May 17 - 19, 2023, B no, Czech Republic, EU
PREPARATION OF PEO COATINGS ON AZ31 MAGNESIUM ALLOY IN THE PRESENCE OF
HUMIC ACID
Leoš DOSKOČIL, Pa lína ŠOMANOVÁ, Ja omí WASSERBAUER, Daniel KAJÁNEK,
Ma in BUCHTÍK
B no Uni e si y o Technology, Facul y o Chemis y, B no, Czech Republic, EU,
[email p o ec ed]
h ps://doi.o g/10.37904/me al.2023.4713
Abs ac
Magnesium ma e ials a e desi able o su ace ea men because o hei low co osion esis ance. The
p epa a ion o coa ings on AZ31 alloy by plasma elec oly ic oxida ion (PEO) in a ba h wi h and wi hou humic
acid was in es iga ed. The coa ings we e cha ac e ized using a scanning elec on mic oscope (SEM) wi h
ene gy dispe si e X- ay spec oscopy (EDS) and Fou ie ans o m in a ed spec oscopy (FTIR). The
co osion p ope ies we e es ed using he po en iodynamic pola iza ion me hod in 0.15 M NaCl. The esul s
show ha he PEO coa ings had signi ican ly highe co osion esis ance han he AZ31 alloy. The PEO coa ing
p epa ed in he p esence o 5 mg/l humic acid (10 min deposi ion) appea ed o ha e sligh ly be e co osion
esis ance han he PEO coa ing p epa ed in he absence o humic acid.
Keywo ds: Magnesium alloys, humic acid, applica ions, co osion
1. INTRODUCTION
Magnesium and i s alloys ha e good mechanical p ope ies such as good machinabili y and low densi y. A
nega i e ea u e o hese alloys is hei low co osion esis ance. Fo his eason, a ious me hods o su ace
ea men (e. g. supe hyd ophobic coa ings, High Veloci y Oxygen Fuel (HVOF) coa ing, Ni-P coa ing) a e
being in es iga ed o imp o e hei co osion esis ance [1].
One o hese su ace ea men s can be plasma elec oly ic oxida ion (PEO). In his p ocess, a coa ing o PEO
is applied om he elec oly ic ba h o he su ace o he sample unde he in luence o ol age and cu en .
The magnesium alloy ea ed in his way p o ides highe co osion esis ance han an un ea ed su ace. The
elec oly ic ba h is usually composed o alkali me al hyd oxides (NaOH, KOH) and simple ino ganic subs ances
such as phospha es, silica es, bo a e e c [2]. O ganic addi i es (e.g. glyce ol, phy ic acid, 8-hyd oxyquinoline,
annic acid) a e also used o modi y PEO elec oly es. Pan e al. [3] epo ed ha he in oduc ion o glyce ol
in o he ba h esul ed in a dense coa ing wi h mo e uni o m po es. Zhang e al. [4] ound ha he addi ion o
8-hyd oxyquinoline (HQ) o he PEO ba h caused he o ma ion o insoluble Mg(HQ)2 compound and educed
he po e size. Eche e y-Rendom e . al. [5] in es iga ed he e ec o hexame hylen e amine and manni ol on
he p ope ies o PEO coa ing. Thei esul s showed ha he as-p epa ed PEO coa ings had be e co osion
esis ance han hose wi hou he use o o ganic subs ances. The g owing in e es in he p epa a ion o PEO
coa ings on magnesium alloys in he p esence o o ganic addi i es is e idenced by a ecen ly published e iew
a icle by Kaseem e al. [6].
In his wo k, he elec oly ic ba h was modi ied by he addi ion o humic acid, which is cha ac e ized by i s
en i onmen al iendliness. Humic acids a e a mix u e o o ganic subs ances wi h a high molecula weigh and
a high con en o oxygen unc ional g oups (ca boxylic and hyd oxyl g oups) [7]. Humic acids a e insoluble in
wa e a a pH below 2. They occu na u ally in wa e , bu also in soil, pea and coal, om which hey can be
isola ed.
May 17 - 19, 2023, B no, Czech Republic, EU
The aim o his wo k was o p epa e a PEO coa ing in he p esence o humic acid on AZ31 magnesium alloy.
The e ec o he deposi ion ime and humic acid concen a ion on he esul ing co osion esis ance was
in es iga ed. Co osion es s we e ca ied ou by po en iodynamic pola iza ion in 0.15 M NaCl.
2. MATERIAL AND METHODS
2.1 Ma e ial
AZ31 magnesium samples we e used in his expe imen . The alloy elemen al composi ion was de e mined by
Glow-Discha ge Op ical Emission Spec oscopy (GDOES; Spec uma GDS 750, Spec uma Analy ik GmbH,
Ho , Ge many) and summa ized in Table 1. Dimensions o hese samples we e 50 mm × 20 mm × 5 mm. The
sample we e su ace ea ed by g inding wi h 800 and 1200 SiC pape , insed wi h deionized wa e and
isop opanol and d ied wi h ho ai . Humic acid was isola ed om Sou h Mo a ian ligni e.
Table 1 Elemen al composi ion o magnesium alloy AZ31
Con en (w .%)
Al
Zn
Mn
Si
Fe
Sn
Zn
Mg
AZ31
3.60
1.34
0.28
0.03
0.002
0.01
-
Bal.
2.2 Coa ing p epa a ion
The PEO coa ing was p epa ed by plasma elec oly ic oxida ion. The elec oly ic ba h consis ed o 12 g Na3PO4
12 H2O, 1 g KOH and a ious concen a ions o humic acid (0 o 50 mg/L). The olume o he p epa ed
elec oly ic ba h was 1.5 L. The g ound AZ31 alloy samples we e placed in he ba h and o med he anode.
S ainless s eel was used as he ca hode. The PEO coa ing was deposi ed unde cons an s i ing o he ba h
a a ol age o 630 V and a cu en o 1.303 mA. The alue o he applied DC cu en was de e mined by
calcula ion based on he a ea o he sample. The deposi ion ime anged om 5-15 min.
Fi s , PEO coa ings we e p epa ed in an elec oly ic ba h con aining 0, 5, 15, 25 and 50 mg/L humic acid o a
deposi ion ime o 10 min. Subsequen ly, ano he se ies o PEO coa ings we e p epa ed in a ba h con aining
0, 5 and 25 mg/L humic acid o 5 and 15 min. A e comple ion o he plasma elec oly ic oxida ion, he samples
we e emo ed om he elec oly ic ba h and insed wi h deionized wa e and isop opanol and d ied wi h ho
ai .
2.3 Cha ac e iza ion
SEM and EDS analysis
A scanning elec on mic oscope (SEM, ZEISS, EVO LS-10) wi h an ene gy dispe si e X- ay spec oscope
(EDS, Ox o d Ins umen s plc, Abingdon, UK) was used o cha ac e ize he su ace mo phology and elemen al
composi ion o he PEO coa ings.
PDP measu emen s
Po en iodynamic pola iza ion (PDP) measu emen s we e pe o med using a Bio-Logic VSP-300 po en ios a
(BioLogic, Seyssine -Pa ise , F ance). The measu ed samples we e used as wo king elec ode. The exposed
a ea o he measu ed samples was 1 cm2. The sa u a ed calomel elec ode (SCE) was used as he e e ence
elec ode and P mesh as he coun e -elec ode. A 0.15 M NaCl solu ion wi hou pH adjus men was used as
a co osi e medium in he co osion cell wi h a olume o 200 ml. Open ci cui po en ial (OCP) we e s abilized
du ing 60 min o exposu e. A e wa ds, PDP measu emen s we e pe o med in he po en ial ange om −150
mV o 500 mV s. OCP a a scan a e o 1 mV/s.
May 17 - 19, 2023, B no, Czech Republic, EU
FTIR spec ome y
Fou ie ans o m in a ed (FTIR) spec ome y (Nicole iS10 spec ome e ) was used o cha ac e ize humic
acid and PEO coa ings p epa ed in he p esence and absence o humic acid. Measu emen s we e pe o med
by a enua ed o al e lec ion (ATR) in he spec al ange 4000-400 cm-1 and a esolu ion o 1 cm-1 wi h an
a e age o 128 scans. A clean and d y diamond c ys al was used o he ATR measu emen s.
3. RESULTS AND DISCUSSION
SEM analysis showed ha he PEO coa ings p epa ed in he ba h wi h and wi hou humic acid a di e en
deposi ion imes had e y simila mo phologies. The e we e open po es and mic opo es on hei su ace, and
no mic oc acks we e obse ed. A la ge numbe o po es was obse ed when a highe concen a ion o humic
acid was used. Figu e 1 shows he ypical su ace appea ance o a coa ing p epa ed in he elec oly e ba h in
he absence o humic acid and in he p esence o 5 mg/L humic acid. A ows in he Figu e 1 show he a ea
o occu ence o po es/mic opo es. These coa ings had he bes co osion esis ance (see below) and we e
p epa ed a a deposi ion ime o 10 min. The simila i y o he mo phology o bo h samples was p obably ela ed
o he ac ha humic acids we e no inco po a ed in o he s uc u e o he PEO coa ing, as shown by he esul s
o FTIR analysis. On he o he hand, EDS analysis e ealed a highe ca bon con en (7.5 a . %) o he PEO
coa ing p epa ed in he p esence o humic acid in compa ed o he PEO coa ing p epa ed in a ba h wi hou
humic acid, which had 5.9 a . % ca bon. This sugges s ha a mino amoun o humic acid may ha e adso bed
on he coa ing su ace om he ba h. I is likely ha he low molecula ac ions o humic acid we e p e e en ially
adso bed.
Figu e 1 SEM analysis o PEO coa ings p epa ed in a ba h wi hou humic acid (le ) and wi h 5 mg/L humic
acid ( igh ). The deposi ion ime was 10 min
Po en iodynamic pola iza ion cu es we e measu ed in 0.15 M NaCl solu ion o un ea ed AZ31 alloy, PEO
coa ing and o PEO coa ings modi ied wi h humic acid in he amoun o 5 – 50 mg/L and a di e en deposi ion
imes o 5 min, 10 min and 15 min. The co osion pa ame e s such as co osion cu en densi y (ico ) and
co osion po en ial (Eco ) we e e alua ed and a e summa ized in Table 2.
F om he esul ing alues o co osion pa ame e s, i is e iden ha he AZ31 alloy ea ed wi h PEO shows
be e co osion esis ance han he un ea ed AZ31 alloy. The PEO coa ing wi hou humic acid was p epa ed
a di e en deposi ion imes (5, 10 and 15 min) o op imize he p epa a ion condi ions. The alues o co osion
pa ame e s show ha he op imal deposi ion ime was 10 min. Samples o PEO coa ing p epa ed in he ba h
wi h 5, 15, 25 and 50 mg/L humic acid we e p epa ed a 10 min o deposi ion ime. The bes alues o co osion
pa ame e s had samples wi h humic acid addi ion o 5 mg/L and 25 mg/L. Fo his eason, hese samples we e
May 17 - 19, 2023, B no, Czech Republic, EU
also p epa ed wi h deposi ion imes o 5 min and 15 min. F om he alues ob ained, i can be seen ha he
op imal deposi ion ime in e ms o co osion esis ance was 10 min, i 5 mg/L o humic acid was added o he
ba h. The imp o emen in co osion esis ance can be mainly indica ed by a posi i e shi in he co osion
po en ial compa ed o he PEO coa ing p epa ed wi hou humic acid.
Table 2 Co osion pa ame e s ob ained om PDP measu emen s. The abb e ia ion HA means humic acid
Sample
ico (µA/cm2)
Eco (V)
Concen a ion o HA
(mg/L)
Time o deposi ion (min)
Un ea ed AZ31
6.7 ± 0.6
-1.48 ± 0.02
0
5
0.66 ± 0.11
-1.66 ± 0.01
10
0.03 ± 0.01
-1.60 ± 0.02
15
0.04 ± 0.01
-1.56 ± 0.01
50
10
0.45 ± 0.09
-1.50 ± 0.02
25
5
0.32 ± 0.10
-1.70 ± 0.02
10
0.08 ± 0.02
-1.56 ± 0.04
15
0.04 ± 0.01
-1.55 ± 0.02
15
10
0.11 ± 0.01
-1.47 ± 0.03
5
5
0.16 ± 0.05
-1.69 ± 0.02
10
0.02 ± 0.01
-1.50 ± 0.01
15
0.04 ± 0.01
-1.53 ± 0.01
Figu e 2 FTIR spec um o humic acid ( ed), PEO coa ing (black) and PEO coa ing p epa ed in he p esence
o 5 mg/L humic acid (g ey)
The FTIR spec um o humic acid is shown in Figu e 2 and i s in e p e a ion was based on he li e a u e [3].
The hyd oxyl g oups we e loca ed in he egion 3390-3270 cm-1. The me hyl and me hylene g oups we e
loca ed in he 2901 and 2982 cm-1 bands. These alipha ic g oups we e also indica ed by he 1450 cm-1 band.
The band in he 1580 cm-1 egion was a ibu ed o a oma ics and he band in he 1380 cm-1 egion was
a ibu ed o me hyl a oma ics. The band a ound 1650 cm-1 was ela ed o ca boxyl g oups. Phenols and e he s
a ound 1230 cm-1 and alcohols a ound 1060 cm-1 we e also p esen in he spec a. A C-H dis o ion in he
May 17 - 19, 2023, B no, Czech Republic, EU
band 900-700 cm-1 co esponding o a oma ics was also ound in he spec a. The spec a o PEO coa ings
p epa ed in he ba h wi h and wi hou humic acid we e he same. This implies ha humic acid was no p esen
in he PEO coa ing. B oad bands cen e ed a ound 1000 cm-1 and 550 cm-1 can be a ibu ed o phospha e
g oups [4,5]. This is in ag eemen wi h he obse a ion o Hadzima e al. [7] who p esen ed ha he PEO
coa ing p epa ed om a ba h o Na3PO4 and KOH was composed o magnesium phospha e. Howe e , EDS
analysis indica ed a highe ca bon con en on he PEO coa ing ha was p epa ed in a ba h con aining humic
acid. This opens he way o expe imen s ela ed o he modi ica ion o he PEO coa ing by humic acid
adso p ion, which could p e en he access o ions o he su ace o he AZ31 alloy due o i s physicochemical
p ope ies.
4. CONCLUSION
The PEO coa ing p epa ed in a ba h en iched wi h 5 mg/L humic acid a a deposi ion ime o 10 min esul ed
in a e y simila alue o co osion cu en densi y as in he case o he PEO coa ing p epa ed wi hou humic
acid a he same deposi ion ime. Howe e , he co osion po en ial shi ed o a mo e posi i e alue. Thus, he
PEO coa ing p epa ed in he p esence o humic acid appea s o be mo e co osion esis an . I s p epa a ion
equi es u he expe imen s. One o he possible di ec ions conce ns he use o speci ic p ope ies o humic
acids o signi ican ly inc ease he co osion esis ance o he coa ing on he AZ31 alloy.
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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