U.S. patent application number 14/646238 was filed with the patent office on 2015-11-19 for chromium-chromium oxide coatings applied to steel substrates for packaging applications and a method for producing said coatings.
The applicant listed for this patent is TATA STEEL IJMUIDEN B.V.. Invention is credited to Arnoud Cornelis Adriaan DE VOOYS, Jan Paul PENNING, Ilja PORTEGIES ZWART, Michiel STEEGH, Jacques Hubert Olga Joseph WIJENBERG.
Application Number | 20150329981 14/646238 |
Document ID | / |
Family ID | 49622838 |
Filed Date | 2015-11-19 |
United States Patent
Application |
20150329981 |
Kind Code |
A1 |
WIJENBERG; Jacques Hubert Olga
Joseph ; et al. |
November 19, 2015 |
CHROMIUM-CHROMIUM OXIDE COATINGS APPLIED TO STEEL SUBSTRATES FOR
PACKAGING APPLICATIONS AND A METHOD FOR PRODUCING SAID COATINGS
Abstract
A coated steel substrate for packaging applications, the
substrate containing (1) a recrystallisation annealed single or
double reduced packaging steel blackplate, or (2) a cold-rolled and
recovery annealed blackplate, wherein one or both sides of the
substrate is coated with a chromium metal-chromium oxide coating
layer produced in a single process step by using a trivalent
chromium electroplating process. A process for obtaining the coated
steel substrate.
Inventors: |
WIJENBERG; Jacques Hubert Olga
Joseph; (AMSTERDAM, NL) ; STEEGH; Michiel;
(SANTPOORT ZUID, NL) ; PENNING; Jan Paul;
('s-GRAVENHAGE, NL) ; PORTEGIES ZWART; Ilja;
(WORMER, NL) ; DE VOOYS; Arnoud Cornelis Adriaan;
(MAARSSEN, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TATA STEEL IJMUIDEN B.V. |
Velsen-Noord |
|
NL |
|
|
Family ID: |
49622838 |
Appl. No.: |
14/646238 |
Filed: |
November 21, 2013 |
PCT Filed: |
November 21, 2013 |
PCT NO: |
PCT/EP2013/074339 |
371 Date: |
May 20, 2015 |
Current U.S.
Class: |
428/626 ;
205/196; 205/287; 205/289 |
Current CPC
Class: |
C25D 3/04 20130101; C25D
7/0614 20130101; C25D 5/48 20130101; Y10T 428/12569 20150115; C25D
9/10 20130101; C25D 3/06 20130101; C25D 9/08 20130101; C25D 5/34
20130101; C25D 11/38 20130101; C25D 5/36 20130101 |
International
Class: |
C25D 3/06 20060101
C25D003/06; C25D 11/38 20060101 C25D011/38; C25D 5/48 20060101
C25D005/48 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2012 |
EP |
12193623.1 |
Dec 3, 2012 |
EP |
12195261.8 |
Claims
1. A process for producing a coated steel substrate for packaging
applications by depositing a chromium metal-chromium oxide coating
on the substrate for packaging applications containing (a) a
recrystallisation annealed single or double reduced packaging steel
blackplate, or (b) a cold-rolled and recovery annealed blackplate,
comprising electrolytically depositing on said substrate said
chromium metal-chromium oxide coating in a single process step from
a plating solution comprising a mixture of a trivalent chromium
compound, a chelating agent, an optional conductivity enhancing
salt, an optional depolarizer, an optional surfactant, to which an
acid or base is optionally added to adjust the pH, wherein the
plating solution does not contain a buffering agent, and wherein a
sufficiently high cathodic current density is being applied to
deposit chromium metal.
2. The process according to claim 1, wherein the chelating agent
comprises a formic acid anion, the conductivity enhancing salt
contains an alkali metal cation and the depolarizer comprises a
bromide containing salt.
3. The process according to claim 1, wherein the chelating agent,
the conductivity enhancing salt and the depolarizer contain a
cationic species and the cationic species in the chelating agent,
the conductivity enhancing salt and the depolarizer is
potassium.
4. The process according to claim 1, wherein the electrolytic
depositing on said substrate said chromium metal-chromium oxide
coating deposits a chromium metal-chromium oxide layer on the
blackplate containing a total chromium content of at least 20
mg/m.sup.2.
5. The process according to claim 1, wherein the electrolytic
depositing on said substrate said chromium metal-chromium oxide
coating deposits a chromium metal-chromium oxide layer on the
blackplate containing a total chromium content of at most 140
mg/m.sup.2.
6. The process according to claim 1, wherein the coated substrate
is further provided on one or both sides with an organic coating,
consisting of a thermosetting organic coating by a lacquering step,
or a thermoplastic single layer, or a thermoplastic multi-layer
polymer by a film lamination step or a direct extrusion step.
7. The process according to claim 1, wherein an anode is chosen
that reduces or eliminates the oxidation of Cr(III) ions to Cr(VI)
ions during the plating step.
8. A coated steel substrate for packaging applications containing
(a) a recrystallisation annealed single or double reduced packaging
steel blackplate, or (b) a cold-rolled and recovery annealed
blackplate, wherein one or both sides of the substrate is coated
with a chromium metal-chromium oxide coating layer produced in a
single process step by using a trivalent chromium electroplating
process, wherein the coated substrate is further provided with an
organic coating, consisting of a thermoplastic single-layer polymer
coating or thermoplastic multi-layer polymer coating.
9. The coated substrate for packaging applications according to
claim 8, wherein the chromium metal-chromium oxide layer contains a
total chromium content of at least 20 mg/m.sup.2.
10. The coated substrate for packaging applications according to
claim 8, wherein the chromium metal-chromium oxide layer contains a
total chromium content of at most 140 mg/m.sup.2.
11. The coated substrate for packaging applications according to
claim 8, wherein the organic coating consists of the thermoplastic
single or multi-layer polymer coating, wherein the thermoplastic
polymer of the thermoplastic single or multi-layer polymer coating
is a polymer coating system comprising one or more layers
comprising polyester and/or copolymers thereof and/or blends
thereof.
12. The process according to claim 1, wherein the electrolytic
deposition deposits a chromium metal-chromium oxide layer on the
blackplate containing a total chromium content of at least 40
mg/m.sup.2.
13. The process according to claim 1, wherein the electrolytic
deposition deposits a chromium metal-chromium oxide layer on the
blackplate containing a total chromium content of at least 60
mg/m.sup.2.
14. The process according to claim 1, wherein the electrolytic
deposition deposits a chromium metal-chromium oxide layer on the
blackplate containing a total chromium content of at most 110
mg/m2.
15. The coated substrate for packaging applications according to
claim 8, wherein the chromium metal-chromium oxide layer contains a
total chromium content of at least 40 mg/m.sup.2.
16. The coated substrate for packaging applications according to
claim 8, wherein the chromium metal-chromium oxide layer contains a
total chromium content of at least 60 mg/m.sup.2.
17. The coated substrate for packaging applications according to
claim 8, wherein the chromium metal-chromium oxide layer contains a
total chromium content of at most 110 mg/m.sup.2.
18. The process according to claim 1, wherein the coated substrate
is further provided on one or both sides with an organic coating,
consisting of a thermosetting organic coating by a lacquering step,
or a thermoplastic single layer, or a thermoplastic multi-layer
polymer by a film lamination step or a direct extrusion step,
wherein the thermoplastic single layer or thermoplastic multi-layer
polymer coating is a polymer coating system comprising one or more
layers comprising a thermoplastic resin selected from the group
consisting of polyesters, polyolefins, acrylic resins, polyamides,
polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene
type resins, ABS resins, chlorinated polyethers, ionomers, urethane
resins and functionalised polymers; and/or copolymers thereof;
and/or blends thereof.
19. The process according to claim 1, wherein a gas diffusion anode
is chosen that reduces or eliminates the oxidation of Cr(III) ions
to Cr(VI) ions during the plating step, such as a anode.
20. The process according to claim 8, wherein the thermoplastic
multi-layer polymer coating is a polymer coating system comprising
one or more layers comprising thermoplastic resins selected from
the group consisting of polyesters or polyolefins, acrylic resins,
polyamides, polyvinyl chloride, fluorocarbon resins,
polycarbonates, styrene type resins, ABS resins, chlorinated
polyethers, ionomers, urethane resins and functionalised polymers;
and/or copolymers thereof; and/or blends thereof.
Description
[0001] This invention relates to chromium-chromium oxide (Cr--CrOx)
coatings applied to steel substrates for packaging applications and
to a method for producing said coatings.
[0002] Tin mill products include tinplate, Electrolytic Chromium
Coated Steel (ECCS, also referred to as tin free steel or TFS), and
blackplate, the uncoated steel. Packaging steels are normally
provided as tinplate, or as ECCS onto which an organic coating can
be applied. In case of tinplate this organic coating is usually a
lacquer, whereas in case of ECCS increasingly polymer coatings such
as PET or PP are used, such as in the case of Protact.RTM..
[0003] Tinplate is characterised by its excellent corrosion
resistance and weldability. Tinplate is supplied within a range of
coating weights, normally between 1.0 and 11.2 g/m.sup.2, which are
usually applied by electrolytic deposition. At present, most
tinplate is post-treated with fluids containing hexavalent
chromium, Cr(VI), using a dip or electrolytically assisted
application process. Aim of this post-treatment is to passivate the
tin surface to stop or reduce the growth of tin oxides, because too
thick oxide layers can eventually lead to problems with respect to
adhesion of organic coatings, like lacquers. It is important that
the passivation treatment should not only suppress or eliminate tin
oxide growth, but should also be able to retain or improve organic
coating adhesion levels. The passivated outer surface of tinplate
is extremely thin (less than 1 micron thick) and consists of a
mixture of tin and chromium oxides.
[0004] ECCS consists of a blackplate product which has been coated
with a metallic chromium layer overlaid with a film of chromium
oxide, both applied by electrolytic deposition. ECCS excels in
adhesion to organic coatings and retention of coating integrity at
temperatures exceeding the melting point of tin (232.degree. C.).
In those cases tinplated material cannot be used. This is important
for producing polymer coated packaging steel because during the
thermoplastic coating application process the steel substrate may
be heated to temperatures exceeding 232.degree. C., with the actual
maximum temperature values used being dependent on the type of
thermoplastic coating applied. This heat cycle is required to
enable initial heat sealing/bonding of the thermoplastic to the
substrate (pre-heat treatment) and is often followed by a post-heat
treatment to modify the properties of the polymer. The chromium
oxide layer is believed to be responsible for the excellent
adhesion properties of thermoplastic coatings such as polypropylene
(PP) or polyester terephthalate (PET) to ECCS. ECCS can also be
supplied within a range of coating weights for both the Cr and CrOx
coating, typically ranging between 20-110 and 2-20 mg/m.sup.2
respectively. ECCS can be delivered with equal coating
specification for both sides of the steel strip, or with different
coating weights per side, the latter being referred to as
differentially coated strip. The production of ECCS currently
involves the use of solutions on the basis of chromium in its
hexavalent state, also known as hexavalent chromium or Cr(VI).
[0005] Hexavalent chromium is nowadays considered a hazardous
substance that is potentially harmful to the environment and
constitutes a risk in terms of worker safety. There is therefore an
incentive to develop alternative metal coatings that are able to
replace conventional tinplate and ECCS, without the need to resort
to the use of hexavalent chromium during manufacturing.
[0006] It is an objective of the invention to provide an
alternative to the use of hexavalent chromium for the passivation
of tinplate.
[0007] It is an objective of the invention to provide an
alternative to conventional tinplate to improve the product
properties e.g. in terms of corrosion performance and sulphur
staining resistance.
[0008] It is also an objective of the invention to provide an
alternative substrate to tinplate and ECCS which provides excellent
dry adhesion to organic coatings in combination with corrosion
protection that does not rely on the use of hexavalent chromium
during manufacturing.
[0009] One or more of these objects are reached by providing a
packaging steel substrate containing: [0010] 1. a recrystallisation
annealed single or double reduced packaging steel substrate (i.e.
blackplate), or [0011] 2. a cold-rolled and recovery annealed
blackplate, wherein one or both sides of the substrate is coated
with a chromium metal-chromium oxide (Cr--CrOx) coating layer
produced in a single plating step by using a trivalent chromium
electroplating process.
[0012] The packaging steel substrate is preferably provided in the
form of a strip.
[0013] For the production of ECCS generally three types of chromium
plating processes are in use throughout the world. The three
processes are "one step vertical process" (V-1), "two step vertical
process" (V-2), and the "one step horizontal high current density
process" (HCD) and based on Cr(VI) electrolytes. The specifications
of ECCS are standardized under Euronorm EN 10202:2001. The two-step
vertical process uses a sulphuric acid free Cr(VI) electrolyte for
applying the chrome oxide layer in the second step. Sulphuric acid
is needed for a good efficiency in applying chrome metal and is
therefore always used for the chrome metal plating step in these
processes. The "one step vertical" and the "one step horizontal
high current density (HCD) process" always have sulphate in the
oxide layer because the chromium metal and chromium oxide are
produced simultaneously in the same electrolyte (Boelen, thesis TU
Delft 2009, page 8-9, ISBN 978-90-805661-5-6). In all cases the
ECCS consists of a chromium oxide layer on top of the chromium
metal.
[0014] In the process according to the invention a coating layer
comprising chromium metal and chromium oxide is deposited, and not
by first depositing a chromium metal layer, and then providing a
chromium oxide layer on top as a conversion layer. The Cr--CrOx
layer should consist of a mixture of Cr-oxide and Cr-metal and the
Cr-oxide should not be present as a distinct layer on the outermost
surface, but mixed through the whole layer Cr--CrOx. Of course
there may be more than one of these single plating steps one after
the other if, for instance, a thicker coating layer comprising
chromium metal and chromium oxide layer is to be deposited. The
phrase single plating step is therefore not limited to mean that
only one of these single plating steps is used.
[0015] The packaging steel substrate is usually provided in the
form of a strip of low carbon (LC), extra low carbon (ELC) or ultra
low carbon (ULC) with a carbon content, expressed as weight
percent, of between 0.05 and 0.15 (LC), between 0.02 and 0.05 (ELC)
or below 0.02 (ULC) respectively. Alloying elements like manganese,
aluminium, nitrogen, but sometimes also elements like boron, are
added to improve the mechanical properties (see also e.g. EN 10
202, 10 205 and 10 239). In an embodiment of the invention the
substrate consists of an interstitial-free low, extra-low or
ultra-low carbon steel, such as a titanium stabilised, niobium
stabilised or titanium-niobium stabilised interstitial-free
steel.
[0016] It was found that a chromium metal-chromium oxide (Cr--CrOx)
coating produced from a trivalent chromium based electroplating
process provides excellent adhesion to organic coatings. In this
aspect, the chromium metal-chromium oxide (Cr--CrOx) coating
produced from a trivalent chromium electrodeposition process has
very similar adhesion properties compared to conventional ECCS
produced via a hexavalent chromium electrodeposition process. By
increasing the thickness of the Cr--CrOx coating layer the porosity
of the coating is reduced and its corrosion resistance properties
improve.
[0017] The Cr--CrOx coating can be applied onto conventional,
non-passivated, electrolytic, and optionally flowmelted, tinplate
(ETP, Electrolytic Tinplate). The Cr--CrOx layer ensures that the
growth of tin oxides is suppressed, i.e. it has a passivation
function. With increasing Cr--CrOx thickness it was unexpectedly
found that the wet adhesion performance, i.e. the organic coating
adhesion after sterilisation, outperforms conventional hexavalent
chromium passivated tinplate. In addition, the resistance to
so-called sulphur staining, i.e. the brown discolouration of
tinplate due to contact with sulphur containing fill-goods, can be
fully suppressed by applying a sufficiently thick Cr--CrOx coating.
The material according to the invention is therefore very suitable
for replacement of hexavalent chromium passivated tinplate,
optionally exceeding the technical performance limits of standard
tinplate. From a process point of view, the fact that the Cr--CrOx
coating layer is applied in a single process step means that two
process steps are combined, which is beneficial in terms of process
economy and in terms of environmental impact.
[0018] Alternatively the Cr--CrOx coating can also be applied
directly onto the blackplate packaging steel substrate, without
prior application of a tin coating, i.e. directly applied onto the
bare steel surface. According to Merriam Webster blackplate is
defined as sheet steel that has not yet been made into tin plate by
being coated with tin or that is used uncoated where the protection
afforded by tin is unnecessary. It was found that the dry adhesion
levels to organic coatings for both thermoset lacquers and
thermoplastic coatings, of this material can approach those
normally associated with the use of ECCS. The material according to
the invention can be used to directly replace ECCS for applications
that require a moderate corrosion resistance.
[0019] The big advantage, both in terms of environmental impact and
health and safety is the fact that with this invention the use of
hexavalent chromium chemistry is prevented, while it is possible to
retain the product performance properties normally attributed to
ECCS and tinplate.
[0020] In an embodiment the Cr--CrOx coating layer applied onto
non-passivated tinplate contains at least 20 mg Cr/m.sup.2, to
create a tin oxide passivating effect. This thickness is adequate
for many purposes.
[0021] In an embodiment the Cr--CrOx coating layer applied onto
non-passivated tinplate contains at least 40 mg Cr/m.sup.2,
preferably at least 60 Cr/m.sup.2, to create a tin oxide
passivating effect and to prevent or eliminate sulphur staining. To
prevent or eliminate sulphur staining, a layer of 20 mg Cr/m.sup.2
was found to be too thin. Starting at thicknesses of about 40 mg
Cr/m.sup.2 the sulphur staining is already much reduced, whereas at
a layer thickness of at least about 60 mg Cr/m.sup.2 sulphur
staining is practically eliminated.
[0022] A suitable maximum thickness was found to be 140 mg
Cr/m.sup.2. Preferably the Cr--CrOx coating layer applied onto
non-passivated tinplate contains at least 20 to 140 mg Cr/m.sup.2,
more preferably at least 40 and/or at most 90 mg Cr/m.sup.2, and
most preferably at least 60 and/or at most 80 mg Cr/m.sup.2.
[0023] These embodiments aim to replace hexavalent chromium
passivated tinplate. The major advantage besides the elimination of
hexavalent chromium from manufacturing is the potential to create a
product with superior sulphur staining resistance and improved
corrosion resistance.
[0024] It was found that the colour of the material changes with
increasing Cr--CrOx layer thickness, with the product becoming
darker (i.e. lower L-value) with increasing coating thickness. As
the optical properties of packaging steels are very important to
create an attractive aesthetic appearance of metal containers, like
aerosol cans, this could be considered a drawback of the invention
for specific applications. However, one way to circumvent these
issues would be to use a differential coating, e.g. to use a low
Cr--CrOx coating weight on one side of the material, while applying
a thicker Cr--CrOx coating weight at the other side. The surface
containing a thicker Cr--CrOx coating weight should be used for the
inside of the container, to make use of the benefits of the
improved corrosion resistance properties. In that case, the surface
with the lower Cr--CrOx coating weight is on the outside of the
container, for which the corrosion resistance requirements are
usually less severe, ensuring optimal optical properties.
[0025] In an embodiment the Cr--CrOx coating layer applied onto
blackplate is at least 20 mg Cr/m.sup.2, to create a material that
approaches the functionality of ECCS (e.g. excellent adhesion to
organic coatings in combination with a moderate corrosion
resistance). Preferably the Cr--CrOx coating layer applied onto
blackplate is at least 40 and more preferably at least 60 mg
Cr/m.sup.2. A suitable maximum thickness was found to be 140 mg
Cr/m.sup.2. Preferably the Cr--CrOx coating layer applied onto
blackplate contains at least 20 to 140 mg Cr/m.sup.2, more
preferably at least 40 mg Cr/m2, and most preferably at least 60 mg
Cr/m.sup.2. In an embodiment a suitable maximum is 110 mg
Cr/m.sup.2.
[0026] The Cr--CrOx coated blackplate aims to replace ECCS. The
major advantage besides the elimination of hexavalent chromium from
manufacturing is the potential to create a product for applications
for which the superior corrosion resistance properties of tinplate
are not required. From a process point of view, the fact that the
Cr--CrOx coating layer is applied in a single process step means
that two process steps are combined, which is beneficial in terms
of process economy and in terms of environmental impact.
[0027] The Cr--CrOx coating can also be applied to a cold-rolled
and recovery annealed blackplate, or to a cold-rolled and recovery
annealed electrolytic, and optionally flowmelted, tinplate. These
substrates have a recovery annealed substrate, rather than the
recystallised single reduced ETP or blackplate or the double
reduced blackplate. The difference in microstructure of the
substrate was not found to materially affect the Cr--CrOx
coating.
[0028] It was found that the material according to the invention
can be used in combination with thermoplastic coatings, but also
for applications where traditionally ECCS is used in combination
with lacquers (i.e. for bakeware such as baking tins, or products
with moderate corrosion resistance requirements) or as a substitute
for conventional tinplate for applications where requirements in
terms of corrosion resistance are moderate.
[0029] In an embodiment the coated substrate is further provided
with an organic coating, consisting of either a thermoset organic
coating, or a thermoplastic single layer polymer coating, or a
thermoplastic multi-layer polymer coating. The Cr--CrOx layer
provides excellent adhesion to the organic coating similar to that
achieved by using conventional ECCS.
[0030] In a preferred embodiment the thermoplastic polymer coating
is a polymer coating system comprising one or more layers
comprising the use of thermoplastic resins such as polyesters or
polyolefins, but can also include acrylic resins, polyamides,
polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene
type resins, ABS resins, chlorinated polyethers, ionomers, urethane
resins and functionalised polymers. For clarification:
[0031] Polyester is a polymer composed of dicarboxylic acid and
glycol. Examples of suitable dicarboxylic acids include
therephthalic acid, isophthalic acid, naphthalene dicarboxylic acid
and cyclohexane dicarboxylic acid. Examples of suitable glycols
include ethylene glycol, propane diol, butane diol, hexane diol,
cyclohexane diol, cyclohexane dimethanol, neopentyl glycol etc.
More than two kinds of dicarboxylic acid or glycol may be used
together.
[0032] Polyolefins include for example polymers or copolymers of
ethylene, propylene, 1-butene, 1-pentene, 1-hexene or 1-octene.
[0033] Acrylic resins include for example polymers or copolymers of
acrylic acid, methacrylic acid, acrylic acid ester, methacrylic
acid ester or acrylamide.
[0034] Polyamide resins include for example so-called Nylon 6,
Nylon 66, Nylon 46, Nylon 610 and Nylon 11.
[0035] Polyvinyl chloride includes homopolymers and copolymers, for
example with ethylene or vinyl acetate.
[0036] Fluorocarbon resins include for example tetrafluorinated
polyethylene, trifluorinated monochlorinated polyethylene,
hexafluorinated ethylene-propylene resin, polyvinyl fluoride and
polyvinylidene fluoride.
[0037] Functionalised polymers for instance by maleic anhydride
grafting, include for example modified polyethylenes, modified
polypropylenes, modified ethylene acrylate copolymers and modified
ethylene vinyl acetates.
[0038] Mixtures of two or more resins can be used. Further, the
resin may be mixed with anti-oxidant, heat stabiliser, UV
absorbent, plasticiser, pigment, nucleating agent, antistatic
agent, release agent, anti-blocking agent, etc. The use of such
thermoplastic polymer coating systems have shown to provide
excellent performance in can-making and use of the can, such as
shelf-life.
[0039] According to a second aspect, the invention is embodied in a
process for producing a coated steel substrate for packaging
applications, the process comprising the electro-deposition of a
chromium metal-chromium oxide coating on the substrate with the
electrolytic deposition on said substrate of said chromium
metal-chromium oxide coating occurring in a single plating step
from a plating solution comprising a trivalent chromium compound,
an optional chelating agent, an optional conductivity enhancing
salt, an optional depolarizer, an optional surfactant and to which
an acid or base can be added to adjust the pH.
[0040] In an embodiment the electro-deposition of the Cr--CrOx
coating is achieved by using an electrolyte in which the chelating
agent comprises a formic acid anion, the conductivity enhancing
salt contains an alkali metal cation and the depolarizer comprises
a bromide containing salt.
[0041] In an embodiment the cationic species in the chelating
agent, the conductivity enhancing salt and the depolarizer is
potassium. The benefit of using potassium is that its presence in
the electrolyte greatly enhances the electrical conductivity of the
solution, more than any other alkali metal cation, thus delivering
a maximum contribution to lowering of the cell voltage required to
drive the electro-deposition process.
[0042] In an embodiment of the invention the composition of the
electrolyte used for the Cr--CrOx deposition was: 120 g/l basic
chromium sulphate, 250 g/l potassium chloride, 15 g/l potassium
bromide and 51.2 g/l potassium formate.
[0043] The pH was adjusted to values between 2.3 and 2.8 measured
at 25.degree. C. by the addition of sulphuric acid.
[0044] According to the invention the chromium containing coating
is preferably deposited from the trivalent chromium based
electrolyte at a bath temperature of between 40 and 70.degree. C.,
preferably of at least 45.degree. C. and/or at most 60.degree.
C.
[0045] Surprisingly, it was found that it is possible to
electro-deposit a chromium metal-chromium oxide coating layer from
this electrolyte in a single process step. From prior art, it
follows that addition of a buffering agent to the electrolyte, like
e.g. boric acid, is strictly required to enable the
electro-deposition of chromium metal to take place. In addition, it
has been reported that it is not possible to deposit chromium metal
and chromium oxide from the same electrolyte, due to this buffering
effect (with a buffering agent being required for the
electro-deposition of the chromium metal but excluding the
formation of chromium oxides and vice versa). However, it was found
that no such addition of a buffering agent was required to deposit
chromium metal, provided that a sufficiently high cathodic current
density is being applied.
[0046] XPS depth profiles were measured and the peaks that are
measured are Fe2p, Cr2p, O1s, Sn3d, C1s. It was observed that the
Cr-layer consists of a mixture of Cr-oxide and Cr-metal and that
the Cr-oxide is not present as a distinct layer on the outermost
surface, but is mixed through the whole layer. This is also
indicated by the 0-peak that is present in the whole Cr-layer. In
all cases the Cr--CrOx layer has a shiny metallic appearance.
[0047] It is believed that a certain threshold value for the
current density must be exceeded for the electro-deposition of
chromium metal to occur, which is closely linked to pH at the strip
surface reaching certain values as a result of the evolution of
hydrogen gas and the equilibration of various (chelated) poly
chromium hydroxide complexes. It was found that after crossing this
threshold value for the current density that the electro-deposition
of the chromium metal-chromium oxide coating layer increases
virtually linearly with increasing current density, as observed
with conventional electro-deposition of metals, following Faraday's
law. The actual value for the threshold current density seems to be
closely linked to the mass transfer conditions at the strip
surface: it was observed that this threshold value increases with
increasing mass transfer rates. This phenomenon can be explained by
changes in pH values at the strip surface: at increasing mass
transfer rates the supply of hydronium ions to the strip surface is
increased, necessitating an increase in cathodic current density to
maintain a specific pH level (obviously higher than the bulk pH) at
the strip surface under steady-state process conditions. The
validity of this hypothesis is supported by results obtained from
experiments in which the pH of the bulk electrolyte was varied
between a value of 2.5 and 2.8: the threshold value for the current
density decreases with increasing pH value.
[0048] Concerning the electro-deposition process of Cr--CrOx
coatings from trivalent chromium based electrolytes, it is
important to prevent/minimise the oxidation of trivalent chromium
to its hexavalent state at the anode and a suitable anode or anode
material must be selected. By using a hydrogen gas diffusion anode
as described below and in copending application EP12193623, the
formation of Cr(IV) can be prevented.
[0049] In an embodiment of the invention the formation of Cr(IV)
can be prevented by using one, more or only hydrogen gas diffusion
anodes at which hydrogen gas (H.sub.2(g)) is oxidised. H.sup.+
(protons) in an aqueous solution bind to one or more water
molecules, e.g. as hydronium ions (H.sub.3O.sup.+). The oxidation
of H.sub.2(g) to H.sup.+(aq) prevents the occurrence of undesirable
oxidation reactions, such as the formation of Cr(IV), which occur
at a higher anodic overpotential when using an anode at which water
(H.sub.2O) is oxidised to oxygen (O.sub.2(g)).
[0050] The reaction H.sub.2(g).fwdarw.2H.sup.+(aq)+2e.sup.- occurs
at an anode potential of 0.00 V (SHE). The reaction
2H.sub.2O.fwdarw.4H.sup.+(aq)+O.sub.2(g)+4e.sup.- occurs at an
anode potential of 1.23 V (SHE). When an anode at which water is
oxidised to oxygen is used, then reactions are possible which would
not have been possible when using an anode at which hydrogen gas is
oxidised.
[0051] One of such undesirable oxidation reactions is the oxidation
of Cr(III) to Cr(VI) and this oxidation reaction can be completely
excluded by using a hydrogen gas diffusion anode (GDA) at which
H.sub.2(g) is oxidised to H.sup.+.
[0052] In an embodiment of the method H.sub.2(g) is oxidised at the
gas diffusion anode to H.sup.+(aq) with a current efficiency of at
least 99%, preferably of 100%. The higher the current efficiency,
the smaller the likelihood of undesirable side reactions. It is
therefore preferable that the current efficiency is at least 99%,
and preferably 100%. Based on thermodynamic and kinetic
considerations it can be argued that using a hydrogen gas diffusion
anode completely eliminates the risk of Cr(III) oxidation as the
anode operating potential is much too low for Cr(III) oxidation to
occur.
[0053] Thermodynamically, under standard conditions (i.e. a
temperature of 25.degree. C. and a pressure of 1 atm) an electrode
potential of >0 V is already sufficient for oxidising H.sub.2(g)
to H.sup.+(aq), whereas an electrode potential of >1.23 V is
required for oxidising H.sub.2O to O.sub.2(g). Cr(III) can only be
oxidised to Cr(VI) when the electrode potential is >1.35 V.
[0054] The electrode potential is measured against the standard
hydrogen electrode. The standard hydrogen electrode (abbreviated
SHE), is a redox electrode which forms the basis of the
thermodynamic scale of oxidation-reduction potentials. Its absolute
electrode potential is estimated to be 4.44.+-.0.02 V at 25.degree.
C., but to form a basis for comparison with all other electrode
reactions, hydrogen's standard electrode potential)(E.degree. is
declared to be zero at all temperatures. Potentials of any other
electrodes are compared with that of the standard hydrogen
electrode at the same temperature.
[0055] The prevailing equilibrium (zero current) potential can be
calculated from the Nernst equation by filling in the appropriate
temperature, pressure and activities of the electro-active species.
The anode operating (non-zero current) potential needed to generate
a specific anodic current is determined by the activation
overpotential (i.e. the potential difference required for driving
the electrode reaction) and the concentration overpotential (i.e.
the potential difference required to compensate for concentration
gradients of electro-active species at the electrode).
[0056] Due to the low anode overpotential required for the
oxidation of H.sub.2(g) to H.sup.+(aq), the anode operating
potential will always stay far below the value at which Cr(III)
oxidation can take place (see FIG. 4 where the current is plotted
against the anode potential in SHE). Firstly this results in a
lower energy consumption of the electrodeposition process.
Secondly, at an anode potential below about 1.35 V oxidation of
Cr(III) to Cr(VI) is not possible (indicated with the crossed
through arrow).
[0057] In an embodiment no depolariser is added to the electrolyte.
When a hydrogen gas diffusion anode is used then the addition of a
depolariser to the electrolyte is no longer needed.
[0058] The use of a hydrogen gas diffusion anode has the added
advantage that the use of a chloride containing electrolyte becomes
possible without the risk of chlorine formation. This chlorine gas
is potentially harmful to the environment and to the workers and is
therefore undesirable. This means that in the case of a Cr(III)
electrolyte the electrolyte could be partly or entirely based on
chlorides. The advantage of using a chloride based electrolyte is
that the conductivity of the electrolyte is much higher compared to
a sulphate only based electrolyte, which leads to a lower cell
voltage that is required to run the electrodeposition, which
results in a lower energy consumption.
[0059] The oxidation reaction of dissolved hydrogen on an active
electrocatalyst surface is a very fast process. As the solubility
of hydrogen in a liquid electrolyte is often low, this oxidation
reaction can easily become controlled by mass transfer limitations.
Porous electrodes have been specifically designed to overcome mass
transfer limitations. A hydrogen gas diffusion anode is a porous
anode containing a three-phase interface of hydrogen gas, the
electrolyte fluid and a solid electrocatalyst (e.g. platinum) that
has been applied to the electrically conducting porous matrix (e.g.
porous carbon or a porous metal foam). The main advantage of using
such a porous electrode is that it provides a very large internal
surface area for reaction contained in a small volume combined with
a greatly reduced diffusion path length from the gas-liquid
interface to the reactive sites. Through this design the mass
transfer rate of hydrogen is greatly enhanced, while the true local
current density is reduced at a given overall electrode current
density, resulting in a lower electrode potential.
[0060] A gas diffusion anode assembly to be used in the proposed
electrodeposition method, typically comprises the use of the
following functional components (see FIG. 5): a gas feeding chamber
1, a current collector 2 and a gas diffusion anode, which consists
of an hydrophobic porous gas diffusion transport layer 3 combined
with an hydrophilic reaction layer 4 (see FIG. 5). The latter is
made up of a network of micropores that are (partly) drowned with
liquid electrolyte. Optionally, the reaction layer is provided with
a proton exchange membrane on the outside 5, like a Nafion.RTM.
membrane, to prevent the diffusion of chemical species (like anions
or large neutral molecules) present in the bulk liquid electrolyte
inside the gas diffusion anode, as these compounds can potentially
poison the electrocatalyst sites, causing degradation in
electrocatalytic activity.
[0061] The main function of the gas feeding chamber is to supply
hydrogen gas evenly to the hydrophobic backside of the hydrogen gas
diffusion anode. The gas feeding chamber needs two connections: one
to feed hydrogen gas and one to enable purging of a small amount of
hydrogen gas to prevent the build-up of gas phase contaminations
potentially present in trace amounts in the hydrogen gas supplied.
The gas feeding chamber often contains a channel type structure to
ensure that hydrogen gas is distributed evenly over the hydrophobic
backside.
[0062] The electrical current collector 2 is (usually) attached to
the hydrophobic backside 3 of the hydrogen gas diffusion anode to
enable the transport of the electrical current generated inside the
anode to a rectifier (not shown in FIG. 5). This current collector
plate must be designed in such a way to enable the hydrogen gas to
contact the backside of the hydrogen gas diffusion anode so it can
be transported to the reactive side inside the gas diffusion anode.
Usually this is accomplished by using an electrically conductive
plate with a large number of holes, a mesh or an expanded metal
sheet made from e.g. titanium.
[0063] The functionality of gas feeding channels and electrical
current collector can also be combined into a single component,
which is then pressed against the hydrophobic back-side of the gas
diffusion anode.
[0064] Once the hydrogen gas diffuses through the hydrophobic
backside of the hydrogen gas diffusion anode it comes into contact
with the electrolyte, which is present in the hydrophilic part of
the anode, i.e. the reaction layer (see FIG. 5, right hand side).
At the gas-liquid interface (between 3 and 4) the hydrogen gas
dissolves into the electrolyte and is transported by diffusion to
the electrocatalytic active sites of the hydrogen gas diffusion
anode. Usually platinum is used as electrocatalyst, but also other
materials like platinum-ruthenium or platinum-molybdenum alloys can
be used. At the electrocatalytic sites the dissolved hydrogen is
oxidised: the electrons that are generated are transported through
the conductive matrix of the gas diffusion anode (usually a carbon
matrix) to the current collector 2, while the hydronium ions
(H.sup.+) diffuse through the proton exchange membrane into the
electrolyte.
[0065] In an embodiment the coated substrate is further provided on
one or both sides with an organic coating, consisting of a
thermosetting organic coating by a lacquering step, or a
thermoplastic single layer, or a thermoplastic multi-layer polymer
by a film lamination step or a direct extrusion step.
[0066] In an embodiment the thermoplastic polymer coating is a
polymer coating system comprising one or more layers comprising the
use of thermoplastic resins such as polyesters or polyolefins, but
can also include acrylic resins, polyamides, polyvinyl chloride,
fluorocarbon resins, polycarbonates, styrene type resins, ABS
resins, chlorinated polyethers, ionomers, urethane resins and
functionalised polymers; and/or copolymers thereof; and/or blends
thereof.
[0067] Preferably the substrate is cleaned prior to Cr--CrOx
electrodeposition by dipping the substrate in a sodium carbonate
solution containing between 1 to 50 g/l of Na.sub.2CO.sub.3 at a
temperature of between 35 and 65.degree. C., and wherein the
cathodic current density of between 0.5 and 2 A/dm.sup.2 is applied
for a period of between 0.5 and 5 seconds.
[0068] Preferably the sodium carbonate solution containing at least
2 and/or at most 5 g/l of Na.sub.2CO.sub.3.
[0069] The invention is now further explained by means of the
following, non-limiting examples and figures.
EXAMPLE 1
[0070] Sheets of conventional, non-passivated, flow melted tinplate
(common steel grade and temper), with a tin coating weight of 2.8 g
Sn/m.sup.2 on both sides, were first given an electrolytic
pre-treatment to minimise the tin oxide layer thickness. This was
done by dipping the sheets into a sodium carbonate solution (3.1
g/l of Na.sub.2CO.sub.3, temperature of 50.degree. C.) and applying
a cathodic current density of 0.8 A/dm.sup.2 for 2 seconds. After
rinsing with de-ionised water, the samples were dipped into a
trivalent chromium electrolyte kept at 50.degree. C. composed of:
120 g/l of basic chromium sulphate, 250 g/l of potassium chloride,
15 g/l of potassium bromide and 51.2 g/l of potassium formate. The
pH of this solution was adjusted to 2.3 measured at 25.degree. C.
by adding sulphuric acid. A Cr--CrOx coating containing between
21-25 mg Cr/m.sup.2 (measured by XRF) was deposited on the surface
by applying a cathodic current density of 10 A/dm.sup.2 for
approximately 1 second, using a platinised titanium anode as
counter electrode. The samples so produced showed a shiny metallic
appearance.
[0071] The study the passivating action of the thin Cr--CrOx
coating on tinplate, the samples were subjected to a long-term
storage test at 40.degree. C. at a static humidity level of 80% RH.
The amount of tin oxide developed on the tinplate surface during
storage is then measured after 2 weeks and after 4 weeks of
exposure, and compared to the amount of tin oxide present on the
sample before the storage test (denoted as `0 weeks`).
Determination of tin oxide layer thickness is done using a
coulometric method, as described in S. C. Britton, "Tin vs
corrosion", ITRI Publication No. 510 (1975), Chapter 4. The tin
oxide layer is reduced by a controlled small cathodic current in a
0.1% solution of hydrobromic acid (HBr) that is freed from oxygen
by scrubbing with nitrogen. The progress of the reduction of the
oxide is followed by potential measurement and the charge passed
for the complete reduction (expressed as Coulomb/m.sup.2 or
C/m.sup.2) serves as a measure of the tin oxide layer thickness.
The results for the sample according to Example 1 are presented in
Table 1, including the performance of the reference material, which
is the same tinplate material that was passivated using hexavalent
chromium, i.e. so-called 311 passivated tinplate.
TABLE-US-00001 TABLE 1 Tin oxide layer thickness (in C/m.sup.2)
Storage at 40.degree. C., ETP - Cr--CrOx according to 80% RH
ETP-311 (ref) Example 1 (25 mg/m.sup.2 Cr) 0 weeks 12 11 2 weeks 12
12 4 weeks 13 11
[0072] The results show that non-passivated tinplate treated
according to the present invention to obtain a light Cr--CrOx
coating shows perfect stability in tin oxide growth and is fully
comparable in performance to traditional 311 passivated
tinplate.
EXAMPLE 2
[0073] Sheets of conventional, non-passivated, flow melted tinplate
(common steel grade and temper), with a tin coating weight of 2.8 g
Sn/m.sup.2 on both sides, were first given an electrolytic
pre-treatment to minimise the tin oxide layer thickness. This was
done by dipping the sheets into a sodium carbonate solution (3.1
g/l of Na.sub.2CO.sub.3, temperature of 50.degree. C.) and applying
a cathodic current density of 0.8 A/dm.sup.2 for 2 seconds. After
rinsing with de-ionised water, the samples were dipped into a
trivalent chromium electrolyte kept at 50.degree. C. composed of:
120 g/l of basic chromium sulphate, 250 g/l of potassium chloride,
15 g/l of potassium bromide and 51.2 g/l of potassium formate. The
pH of this solution was adjusted to 2.3 measured at 25.degree. C.
by adding sulphuric acid. A Cr--CrOx coating containing between
65-75 mg Cr/m.sup.2 (measured by XRF) was deposited on the surface
by applying a cathodic current density of 15 A/dm.sup.2 for
approximately 1 second, using a platinised titanium anode as
counter electrode. All samples so produced showed a shiny metallic
appearance. A typical SEM image is shown in FIGS. 1 & 2, which
shows the deposition of very fine grains of chromium metal-chromium
oxide on the tin surface.
[0074] The sheets were subsequently lacquered, applying a
commercially available epoxy-anhydride lacquer system (Vitalure.TM.
120 supplied by AkzoNobel). Subsequently, the lacquered sheets were
locally deformed by Erichsen cupping.
[0075] To analyse the performance of the chromium-chromium oxide
coated tinplate several sterilisation tests were done to assess the
wet adhesion performance on flat and deformed material. In total 5
different sterilisation media were used during these tests, as
shown in Table 2.
TABLE-US-00002 TABLE 2 Conditions of sterilisation tests
Temperature Time Type Sterilisation medium [.degree. C.] [min]
Saline 3.6 wt % NaCl 121 90 Acetic acid 1 wt % CH.sub.3COOH 121 90
Cysteine 3.56 g/l KH.sub.2PO.sub.4 + 121 90 7.22 g/l
Na.sub.2HPO.sub.4.cndot.2H.sub.2O + 0.5 g/l
C.sub.3H.sub.7NO.sub.2S.cndot.HCl.cndot.H.sub.2O (in buffer
solution, pH = 7) Salt-Acid 18.7 g/l NaCl + 121 60 30 g/l
CH.sub.3COOH Lactic acid 22.5 g/l C.sub.3H.sub.6O.sub.3 121 60
[0076] After sterilisation the level of lacquer adhesion of the
panels was evaluated (by the Cross-cut and tape test (ISO
2409:1992(E)), blister formation (size and number of blisters) and
visual discolouration. The overall results are presented in Table
3, including the performance of the reference material, which is
the same tinplate material that was passivated using hexavalent
chromium, i.e. so-called 311 passivated tinplate. The performance
ranking is on a scale from 0 to 5, with 0 being an excellent
performance and 5 a very bad performance. The results are averaged
over a number of observations, leading to scores with a decimal
value.
TABLE-US-00003 TABLE 3 Results of lacquer adhesion tests
Sterilisation type ETP-311 (ref) ETP - Cr--CrOx Flat Saline 2 1.5
Acetic acid 4 1.5 Cysteine 1 1 Salt-Acid 5 1 Lactic acid 3 2 Dome
Saline 2 1 Acetic acid 3.5 1.5 Cysteine 4.5 0.5 Salt-Acid 4 0.5
Lactic acid 3 2.5
[0077] The inventors found that the tinplate variant manufactured
according to the invention performed consistently equal or better
compared to the standard tinplate that is passivated using
hexavalent chromium (i.e. the reference). Striking is the fact that
no sulphur staining was found for the material according to the
invention, which is difficult to achieve with conventional
passivated tinplate and notoriously difficult to achieve with
alternative passivations for tinplate that are free of hexavalent
chromium.
EXAMPLE 3
[0078] A coil of blackplate (common steel grade and temper), not
containing any metal coating, was treated in a processing line
running at a line speed of 20 m/min. The processing sequence
started with alkaline cleaning of the steel by running the strip
for approximately 10 seconds through a solution containing 30 ml/l
of a commercial cleaner (Percy P3) and 40 g/l of NaOH, which was
kept at 60.degree. C. During cleaning of the strip an anodic
current density of 1.3 A/dm.sup.2 was applied. After rinsing with
de-ionised water, the steel strip was passed through an acid
solution for approximately 10 seconds, to activate the surface. The
acid solution consisted of 50 g/l H.sub.2SO.sub.4, which was kept
at 25.degree. C. After rinsing with de-ionised water, the steel
strip was passed into an electroplating tank containing the
trivalent chromium based electrolyte kept at 50.degree. C. This
electrolyte consisted of: 120 g/l of basic chromium sulphate, 250
g/l of potassium chloride, 15 g/l of potassium bromide and 51.2 g/l
of potassium formate. The pH of this solution was adjusted to 2.3
measured at 25.degree. C. by adding sulphuric acid. The
electroplating tank contained a set of anodes consisting of
platinised titanium. During processing of the strip a cathodic
current density of approximately 17 A/dm.sup.2 was applied for just
over 1 second to electro-deposit a chromium-chromium oxide coating
of 60-70 mg Cr/m.sup.2 (measured by XRF) onto the blackplate
surface. All samples so produced showed a shiny metallic
appearance. A typical SEM image is shown in FIGS. 1 and 2, which
shows the deposition of very fine grains of chromium metal-chromium
oxide on the steel surface.
[0079] The material so produced, was passed through a coating line
to apply a commercially available 20 micrometer thick PET film,
through heat sealing. After film lamination, the coated strip was
post-heated to temperatures above the melting point of PET, and
subsequently quenched in water at room temperature, as per a usual
processing method for the PET lamination of metals. The same
procedure was followed for the manufacturing of reference material,
using a commercially produced coil of ECCS.
[0080] The laminated materials were used to produce standard food
DRD cans (211.times.400). In all cases the dry adhesion of the PET
film to the can wall was excellent. This was confirmed by measuring
the T-peel forces of the PET film on the can wall, which showed
similar values for the PET film applied to both the material
according to the invention and commercial ECCS (.about.7 N/15
mm).
[0081] The DRD cans were subsequently filled with different media,
closed and exposed to a sterilisation treatment. Some cans were
processed that contained a scratch made on the can wall, to
simulate and observe the effect of incidental coating damage. An
overview of the type of sterilisation tests done is presented in
Table 4.
TABLE-US-00004 TABLE 4 Conditions of sterilisation tests
Temperature Time Type Sterilisation medium [.degree. C.] [min]
Saline 3.6 wt % NaCl 121 60 Acetic acid 1 wt % CH.sub.3COOH 121 60
Cysteine 3.56 g/l KH.sub.2PO.sub.4 + 130 60 7.22 g/l
Na.sub.2HPO.sub.4.cndot.2H.sub.2O + 0.5 g/l
C.sub.3H.sub.7NO.sub.2S.cndot.HCl.cndot.H.sub.2O (in buffer
solution, pH = 7)
[0082] After the sterilisation treatment the DRD cans were cooled
to room temperature, emptied, rinsed and dried for one day. The
bottom and can wall were judged visually on the presence of
corrosion spots and blisters. The results, as presented in Table 5,
show that the sterilisation performance of the material according
to the invention is in general somewhat less compared to the ECCS
reference. The material seems especially more susceptible to
corrosion/coating delamination after coating damage. However, these
sterilisation tests are quite severe, so in practice the material
according to the invention can be used in specifically selected
applications involving sterilisation.
[0083] The performance ranking is on a scale from 0 to 5, with 0
being an excellent performance and 5 a very bad performance.
TABLE-US-00005 TABLE 5 Results of sterilisation tests Sterilisation
type ECCS (ref) BP + Cr--CrOx Saline 1 (1)* 1 (4)* Acetic acid 1 3
Cysteine 0 0 *Symbol in brackets relates to DRD cans with a scratch
on the can wall.
EXAMPLE 4
[0084] A coil of blackplate (common steel grade and temper), not
containing any metal coating, was treated in a processing line
identical to that described in the previous example to apply a
Cr--CrOx coating.
[0085] The sheets cut from this coil were subsequently lacquered,
applying a commercially available epoxy-phenol lacquer system
(Vitalure.TM. 345 supplied by AkzoNobel). Subsequently, the
lacquered sheets were locally deformed by Erichsen cupping.
[0086] To analyse the performance of the chromium-chromium oxide
coated blackplate several sterilisation tests were done to assess
the wet adhesion performance on flat and deformed material. In
total 5 different sterilisation media were used during these tests,
as shown in Table 6.
TABLE-US-00006 TABLE 6 Conditions of sterilisation tests
Temperature Time Type Sterilisation medium [.degree. C.] [min]
Saline 3.6 wt % NaCl 121 60 Acetic acid 1 wt % CH.sub.3COOH 121 60
Cysteine 3.56 g/l KH.sub.2PO.sub.4 + 130 60 7.22 g/l
Na.sub.2HPO.sub.4.cndot.2H.sub.2O + 0.5 g/l
C.sub.3H.sub.7NO.sub.2S.cndot.HCl.cndot.H.sub.2O (in buffer
solution, pH = 7) Salt-Acid 18.7 g/l NaCl + 121 60 30 g/l
CH.sub.3COOH Lactic acid 22.5 g/l C.sub.3H.sub.6O.sub.3 100 30
[0087] After sterilisation the panels were evaluated with respect
to the level of lacquer adhesion (by the Cross-cut and tape test
(ISO 2409:1992(E))), blister formation (size and number of
blisters) and visual discolouration. The overall results are
presented in Table 7, including the performance of the reference
material, for which commercially available ECCS was used. The
performance ranking is on a scale from 0 to 5, with 0 being an
excellent performance and 5 a very bad performance.
TABLE-US-00007 TABLE 7 Results of sterilisation tests Sterilisation
type ECCS (ref) BP + Cr--CrOx Flat Saline 0 0 Acetic acid 0 0
Cysteine 0 0 Salt-Acid 0 0 Lactic acid 0 0 Dome Saline 0 0 Acetic
acid 5 4 Cysteine 0 0 Salt-Acid 0 0 Lactic acid 0 0
[0088] The inventors found that the Cr--CrOx coated blackplate
material manufactured according to the invention performed
consistently similar to conventional ECCS.
BRIEF DESCRIPTION OF DRAWINGS
[0089] FIGS. 1 and 2 show typical SEM images, which show the
deposition of very fine grains of chromium metal-chromium oxide
onto the surface. FIG. 1 relates to a tinplate substrate and FIG. 2
relates to a blackplate substrate.
[0090] FIG. 3 shows an overview of various packaging applications.
On the X-axis are packaging steel grades, and on the Y-axis a
typical thickness range is shown for these applications for which
the packaging steel substrate according to the invention could be
used.
[0091] FIG. 4 shows where the current is plotted against the anode
potential in SHE and FIG. 5 shows a schematic drawing of a gas
diffusion anode.
* * * * *