Zairyo-to-Kankyo, 52, 316-318 (2003)

ノ ー ト

Corrosion Prevention of Pure Iron Using Self-Assembled

Monolayer Coating

Hideaki Ohkubo*, Jun Itoh*, Hiroo Nagano*, Toshio Koura**, Masatoshi Kawase** and Takayuki Takahagi***

Venture Business Laboratory, Hiroshima UniversGraduate School of Letters, Hiroshima Univers

Graduate School of Advanced Sciences of Matter, Hiroshima Univers

A 1, 2-bis (triethoxysilyl) ethane (BTSE) is coated to build up self-assembled monolayers (SAM) on the sur-

face of iron for protecting from in-door atmospheric corrosion. To close the terminals of SAM molecule, a

water repellent agent octadecyltrimethoxysilane (ODTMS) is coated further on BTSE coated film. For corro-

sion test to evaluate corrosion-resistant property, accelerated corrosion test was performed in a thermo-

hygrostat kept at 40•Ž and 40% relative humidity. Also, anodic polarization measurement was performed. It

is found that the ODTMS coating on BTSE coated film shows high corrosion resistance by anodic polariza-

tion measurement and an accelerated corrosion testing.

Key words: corrosion, self-assembled monolayer (SAM) ,1, 2-bis (triethoxysilyl) ethane (BTSE), octade-

cyltrimethoxysilane (ODTMS), coating, iron

1. Introduction

The cultural properties made of metal are subject to corro-

sion, discoloring, etc. due to acid rain and other causes, and

the surfaces are often covered with rust. Even when the cul-

tural properties are kept in storage in a weatherproof muse-

um, it is necessary to protect them from rusting, which may

occur under condensed water.

Conventionally, the object is coated with acrylic resin

under reduced pressure, or tannin is permeated into the rust,

and then organic film of several tens of ƒÊm is coated to pre-

vent rusting. Although corrosion may be prevented by such a

thick coating, it often happens that original external appear-

ance of cultural properties is impaired. Because cultural prop-

erties are widely appreciated by many thousands of people, it

is an important problem to develop a new coating method to

protect them from corrosion by maintaining external appear-

ance as really as possible to the original state.

Organofunctional silanes were mainly used as coupling

agents between metals and polymer paint, and were studied

extensively1)-4). In this study as a new attempt, molecules of

the organofunctional silanes are coated to build up self-

assembled monolayers (SAM) on the surface of metals for

protecting them from in-door atmospheric corrosion.

Further, the film is coated to the thickness of only several

tens of nm in order to reject light reduction through coatings.

Therefore, efforts are made to try to develop a coating film,

which is suitable to maintain external appearance of cultural

properties not different from the original state.

2. Experimental

As the metal of the base material, pure iron with purity of

99.9% was used. The surface was mirror-polished using alu-

mina powder to reduce surface irregularities to less than 1

m. Then, ultrasonic cleaning was performed using acetone

to be defatted, and blow dried with compressed air.

As a principal material for the self-assembled film, the solu-

tion was made by mixing 1, 2-bis (triethoxysilyl) ethane

(BTSE) [(CH3CH2O)3-Si-CH2-CH2-Si-(OCH2CH3)3]

(4%) , ethanol (89.6%), water (6%) and acetic acid (0.4%)

by volume3). Ethoxyl group is present each at both ends of a

molecular chain, and when it is brought into contact with

water molecules, it is hydrolyzed, forming a silanol group

having-Si-OH. When this is heated up, condensation

reaction occurs, and a firm siloxane bond of O-Si-O is

obtained (Fig. 1 (a))5). The self-assembled film is formed by

this reaction. To coat this on a metal specimen, simple dip-

ping method is adopted in BTSE solution.

In experiment, the specimen was dipped in BTSE solution

for one hundred seconds, followed by the coating with

octadecyltrimethoxysilane (ODTMS) [(CH3O)3-Si-(CH2)17-

CH3] was coated, where terminals of the self-aseembled mol-

ecule were closed (Fig. 1 (b)). Since the terminals of

ODTMS terminated with alkoxy group and -CH3, it had

showed high water repellent property. ODTMS was diluted

with ethanol with 4vol% ODTMS. After BTSE and ODTMS

were coated, thermal treatment was performed for one hour

in air at 80•Ž.

Accelerated corrosion test was performed in a thermo-

hygrostat kept at 40•Ž and 40% relative humidity intermit-

tent of spraying 0.1mol/l sodium sulfate aqueous solution at

4 hour regular interval. Also, anodic polarization measure-

ment was performed in 0.1mol/l (0.1M) sodium sulfate aque-

ous solution. Potential sweeping rate for anodic polarization

measurement was set of 20mV/min. As one of the properties

of the coated film, contact angles with water for different film

thickness were measured.

*2-313 , Kagamiyama, Higashi-Hiroshima, 739-8527 Japan**1-2-3 , Kagamiyama, Higashi-Hiroshima, 739-8522 Japan

***1-3-1 , Kagamiyama, Higashi-Hiroshima, 739-8530 Japan

Vol.52, No.6 317

Fig. 1 Illustration of interaction between metals and BTSE mole-cules.(a) BTSE films is connected to metal substrates by forming

strong siloxane bonds with metals.(b) ODTMS is coated on BTSE molecules. The terminals of

self-assembled molecule were closed.

Table 1 Results of measurement, i.e. film thickness measured and contact angles with water for the organizing film.

3. Results and Discussion

After BTSE solution was coated, the reflectance of the iron itself was not lost on the surface of the specimen. The BTSE coating gave a colorless, transparent film enough to maintain original external appearance of the cultural properties.

Table 1 summarizes the results of measurement, i.e. film thickness measured using ellipsometer and contact angles with water. Films 50 to 80nm thick was detected after some of the treatment procedures. From the results of contact angles, it was found that the hydrophobic property was increased after the coating of ODTMS. The specimen with-out coating after polishing showed a high contact angle, and this was because of the formation of a FeO(OH) (ferric oxy-hydroxide) of about 2nm thick on the surface of the speci-men6). After polishing, the oxyhydroxide substances tend to be formed on iron specimen immediately. Therefore, it is not easy to remove these substances for performing a good coat-ing. Since, cultural properties are generally covered with rust, it is very important to develop a special coatings that are formed easily to rusted metal surfaces.

Next, in Fig. 2 (a) and Fig. 2 (b), the results of polarization

Fig. 2 (a) Anodic polarization curve for iron with or without

BTSE coating.

(b) Anodic polarization curve for iron with BTSE and ODTMS coating.

measurement in the anodic direction are shown. A specimen

coated with BTSE coating was dipped in 0.1mol/l sodium

sulfate aqueous solution for 30 minutes, and polarization was

measured in the anodic direction from the spontaneous

potential. In Fig. 2 (a), comparison is made in changing the

type of alcohol used for dilution of BTSE specimens. When

methanol was used, the anodic current density for the iron

specimen coated with BTSE was higher than the uncoated

one, indicating less corrosion resistance. According to previ-

ous works3), it is reported that the formation of the self-

assembled film using methanol is impaired because the boil-

ing point of methanol is lower than 80•Ž. When thermal treat-

ment is performed at 80•Ž, ethanol seems to be more suitable

than methanol as the alcohol to dilute BTSE.

Fig. 2 (b) shows the results when ODTMS was coated fur-

ther on the BTSE-coated film. Comparison was made in

changing the dipping time in the ODTMS solution for 100

seconds, 30 minutes and one hour, respectively. Specimens

318 Zairyo -to-Kankyo

Fig. 3 Results of accelerated corrosion test for 3 days by spraying 0 .1

mol/l sodium sulfate aqueous solution at 4 hour regular interval in

a thermo-hygrostat maintained at 40•Ž and 40% relative humidity.

Fig. 4 Optical micrograph of a specimen coated.

(a) BTSE coating, (b) ODTMS coating on BTSE film.

treated for the dipping time of 100 seconds and 30min

respectively, showed nearly same polarization curve. The

molecules having high water repellent property were formed

on the terminals of BTSE molecules. The specimen exhibited

hydrophobic property, and showed higher spontaneous

potential than the specimen coated with BTSE without

ODTMS. This shows the corrosion preventive effect of

ODTMS coating. When ODTMS treatment time exceeds one

hour, methoxyl group of ODTMS excessively reacts with

silanol group of BTSE. This impairs self-organizing of BTSE

and decreases corrosion-resistant property. This suggests

that the dipping time in the ODTMS solution for about 100

seconds would be the most appropriate.

Fig. 3 shows results of an accelerated corrosion testing.

The test was carried out for 3 days by operating 0.1mol/l sul-

fate solution at the 4 hour interval in a thermo-hygrostat

maintained at 40•Ž and 40% relative humidity. On the speci-

men without coating after polishing, the entire surface was

covered with brown rust within 3 days. The specimen coated

with BTSE was covered with less rust than the one without

BTSE coating. This reveals that rusting can be prevented by

the coating of BTSE in the thickness of several tens of nm.

When the conventional coating method is used, it is not easy

to have such a thin film as the BTSE treatment. When

ODTMS was coated further on the film composed of BTSE,

surface area covered with rust apparently decreased. It

means that rusting could be prevented to a considerable

extent, and high corrosion protection effect could be con-

firmed. This suggests that the method to coat ODTMS on

the BTSE film on iron has the highest corrosion preventive

effect from the results of both anodic polarization curves and

the accelerate corrosion test using the thermo-hygrostat.

Fig. 4 (a), (b) shows an optical micrograph of the rusted

area of a specimen coated with BTSE and ODTMS-on-BTSE

film, respectively. The rust structure and pit diameter of both

BTSE and ODTMS-on-BTSE film seems to be appar-

ently the same. On the specimen coated with BTSE,

the non-rusted surface maintained initial brightness

of the iron, and no defect such as stripping of BTSE

was observed. Under optical microscope, needle-

shaped pits remained at the center of rusting, indicat-

ing that pitting occurred at the early stage of corro-

sion. In general, pitting is often observed on a materi-

al with highly corrosion resistant alloys such as stain-

less steel, therefore, the above finding indicates high corro-

sion-resistant property of the BTSE film. In the corrosion of

the BTSE-coated specimen, pitting initiates from the defects

of the coated film, leading to corrosion. Corrosion propagates

beneath the coating film, resulting in the deterioration of the

BTSE-coated film. Prevention of pitting initiation -in other

words, the elimination of the defects of the BTSE-coated film

where rusting occurs -is the problem to be solved in the

future.

4. Conclusion

In the present study, a 1, 2-bis (triethoxysilyl) ethane

(BTSE) thin film of self-assembly was formed on pure iron,

and improvement in its corrosion-resistance was evaluated.

The results of the experiment are as follows,

1. A water repellent agent octadecyltrimethoxysilane

(ODTMS) was coated further on BTSE coated film on iron

in the thickness of tens of nm shows high corrosion resis-

tance.

2. For the storage of cultural properties in museum, a self-

assembled coating of BTSE-ODTMS film is recommended

for corrosion prevention and the maintenance of good

external appearance of the cultural properties.

3. In the present study, a self-assembled film of BTSE was

formed on mirror-polished pure iron. Further study is nec-

essary to evaluate whether a self-assembled film can be

formed on rusted steel surface, and how durable the coat-

ing form the view point of the protection of cultural proper-

ties in museums.

Acknowledgements

The authors want to express thanks Dr. K. Kikukawa in

Hiroshima University for valuable discussion during this

investigation. Help in sample preparation, given by laboratory

coworkers, is also gratefully acknowledged. This research

was supported by a budged for literature and science joint

project of Hiroshima university.

Reference

1) K. Aramaki, Corrosion, 56 [9] 901 (2000).2) K. Aramaki, Corrosion Science, 42, 2023 (2000).

3) V. Subramanian and W.J. Ooji, Corrosion, 54 [3] 204 (1998).

4) W.J. van Ooji and D. Zhu, Corrosion, 57 [5] 413 (2001).5) B. Arkles, Chem. Tech., 765, December (1977).

6) J.C. Bolger, in K.L. Mittal (ed.), •gAdhesion Aspects of Polymeric Coatings•h, Olenum Press, New York, p. 3, (1983).

(Manuscript received September 27, 2002;

in final form February 24, 2003)