Innovation Corundum Thin Film Coating Technology for 30 years of life for Fasteners and metal hardware as per ISO10683/ASTMF3393: up to 6000 hours SST
Cathodic Corrosion protection mechanism analysis in scratched condition
Author:
Kalyan Dhakane
EFFCO Finishes & Technologies Pvt Ltd
Pune India
Contact: +91 9867289420
kdhakane@effcoindia.com
Co-Author
P. P. Deshpande
Dept of Metallurgy and Material Science
College of Engineering, University of Pune. India
Abstract
ECOMET corundum coated bolts having 0.5 mm and 1 mm cut in the coating do not show sign of corrosion even in the scratch.
The ECOMET corundum coated bolt having 1.5 - 2 mm cut shows rusting in the cut section within 1-2 days after the scratch is made in the coating.
At the cut edges, zinc corrodes preferentially providing sacrificial protection to the base steel. The resulting product then fills the cavities in the coating and inhibits further corrosion.
However, as the size of cut gets increased beyond critical value, there is reduction in Zn content in the surface layer of the scratched portion of the coating. This leads to the corrosion in the scratched region.
Introduction
Low carbon steels corrode in most of the atmospheric environments when the relative humidity exceeds sixty per cent. Effective coatings, used to prevent corrosion, contain environmental hazardous pigments.
Recently EFFCO Finishes and Technologies Pvt Ltd Pune 411057 (M.S.) India introduced nontoxic and environmentally friendly paint formulation under trade name ECOMET corundum. It is found that the ECOMET corundum painted bolts having scratch 0.5 and 1 mm do not corrode where as the bolt having scratch 1.5 mm corrode within 1-2 days after the scratch is made. The aim of these investigations is to find out reasons of corrosion in the scratched region of the coated steel bolt.
Experimental Characterization
Scanning electron microscopy using field emission scanning electron microscope (FE-SEM, SIGMA IV Carl Zeiss U.K.) was conducted at 3000 X to examiner morphology of the coating. X- Ray diffraction (Bruker AXS DS Advance, Germany) was carried out to study the composition of the compound formation at the scratch.
Corrosion studies
A corrosion cell having three-electrode geometry of coated steel bolt as working electrode, platinum as counter electrode and saturated calomel electrode (SCE; pH Products, Hyderabad, India) as a reference electrode was used. The cell related to Gamry reference system 1000(Wilmington, USA) for electrochemical measurements.
Corrosion performance of the coated samples was assessed by electrochemical impedance spectroscopy over frequency range 100 KHz to 0.1 Hz using amplitude signal 10 mV rms (ASTM G106 and ASTM 2005b).For all the electrochemical experiments, 3.5 wt % NaCl aqueous solution was used. All measurements were carried out five times to obtain good reproducibility of the results and these results are reported in this work.
Results and discussion
Scanning electron microscopic image and XRD of the 0.5mm, 1 mm and 1.5 to 2 mm cut sectioned Ecomet corundum samples are shown in figures 1, 2, 3,4, 5 and 6 respectively .
Figure 1: SEM of the Ecomet corundum 0.5mm cut sample at 500 X.
Figure 2 XRD of the Ecomet corundum 0.5mm cut sample
Table 1: Semi Quantitative Analysis of the Ecomet corundum 0.5mm cut sample
M10 Hex bolts coated with ECOMET Corundum. Cut made until metal with 500 microns width. Picture taken by Apple 12 Pro camera, 10X
Above Picture after 1200 Hours SST as per ASTM117. No metal rust observed even in cut sections. Very high cathodic protection compared to only 72 hours mentioned under ISO10683. Coating applied ECOMET Corundum by dip spin method.
Figure 3: SEM of the Ecomet corundum 1 mm cut at 500 X.
Figure 4: XRD of the Ecomet corundum 1 mm cut
Table 2 :Semi Quantitative Analysis of the Ecomet corundum 1 mm cut sample
Figure 5: SEM of the Ecomet corundum 1.5 to 2 mm cut at 500 X.
Figure 6 : XRD of the Ecomet corundum 1.5 to 2 mm cut.
Table 3 :Semi Quantitative Analysis of the Ecomet corundum 1.5 to 2 mm cut sample
Picture of tested bolts with 5 times torquing as per ISO16047 with lower COF values. Coating system: ECOMET Corundum + ECOMET Top clear with sol gel technology. Coating done by dip spin method.
ECOMET Corundum+ EOCMET Top Clear sol gel technology coated and done 5 times torquing test as per ISO16047. Afterwards done SST as per ASTM117 for 1200 hours. No metal rust observed.
Electro chemical investigations
Table 4 contains results of the electrochemical impedance spectroscopic investigations just after immersion and open circuit potential measurements in 3.5 % NaCl .
Table 4 : Impedance parameters
Table 4 reveals decrease in impedance values of the coating with increase in the size of the cut section. This points out corrosion tendency of the coated bolt as a function of scratch size. Negative shift in open circuit potential confirm cathodic protection of the bolt steel.
Corrosion Protection mechanism
It is suggested that corundum coating functions by three mechanisms- barrier protection, galvanic protection and the formation of corrosion inhibiting products. The coating, being barrier, precludes water and oxygen from reaching the metal substrate.
Al-oxide imparts dielectric properties to the coating, opposes the passage of corrosive species and improves the barrier action. The barrier mechanism is effective as long as the coating is intact. When the scratch is made, at the cut edges, zinc corrodes preferentially providing sacrificial protection to the steel.
The resulting product then fills the cavities in the coating and inhibits further corrosion. As the exchange current density of zinc dissolution ( 10 - 7 A/ cm 2 ) is lower than that of the steel ( 10 - 6 A/ cm2), it begins to corrodes first in case of the damage of the coating and it corrodes more slowly than the steel, and thereby protecting the steel for longer period. This is the basis of galvanic protection of steel.
It should be noted that Zn is sufficient in surface layer on the bolt to provide electrical contact up to the certain scratch size only.
As the scratch size increases beyond critical value, Zn is not adequate to impart sacrificial protection to the base steel (Table 3).
The underlying steel gets exposed due to increased scratch size. Since Al is in oxide form, it works as cathode with respect to the steel. Hence, steel begins to corrode and thereby leads to the formation of an iron oxide i.e. red rust. Coating thickness should be increased to impart corrosion protection to the bolt if the scratch size is 1.5 to 2 mm.
