TITLE; REACTIVE CHROMATE PASSIVATION FOR GALVANIZED MATERIAL
FIELD OF INVENTION
This present invention relates to development of a reactive-chrornate-based passivation
formulation which can be applied as a post treatment coating on the galvanized sheet
surface in continuous galvanizing line. This passivation enhances the white rust
resistance as well as gives an excellent resistance to darkening and fretting corrosion
on galvanized surface.
BACKGROUND OF THE INVENTIONS:
In tiie engineering and construction sectors, hot dip galvanized steel sheets are
preferred by the user for tiieir appearance and long term corrosion protection.
However, the freshly produced galvanized sheets are highly susceptible to external
factors that lead to surface degradation during storage and transport. These surface
defeete are mainly in the form of white rust, darkening and fretting corrosion. The
presence of any one or their combination causes poor acceptance of the material by the
customer. For these reasons, practically all galvanized sheets are given a chromate
passivation treatment after galvanizing in a continuous line. A common passivating
agent used is the chromic acid leading to a complex chromate film of Cr (IV) and Cr
(IE) in compound form on a galvanized surface. Although the generation of white rust
is effectively prevented by subjecting galvanized steel sheets to a chromate treatment
but the other surface defects like blackening and / or fretting corrosion occurs in a
relatively short time during storage, usage or transport. To overcome these surface
defects, we need to understand these defects.

White rust: This is found particularly on fresh galvanized, bright surface and especially
in crevices between closeiy packed sheets. White rust is a white, crumbly, and porous
coating consisting of 2ZnC03,3Zn(OH)2 together with ZnO and voluminous B - Zn(OH)2.
When zinc coating corrode in open air, zinc oxide and zinc hydroxide are normally
formed. If the supply of air to the surface of the zinc is restricted, as in a narrow
crevice, there is not enough carbobn dioxide for the subsequent formation of a zinc
carbonate layer. The layer of zinc oxide and zinc hydroxide is voluminous and porous
and adheres only loosely to the zinc surface. Consequently, it does not protect the zinc
surface against oxygen in the water. Corrosion can, therefore, proceed as long as there
is moisture left on the surface.
Darkening: Darkening is more liable to develop when GI sheets are stored in stacked
form after cutting into pieces or in coiled form than when these are left standing in bare
form in a room. It blackens on usages as well in the open, or in enclosed of normal air.
It has been recognized that this phenomenon occurs more readily in steel sheets in
which the coating surface has been activated by skin pass rolling (usually 1-3% draft)
following zinc coating, and in steel sheets coated with a molten zinc-aluminum alloy
containing several % of aluminum. Also on hot-dipped galvanized steel sheets (non-
skin-passed sheets), black tarnish due to the spangles is occasionally observed in spotty
form during ling-term storage, literature suggests that the larger impurity diffusion in
dull spangle is responsible for its lower resistance to corrosion attack and the
blackening is produced by diffusion and oxidation of antimony /lead that is used as
surfactant.

The galvanized sheet looks black, apparently because the basic zinc carbonate of which
it is composed, and which is represented as (ZnCO.sub.3)x.[Zn(OH).sub.23y, like white
rust, has a particle diameter felling within the visible light wavelength range of 400 to
700 nm, and is, therefore, very likely to scatter and absorb light. The black patina is
considered as a product of corrosion formed in an environment lacking oxygen, and
particularly with the progress of corrosion from the grain boundary.
Fretting Corrosion: The fretting corrosion appears as light to dark black spots / marks
distributed over the surfeee. The mechanism of fretting corrosion is that asperities
rubbing together cause attrition and the debris so formed further aids the process. The
conditions necessary are that the small surface shall be in finer contact or shall move
only over very small) amplitude. This condition occurs during transporting the coils as
they normally stand on the circumference that initiate the fretting to take place. It has
been recognized that this phenomenon occurs more readily in steel sheets coated with
a molten zinc-aluminum alloy containing several % of aluminum. The surface is entirely
covered by a thin film containing aluminium and oxygen in approximately a 2:3 atom
ratio and the film thickness was of the order of 5nm. This aluminium oxide layer
aggravates the fretting corrosion during the transportation.
PRIOR ART:
The above problems are experienced by most of the hot dip galvanized coil producer
and solved by their own way. A large number of literatures and patents are already
made in this area and few are mentioned here.

Japanese Patent Application Laid-Open No. 114695/82 discloses a process for producing
galvanized steel sheets superior resistance to black tarnishing and intergranular
corrosion. This technique comprises treating the galvanized sheets with an alkali metal
carbonate solution, followed by electroplating the treated sheets with zinc. According to
this patent application, the oxides of In, Al, and Fe formed on the hot-dipped
galvanized surfeee are completely dissolved by the treatment with an alkali metal
carbonate solution and this permits the black tarnishing and the intergranular corrosion
to be prevented even when the subsequent zinc electroplating produced a thin film.
Japanese Patent Application Laid-Open No. Sho 59-177381 proposed flashing treatment
with an aqueous solution containing Ni or Co ions as a method of preventing a black
patina from appearing after ehromate freatment The surfeee of a zinc or zine-alioy
plated steel sheet is given flashing treatment with an aqueous solution having a pH of 1
to 4, or 11 to 13, and containing Ni or Co ions, or both prior to its ehromate treatment,
after it is washed with water, a ehromate film is formed thereon.
A similar theory is patented by the ISA patent 4663245, a steel sheet coated by hot
dipping with zinc or with a zinc alloy containing low concentrations of A1 and Mg is
immersed for a short period of time in a solution of Ni ions and / or Co ions or sprayed
with this solution and is subjected to the usual chromating, thereby achieving a superior
effect of preventing black tarnish.

USA patent 6280535 discusses about a chromating process which combines both the
flashing process and chromating process as a single step process by incorporating the
metal ion which is used in the flashing treatment (Ni or Co) into the chromatining bath.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a chromate treatment
coating on the galvanized sheet surface which eliminates the disadvantage of prior Art.
Another object of the present invention is to propose a chromate treatment coating on
the galvanized sheet surfcce which reduces the formation of white rust on the
galvanized sheet surface,
A further object of the present invention is to propose a chromate treatment coating on
the galvanized sheet surface which reduces the formation of darkening on the
galvanized sheet.
A still further object of the present invention is to propose a chromate treatment
coating on the galvanized sheet surfece which reduces the formation of fretting
corrosion.
As yet further object present invention is to propose a chromate treatment coating on
the galvanized sheet surfece which is eco-friendly.

DETAILED DESCRIPTION OF THE INVENTION:
The chromate treatment was given on the galvanized sheet in a continuous hot-
dip galvanizing line. Regular spangles galvanized sheet material was produced in
commercial line and were chromated by the invented chromate solution.
The invented chromate solution comprising:-
total chromium 3.4 - 17.0 gram/litre;
total nitrate ion 1.3 - 6.7 gram/litre;
total cobalt ion 0.2 - 0.4 gram/litre;
total fluorosilicic acid 0.3 - 1.5 gram/litre.
The chromic acid was used to make chromium formulation, the nitrate ions
produced from nitric acid and cobalt nitrate, the cobalt ions produces from cobalt
nitrate and the 30% concentration of fluorosilicic acid were used.
For comparison study, the galvanized sheet material treated by commercially
available chromate passivation solution was taken.
The pH of the invented chromate solution is maintained in the range of 0.7 to
2.5 and the temperature of the chromate solution is maintained in the range of
25 to 65 degree centigrade. The chromate film formed on the galvanized surface
having a coating weight of 5 - 40 mg/m2 in each side of surface in term of total
chromium formation. The duration of wet coating on the surface is in the range
of 1 - 10 seconds for the running strip before drying.

The Chromate passivation Performance was measured by salt spray test as per ASTM B
117. According to this specification, 5% sodium chloride solution is atomized in a salt
spray chamber at 35°C with the solution pH around 7. the test panels are placed at an
angle of 45° in the chamber, exposed to the salt for a certain period and the initiation
of white rust formation on the sample was recorded, The 5% white rust on the
galvanized surface was considered as a cut-off value for the chromate passivation
performance evaluation.
The aluminium leaching from the galvanizing surface after the chromate passivation
was considered as effectiveness of the chromate film in reducing the fretting corrosion.
This galvanized surface analysis was conducted by Scanning Electron Microscope (SEM)
attached with Energy Dispersive Spectroscope (EDS) detector to measure the
aluminium wt. % present in the surface. It was observed that the aluminium % was
less on surface in invented chromate treated product compared to commercially
available chromate treatment. The aluminium leaching value depends on the aluminium
present on the galvanized surface therefore it was not quantified. However, the
reduction in aluminium has been qualitatively evaluated.

TABLE1: COMPARATIVE STUDY OF galvanized sheets treatment with
commercial and invented chromate solution.


WE CLAIM:
1. A reactive chromate passivation solution for galvanized sheet surface to
decorate protecting coating, the chromate passivation solution comprising:
total chromium 3.4 - 17.0 gram/litre;
total nitrate ion 1.3 - 6.7 gram/litre;
total cobalt ion 0.2 - 0.4 gram/litre;
total fluorosilicic acid 0.3 to 1.5 gm/litre;
the PH of chromate solution is 0.7 - 2.5; and
characterized in that chromate ion present in the form of hexavalent
and trivalent in chromate solution along with nitrate and cobalt ion
suppresses the chemical reactivity of aluminum present in the
galvanized sheet which in turn prevents the formation of white rest,
darkening and fretting corrosion.
2. The chromate solution as claimed in claim 1 wherein working temperature
applicable in the range of 25-65°C.
3. The chromate solution as claimed in claim 1 wherein chromate film formed
on the galvanized surface having a coating weight of 5-40 mg/m2 in each
side of surface in term of total chromium formation.
4. The chromate solution as claimed in claim 1 wherein the duration of wet
coating on the surface is in range of 1-10 seconds for the running strip
before drying.

ABSTRACT

REACTIVE CHROMATE PASSIVATION FOR GALVANIZED
MATERIAL
The present invention is provided with a reactive chromate passivation solution
for galvanized sheet surface to decorate protecting coating, the chromate
passivation solution comprising total chromium 3.4 - 17.0 gram/litre; total nitrate
ion 1.3 - 6.7 gram/litre; total cobalt ion 0.2 - 0.4 gram/litre; total fluorosilicic
acid 0.3 to 1.5 gm/litre; the PH of chromate solution is 0.7 - 2.5; and
characterized in that chromate ion present in the form of hexavalent and
trivalent in chromate solution along with nitrate and cobalt ion suppresses the
chemical reactivity of aluminum present in the galvanized sheet which in turn
prevents the formation of white rest, darkening and fretting corrosion.