CA1198037A

CA1198037A - Nitric acid modified low temperature phosphate coating for electrical steel

Info

Publication number: CA1198037A
Application number: CA000427783A
Authority: CA
Inventors: Norman M. Pavlik; John Sefko
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1982-05-12
Filing date: 1983-05-10
Publication date: 1985-12-17
Also published as: JPS5925538A; JPH0256020B2; FR2526814B1; FR2526814A1; MX159679A; ZA833072B; ES522300A0; KR910003743B1; BE896728A; KR840004793A; IN158643B; ES8608590A1; IT8321050A0; IT1171065B; IT8321050A1

Abstract

ABSTRACT OF THE DISCLOSURE
A low temperature curable, phosphate coating on ferrous sheet steel, is made by applying an admixture of aqueous, aluminum phosphate solution and from about 1 vol.% to about 15 vol.% of nitric acid, and heating at a sheet steel temperature of from about 90°C to about 130°C.

Description

1 50,~85 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 , "
~,' ~ 50,285 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.

?~3~33~7 3 50,28S
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.

'~

4 50,28S
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 OJ
50,285 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.

~.~ Jl~_t ~
6 S0,285 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:

7 50,285 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 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.

Claims

We claim:

1. An article of manufacture comprising a ferrous sheet steel member and a cured, insulating phos-phate coating thereon, said coating derived from an aqueous solution comprising:
(A) 100 volumes of aqueous, aluminum phosphate, and (B) about 1 volume to about is volumes of nitric acid.

2. An article of manufacture comprising a ferrous sheet steel member and a heat cured, insulating phosphate coating thereon, said coating derived from an aqueous composition comprising an admixture of:
(A) phosphoric acid, (B) hydrated alumina, (C) water, and (D) from about 1 volume to about 15 volumes of nitric acid, based on the volume of (A), (B) and (C).

3. The article of claim 2, where from about 3 volumes to about 8 volumes of nitric acid are used.

4. An article of manufacture comprising a ferrous sheet steel member and a cured, insulating phos-phate coating thereon, said coating derived from an aqueous solution comprising:
(A) 100 volumes of a phosphate coating composi-tion consisting essentially of:
(i) about 28 wt.% to about 38 wt.% P205, (ii) about 5 wt.% to about 12 wt.% A1203, and (iii) about 50 wt.% to about 70 wt.% H20, and (B) About 1 volume to about 15 volumes of nitric acid; where the applied coating is capable of cure at relatively low sheet steel temperatures.

5. An article of manufacture comprising a ferrous sheet steel member and a cured, insulating phos-phate coating thereon, said coating derived from an aqueous solution consisting essentially of (A) 100 volumes of a phosphate coating composi-tion consisting essentially of:
(i) about 30 wt.% to about 35 wt.% P203, (ii) about 7 wt.% to about 10 wt.% A1203, and (iii) about 50 wt.% to about 70 wt.% H20, (B) about 50 volumes to about 400 volumes of H20, (C) an effective amount of a wetting agent, and (D) about 3 vol.% to about 12 vol.%, based on the volume of components (A) + (B), of nitric acid; where the applied coating is capable of cure at sheet steel temperatures of from about 90°C to about 130°C.

6. The article of claim 5, where the cure causes a reaction to take place between the surface of the ferrous member and the applied coating, and the amount of nitric acid used is from about 3 vol.% to about 8 vol.%.

7. The article of claim 5, where the coating contains up to about 1.5 vol.% of wetting agent, and the phosphate coating composition is free of nickel and zinc components.

8. The article of claim 5, being selected from the group consisting of a magnetic core punching lamina-tion and an electrical steel strip.

9. An article of manufacture comprising a ferrous sheet steel member and a cured, insulating phos-phate coating thereon, the coating derived from an aqueous solution consisting essentially of:

(A) 100 volumes of a phosphate coating composi-tion consisting essentially of:
(i) about 30 wt.% to about 35 wt.% P205, (ii) about 7 wt.% to about 10 wt.% A1203, and (iii) about 50 wt.% to about 70 wt.% H20, (B) about 100 volumes to about 250 volumes of H20, (C) an effective amount of a wetting agent, and (D) about 3 vol.% to about 8 vol.%, based on the volume of components (A) + (B), of nitric acid.

10. The article of claim 9, being selected from the group consisting of a magnetic core punching lamina-tion, and an electrical steel strip.

11. In the process of providing an insulating phosphate coating on ferrous sheets wherein an aqueous composition of phosphoric acid and hydrated alumina is deposited on the sheets and heated thereon to form the coating, the improvement comprising including from about 1 vol.% to about 15 vol.% of nitric acid in the aqueous composition.

12. In the process of claim 11, the improvement comprising including from about 3 vol.% to about 8 vol.%
of nitric acid in the aqueous composition.

13. In the process of providing an insulating phosphate coating on ferrous sheet steel members, the steps comprising applying to the surface of the ferrous members an aqueous coating consisting essentially of:
(A) 100 volumes of phosphate coating composi-tion consisting essentially of:
(i) about 28 wt.% to about 38 wt.% P205, (ii) about 5 wt.% to about 12 wt.% A1203, and (iii) about 50 wt.% to about 70 wt.% H20, (B) about 50 volumes to about 400 volumes of (C) an effective amount of a wetting agent, and (D) about 1 vol.% to about 15 vol.%, based on the volume of components (A) + (B), of nitric acid; and heating the applied coating at a sheet steel temperature of from about 90°C to about 130°C, for a period of time sufficient to drive off water to produce an adherent cured insulating coating.

14. In the method of claim 13, the ferrous member being selected from the group consisting of a magnetic core punching lamination, and an electrical steel strip, and the amount of nitric acid added is from about 3 vol.% to about 8 vol.%.