Corrosion Protection of High Carbon Steel with Duplex Coating ...

Such duplex coatings can achieve significant corrosion resistance combined with high
hardness and low friction. In short, a secondary layer can drastically improve corrosion
resistance and lubricity of a steel surface without increasing the cost of the duplex
coating significantly.
Termosil ® (Termosil Asia Sdn Bnd 1997) is a similar solution to Dacromet ® –
A/B (Metal Coating International Inc. 1998; Ridder J.A. 1995). Termosil is a waterbased
solution with dispersed Zn- and Al-flakes, proprietary organic, Cr 6+ , which is
reduced to Cr 3+ into the sinter oven at ~300 0 C. Small additions of inorganic
components and metal oxides are present in Termosil (White R.W. 1995). After
sintering, Termosil-coatings provide outstanding corrosion protection for ferrous
materials (1,000-1,200 hrs under salt spray test without formation of red rust in
comparison with 500 hrs for Dacromet), good bimetallic corrosion resistance for Al,
high electrical conductivity (electrical resistance – 0.6 ), and low coefficient of
friction (0.09-0.1 with special lubrication agent on the top).
Termosil passivating property is realized by the addition of metal oxides, which
slow down corrosion reactions of zinc on steel. Damaged areas in the Termosil coating
repair themselves by filling with Zinc oxides and carbonates. The final coating contains
no Cr 6+ chemicals. The Zn-flakes stratify well forming a metallic sacrificial coating
with Al-flakes and metal oxide dispersant.
However, Termosil’s appearance is inferior to Dacromet in two respects. These
are that Termosil scuffs more easily and is not available in as many colours as
Dacromet. It is thought that Termosil’s excellent corrosion resistance may be due to the
inclusion of 15% Al in the coating composition, which reduces corrosion rate due to the
impermeability of alumina to oxygen.
For selection a primary layer for duplex coatings, Ni-electroless was chosen as
expensive coating having good hardness and wear resistance, but not satisfying
corrosion resistance requirements by virtue of its inherent porosity (Gawrilow G.G.
1979). Red rust on the AISI-52100 steel surface coated with Ni-electroless appears
after 7 hours under salt spray test. The frictional coefficient of Ni-electroless is also
unsatisfactory (µ NiP =0.38-0.45) for sliding steel joints.
It would be very useful to develop the duplex system of Termosil for NiPelectroless
as a primary layer in order to enhance corrosion resistance of NiP-Termosil
system on steels. The aim of this research and development is to improve corrosion
resistance of AISI-52100 (high carbon steel) coated with NiP-electroless coatings,
which is used in the automotive industry, fastener manufacturing, etc, applying
Termosil as the topcoat.
2. Experimental procedure and corrosion testing
Corrosion Protection of High Carbon Steel with Duplex Coating System:
Ni-electroless and Termosil
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