1. The 7th International Color and Coating Congress 19-20 December 2017
ICCC 2017
The 7th International Color and Coating Congress December 2017
Use of 8-hydroxyquinoline corrosion indicator in investigation of epoxy coated St-37
Sh. Roshan1, A.A. Sarabi Daryani2
1. M. Sc. Student, Faculty of Polymer Engineering & Color Technology, Amirkabir University of Technology
2. Assistant Professor, Faculty of Polymer Engineering & Color Technology, Amirkabir University of Technology
Abstract
A coating with 30 5 micron thickness was applied on polished surface of St-37 using a film applicator. Coating was cured at room temperature for 1 week. Sample was immersed in 3.5% sodium chloride solution and the monitoring of the corrosion was done by fluorescence microscopy
Fluorescent indicator can be used to detect the underlying corrosion on metal substrate based on forming complex with metal ions such as those generated from corrosion reactions. In the present work 8-hydroxyquinoline (8HQ) was chosen as fluorescent indicator. The ion sensitivity of the indicator was investigated by fluorescent spectroscopy. Fluorescence emission intensity was observed to increase with increasing iron ions concentration. By incorporating 8HQ in to epoxy coated- St-37 system, it provided an early warning of corrosive conditions at the metal surface. The initial state of the coating system was non-fluorescent under UV light and it changes to fluorescent at the corrosion site. Bright corrosion sites were detected by fluorescent microscopy.
Results & Discussions
The effect of iron ion concentration on fluorescence emission intensity and excitation and emission wavelength is illustrated in table 1. It is obviously found that the increase in iron ion concentration cause increase in fluorescence emission intensity. The results confirmed that 8HQ was formed complex with iron ions. An excessive concentration of ferric chloride in solution caused decrease in emission intensity because of self absorption phenomenon. Figure 1 shows the fluorescent images of coated St-37 during immersion time, which were captured by fluorescence microscope. The fluorescent indicator in the coating should result in turning-on by forming complex with iron ions which were generated at corrosion sites. It requires showing no-fluorescence intensity in coating after the 8HQ is added to coating at initial time. Figure 1a shows initial intensity of the primer plus indicator. After 4 days a bright spot was appeared on the surface of St-37 but no visible sign was observed by naked eyes. It is found that corrosion is occurred under coating at that region (figure 1b). 2days later another bright spot was appeared on the surface (figure 1c). Results show that after 10 days a dark spot was appeared at the center of first bright spot. One possible reason for appearance of dark spot is the deposition of corrosion products at corrosion site (Figure 1d). A magnified image of spot was demonstrated by red circle is illustrated in figure 2.
Introduction
Corrosion has become an important issue so detecting corrosion at its initial stages has involved both economical and safety factors. Recently, corrosion sensing-coatings have been developed to provide early corrosion detection. These coatings can prepare in different ways such as incorporate corrosion sensitive chemical compounds in to coating. The chemical compounds are usually sensitive to variation in pH or metal ions which are generated from corrosion reactions. Some indicators are color change indicators and some of them are fluorescent compounds. The other common method is incorporating optical fibers in to coatings. Since these fibers are fragile, this method is less useful than the other method. Corrosion-sensing coatings can detect corrosion even in remote locations such as deep within the lap joints of metal structures. Johnson and Agarwala investigated corrosion by incorporating different fluorescent chemical in to paint. The change in their fluorescence property occurred upon oxidation or upon complexation with metal ions generated during the corrosion process.
The basic idea of present work is to modify epoxy coating, to make it sensor for corrosion detection by using fluorescent indicator. Having turn-on mechanism without any reactions with coating functionality make 8HQ useful for this purpose. Fluorescent spectroscopy and fluorescent microscopy confirmed the positive performance of 8HQ as fluorescent corrosion indicator.
Conclusion
8-hydroxyquinoline was able to form complex with iron ions and become fluorescent. It showed acceptable performance as corrosion indicator even in epoxy coating. It had the ability to reveal the underlying corrosion before any visible sign has appeared on the surface of the coating. 
References
1. V. S. Agarwala, Proceedings of the International Conference on Corrosion CONCORN, 97, 140 ,1997
2. R. E. Johnson and V. S. Agarwala, Corrosion (Houston), Paper no. 304, NACE
3. P. Sibi, Zh.Zong, Prog .org. coatings, 47,8-15,2003
4. A. Augustyniak, J. Tsavalas, W. Ming, American Chemical Society, 1: 2618–2623 ,2009
5. A.Augustyniak and W. Ming. Prog. Org. Coatings.71, 406–412,2011
Table 1: Effect of iron ion concentration on emission intensity
Mole ratio of Fe3+/8HQ
Excitation wavelength(nm)
Emission wavelength(nm)
emission intensity(a.u)
0.1
289
567 48
0.5
291
572
140
0.75
294
573
270 1
298
576
399 2
296
575
354
Experimental
Methanol, Ferric chloride hexahydrate and 8-hydroxyquinoline were purchased from Merck company. epoxy resin (Epon 828), based on diglycidyl ether of bisphenol A (DGEBA) with solid content, epoxy value and density of 95–100%, 185–192 and 1.17 g cm−3 respectively and amine containing hardener (Epikure F205) were supplied from Shell Chemicals. DELTA-FC® 1040 used as silicon defoamer and DELTA-SC® 2777 as a free silicone additive utilized as leveling agent in formulation of coatings which were received from Delta specialties company. St-37 sheets are chosen as substrates were purchased from Foolad Mobarakeh company.
Fluorescence excitation and emission spectra was obtained from Perkin-Elmer LS-5S fluorescence spectrophotometer and fluorescent image was taken by Nikon eclipse E-800 fluorescent microscope with nm excitation. In order to measuring the sensitivity of 8HQ to iron ions, solutions with different concentration of ferric chloride were prepared and were added to 8HQ solution. To investigate the performance of 8HQ in coating as corrosion sensor, 0.1wt% of 8HQ was added to the coating. The Coating was prepared by mixing amine and epoxide components.
Figure2: fluorescent image of coated-St-37 after 10 days
Figure 1: fluorescent image of coated-St-37 during immersion time: (a) at initial time (b) after 4 days (c) after 6 days (d) after 10 days
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