Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to tinplate container component
and methods of making them. It is especially concerned with
tinplate components suitable for, but not confined to foodstuff
containers which are partly or entirely of tinplate, and par-
ticularly to components for containers for milk products.
A typical foodstuff container consists of a can having
either two or three components, comprising a can body with a
separate can end member seamed on to each end, or a can body
closed integrally at one end and having a separate end memb0r
seamed onto the other end. Any, both or all components may be
; of tinplate, formed from sheet consisting of steel having a
flow-brightening coating comprising an outer layer of metallic
tin and usually also a relatively thin layer of alloy between
, the steel and the metallic tin outer layer. The alloy layer,
which is formed during the flow-brightening process, consists
entirely or mainly of the inter-metallic compound FeSn2. In
addition a relatively thin passivation layer containing chromium
may be applied over the tin coating, and it is common in compo- ;
nents for foodstuff containers to apply a suitable protective
lacquer over the tin coating (or over the passivation layer
i~ present).
There are various ways of preventing corrosion of the
tin coating by the contents of a container, beyond the mere use
of lacquer and passivation layers, which in many instances may
not be sufficient in themselves. Among such additional methods
may be mentioned the use of a reducing or substantially chem~cally~
! inert atmo~phere in the container headspace, and the modification
of the lacquer by the use of additives. Nevertheless, such
expedients do tend to add to the cost of packaging the container
contents and to be somewhat limited as to their application.
We have discovered, ~urprisingly, that there is a posi-
tive relation~hip between, on the one hand, the grain orientation
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of the metallic tin in commercially-produced flow-brightened
electrolytic tinplate and, on the other hand, the resistance
of the tin outlet layer itself to corrosion when used in com-
ponents for containers for certain foodstuffs, in particular
certain milk products. This grain orientation is usually
referred to as the texture of the metal, and this is the term
that will be used hereinafter. We have also discovered that
the texture of the tin coating as deposited electrolytically
undergoes a marked change due to the flow-brightening process,
and that it i9 the extent to which this change has taken place
that can be correlated with the said corrosion performance.
The invention accordingly provides a method of making
a tinplate container component (particularly though not neces-
i sarily for use with milk products), which method includes the
selection of tinplate sheet having certain predetermined
characteristics, and in particular the textural characteristics
which we have found prevent or substantially prevent corrosion
of the tin outer layer (de-tinning by milk products).
These textural characteristics are best defined in
terms of test results which may conveniently be those obtained
using known techniques, in particular those of the measurement
of the texture of the metallic tin outer layer by X-ray diffrac-
tion. These X-ray diffraction measurements can be conveniently
displayed in various ways, for example on a pole figure, from
which both qualitative and quantitative information regarding
the characteristics of the texture can be drawn. Quantitive
information as to the texture can be ohtained from measurements
of the inten~itie~ of selected certain reflection5 or in terms
of the ratio of the peak intensities of ~elective reflections
obtained u~ing an X-ray diffractometer. The latter type of
measurement i~ more convenient, quicker and cheaper than pole
figure determination~,
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The purpose of these texture measurements is to deter-
mine the degree to which the texture of the original as-
electrolytically deposited tin (hereinaf~er called the " as-
depositedl' tin coating) has been transformed to the texture
which is characteristic of fully flow-brightened tin. The term
' ~'partially flow-brightened" means that the tin coating has been
~ubjected to an incomplete flow-brightening process, for example,
by passing it between the brightening rolls with the latter
set to roll the tin coating so lightly as to change its grain
`, 10 orientation to a certain extent in plastic flow without melting
~, it. The tin coatings which are found to be substantially resis-
! tant to corrosion by milk products have textures which are ' ~ '
neither one o~ these two extremes but rather are interrnediate
between them;this intermediate texture can be expressed in terms
of the aforesaid texture measurements~ ~ ,
We have al~io found that there is a relationship
between these texture measurements using on the one hand, X- ''
, ray diffraction and, on the other, oxide reduction potential ,-
measurements, so that oxide reduction potential measurements
may also be u~ed to express the degree of flow-brightening
achieved. We have found a satiisfactory correlation between
the result3 obtained in corrosion tests using milk products
on variou~ samples of tinplate, on the one hand, and on the
other , the reqults of experiments to determine the degree of
flo~-brightening in terms of the texture and other characteris-
tics of the outer layer by both of the above-mentioned techniques. "~
Accordingly, in a first aspect the invention provides ~ ,
a method of making a tinplate container component, comprising
the steps of (i) 3electing tinplate sheet having predetermined
characteristic3 and (ii) forming the,component therefrom, the
isaid characteristici~ being that:
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a) The tin coating of the tinplate includes an outer
layer of unalloyed, electrolytic beta-tin, partially flow-
brightened ~as hereinbefore defined), the degree of severity
of flow-brightening being defined by (b) as follows:
b) X-ray diffraction measurement shows that the
preferred grain orientation of the tin in said outer layer is
intermediate between, and different from, that of fully flow-
brightened t~n (as hereinbefore defined) and that of as-deposited
tin, and
j 10 c) The tin coating is substantially resistant to
corrosion by milk products.
The texture of the outer layer is specified by means
of X-ray diffraction measurements so as to define the required
degree of transformation of the texture from that obtaining
for the as-deposited (non-flow brightened) tin coating to that -
obtaining for the required partially-flow brightened tin coating.
What i~ shown by the diffraction measurement is most
conventionally expressed in terms of the ratio between the
inten~ities of two preselected reflections of the metallic
tin outer layer. Preferably the intensity ratio af the two
preselected reflections obtained by X-ray diffraction is such
a~ to indicate a texture of said outer layer intermediate
between, and different from, both that of as-deposited tin and
that of fully flow-brightened tin (as hereinbefore defined).
Preferably, said ratio has a value greater than 0.1 where the
said reflections are the (101) and (200) reflections.
In a second aspect, the invention provides a method
of making a tinplate container component, comprising the steps
of ~i) selecting tinplate sheet having predetermined characte
ristics and (ii) forming the component therefrom, the said ~;
characteri~tic~ being that:
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a) the tin coating of the tinplate includes an outer
layer of unalloyed partially flow-brightened electrolytic
; beta-tin,
b) the tin coating has reduction characteristics such
that after oxidation, from a substantially oxide-free state,
for 24 hours at iimbient temperature, a first current peak will
occur at a reduction potential measured from a set potential
of -600 mV in the range -600 mV ta -930 mV; and
c) the tin coating is substantially resistant to
corrosion by milk products.
According to the invention in a third aspect, there
i8 provided a container component of tinplate whose tin coating :
includes an outer layer of unalloyed, electrolytic beta-tin,
substantially resistant to corrosion by milk products and
flow-brightened (as hereinbefore defined) to a degree of severity
~uch that X-ray diffraction ~how3 that the preferred grain orien- .
tation of the tin in said outer layer is intermediate between,
and different from, that of fully flow-brightened tin (as here-
inbefore defined) and that of as-deposited tin.
According to the invention in a fourth aspect, there :
is provided a container component of tinplate whose tin coating
includes an outer layex of unalloyed, electrolytic beta-tin,
substantially resistant to corrosion by milX products and
partially flow-brightened (as hereinbefore defined), said outer
layer having reduction characteristics such that, after oxidation
from a substantially oxide-free state for 24 hours at ambient
temperature, a first current peak will occur at a reduction
potential, mea~ured from a set potential of -600 mV, in the range
-600 mV to -930 mV.
It is to be understood that in each of the above
definition~ of the method of the invention in its first and
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second aspect~, the determination or evaluation of characteristics
of the metallic tin outer layer under (b) is not a step in the
method, but rather as a convenient way of defining the charac-
teristics concerned.
Embocliments of the invention will now be described
by way of example only, and partly with reference to the
accompanying drawings, of which:
, Figure l is a pole figure for the (321) re~lection
of the metallic tin outer layer of a sample of as-deposited
tinplate,
Figure 2 is a pole figure for the (200) reflection of
the metallic tin outer layer of a sample of as-deposited
tinplate,
Figure 3 is a pole figure for the (200) reflection
of the metallic tin outer layer of a sample of fully flow-
brightened tinplate not having the characteristics required
for making a component according to the invention,
Figure 4 i8 a pole figure for the (200) reflections
of the metallic tin outer layer of a ~ample of tinplate having
the characteristics for such a component and not being fully
flow-brightened, and
; Figure 5 i5 a general view of a typical tinplate
container.
In a methocl of making a tinpla-te can end member lO,
Flgure 5, according to the invention, the can end is made from
~heet material, by any convenient known technique, which
u~ually means stamping in a conventional press and which need
not be described further here. A can body ll may be made of
similar material to the end member lO, and a complete container
i~ made by securing the member 10 onto the open end of the body
11 by conventional means well known in the art.
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The sheet material selected for the can end 10 is - -
electrolytic tinplate in which the tin coating includes an
outer layer of unalloyed metallic beta-tin which has been
partially flow-brightened for example as discussed earlier
herein. There will usually also be a relatively thin layer
of the iron-tin alloy FeSn2 between the metallic tin outer
layer and the steel substract. The sheet material is further
cho~en so as to be substantially resistant to corrosion by
milk product~, and it preferably has a coating of a suitable
protective lacquer over the surface of the tin outer layer,
with a very thin passivation layer containing chromium between
the tin and the lacquer.
The material is selected for its corrosion resistance
according to the grain orientation characteristics of the tin
; in the outer layer. This may be determined by various methods
as already set forth herein. Specific examples of these methods
will now be described. In each case the samples used are of
unlacquered, partially flow-brightened electrolytic tinplate
whose tin coating has an outer layer of beta-tin as above described.
; 20 Example 1
In this example X-ray diffraction techniques ha~e been
used, and their results are expressed in pole figures,four of
which are represented by Figures 1 to 4. The numerals marked
on the pole figures are "contours'7 on a scale of 1 to 10,
10 being the maximum. These contours are directly proportional
in each Figure to the numberof counts per second in the X-ray
test concerned.
Figure 1 is a pole figure for the (321) reflection for
a sample of an as-deposited non-flow-brightened tinplate. The
contour scale 1 to 10 represents a range of 1 to 14 counts
per second. A pole figure using this reflection is quite
useful for characterising the texture of the outer layer
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of the tin coating of tinplate in this condition because the
loops of contours at its centre which are close together indicate - -
~that there is a si~nificant proportion of the tin grains with
their (321) plane parallel to the surface of the specimen.
Figure 2 is a pole figure for the (200) reflection for
a sample of an as-deposited non-flow-brightened tinplate (the
contour scale of 1 to 10 representing a range of 1 to 60 counts
per second) which is useful for following the transformation of
the texture of the outer layer with the flow-brightening process,
since for this sample the minimum in the centre of the pole
figure indicates that a high proportion of the tin grains do
not have the (100) planes parallel to the surface of the specimen,
whereas:
Figure 3 which is a pole figure for the t200) reflection
for a sample of fully flow-brightened tinplate, (as hereinbefore -
defined) with the contour scale 1 to 10 representing a range of
1 to 180 counts per second, shows a pronounced maximum at the
centre of the pole figure, indicating that a significant proportion
` , of the tin grains in the outer layer have the (100) planes
parallel to the surface of the specimens.
The sample which was the subject of Figure 3 was
proved to have unsatisfactory resistance to corrosion by milk
product~.
Figure 4 is a pole figure for the (200) reflection
for a sample of tinplate which proved to have satisfactory re-
sistance to corrosion by milk product5. The contour scale 1
to 10 represents a range of 1 to 80 counts per second.
Thi~ pole figure i~ le~s clear cut than the others,
indicating a more complex texture in the tin outer layer situation.
Thereis a maximum near the centre of the pole figure, but it is
markedly less prononced than that in Figure 3 and there are also
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contours over most of the figure which is not the case in
Figure 3.
Example 2
A number of different tinplate samples were subjected
to conventional X-ray diffraction analysis. The same samples
were also each subjected to a standard electrochemical test
carried out as follows.
The chromium passivation layer was removed electro-
chemically from each sample in an electrolyte bath under
controlled conditions. The samples were rinsed and cathodically
cleaned in a separate electrolyte bath under known conditions ;~
to remove any tin oxides present, before being rinsed and dried
in warm air. The samples were then exposed to air at ambient
temperature for 24 hours, at the end of which the resulting
oxide layer was measured in a mixed borate solution under
deaerated conditions. The reduction peak was measured poten-
tio-dynamically from a set potential of -600 mV with respect
to a N/10 calomel reference electrode, scanning in a cathodic
direction. The potential at which the first current peak
occurred i.e. the so-called reduction potential at which the
; oxide was reduced and current rose to a peak value and then
fell, waa recorded.
In the following table, the second column indicates
re~is~ance of the relevant sample to corrosion by milk products.
; The third column shows the ratio R between the maximum
intensity of preferred grain orientation in the outer layer of
metallic tin, obtained by the X-ray tests in the (101) plane,
to that obtained by the X-ray tests in the (200) plane.
The fourth column shows the potential at which the
first reduction peak, above mentioned,03Curred for each sample
in the electrochemical tests.
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Sample Performance R Reduction :
~o. _ Potential (mV) :~
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1 Satisfactory 0.80 -917 to -920
2 Satisfactory 0.47 -920
3 Satisfactory 0.34 -917
Satisfactory 0.18 -910 to -915 ~
Unsatisfactory 0.09 -940
6 Unsatisfactory 0.07 -935
7 Unsatisfactory 0.03 -933 to -935
. . Unsatisfactory 0.02 -932
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From the table it can be seen that for the samples
with satisfactory corrosion resistance, the ratio R was over
0.10 with reference to the two planes chosen. Similarly, it
will be seen that the reduction potential at which the current -
peak occurs lie~ within a predetermined range when the required
characteri~tics are present, being, in the above examples, in
the range -910 to -920 mV and in any event within the range .
-600 mV to -930 mV.
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