Process for the zinc/calcium phosphatizing of metal surfaces at low treatment temperatures

The invention relates to a process for phosphatizing metal surfaces with an acidic phosphatizing solution containing zinc ions, calcium ions, phosphate ions and accelerators and, optionally, other additives, characterized in that, after degreasing and without activation, the metal surfaces are brought into contact at 30.degree. to 65.degree. C. with solutions which contain Zn.sup.2+, Ca.sup.2+, PO.sub.4.sup.3- and, as accelerators, nitrate and/or nitrite and/or chlorate ions and/or an organic nitro compound and/or H.sub.2 O.sub.2 or an inorganic peroxide and which have a pH-value of from 2.2 to 3.8, a ratio of free acid to toal acid of from 1:10 to 1:60 and a ratio by weight of (Ca.sup.2+ +Zn.sup.2+) to PO.sub.4.sup.3- of from 1:>8 to 1:40, and to the use of this process for pretreating the metal surfaces for painting by electrodeposition, particularly by cathodic electrodeposition.

This invention relates to a process for the phosphatizing of metal 
surfaces, particularly surfaces of iron, steel, zinc and/or aluminum, with 
a phosphatizing solution containing zinc, calcium, phosphate and 
accelerators and, optionally, other additives and to the use of this 
process for pretreating the metal surfaces for painting by 
electrodeposition, particularly by cathodic electrodeposition. 
The protection of metal surfaces, particularly the protection of iron and 
steel surfaces, by phosphate-containing coatings has been known for some 
time. In this connection, a distinction is drawn between so-called 
"non-layer-forming phosphatizing", i.e. the use of alkali metal and/or 
ammonium orthophosphate solutions to produce iron phosphate coatings in 
which the iron ion emanates from the metal surface to be coated, and 
so-called "layer-forming phosphatizing", in which zinc phosphate layers or 
zinc/calcium phosphate layers are formed on metal surfaces using zinc or 
zinc/calcium phosphate solutions. 
Phosphate coatings such as these not only improve the corrosion inhibition 
of the metal surfaces, they also increase the adhesion of lacquers 
subsequently applied to the surface. In addition, they are able in certain 
cases to contribute towards improving the properties of metal sheets for 
cold forming and for deep drawing. Zinc/calcium phosphate solutions are 
being used to an increasing extent, particularly for the phosphatizing of 
metal surfaces which are to be subsequently coated with electrodeposition 
lacquers. In this connection, experience of the composition of the bath 
solutions on the one hand and basic knowledge of the structure of 
phosphatizing layers on the other hand (A. Neuhaus and M. Gebhart, 
Werkstoffe und Korrosion, 567 (1966)) have shown that the structure of 
uniform, uninterrupted phosphate coatings depends not only on the 
composition of the bath solutions, but also upon the pretreatment of the 
starting metal sheets, upon the activation before the phosphatizing steps, 
upon the choice of a suitable accelerator and upon other process 
parameters. 
German published application DE-OS No. 15 21 818 and its corresponding 
British patent GB PS No. 1,040,020 describes aqueous phosphatizing 
solutions which contain as their principal constituents zinc, calcium, 
nickel, phosphate, nitrite and nitrate ions and which may be used for 
phosphatizing galvanized iron surfaces at elevated temperatures. However, 
one disadvantage of this known phosphatizing solution is that the 
temperature at which the phosphatizing step is carried out has to be 
relatively high if the solutions are to be able to be applied quickly in 
economically reasonable times. For total immersion times of from 1 to 20 
seconds, the temperatures are in the range from 66.degree. to 116.degree. 
C. The solutions used have very high contents of zinc and calcium ions for 
a low phosphate ion content. The ratio by weight of the sum of zinc and 
calcium ions to phosphate ions is in the range from 1:3.5 to 1:7.1. 
Aqueous phosphatizing solutions containing zinc, calcium, phosphate and, 
optionally, also nickel ions and, in addition, H.sub.2 O.sub.2 as 
accelerator are described in BE-PS No. 811 220. However, the temperatures 
at which solutions such as these are applied are again relatively high. In 
relation to the phosphate content, the content of zinc and calcium ions is 
very high. 
Processes for phosphatizing metal surfaces with acidic zinc phosphate 
solutions containing oxidizing agents in which a comparatively low zinc 
content contrasts with a distinctly higher phosphate content and which may 
contain other divalent metal ions for example even Ca.sup.2+ -ions, are 
described in DE-AS No. 22 32 067 and in DE-OS No. 31 18 375, corresponding 
to U.S. Pat. No. 4,419,199. Although it is possible by the process 
according to DE-AS No. 22 32 067 to produce high-quality phosphate 
coatings with fresh phosphate solutions, the quality of corrosion 
prevention deteriorates after a relatively large number of sheets have 
been treated on account of irregular phosphate coatings. In some cases, 
the protective coatings formed are of no use whatever. 
Another disadvantage of most known phosphatizing processes lies in the fact 
that the quality of the heavy metal phosphate coatings formed in the 
phosphatizing step depends to a very large extent upon the degreasing 
pretreatment of the metal surfaces and also upon their activation. In 
particular, considerable significance is attributed to the activation step 
insofar as it represents the basis for the adhesion of the subsequent 
phosphate coatings and hence has a considerable bearing upon the quality 
of the phosphate coatings formed. The desired formation of thin, 
fine-grained crystalline phosphate coatings is only possible after 
adequate activation by suitable activating agents, for example 
phosphate-containing activating solutions. In this connection, there is 
the particular difficulty of avoiding speckle formation which adversely 
affects the quality of the phosphate coating. 
OBJECTS OF THE INVENTION 
An object of the present invention is to develop a process for obtaining 
thin, fine-grained crystalline zinc/calcium phosphate coatings of high 
homogeneity on metal surfaces, at low treatment temperatures. 
Another object of the present invention is the development of a process for 
phosphatizing metal surfaces with an acidic phosphatizing solution 
containing zinc ions, calcium ions, phosphate ions and accelerators 
consisting essentially of contacting said metal surfaces after degreasing 
and without activation with an aqueous solution at a temperature of from 
30.degree. to 65.degree. C., said aqueous solution containing 
(a) more than 0.5 to 1.5 g.l.sup.-1 of Ca.sup.2+, 
(b) 0.5 to 1.5 g.l.sup.-1 of Zn.sup.2+, 
(c) 10 to 50 g.l.sup.-1 of PO.sub.4.sup.3-, 
(d) at least one accelerator selected from the group consisting of: 
0.5 to 30 g.l.sup.-1 of NO.sub.3.sup.-, 
0.01 to 0.6 g.l.sup.-1 of NO.sub.2.sup.-, 
0.2 to 10 g.l.sup.-1 of ClO.sub.3.sup.-, 
0.1 to 2 g.l.sup.-1 of an organo nitro compound, 
0.01 to 0.5 g.l.sup.-1 of an inorganic peroxide or hydrogen peroxide, 
and mixtures thereof 
which aqueous solution having a pH of from 2.2 to 3.8, a ratio of free acid 
to total acid of from 1:10 to 1:60 and a ratio by weight of (Ca.sup.2+ 
+Zn.sup.2+) to PO.sub.4.sup.3- of from 1:&gt;8 to 1:40. 
A yet further object of the present invention is the obtaining of an 
aqueous acidic phosphatizing solution for treating metal surfaces after 
degreasing and without activation at a temperature of from 30.degree. to 
65.degree. C. consisting essentially of 
(a) more than 0.5 to 1.5 g.l.sup.-1 of Ca.sup.2+, 
(b) 0.5 to 1.5 g.l.sup.-1 of Zn.sup.2+, 
(c) 10 to 50 g.l.sup.-1 of PO.sub.4.sup.3-, 
(d) at least one accelerator selected from the group consisting of 
0.5 to 30 g.l.sup.-1 of NO.sub.3.sup.- 
0.01 to 0.6 g.l.sup.-1 of NO.sub.2.sup.-, 
0.2 to 10 g.l.sup.-1 of ClO.sub.3.sup.-, 
0.1 to 2 g.l.sup.-1 of an organo nitro compound, 
0.01 to 0.5 g.l.sup.-1 of an inorganic peroxide or hydrogen peroxide, 
and mixtures thereof, 
which aqueous solution has a pH of from 2.2 to 3.8, a ratio of free acid to 
total acid of from 1:10 to 1:60 and a ratio by weight of (Ca.sup.2+ 
+Zn.sup.2+) to PO.sub.4.sup.3- of from 1:&gt;8 to 1:40. 
These and other objects of the invention will become more apparent as the 
description thereof proceeds. 
DESCRIPTION OF THE INVENTION 
It has now surprisingly been found that the above objects can be achieved 
and that thin, fine-grained crystalline zinc/calcium phosphate coatings of 
high homogeneity can be obtained, even at low treatment temperatures, if 
the metal surfaces are treated with acidic aqueous solutions containing 
zinc, calcium and phosphate ions and also one or more accelerators, where 
a narrow pH-range, a certain acid ratio and a predetermined ratio by 
weight of the sum of calcium and zinc ions to phosphate ions, have to be 
maintained. 
Accordingly, the present invention relates to a process for phosphatizing 
metal surfaces, particularly surfaces of iron, steel, zinc and/or 
aluminum, with an acidic phosphating solution containing zinc ions, 
calcium ions, and phosphate ions and accelerators and, optionally, other 
additives, characterized in that, after degreasing and without activation, 
the surfaces are brought into contact at 30.degree. to 65.degree. C. with 
solutions which contain more than 0.5 to 1.5 g.l.sup.-1 of Ca.sup.2+, 0.5 
to 1.5 g.l.sup.- of Zn.sup.2+, 10 to 50 g.l.sup.- of PO.sub.4.sup.3- and, 
as accelerator, 0.5 to 30 g.l.sup.-1 of NO.sub.3.sup.- and/or 0.01 to 0.6 
g.l.sup.- of NO.sub.2.sup.- and/or 0.2 to 10 g.l.sup.- of ClO.sub.3.sup.- 
and/or 0.1 to 2 g.l.sup.-1 of an organic nitro compound and/or 0.01 to 
0.5 g.l.sup.- of H.sub. 2 O.sub.2 or of an inorganic peroxide and which 
solutions have a pH value of from 2.2 to 3.8, a ratio of free acid to 
total acid of from 1:10 to 1:60 and a ratio by weight of (Ca.sup.2+ 
+Zn.sup.2+) to PO.sub.4.sup.3- of from 1:&gt;8 to 1:40. 
More particularly, the present invention relates to a process for 
phosphatizing metal surfaces with an acidic phosphatizing solution 
containing zinc ions, calcium ions, phosphate ions and accelerators 
consisting essentially of contacting said metal surfaces after degreasing 
and without activation with an aqueous solution at a temperature of from 
30.degree. to 65.degree. C., said aqueous solution containing 
(a) more than 0.5 to 1.5 g.l.sup.-1 of Ca.sup.2+, 
(b) 0.5 to 1.5 g.l.sup.-1 of Zn.sup.2+, 
(c) 10 to 50 g.l.sup.-1 of PO.sub.4.sup.3-, 
(d) at least one accelerator selected from the group consisting of: 
0.5 to 30 g.l.sup.-1 of NO.sub.3.sup.-, 
0.01 to 0.6 g.l.sup.-1 of NO.sub.2.sup.-, 
0.2 to 10 g.l.sup.-1 of ClO.sub.3.sup.-, 
0.1 to 2 g.l.sup.-1 of an organo nitro compound, 
0.01 to 0.5 g.l.sup.-1 of an inorganic peroxide or hydrogen peroxide, 
and mixtures thereof 
which aqueous solution having a pH of from 2.2 to 3.8, a ratio of free acid 
to total acid of from 1:10 to 1:60 and a ratio by weight of (Ca.sup.2+ 
+Zn.sup.2+) to PO.sub.4.sup.3- of from 1:&gt;8 to 1:40.; as well as the 
aqueous, acidic phosphatizing solution. 
The present invention also relates to the use of this process for 
pretreating the metal surfaces for painting by electrodeposition, more 
particularly by cathodic electrodeposition. 
The process according to the invention is particularly suitable for 
phosphatizing metal surfaces of iron, steel and zinc. However, surfaces of 
aluminum may also be coated with zinc/calcium layers by the process 
according to the invention. 
It has been found that the treated metal surfaces become coated with a 
layer of dizinc calcium phosphate dihydrate (scholzite). Although zinc and 
calcium ions are incorporated in the phosphate coating in a molar ratio of 
2:1, it is crucial to the composition of the bath solutions used in the 
process according to the invention that they contain zinc and calcium ions 
in a ratio by weight of from 1:0.5 to 1:1.5 preferably in a ratio of 1:1 
to 1:1.5 and particularly in a ratio of 1:1. 
To this end, suitable water-soluble zinc and calcium salts or solutions are 
added to the solutions so that the content of Zn.sup.2+ amounts to 
between 0.5 and 1.5 g.l.sup.-1, preferably from 0.7 to 1.4 g.l.sup.- of 
the phosphatizing solution and the content of Ca.sup.2 + amounts to more 
than 0.5 to 1.5 g.l.sup.-, such as 0.52 to 1.5 g.l.sup.-, preferably 0.6 
to 1.3 g.l.sup.-1 of the phosphatizing solution. More particularly, ZnO, 
phosphoric acid and Ca(NO.sub.3).sub.2 :4H.sub.2 O are used as starting 
compounds. 
The proportion of PO.sub.4.sup.3- -ions, adjustable through phosphoric 
acid, in the phosphating solutions according to the invention is 
considerably higher, amounting to between 10 and 50 g.l.sup.-1, preferably 
between 20 and 35 g.l.sup.-1. 
Accordingly, the indicated quantities of the active ions forming the 
principal constituents of the phosphatizing solutions according to the 
invention are characterized in that the ratio by weight of the sum of 
calcium and zinc ions to phosphate ions is always in the range from 1:&gt;8 
to 1:40, preferably from 1:8.2 to 1:20. This ensures that homogeneous 
scholzite layers are formed on all the metal surfaces treated. The 
formation of, for example, tertiary zinc phosphate tetrahydrate (hopeite) 
or dizinc iron phosphate tetrahydrate (phosphophyllite), which together 
with scholzite would lead to a less homogeneous and less firmly adhering 
protective layer, is not observed. 
Another important process parameter is the molar ratio of free acid to 
total acid (acid ratio) which has to be adjusted to values of from 1:10 to 
1:60, preferably from 1:15 to 1:50. This means in particular that a 
relatively low value for the concentration of free acid is particularly 
important to the formation of good scholzite layers. 
The pH of the acidic phosphatizing solutions is maintained between 2.2 and 
3.8, preferably between 2.8 and 3.7. 
From 0.5 to 30 g.l.sup.-1, preferably 2 to 10 g.l.sup.-1, of nitrate ions 
and/or from 0.01 to 0.6 g.l.sup.-1, preferably 0.05 to 0.2 g.l.sup.-1, of 
nitrite ions, and/or from 0.2 to 10 g.l.sup.-1, preferably 0.5 to 4 
g.l.sup.-1 of chlorate ions, and/or from 0.1 to 2 g.l.sup.-1, preferably 
0.4 to 1 g.l.sup.-1 of organic nitro compounds, and/or from 0.01 to 0.5 
g.l.sup.-1 of inorganic peroxides or H.sub.2 O.sub.2 are added as 
accelerating oxidizing agents to the phosphatizing solutions for the 
process according to the invention. 
If aluminum surfaces are to be phosphatized by the process according to the 
invention, simple and/or complex fluorides may be added to the bath 
solutions in a quantity of from 0.01 to 2 g.l.sup.-1 in order to complex 
even very small quantities of aluminum, which could enter the bath from 
the metal surface and impair its effectiveness, by fluoride ions. 
The phosphating solutions with which metal surfaces are phosphated by the 
process according to the invention may also contain other metal ions, for 
example Ni.sup.2+. Their content is in the range from 0.01 to 1.5 
g.l.sup.-1 of bath solution. 
The treatment of the iron, steel, zinc and/or aluminum surfaces by the 
process according to the invention may be carried out by spraying, 
immersion or even flooding. However, combined processes, such as 
spray-immersion for example, may be used with equally good results. The 
times for which the phosphating solutions are in contact with the metal 
surfaces are between 60 and 240 seconds. In the case of spraying for 
example, the contact times are between 60 and 180 seconds and, in the case 
of immersion, between 90 and 240 seconds. However, considerably shorter 
treatment times are also possible. 
According to the invention, the metal surfaces are treated with the 
phosphatizing solutions at temperatures in the range from 30.degree. to 
65.degree. C. Preferred treatment temperatures are in the range from 
48.degree. to 57.degree. C. 
One of the major advantages of the process according to the invention is 
that the scholzite layers are formed on the metal surfaces completely 
irrespective of the method used to clean them before the phosphatizing 
step. Where the process according to the invention is applied, therefore, 
there is complete freedom of choice in regard to the degreasing and 
cleaning agents used. 
Another advantage lies in the fact that particularly thin, fine-grained 
crystalline phosphate coatings are obtained even without the use of 
activating agents of the type commonly used in known processes. Not only 
does this save at least one process step before the phosphatizing step, it 
also saves the raw materials required for that process step, such as for 
example titanium phosphates which are used as activating agents. 
The process according to the invention makes it possible to obtain 
excellent protective layers between 0.5 and 5 .mu.m thick which are 
eminently suitable for use as a substrate for electrodeposition paints, 
particularly cathodic electrodeposition paints of the type being used to 
an increasing extent in the automotive industry. However, the phosphate 
coatings obtained in accordance with the invention are also suitable for 
use as a substrate for other organic protective surface layers.

The invention is illustrated by the following Examples. 
The phosphatizing solutions according to the invention were prepared in 
known manner by combining the required components, particularly 
concentrates containing zinc oxide, phosphoric acid and calcium nitrate 
tetrahydrate, salts and solutions, followed by dilution with water to the 
concentrations according to the invention. 
EXAMPLE 1 
A phosphatizing solution containing 
1.0 g.l.sup.-1 of Ca.sup.2+ 
1.2 g.l.sup.-1 of Zn.sup.2+ 
29.5 g.l.sup.-1 of PO.sub.4.sup.3- 
1.0 g.l.sup.-1 of ClO.sub.3.sup.- 
3.2 g.l.sup.-1 of NO.sub.3.sup.- 
0.1 g.l.sup.-1 of NO.sub.2.sup.- 
was prepared. The phosphating solution had the following characteristics: 
pH-value: approx. 3.1 
Acid ratio: approx. 1:19 
.SIGMA.Ca.sup.2+ +Zn.sup.2+ :PO.sub.4.sup.3- =1:13.4 
Steel sheets which had been cleaned by immersion for 3 minutes at 
50.degree. C. in an alkaline cleaning solution and then rinsed with water 
were immersed in the above-mentioned phosphatizing solution for 4 minutes 
at 55.degree. C. They were rinsed with water and distilled water and 
dried. 
The phosphate coatings obtained were finely crystalline and non-porous. 
The sheets were then coated with a cathodic electrodeposition paint and 
dried for 20 minutes by heating at 185.degree. C. The dry film thickness 
of the paint amounted to 18 .mu.m. 
The sheets were then provided with a single cut in accordance with DIN 
53167 and salt-spray tested for 480 h in accordance with DIN 50021. 
Evaluation in accordance with DIN 53167 revealed a creepage value of &lt;0.1 
mm. 
This Example shows that the process according to the invention gives good 
phosphate coatings. 
EXAMPLE 2 
A phosphatizing solution containing: 
0.6 g.l.sup.-1 of Ca.sup.2+ 
0.7 g.l.sup.-1 of Zn.sup.2+ 
22.6 g.l.sup.-1 of PO.sub.4.sup.3- 
0.3 g.l.sup.-1 of Ni.sup.2+ 
2.7 g.l.sup.-1 of NO.sub.3.sup.- 
0.5 g.L.sup.-1 of F.sup.- 
0.1 g.l.sup.-1 of NO.sub.2.sup.- 
was prepared. The phosphatizing solution had the following characteristics: 
pH-value: approx 3.3 
Acid ratio: approx. 1:39 
.SIGMA.Ca.sup.2+ +Zn.sup.2+ :PO.sub.4.sup.3- =1:17.4 
Steel plates which had been sprayed with an alkaline cleaning solution for 
60 seconds at 45.degree. C. were sprayed with the above-mentioned 
phosphatizing solution for 90 seconds at 48.degree. C. They were rinsed 
with water and distilled water and dried with compressed air. 
The phosphate coatings obtained were finely crystalline and non-porous. 
The sheets were then coated with a cathodic electrodeposition paint and 
dried by heating for 20 minutes at 185.degree. C. The dry film thickness 
of the paint amounted to 18 .mu.m. 
The sheets were then provided with a single cut in accordance with DIN 
53167 and salt-spray tested for 480 h in accordance with DIN 50021. 
Evaluation in accordance with DIN 53167 produced a creepage value of &lt;0.1 
mm. 
This Example demonstrates that the process according to the invention gives 
good phosphate coatings. 
EXAMPLE 3 
A phosphatizing solution containing 
1.3 g.l.sup.-1 of Ca.sup.2+ 
1.3 g.l.sup.-1 of Zn.sup.2+ 
21.2 g.l.sup.-1 of PO.sub.4.sup.3- 
1.0 g.l.sup.-1 of Ni.sup.2+ 
2.2 g.l.sup.-1 of ClO.sub.3.sup.- 
0.6 g.l.sup.-1 of sodium nitrobenzene sulfonate 
was prepared. The phosphatizing solution had the following characteristics: 
pH-value: approx. 2.9 
Acid ratio: approx. 1:18.6 
.SIGMA.Ca.sup.2+ +Zn.sup.2+ :PO.sub.4.sup.3- =1:8.2 
Electrogalvanized steel sheets which had been immersed for 3 minutes at 
50.degree. C. in an alkaline cleaning solution and subsequently rinsed 
with water were immersed in the above-mentioned phosphatizing solution for 
3 minutes at 57.degree. C. They were then rinsed with water and distilled 
water and dried with compressed air. 
The phosphate coatings produced were finely crystalline and non-porous. 
The sheets were then coated with a cathodic electrodeposition paint and 
dried by heating for 20 minutes at 185.degree. C. The dry film thickness 
of the paint amounted to 18 .mu.m. 
The sheets were then provided with a single cut in accordance with DIN 
53167 and salt-spray tested for 480 h in accordance with DIN 50021. 
Evaluation in accordance with DIN 53167 produced a creepage value of &lt;0.1 
mm. 
This Example shows that the process according to the invention gives good 
phosphate coatings. 
EXAMPLE 4 
A phosphatizing solution containing 
1.0 g.l.sup.-1 of Ca.sup.2+ 
1.4 g.l.sup.-1 of Zn.sup.2+ 
26.6 g.l.sup.-1 of PO.sub.4.sup.3- 
3.2 g.l.sup.-1 of NO.sub.3.sup.- 
0.1 g.l.sup.-1 of NO.sub.2.sup.- 
was prepared. The phosphatizing solution had the following characteristics: 
pH-value: approx. 3.6 
Acid ratio: approx. 1:48 
.SIGMA.Ca.sup.2+ +Zn.sup.2+ :PO.sub.4.sup.3- =1:11.1 
Steel sheets which had been sprayed with an alkaline cleaning solution for 
60 seconds at 45.degree. C. were sprayed with the above-mentioned 
phosphatizing solution for 120 seconds at 35.degree. C. They were then 
rinsed with water and distilled water and dried with compressed air. 
The phosphate coatings formed were finely crystalline and non-porous. 
The sheets were then coated with a cathodic electrodeposition paint and 
dried by heating for 20 minutes at 185.degree. C. The dry film thickness 
of the paint amounted to 18 .mu.m. 
The sheets were then provided with a single cut in accordance with DIN 
53167 and salt-spray tested for 480 h in accordance with DIN 50021. 
Evaluation in accordance with DIN 53167 produced a creepage value of 0.2 
mm. 
This Example shows that the process according to the invention gives good 
phosphate coatings. 
COMISON EXAMPLE 
A phosphatizing solution was prepared in accordance with GB-PS No. 10 40 
020, page 3 (solution B). 
Steel sheets which had been cleaned with an alkaline cleaning solution for 
30 seconds at 72.degree. C. were sprayed with the above-mentioned 
phosphatizing solution for 60 seconds at 66.degree. C. They were then 
rinsed with water and distilled water and dried with compressed air. The 
phosphate coatings obtained were coarsely crystalline and not entirely 
non-porous. 
The sheets were then coated with a cathodic electrodeposition paint and 
dried by heating for 20 minutes at 185.degree. C. The dry film thickness 
of the paint amounted to 18 .mu.m. 
The sheets were then provided with a single cut in accordance with DIN 
53167 and salt-spray tested for 480 h in accordance with DIN 50021. 
Evaluation in accordance with DIN 53167 produced a creepage value of from 
4 to 6 mm. 
This comparison Example shows that, in contrast to the Examples according 
to the invention, distinctly inferior corrosion prevention is obtained. 
The preceding specific embodiments are illustrative of the practice of the 
invention. It is to be understood however, that other expedients known to 
those skilled in the art or disclosed herein may be employed without 
departing from the spirit of the invention or the scope of the appended 
claims.