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

The AISI-52100 steel samples were coated with a single layer of Termosil and
dried at room temperature (25 0 C). These samples have shown a very low corrosion
protection in salt spray test (24 hours without formation of RR). It means that Termosil
cannot be used as a raw composition without sintering for corrosion protection.
In the next stage, the samples with one-layer-Termosil dried at 25 0 C were
sintered at 250 0 C, 350 0 C, 300 0 C, and 470 0 C for 15 minutes. The samples were subject
to salt spray test. The results are displayed in Figure 3.2.
Temperature curing => Dried (25 0 C) 250 0 C 350 0 C 470 0 C
Termosil (1 layer) 18±6 42±6 42±6 210±6
Figure 3.2 – Corrosion resistance (hrs without formation of red rust) of one-layer-
Termosil over the bare AISI-52100 steel followed by sintering for 15 minutes.
It is clear that any chemical transformation takes place during sintering at
470 0 C. This temperature is significantly higher that is recommended (Termosil Asia
Sdn Bnd 1997). The curing changes initial raw composition of Termosil to the final
composition, as shown in Figure 2, at 470 0 C only. Corrosion protection achieved with
Termosil corresponds to 210 hrs under the NSST. However, this is not as remarkable as
Termosil used in combination with a fine intermediate layer of Zn-mechanically plated
(Termosil Asia Sdn Bnd 1997).
Application of Termosil to improve lubricity and corrosion protection as
secondary layer over Ni-electroless coating was realised (e.g. Figure 3.3).
Temperature curing => 470 0 C 350 0 C
Ni-electroless (primary layer) +
one-layer-Termosil (secondary layer) 546±6 210±6
Ni-electroless (primary layer) +
two-layer-Termosil (secondary layer) 498±6 210±6
Figure 3.3 – Corrosion resistance (hrs without formation of red rust) of duplex
coating (Ni-electroless + Termosil) sintered at 350 0 C and 470 0 C.
One- and two-layer-Termosil over Ni-electroless were sintered at 350 0 C and
470 0 C. Maximum corrosion protection was achieved with one-layer-Termosil over Nielectroless.
Up to 552 hrs without red rust under salt spray test can be achieved with
this combination of duplex coating systems.
Ni-electroless plating costs 22-28 times more than Zn-electroplating, and
Termosil application costs equal to Zn-electroplating costs. Therefore, Termosil coating
over Ni-electroless does not add significant cost to this duplex coating system, i.e. only
Corrosion Protection of High Carbon Steel with Duplex Coating System:
Ni-electroless and Termosil
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