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

Corrosion Protection of High Carbon Steel with Duplex Coating
System: Ni-electroless and Termosil ®
by
Gregory Demyashev, IRIS
Dr. Alexander Taube, IRIS
Abstract
The aim of this R&D is to develop duplex coatings that are suitable for automotive
applications. A topcoat of Termosil ® as the second layer of a duplex system with Nielectroless
as the primary layer has allowed increased corrosion resistance of Nielectroless
from 7 hours to 552 hours without formation of red rust (RR). This
significant increase in corrosion resistance does not result in increased cost of the
duplex coating system in comparison with Ni-electroless plating.
1. Introduction
Corrosion resistance limitations of one-component coatings can be overcome by
combining multi-layer coating systems. It is possible to produce a complex finish with
increased corrosion resistance (Dong H. et al., 1997; Fazio C. et al., 1998; Chang, S.,
1985) and low coefficient of friction. In the simplest case, a multi-layer coating system
consists of two different layers. Double-layer (or duplex) coating systems are defined
as two finishes used in conjunction in order to create superior properties for a combined
finish (Bodycote, 1998). The secondary layer is practical to preliminary treated/coated
steel surface with sherardising, mechanical plating, electroplating, etc. A second layer
can be applied by means of immersion and following topcoat curing.
Surface engineering becomes simpler if wear-resistant and corrosion-resistant
properties of coatings are separated by use of two different technologies consecutively
for obtaining a duplex coating system exhibiting both corrosion and wear resistance.
The technologies are subjected to process-oriented optimization of duplex coatings for
corrosion/wear protection of steels.
Architecture of duplex coating systems possessing both corrosion and wear
protection can be realised as following:
• Primary layer – usually expensive wear-resistant coatings, e.g. Ni-electroless,
TiN, etc, having poor corrosion protection (Korhonen A.S. 1994; Drees D. et al.
1994; Erdemir A. et al. 1985; Motojima S. & Kohno M. 1986)
• Secondary layer – usually an inexpensive finish possessing high corrosion
resistance and low coefficient of friction, such as Termosil, Dacromet,
Dorrltech, etc, but having low hardness and wear resistance.
Corrosion Protection of High Carbon Steel with Duplex Coating System:
Ni-electroless and Termosil
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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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Steel surfaces that have to be plated with Termosil must be clean, rust free, and
in grease free condition. The steel surface was cleaned with solution approximately 1
liter of water and 1 ml activator for 10 minutes.
Industrial procedure was used for application of Termosil coatings
(International Standard Organisation 1993). Termosil coatings were applied on steel
surfaces with preliminary Ni-electroless coating. After centrifuging, such coated
samples were pre-dried by hot air. Further, the raw Termosil coating over Ni-electroless
was cured within range of +200 0 C and +470 0 C. The heat treatment is necessary to
reduce hazardous hexavalent chromium content to safe trivalent chromium system.
This trivalent chromium fills out the main gaps between flakes, increasing the
resistance of moisture and oxidants penetration.
Such prepared samples were ready for a testing. Neutral Salt Spray Test (NSST)
was chosen as a method for testing of corrosion resistance of the samples. The NSST is
described elsewhere (Baboian R. 1997; ASTM 1986; ASTM 1987) and widely used in
the fastener and automotive industry (Crotty D. 1996) for evaluating of corrosion
resistance.
The salt spray apparatus represents an environmentally controlled chamber
containing a salt fog (100% humidity) of solution of 5±1 wt % NaCl with not more than
0.3% of total impurities in distilled water at a temperature held at 45±2 0 C. The size and
detailed construction of the apparatus provide the conditions for this method (ASTM
1986). Sea water is a saline 3.4%-solution of sodium chloride (NaCl) and magnesium
chloride (MgCl), and is slightly alkaline (pH 8). There are many other solution
minerals, which are present in very small quantities. 5%-solution of NaCl, which has
maximum corrosive ability compared to higher/lower NaCl-concentrations, is
considered equivalent to sea water.
3. Results and discussion
At the first stage, it was important to find out the correct composition of
Termosil. Atomic Emission Spectroscopy was used for the aim. Results of the
quantitative analysis of Termosil have been presented (e.g., Figure 3.1). Termosil is
composed predominantly of Zn with addition of aluminium, tin, chromium, and iron.
These aluminium-filled ceramics have electrochemical potential (-0.74 V) in aerated
3.4% NaCl-solution similar to cadmium-plated mild steel (-0.76 V) (Mosser M.F. &
McMordie B.G. 1994). Therefore, this particular composition, like cadmium, is optimal
for corrosion protection.
Zn ( at %) Al (at %) Sn (at %) Cr (at %) Fe (at %)
37-51 6-10 8-18 6-7 2-10
Figure 3.1 – Composition of Termosil coating after sintering.
Corrosion Protection of High Carbon Steel with Duplex Coating System:
Ni-electroless and Termosil
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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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3-4% of cost of Ni-electroless. However, the achievement in corrosion resistance and
lubricity is significant.
5. Conclusion
Application of Termosil as the second layer of duplex system in the aggregate
with Ni-electroless has allowed increasing corrosion resistance of Ni-electroless from 7
hours until 552 hours without formation of red rust. Termosil coating over Nielectroless
increases the cost of this duplex coating system by only 3-5% of the initial
cost of Ni-electroless.
References
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Ni-electroless and Termosil
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Ni-electroless and Termosil
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