa e , and alcohol, and ai -d ied.
The su ace mo phology and chemical composi ion o he p epa ed coa ings we e analyzed using Zeiss EVO
LS-10 scanning elec on mic oscope (SEM) (Ca l Zeiss L d., Camb idge, UK) wi h ene gy dispe si e
spec ome e (EDS) Ox o d Ins umen s Xmax 80 mm2 (Ox o d Ins umen s plc, Abingdon, UK). AZ ec
so wa e ( e sion 2.4, Ox o d Ins umen s, High Wycombe, UK) was used o analyze he ob ained EDS da a.
3. RESULTS AND DISCUSSION
Figu e 2 shows he SEM mo phology o deposi ed CaP coa ings upon he addi ion o he a ious
concen a ions o a chela ing agen (ci ic acid) o selec ed pH alues (5, 7, and 9). By compa ing he indi idual
s uc u es, we can see a signi ican e ec o he concen a ion o ci ic acid and pH alue on he mo phology
o CaP coa ings.
In he i s se ies, a CaP coa ing was p epa ed wi hou he addi ion o a chela ing agen . The coa ing was
p epa ed only om po assium dihyd ogen phospha e and calcium ni a e p ecu so s. As can be seen om
Figu e 2, a pH 5, 7, and 9, a inely s uc u ed coa ing consis ing o lamella c ys als was o med. By compa ing
he indi idual coa ings, i is clea ha he coa ing deposi ed a pH 5 is sligh ly di e en om he coa ing
p epa ed a pH 7 and 9. The coa ing deposi ed a pH 5 is ela i ely homogeneous om a mac oscopic poin
o iew, wi h no s uc u al de ec s (such as c acks, lakings, e c.). On a close inspec ion, a lamella subs uc u e
consis ing o ine ibbon-like CaP pa icles is e iden . A pH 7 and 9 (Figu es 2b-c), he coa ings we e simila ,
and no no iceable di e ence was obse ed. F om de ail, i is clea ha he coa ings a pH 7 and 9 a e sligh ly
dense han he coa ing deposi ed a pH 5 and he indi idual CaP lamellae a e coa se . In addi ion, a pH 9
(Figu e 2c), he p esence o 10-15 um ine globula o ma ions could be obse ed on he su ace o he coa ing.
These a e likely o be adhe en p ecipi a es o CaP pa icles om he coa ing mix u e. These o ma ions
a e composed o e y ine needles. The EDS analysis o all deposi ed coa ings wi hou ci ic acid
is summa ized in Table 1. Based on he a ailable li e a u e and he EDS analysis pe o med, i was ound ha
he coa ing deposi ed a pH 5 would mos closely co espond in composi ion o he icalcium phospha e (TCP)
phase o o mix u es o TCP and dicalcium phospha e dihyd a e (DCPD). In he case o he coa ing deposi ed
a pH 7 and 9, he a omic composi ion is be ween HAp and OCP (Table 1) [9,10].
May 17 - 19, 2023, B no, Czech Republic, EU
pH 5
pH 7
pH 9
0 mM
5 mM
50
mM
500
mM
Figu e 2 Su ace mo phology o p epa ed CaP coa ings
Table 1 EDS analysis o laye s p epa ed a pH 5, 7 and 9 wi hou ci ic acid
pH
Ca
P
O
O he elemen s
5
20.39
13.98
61.79
3.84
7
22.91
15.58
57.21
4.30
9
22.67
15.82
56.93
4.58
In o de o es ima e he o ma ion o CaP phases on he Mg su ace, a simula ion o possible chemical
equilib ia was pe o med using Hyd a-Medusa so wa e. Acco ding o he Hyd a-Medusa so wa e, DCPD;
HAp wi h HPO42-,H2PO4- (in mix u e); and HAp wi h HPO42- (in mix u e) a e o med a pH 5, 7, and 9
espec i ely unde equilib ium condi ions (Figu e 3a).
I can be due o he ac ha he Hyd a-Medusa so wa e indica es he p esence o only s able phases.
Deposi ion o CaP coa ings akes place unde non-equilib ium condi ions, which can lead o he o ma ion o
May 17 - 19, 2023, B no, Czech Republic, EU
uns able phases like TCP, OCP (oc acalcium phospha e), e c. Acco ding o he diag am om Hyd a-Medusa
so wa e (Figu e 3a-d), he deposi ed coa ing a pH 7 and 9 should consis o HAp phase. As desc ibed abo e
he OCP phase is no eco ded in he diag am due o equilib ium condi ions.
By he addi ion o 5 mM o ci ic acid in o he eac ion mix u e wi h pH 5, a coa ing consis ing o app ox. 30 μm
globula o ma ions we e deposi ed on he subs a e su ace (Figu e 2d). These o ma ions did no co e
he en i e su ace. S uc u al de ec s, mos ly c acks, could be obse ed. F om he de ail, i is clea ha hese
globula o ma ions ha e a lamella subs uc u e. A pH 7, he o med coa ing was e y hin (Figu e 2e).
F om he mac oscopic poin o iew, c acks in he coa ing and local peeling o pa s o he coa ing a e e iden .
A a close look, he laye is composed o ine lamellae. F om a mac oscopic poin o iew, no o med coa ing
can be obse ed on he subs a e deposi ed a pH 9 (Figu e 2 ). EDS analysis o all deposi ed coa ings
p epa ed by addi ion o 5 mM o ci ic acid is summa ized in Table 2. P obably a highe pH alue, he HAp
pa icles we e al eady p ecipi a ed in he eac ion mix u e and we e no deposi ed on he Mg subs a e.
Table 2 EDS analysis o laye s p epa ed a pH 5, 7 and 9 and concen a ion o ci ic acid 5 mM
pH
Ca
P
O
Mg
C
O he elemen s
5
20.88
18.70
60.42
0.00
0.00
0.00
7
19.36
15.09
61.78
3.22
0.00
0.55
9
0.00
0.29
62.56
33.09
3.85
0.21
Inc easing he concen a ion o 50 mM esul ed in a change o s uc u e. The coa ing deposi ed a pH 5
(Figu e 2g) o med globula o ma ions simila o he o ma ion in he case o 5 mM o ci ic acid (Figu e 2d).
Howe e , a close look shows ha he subs uc u e o he globula o ma ions is di e en . The globula
o ma ions did no o m a uni o m coa ing on he su ace o he subs a e. The su ace be ween he globula
o ma ions is co e ed by a hin ilm ha is se e ely dis u bed by c acks. The deposi ed coa ing a pH 7 o med
a ine, s ongly c acked ilm ha co e ed he subs a e su ace (Figu e 2h). A pH 9 (Figu e 2ch), as in he
p e ious case (pH 9, 5 mM o ci ic acid), no coa ing was o med.
Acco ding o he EDS esul s (Table 3), a pH 5, he coa ing is no o med by CaP, because he analysis did
no de ec he p esence o P. Highe con en o ca bon was also de ec ed. Acco ding o he Hyd a-Medusa
diag am, a pH 5, unde equilib ium condi ions, mo e ci a e compounds (p obably calcium ci a e) should
be o med in he mix u e, bu also he DCPD phase. I can be assumed ha he e was a o ma ion o ci a e
on he su ace. Based on EDS analysis o he deposi ed coa ing a pH 7 i was de e mined ha he deposi ed
coa ing is composed o he DCPD phase o a mix u e o DCPD and OCP. Acco ding o Hyd a-Medusa
(Figu e 3c), ci a e, and HAp should be o med a pH 7 and 9. The p esence o ci a e may be due o he
highe ca bon con en .
Table 3 EDS analysis o laye s p epa ed a pH 5, 7 and 9 and concen a ion o ci ic acid 50 mM
pH
Ca
P
O
Mg
C
O he elemen s
5
13.68
0.00
46.46
0.00
39.86
0.00
7
15.00
12.13
53.39
7.35
10.80
1.33
9
0.00
0.11
64.79
34.69
0.00
0.41
Inc easing he concen a ion o he addi ion o he chela ing agen ci ic acid o 500 mM led o a change in he
mo phology o he deposi ed coa ing (Figu e 2). The coa ing deposi ed a pH 5 o med a smoo h ilm
(Figu e 2i). No dis inc subs uc u e o he deposi ed coa ing can be obse ed om he de ailed image and he
coa ing may be o med on an o ganic basis based on i s appea ance. The SEM images e ealed dis inc
May 17 - 19, 2023, B no, Czech Republic, EU
s uc u al c acks. The deposi ed coa ing a pH 7 (Figu e 2j) was oughened and o med by c acks and po es.
The coa ing o med a pH 9 (Figu e 2k) was e y hin. The e was minimal o e lap o he g oo es om
he g ound Mg subs a e. Conside ing he esul s o he EDS analysis (Table 4), he deposi ed coa ing is mainly
composed o calcium ci a e phase and possibly a small amoun o Hap (Figu e 3d).
Figu e 3 The modynamic calcula ions om Hyd a-Medusa chemical equilib ium so wa e
Table 4 EDS analysis o laye s p epa ed a pH 5, 7 and 9 and concen a ion o ci ic acid 500 mM
pH
Ca
P
O
Mg
C
O he elemen s
5
7.41
5.36
50.25
0.50
30.01
6.47
7
10.96
5.21
50.10
0.63
23.77
9.33
9
2.21
1.50
49.98
0.31
29.11
16.89
4. CONCLUSION
Based on he abo e p esen ed indings, he ollowing conclusions can be d awn:
• Ci ic acid, as a chela ing agen , a ec s he s uc u e and composi ion o deposi ed coa ings - wi hou
he addi ion o he chela ing agen , he deposi ed CaP coa ings consis ed o lamella c ys als.
• The addi ion o 5 mM ci ic acid in he eac ion mix u e led o a change in he s uc u e o he CaP
coa ings. The o iginal lamella o ma ions ga e ise o coa se and dense globula o ma ions consis ing
o a pla e-like subs uc u e a pH 5. A pH 7, a coa ing wi h a e y ine subs uc u e was o med, bu
de ec s - c acks, laking - we e obse ed in his coa ing.
• Concen a ions o 50 and 500 mM ci ic acid in he eac ion mix u e did no lead o he o ma ion
o a homogeneous coa ing wi hou s uc u al de ec s a all. The deposi ed coa ings we e no con inuous,
hey we e o med by c acks and peeling occu ed. Mo eo e , i was shown ha a highe concen a ion
o ci ic acid in he eac ion mix u e led o he o ma ion o ci a e on he su ace.
• The addi ion o a chela ing agen caused ha a pH 9 no coa ing a all was o med on he su ace o he
Mg subs a e due o he p ecipi a ion o CaP al eady in he eac ion mix u e.

May 17 - 19, 2023, B no, Czech Republic, EU
• The coa ings deposi ed a pH 5 and 5 mM ci ic acid concen a ion in he eac ion mix u e showed he
mos compac s uc u e and can be expec ed o ha e he bes co osion p ope ies and biodeg adable
beha io .
ACKNOWLEDGEMENTS
This wo k was suppo ed by Speci ic Uni e si y Resea ch a FCH BUT, P ojec N . FCH-S-23-8208,
Minis y o Educa ion, You h and Spo s o he Czech Republic.
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