EuroFasteners March 2023

Technical Article

Initial open circuit potential, mV

Final open circuit potential, mV


Z mod . ohm- cm 2

Z real ohm- cm 2

0.5mm cut 1mm cut 1.5-2mm cut

161.5 79.12 38.75

162.5 72.19 22.93

-982.5 -955.6 -823.4

-987.5 -960.7 -884.3

Table 4: Impedance parameters

Electro chemical investigations

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 (10-7A/cm 2 ) is lower than that of the steel (10-6A/cm 2 ), it begins to corrode first in the case of the damage of the coating, and it corrodes more slowly than the steel, thereby protecting the steel for a longer period. This is the basis of galvanic protection of steel. It should be noted that Zn is sufficient in the surface layer on the bolt to provide electrical contact up to a certain scratch size only. As the scratch size increases beyond the critical value, Zn is not adequate to impart sacrificial protection to the base steel ( Table 3 ). The underlying steel gets exposed due to the increased scratch size. Since Al is in oxide form, it works as a cathode with respect to the steel. Hence, steel begins to corrode and thereby leads to the formation of iron oxide, ie red rust. Coating thickness should be increased to impart corrosion protection to the bolt if the scratch size is 1.5 to 2mm.

Table 4 the electrochemical impedance spectroscopic investigations just after immersion and open circuit potential measurements in 3.5 % NaCl. Table 4 reveals a decrease in impedance values of the coating with the increase in the size of the cut section. This points out the corrosion tendency of the coated bolt as a function of scratch size. Negative shift in open circuit potential confirms cathodic protection of the bolt steel. contains results of

Figure 3: SEM of the Ecomet corundum 1m cut at 500 X

Figure 4: XRD of the Ecomet corundum 1mm cut

Compound name


Formula Wt %

Blue Red

Iron Zinc

Fe Zn


39.7 39.3

Corundum Al 2 O 3


Table 2: Semi quantitative analysis of the Ecomet corundum 1mm cut sample

After 1,200 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 5: SEM of the Ecomet corundum 1.5 to 2mm cut at 500 X

Figure 6: XRD of the Ecomet corundum 1.5 to 2mm cut

Compound name


Formula Wt %





Corundum Al 2 O 3



Blue Pink



15.5 10.6

Tested bolts with five times torqueing as per ISO16047 with lower COF values. Coating system: Ecomet corundum + Ecomet Top Clear with sol gel technology. Coating done by dip spin method

Effco Finishes & Technologies Pvt Ltd Pune, India Website:

Fe 2 O 3


Table 3: Semi quantitative analysis of the Ecomet corundum 1.5 to 2mm cut sample

EF 10/64

March 2023

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