Note: Descriptions are shown in the official language in which they were submitted.
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FLUX COMPOSITIONS FOR STEEL GALVANIZATION
FIELD OF THE INVENTION
The present invention relates to the field of galvanization, more specifically
hot
dip galvanization or hot-dip zinc coating. In particular the present invention
relates to
the galvanization of ferrous materials such as, but not limited to, iron, cast
iron, steel
and cast steel. More particularly the present invention relates to a range of
flux
compositions for treating the surface of a ferrous material such as iron and
steel before
it is dipped into a zinc-based molten bath. The present invention also relates
to (1)
galvanization processes, in particular hot dip galvanization, making use of
the flux
compositions in at least one process step, and (2) galvanized products,
including
galvanized ferrous products (e.g. steel flat and long products), made by a
process
wherein the product surface is treated with the novel flux compositions.
BACKGROUND OF THE INVENTION
The importance of providing protection against corrosion for ferrous (e.g.
iron or
steel) articles used outdoors such as fences, wires, bolts, cast iron elbows
and
automobile parts is well known, and coating a ferrous material with zinc is a
very
effective and economical means for accomplishing this goal. Zinc coatings are
commonly applied by dipping or passing the article to be coated through a
molten bath
of the metal. This operation is termed "galvanizing", "hot galvanizing" or
"hot-dip
galvanizing" (HOG) to distinguish it from zinc electroplating processes. In
this process,
a solidified layer of zinc is formed on the article surface and the zinc
coating layer
formed as a result is strongly adhered to the surface of the article by an
iron/zinc
intermetallic alloy which forms during galvanizing. Oxides and other foreign
materials
("soil") on the surface of the steel article interfere with the chemistry of
the galvanizing
process and prevent formation of a uniform, continuous, void-free coating.
Accordingly,
various techniques and combinations of techniques have been adopted in
industry to
reduce, eliminate, or at least accommodate, oxides and soil as much as
possible.
Improvement in the properties of galvanized products can be achieved by
alloying zinc with aluminum and/or magnesium. Addition of 5 wt.% aluminum
produces
an alloy with a lower melting temperature (eutectic point at 381 C) which
exhibits
improved drainage properties relative to pure zinc. Moreover, galvanized
coatings
produced from this zinc-aluminum alloy have greater corrosion resistance,
improved
formability and better paintability than those formed from essentially pure
zinc.
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However, zinc-aluminum galvanizing is particularly sensitive to surface
cleanliness so
that various difficulties, such as insufficient steel surface wetting, are
often encountered
when zinc-aluminum alloys are used in galvanizing.
Many techniques and combinations thereof have been adopted in industry to
reduce, eliminate, or at least accommodate, oxides and soil as much as
possible. In
essentially all these processes, organic soil (i.e. oil, grease, rust
preventive
compounds), is first removed by contacting the surface to be coated with an
alkaline
aqueous wash (alkaline cleaning). This may be accompanied by additional
techniques
such as brush scrubbing, ultrasound treatment and/or electro-cleaning. Then
follows
rinsing with water, contacting the surface with an acidic aqueous wash for
removing
iron fines and oxides (pickling), and finally rinsing with water again. All
these cleaning-
pickling-rinsing procedures are common for most galvanizing techniques and are
industrially carried out more or less accurately.
Another pre-treatment method used for high strength steels, steels with high
carbon contents, cast iron and cast steels is a mechanical cleaning method
called
blasting. In this method, rust and dirt are removed from the steel or iron
surface by
projecting small shots and grits onto this surface. Depending on the shape,
size and
thickness of the parts to be treated, different blasting machines are used
such as a
tumble blasting machine for bolts, a tunnel blasting machine for automotive
parts, etc.
There are two main galvanizing techniques used on cleaned metal (e.g. iron or
steel) parts: (1) the fluxing method, and (2) the annealing furnace method.
The first galvanizing technique, i.e. the fluxing method, may itself be
divided into
two categories, the dry fluxing method and the wet fluxing method.
The dry fluxing method, which may be used in combination with one or more of
the above cleaning, pickling, rinsing or blasting procedures, creates a salt
layer on the
ferrous metal surface by dipping the metal part into an aqueous bath
containing
chloride salts, called a "pre-flux". Afterwards, this layer is dried prior to
the galvanizing
operation, thus protecting the steel surface from re-oxidation until its
entrance in a
molten zinc bath. Such pre-fluxes normally comprise aqueous zinc chloride and
optionally contain ammonium chloride, the presence of which has been found to
improve wettability of the article surface by molten zinc and thereby promote
formation
of a uniform, continuous, void-free coating.
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The concept of wet fluxing is to cover the galvanizing bath with a top flux
also
typically comprising zinc chloride, and usually ammonium chloride, but in this
case
these salts are molten and are floating on the top of the galvanizing bath.
The purpose
of a top flux, like a pre-flux, is to supply zinc chloride and preferably
ammonium
chloride to the system to aid wettability during galvanizing. In this case,
all surface
oxides and soil which are left after cleaning-pickling-rinsing are removed
when the steel
part passes through the top flux layer and is dipped into the galvanizing
kettle. Wet
fluxing has several disadvantages such as, consuming much more zinc than dry
fluxing, producing much more fumes, etc. Therefore, the majority of
galvanizing plants
today have switched their process to the dry fluxing method.
Below is a summary of the annealing furnace method. In continuous processes
using zinc or zinc-aluminum or zinc-aluminum-magnesium alloys as the
galvanizing
medium, annealing is done under a reducing atmosphere such as a mixture of
nitrogen
and hydrogen gas. This not only eliminates re-oxidation of previously cleaned,
pickled
and rinsed surfaces but, also actually removes any residual surface oxides and
soil that
might still be present. The majority of steel coils are today galvanized
according to this
technology. A very important requirement is that the coil is leaving the
annealing
furnace by continuously going directly into the molten zinc without any
contact with air.
However this requirement makes it extremely difficult to use this technology
for shaped
parts, or for steel wire since wires break too often and the annealing furnace
method
does not allow discontinuity.
Another technique used for producing zinc-aluminum galvanized coatings
comprises electro-coating the steel articles with a thin (i.e. 0.5 - 0.7 pm)
layer of zinc
(hereafter "pre-layer"), drying in a furnace with an air atmosphere and then
dipping the
pre-coated article into the galvanizing kettle. This is widely used for hot-
dip coating of
steel tubing in continuous lines and to a lesser extent for the production of
steel strip.
Although this does not require processing under reducing atmospheres, it is
disadvantageous because an additional metal-coating step required.
Galvanizing is practiced either in batch operation or continuously. Continuous
operation is typically practiced on articles amenable to this type of
operation such as
wire, sheet, strip, tubing, and the like. In continuous operation, transfer of
the articles
between successive treatments steps is very fast and done continuously and
automatically, with operating personnel being present to monitor operations
and fix
problems if they occur. Production volumes in continuous operations are high.
In a
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continuous galvanizing line involving use of an aqueous pre-flux followed by
drying in a
furnace, the time elapsing between removal of the article from the pre-flux
tank and
dipping into the galvanizing bath is usually about 10 to 60 seconds, instead
of 10 to 60
minutes for a batch process.
Batch operations are considerably different. Batch operations are favored
where
production volumes are lower and the parts to be galvanized are more complex
in
shape. For example, various fabricated steel items, structural steel shapes
and pipe
are advantageously galvanized in batch operations. In batch operations, the
parts to be
processed are manually transferred to each successive treatment step in
batches, with
little or no automation being involved. This means that the time each piece
resides in a
particular treatment step is much longer than in continuous operation, and
even more
significantly, the time between successive treatment steps is much wider in
variance
than in continuous operation. For example, in a typical batch process for
galvanizing
steel pipe, a batch of as many as 100 pipes after being dipped together in a
pre-flux
bath is transferred by means of a manually operated crane to a table for
feeding, one at
a time, into the galvanizing bath.
Because of the procedural and scale differences between batch and continuous
operations, techniques particularly useful in one type of operation are not
necessarily
useful in the other. For example, the use of a reducing furnace is restricted
to
continuous operation on a commercial or industrial scale. Also, the high
production
rates involved in continuous processes make preheating a valuable aid in
supplying
make-up heat to the galvanizing bath. In batch processes, delay times are much
longer
and moreover production rates, and hence the rate of heat energy depletion of
the
galvanizing bath, are much lower.
There is a need to combine good formability with enhanced corrosion protect-
tion of the ferrous metal article. However, before a zinc-based alloy coating
with high
amounts of aluminum (and optionally magnesium) can be introduced into the
general
galvanizing industry, the following difficulties have to be overcome:
- zinc alloys with high aluminum contents can hardly be produced using the
standard zinc-ammonium chloride flux. Fluxes with metallic Cu or Bi deposits
have been proposed earlier, but the possibility of copper or bismuth leaching
into the zinc bath is not attractive. Thus, better fluxes are needed.
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- high-aluminum content alloys tend to form outbursts of zinc-iron
intermetallic
alloy which are detrimental at a later stage in the galvanization. This
phenomenon leads to very thick, uncontrolled and rough coatings. Control of
outbursts is absolutely essential.
wettability issues were previously reported in Zn-Al alloys with high-aluminum
content, possibly due to a higher surface tension than pure zinc. Hence bare
spots due to a poor wetting of steel are easily formed, and hence a need to
lower the surface tension of the melt.
- a poor control of coating thickness was reported. in Zn-Al alloys with high-
aluminum content, possibly depending upon parameters such as temperature,
flux composition, dipping time, steel quality, etc.
WO 02/42512 describes a flux for hot dip galvanization comprising 60-80 wt.%
zinc
chloride; 7-20 wt.% ammonium chloride; 2-20 wt.% of at least one alkali or
alkaline
earth metal salt; 0.1-5 wt.% of a least one of NiCl2, CoCl2 and MnC12; and 0.1-
1.5 wt.%
of at least one of PbCl2, SnCl2, SbCI3 and BiCI3. Preferably this flux
comprises 6 wt.%
NaCI and 2 wt.% KCI. Examples 1-3 teach flux compositions comprising 0.7-1
wt.%
lead chloride.
WO 2007/146161 describes a method of galvanizing with a molten zinc-alloy
comprising the steps of (1) immersing a ferrous material to be coated in a
flux bath in
an independent vessel thereby creating a flux coated ferrous material, and (2)
thereafter immersing the flux coated ferrous material in a molten zinc-
aluminum alloy
bath in a separate vessel to be coated with a zinc-aluminum alloy layer,
wherein the
molten zinc-aluminum alloy comprises 10-40 wt.% aluminum, at least 0.2 wt.%
silicon,
and the balance being zinc and optionally comprising one or more additional
elements
selected from the group consisting of magnesium and a rare earth element. In
step (1),
the flux bath may comprise from 10-40 wt.% zinc chloride, 1-15 wt. A ammonium
chloride, 1-15 wt.% of an alkali metal chloride, a surfactant and an acidic
component
such that the flux has a final pH of 1.5 or less. In another embodiment of
step (1), the
flux bath may be as defined in WO 02/42512.
JP 2001/049414 describes producing a hot-dip Zn-Mg-Al base alloy coated
steel sheet excellent in corrosion resistance by hot-dipping in a flux
containing 61-80
wt.% zinc chloride, 5-20 wt.% ammonium chloride, 5-15 wt. % of one or more
chloride,
fluoride or silicafluoride of alkali or an alkaline earth metal, and 0.01-5
wt.% of one or
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more chlorides of Sn, Pb, In, TI, Sb or Bi. More specifically, table 1 of JP
2001/049414
discloses various flux compositions with a KCl/NaCI weight ratio ranging from
0.38 to
0.60 which, when applied to a steel sheet in a molten alloy bath comprising
0.05-7
wt.% Mg, 0.01-20 wt.% Al and the balance being zinc, provide a good plating
ability, no
pin hole, no dross, and flat. By contrast, table 1 of JP 2001/049414 discloses
a flux
composition with a KCl/NaCI weight ratio of 1.0 which, when applied to a steel
sheet in
a molten alloy bath comprising 1 wt.% Mg, 5 wt.% Al and the balance being
zinc,
provides a poor plating ability, pin hole defect, some dross, and poorly flat.
Chinese patent application No. 101948990 teaches an electrolytic flux for hot
dip
galvanization of a steel wire, comprising g/L 30-220 g/L zinc chloride, 2-90
g/L
ammonium chloride, 0-150 g/L potassium chloride, 0-150 g/L sodium chloride, 0-
100
g/L boric acid, 0-70 g/L acetic acid,1-25 g/L sodium fluoride, 2-50 g/L cerium
chloride,
0-50 g/L potassium fluozirconate, 0-50 methanol, 0.5-20 g/L hydrogen peroxide,
and
the balance water. Hydrogen peroxide is used as an oxidant and, since the pH
value is
Thus, the common teaching of the prior art is a preferred KCl/NaCI weight
ratio
below 1.0 in fluxing compositions with major proportions (more than 50 wt.%)
of zinc
chloride. However the prior art has still not resolved most of the technical
problems
outlined hereinbefore. Consequently there is still a need in the art for
improved fluxing
compositions and galvanizing methods making use thereof.
The object of the present invention is to provide a flux composition making it
possible to produce continuous, more uniform, smoother and void-free coatings
on
metal articles, in particular iron or steel articles, of any shape and size by
hot dip
galvanization with pure zinc or zinc alloys, in particular zinc-aluminum
alloys and zinc-
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defined in claim 6. Specific embodiments of this invention are defined in
dependent
claims 2-5 and 7-15.
DETAILED DESCRIPTION OF THE INVENTION
As defined in claim 1, the essential feature of this invention is the
recognition
that huge improvements in galvanization of metals, in particular iron and
steel, can be
achieved when starting from a flux composition having a set of at least two
alkali metal
chlorides including sodium chloride and potassium chloride, provided that the
KCl/NaCI
weight ratio of said set of at least two alkali metal chlorides ranges from
2.0 to 8Ø This
feature is associated with specific amounts of other flux components.
Definitions
The term "hot dip galvanization" is meant to designate the corrosion treatment
of a metal article such as, but not limited to, an iron or steel article by
dipping into a
molten bath of pure zinc or a zinc-alloy, in continuous or batch operation,
for a
sufficient period of time to create a protective layer at the surface of said
article. The
term "pure zinc" refers to zinc galvanizing baths that may contain trace
amounts of
some additives such as for instance antimony, bismuth, nickel or cobalt. This
is in
contrast with "zinc alloys" that contain significant amounts of one or more
other metals
such as aluminum or magnesium.
In the following the different percentages relate to the proportion by weight
(wt.%) of each component with respect to the total weight (100%) of the flux
compo-
sition or zinc-based bath. This implies that not all maximum or minimum
percentages
can be present at the same time, in order for the sum to match to 100 wt.%.
In one embodiment of this invention, the specified KCl/NaCI weight ratio is
associated with the presence of lead chloride in the flux composition. The
proportion of
lead chloride may be at least 0.1 wt.%, or at least 0.4 wt.% or at least 0.7
wt.% of the
flux composition. In another embodiment of this invention, the proportion of
lead
chloride in the flux composition may be at most 2 wt.%, or at most 1.5 wt.% or
at most
1.2 wt.%. In a specific embodiment of this invention, the proportion of lead
chloride in
the flux composition is from 0.8 to 1.1 wt.%.
In one embodiment of this invention, the specified KCl/NaCI weight ratio is
associated with the presence of tin chloride in the flux composition. The
proportion of
tin chloride in the flux composition may be at least 2 weight % or at least
3.5 weight %
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or at least 7 weight %. In another embodiment of this invention, the
proportion of tin
chloride in the flux composition is at most 14 weight %.
In one embodiment, the combined amounts of lead chloride and tin chloride
represent at least 2.5 wt.%, or at most 14 wt.% of the flux composition. In
another
embodiment, the flux composition may further comprise other salts of lead
and/or tin,
such as the fluoride, or other chemicals that are inevitable impurities
present in
commercial sources of lead chloride and/or tin chloride.
In one aspect of this invention, the specified KCl/NaCI weight ratio is
combined
with specified proportions of other chlorides that make it possible to produce
continuous, more uniform, smoother and void-free coatings on metal, in
particular iron
or steel, articles by galvanization, in particular hot dip galvanization,
processes with
molten zinc or zinc-based alloys, especially in batch operation or
continuously.
For instance, the specified KCl/NaCI weight ratio in the flux composition is
combined with more than 40 and less than 70 wt.% zinc chloride. In one
embodiment
of this invention, the proportion of zinc chloride in the flux composition is
at least 45
wt.% or at least 50 wt.%. In another embodiment, the proportion of zinc
chloride in the
flux composition is at most 65 wt.% or at most 62 wt.%. These selected
proportions of
ZnCl2 are capable, in combination with the specified KCl/NaCI weight ratio in
the flux
composition, to ensure a good coating of the metal article to be galvanized
and to
effectively prevent oxidation of the metal article during subsequent process
steps such
as drying, i.e. prior to galvanization itself.
In one aspect of this invention, the specified KCl/NaCI weight ratio in the
flux
composition is combined with 10-30 wt.% ammonium chloride. In one embodiment,
the
proportion of NFI4C1 in the flux composition is at least 13 wt.% or at least
17 wt.%. In
another embodiment, the proportion of ammonium chloride in the flux
composition is at
most 26 wt.% or at most 22 wt.%. The optimum proportion of NH4C1 may be
determined by the skilled person, without extensive experimentation and
depending
upon parameters such as the metal to be galvanized and the weight proportions
of the
metal chlorides in the flux composition, by simply using the experimental
evidence
shown in the following examples, to achieve a sufficient etching effect during
hot
dipping to remove residual rust or poorly pickled spots, while however
avoiding the
formation of black spots, i.e. uncoated areas of the metal article. In some
circumstances it may be useful to substitute a minor part (e.g. less than 1/3
by weight)
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of NH4CI with one or more alkyl quaternary ammonium salt(s) wherein at least
one
alkyl group has from 8 to 18 carbon atoms such as described in EP 0488.423,
for
instance an alkyl-trimethylammonium chloride (e.g. trimethyllauryl-ammonium
chloride)
or a dialkyldimethylammonium chloride.
In one aspect of this invention, the specified KCl/NaCI weight ratio in the
flux
composition is further combined with the presence of suitable amounts of one
or more
alkali or alkaline earth metal halides, in particular optional halides from
alkali or alkaline
earth metals other than K and Na. These halides are preferably or
predominantly
chlorides (bromides and iodides may be useful as well), and the other alkali
or alkaline
earth metals may be selected (sorted in decreasing order of preference in each
metal
class) from the group consisting of Li, Cs, Mg, Ca, Sr and Ba. Preferably,
fluorides
should be avoided for safety and/or toxicity reasons, i.e. the flux
compositions should
be fluoride salts-free. In one embodiment, the set of at least two alkali
metal chlorides,
optionally together with halides from alkali or alkaline earth metals other
than K and Na,
represents 6-30 wt.% of the flux composition. In another embodiment, the set
of at
least two alkali metal chlorides includes sodium chloride and potassium
chloride as
major or only components. In another embodiment, the set of at least two
alkali metal
chlorides (e.g. including sodium chloride and potassium chloride as major or
only
components) represents at least 12 wt.% or at least 15 wt.% of the flux
composition. In
another embodiment, the set of at least two alkali metal chlorides (e.g.
including
sodium chloride and potassium chloride as or only major components) represents
at
most 25 wt.%, or at most 21 wt.%, of the flux composition. NaBr, KBr, MgCl2
and/or
CaCl2 may be present as minor components in each of the above stated
embodiments.
In one aspect of this invention, the specified KCl/NaCI weight ratio in the
flux
composition is further combined with the presence of suitable amounts of one
or more
other metal (e.g. transition metal or rare earth metal) chlorides such as, but
not limited
to, nickel chloride, cobalt chloride, manganese chloride, cerium chloride and
lanthanum
chloride. For instance, some examples below demonstrate that the presence of
up to 1
wt.% (even up to 1.5 wt.%) nickel chloride is not detrimental to the behavior
of the flux
composition of the present invention in terms of quality of the coating
obtained after hot
dip galvanization. Other metal chlorides that may be present include bismuth
chloride,
antimony chloride and the like.
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In order to solve the stated problems and achieve the stated advantages, the
KCl/NaCI weight ratio is important. In anyone embodiment of this invention,
the
KCl/NaCI weight ratio may for instance be from 3.5 to 5.0, or from 3.0 to 6Ø
In other aspects of this invention, the specified respective KCl/NaCI weight
ratio
in the flux composition is further combined with the presence of other
additives,
preferably functional additives participating in tuning or improving some
desirable
properties of the flux composition. Such additives are presented below.
For instance the flux composition of this invention may further comprise at
least one
nonionic surfactant or wetting agent which, when combined with the other
ingredients,
is capable of achieving a predetermined desirable surface tension. Essentially
any type
of nonionic surfactant, but preferably liquid water-soluble, can be used.
Examples
thereof include ethoxylated alcohols such as nonyl phenol ethoxylate, alkyl
phenols
such as Triton X-102 and Triton N101 (e.g. from Union Carbide), block
copolymers of
ethylene oxide and propylene oxide such as L-44 (from BASF), and tertiary
amine
ethoxylates derived from coconut, soybean, oleic or tallow oils (e.g. Ethomeen
from
AKZO NOBEL), polyethoxylated and polypropoxylated derivatives of alkylphenols,
fatty
alcohols, fatty acids, aliphatic amines or amides containing at least 12
carbon atoms in
the molecule, alkylarene-sulfonates and dialkylsulfosuccinates, such as
polyglycol
ether derivatives of aliphatic and cycloaliphatic alcohols, saturated and
unsaturated
fatty acids and alkylphenols, said derivatives preferably containing 3-10
glycol ether
groups and 8-20 carbon atoms in the (aliphatic) hydrocarbon moiety and 6-18
carbon
atoms in the alkyl moiety of the alkylphenol, water-soluble adducts of
polyethylene
oxide with poylypropylene glycol, ethylene-diaminopolypropylene glycol
containing 1-10
carbon atoms in the alkyl chain, which adducts contain 20-250 ethyleneglycol
ether
groups and/or 10-100 propyleneglycol ether groups, and mixtures thereof. Such
compounds usually contain from 1-5 ethyleneglycol (EO) units per
propyleneglycol unit.
Representative examples are nonylphenol-polyethoxyethanol, castor oil
polyglycolic
ethers, polypropylene-polyethylene oxide adducts, tributyl-phenoxypolyethoxy-
ethanol,
polyethylene-glycol and octylphenoxypolyethoxyethanol. Fatty acid esters of
polyethylene sorbitan (such as polyoxyethylene sorbitan trioleate), glycerol,
sorbitan,
sucrose and pentaerythritol, and mixtures thereof, are also suitable non-ionic
surfactants. Low foaming wetting agents such as the ternary mixtures described
in U.S.
Patent No. 7,560,494 are also suitable. Commercially available non-ionic
surfactants of
the above-mentioned types include those marketed by Zschimmer & Schwarz GmbH &
CA 02831049 2013-10-23
Co KG (Lahnstein, Germany) under the trade names OXETAL, ZUSOLAT and
PROPETAL, and those marketed by Alfa Kimya (Istanbul, Turkey) under the trade
name NETZER SB II. Various grades of suitable non-ionic surfactants are
available
under the trade name MERPOL.
The hydrophilic-lipophilic balance (HLB) of said at least one nonionic
surfactant is
not a critical parameter of this invention and may be selected by the skilled
person
within a wide range from 3 to 18, for instance from 6 to 16. E.g. the HLB of
MERPOL-A
is 6 to 7, the HLB of MERPOL-SE is 11, and the HLB of MERPOL-HCS is 15.
Another
feature of the nonionic surfactant is its cloud point (i.e. the temperature of
phase
separation as may me determined e.g. by ASTM D2024-09 standard test method;
this
behavior is characteristic of non-ionic surfactants containing polyoxyethylene
chains,
which exhibit reverse solubility versus temperature in water and therefore
"cloud out" at
some point as the temperature is raised; glycols demonstrating this behavior
are known
as "cloud-point glycols") which should preferably be higher than the flux
working
temperature as defined below with respect to the use of a fluxing bath in a
hot dip
galvanization process. Preferably the cloud point of the nonionic surfactant
should be
higher than 90 C.
Suitable amounts of nonionic surfactants are well known from the skilled
person and
usually range from 0.02 to 2.0 wt.%, preferably from 0.5 to 1.0 wt.%, of the
flux
composition, depending upon the selected type of compound.
The flux compositions of the invention may further comprise at least one
corrosion inhibitor, i.e. a compound inhibiting the oxidation of steel
particularly in
oxidative or acidic conditions. In one embodiment, the corrosion inhibitor
includes at
least an amino group. Inclusion of such amino derivative corrosion inhibitors
in the flux
compositions can significantly reduce the rate of iron accumulation in the
flux tank. By
"amino derivative corrosion inhibitor" is meant herein a compound which
inhibits the
oxidation of steel and contains an amino group. Aliphatic alkyl amines and
quaternary
ammonium salts (preferably containing 4 independently selected alkyl groups
with 1-12
carbon atoms) such as alkyl dimethyl quaternary ammonium nitrate are suitable
examples of this type of amino compounds. Other suitable examples include
hexamethylenediamines. In another embodiment, the corrosion inhibitor includes
at
least one hydroxyl group, or both a hydroxyl group and an amino group and are
well
known to those skilled in the art. Suitable amounts of the corrosion inhibitor
are well
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known from the skilled person and usually range from 0.02 to 2.0 wt.%,
preferably 0.1-
1.5 wt.%, or 0.2-1.0 wt.%, depending upon the selected type of compound. The
flux
compositions of the invention may comprise both at least one corrosion
inhibitor and a
nonionic surfactant or wetting agent as defined hereinabove.
In anyone of the above embodiments, the flux compositions of the invention are
preferably free from volatile organics, e.g. acetic acid, boric acid and
methanol,
especially those banned from galvanization units by legislation (safety,
toxicity).
The flux compositions of the invention may be produced by various methods.
They can simply be produced by mixing, preferably thoroughly (e.g. under high
shear),
the essential components (i.e. zinc chloride, ammonium chloride, alkali metal
chlorides)
and, if need be, the optional ingredients (i.e. lead chloride, tin chloride,
alkyl quaternary
ammonium salt(s), other transition or rare earth metal chlorides, other alkali
or alkaline
earth metal halides, corrosion inhibitor(s) and/or nonionic surfactant(s)) in
any possible
order in one or more mixing steps. The flux compositions of the invention may
also be
produced by a sequence of at least two steps, wherein one step comprises the
dissolution of lead chloride in ammonium chloride or sodium chloride or a
mixture
thereof, and wherein in a further step the solution of lead chloride in
ammonium
chloride or sodium chloride or a mixture thereof is then mixed with the other
essential
components (i.e. zinc chloride, potassium chloride) and, if need be, the
optional
ingredients (as listed above) of the composition. In one embodiment of the
latter
method, dissolution of lead chloride is carried out in the presence of water.
In another
embodiment of the latter method, it is useful to dissolve an amount ranging
from 8 to 35
g/I lead chloride in an aqueous mixture comprising from 150 to 450 g/I
ammonium
chloride and/or or sodium chloride and the balance being water. In particular
the latter
dissolution step may be performed at a temperature ranging from 55 C to 75 C
for a
period of time ranging from 4 to 30 minutes and preferably with stirring.
A significant advantage of a flux composition of the invention is its broad
field of
applicability (use). The present flux compositions are particularly suitable
for batch hot
dip galvanizing processes using a wide range of zinc alloys but also pure
zinc.
Moreover, the present flux can also be used in continuous galvanizing
processes using
either zinc-aluminum or zinc-aluminum-magnesium or pure zinc baths, for
galvanizing
a wide range of metal pieces, e.g. wires, pipes, tubes or coils (sheets),
especially made
from ferrous materials like iron and steel (e.g. steel flat and long
products).
12
CA 02831049 2013-10-23
According to another aspect, the present invention thus relates to a fluxing
bath
for galvanization, in particular hot dip galvanization, wherein a suitable
amount of a flux
composition according to any one of the above embodiments is dissolved in
water or
an aqueous medium. Methods for water-dissolving a flux composition based on
zinc
chloride, ammonium chloride, alkali metal chlorides and optionally one or more
chlorides of a transition or rare earth metal (e.g. lead, tin, nickel, cobalt,
cerium,
lanthanum) are well known in the art. The total concentration of components of
the flux
composition in the fluxing bath may range within very wide limits such as 200-
750 WI,
preferably 350-750 g/I, most preferably 500-750 g/I or 600-750 g/I. This
fluxing bath is
particularly adapted for hot dip galvanizing processes using zinc-aluminum
baths, but
also with pure zinc galvanizing baths, either in batch or continuous
operation.
For use in hot dip galvanization processes (whether batch or continuous), the
fluxing bath of this invention should advantageously be maintained at a
temperature
within a range of 50 C-90 C, preferably 60 C-90 C, most preferably 65 C-85 C.
The
process comprises a step of treating (fluxing), e.g. Immersing, a metal
article in a
fluxing bath according to any one of the above embodiments. Preferably, in
discontinuous (batch) operation, said treatment step is performed at a speed
output in
the range of 1-12 m/min. or 2-8 m/min, for a period of time ranging from 0.01
to 30
minutes, or 0.03 to 20 minutes, or 0.5 to 15 minutes, or 1 to 10 minutes
depending
upon operating parameters such as the composition and/or temperature of the
fluxing
bath, the composition of the metal (e.g. steel) to be galvanized, the shape
and/or size
of the article. As is well known to the skilled person, the treatment time may
widely vary
from one article to the other: the shorter times (close to or even below 0.1
minute) are
suitable for wires, whereas the longer times (closer to 15 minutes or more)
are more
suitable for instance for rods. In continuous operation, the metal treatment
step, i.e.
immersion in the fluxing bath, may be performed at a speed from 0.5 to 10
m/minute, or
1-5 m/minute. Much higher speeds of 10-100 m/min, e.g. 20-60 m/min, can also
be
achieved.
Practically, any metal surface susceptible to corrosion, for instance any type
of
iron or steel article may be treated this way. The shape (flat or not),
geometry (complex
or not) or the size of the metal article are not critical parameters of the
present
invention. The article to be galvanized may be a so-called long product. As
used herein
the term "long product" refers to products with one dimension (length) being
at least 10
times higher than the two other dimensions (as opposed to flat products
wherein two
13
CA 02831049 2013-10-23
dimensions (length and width) are at least 10 times higher than thickness, the
third
dimension) such as, wires (coiled or not, for making e.g. bolts and fences),
rods,
bobbins, reinforcing bars, tubes (welded or seamless), rails, structural
shapes and
sections (e.g. l-beams, H-beams, L-beams, T-beams and the like), or pipes of
any
dimensions e.g. for use in civil construction, mechanical engineering, energy,
transport
(railway, tramway), household and furniture. The metal article to be
galvanized may
also be, without limitation, in the form of a flat product such as plates,
sheets, panels,
hot-rolled and cold-rolled strips (either wide 600 mm and above, or narrow
below 600
mm, supplied in regularly wound coils or super imposed layers) being rolled
from slabs
(50-250 mm thick, 0.6-2.6 m wide, and up to 12 m long) and being useful in
automotive, heavy machinery, construction, packaging and appliances.
It is important in any galvanizing process for the surface of the article to
be
galvanized to be suitably cleaned before performing the fluxing step.
Techniques for
achieving a desirable degree of surface cleanliness are well known in the art,
and may
be repeated, such as alkaline cleaning, followed by aqueous rinsing, pickling
in acid
and finally aqueous rinse. Although all of these procedures are well known,
the
following description is presented for the purpose of completeness.
Alkaline cleaning can conveniently be carried out with an aqueous alkaline
composition also containing phosphates and silicates as builders as well as
various
surfactants. The free alkalinity of such aqueous cleaners can vary broadly.
Thus at an
initial process step, the metal article is submitted to cleaning (degreasing)
in a
degreasing bath such as an ultrasonic, alkali degreasing bath. Then, in a
second step,
the degreased metal article is rinsed. Next the metal article is submitted to
one or more
pickling treatment(s) by immersion into an aqueous strongly acidic medium,
e.g.
hydrochloric acid or sulfuric acid, usually at a temperature from 15 C to 60 C
and
during 1-90 minutes (preferably 3-60 minutes), and optionally in the presence
of a
ferrous and/or ferric chloride. Acid concentrations of about 5 to 15 wt.%,
e.g. 8-12
wt.%, are normally used, although more concentrated acids can be used. In a
continuous process the pickling time typically ranges from 5 to 30 seconds,
more
typically 10 to 15 seconds. In order to prevent over-pickling, one may include
in the
pickling bath at least one corrosion inhibitor, typically a cationic or
amphoteric surface
active agent, typically in an amount ranging from 0.02 to 0.2 wt.%, preferably
0.05-0.1
wt.%. Pickling can be accomplished simply by dipping the article in a pickling
tank.
Additional processing steps can also be used. For example, the article can be
agitated
14
CA 02831049 2013-10-23
either mechanically or ultrasonically, and/or an electric current can be
passed through
the article for electro-pickling. As is well known these additional processing
means
usually shorten pickling time significantly. Clearly these pre-treatment steps
may be
repeated individually or by cycle if needed until the desirable degree of
cleanliness is
achieved. Then, preferably immediately after the cleaning steps, the metal
article is
treated (fluxed), e.g. immersed, in a fluxing bath of the invention,
preferably under the
total salt concentration, temperature and time conditions specified above, in
order to
form a protective film on its surface.
The fluxed metal (e.g. iron or steel) article, i.e. after immersion in the
fluxing
bath during the appropriate period of time and at the suitable temperature, is
preferably
subsequently dried. Drying may be effected, according to prior art conditions,
by
transferring the fluxed metal article through a furnace having an air
atmosphere, for
instance a forced air stream, where it is heated at a temperature from 220 C
to 250 C
until its surface exhibited a temperature between 170 C and 200 C, e.g. for 5
to 10
minutes. However it has also been surprisingly found that milder heating
conditions
may be more appropriate when a fluxing composition of the invention, or any
particular
embodiment thereof, is used.
Thus it has been found that it may be sufficient for the surface of the metal
(e.g.
steel) article to exhibit a temperature from 100 to 200 C during the drying
step. This
can be achieved for instance by using a heating temperature ranging from 100 C
to
200 C. This can also be achieved by using a poorly oxidative atmosphere during
the
drying step. In one embodiment of the invention, the surface temperature of
the metal
article may range from 100 C to 160 C, or 125-150 C, or 140-170 C. In another
embodiment, drying may be effected for a period of time ranging from 0.5 to 10
minutes, or 1-5 minutes. In another embodiment, drying may be effected in
specific gas
atmospheres such as a water-depleted air atmosphere, a water-depleted nitrogen
atmosphere, or a water-depleted nitrogen-enriched air atmosphere (e.g. wherein
the
nitrogen content is above 20%).
At a next step of the galvanization process, the fluxed and dried metal
article is
dipped into a molten zinc-based galvanizing bath to form a metal coating
thereon. As is
well known, the dipping time may be defined depending upon a set of parameters
including the size and shape (e.g. flat or long) of the article, the desired
coating
thickness, and the exact composition of the zinc bath, in particular its
aluminum content
(when a Zn-Al alloy is used as the galvanizing bath) or magnesium content
(when a
CA 02831049 2013-10-23
Zn-Al-Mg alloy is used as the galvanizing bath). In one embodiment, the molten
zinc-
based galvanizing bath may comprise (a) from 4 to 24 wt.% (e.g. 5 to 20 wt.%)
aluminum, (b) from 0.5 to 6 wt% (e.g. 1 to 4 wt.%) magnesium, and (c) the rest
being
essentially zinc. In another embodiment, the molten zinc-based galvanizing
bath may
comprise tiny amounts (i.e. below 1.0 wt.%) or trace amounts (i.e. unavoidable
impurities) of other elements such as, but not limited to, silicium (e.g. up
to 0.3 wt.%),
tin, lead, titanium or vanadium. In another embodiment, the molten zinc-based
galvanizing bath may be agitated during a part of this treatment step. During
this
process step the zinc-based galvanizing bath is preferably maintained at a
temperature
ranging from 360 C to 600 C. It has been surprisingly found that with the flux
composition of the invention it is possible to lower the temperature of the
dipping step
whilst obtaining thin protective coating layers of a good quality, i.e. which
are capable
of maintaining their protective effect for an extended period of time such as
five years
or more, or even 10 years or more, depending upon the type of environmental
conditions (air humidity, temperature, etc). Thus in one embodiment, the
molten zinc-
based galvanizing bath is kept at a temperature ranging from 350 C to 550 C,
or 380-
520 C, or 420-520 C, the optimum temperature depending upon the content of
aluminum and/or magnesium optionally present in the zinc-based bath. In
another
particular embodiment of the galvanization process of the invention, dipping
is
performed at a temperature ranging between 380 C and 440 C, and said molten
zinc-
based galvanizing bath comprises (a) from 4 to 7 weight % aluminum, (b) from
0.5 to 3
weight % magnesium, and (c) the rest being essentially zinc.
In one embodiment, the thickness of the protective coating layer obtained by
carrying out the dipping step on a metal article, e.g. an iron or steel
article, that has
been treated with the flux composition of this invention may range from 5 to
50 pm, for
instance from 8 to 30 pm. This can be appropriately selected by the skilled
person,
depending upon a set of parameters including the thickness and/or shape of the
metal
article, the stress and environmental conditions that the metal article is
supposed to
withstand during its lifetime, the expected durability in time of the
protective coating
layer formed, etc. For instance a 5-15 pm thick coating layer is suitable for
a steel
article being less than 1.5 mm thick, and a 20-35 pm thick coating layer is
suitable for a
steel article being more than 6 mm thick.
16
CA 02831049 2013-10-23
Finally, the metal, e.g. iron or steel, article is removed from the
galvanizing bath
and cooled. This cooling step may conveniently be carried out either by
dipping the
galvanized metal article in water or simply by allowing it to cool down in
air.
The present hot dip galvanization process has been found to allow the
continuous or batch deposition of thinner, more uniform, smoother and void-
free,
protective coating layers on iron or steel articles (both flat and long
products),
especially when a zinc-aluminum or zinc-aluminum-magnesium galvanizing bath
with
not more than 95% zinc was used. Regarding roughness, the coating surface
quality is
equal to or better than that achieved with a conventional HOG zinc layer
according to
EN ISO 1461 (i.e. with not more than 2% other metals in the zinc bath).
Regarding
corrosion resistance, the coating layers of this invention achieve about 1,000
hours in
the salt spray test of ISO 9227 which is much better than the about 600 hours
achieved with a conventional HOG zinc layer according to EN ISO 1461.
Moreover,
pure zinc galvanizing baths may also be used in the present invention.
Moreover the process of the present invention is well adapted to galvanize
steel
articles of any shape (flat, cylindrical, etc.) such as wires, sheets, tubes,
rods, rebars
and the like, being made from a large variety of steel grades, in particular
articles made
from steel grades having a carbon content up to 0.30 wt.%, a phosphorous
content
between 0.005 and 0.1 wt.% and a silicon content between 0.0005 and 0.5 wt.%,
as
well as stainless steel. The classification of steel grades is well known to
the skilled
person, in particular through the Society of Automotive Engineers (SAE). In
one
embodiment, the metal may be a chromium/nickel or chromium/nickel/molybdenum
steel susceptible to corrosion. Optionally the steel grade may contain other
elements
such as sulfur, aluminum, and copper. Suitable examples include, but are not
limited
to, the steel grades known as AISI 304 (*1.4301), AISI 304L (1.4307, 1.4306),
AISI 316
(1.4401), AISI 316L (1.4404, 1.4435), AlS1316Ti (1.4571), or AISI 904L
(1.4539)
[*1.xxxx = according to DIN 10027-2). In another embodiment of the present
invention,
the metal may be a steel grade referenced as S235JR (according to EN 10025) or
S460MC (according to EN 10149) or 20MnB4 (*1.5525, according to EN 10263).
The following examples are given for understanding and illustrating the
invention and should not be construed as limiting the scope of the invention,
which is
defined only by the appended claims.
17
CA 02831049 2013-10-23
EXAMPLE 1 ¨ aeneral procedure for aalvanization at 440 C
A plate (2 mm thick, 100 mm wide and 150 mm long) made from the steel grade
S235JR (weight contents : 0.114% carbon, 0.025% silicium, 0.394% manganese,
0.012% phosphorus, 0.016% sulfur, 0.037% chromium, 0.045% nickel, 0.004%
molybdenum, 0.041% aluminum and 0.040% copper) was pre-treated according the
following pre-treatment sequential procedure:
- first alkaline degreasing by means of SOLVOPOL SOP (50 g/1) and a tenside
mixture EMULGATOR SEP (10 g/1), both available from Lutter Galvanotechnik
GmbH, at 65 C for 20 minutes;
- rinsing with water;
- first pickling in a hydrochloric acid based bath (composition: 10 wt% HCI,
12
wt% FeCl2) at 25 C for 1 hour;
- rinsing with water;
- second alkaline degreasing for 10 minutes in a degreasing bath with the
same
chemical composition as in the first step;
- rinsing with water;
- second pickling for 10 minutes in a pickling bath with the same chemical
composition as above;
- rinsing with water,
- fluxing in a flux composition as described in one of the following tables
for 180
seconds at a concentration of 650 g/I and 0,3% by weight Netzer 4 (a non-ionic
wetting agent commercially available from Lutter Galvanotechnick GmbH);
- drying at 100¨ 150 C for 200 seconds;
- galvanizing for 3 minutes at 440 C at a dipping speed of 1.4 m/minute in
a zinc-
based bath comprising 5,0% by weight aluminum, 1,0% by weight magnesium,
trace amounts of silicium and lead, the balance being zinc; and
- cooling in air.
18
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EXAMPLES 2 to 17¨ steel treatment with illustrative flux compositions of this
invention
before galvanizing at 440 C
The experimental procedure of example 1 has been repeated with various flux
compositions wherein the proportions of the various chloride components are as
listed
in table 1. The coating quality has been assessed by a team of three persons
evaluating the percentage (expressed on a scale from 0 to 100) of the steel
surface
that is perfectly coated with the alloy, the value indicated in the last
column of table 1
below being the average of these three individual notations. The coating
quality has
been assessed while keeping the fluxing bath at 72 C (examples 1 to 10, no
asterisk)
or at 80 C (examples 11 to 17, marked with an asterisk).
Table 1
. _____________________________________________________________ ,
Ex. ZnCl2 NH4CI NCI % KCI % SnCl2 PbCl2 Coating
A ok ok % quality
:1* i 59 ' 20 3 12 4 ' 1 75
2 60 20 3 12 4 1 90
3* 52.5 17.5 3 12 13 1 75
.. 4 53 ' 18 3 12 13 1 80
5 * 52 21 4 17 4 ' 1 70
6 . 52.5 21.5 4 17 ' 4 ' 1 60
7 60.5 12 4.5 18 f4 1 60
_
8 57 19 ¨ 3 ' 12 8 1 85
- ,
'9 59 20 4.5 11.5 4 1 70
10 59 20 2.5 13.5 4 1 70
11 61.3 20.4 3.1 12.3 2 1 95 *
.4
12 55 ' 25 3 12 4 1 95*
________________________________________________________________ f
13 56.1 25.5 ' 3.1 12.2 2 1 90*
_
19
CA 02831049 2013-10-23
14 50 30 3 12 4 1 60*
15 54.1 18 2.7 20.7 3.6 0.9 70*
16 62.5 20.8 3.2 12.5 0 1 80*
17 57.3 26 3.2 12.5 0 1 85*
Table 1 (end)
= The flux compositions of examples 1, 3 and 5 additionally contain 1 wt.%
NiCl2
to match up to 100% by weight.
COMPARATIVE EXAMPLE 18
The experimental procedure of example 1 has been repeated with a flux
composition comprising 60 wt% zinc chloride, 20 wt% ammonium chloride, 10 wt%
sodium chloride, 5 wt% potassium chloride and 5 wt% tin chloride,. The coating
quality
has been assessed by the same methodology as in the previous examples and has
found been found 20%. This comparative example demonstrates that when a
KCl/NaC1
weight ratio of 1/3 is used as in the prior art, then the coating quality is
significantly
lower than for examples 1 to 17.
EXAMPLE 19 ¨ general procedure for galvanization at 520 C
The sequential procedure of example 1 is repeated, the treatment step with a
fluxing composition being performed at 80 C, except that in the penultimate
step
galvanizing was effected at 520 C at a dipping speed of 4 m/minute in a zinc-
based
bath comprising 20.0 wt.% aluminum, and 1.0 wL% magnesium, trace amounts of
silicium and lead, the balance being zinc.
EXAMPLES ?(:) to 26 ¨ steel treatment with illustrative flux crynci9sitkms of
this
invention before galvanizing at 520 C
The experimental procedure of example 19 has been repeated with various flux
compositions wherein the proportions of the various chloride components are as
listed
in table 2 below. The coating quality has been assessed by the same
methodology as
in the previous examples.
CA 02831049 2013-10-23
Table 2
Ex. ZnCl2 NH4CI- NaCI % KCI % SnCl2 PbCl2 Coating
quality
20 60 20 3 12 4 1 95
21 57 19 3 12 8 1 80
22 61.3 20.4 3.1 12.3 2 1 85
23 55 25 3 12 4 1 80
24 56.1 " 25.5 3.1 " 12.2 2 1 85
25 54.1 18 2.7 20.7 3.6 0.9 75
Table 2 (end)
EXAMPLE 26 ¨ oeneral procedure for oalvanization at 460 C
The sequential procedure of example 1 was repeated, the treatment step with a
fluxing composition being performed at 80 C, except that in the penultimate
step
galvanizing was effected at 460 C at a dipping speed of 4 m/minute in a zinc-
based
bath comprising 11.0 wt.% aluminum, 3,0 wt.% magnesium, trace amounts of
silicium
and lead, the balance being zinc.
EXAMPLES 27 to 29 ¨ steel treatment with illustrative flux compositions of
this
invention before oalvanizing at 460 C
The experimental procedure of example 26 has been repeated with various flux
compositions wherein the proportions of the various chloride components are as
listed
in table 3 below. The coating quality has been assessed by the same
methodology as
in the previous examples.
21
CA 02831049 2013-10-23
Table 3
Ex. ZnCl2 NH4CI NaCI % KCI % SnCl2 PbCl2 Coating
quality
27 61.3 20.4 3.1 12.3 2 1 95
28 55 25 3 12 4 1 95
29 56.1 25.5 3.1 12.2 2 1 95
As a summary, examples 20-25 and 27-29 demonstrate that the present
invention achieves outstanding coating quality whatever the composition of the
zinc-
based galvanization bath may be.
EXAMPLE 30 ¨ galvanization of steel plates at 510 C
A steel plate (thickness 2.0 mm) from a steel grade S235JR (composition as
defined in example 1) was treated according the following procedure:
- first alkaline degreasing at 60 C by means of SOLVOPOL SOP (50 g/l) and a
tenside mixture Emulgator Steal (10 WI), both available from Lutter
Galvanotechnik GmbH, for 30 minutes;
- rinsing with water;
- first pickling in a hydrochloric acid based bath (composition: 12 wt% HCI,
15
wt% FeCl2, 1 wt% FeCl3, 2 m1/I of inhibitor HM and 2.5 m1/I Ennulgator C75
from Lutter Galvanotechnik GmbH) at 25 C for 60 minutes;
- rinsing with water;
- second alkaline degreasing bath at 60 C for 5 minutes in a degreasing
bath with
the same chemical composition as in the first step;
- rinsing with water;
- second pickling in a hydrochloric acid based bath with the same
composition as
in the first pickling step at 25 C for 5 minutes;
- rinsing with water;
- fluxing the steel plate at 80 C for 3 minutes in a flux composition
(comprising 60
wt.% zinc chloride, 20 wt.% ammonium chloride, 3 wt.% sodium chloride, 12
22
CA 02831049 2013-10-23
wt.% potassium chloride, 4 wt.% tin chloride and 1 wt.% lead chloride) with a
total salt concentration of 750 g/I and in the presence of 1 m1/I Netzer 4 (a
wetting agent from Lutter Galvanotechnik GmbH), by using an extraction speed
of 4m/min or higher;
- drying until the steel plate surface temperature reaches 120 C;
- galvanizing the fluxed steel plate for 3 minutes at 510 C in a zinc-
based bath
comprising 20.0 wt.% aluminum, 4,0 wt.% magnesium, 0,2 wt.% silicium, trace
amounts of lead, the balance being zinc; and
- cooling down the galvanized steel plate in air.
This procedure has been found to provide a superior coating quality similar to
example 20. The following variants of this procedure also provide superior
coating
quality:
= !dem but 650 g/ltotal salt concentration, 2 m1/I Netzer 4 in flux,
and galvanizing in the zinc-based bath at 490 C,
= 'dem but 650 g/I total salt concentration, 2 m1/I Netzer 4 in flux, and
galvanizing
in the zinc-based bath at 500 C during 1 minute,
= ldem but 650 g/I total salt concentration, fluxing for 5 minutes with 2
m1/I Netzer
4 in flux, and galvanizing in the zinc-based bath at 510 C during 10 minutes,
= !dem but 650 g/I total salt concentration, fluxing for 5 minutes with 2
ml/1 Netzer
4 in flux, and galvanizing in the zinc-based bath at 530 C during 5 minutes,
and
= ldem but 650 g/I total salt concentration, fluxing for 5 minutes with 2
m1/I Netzer
4 in flux, and galvanizing in the zinc-based bath at 530 C during 15 minutes.
EXAMPLE 31 ¨ galvanization of steel plates at 520 Q
A steel plate (thickness 2.0 mm) from a steel grade S235JR (composition
asdefined in example 1) was treated according the same procedure as in example
30,
except for the following operating conditions:
- in the fluxing step, a total salt concentration of 650 g/I in the
presence of 2 m1/I
Netzer 4, and
- a galvanizing step of 3 minutes at 520 C in a zinc-based bath comprising
20.0
wt.% aluminum, 2.0 wt.% magnesium, 0.13 wt.% silicium, trace amounts of
lead, the balance being zinc.
This procedure has been found to provide a superior coating quality similar to
example 20.
23
