Surface pre-treatment of aluminium by cleaning, chemical ething and conversion coating

Abstract

Surface pre-treatment is one of the critical treatments for surface modification of aluminium (Al). In this study, pre-treatment of Al surface involved three stages; (1) cleaning (polishing and degreasing), (2) chemical etching (alkaline and acid) and (3) conversion coating (ie: zincate treatment). Cleaning process of Al was conducted by polishing and degreasing with acetone while etching process was done by immersion in 1.25 M NaOH solution (i.e: alkaline etching) followed with acid etching using 8 M HNO3 solution. The zincate treatment was conducted via electroless coating method by immersion of Al into a bath solution containing 0.5 M Zn(NO3)2, 0.1 M HNO3 and 0.2 M NaBH4 (reducing agent) for one hour. Different temperatures (ie: 25 °C, 50 °C, 75 °C, 90 °C) of bath solutions at pH 4 were used to investigate the effect of temperature on zincate treatment. Surface morphology and chemical composition of the pre-treated Al were characterized using Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersion X-ray analysis (EDX), respectively. The results showed that oxide layer on Al surface decreased after chemical etching process. Temperature of zincate solution has significantly affected the conversion coating process of aluminium. It was found that zinc oxide (ZnO) and zinc borate (ZnO.B2O3) were dominantly formed after zincate treatment at high temperature (ie:90 °C) with curved blade-like structure and composition of Zn, B and O with 13.70 wt.%, 3.52 wt.% and 54.39 wt.%, respectively. However, zincate treatment at low temperature (ie:<50 °C) has produced low metallic Zn.

Full text

Surface pre-treatment of aluminium by cleaning, chemical ething and conversion
coating
Mohammad Hafizudden Mohd Zaki, Yusairie Mohd, and Nik Norziehana Che Isa
Citation: AIP Conference Proceedings 1901, 120006 (2017);
View online: https://doi.org/10.1063/1.5010556
View Table of Contents: http://aip.scitation.org/toc/apc/1901/1
Published by the American Institute of Physics

Surface Pre-treatment of Aluminium by Cleaning, Chemical
Ething and Conversion Coating
Mohammad Hafizudden Mohd Zaki a), Yusairie Mohd b)
and Nik Norziehana Che Isa c)
Faculty of Applied Sciences, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia
b) Corresponding author:yusairie@salam.uitm.edu.my
a) hafizudden@yahoo.com
c) niknorziehanacheisa@yahoo.com
Abstract. Surface pre-treatment is one of the critical treatments for surface modification of aluminium (Al). In this study,
pre-treatment of Al surface involved three stages; (1) cleaning (polishing and degreasing), (2) chemical etching (alkaline
and acid) and (3) conversion coating (ie: zincate treatment).Cleaning process of Al was conducted by polishing and
degreasing with acetone while etching process was done by immersion in 1.25 M NaOH solution (i.e: alkaline etching)
followed with acid etching using 8 M HNO3 solution. The zincate treatment was conducted via electroless coating method
by immersion of Al into a bath solution containing 0.5 M Zn(NO3)2 , 0.1 M HNO3 and 0.2 M NaBH4 (reducing agent) for
one hour. Different temperatures (ie: 25 °C, 50 °C, 75 °C, 90 °C) of bath solutions at pH 4 were used to investigate the
effect of temperature on zincate treatment. Surface morphology and chemical composition of the pre-treated Al were
characterized using Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersion X-ray analysis (EDX),
respectively. The results showed that oxide layer on Al surface decreased after chemical etching process. Temperature of
zincate solution has significantly affected the conversion coating process of aluminium. It was found that zinc oxide (ZnO)
and zinc borate (ZnO.B2O3) were dominantly formed after zincate treatment at high temperature (ie:90 °C) with curved
blade-like structure and composition of Zn, B and O with 13.70 wt.%, 3.52 wt.% and 54.39 wt.%, respectively. However,
zincate treatment at low temperature (ie:<50 °C) has produced low metallic Zn.
INTRODUCTION
Aluminium (Al) is widely used in domestic and industrial purpose such as in cans, doors, automotive and
construction due to its low density and good mechanical properties. Today, Al is surpassed only by steel in its use as
structural material. Metallic Al is naturally covered with a protective oxide layer (ie: Al2O3) when exposed to
atmospheric environment. However, the presence of oxide layer on the Al causes difficulty in surface modification
and becomes a big challenge for researchers due to the high electrical resistance surface [1-5].
The natural oxide layer must be removed in order to get a good surface modification of Al [6]. Thus, surface pre-
treatment by cleaning and chemical etching is the most successful methods that can be used to remove the natural
oxide layer from the Al surface. In surface modification of Al, conversion coating is commonly introduced during
pre-treatment process in order to cover the etched Al surface with second metal. The conversion coating functions for
improving wear resistance, hardness, decorative purposes and to obtain high adhesive strength [7].
Chromate and phosphate are the most popular types of chemicals used for conversion coatings and had been used
globally for surface treatment of Al and its alloys. Nevertheless, the use of chromate and phosphate for conversion
coating has gradually reduced due to their highly toxicity to environment and carcinogenic nature [8]. Thus, a safer
conversion coating process should be used such as zincate. In this study, Al substrate has undergone a three-stage
surface pre-treatment process; cleaning, chemical etching and conversion coating and the effect of each process was
investigated.
Advanced Materials for Sustainability and Growth
AIP Conf. Proc. 1901, 120006-1–120006-6; https://doi.org/10.1063/1.5010556
Published by AIP Publishing. 978-0-7354-1589-8/$30.00
120006-1

EXPERIMENTAL
The experimental study of surface pre-treatment of aluminium (Al) was carried out in three stages; cleaning,
chemical etching and conversion coating. For cleaning stage, Al specimens of 2 cm x 2 cm were mechanically
polished using different grits of silica carbide (SiC) paper (P800, P1200, P2000). The polished Al specimens were
rinsed using distilled water and ultrasonically degreased in acetone for 15 minutes. For chemical etching process, the
degreased Al specimens were dipped into 1.25 M sodium hydroxide (NaOH) solution at 80 °C for 5 minutes for
alkaline etching process. The etched samples were rinsed with ultra-pure water. The samples were then undergone
acid etching (desmutting) process by immersion in solution containing 8 M nitric acid (HNO3) solution for one minute
at ambient temperature. The desmutted samples were washed with ultra-pure water. All etched Al specimens were
then undergone conversion coating process (i.e: zincate treatment) via electroless deposition by dipping the specimens
into zincate solution containing 0.5 M zinc nitrate (Zn(NO3)2), 0.1 M nitrate acid (HNO3) and 0.2 M sodium
borohydride (NaBH4) as reducing agent. The zincate treatment was carried out at various temperatures of 25 °C, 50
°C, 75 °C and 90 °C at pH 4 for one hour, followed by rinsing with ultra-pure water and dried with acetone. The surface
morphology of the treated specimens were visualized and imaged using Field Emission Scanning Electron Microscope
(FESEM, Carl Zeiss SMT Supra 40VP) while the chemical composition of samples were analyzed by Energy
Dispersive X-Ray spectroscopy (EDX) controlled by a software (the Oxford INCA X-max 51-XMX 0021).
RESULTS AND DISCUSSION
The surface morphology of Al after the cleaning process and chemical etching (alkaline and acid etching) was
studied by FESEM analysis and the results obtained are shown in Figure 1 and Figure 2, respectively.
FIGURE 1. FESEM images of Al surface (a) before and (b) after polished (5000x magnification)
Figure 1(a) shows a rough surface of unpolished Al with imperfection due to contamination which can be extrinsic;
composed of organic debris, mineral and dust from the environment. After polishing process and degreasing with
acetone, the Al surface became smoother and free from the larger imperfections as in Figure 1(b). Only fine
intermetallic particles appear as white spots along with scratches on the Al surface due to the presence of oxide layer.
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FIGURE 2. FESEM images of Al samples after (a) alkaline etching and (b) acid etching (5000x magnification).
Following the etching process, the surface roughness of Al increased. Besides that, it can be seen that the
intermetallic particles of Al surface became more obvious after alkaline etching (Figure 2(a)). The scalloped
appearance was also revealed after alkaline etching [6]. While, surface like grain boundary was formed after acidic
cleaning with HNO3 solution (Figure 2 (b)) and it can be related to the removal of intermetallic particles from the
surface due to local dissolution of Al [9,10]. The surface morphology of Al samples has significantly changed after
alkaline etching and acidic cleaning. The chemical composition of samples was analyzed using Energy Dispersive X-
Ray spectroscopy (EDX). The weight percentages of various chemical composition elements on Al substrates are
tabulated in Table 1.
It can be seen in Table 1 that the presence of oxygen (O) content was high with 7.14 wt% before alkaline etching
indicating the presence of oxide layer (Al2O3) on the Al surface. After alkaline etching, oxygen content on the Al
substrate was lower than untreated Al.
TABLE 1. The chemical composition of Al after cleaning and etching processes
Process
Elements (weight%)
Al
O
C
Cleaning
87.56
7.14
5.29
Alkaline Etching
90.89
6.44
2.67
Acidic Cleaning
97.47
1.35
1.35
Also, the composition of intermetallic particle (i.e:carbon (C)) significantly decreased after alkaline etching. The
decreasing of intermetallic particle after alkaline etching showed that the oxide layer was removed during the process
[6]. Nevertheless, the oxygen content was still high with 6.44 wt% after alkaline etching. It may be attributed to the
formation of corrosion product namely aluminium hydroxide (Al(OH)3) during alkaline etching [10]. The chemical
reactions involved on the Al surface during alkaline etching are shown in Equation (1)-(3)[9]:
ʹܱܪି൅͵ܪ
ଶܱ ՜ ʹሺܣ݈ሺܱܪሻସሻି (1)
ʹܣ݈ ൅ ʹܱܰܽܪ  ൅  ʹܪଶܱ՜ʹܰܽܣ݈ܱ
ଶ൅͵ܪ
ଶ ՛ (2)
ܰܽܣ݈ܱଶ൅ʹܪ
ଶܱ՜ܣ݈ሺܱܪሻ
ଷ՝  ൅ܱܰܽܪ (3)
Alkaline-based etching process on the Al surface to produce sodium aluminate (NaAlO2) and hydrogen gas (H2)
is based on Equation 2. In this reaction, excess free NaOH is required in the etching solution to provide a powerful
force for the reaction. The chemical reaction of Al etching with NaOH etchant is mainly completed after three steps
in which NaOH attacks the Al. The etchant was reacting with the material surface in which electron transfer in starts
to occur. Lastly, the corroded Al ions and the elements of etched materials diffuse into the etching solution [13].
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Meanwhile, it was found that, the oxygen content was decreased to 1.35 wt% and Al content was increased to 97.47
wt% after acidic cleaning (ie:desmutting). It was found that,the oxygen content is still present after acidic cleaning
because the oxide film can easily form within seconds when exposed to air, continuation of growth due to environment,
being accelerated by increasing the humidity and temperature [1]. The intermetallic particle (i.e: C) content decreases
after the acidic cleaning showing that the intermetallic particle was removed from the Al surface [10,11].
The increase of Al and the decrease of oxygen content after acidic cleaning can be explained using Equation (4).
The equation shows the chemical reactions occurring on the Al surface during cleaning process [11,12]. The acidic
cleaning process has been proven to remove corrosion products formed on Al surface and also the intermetallic
particles after alkaline etching. Thus, the composition of Al increased with decreasing oxygen content.
ܣ݈ሺܱܪሻଷ ൅͵ܪܱܰ
ଷ՜ܣ݈ሺܱܰ
ଷሻଷ൅͵ܪ
ଶܱ (4)
For conversion coating of etched Al, the effect of different temperature of zincate solutions containing Zn(II) ions
at a fixed pH 4 was investigated for 1 hour immersion time with the presence of NaBH4 as reducing agent. The surface
morphology of conversion coatings formed after zincate treatment was imaged by FESEM. The FESEM images of
Zn deposits obtained at different zincate solution temperatures are shown in Figure 3. It can be seen that the surface
morphology of Al (Figure 3(a)) after zincate treatment at 25 °C has not significantly changed as compared to after
acidic cleaning (Figure 2(b)).
Meanwhile, after conversion coating at 50 °C, the Al surface has gradually changed to form sphere-like spots as
shown in Figure 3(b). The surface morphology was significantly modified to form crack dry riverbed-like structure
with initial germinated curved blade-like when the temperature increased to 75 °C as shown in Figure 3 (c). However,
the increase of temperature to 90 °C enables a clear change in the surface morphology. Compact and uniform like
curved blade morphology was covering the surface of the treated Al as shown in Figure 3 (d).
The chemical composition of Al surface after zincate treatment at various temperatures are shown in Table 2.
Analysis of the surface reveals that aluminium (Al), zinc (Zn), oxygen (O), carbon (C) and boron (B) are present as
the major elements.
FIGURE 3. FESEM images of Al after zincate treatment at pH 4 solution for 1 hour at (a) 25 oC, (b) 50 oC, (c) 75 oC and (d)
90 oC (5000x magnification)
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TABLE 2. The chemical composition of Al after zincate treatment at different temperatures
Temperature (
o
C)
Element (Weight %)
Al
Zn
O
C
B
25
94.07
0.00
2.77
3.16
0.00
50
93.91
0.42
2.29
3.38
0.00
75
35.29
12.01
52.70
0.00
0.00
90
26.72
13.70
54.39
1.67
3.52
From the results obtained, the presence of Zn, O and B could be accounted for the formation of metallic zinc (Zn),
zinc borate (ZnO.B2O3) and zinc oxide (ZnO) on the Al surface. The presence of Al was originated from the base
metal and because of conversion coating itself, which is not sufficiently thick or not fully covered [15]. Zn is not
detected after 1 hour conversion coating at 25 °C indicating that no Zn was formed on the treated Al surface. As the
temperature increased to 50 °C, the Zn content was 0.42 wt.% and O content decreased from 2.77 wt.% to 2.29 wt.%,
implying the formation of Zn metal on the treated Al surface.
In contrast, the O content increased to 52.70 wt.% as the temperature increased to 75 °C and the amount of Zn
also increased as much as 12.01 wt.%. Thus, it can be assumed that ZnO was formed on the treated Al surface at 75
°C. At 90 oC, the amount of Zn ascended with the presence of B implying the possible formation of ZnO and ZnO.B2O3.
The increase of zincate solution temperature will increase the speed of plated metals diffusion on the Al surface [14].
However, with increasing temperature, Zn was prone to react with water to generate Zn(OH)2 . Equations (5)-(8) show
the possible chemical reactions occurring during zincate treatment.
ܼ݊ଶା ൅ʹ݁ି՜ ܼ݊(5)
ܼ݊ ൅  ʹܪଶܱ ՜ ܼ݊ሺܱܪሻଶ ൅ܪ
ଶ ՛    (6)
ܼ݊ሺܱܪሻଶ՜ ܼܱ݊ ൅ ܪ
ଶܱ(7)
ܼܱ݊ ൅ Ͷܪଶܱ൅ʹܤܪ
ସ
ି՜ܼ݊ͲǤܤ
ଶܱଷǤܪ
ଶܱ ൅ ͳͶܪା൅ ͳ͸݁ି(8)
CONCLUSIONS
The surface pre-treatment of Al was carried out using three different procedures. Each pre-treatment procedure
has affected the Al surface morphology and composition. The oxide layer on Al surface after acid etching process
decreased to 1.35 wt.% compared to 7.14 wt.% after cleaning and degreasing processes. It showed that chemical
etching process caused the natural oxide layer deterioration and a degree of oxide layer was removed. For conversion
coating process, zincate solution temperature has affected the formation of zinc products on Al surface. The amount
of coating deposited on the Al surface is increased with increasing zincate solution temperature. It was found that Zn,
ZnO and ZnO.B2O3 were formed on the pre-treated Al surface when zincate treatment was carried out at 90 oC.
Meanwhile, zincating at 50 oC has produced low metallic Zn deposit on Al surface and no Zn was detected after
zincate treatment at 25 oC.
ACKNOWLEGMENTS
The authors wish to acknowledge the Ministry of Higher Education (Malaysia) for the financial support through
Fundamental Research Grant Scheme (FRGS) 600-RMI/FRGS 5/3 (139/2015) and Faculty of Applied Sciences,
Universiti Teknologi MARA (UiTM) Shah Alam, Selangor for the facilities provided.
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