EuroFasteners March 2023

Technical Article

Corundum thin film coating technology for 30 years of life for fasteners and metal hardware as per ISO10683/ASTMF3393: up to 6,000 hours SST

By Kalyan Dhakane, Effco Finishes & Technologies Pvt Ltd Co-author PP Deshpande, Dept of Metallurgy and Material Science, College of Engineering, University of Pune, India

Corrosion performance of the coated samples was assessed by electrochemical impedance spectroscopy over a frequency range of 100 kHz to 0.1 Hz using amplitude signal 10 mV rms (ASTM G106 and ASTM 2005b). For all of 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 images and XRD of the 0.5mm, 1mm and 1.5 2mm cut sectioned Ecomet corundum samples are shown in Figures 1, 2, 3, 4, 5 and 6 , respectively.

Ecomet corundum-coated bolts having 0.5mm and 1mm cuts in the coating do not show sign of corrosion, even in the scratch. The Ecomet corundum-coated bolt having a 1.5 to 2mm cut shows rusting in the cut section within one to two 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 atmospheric environments when the relative humidity exceeds 60 per cent. Effective coatings used to prevent corrosion contain environmentally hazardous pigments. Effco Finishes and Technologies Pvt Ltd (Pune, India) introduced a non-toxic and environmentally friendly paint formulation under the trade name Ecomet corundum. It is found that the Ecomet corundum-painted bolts having 0.5 and 1mm scratches do not corrode, whereas a bolt having a 1.5mm scratch corrodes within one to two 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.

Ecomet corundum + Ecomet Top Clear sol gel technology coated, with five times torqueing test as per ISO16047, followed by SST as per ASTM117 for 1,200 hours. No metal rust observed

Experimental characterisation Scanning electron microscopy using a field emission scanning electron microscope (FE-SEM, Sigma IV Carl Zeiss UK) was conducted at 3,000 X to examine the 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.

Figure 1: SEM of the Ecomet corundum 0.5mm cut sample at 500 X

M10 Hex bolts coated with Ecomet Corundum. Cut made until 500 microns wide

Figure 2: XRD of the Ecomet corundum 0.5mm cut sample

Compound name

Colour

Formula Wt %

Blue Red

Iron Zinc

Fe Zn

32.4 27.5 40.1

Corundum Al 2 O 3

Green

Table 1: Semi quantitative analysis of the Ecomet corundum 0.5mm cut sample

EF 9/63

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March 2023