Note: Descriptions are shown in the official language in which they were submitted.
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NITRIC ACID MO~IFIED LOW TEMPERATURE
PHOSPHATE COATING FOR ELECTRICAL STEEL
BACKGR~U~D OF THE INVENTION
The magnetic cores utilized in ger~erators,
transformers, electric motors, etc. are preferably made of
laminated sheet steel punchings, with a coating of insu-
lating material on each lamination. As is well known inthe art, these cores are of laminate form instead of solid
steel, ~o that undesirable eddy currents are reduced to a
~i~;ml1m d~ring use of the cores.
Al-~r;n-l~ orthophospha~e, also called mono alumi-
num phosphate, is an inorganic coating solution which isused in diluted form to produce an interlaminar insulative
medium on ferrous sheet steel strip and punchings. It is
used to provide resistance between core laminations of
motors, generators and transformers, to additionally
minimize the eddy current lo~ses in the machines. The
coating solution, in undiluted concentration, consists of
a mixture of phosphoric acid and hydrated alumina, and
contains approximately 33 wt.% P205, 8 wt.% A1~03, and 59
wt.% H20. This type of coating solution is commercially
available, for example, under the tr~n~ e Alkophos, and
is used extensively in the electrical equipment industry.
It is applied to the electrical ste~l by the equipment
manufacturer or by the electrical steel supplier.
The mono alll~; n~m phosphate solution described
above is generally diluted further with water prior to
, "
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usage. The useful concentrations u~ually consist of about
0.5 to 4 volume of H20 to 1 volume of mono aluminum phos-
phate solution, depending on the resistance and thickness
de~ired, and on the coating mechanics which are used to
apply the coating. This so.lution is applied to the steel
with felt or rubber roll applicators, then cured in a
t~ndem oven. These types o~ phosphate coatings are well
known in the art, and axe used a~ coatings primarily for
hot rolled steel by Steinherz, in U.S. Patent 2,743,203.
Steinherz suggests curing at 135C to 500C
sheet steel temperatures, where the heat drives of ~he
water, leaving a thin coating of pho~phate reacted wi~h
the Xerrous metal surface o the sheet. The resulting
coating has high insulation vaLue and a good space factor.
The coating also has proven ability to resist various
mechanical, thermal and chemical hazards which are en~
cou~tered in electrical machines or in processing steps
~uring production.
In today's practice, however, where cold rolled
steel, having a low reactive surface, is generally used,
khe commercial ~heet steel temperature re~uired to cure
this unmodified phosphate coating is about 340C. In
order to attain this sheet steel temperature, which will
also be the temperature of the phosphate coating, the oven
temperature must be set at a much higher level, g~nerally
in the 425C to 650C range, dep~n~;ng on furnace design,
conveyor speeds, and lamination thickness. Such high
curing te~peratures result in a costly process, due to the
high co~t of natural gas or electricity for the ovens,
high labor cost due to the slow coating rates using the
unmodified material, high equipment maintenance costs,
plu5 additional problemq, such as high heat feedback to
coating rolls, and an increased chance for undercuring.
There has been a long felt need then, for a new
coating, or an improved process, that is simple and inex-
spensive, yet which will provide interlaminar coating,
insulation, and space factor characteristics equivalent to
the prior coating or process.
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SUMMARY OF THE INVENTION
The above needs have been met and the above
problems solved by providing a nitric acid modified, low
temperature, aqueous, aluminum phosphate solution coating.
This phosphate co~ting is an admixture of diluted alllminl~
phosphate solution, i.e, usually 1 volume of aqueous mono
aluminum phosphate solution plus about O . 5 to 4 volumes of
additional dilution water; and from about 1 vol.% to about
15 vol.%, based on the volume o diluted mono aluminum
phosphate solution, of nitric acid.
Unexpectedly, this addition of nitric acid can
lower the commercial sheet steel temperature required to
cure the phosphate coating to about 9QC, while maintain-
ing the coating, insulation and space factor characteris~
tics of the unmodlfied phosphate coating. The lower
curing temperature also has the added benefits of mini-
mizing handling problems and reducing energy requirements.
DESCRIPTION OE TH~ PREFERRED EMBODIMENTS
Sheets of various ferrous base metal~ may be
treated in accordance with the invention. Thus silicon-
iron sheets having up to about 7% silicon may be coated
therewith. Nickel-iron magnetic sheets with up to 85%
nlckel may b~ treated. Other magnetic sheets containing a
high proportion of iron, alloyed with one or more other
25 metals may be treated. Generally, all such sheets are
continuously cold rolled. The term "ferrous sheet steel"
will be employed herein to include any of these.
The cured phosphate coating of this invention is
derived from an admixture of diluted, aqueous aluminum
phosphate, and from about 1 vol.% to about 15 vol.%,
preferably from about 3 vol.% to about 12 vol.%, most
preferably from about 3 vol.% to about 8 vol.%, ba~ed on
the volume of diluted all~m;nl1~ phosphate solution, of
HN03. Under 1 volO% HN03, there is no improved curing.
35 Over 15 v31.% HN03, the coating solution may be overly
corrosive.
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The diluted aqueous aluminum orthophosphate
composition consists of 10~ volumes of a~ueous alllminl~m
phosphate solution. This aluminum phosphate composition
is generally an admixture of phosphoric acid and hydrated
alumina, which can be described in terms of the P205 and
A1203 active ingredient content. The aluminum phosphate
composition contains about 28 wt.% to about 38 wt.%,
preferably about 30 wt.% to about 35 wt.%, P205, about 5
wt.% to about 12 wt~%, preferably about 7 wt.% to about 10
wt.% A1203, and about 50 wt.% to about 70 wt.% water; and
p~eferably, ~bout 50 volumes to about 400 volumes, most
preferably about 100 to about 250 volumes, of additional
dilution water. Use of under 50 volumes of additional
dilution water provides coatings that can be difficult to
apply evenly to electrical steel. Over 400 volumes of
additional dilution water provides coatings that may be
too thin, causing a loss of insulative effect.
Thus, the aqueous mono aluminum phosphate com-
position consists of a 33% to 50% solids content of P205
plus A1203, where other materials such as zinc, zinc
phosphate, and nickel are specifically excluded. However,
an effective amount of non-ionic wetting agent for the
steel, usually up to about 1.5 vol.% but preferably from
0.5 vol.% to about 1.2 vol.%, such as materials containing
a medium chain alkyl group attached to an
OR
I
-- O -- P = O
OR
group, where R is a water solubilizing group, can be used,
as is well known in the art. Under 0.5 vol.% wetting
agent, the steel is not wetted or made adherent for the
phosphate coating.
A typical formulation for the low temperature,
ni.tric acid modified phosphate coating consists of: 10
liters of all~;ntlm phosphate solution consisting of 33
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wt.% P205, 8.6 wt.~ Al203 and 58.4 wt.% H~0, 20 llters of
additional dilution H20, about 0.3 liter of non-ionic
wetting agent, and 1.5 liters of HM03, i.e., about 5 vol.%
HN03 based on the combination o the above ingredients.
The term "nitric acid" as defined herein is HN03 at over
65% concentration, i.e., reagent grade, usually 69% to 71%
concentrated HN03.
This nitric acid modified phosphate coating
composition is applied to eLectrical steeL, generally of
the cold rolled variety, by roller application techniques,
utilizing ei-th~r grooved rubber or felt applicator rolls.
When this compositio~ is applied to the sur~ace of an
~mderlying stael sheet and thereafter the steel sheet and
therefore the phosphate coating is heated to a sheet steel
temperature of between about 90C and about 130C for a
time suficient to drive off most of the water, the ap~
plied coating cures into an adherent unitary mass with the
substrate. This adherent coating imparts to ~he substrate
a good interlaminar resistance with improved eddy current
losses when the individual lamina-tions are stacked for use
in electroma~netic apparatus. Thi~ nitric acid modiied
phosphate coating can be used in plural layers, and can be
applied over other base phosphate coatings. The term
"sheet steel temperature" means the temperature of the
steel and coating measured by thermocouple or the like and
not the generally much higher oven temperature.
This nitric acid modiied phosphate coating
provides a density after curing of between about 1 and
a~out 1.5 grams per cubic centimeter. As cured it has
been found that ~he coating provides at least a 2 ohm
cm2/lam and as much as 64U ohm cm2/lam value in the ASTM
A~717 Franklin test. The term "cured" as herein used,
means that the phosphate coating is a non-tacky solid.
During cure it is thought that the HN03 increases ferrous
surface activity, allowing much faster reaction of the
coating with the ferrous sheet steel to form, ~or example,
iron phosphate, and thereby causi~g a precipitation of
alumin~m phosphate.
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EXAMPLE 1
A nitric acid modified phosphate coating com-
position was made, consisting of the admixture of 10
volumes of mono aluminum phosphate solution having an
5 analysis of 33.1 wt.% P205, 8.6 wt.% A1203 and 58.g wt.%
H20, having a viscosity of about 40 cps. at 25C, a speci~
ic gravity of 1.47, and a molecular weight of about 318
(sold commercially by Monsanto Chemical Co. under the
tradename Alkophos C), 20 volumes of additional dilution
H20, and 5 vol.% concentratad 71% HN03, i.e., 0.05 ~ 30
volumes = 1.5 volumes of EN03. This was mixed at 25C,
with acid slowly added as the last component. About 1
vol.~ o a non-ionic, phosphorous containing, medium alkyl
group wetting agent (sold commercially by Victor Chemical
Co. under the tradename Victawet #12) was added to the
phosphate coating composition.
A comparative, non-modified phosphate coating
solution was alæo made, and included the same amounts and
ingredients except that HN03 was axcluded. Both of these
coatings were then roller applied to cold rolled electri-
cal steel and heated to cure, after which insulation
characteristics were measured. They were also coated on
armature punchings and tested for interlaminar resistance.
The results are shown in Tables 1 and 2 below:
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TABLE 1
Interlaminar Insu ation Characteristi~s on 0.018" ~122 Electrical Steel
Franklin
Thickness Franklin ohmcm2/lam Space
Cure Temp. mil/side Amps(ave) (ave) Factor
HN03 modified 93C 0.068 0.048 128 98%
phosphate c~ating
*~nmodified 340C 0.087 0.042 147 ~7.9%
ph~sphate coating
lO *comparative sample
TABLE 2
I~erlami~a~ Resistance o~ 0.025" thick 6~" diame~er Armature Pl~nchin~
HN03 modified 9.4 ~hm cm /lam (avP)
phosphate coating
15 *unmodified 9.4 ohm c~2/lam (ave)
phosphate coa~ing
*comparati~e sample
As can be seen, insulation and space factor
characteristics of the HN03 modiied phosphate coating of
20 this invention remain outst~n~ing, while cure temperature
has been dropped about 250C from that of the standard
unmodified phosphate coating.
