Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Nanometric silica as a new surface treatment for the corrosion protection of steels and galvanized steels

The main outcomes of the SILACOR project were the following:

a) Synthesis of silica-based compounds:
- Two suitable basic formulations of silica sols were elaborated and currently used during the research. They were referred as:
-- UFC sol: FZ1 is the main example.

-- Coventya sol: 600/200/1 is the main representative.

They differed principally in the kind of the added silicate stoechiometry (SiO2/Na2O)

--Silica powders were prepared from silica sols, modified and tested successfully as inhibitors in paints by SNCZ.

-- The silica synthesis was upscaled at PRAYON (Batches up to 100 kg).

b) Deposition technology and ranking silica sols:
-Different treatment sequences were defined, depending on the kind of substrate and deposition technology (dip coating, roll and bar coatings, electrophoresis). They produced silica-based films with protection comparable to that of Cr(VI).

c) Reactivity of particles and substrates:
- Polymerisation and fictionalisation onto steel and zinc substrates did not produce any significant enhancement of the silica protection.

- Sol properties (ionisation, adsorption, solubilisation-polymerisation, aggregation, viscosity etc.) were carefully measured.

- The adhesion strength and kinetics of film formation were measured vs. operational parameters

The growth and structure of the silica film were modelized.

d) Comprehension of the corrosion protection:
- A zinc corrosion model was established in relation with the experimental impedance diagram (I.D.).

- The correspondence was established between the nature of corrosion and the I.D. evolution.

- Corrosion inhibition by silica was related to specific evolutions of the voltamograms and I.D.

- Effects of formulation, deposition parameters, coating thickness were systematically measured by R.p., I.D.

- Corrosion of damaged coupons and self healing was investigated on silica treated steel and zinc coated steel.

- Well protected EG coupons exhibited a final I.D. and Rp of the same order of those from Cr(VI) passivation.

- SEM/EDS, TEM and SIMS analysis of oxidized coupons at high temperature produced data on the interdiffusion process (O, Cr, Fe and Si) showing that the protection of silica results from the reduction of the outward flux of Cr.

e) Industrial target:
The main conclusions from the industrial partners are (OCAS report):
- Automotive applications: corrosion protection is of the same order than Cr(VI) passivation and comparable to other alternatives. Good weldability and adhesive bonding. The lack of lubricating properties calls for other additives in the bath formulation.
- Building and appliances:
Promising results fulfilling building requirements (AFNOR XP 34-401, class 4) were obtained in combination with polyester/melamine primer (several pigmentation) and topcoat. Some actions still needed before industrial coil-coating trial.

OCAS is planning an industrial trial with silica-silicate based coating as Cr-free passivation treatment. The formulation FZ1-NH4-HEC would be the best alternative (containing Silica, silicate, ammonium counterions and Hydroxy ethyl cellulose).

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