J. LI et al.: THE EFFECT OF A CR-FREE FINGERPRINT-RESISTANT PASSIV ATION FILM ON THE PERFORMANCE OF ...
511–520
THE EFFECT OF A Cr-FREE FINGERPRINT-RESISTANT
PASSIV ATION FILM ON THE PERFORMANCE OF HOT-DIP
55 % Al-Zn COATED STEEL
VPLIV PROTI PRSTNIM ODTISOM ODPORNEGA
PASIV ACIJSKEGA FILMA BREZ Cr NA LASTNOSTI
OPLA[^ENEGA JEKLA PREVLE^ENEGA S PREVLEKO IZ 55 %
Al-Zn IZDELANO S POSTOPKOM VRO^EGA POTAPLJANJA
Jian Li
1‡
, Zhanbiao Zhao
1‡
, Xingchang Tang
2
,YiWang
3
, Youzhi Cao
3
, Yang Li
3
,
Xiaofeng Yuan
3
, Deyi Zhang
3*
1
Gansu Jiu Gang Group Hongxing Iron and Steel Co., Ltd. Gansu 73500 China
2
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology,
Lanzhou 730050, China
3
School of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, China
Prejem rokopisa – received: 2024-04-22; sprejem za objavo – accepted for publication: 2024-07-02
doi:10.17222/mit.2024.1160
Cr-free fingerprint-resistant hot-dip 55 w/% Al-Zn coated steel (CFAZCS) is a upmarket steel plate product that is widely used
in consumer goods with high added value, such as LCD back panels for monitors, as well as various electrical and electronic
products. Passivation is a crucial process in the production of CFAZCS, greatly affecting the overall performance of the
CFAZCS product. A comprehensive evaluation of passivation films from different manufacturers on the performance of
CFAZCS can assist production enterprises in optimizing and controlling the product quality according to the requirements of
target customers. This article comprehensively tests and evaluates the performance of corrosion resistance, acid/alkali resis-
tance, anti-yellowing/blackening, paint adhesion, abrasion resistance, fingerprint resistance, and the conductivity of mainstream,
commercially available, Cr-free, fingerprint-resistant, passivation solutions, providing guidance for the selection of passivation
solutions in production processes.
Keywords: Cr-free fingerprint-resistant passivation film, hot dip 55 w/% Al-Zn coated steel, performance evaluation
Avtorji v ~lanku opisujejo potapljanje jeklene plo{~e v talini s 55 w/% Al in 45 w/% Zn za njeno za{~ito proti prstnim odtisom
(CFAZCS; angl.: Cr-free fingerprint-resistant hot-dip Al-Zn coated steel). Izbrano jeklo z visoko dodano vrednostjo se na trgu
zelo uporablja za hrbti{~a LCD panelov za monitorje, kakor tudi za razli~ne druge elektri~ne in elektronske proizvode.
Pasivacija je najbolj pomemben postopek pri proizvodnji CFAZCS, ki mo~no vpliva na lastnosti panelov in seveda posledi~no
na njihovo kon~no vrednost oz. ceno. Avtorji so izvedli obse`no ovrednotenje pasivacijskih filmov na CFAZCS izdelkih
razli~nih proizvajalcev, ki je pomagala pri optimizaciji in kontroli kakovosti te vrste izdelkov v skladu z zahtevami ciljanega
kupca. V tem ~lanku avtorji opisujejo obse`no testiranje in ovrednotenje proti-korozijskih lastnosti, odpornost proti kislinam in
lugom, odpornost proti porumenitvi in ~rnjenju, adhezijo barve (odpornost proti lu{~enju), odpornost proti prstnim odtisom in
prevodnost vseh glavnih proizvajalcev ustreznih in komercialno dosegljivih jeklenih izdelkov opla{~enih s tanko plastjo
(filmom) brez kroma in odporno proti prstnim odtisom. Rezultati analiz in ovrenotenje le-teh so slu`ili avtorjem tega ~lanka kot
vodilo za izbiro optimalnih re{itev postopka pasivacije in tudi celotnega proizvodnega procesa.
Klju~ne besede: proti prstnim odtisom odporen pasivacijski film brez kroma, vro~e potapljanje oziroma opla{~enje jeklene
plo{~e v talini s 55 w/% Al in 45 w/% Zn, ovrednotenje lastnosti
1 INTRODUCTION
Hot-dip 55 w/% Al-Zn coated steel (HDAZCS) is
widely used in industries such as household appliances,
the auto industry, power-transmission equipment, and the
construction industry due to its excellent corrosion resis-
tance and weather resistance. According to a study by
Global Info Research, the global market size of
HDAZCS was valued at $620 billion in 2023 and it is
projected to reach around $746 billion by 2030 with a
compound annual growth of 2.7 %. The coating layer of
HDAZCS consists of 55 % Al, 43.4 % Zn, and 1.6 % Si,
which is known for its corrosion resistance under high
temperature, smooth surface, and excellent appearance.
1,2
However, the chemical properties of metallic zinc are ac-
tive. If the hot-dipped coating layer does not undergo a
passivation treatment, it will quickly darken while gener-
ating white corrosion products, such as
(Zn(OH)
2
)
3
·ZnSO
4
·nH
2
O, NaZn
4
(SO
4
)(OH)
6
Cl·6H
2
O,
and Zn
5
(OH)
8
Cl
2
·H
2
O.
3,4
The passivation process is a
critical step in the production of HDAZCS, affecting the
overall performance of the HDAZCS product.
5
There are
primarily two types of passivation processes for the sur-
face of HDAZCS: Cr-containing and Cr-free passivation
processes. Due to the environmental toxicity associated
with Cr, the Cr-containing passivation process has gradu-
ally been phased out, and currently, most manufacturing
Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520 511
UDK 669.15:669.058.6 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(4)511(2024)
*Corresponding author's e-mail:
lzdeyizhang@gmail.com (Deyi Zhang)
‡Authors contributed equally to this work.

enterprises primarily employ Cr-free passivation pro-
cesses.
6,7
With the intensification of competition, down-
stream manufacturing enterprises increasingly require a
Cr-free passive film with superior performance. In addi-
tion to excellent corrosion resistance, HDAZCS with a
Cr-free passive film are also required to have good per-
formance in anti-yellowing/blackening, acid/base resis-
tance, paint adhesion, abrasion resistance, fingerprint re-
sistance, and conductivity.
Cr-free fingerprint-resistant hot-dip 55 w/% alumi-
num-zinc steels (CFAZCS) are generally used in indus-
tries such as LCD back panels for monitors, as well as
various electrical and electronic products, etc., and are
classified as upmarket hot-dip aluminum-zinc steel prod-
ucts.
8
Currently, the core producers of CFAZCS in thw
world include ArcelorMittal, Shougang, Nippon Steel,
POSCO, Baosteel, Pangang Group, JISCO, FE Steel,
BlueScope, Doowo Steel, et al. As a core material for the
production of CFAZCS, the Cr-free fingerprint-resistant
passivation solution is currently dominated by Henkel. In
order to break the single source of Cr-free fingerprint-re-
sistant passivation solution, some enterprises including
Desytek and Xinyu-Ecosil have actively developed
Cr-free fingerprint-resistant passivation solutions to pro-
vide alternatives for CFAZCS production enterprises.
However, due to the lack of actual usage data and com-
prehensive performance evaluations, these substitutes for
the Cr-free, fingerprint-resistant, passivation solution
produced by Henkel have not yet been widely applied to
CFAZCS products in mainstream steel enterprises.
Therefore, it is particularly important to comprehen-
sively evaluate and compare the performance of the
chromium-free fingerprint-resistant passivation solutions
produced by different manufacturers. This will provide
guidance for CFAZCS producers to select Cr-free, fin-
gerprint-resistant, passivation solutions based on cus-
tomer demands.
In this paper, the performance of Cr-free, finger-
print-resistant, passivation solutions produced by
Henkel, Desytek and Xinyu-Ecosil was investigated
comprehensively. The performance of corrosion resis-
tance, acid/alkali resistance, anti-yellowing/blackening,
paint adhesion, abrasion resistance, fingerprint resis-
tance, and conductivity of the passivation films was eval-
uated and compared based on the relevant China Na-
tional Standard (GB). This work will provide guidance
for relevant manufacturing enterprises in the selection of
passivation solutions and the adjustment of product per-
formance, helping them to improve product quality while
reducing production costs, thereby gaining a greater ad-
vantage in an intense market competition.
2 EXPERIMENTAL PART
2.1 Materials
The DC53D+AZ-type, hot-dip, aluminum-zinc-
coated carbon steel produced by JISCO was used as the
experimental substrate (thickness 1.0 mm, with Al and
Zn content in the surface coating at 55 % and 43.6 % re-
spectively). As required, the substrate metal was la-
ser-cut into specimens with a size of (150 × 100 × 1) mm
J. LI et al.: THE EFFECT OF A CR-FREE FINGERPRINT-RESISTANT PASSIV ATION FILM ON THE PERFORMANCE OF ...
512 Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520
Table 1: Composition and content of elements in passivation films
Elements (mol %) Fe C O N Si P S Al Zn Na
HG 0.749 46.901 16.869 29.398 0.656 0.222 0.458 0.294 4.36 0.094
DS 1.462 44.801 22.716 6.531 21.822 1.584 0.181 0.254 0.774 0.038
XSR 0.909 46.786 17.895 28.107 0.741 0.317 0.382 0.337 4.447 0.098
Table 2: Methods for performance testing and evaluation standards
Performance Testing method
Evaluation
methodology
Criterion of acceptability
Corrosion resis-
tance
Neutral Salt Spray Test, referring to GB/T 10125-2012, us-
ing salt spray chamber model LYW-015
Corrosion
Area/Rating
Corrosion area less than
5 % after 72 h
(GB/T 6461-2002)
Acid resistance
Referring to GB/T 13448-2006, immersion in HCl solution
with pH=4 at room temperature for 2 minutes
$E* measurement $E*  3
Alkali resistance
Referring to GB/T 13448-2006, immersion in NaOH solu-
tion with pH=12 at room temperature for 2 minutes
$E* measurement $E*  3
Anti- yellowing
Referring to GB/T 1740-2007, constant temperature and
humidity test chamber model HW-50L
$E* measurement $E*  3
Anti-blackening
Referring to GB/T 1740-2007, constant temperature and
humidity test chamber model HW-50L
$E* measurement $E*  3
Paint adhesion
Scratch method, referring to GB/T 13448-2006 and GB/T
9286-1998
Visual observation No peeling
Fingerprint resis-
tance
Coating with Vaseline at room temperature for 30 minutes $E* measurement $E*  3
Abrasion resis-
tance
Referring to GB/T 1768-2006, 500 cycles of friction
Abrasion marks
observation
—
Conductivity — Surface resistivity  0.8 m%

or (100 × 70 × 1) mm. The specimens were then
deburred using a trimming machine to remove edge
burrs, activated by soaking in acetone, rinsed thoroughly
with distilled water, and wipe dried with lint-free cotton
for subsequent use. The Cr-free fingerprint-resistant
passivation solutions were provided by Henkel
(Granocoat 621, HG), Desytek (DS981LX, DS), and
Xinyu-Ecosil (X220, XSR). The element composition
and contents in the passivation solution are listed in Ta-
ble 1. All other reagents used in the tests were of analyti-
cal grade.
2.2 Experimental Methods
The passivation solutions were uniformly coated to
the substrate surface using a coating bar (OSP-04, Ja-
pan), with the coating thickness (dry film) controlled at
0.8–1.2 g/m
2
, resulting in a physical film thickness of ap-
proximately 1.0 μm. The drying temperature was set at
130 °C, and the drying time was 10 minutes. Perfor-
mance testing and evaluation methods were described in
Table 2. The color difference ($E*) was calculated us-
ing the formula $E*=[($L)
2
+( $a)
2
+( $b)
2
]
1/2
, where
L, a, and b are the tristimulus color coordinates.
2.3 Characterization Methods
The surface morphology and 3D profile of the speci-
mens after the performance testing were observed using
a metallographic microscope (Leica DM2700 M), scan-
ning electron microscope (SEM, Hitachi SU8600), and
white-light interferometer (WLI, Mahr MarSurf WM
1003D). The thickness of the passivation film were ana-
lyzed using a glow-discharge optical emission spectrom-
eter (GD-OES, HORIBA GD-Profiler2). The elements
C, O, and N were determined using an Organic Element
Analyzer (OEA, EA1112, Thermo FlashSmart, Amer-
ica), while other elements were determined using Induc-
tively Coupled Plasma Mass Spectrometry (ICP-MS,
ELEMENT2, Thermo Scientific, America). The $E*
was measured using a colorimeter (KONICA CR-10
Plus).
3 RESULTS AND DISCUSSION
3.1 Morphological characterization of the passivation
film
Figures 1a to 1h show the metallographic images of
the surfaces of HDAZCS with and without the Cr-free,
fingerprint-resistant passivation film (CrFPF) coating.
The surface of the HDAZCS without passivation-film
coating exhibits obvious zinc-flower structures with den-
dritic stripes. The zinc flowers are formed by the natural
solidification process of the zinc layer during the pull-
ing-out process of the hot-dip, aluminum-zinc coating in
the zinc pot (Figures 1a and 1b).
9
After coating with
CrFPF, the zinc-flower structures on the surface of
J. LI et al.: THE EFFECT OF A CR-FREE FINGERPRINT-RESISTANT PASSIV ATION FILM ON THE PERFORMANCE OF ...
Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520 513
Figure 1: Metallographic images of the surface of HDAZCS without passivation film coating (a, b) and with CrFPF coating (HG: c, d; DS: e, f;
XSR: g, h); SEM image of a typical DS passivation film (i); Depth distribution curve of Al element content (j)

HDAZCS become less prominent, and the dendritic
stripes widen and form a 3D interconnected network
structure. The stripes of the HG and DS passivation films
exhibit rectangular microstructures that are intercon-
nected. The rectangular microstructure formed by the
HG passivation film is more regular than that formed by
the DS passivation film, but the spacing between the
stripes is larger in the HG passivation film than in the DS
passivation film (Figures 1c–1f). The stripes of the XSR
passivation film exhibit an irregular network structure,
with a higher density compared to the HG and DS
passivation films (Figures 1g and 1h). Figure 1i shows
an SEM image of the DS passivation film. It can be ob-
served that the thickness of the hot-dip aluminum-zinc
coating is approximately 20–40 μm, with the passivation
film covering its surface being around 1.0 μm thick.
Analysis of the longitudinal distribution of the Al con-
tent along the specimen surface was performed using
GD-OES. Since all three passivation solutions do not
contain Al elements, a sharp increase in the Al element
content indicates that the argon-ion beam has penetrated
the passivation film and reached the surface of the
hot-dip, aluminum-zinc coating. Therefore, the depth
corresponding to the initial sharp increase in Al element
content corresponds to the thickness of the respective
passivation film.
10
As shown in Figure 1j, when the
loading mass is 1.2 g/m
2
, the thickness of the HG, DS,
and XSR passivation films is 1.15 μm, 1.02 μm, and
0.91 μm, respectively.
3.2 Evaluation of corrosion resistance of passivation
films
Referring to the GB/T 10125-2012 (Artificial atmo-
sphere corrosion test – Salt spray test), specimens with
and without a passivation film coating were subjected to
a 72-h neutral salt-spray test. Simultaneously, the perfor-
mance of the specimens was evaluated using the method
specified in GB/T 6461-2002 (Rating of the specimens
and specimens corroded on metal substrates and metal
and other inorganic coatings). Based on the corrosion
area and appearance of the specimens after the test, the
protection rating (Rp) and appearance rating (RA) were
determined. After the 72-h neutral salt-spray test, the
surface of the specimens coated with the HG passivation
film exhibited a moderate color change due to the corro-
sion of the aluminum-zinc layer.
11
More than 1.6 % of
the total surface area of the specimen suffered from cor-
rosion due to the breakage of the passivation films, while
over 37 % of the total surface area exhibited a severe
color change due to the damage of the passivation
films.
12
The corresponding corrosion resistance rating of
the HG passivation film was 6/1 m A. The corrosion area
was less than 5 % after a 72-h test, indicating acceptable
corrosion-resistance performance for the HG passivation
film (Figure 2a). The specimen coated with the DS
passivation film showed only a small amount of pitting
corrosion on the surface after the salt spray test, with the
corroded area of the base Al-Zn coating for only 0.03 %
of the total area. There was no significant darkening area
caused by extensive damage to the passivation film. The
performance rating is 9/10 vs B, indicating good corro-
sion resistance (Figure 2b). The specimen coated with
the XSR passivation film displayed more pronounced
pitting corrosion and surface darkening compared to
specimens coated with HG and DS passivation films.
The corrosion area of the base Al-Zn coating exceeded
1.6 % of the total surface area, with the remaining sur-
face showing a severe color change due to extensive
damage to the passivation film, which covered over 25 %
of the total area. The performance rating was 5/1 x A.
Despite the corrosion area being less than 5 % after a
72-h test, the passivation film showed discoloration and a
deeper color, indicating a less-than-ideal corrosion resis-
tance performance. However, overall, the specimen still
met the requirements for corrosion resistance (Figure
2c).
The corrosion morphology caused by the neutral
salt-spray corrosion on the surface of passivation films
can be observed through metallographic images. The di-
ameters of the pitting corrosion pits on the surfaces of
the specimens coated with HG, DS, and XSR passivation
films were approximately 18–29 μm, 33 μm, and 15–23
μm, respectively. The specimen coated with the DS
passivation film has the largest diameter of pitting corro-
sion, but the quantity was relatively small. On the other
hand, the specimen coated with the DS passivation film
exhibited the densest distribution of pitting corrosion,
with individual pit diameters being the smallest (Figures
2d–2f). The blackened areas on the specimen surfaces
were observed by SEM. It was evident that the
passivation-film network on the surface of the specimen
coated with HG passivation film remained relatively in-
tact. However, it was noticeable that the underlying
Al-Zn coating was visible beneath the passivation film,
indicating that the neutral salt spray had caused thinning
of the passivation film, resulting in partial loss of protec-
tion in some areas (Figure 2g). The passivation film net-
work structure in the blackened areas of specimen coated
with DS passivation film had undergone slight damage,
but the protection remained intact (Figure 2h). Mean-
while, the passivation film in the blackened areas of the
specimen coated with XSR passivation film still exhib-
ited a network structure, but with reduced network den-
sity. In some areas, the network structure has disap-
peared, exposing the underlying Al-Zn coating and
weakening the protection of the substrate (Figure 2i).
These results indicated that DS passivation film exhibits
the best corrosion-resistance performance, while the
XSR passivation film shows the poorest corrosion-resis-
tance performance, although still meeting the national
standard requirements.
The morphological characteristics of the corroded ar-
eas were further determined using WLI. As shown in
Figure 2j, after 72 h of neutral salt-spray corrosion, the
J. LI et al.: THE EFFECT OF A CR-FREE FINGERPRINT-RESISTANT PASSIV ATION FILM ON THE PERFORMANCE OF ...
514 Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520

specimen coated with the HG passivation film exhibited
regular pitting corrosion (blue area) on the surface. The
depth of the pitting corrosion pits reached up to approxi-
mately 12 μm, covering 1.0 % of the observed area. The
depth of the pits exceeded the thickness of the
passivation film, indicating substantial damage to the
passivation film in the pitting-corrosion area. Addi-
tionally, the underlying Al-Zn coating began to corrode,
although the corrosion did not reach the surface of the
steel. The height variation of the passivation film outside
the pitting corrosion area was minimal, indicating effec-
tive protection provided by the passivation film. On the
other hand, the blackened area on the surface of the spec-
imen coated with the DSR passivation film exhibited a
corrosion morphology characterized by a mixture of strip
corrosion and pitting corrosion, with the maximum depth
of the corrosion pits reaching approximately 12 μm, cov-
ering 2.27 % of the observed area (Figure 2l). In con-
trast, the specimen coated with the DS passivation film
showed only a small amount of strip corrosion and pit-
ting corrosion, with the depth of the corrosion pits
around 5 μm and the corrosion area occupying only
0.44 % of the observed area (Figure 2k). These results
indicated that the DS passivation film provided superior
protection to the base Al-Zn coating. Tafel curves further
demonstrate that the DS passivation film exhibited supe-
rior corrosion resistance compared to both the HG and
XSR passivation films. As shown in Figure 3, the corro-
sion potentials of the specimens coated with HG, DS,
and XSR passivation films are –0.64, 0.18, and –0.64 V ,
while the corrosion current density for the specimens
coated with HG, DS, and XSR passivation films were
1.031×10
–4
, 6.346×10
–7
and 8.611×10
–5
Ac m
–2
, respec-
J. LI et al.: THE EFFECT OF A CR-FREE FINGERPRINT-RESISTANT PASSIV ATION FILM ON THE PERFORMANCE OF ...
Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520 515
Figure 2: Optical (a-c), metallographic (d-f), and WLI (j-l) images of the specimens coated with different passivation films after 72 hours of
salt-spray corrosion
Figure 3: Tafel curves of specimens coated with different passivation
films

tively (Figure 3). A higher corrosion potential and low
corrosion-current density indicate a stronger obstruction
against Cl
-
ions in the solution for coatings deposited on
specimens. This is advantageous for suppressing the oc-
currence of electrochemical corrosion because the in-
creased charge transfer resistance reduces the electron
transfer rate between the anode and the cathode, thereby
slowing down the corrosion reaction.
6,13
The corrosion
potentials of the specimens coated with passivation films
were generally higher than these of the specimen without
a passivation film, indicating that passivation films of
HG, DS, and XSR all contribute to enhancing the corro-
sion resistance of the zinc-aluminum coatings. Among
them, the corrosion resistance of the DS passivation film
was the best.
3.3 Evaluation of the acid/base resistance performance
of passivation films
Following the GB/T 13448-2006 (Test methods for
color-coated steel sheets and steel strips), the acid/base
resistance of the passivation films was tested. Addi-
tionally, according to the GB/T 11186.3-1989 (Measure-
ment method for coating color – Part 3: calculation of
color difference), the color difference ($E*) of the speci-
mens before and after testing was measured and calcu-
lated. A $E* value of  3 % is considered acceptable.
After the acid-resistance test, optical photographs
showed a slight darkening of the surfaces of specimens
coated with HG, DS, and XSR passivation films. The
$E* before and after testing respectively reached
0.78 %, 1.06 %, and 0.56 % for the HG, DS, and XSR
passivation films, all below the acceptable standard of
less than3%( Figure 4a–4c). Metallographic photo-
graphs revealed no significant changes on the surfaces of
specimens coated with the HG, DS, and XSR passivation
films after the acid-resistance test. The network structure
formed by the passivation film remained intact and clear.
In the DS passivation film, there were a few black spots
appearing, and the color of the protruding part of the
passivation film darkened slightly. Although the network
structure of the HG passivation film remained clear, the
color  of  the  protruding  parts  also  darkened  slightly.
Meanwhile, the XSR passivation film showed a minimal
change after the acid-resistance test (Figure 4d–4f). It
was evident that the XSR passivation film exhibited the
best acid resistance performance.
After the alkali resistance test, the optical photo-
graphs of specimens coated with HG, DS, and XSR
passivation films showed no significant changes com-
pared to before the test. The corresponding $E* were
1.22 %, 0.32 %, and 0.76 %, respectively, all below the
acceptable standard of less than3%( Figure 4g–4i).
Metallographic photographs revealed that the HG, DS,
and XSR passivation films all exhibited a large number
of uniformly distributed black spots after the alkali-resis-
tance test. This may be due to the reaction between some
metal oxides in the passivation solution and the alkaline
solution.
6
The black spots in the HG passivation film
J. LI et al.: THE EFFECT OF A CR-FREE FINGERPRINT-RESISTANT PASSIV ATION FILM ON THE PERFORMANCE OF ...
516 Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520
Figure 4: Optical (a-c) and metallographic (d-f) images of specimens coated with different passivation films after acid-resistance testing; Optical
(g-i) and metallographic (j-l) images of specimens coated with different passivation films after alkali resistance testing

were the most densely distributed but have smaller diam-
eters, whereas those in the XSR passivation film were
sparser but have larger diameters. The test results indi-
cated that the alkali-resistance performance of the DS
passivation film was superior to that of the XSR and HG
passivation films.
3.4 Evaluation of the anti-yellowing/blackening perfor-
mance of passivation films
Referencing GB/T 1740-2007 (Test method for resis-
tance to humidity and heat of coating films), the anti-yel-
lowing/blackening performance of the passivation films
was determined. Additionally, according to GB/T
11186.3-1989, the $E* was measured and calculated. A
$E* value of =3 % is considered acceptable. From Fig-
ure 5a–5c it can be observed that after 80 minutes of
standard anti-yellowing testing, the surfaces of the speci-
mens coated with HG, DS, and XSR passivation films all
exhibited varying degrees of yellowing. Among them,
the XSR passivation film showed the most pronounced
yellowing, with large areas of yellowing spots, which
were deeper in color. The DS passivation film showed
lighter and more uniform yellowing, while the HG
passivation film exhibited slight yellowing, but with lo-
calized areas of deeper yellowing spots (Figure 5a–5c).
The $E* before and after testing for the HG, DS, and
XSR passivation films reached 5.29 %, 2.51 %, and
5.25 %, respectively. Only the $E* value of the DS
passivation film was below the acceptable standard of
less than 3 %. The color differences of the HG and XSR
passivation films did not meet the acceptable standard.
From the metallographic photographs (Figure 5d–5f), it
can be observed that although the network structure of
the HG and XSR passivation films remained intact after
the yellowing resistance test, a large number of black
spots appeared within the passivation film. Compared to
the HG and XSR passivation films, the density of black
spots in the DS passivation film was lower, demonstrat-
ing superior yellowing resistance performance.
Figure 5g–5i presents the optical photographs of
specimens coated with HG, DS, and XSR passivation
films after the anti-blackening test. It can be observed
that the color of all the samples darkened, and numerous
darker black spots appeared after the blackening test.
The $E* before and after testing for the HG, DS, and
XSR passivation films reached 1.24 %, 1.94 %, and
4.44 %, respectively. The $E* values of the HG and DS
passivation films were both below the acceptable stan-
dard of less than 3 %, while the $E* value of the XSR
passivation film did not meet the acceptable standard of
less than 3 %. The metallographic photographs showed
that after the anti-blackening test, the black spots in the
XSR passivation film were the most pronounced and
dense, but the passivation film remained intact and still
provides protection to the substrate.
To investigate the thermal weight-loss behavior of the
different passivation films, the passivation solutions was
vacuum-dried at 80 °C for 12 h to remove the solvent.
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Figure 5: Optical (a-c) and metallographic (d-f) images of specimens coated with different passivation films after the yellowing-resistance test;
Optical (g-i) and metallographic (j-l) images of specimens coated with different passivation films after the blackening-resistance test

The obtained solids were ground into powder and sub-
jected to thermal weight-loss testing. As shown in Fig-
ure 6, when the temperature rose to 300 °C at a rate of
5 °C/min in an inert atmosphere, the powder obtained
from drying the HG passivation solution exhibited a slow
weight loss of 2.84 % below 96.3 °C. In the temperature
range of 96.3 to 152.7 °C, the first rapid weight-loss
zone was observed, reaching 9.39 % weight loss. Subse-
quently, in the range of 152.7 to 202 °C, the second slow
weight-loss zone was observed, with a weight loss of
3.37 %. The second rapid weight-loss zone occurred be-
tween 202 °C and 263 °C, with a weight loss of 7.82 %.
This was followed by intensified weight loss due to the
thermal decomposition and carbonization of the organic
passivation film, reaching a total weight loss of 39.52 %
at 300 °C
14
. The first rapid weight loss zone corresponds
to the dehydration of the film-forming agent, while the
second rapid weight loss zone is likely due to the decom-
position of the organic polymer film, which is the main
cause of yellowing in the HG passivation film. For the
powder obtained from drying the DS passivation solu-
tion, a continuous and slow weight loss of 7.2 % was
observed as the temperature remains below 246.4 °C, af-
ter which a sharp weight loss occurs. The slow weight
loss below 246.4 °C is attributed to the increasing poly-
merization degree of the main film-forming component,
the silane coupling agent.
15
The sharp weight loss ob-
served above 246.4 °C is due to the thermal decomposi-
tion and carbonization of the organic components. The
powder obtained from drying the XSR passivation solu-
tion exhibited a slow weight loss rate below 217.5 °C,
but a sharp weight loss occurred when the temperature
exceeded 217.5 °C. This indicated that the thermal sta-
bility temperature of the XSR passivation film was lower
than the test temperature of the yellowing resistance (240
°C), and the rapid yellowing of the passivation film
caused by its thermal decomposition resulted in its poor
yellowing-resistance performance.
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Figure 7: Metallographic (a-c) and WLI (d-f) images of specimens coated with different passivation films
Figure 6: Thermal weight-loss curves of different passivation films

3.5 Evaluation of the paint adhesion of the passivation
films
The evaluation of the paint adhesion of the
passivation films was conducted in accordance with
GB/T 13448-2006 (Test methods for color-coated steel
sheets and strips) and GB/T 9286-2021 (Cross-cut test
for paints and varnishes). Grid patterns were cut into the
specimen surface using a gridded knife, and then adhe-
sive tape was used to peel off the coating in the grid area
following the standard method. The area of coating de-
tachment was observed to assess the coating perfor-
mance of the passivation films. The metallographic im-
ages and surface morphology measured by WLI showed
clear grid marks in the grid areas of the HG, DS, and
XSR passivation films. The passivation films remained
intact, and no detachment of the passivation films was
observed (Figure 7). The results of the paint-adhesion
test indicated that the coating performance of the HG,
DS, and XSR passivation films all meet the standard for
grade 0, which complies with the performance require-
ment of  3.
3.6 Evaluation of the abrasion-resistance performance
of the passivation films
Evaluation and comparison of the abrasion-resistance
performance of passivation films were conducted follow-
ing GB/T 1768-2006 (Test method for wear resistance of
paints and varnishes – Part 2: rotating rubber abrasion).
The abrasion wheel used was calibrated with an elastic
rubber CS-10 wheel, with an abrasion load of 250 g. Af-
ter 500 cycles of abrasion on the Taber Abraser, the wear
amounts of specimens coated with the HG, DS, and XSR
passivation films were similar, measuring (3.5, 3.4, and
3.7) mg, respectively. Optical, SEM, and WLI images of
all passivation films after 500 cycles of abrasion testing
displayed varying degrees of damage with evident traces
of abrasion (Figure 8). Among them, the damage to the
XSR passivation film was the most pronounced, with the
passivation film nearly disappearing in the abrasion area.
HG and DS passivation films remained relatively intact
after the abrasion experiment, with over 80 % of the ar-
eas still protected by the passivation film. Metallographic
images revealed varying degrees of damage to the
passivation films after abrasion testing, but the passiv-
ation film network still existed, maintaining its protective
function on the Al-Zn coating. 3D-profile images show
that the scratches on the HG passivation film were more
densely distributed, but generally shallower, while those
on the DS passivation film were more sparse but deeper.
The results of the abrasion-resistance test indicated that
HG, DS, and XSR passivation films all exhibited good
abrasion-resistance performance, with the HG and DS
passivation films slightly outperforming the XSR
passivation film.
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Materiali in tehnologije / Materials and technology 58 (2024) 4, 511–520 519
Figure 8: shows Optical (a-c), SEM (d-f), and WLI (g-i) images of specimens coated with different passivation films

3.7 Other performance evaluations
The fingerprint-resistance performance is an impor-
tant index for evaluating the Cr-free, fingerprint-resistant
passivation film. Fingerprint marks are typically caused
by the sweat from operators’ fingers, so the finger-
print-resistance performance test used white Vaseline as
a simulated medium for human sweat. A small amount
of Vaseline was evenly applied to approximately half of
the sample surface using a clean cotton ball. After 30
minutes, the Vaseline on the surface was wiped off with
a clean cotton ball, and the color difference ($E*) of the
coated surface was measured, with $E*  3.0 considered
as the pass criterion. The color differences of the sam-
ples coated with the HG, DS, and XSR passivation films
after the fingerprint-resistance test were 0.41, 0.66, and
1.14, respectively, all less than 3.0, meeting the quality
requirements. However, the fingerprint-resistance perfor-
mance of the HG and DS passivation films was signifi-
cantly better than that of XSR passivation film.
A low surface resistance is beneficial for the rapid
transfer and diffusion of electrons, reducing electrostatic
buildup and lowering safety risks. Typically, the surface
resistance of CFAZCS is required to be less than
0.8 m%. The surface resistance of the samples coated
with the HG, DS, and XSR passivation films was mea-
sured using the Mitsubishi Electric MCP-T370 resistivity
meter. The results indicated that the surface resistance of
the samples coated with the HG, DS, and XSR passiv-
ation films was 0.04 m%, meeting the performance re-
quirements.
4 CONCLUSIONS
The evaluation results of the passivation films dem-
onstrated that the DS passivation film met all the stan-
dard requirements for corrosion resistance, acid/base re-
sistance, anti-yellowing/blackening, paint adhesion,
abrasion resistance, fingerprint resistance, and surface
resistance performances. Except for its poor anti-yellow-
ing performance, the HG passivation film met the stan-
dards for all other aspects. However, the anti-yellow-
ing/blackening performances of the XSR passivation
film did not meet the standards. The DS passivation film
showed optimal performance in corrosion resistance, al-
kali resistance, and anti-yellowing performances, while
the HG passivation film excelled in anti-blackening and
abrasion resistance. The XSR passivation film performed
best in acid resistance.
Acknowledgment
This work was supported by the Major Scientific and
Technological Special Project of Guizhou Province,
China (Grant No. 22ZD6GB019).
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