Effect of Nano Clay on Corrosion Protection of Zinc-rich Epoxy Coatings on Steel 37

doi:10.1016/j.jmst.2016.08.017

Abstract

Abstract:

Epoxy zinc rich coatings containing clay nanoparticles were prepared and the effect of clay content on the cathodic protection performance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) and immersion test. Open circuit potential (OCP) measurements and immersion tests were also carried out to better understand the behavior of zinc rich coating. EIS and OCP measurements showed that addition of 1 wt% clay improved the cathodic protection duration and sacrificial properties of the epoxy zinc rich coating. Transmission electron microscopy (TEM) photographs confirmed that clay nanoparticles were successfully dispersed in the coating matrix loaded with 1 wt% clay. Immersion test results indicated that addition of 1 wt% clay nanoparticles in zinc rich epoxy coatings increased the cathodic protection ability of coatings.

Key words: Epoxy coatings, Zinc rich, Impedance, Cathodic protection, Clay

Tohidi Shirehjini Farhad,Danaee Iman,Eskandari Hadi,Zarei Davood. Effect of Nano Clay on Corrosion Protection of Zinc-rich Epoxy Coatings on Steel 37[J]. J. Mater. Sci. Technol., 2016, 32(11): 1152-1160.

Figures/Tables 13

Table 1. Specification of the coatings with different clay loading

Type of coating	A	B	C	D
Weight ratio of zinc (%)	85	85	85	85
Weight ratio of binder (resin + hardener + clay) (%)	15	15	15	15
Solid content of resin + hardener	9	9	9	9
Weight ratio of clay/binder	0	0.01	0.02	0.03
Volume percent of zinc (%)	-	46.82	46.91	46.97
Volume percent of resin and hardener (%)	-	52.80	53.36	51.92
Volume percent of clay (%)	-	0.38	0.73	1.11
PVC	-	47.19	47.64	48.08
CPVC	-	62.25	62.13	61.88

Fig. 1. Variation of open circuit potentials vs. immersion time of the epoxy zinc rich coatings containing different amounts of clay in 3.5 wt% NaCl solution.

Fig. 2. Bode resistance and Bode phase diagrams of epoxy zinc rich coatings at different clay content in 3.5 wt% NaCl solution: (a, b) 0, (c, d) 1%, (e, f) 2%, and (g, h) 3%.

Fig. 3. Variations of (a) low frequency impedance and (b) high frequency phase angle vs immersion time for the different zinc rich coatings obtained from Bode plots.

Fig. 4. TEM image of clay nanolayers in epoxy matrix of coating B in the absence of zinc.

Fig. 5. TEM image of clay nanolayers between zinc particles in epoxy matrix of coating B.

Fig. 6. Surface images without magnification of (a) coating A, (b) coating B, (c) coating C and (d) coating D after 1400 h immersion in 5 wt% NaCl solution.

Table 2. Immersion test results obtained during 1400 h exposure to 5 wt% NaCl solution

Samples	Parameters ------------------------------------------------------
	Blister ------------------------		Corrosion at scribed marks	Corrosion spot on the surface of coating
	Time of appearance	Size and density after 1400 h	Corrosion at scribed marks	Corrosion spot on the surface of coating
Coating A	After 288 h	2MD	No red rust	No red corrosion spot
Coating B	After 800 h	8F	No red rust	No red corrosion spot
Coating C	After 500 h	4F	No red rust	No red corrosion spot
Coating D	After 670 h	0F	Red rust after 680 h	No red corrosion spot

Fig. 7. SEM micrographs of surface of (a) coating A, (b) coating B, (c) coating C and (d) coating D before immersion test.

Fig. 8. SEM micrographs of surface of (a) coating A, (b) coating B, (c) coating C and (d) coating D after immersion test.

Fig. 9. SEM micrographs of scribed area of (a) coating A, (b) coating B, (c) coating C and (d) coating D after immersion test with low magnification.

Fig. 10. EDX analysis around scribed area of (a) coating A, (b) coating B, (c) coating C and (d) coating D after immersion test.

Table 3. Energy dispersive X-ray analysis (wt%) of different coating surfaces after 1400 h exposure to 5 wt% NaCl solution

	C	O	Na	Al	Si	Cl	Zn	Mg	Ca	Fe
A	4.89	24.66	6.16	0.65	0.83	7.97	54.85	-	-	-
B	4.73	27.91	7.84	0.92	0.99	2.48	55.13	-	-	-
C	30.81	32.89	2.30	0.92	2.20	0.46	29.42	1.00	-	-
D	4.24	27.48	2.56	0.88	2.31	1.95	8.42	-	2.02	50.14

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