PEO coatings with active protection based on in-situ formed LDH-
nanocontainers
M. Serdechnova
1
, M. Mohedano
1
, B. Kuznetsov
2
, C.L. Mendis
1
, M. Starykevich
3
, S.
Karpushenkov
2
, J. Tedim
3
, M.G.S. Ferreira
3
, C. Blawert
1
, M.L. Zheludkevich
1,3,*
1
Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straβe 1, 21502 Geesthacht,
Germany
2
Belarusian State University, Faculty of Chemistry, 4, Nezavisimosti avenue, 220030, Minsk, Belarus
3
Department of Materials and Ceramic Engineering, CICECO – Aveiro Institute of Materials, University of Aveiro,
3810-193 Aveiro, Portugal
Abstract
In the present work, for the first time Zn-Al layered double hydroxide (LDH) nanocontainers
were grown in-situ on the surface and in the pores of plasma electrolytic oxidation (PEO) layer
and then loaded with a corrosion inhibitor to provide an active protection. The developed LDH-
based conversion process ensures partial sealing of the pores and provides an effective corrosion
inhibition on demand leading to increased fault-tolerance and self-healing properties. The
structure, morphology and composition of the LDH-sealed PEO coatings on 2024 aluminum
alloy were investigated using SEM, TEM/FIB, XRD and GDOES. Electrochemical impedance
spectroscopy and scanning vibrating electrode techniques show a remarkable increase in the
corrosion resistance and fault tolerance when PEO coating is sealed with a LDH-inhibitor
treatment.
Keywords
AA2024; Plasma electrolytic oxidation; Layered double hydroxides; Corrosion inhibitor;
* Corresponding author: Prof. Mikhail Zheludkevich, mikhail.zheludkevich@hzg.de
Highlights
Zn–Al LDH was grown on PEO treated 2024 aluminum alloy using a conversion process
The loading of vanadate into LDH nanocontainers is achieved via an anionic exchange reaction
The in-situ grown LDH-nanocontainers confer remarkable active corrosion protection and
increased fault-tolerance to the PEO coatings
1. Introduction
Plasma electrolytic oxidation (PEO) is an advanced anodizing process which leads to the
formation of ceramic-like coatings on the surface of many light alloys. The coatings form on the
surface as a result of short-lived micro-discharges at high voltages in low-concentrated eco-
friendly electrolytes [
1
]. The oxide layers developed by PEO are usually hard, strongly-adherent
to the substrate and confer both corrosion and wear resistance [
2
].The properties of PEO layers
can be tuned for various applications such as biomedical, photocatalytic, thermal and decorative
playing with composition of electrolytes and electrical parameters [
3
,
4
,
5
]. In spite of many
advantages, the PEO coatings are usually composed of relatively porous layers as a result of
discharge breakdowns and gas evolution during the coating growth. Such an intrinsic porosity of
the layer often compromises the barrier properties of even relatively thick coatings. Moreover,
thicker is the coating larger are the pores in many cases. Several attempts have been made to
reduce or seal such porosity. Optimization of current/voltage regimes [
6
,
7
], changing the
electrolyte composition including systems with particles [
8
,
9
], different post-treatments [
10
,
11
] and
duplex coatings [
12
,
13
] were tried among other strategies. However, in spite of offering certain
improvement to the barrier properties none of these approaches ensures an active protection.
Without the active protection the acceptance of PEO coatings for many high demanding
applications such as aeronautics is limited. In previous works, a post-treatment immersion of
PEO coated Mg alloys into inhibitor containing solution was tried to achieve the active
protection. Ce
3+
[
14
] and 8-hydroxyquinoline [
15
] were used as corrosion inhibitors. Certain active
protection effect was reported though the important issues related to uncontrollable release of
inhibiting species were not considered.
Recently, layered double hydroxides (LDH) have been widely investigated as environmentally-
friendly containers for active corrosion protection of metals, in the form of conversion films
[
16
,
17
,
18
,
19
] and as inhibiting pigments being incorporated into the polymer coatings [
20
]. The
LDH particles offer a twofold effect absorbing excess of chloride anions from the corrosive
environment and releasing the corrosion inhibitors on demand [
21
,
22
,
23
]. The release of
incorporated inhibitors can also be triggered by local increase of pH at cathodic sites [
24
,
25
].
Typically LDHs are composed by positively-charged mixed metal M
II
-M
III
hydroxide layers and
interlayers filled by the charge compensating anions (A
y-
) and H
2
O molecules [
26
]. The general
formula for the most common LDHs is [M
II
1-x
M
III
x
(OH)
2
]
x+
(A
y-
)
x/y
·zH
2
O [
27
,
28
].
In several recent studies, LDH films were prepared on bare aluminum alloys as a result of
conversion process [
29
,
30
] to provide an additional active protection. The aluminum substrate
was immersed in a M
2+
containing solution leading to the formation of the LDH film, containing
M
2+
and Al
3+
. The LDH layer was successfully loaded with a corrosion inhibitor via anion-
exchange process forming a nano-structured layer next to the metal surface [
31
,
32
]. The
application of LDH-based conversion process for sealing the pores in a TSA anodic layer on
aluminum was also reported in a recent work [
33
]. In this case the LDH structures are formed in
the pores of anodic layer and on its surface. The barrier properties of the anodic coating are
drastically improved similarly to the hot-water sealing but an active protection effect is also
additionally achieved when vanadate anions are placed between the LDH layers via an ion-
exchange reaction. The superior performance of such a sealing process was demonstrated in
accelerated corrosion tests and using electrochemical methods.
In the present work for the first time we propose an inhibitor-containing LDH-based post
treatment for sealing the PEO layers aiming at additional active protection acting on demand and
improved barrier properties. For this purpose the PEO treated AA2024 aluminum alloy was
sealed with Zn-Al LDH and vanadate as an inhibitor (with possible efficiency of almost 100% in
comparison with chromate treatment [
34
]) was intercalated into the LDH galleries. This system
was chosen as a model one for the aluminum protection, due to the combination of effective
barrier properties, provided by PEO layer, and active protection, provided by LDH with
inhibitor. The structure, morphology and properties of the PEO-LDH layer were investigated
using diffraction and microscopic techniques, while the active corrosion protection and increased
fault tolerance were studied by combination of integral and localized electrochemical techniques.
2. Experimental
2.1. Chemicals
The chemicals used in this work are: zinc nitrate hexahydrate (Zn(NO
3
)
2
∙6H
2
O, >99%, CarlRoth,
Germany), ammonium nitrate (NH
4
NO
3
, >98.5%, Bernd Kraft, Germany), ammonia solution
(NH
3
H
2
O, 25%, Merck KGaA, Germany), sodium vanadium oxide (NaVO
3
, 96%, AlfaAesar,
Germany), sodium metasilicate (Na
2
SiO
3
, 44-47% SiO
2
, Sigma-Aldrich Chemie GmbH,
Germany), sodium hydroxide (NaOH, >99%, Merck KGaA, Germany), sodium dihydrogen
phosphate (Na
2
H
2
P
2
O
7
, 98%, Chempur, Germany), nitric acid (HNO
3
, 65%, Merck KGaA,
Germany) and sodium chloride (NaCl, 99.98%, Fisher Chemical, UK). The solvent was
deionized water.
2.2. Specimens preparation
The used substrate was 2024-T3 aluminum alloy with a nominal composition in wt.% : 3.8–4.9
Cu, 0.5 Fe, 0.1 Cr, 1.2–1.8 Mg, 0.3–0.9 Mn, 0.5 Si, 0.15 Ti, 0.25 Zn, 0.15 others and Al balance.
The samples were cut from sheets into 20 mm × 30mm × 2mm coupons. Prior to the PEO
processing, specimens were etched in 20 wt.% sodium hydroxide solution for 60 s, rinsed in
deionized water, desmutted in 65 wt.% nitric acid solution for 60 s, rinsed in deionized water
again and, finally, dried with warm air.
The PEO processing was conducted at a constant voltage of 400 V for 15 min using a pulsed DC
power supply with a pulse ratio of t
on
: t
off
=1ms : 9ms and a current density limit of 70 mA cm
-2
(rms). The electrolyte, containing 9 g L
− 1
Na
2
SiO
3
, 2g L
− 1
NaOH and 11g L
− 1
Na
2
H
2
P
2
O
7
dissolved in deionized water (methodology adopted from [
35
,
36
]), was continuously stirred during
the treatment and kept at 20 ± 2 °C by a water cooling system. The counter-electrode was made
of stainless steel. After PEO treatment, the specimens were rinsed in deionized water and dried
in warm air.
Zn-Al LDH-nitrate (LDH-NO
3
) were grown on the surface of PEO covered AA2024 using the
methodology adapted from the previous work on sealing TSA anodized aluminum [
33
]. Briefly,
Zn(NO
3
)
2
·6H
2
O (0.01 mol) and NH
4
NO
3
(0.06 mol) were dissolved in deionized water (100 ml);
then the pH of the solution was adjusted to 6.5 by drop wise addition of 1 % ammonia. The
specimens were immersed in the solution at 95 °C for 30 minutes under continuous stirring then
rinsed in deionized water and dried in air at room temperature.
The inhibitor loading was performed by anion-exchange reaction between the nitrate from LDH
galleries and vanadate from the solution. For the anion-exchange reaction 0.1 M NaVO
3
solution
(pH 8.4) was prepared. The specimens coated with PEO and sealed with LDH-NO
3
were
immersed in this solution at 50°C for 30 min. After the exchange, the samples were rinsed with
deionized water and dried at room temperature in air. The product of anionic-exchange reaction
between LDH-NO
3
and
vanadate in obtained and called LDH-VO
x
in frame of this work. Its
structure and properties for active corrosion properties for PEO covered AA2024 are further
discussed.
2.3. Techniques
Tescan Vega3 SB scanning electron microscope (SEM) equipped with energy dispersive X-ray
(EDX) spectrometer, operating at 17 kV was used to evaluate morphology and composition of
the coatings. Cross-sections were prepared using standard metallographic techniques.
Crystalline phases formed during PEO and LDH sealing were characterized using a PANalytical
X’Pert Powder diffractometer (XRD, Ni-filtered Cu Kα radiation, step size 0.02
o
, dwell time
~1.5 s) at room temperature.
