CORROSION RESISTANT, WELDABLE COATING COMPOSITIONS

COMPOSITIONS DE REVETEMENT SOUDABLES ET RESISTANTES A LA CORROSION

English Abstract

A zinc-based anti-corrosive coating is disclosed. The coating includes zinc,
iron phosphide, and potassium silicate having a molar
ratio of silicate to potassium of 4.1 to 6Ø

French Abstract

Revêtement anti-corrosif à base de zinc. Le revêtement est composé de zinc, de phosphure de fer et de silicate de potassium présentant un rapport moléculaire silicate/potassium compris entre 4,1 et 6,0.

Claims

CLAIMS
1. An anticorrosive coating composition
comprising:
(a) from 8 to 40 parts by weight of a
pigment, said pigment comprising a mixture of
from 35 to 90 percent by weight of zinc and from
65 to 10 percent by weight iron phosphide;
(b) from 2 to 5 parts by weight of
potassium silicate of formula K2O.cndot.nSiO2 wherein n
is from 4.1 to 6.0; and
(c) from 7 to 12 parts by weight water.
2. A composition according to claim 1 in which
said zinc and said iron phosphide are of median
particle size between 2 µm and 20 µm.
3. A composition according to claim 2 in which
said zinc and iron phosphide are of 3 to 8 µm median
particle size.
4. A composition according to claim 1 wherein
n is from 4.5 to 6Ø
5. A composition according to claim 1 wherein
n is from 5.0 to 6Ø
6. A composition according to claim 1 wherein
n is from 5.3 to 6Ø

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7. A composition according to claim 1 wherein
n is from 5.0 to 5.3.
8. A composition according to claim 1
additionally comprising lithium silicate Li2O.cndot.SiO2
wherein p is from 3.0 to 8Ø
9. A composition according to claim 1
additionally comprising sodium silicate Na2O.cndot.mSiO2
wherein m is from 2.0 to 4Ø
10. A composition according to claim 1
additionally comprising a filler.
11. A composition according to claim 10 wherein
said filler is mica.
12. A composition according to claim 1
comprising:
(a) from 23 to 28 parts by weight of a
pigment, said pigment comprising a mixture of
from 45 to 55 percent by weight of zinc and from
45 to 55 percent by weight iron phosphide;
(b) from 3 to 4 parts by weight of
potassium silicate of formula K2O.cndot.nSiO2 wherein n
is from 5.0 to 5.5; and
(c) from 9 to 10 parts by weight water.

Description

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CORROSION RESISTANT, WELDABLE COATING COMPOSITIONS
FIELD OF THE INVENTION
The invention relates to an anticorrosive
coating composition that does not interfere with
welding. The composition includes zinc, iron
phosphide and an aqueous potassium silicate solution
containing a high ratio of silicate to potassium.
BACKGROUND OF THE INVENTION
In marine and industrial construction, it is
usually desirable to pre-paint steel with a zinc-rich
primer before fabrication, and many such coating
compositions are known. Many of these compositions
include zinc powder in an organic resin such as an
epoxy resin, a chlorinated rubber, a polystyrene
resin or a silicone resin. Coatings based on these
organic binders are not well suited for coating steel
that must ultimately be welded because the binder
tends to decompose from the heat of the weld,
resulting in damaged coatings and loss of adhesion.
As an alternative to coatings based on organic
binders, zinc coatings based on inorganic binders are
also known. For example, Schutt (U. S. patent
3,620,784) discloses a coating composition containing
zinc dust carried in a silicate vehicle. The
particular formulation disclosed by Schutt is said to
avoid many of the problems commonly encountered with
inorganic coatings: weather wear, cracking, crazing
and non-uniform adherence before and after baking.

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However, even if one surmounts the problems
associated with the physical characteristics of the
applied silicate coating, as Schutt claims to have
done, one is still faced with the problem of welding
steel that has been coated with zinc dust primer in a
silicate vehicle. During welding the vaporized zinc
can penetrate the weld root and become entrapped,
creating a porous weld. It is sometimes possible to
partially overcome the porosity problem by reducing
the welding speed sufficiently to allow the zinc to
become completely volatized ahead of the weld. Even
when this technique is successful, however, it
results in increased fabrication costs due to reduced
welding speed.
It is known that a portion of zinc in zinc-rich
primers can be replaced by iron phosphide of suitable
particle size. The resulting zinc-coated steel,
while retaining its anti-corrosive properties, can be
more readily and efficiently welded.
As an example of this approach, Makishima et al.
(U. S. patent 4,011,088) have proposed an anti-
corrosive coating composition comprising 5 to 800 of
a binder, which is either potassium silicate or
ammonium silicate, and 20 to 950 of a pigment mixture
of zinc powder and iron phosphide or zinc powder and
nickel phosphide. In the embodiment in which
potassium silicate is used as the binder, Makishima
et al. indicate that the ratio of silicate to
potassium should be between 2.5 and 4Ø If the mole
ratio is smaller than 2.5, the film forming property
is said to be insufficient and if the mole ratio is

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larger than 4.0, the stability of the binder is said
to be reduced.
I have now discovered an improved zinc based
primer that provides a durable, corrosion-resistant
coating, combined with excellent weldability.
Moreover, the composition is water based and thereby
avoids the environmental problems associated with
solvent-based primers.
SUMMARY OF THE INVENTION
The invention relates to an anticorrosive
coating composition comprising: (a) from 8 to 40
parts by weight of a pigment, said pigment comprising
a mixture of from 35 to 90 percent by weight of zinc
and from 65 to 10 percent by weight iron phosphide;
(b) from 2 to 5 parts by weight of potassium
silicate of formula KZO~nSiOz wherein n is from 4.1 to
6.0; and (c) from 7 to 12 parts by weight of water.
Preferably the zinc and the iron phosphide are of
median particle size between 2 ~.m and 20 ~,m; most
preferably 3 to 8 ~,m median particle size. The ratio
of silicate to potassium in the potassium silicate of
the inventive composition may be from 4.5 to 6.0;
more preferably 5.3 to 6.0; and most preferably 5.0
to 5.3. Compositions according to the invention may
additionally include lithium silicate Li20~pSi02
wherein p is from 3.0 to 8.0 or sodium silicate
Na20~mSi02 wherein m is from 2.0 to 4.0, or a
combination of the two. Compositions according to
the invention may also include a filler such as mica.

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In a preferred embodiment, the invention
comprises (a) from 23 to 28 parts by weight of a
pigment; (b) from 3 to 4 parts by weight of potassium
silicate wherein n is from 5.0 to 5.5; and (c) from 9
to 10 parts by weight water. The pigment in this
case is a mixture of from 45 to 55 percent by weight
of zinc and from 45 to 55 percent by weight iron
phosphide;
DETAILED DESCRIPTION OF THE INVENTION
The coating composition of the invention
comprises three basic elements: (a) a zinc-iron
phosphide pigment; (b) a potassium silicate binder;
and (c) an amount of water to provide the appropriate
rheology. The compositions may also contain
compatible fillers and dyes, which should not,
however, adversely affect the durability or
weldability of the resultant composition.
The zinc-iron phosphide pigment consists of a
mixture of metallic zinc in the form of a powder or
flakes and particles of roughly the same size of iron
phosphide. Zinc dust is preferred, and it is readily
commercially available. I have found that UP6 zinc
dust from North American Oxide (Clarksville,
Tennessee) having an average particle size of 6 to 7
microns is suitable for the composition of the
invention. Iron phosphide is alsc commercially
available, and I have found that G:.~de 2131
Ferrophos°, available from Occidental Chemical
Corporation (Niagara Falls, New York) having a mean
particle size between 3 and 5 microns is well suited
for the compositions of the invention. Ferrophos° is

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primarily iron phosphide (Fe2P) with minor amounts of
Si02. Details of its preparation and constitution may
be found in US patent 3,562,1:?4 (see column 3, line
45 to column 4, line 33) .
The weight ratio of zinc in the pigment should
be greater than 35% in order to provide adequate
protection against corrosion, but it cannot be higher
than 90% without giving rise to porous welds. Other
pigments and fillers may be added to composition of
the invention as long as the percentages of zinc and
iron phosphide are maintained within the stated
range. Examples of fillers that could be added
include diatomaceous earth, bentonite clay,
kaolinite, wollastonite, talc .and mica.
The binder is potassium silicate having a mole
ratio of silica to potassium o:Kide greater than 4Ø
Aqueous solutions of potassium silicate are
commercially available and are prepared by the method
disclosed in U.S. patent 4,162,.169.
I have found that t:he commercially
available K-silicateT"' from Polyset Chemical Co.
(Mechanicville, New York) is well suited to the
compositions of the invention. This material is
about 23-28% by weight potassium silicate in water
(i.e. 23-28o solids content) ar.~d the mole ratio of
silicate to potassium is about 5.2.
While not necessary to produce coating
compositions of the invention, other alkali metal
silicates may be added to the binder. In this regard

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one may add either or both of lithium silicate
(LizO ~ pSiOz wherein p is from 3 . 0 to 8 . 0 ) and sodium
silicate (Na20~mSi02 in which m is from 2.5 to 4.0) .
A composition of the invention is prepared by
blending 3.17 kilograms of UP6 zinc powder to
homogeneity with 3.17 kilograms of 2131 Ferrophos°
(iron phosphide) in a high speed blender. The
homogenous mixture is combined with 3.31 kilograms of
K-silicateTM (potassium silicate) and stirred to
produce 3.78 liters of a homogeneous mixture
according to the invention.
As discussed above, if a filler such as mica is
to be added, it would be blended together with the
zinc and iron phosphide before mixing with the
silicate vehicle. Similarly, if a lithium silicate
or sodium silicate were to be added to the
composition, it would be mixed with the potassium
silicate aqueous vehicle before combining with the
pigment.
The substrates to which the compositions of the
invention are applied will commonly be metal.
Indeed, the major advantage of the present
compositions, weldability, is observed when the
substrate is steel. For steel substrates, the
substrate is usually cleaned with phosphoric acid or
sand blasting immediately before application of the
coating. The coating is sprayed, brushed or rolled
on the substrate by conventional means. It spreads
well and adheres uniformly. The resulting coating
resists corrosion and abrasion; it does not crack or
peel and it weathers well.