FIELD OF INVENTION
The present invention relates to a flux composition adaptable in a process of
single-step hot dip coating on steel tubes in a Zn-5%AI alloy bath at an
atmospheric environment.
BACKGROUND OF INVENTION
Modern hot dip galvanisation of steel tubes is an essential operation just after
pickling of the steel tubes to protect the steel surface from oxidation.
Immediately after the pickling operation is completed, a galvanising flux material
is used to protect the steel tubes during the intervening period of pickling and
dipping of the tubes in the zinc or zinc alloy bath.
The galvanising flux also dissolves the traces of metallic oxides that may form on
the steel substrate. To prevent the formation of metallic oxide on the steel
surface, a mixture of inorganic chloride salts are used. The most important
features of the process is to achieve an extremely smooth and shining coating on
the steel surface without any defect, by selecting a ratio in which zinc chloride
and ammonium chloride salts are mixed.
Although the improved corrosion resistance by Zn-AI alloy coatings compared to
standard galvanised coatings on steel is a proven process, its application on
tubes through a single dip process is not possible mainly due to non-availability
of a suitable flux.
Although some patents exist for the flux materials in tube galvanising, there is
hardly any patent available which teaches a suitable flux in a single dip Zn-AI
coating on tubes. US patent 2097194 suggests a mixture of zinc chloride (44%)
and ammonium chloride (56%) that acts well as a galvanising flux. A mixture of
zinc chloride, stannous chloride or alkali metal chloride, alkaline earth metal
chlorides, and alkyl quaternary ammonium salt can also be used for zinc as well
as zinc aluminium alloy coating on steel substrate according to US patent
6921439 B2 and US patent 5292377. The gradual decrease in the weight percent
of ammonium chloride up to 15% is also disclosed in US patent 4911764.
According to US patent 4802932, zinc as well as zinc aluminium bath for
galvanising a steel substrate is used, wherein the composition for flux salts varies
between 80-20% of zinc chloride and 90-10% of ammonium chloride.
Earlier a salt ZnCI2.2NH4CI (mol.wt-243.27) having composition 55.5% ZnCI2
and 44.5% NH4CI was used for fluxing. But, as this 1:2 type salt is very
hygroscopic in nature, it has not given satisfactory results whereas the triple
salt(l:3) having composition 45% ZnCI2 and 51% NH4CI is more stable and
more preferable for the fluxing process.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a flux composition which is
enabled to protect the tube surfaces during the intervening period of pickling and
galvanizing.
Another object of the invention is to propose a flux composition which has a
lower melting point including a high specific heat, and low specific gravity.
A further object of the invention is to propose a flux composition adaptable to
hot dip coating of steel tubes, which possesses low viscosity and surface tension.
SUMMARY OF INVENTION
When the bath of 95% zinc and 5% aluminium is used, the best result is
observed by decreasing the percentage of ammonium chloride, having 80-85%
of zinc chloride and 15-20% of ammonium chloride with the specific gravity of
the mixture at 1.2-1.3.
According to the invention a composition of flux is proposed for a single-step hot
dip coating on steel tubes in Zn-5%AI alloy bath maintained under atmospheric
environment. The flux composition comprising 80-85% of zinc chloride (ZnCI2)
by weight; 15-20% of ammonium chloride (NH4CI) by weight; wherein the zinc
chloride and ammonium chloride salts being dissolved in water and mixed in hot
condition maintained at a temperature between 70-80 °C, wherein the solution of
said flux is heated to achieve a specific gravity of 1.2-1.3, and wherein the pH of
the fluxing solution maintained at 3.5-4.5. The steel tubes after coating develops
a smooth and silvery coating without any bare spot. The selected flux range have
the lowest melting point with a high specific heat in relation to iron, as well as
low specific gravity. The viscosity and the surface tension of the flux composition
is also low which provides an improved wetting of the surface including
dispersion of the hard particles. With this composition the evolution of white
fumes during immersion in molten zinc bath is reduced by 50-70% compared to
the conventional flux.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure - 1 - shows a process flow chart depicting the fluxing process and hot dip
coating on steel tube in a Zn 5%AI alloy bath maintained under atmospheric
environment, according to the invention.
Figure - 2 - shows a photographic view of a Zn-AI coated tube with 85% Zncb
and 15% NH4CI of specific gravity of 1.20 according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The invention proposes a Zinc chloride and ammonium chloride based flux
composition adaptable to flux the steel tubes for protection of the tube surface
from being oxidized during the intervening period of pickling the tubes and
immersing the tubes for coating in the Zn-5%AI alloys bath maintained under
atmospheric environment. The flux composition of the invention is water-based
and having a specific gravity in the range of 1.2 to 1.3.
Figure-1 shows the process steps of the single-step coating of steel tubes in a
galvanising bath including the intermediate step of fluxing the tubes.
After manufacturing of the steel tubes, a step of degreasing the tube (step 101)
at a temperature around 70°C is undertaken using a solution containing at least
10% NaOH. The steel tubes are rinsed with demineralized water (DM-water) at
step 102. After degreasing and rinsing, the steel tubes undergo a pickling step
for atleast 5 to 10 minutes with a solution consisting at least 10% HCL (step
103). At. step 104, the pickled steel tubes are first rinsed with DM-water, and
then fluxed (step 105) with zinc ammonium chloride solution of the invention for
about 2-minutes. At step 106, the pickled and fluxed steel tubes are dried before
galvanizing.
Figure-2 shows a steel tube sample fluxed and coated in a single step with the
flux composition of the invention.
WE CLAIM
1. A flux composition for a single step hot dip coating one steel tubes in a Zn-
5%AI alloy bath maintained under atmospheric environment, comprising:
- 80-85% of zinc chloride (ZnCI2) by weight;
- 15-20% of ammonium chloride (NH4CI) by weight;
wherein the zinc chloride and ammonium chloride salts being dissolved in
water and mixed in hot condition maintained at a temperature between 70-
80°C, wherein the solution of said flux is heated to achieve a specific gravity
of 1.2-1.3, and wherein the pH of the fluxing solution maintained at 3.5-4.5.

The invention relates to a flux composition for a single step hot dip coating one
steel tubes in a Zn-AI alloy bath maintained under atmospheric environment,
comprising: 80-85% of zinc chloride (ZnCI2) by weight; 15-20% of ammonium
chloride (NH4CI) by weight; wherein the zinc chloride and ammonium chloride
salts being dissolved in water and mixed in hot condition maintained at a
temperature between 70-80°C, wherein the solution of said flux is heated to
achieve a specific gravity of 1.2-1.3, and wherein the pH of the fluxing solution
maintained at 3.5-4.5.