This invention relates to a fluxing solution for zinc-aluminium (Zn-AI) alloy casting on steel surfaces through single hot dip process.
This invention further relates to a fluxing solution for Zn-AI alloy coatings through single hot dip process on steel substrate with
significant improved corrosion resistance in all environments, compared to pure zinc coating.
BACKGROUND OF THE INVENTION:
Building materials, structures, white goods and automobiles made of iron and steel are coated with molten zinc for protection against
corrosion. Cathodic protection (CP) reduces corrosion by changing the thermodynamics of the steel, Le. the chemical potential of the steel is changed to make it more inert. More electropositive material like Zn and
Al have a tendency to give sacrificial protection to base steel where the
coating material is likely to be damaged and the base metal remains free
from corrosion. A series of long term exposures test reported that galvanized coating delays the onset of corrosion in marine environment,
but does not prevent it completely. An eleven-year exposure programme in marine environment revealed that the zinc coating suffered 2 to 3 mil loss in thickness of the original zinc layer due to corrosion. Zinc is an amphoteric metal, it stable in a specific range of pH 6-12 only and corrosion behavior of galvanized steel in the presence of chlorides is controlled by the medium pH. Ductility of the coating of pure zinc coated material is very poor due to presence of thick brittle phase zeta. Alloying
elements like copper and cadmium is harmful for ductility of the coating.
Addition of nickel in zinc coating reduces the hydrogen evolution reaction as well increase the corrosion resistance property against chloride. The minimum requirement of nickel in a zinc-nickel alloy is 12 wt % for drastic improvement in corrosion resistance against chloride ions. Alloy making with such composition is cost prohibitive and difficult due to wide difference of their melting temperatures.
Therefore, the need exists in the industry, for a Zn alloy coating which prevents the corrosion of steel completely and at the same time does not affect the properties of steel.
OBJECTS OF THE INVENTION:
It is therefore an object of this invention to propose a fluxing solution for
a zinc-aluminium (Zn-Al)alloy coasting on steel surfaces, which prevents
corrosion of the base metal completely.

It is a further object of this invention to propose a fluxing solution for a
zinc-aluminium (Zn-AI)alloy coasting on steel surfaces, which does not
affect the properties of the base metal.
Another object of this invention is to propose a fluxing solution for a
zinc-aluminium (Zn-AI) alloy coating on steel surfaces, which is cost
effective and simple to produce.
These and other objects and advantages of the invention will be apparent
from the ensuing description.
DESCRIPTION OF THE INVENTION:
This invention relates to a Zinc Aluminium coating on steel substrates.
In accordance with this invention, suitable fluxing solution has been
developed for single hot-dip Zn-Al alloy coating on steel substrate with
significant improvement in coating properties than the conventional pure
Zn coating. The fluxing solution is an aqueous mixture chloride salts.
The chloride salts are selected from zinc chloride, ammonium chloride,
sodium chloride, lead chloride, cobalt chloride and stannous chloride,
the preferred salts being ammonium chloride and zinc chloride. All
grades of water such as tap water, distilled and double distilled water,
and all grades of ammonium and zinc salt can be used. The water to
total salt ratio can be such that 300 to 1000 gms of salt may be used per
litre of water. The major constituent is zinc salt which is present in a
proportion of 44 to 85% by weight of the solution and ammonium salt is
used in 12 to 55% by weight of the solution.
In a preferred embodiment, the total water to salt ratio is 1, ammonium
chloride is 55% and zinc chloride is 45%. Temperature of the fluxing
solution can be varied widely from room temperature to as high as
boiling temperature. Most favorable fluxing solution temperature is in the
range of 85-90°C.
Other salts besides zinc and ammonium salt may be made to vary in the
range of 1 to 5%.
The Zinc-Aluminium alloy includes a Zinc-Aluminium ratio in the range
of 3: 1 to 99: 1. To get a coating at low temperature, the ratio of 19:1 is
most suitable. The dipping time in the fluxing solution can be varied
from a fraction of a second to several minutes and the alloy bath
temperature depends on the alloy composition.
The bath temperature can be varied from 400 to 600°C depending on
alloy composition. Most suitable bath temperature is 40°C above the
melting point of that particular alloy.
Addition of a small amount of Misch metal at 0.05 to 0.15 wt% can
improve the fluidity of the alloy bath.
The coating may be applied in a batch as well as in a continuous
process.
All types of wiping arrangements are suitable for the alloy coating.
Normal air, force air, cooled air, oil or water quench can be used as
cooling media, after alloy coating.
The coating may be applied on steel surfaces which includes wire, pipe,
structure hollow section rebar and sheet metal. The sheet metal includes
cold rolled and hot rolled steel sheet. The cold rolled steel sheet includes
CQ, EDD (extra deep drawable), IF (interstitial free), IF-HS (interstitial
free high strength), DP (Dual phase).

The wires on which the coating may be applied include high, medium
and low carbon wires and all forms of drawn and normalized wires. The
pipes and hollow sections on which the coating may be applied, includes
all sizes and compositions, and the rebar includes TMT (thermo
mechanically treatment), TMT-CRS (corrosion resistance steel) and CTD
(cold twisted deformation) steel.
In the coated surfaces, the corrosion resistance against aggressive saline
environment increased up to 8 times than pure Zn coating for same
coating thickness and the corrosion resistance increased upto 16 times
against industrial environment, for the same coating thickness. The
resistance was found to increase 13 times against white rust formation,
for the same coating thickness.
The tensile and ductility parameters were also found to be much better
for the alloy coated steel rather than for pure zinc coated steel.

WE CLAIM:

1. A fluxing solution for zinc-aluminium alloy casting on steel
surfaces through single hot dip process, said fluxing solution being
an aqueous solution comprising a mixture of chloride salts.
2. The fluxing solution as claimed in claim 1, wherein the chloride
salts are selected from zinc chloride, ammonium chloride, sodium
chloride, lead chloride, cobalt chloride and stannous chloride.
3. The fluxing solution as claimed in claim 1, comprising 300 gms to
1000gms of salt per liter of fluxing solution.
4. The fluxing solution as claimed in claim 1, comprising ammonium
chloride in a proportion of 12 to 55% by weight of the solution.
5. The fluxing solution as claimed in claim 1, comprising Zinc
chloride salt in a proportion of 44 to 85% by weight of the solution.
6. The fluxing solution as claimed in claim 1, wherein the other salts
are present in 1 to 5% by weight of the fluxing solution.
7. The fluxing solution as claimed in claim 1, wherein the aqueous
solution is made using water selected from tap water, single distilled water and double distilled water.
8. A process for coating a substrate with Zn-AI alloy by the fluxing
route, characterized in that the alloy is deposited in a single dip,
and wherein the fluxing solution is an aqueous solution of a mixture of chloride salts.
9. The process as claimed in claim 8, wherein the chloride salts are present in 300 to 1000 gms of salt per liter of fluxing solution.
10. The process as claimed in claim 8, wherein ammonium chloride is present in 12 to 55% by weight.
11. The process as claimed in claim 8, wherein zinc chloride is present in 44 to 85% by weight.
12. The process as claimed in claim 8, wherein the temperature of the fluxing solution varies from room temperature to boiling point of
the solution, preferably in the range of 85 to 90°C.
13. The process as claimed in claim 8, wherein the dipping time in the fluxing solution varies from a fraction of a second to several
minutes.
14. The process as claimed in claim 8, wherein the ratio of Zn and aluminium is in the range of 3: 1 to 99: 1.
15. The process as claimed in claim 8, wherein zinc alloy bath temperature is in the range of 400 to 600°C, preferably 40°C above the melting point of the alloy.
16. The process as claimed in claim 8, wherein the substrate is selected from wire, pipe, structural hollow section rebar, sheet
metal etc.
17. The process as claimed in claim 16, wherein the sheet metal includes hot and cold rolled steel sheet.
18. The process as claimed in claim 17, wherein the cold rolled sheet includes CQ, EDD (extra, deep drawable), IF (interstitial free), IFHS
(interstitial free high strength), DP (Dual phase).
19.The process as claimed in claim 16, wherein the wire includes high, medium low carbon wires, drawn wires and normalized wires.
20.The process as claimed in claim 16, wherein the rebar includes TMT (thermo mechanically treatment), TMT-CRS (corrosion resistance steel) and CTD cold twisted deformation steel.
21.The process as claimed in claim 8, wherein 0.05 to 0.15 wt% of misch metal is added to the alloy bath.
22.The process as claimed in claim 8, wherein the process is a batch process or a continuous process.