FIELD OF THE INVENTION;
This invention relates to a process for the reduction of fretting corrosion
of galvanised sheets.
BACKGROUND OF THE INVENTION:
Fretting is a common phenomenon of corrosion occurring at contact
areas between materials under load subjected to vibration and slip which
can be reduced by lowering the surface friction.
Fretting corrosion is observed in galvanized products the root cause for
this is the presence of higher amount of Al2O3 at the surface.
The fretting corrosion appears as light to dark black spots/marks
distributed over the surface. 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 is has been recognized that his phenomenon occurs more readily
in steel sheets coated with a molten zinc-aluminium alloy containing
several % of aluminum.
Aluminium is added in different proportions to the hot-dip galvanizing
bath to produce specific properties, in the coated product. The A1
addition in the Zinc bath helps in formation of Fe2Al5 at the steel-molten

Zn interface and controls the diffusion of Fe in to the Zn. Beside this;
rest of the Al comes at the surface of the Zn coating and forms a
continuous Al2O3 layer in a approximately 2:3 atoms ratio. The
continuous Al2O3 layer on the coating surface inhibits further oxidation
by acting as a barrier to diffusion of zinc and oxygen atoms. Al2O3 is
abrasive in nature and the surface Al2O3 increases the surface roughness
and affects the friction as well as fretting wear properties of the
galvanized steel sheets. Al2O3 causes higher amount of abrasion at the
surface which leads to higher fretting corrosion. This aluminium oxide
layer aggravates the fretting corrosion during the transportation.
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 Zn, Al and Fe formed on the hot-dipped
galvanized surface are completely dissolved by the treatment with an
alkali metal carbonate solution and this permits the black tarnishing an
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 chromate
treatment. The surface of a zinc or alloy 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 chromate
treatment, after it is washed with water, a chromate film is formed
thereon.
A similar theory is patented by the USA patent 4663245, a steel sheet
coated by hot dipping with zinc or with a zinc alloy containing low
concentrations of Al 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 with
combines both 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 this invention to propose a process for the
reduction of fretting corrosion of galvanised sheets which is cost effective.
It is therefore an object of this invention to propose a process for the
reduction of fretting corrosion of galvanised sheets, which is simple and
uses easily available raw materials.
These and other objects and advantages of the invention will be apparent
from the ensuing description.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
The invention will now be explained in greater detail with the help of the
accompanying drawings where
Figure 1 shows the effect of hydrofluorosilic-acid (H2SiF6) on surface
alumina on galvanized samples.
Figure 2 shows the effect of hydrofluorosilic acid (H2SiF6) on frictional
behaviour of galvanized samples.
DESCRIPTION OF THE INVENTION;
Thus according to this invention is provided a process for the reduction
of fretting corrosion of galvanised sheets. In accordance with this
invention, the amount of alumina (Al2O3) present on the surface of
galvanised sheets is measured.
Hydrofluro silicic acid (H2SiF6) is applied at different concentration along
with conventional chromate solution. The main compound of chromating
bath is chromic acid or bichromate, and the addition of activators to the
bath is required.
As chromic acid is a very strong oxidizing agent, it passivates the metal
surface and a chromate film is not formed. To inhibit the passivation of a
base metal surface, activators are added to the chromate bath. Activators

axe selected from sulphuric acid, hydrochloric acid, nitric acid,
phosphoric acid, and acetic acid etc., and their combinations and
concentrations have an effect on the properties of chromate films.
Chromating coating is composed of a continuous layer that consists of
insoluble trivalent chromium compounds and soluble hexavalent
chromium compounds, which originate from soluble chromate
compounds such as chromic acid, sodium dichromate etc. The coating
structure provides a secondary barrier and inhibiting action. This
chromium coating gives better white rust prevention of galvanized sheets.
In this case in place of other activators, hydrofluro silicic acid is used at
different concentration. A proportion of 3 to 10% by weight of
hydrofluorosilicic acid (H2SiF6) is used in chromate solution which has a
chromate concentration of 3.4 to 17.0 gms/litre of solution, which
includes hexavalent and trivalent chromate ions.
Different concentration of hydrofluoro silicic is added to chromate
solution and the amount of Al2O3 present at the surface is measured. It
is found that with increase in concentration of H2SiF6 in chromate
solution, the amount of Al2O3 present at the surface keep on decreasing
to the amount of Al2O3 decreases, the surface coefficient of friction
decreases. The coefficient friction drops down from 0.9 to 0.2 after
removal of Al2O3. As the fretting corrosion is dependent on surface Al2O3,
subsequent reduction of surface Al2O3 reduces the fretting corrosion of
galvanized sheets.

A 3-10% H2SiF6 in chromate solution reduces the surface Al2O3 of the
galvanized samples in various extents. In an exemplary embodiment the
surface concentration of 1.67 wt% Al2O3 is reduced to about 0.1 wt% as
the H2SiF6 concentration increases to 10%.
The effect of hydrofluorosilicic acid on alumina (Al2O3) content of
surfaces is measured and the results are shown in Figure 1. Different
concentrations of hydrofluorosilic are added to chromate solution and
amount of alumina present on the surface is measured. It may be
concluded that with increase in concentration of H2SiF6 in chromate
solution the amount of Al2O3 present at the surface keeps on decreasing.
As the amount of Al2O3 decreases, the surface co-efficient of friction
decreases which is shown in Figure 2.

WE CLAIM:
1. A composition for the reduction of fretting corrosion in
galvanised steel sheet comprising hydrofluorosilicic acid
(H2SiF6) in a chromate solution.
2. The composition as claimed in claim 1, wherein the chromate
ions present in solution originate from soluble chromate
compound such a chromic acid, sodium dichromate etc.
3. The composition as claimed in claim 1, wherein the chromate
ion is present in a proportion of 3.4 to 17.0 gms/litre.
4. The composition as claimed in claim 1, wherein hydrofluorosilic
acid is present in a proportion of 3 to 10% by weight.

A composition for the reduction of fretting corrosion in galvanised steel sheet comprising hydrofluorosilicic acid (H2SiF6) in a chromate solution.