Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy
The 5083 aluminum alloy surfaces were coated with chromate conversion coatings. Then epoxy paints were applied on the chromate and non-chromate treatment surfaces. The adhesion, hardness and corrosion resistance of the coated samples were evaluated using adhesion tape test, pencil hardness test, sal...
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України
2012
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| Цитувати: | Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy / Eshaghi Ameneh, Eshaghi Akbar // Фізико-хімічна механіка матеріалів. — 2012. — Т. 48, № 2. — С. 47-50. — Бібліогр.: 10 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
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irk-123456789-1393532018-06-21T03:06:54Z Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy Eshaghi Ameneh Eshaghi Akbar The 5083 aluminum alloy surfaces were coated with chromate conversion coatings. Then epoxy paints were applied on the chromate and non-chromate treatment surfaces. The adhesion, hardness and corrosion resistance of the coated samples were evaluated using adhesion tape test, pencil hardness test, salt spray and electrochemical technique. The results indicated that the adhesion of the coating systems including chromate treatments was better than that of non-chromate conversion coatings, whereas the hardness of the coating systems was equal. Salt spray results showed that corrosion propagation from the scratched area in the painted samples with chromate treatment was lower than that of nonchromate coated surfaces. In addition, electrochemical corrosion measurements indicated that the coating system including chromate treatment presented lower corrosion current density, bigger polarization resistance and less negative corrosion potential than those of the non-chromate conversion coated surfaces. Досліджено вплив хроматної конверсійної обробки алюмінієвого сплаву 5083 на адгезію епоксидних лакофарбових покривів та їх захисні властивості. Встановлено, що адгезія покривів по поверхні сплаву з хроматним конверсійним шаром більша, ніж за безхроматної обробки, а їх твердість в обох випадках однакова. Корозія металу під плівкою біля дефекту лакофарбового покриву за умов попереднього хроматування протікає не так інтенсивно, як після безхроматної обробки. Зразки сплаву зі системою покриву, яка охоплювала хроматну обробку, мали нижчі густини струму корозії, більший поляризаційний опір та менш негативний потенціал корозії, ніж безхроматно оброблені. Исследовано влияние хроматной конверсионной обработки алюминиевого сплава 5083 на адгезию эпоксидных лакокрасочных покрытий и их защитные свойства. Установлено, что адгезия покрытий по поверхности сплава с хроматным конверсионным слоем больше, нежели после безхроматной обработки, а их твердость в обеих случаях одинакова. Коррозия металла под пленкой возле дефекта лакокрасочного покрытия после предварительного хромирования протекает не так интенсивно, как после безхроматной обработки. Образцы сплава с системой покрытия, которая охватывала хроматную обработку, имели меньшую плотность тока коррозии, большее поляризационное сопротивление и менее негативный потенциал коррозии, нежели безхроматно обработанные. 2012 Article Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy / Eshaghi Ameneh, Eshaghi Akbar // Фізико-хімічна механіка матеріалів. — 2012. — Т. 48, № 2. — С. 47-50. — Бібліогр.: 10 назв. — англ. 0430-6252 http://dspace.nbuv.gov.ua/handle/123456789/139353 en Фізико-хімічна механіка матеріалів Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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The 5083 aluminum alloy surfaces were coated with chromate conversion coatings. Then epoxy paints were applied on the chromate and non-chromate treatment surfaces. The adhesion, hardness and corrosion resistance of the coated samples were evaluated using adhesion tape test, pencil hardness test, salt spray and electrochemical technique. The results indicated that the adhesion of the coating systems including chromate treatments was better than that of non-chromate conversion coatings, whereas the hardness of the coating systems was equal. Salt spray results showed that corrosion propagation from the scratched area in the painted samples with chromate treatment was lower than that of nonchromate coated surfaces. In addition, electrochemical corrosion measurements indicated that the coating system including chromate treatment presented lower corrosion current density, bigger polarization resistance and less negative corrosion potential than those of the non-chromate conversion coated surfaces. |
| format |
Article |
| author |
Eshaghi Ameneh Eshaghi Akbar |
| spellingShingle |
Eshaghi Ameneh Eshaghi Akbar Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy Фізико-хімічна механіка матеріалів |
| author_facet |
Eshaghi Ameneh Eshaghi Akbar |
| author_sort |
Eshaghi Ameneh |
| title |
Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy |
| title_short |
Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy |
| title_full |
Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy |
| title_fullStr |
Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy |
| title_full_unstemmed |
Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy |
| title_sort |
effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy |
| publisher |
Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| publishDate |
2012 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/139353 |
| citation_txt |
Effect of chromate conversion coatings on the adhesion and corrosion resistance of painted 5083 aluminum alloy / Eshaghi Ameneh, Eshaghi Akbar // Фізико-хімічна механіка матеріалів. — 2012. — Т. 48, № 2. — С. 47-50. — Бібліогр.: 10 назв. — англ. |
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Фізико-хімічна механіка матеріалів |
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47
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2012. – ¹ 2. – Physicochemical Mechanics of Materials
EFFECT OF CHROMATE CONVERSION COATINGS ON THE ADHESION
AND CORROSION RESISTANCE OF PAINTED 5083 ALUMINUM ALLOY
AMENEH ESHAGHI 1, AKBAR ESHAGHI 1, 2
1 Sari Branch, Islamic Azad University, Sari, Iran;
2 Maleke Ashtar University of Technology, Shahinshahr, Isfahan, Iran
The 5083 aluminum alloy surfaces were coated with chromate conversion coatings. Then
epoxy paints were applied on the chromate and non-chromate treatment surfaces. The ad-
hesion, hardness and corrosion resistance of the coated samples were evaluated using ad-
hesion tape test, pencil hardness test, salt spray and electrochemical technique. The results
indicated that the adhesion of the coating systems including chromate treatments was
better than that of non-chromate conversion coatings, whereas the hardness of the coating
systems was equal. Salt spray results showed that corrosion propagation from the scra-
tched area in the painted samples with chromate treatment was lower than that of non-
chromate coated surfaces. In addition, electrochemical corrosion measurements indicated
that the coating system including chromate treatment presented lower corrosion current
density, bigger polarization resistance and less negative corrosion potential than those of
the non-chromate conversion coated surfaces.
Keywords: adhesion, aluminum, chromating, corrosion, paint.
Aluminum-magnesium alloys of the 5xxx series are commonly used in marine
applications where low density, favorable mechanical properties and relatively good
corrosion resistance are desired [1, 2]. The electrochemical behavior of Al and its
alloys has attracted the attention of many investigators. However, the natural oxide
film on aluminum alloys does not offer sufficient protection against aggressive anions
in seawater environment such as chloride ions leading to the formation of localized
corrosion on the aluminum surface [2, 3]. The protection by organic coatings is the
most dominant among the various surface-protection methods due to their perfor-
mance, commercial feasibility, and simple application [4]. However, the formation of
adhesive and lasting organic coatings on aluminum alloys depends on the surface pre-
treatment prior to painting. Chromate conversion coatings (CCCs) are highly effective
methods of aluminum alloys surface pretreatment for service in corrosive environments
[5, 6]. In spite of the toxicity, CCCs provide further advantages such as low cost, good
paint adhesion, and quick and easy application [7, 8]. Another important aspect of
CCCs is the dynamic repair of defects on coated metals, which is called self-healing
effect. Self-healing arises with a storage of soluble Cr(VI) species located in the outer
layer of mainly Cr(III) hydrated oxide/hydroxide coating. Upon contact with an elec-
trolyte, the labile Cr(VI) is released into solution and is transported to damage areas
where it is reduced and precipitated to form an insoluble Cr(III) hydroxide barrier [9,
10]. However, the main attractions for chromate conversion coatings in industrial
applications are paint adhesion improvement on Al alloys and self-healing effect in the
scratched area exposed to corrosive environments. The effect of chromate conversion
coating on the adhesion and corrosion resistance of 5083 aluminum alloy painted was
investigated in the present work.
Corresponding author: A. ESHAGHI, e-mail: eshaghi.akbar@gmail.com
48
Experimental. The 5083 aluminum alloy specimens (20×10 cm) with composi-
tion 4…4.9% Mg, 0.4…1% Mn, 0.05…0.25% Cr, 0.4% Si, 0.4% Fe, 0.1% Cu, 0.25%
Zn, and 0.15% Ti were used as the substrate material for the current study. The spe-
cimens were degreased by wiping with a paper towel wetted with acetone, rinsed in
deionized water, alkaline pickled 30 s in NaOH (12.5 N) at 70°C, rinsed, acid pickled
60 s in 50% HNO3 at 30°C, rinsed, dried and stored in an inert grease-free atmosphere
for at least 24 h. Some of these specimens were chromate conversion coated and others
were the non-chromate coated.
For the chromate treatment, the specimen was immersed in the chromate bathe:
64 g/l H3PO4, 10 g/l CrO3, 5 g/l NaF, pH 1.5, T = 30°C, for 2 min. After that, samples
were rinsed with de-ionized water and dried at room temperature.
After proper surface preparation, epoxy paints (epoxy primer HB, MIL-P-24441/1,
thickness 50 µm and epoxy finish HB, MIL-P-24441/2, thickness 153 µm) were deposited
on the chromate (thickness 2 µm) and non-chromate treatment samples by a brush method.
The adhesion of coating systems
(before and after of salt spray test) was
measured according to the ASTM D3359
standard (Tape test). Hardness of coatings
was measured according to the ASTM
D3363 by using a pencil design (from 6B
to 6H). This method is one of the simplest
to measure the coatings hardness.
The salt spray test was conducted on
the coated specimens according to the
ASTM B117 standard. For this test, the
coated specimens were scratched diago-
nally (Fig. 1) and exposed in the salt spray
fog chamber containing 5 wt.% sodium
chloride solution at 35°C for a period of 30 days. Five percent sodium chloride solution
was sprayed using a compressor with a pressure of 10…25 Psi. The observation of the
coated specimens was taken periodically. The purpose of this test was the corrosion
propagation measurement and the determination of coating blistering around the scra-
tched area on the painted samples.
Electrochemical corrosion measurements were carried out in a 3.5 wt.% sodium
chloride aqueous solution. A conventional three-electrode electrochemical cell was uti-
lized. In this cell, a SCE as the reference electrode, a platinum foil as the counter elec-
trode, and the 5083 aluminum alloy coated and non-coated surfaces (non-scratched)
were the working electrodes with the exposed area 1.0 cm2. The electrolyte solution
was not stirred or aerated. Tafel plots were generated by scanning the potential from
Ecorr to ±250 mV. The scans were started at 250 mV vs SCE with a sweep rate of
5 mV/s. Corrosion current (Icorr) was determined by superimposing a straight line along
the linear portion of cathodic or anodic curve and extrapolation it through Ecorr.
Results and discussion. Table 1 presents the results of adhesion tape test. By
comparing the results, it can be seen that adhesion of the paint of the chromating sur-
faces is better than that of the non-chromating. Thus, the role of the surface pretreat-
ment in the adhesion of coating on the aluminum alloy is determined. Aluminum has a
flat surface, thus it cannot adsorb and keep the paints. Application of the chromate con-
version coatings generate in the aluminum surfaces some micro voids which act as
capillary tubes and paint penetrates into these voids and traps. Therefore, chromate
conversion coating has capillaries and micro cavities, which provide mechanical inter-
locking. As a result, adhesion mechanism of paints in the presence of chromate conver-
sion coating could be explained by the mechanical locking phenomena.
Fig. 1. Scratched sample in salt spray chamber.
49
Table 1. Adhesion and hardness results of coating systems
Coating system Total thickness,
µm
Adhesion
(before salt spray)
Adhesion
(after salt spray) Hardness
1-Chromate layer
Epoxy primer
Epoxy finish
202 5B 5B HB
2-Epoxy primer
Epoxy finish 200 3B 0B HB
The hardness of coating depends on the topcoat resin kinds. In two coating sys-
tems, topcoats were equal and therefore, two coating systems showed equal hardness.
The hardness of coatings does not depend on the surface pretreatment prior to painting.
The results of the salt spray test are shown in the Table 2. The most effective pro-
perties of organic coating systems were the resistance to penetration of corrosive agents
and corrosion propagation.
Table 2. Results of salt spray test
Exposure time, h
120 240 360 480 600 720 Coating system
Corrosion propagation, mm
1 0 0.03 0.07 0.12 0.16 0.2
2 0.15 0.33 0.68 0.97 1.27 1.5
According to salt spray results (Fig. 2) corrosion propagation (lcorr) from the scra-
tched area into the coating systems including chromate treatment was lower than that
of the non-chromate coating. The reason of this phenomenon can be attributed to the
self-healing effect of chromate treatment.
The hexavalent Cr particles in the chro-
mate coatings dissolved due to arriving of
moisture and these particles migrated to
the scratched area and repaired the injury
[9, 10]. Then corrosion propagation re-
duces or stops. Furthermore, in the non-
chromate treatment the blistering was
seen around the scratched area, which is 5
mm in the diameter. This phenomenon
occurs due to weak adhesion of coating
and moisture, which easily penetrates
under the coating and causes blistering.
Thus, chromating treatments have several
objectives for the painted surfaces: (1) to
act as a barrier layer providing corrosion
protection, and (2) to improve the adhesion between the organic coating and the metal
surface so that it avoids undercoat corrosion when it is applied before the organic
coating and (3) a self-healing effect.
Table 3 presents the results of electrochemical corrosion measurement. Applica-
tion of coatings on the 5083 aluminium alloy decreases the Icorr value. The polarization
resistance shows maximum for the coatings including chromate treatment and declines
for the non-coated surfaces. These results indicate that the coatings including chromate
layer protect aluminium better than the non-chromate layer.
Fig. 2. Time dependence of corrosion propa-
gation from the scratch on the specimen with
chromate (1) and non-chromate (2) coating.
50
Table 3. Results of potentiodynamic polarization measurements
Sample Coating system Ecorr, mV Icorr, µА/cm2 Rp, Ω Corrosion rate, mpy
5083 Al – –824 8.495 4.9 3.7
5083 Al 1 –758 1.849 62.2 0.81
5083 Al 2 –760 5.123 16.25 2.23
CONCLUSION
The effect of chromate treatment on the adhesion and corrosion resistance of alu-
minium alloy painted was investigated. Adhesion test results indicated that paint adhe-
sion on the aluminium alloy was improved by chromating treatment. Salt spray test
showed that the samples with chromate treatment were more resistance to corrosion in
the painted surface, which is congruent to the electrochemical evaluation.
РЕЗЮМЕ. Досліджено вплив хроматної конверсійної обробки алюмінієвого сплаву
5083 на адгезію епоксидних лакофарбових покривів та їх захисні властивості. Встановле-
но, що адгезія покривів по поверхні сплаву з хроматним конверсійним шаром більша, ніж
за безхроматної обробки, а їх твердість в обох випадках однакова. Корозія металу під
плівкою біля дефекту лакофарбового покриву за умов попереднього хроматування проті-
кає не так інтенсивно, як після безхроматної обробки. Зразки сплаву зі системою покриву,
яка охоплювала хроматну обробку, мали нижчі густини струму корозії, більший поляри-
заційний опір та менш негативний потенціал корозії, ніж безхроматно оброблені.
РЕЗЮМЕ. Исследовано влияние хроматной конверсионной обработки алюминие-
вого сплава 5083 на адгезию эпоксидных лакокрасочных покрытий и их защитные свой-
ства. Установлено, что адгезия покрытий по поверхности сплава с хроматным конверси-
онным слоем больше, нежели после безхроматной обработки, а их твердость в обеих слу-
чаях одинакова. Коррозия металла под пленкой возле дефекта лакокрасочного покрытия
после предварительного хромирования протекает не так интенсивно, как после безхро-
матной обработки. Образцы сплава с системой покрытия, которая охватывала хроматную
обработку, имели меньшую плотность тока коррозии, большее поляризационное сопро-
тивление и менее негативный потенциал коррозии, нежели безхроматно обработанные.
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Received 28.02.2011
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