Note: Descriptions are shown in the official language in which they were submitted.
103~5
The present invention relates to a process o~ improving
the anti-corrosion properties of ferrous metals, e.g. steel articles,
with a surface layer containing nickel andfor cobalt and the article s
produced thereby.
It is known to coat steel already coated with a layer of ;
nickel with another metal layer, e.g. zinc or chromium. In this way,
~n improvement in anti-corrosion properties of the steel is obtained.
It is also known to coat steels with a layer of nickel and then to
subject the coated steel to a diffusion heat treatment at a temperature
in excess of 725C for a relatively short time in order to effect
an improvement in the anti-corrosion properties thereof. In this
connection, reference is made to U.S. Patent No. 2,731,403.
An object of one aspect of this invention is to provide a
process for improving the corrosion resistant properties of a ferrous
metal articleg e.g. a steel substrate or article, already coated with
a surface layer of nickel andior cobalt. - -
An object of another aspect of this invention is to provide
a coated steel article characterized by improved resistance to corro--
sion.
It has now surprisingly been discovered that a combination
of the known processes described above provides a considerably
improved effect with respect to the anti-corrosion properties of the
steel;
- Accordingly, by one broad aspect the present invention
provides for properties of a steel article which process comprises
applying to said article a first layer of at least one metal selected
from the group consisting of nickel and cobalt, subjecting said coated
article to a bonding heat treatment at an austenitizing temperature
in the range of 725C - 1000C for a time less than 45 minutes, and
then applying as a second layer at least one metal seIected from the
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group consisting of chromium, tin~ lead, zinc, copper and cadmium to
~aid first layer~ whereby a coated article is obtained characterized
by improved resistance to corrosion.
By one variant of this process, the period of heat treatment
is less than 15 minutes, preferably less than 5 minutes.
By yet another variant, the first layer is applied at a
thickness of over 2 microns and wherein said second mètal layer is
also applied at a thickness of over 2 microns.
By a still further variant, the thicknesses of the first
layer and of the second layer is 5-25 microns.
By another variant~ the steel is a carbon steel containing
up to 0O9~ carbon.
- ~ By still another variant, the steel article contains 0.2%
to 0.6% carDon and wherein following the bonding heat treatment at
said austenitizing temperature, the article is rapidly cooled to form
a martensitic structure.
By another a,spect of'this invention, a process is provided
for improving the corrosion resistance of a steel article which compris-
es, applying to said article a first thin layer of at least one metal
selected from the group consisting of nickel and cobalt over a
thickness range of over 2 to 50 microns, heat treating said coated
article at an austenitizing temperature falling within the range of
725C to 1000C at substantially less than 45 minutes, and then
applying a second thin layer of at least one metal selected from the
group consisting of chromium 9 tin, lead, zinc, copper and cadmium to
said first layer of a thickness of o~er 2-50 microns, whereby a coated
article is obtained characterized by improved resistance to corrosion.
By a variant thereof, the period of heat treatment is less
than 15 minutes, preferably less than 5 minutes.
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1038Z~s
By another varian~ ther~of, the steel article con~ains
up to 0.9~ carbon by weight and wherein following the bonding heat
treatment at said austenitizing temperature, the article is rapidly
cooled to form a martensitic structure.
By another aspect of this invention, a novel article
of manufacture of provided comprising a steel substrate having bonded
thereto a first layer of a metal selected from the group consisting
of nickel and cobalt at a thickness ranging from over 2 to 50
microns and a second metal layer selected from the group consisting
of chromium, tin, lead, zinc, copper and cadmium bonded to said
first layer at a thickness ranging from over 2 to S0 microns, said
article being heat *reated at a temperature of 725-1000C at which
- phase transition to austentic structure of carbon begins before the
deposition of said second metal 7 said steel substrate being characteri-
zed by markedly improved resistance to corrosion.
By one variant, the steel is a carbon steel containing
up to 0.9~ carbon, preferably 0.2 - 0.6% carbon.
The heat treatment of the nickel and/or cobalt layer
comprises the first stage of the process of an aspect of the present
invention. This heat treatment should take place within such limits
with respect to temperature and time that the diffusion of nickel and/
or obalt into the steel substrate is sufficiently inhibited and
preferably does not exceed a maximum of a few microns. In this
manner, diffusion of iron into the nickel and/or cobalt layer will also
be sufficiently inhibited. ThiS is achieved by carrying out the
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1038245 ~
heat treatment at a temperature in excess of b7~t 725C for,a
time lèss than 45 minutes, Actually, the bèst results are
obtained as regards the anti-corrosion properties of the final
surface layer if the heat treatment of the steel object,pre- - L
coated with nickel and/ox cobalt is carried out for a time sub-
s~antially less than 30 minutes. Thus, it has been found that
diffusion of nickel and/or cobalt into the substrate is further ~ -,
inhibited if the heat treatment is less than 15 minutes. Tests
~ - - .
have indicated that better anti-corrosion properties are obtained ~
at less,than 15 minutes of heat treatment as compared to heat - ~r-~
treatment tim2s of be~een 15 and 30 minutes and that periods '~
' of heat treatment of even less than ' 5 minutes provide the ,
i best properties in the final surface layer~ The-time at which -
, 'the temperature of treatment of the surface layer amounts to or
, ' ,~ .
15' ' exceeds 725C is referred to as ~he period of heat treatment. ~ - -
P2rio~s of heat treatment in excess of 45 minutes have
been found to cause diffusion of such-an order that the proper-
- ties,of the surface layer are markedly deteriorated in one or
' more respects. It has been found th'at heat treatment at a tem-
perature belo-~ 725C, i.e. below the temperature at which a phase
'' transition to austenitic structure of carbon steels b2gins, does
not provide goo~ adhesion of the nickel and/or cobalt surface
layer.
It has been observed that a heat treatment at a te~- ~'
p~ .
-25 perature in excess of 725C for a time shorter t,han 1 minute
t .~ . .
~` 1038z4s
provides an excellent product which is quite superior to nonheat treated
products and to products which have been treated for a time in excess of
45 minutes.
The article (of steel or iron~ to which the surface layer of
nickel and/or cobalt is applied can be of arbitrary form and composition.
Conventional carkon steel in the form of plates, pip~s or bands is suitable
for many purposes. According to aspects of the present invention, it has
also been found advantageous to apply the surface layers to such finished
steel objects as bolts, screws and castings.
Ihe very short time of treatment is believed to be due to the
fact that the interlayer between the surface layer and the substrate re-
crystallizes very rapidly because of high inherent surface energy and that
an atomic bond is achieved to the substrate by this recrystallization. If
; the time of heat treatment is too long, erg~ substantially more than 45
minutes at temperature, diffusion occurs which gradually impaLrs the
0rrosion resistant properties of the surface layer.
In ths electr~lytic coatins of steel with a first layer of
nickel or cobalt, a certain amDunt of pores is apt to form passing
j through the whole layer and thus expose the steel surface. If a new metal
layer, e.g. zinc, is applied on top of this layer, atomic hydrogen is
precipitated in the pores ~Ihich diffuses into the steel and causes so-calle
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1038245
gen embrittlement. By the heat treatment according to the process
of an aspect of the present inven~ion, two positive effects are
achieved with respect to avoiding hydrogen embrittlement. For example,
the hydrogen, which penetrates the steel during pickling and during
the electrolytic coating of nickel or cobalt, is released by diffusion,
and the pores formed in the nickel or cobalt layer tend to close
themselves so that the steel surface is no longer exposed. This
prevents additional hydrogen absorption by the steel during the
application of the next metal layer.
The heat treatment according to the process of an aspect
of the present invention is usually carried out in a neutral or
reducing atmosphere. Such conditions may preferably comprise a
- protective gas 9 e.g. argon, or a reducing gas, e.g. hydrogen or carbon
monoxide. The heat treatment can, if desired, also be carried out in
air, especially if a coloured surface is desired.
In order to assure optimum improvement of the anti-corrosion
properties in practice, the temperature during heat treatment should be
at least 72soc at` the surface layer, as stated above. A usable range
of temperature has been found to be 725c to 1000C. It is preferred
that the heat treatment be carried out at a temperature at which there
is a complete transformation into austenitic structure. It appears
that temperature below 7?5 result in such a slow crystallization in
the interlayer that no adhesion occurs, unless longer periods of
heat treatment are used. If, on the other hand, long periods of heat
treatment are used 7 the properties of the substrate material tend to
deteriorate. Thus, a very essential advantage of the process according
to aspects of this ~ention is that the substrate material is not
changed appreciably by the short time of treatment used. This is of
great importance in the manufacture of wire, among other products, as
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-the microstructure is very importan-t for assuring the desired mechani-
cal properties of the wire, e.g. Alasticity and fatigue strength.
One of the advantages of the process of aspects of this
invention is that hardenable steels which have been surface coated
with nickel and/or cobalt can be hardened in connection with the
present heat treatment by cooling the article rapidly following
completion of the heat treatment so that an austenitic decomposition
product is obtained comprising a martensitic structure, provided that,
during heat treatment, the steel arti~le is heated to an austenitizing
temperature in excess of 725C. Such steels may contain up to
0.9~ by weight, for example, at least 0.05~ carbon, and preferably
0.2% to 0.6% carbon. The term "steel" used herein is meant to cover
iron as well.
The surface layer containing nickel and/or cobalt is `
applied to the article (of steel or iron) in conventional manner by
electrolysis or chemically. Plating baths are disclosed in the
book entitled Modern Electroplating (Second Edition, 1953) published
by John Wiley and Sons, for example on pages 141 to 146, 260 to 263
and 282 to 286.
To the surface layer of nickel and~or cobalt following
heat treatment, a thin additional layer of one or more of the metals
chromium, tin, lead, zinc, copper and cadmium is then applied. This
layer can also be applied in a conventional manner, e.g. by electrolysis,
by spraying of molten metal on the steel substrate or by chemical
deposition. Such methods are well known to those skilled in-the art.
Markedly improved results have been obtained with the
coating of articles characterized by uniform smooth surfaces, for
example, steel pipes, steel plates, strip and the like. Thus, an iron
pip~ (0.01%C by weig~t) coated inside and outside with nickel in
accordance with the process of an aspect of this invention (e.g. a
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1038245
nickel electroplate of 10 ~icrons -thick and heat treated at 870C
for 20 minutes) and then coated with a zinc electroplate of 10
microns exhibited a resistance to corrosion under the salt spray tes.
(ASTM B-117) for a very long period of time as evidenced by the
appearance of red rust a~ter 800 to 1000 hours.
In tests conducted on threaded steel articles or other
complicated shapes, it has been found that a normal carbon steel
(0.4~ carbon) coated with a layer of nickel or cobalt of thi~kness of
10 microns and which has not been subsequently heat treated, exhibited
10 red rust attach after 10 hours in a salt spray test (ASTM B-117). These
same articles coated with a zinc layer alone of 10 microns thick
exhibited red rust in the same test after 90 to 100 hours. On the
; other hand, when such threaded carbon steel articles were fi~st coated
with a nickel layer of 10 microns ~hick and then heat treated accord-
ing to the process of an asp,ect'of this invention followed by coating
with a zinc layer of 10 microns thick, red rust did not appear until
- after 200 to 300 hours under the same testing conditions. This is
- unusual for such complicated shapes. Similar improved results are
obtainable with cobalt as the first layer closest to the steel and with
metals other than zinc as the second or outer layer. Thus, markedly
improved properties with respect to resistance to corrosion have been
obtained'for the following coating combinations: nickel-cadmium, and
nickél-copper. It wilI be observed from these tests that it is very
important that the original nickel or cobalt layer be subjected to
heat treatment according to the process of aspects of this invention
before the second metal layer is applied. It has also been noted that
a nickel layer having a thickness of 10 microns and which has not been
heat-treated, does not provide the ma~ked improvements of the teachings
of aspects of th~s invention as regards anti-corrosion properties, even
when a zinc layer is applied to -the nickel layer,
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1038Z4s
Si~ilarly~ in cases whcre the nickel layer has been subjected to too
long a heat -treatment of more than 45 minutes at a temperature in
excess of 725C, inferior properties have been obtained. It is
believed that apparently iron diffuses into the nickel ~hich results
in red rust appearing much sooner. In the conventional hot-dip
coating of zinc,some amount of dissolved
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iron will a~pear in the zinc lt~yer which has a deteriorating ef-
fect on the anti-corrosion properties.
The preferre~ as~ects of the invention will be apparent
from the following examples. ~ -
Example 1
For comparlson with known techni~ues, the following sur-
face coatings were made on a 1.5 ~ (r~ 0O06 inch) steel wirc
~ of carbon steel ( 0O7% carbon)O
; Ao Coating with 1 micron tin layer alone by hot-dip
:-. - . _,
3 coating.
B~ Coating with 15 micron zinc layer alone by hot-dip -
coating- k
C. Coating with 10 micron nickel layer-by electro-
lysis and heat treated. ._~
D. Coating with 10 micron-nickel layer by electro-
~- lysis, heat treated at 850C for 4 minutes and
then coated with 12 micron zinc layer on top of
the nickel layer by electrolysis according to
the present invention.
~0 Corrosion attack as evidenced by the appearance of red
rust was evaluated a ter testing each specimen in the salt spray r
-~ atmosphere according to ASTM B-117 and the following results were
; o~tained:
A. Red rust o-curred after a testing period of
~5 5 hours.
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103~24s
- B. Red rust occurred after a testin~ period of 35 h~urs.
C. Red rust occurr~d after a testin~ period of 20 hours.
D. `~o attack due to red rust occul-red after a testing period of 300 hours.
As will be noted, markedly improved results are obtained with
S the invention ~i.e. D.).
Example 2
Bolts of carbon steel (0.4% carbon) were coated with a 10 micron
layer of nickel by electr~lysis by barrel plating in a nickel salt solution
(from a Watts Bath at lamp/dm2 and 50C for 50 minutes) and were then hard-
ened by heating to 850C for 10 mlnutes and by cooling Ln oil. The bolts
were coated with a 10 micron zinc layer electrolytically according to a
known process comprising 15 to 20 gPl ~grams per liter) zinc, 25 b~ 45 gpl
of sodium cyanide, 80 gpl NaO~, the plating being carried out at a current
d~nsity of 1 a~p/dm2 for 60 minutes at rDom temperature. For comparison
purposes, the same bolts were hardened without nickel layers and thencoated with a lo micron zinc layer.
- Improved results were obtained with the bolts coated according
bo the process of the present invention in that red rust formed after a
salt spray testing period of 200 hours; whereas red rust was
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~038Z45
obserYed after a shorter period of 95 hours for the bolts 5
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electrolytically coate~ with zinc alone in the conventional man-
ner. Bolts coa.ed only with a 10 micron nickel layer (followe~
by heat treating at 870CC for 20 minutes) corro~ed after a very
short period of 10 hours.
ExamPle 3
Tests were conducted with 100 steel or iron pipe sam- j
ples ( 0.01% C) m~asuring 200 mm long and 10 mm in aiameter. ~
The pipes were coated with a 10-micron layer of nickel from a
"~ ~ - ~5`7'
l Watts Bath at a curren. density of ' - 4 amps/dm2 at room te~- :
perature. The coated pipes were then sub~ected to a bonding
heat treatment at 870C for 20 minutes in argon, 50 of the pipe
samples being then coated with zinc according- to the process
described in Example 2, while the remaining 50 were not. In addi- -
: ::
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tion, 50 pipe samples without the nickel coating were coated with
` a 10-micron layer of zinc as in Example 2.
-All of the pipes were subjected to a salt spray test
referred to as ASTM B-117 and the following results obtained:
- ~ Av~rage Time For
Pipe SamPles Red Rust to Appear
(1) 50 pipes ~i-coated and heat
7 treated 10 hours
(2) 50 pipes zinc-coated only 110 hours r
~3) 50 pipes Ni-coated (heat treated)
and then zinc coated 900 hours
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1038Z45 ~`
Broadly speaking, the first coating metal nickel and/or
cobalt may be applied at a thickness of at least several microns
and may range from over 2 - 3 to 25 or even to SO microns, depend-
ing upon the economics of the pro_ess and the configuration of
the articles. The second coating metal selected from the group
consisting o Cr, Sn, Pb, Zn, Cu and Cd may be applied similarly
at a thickness of over several microns and range from over 2 - 3
to 25 or even to SO or more microns depending upon the economics
of the process and the configuration of the article. A prefer-
red range of thicknesses of the foregoing coatings is 5 to 25
micronsr . i :
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