Note: Descriptions are shown in the official language in which they were submitted.
216~i60
.
WO 94/26859 Translation PCT/EP94/01386
Corrosion inhibitors for sil~-er (I)
It is a generally known fact that silver "tarnishes"
even when it is not in use. It is only a matter of time
before it develops dark, brownish, bluish to blue-black
stains or completely discolors and, hence, is said in
common usage to have "tarnished".
In practice, the machine washing of table silver
also involves recurring problems in the form of tarnish-
ing and discoloration of the silver surfaces. In this
case, silver can react to sulfur-containing substances
which are dissolved or dispersed in the wash liquor
because, in domestic dishwashing machines (DDWM), food
residues, including mustard, peas, egg, and other sulfur-
containing compounds, such as mercaptoamino acid, are
introduced into the wash liquor. The much higher temper-
lS atures prevailing in dishwashing machines and the longercontact times with the sulfur-containing food residues
promote the tarnishing of silver by comparison with
manual dishwashing. In addition, through the intensive
cleaning process in dishwashing machines, the silver
surface is completely degreased and, hence, becomes more
sensitive to chemical influences.
Where detergents containing active chlorine are
used, tarnishing by sulfur-containing compounds can
largely be prevented because these compounds are reacted
to sulfones or sulfates by oxidation of the sulfidic
functions in a secondary reaction.
However, the problem of tarnishing in the case of
silver became topical again when active oxygen compounds,
such as sodium perborate or sodium percarbonate for
example, were used as an alternative to active chlorine
compounds to eliminate bleachable soils, for example tea
stains/tea coatings, coffee residues, dyes from vegetab-
2162160
les, lipstick residues and the like.
These active oxygen compounds are used ir. cor,junc-
tion with bleach activators above all in modern low-
alkali machine dishwashing detergents of the new genera-
tion. These modern detergents generally consist of thefollowing functional components: builder component
(complexing agent/dispersant), alkali carrier, bleaching
system (bleaching agent + bleach activator), enzymes and
wetting agents (surfactants).
Basically, the silver surfaces react more sensitive-
ly to the modified formulation parameters of the new-
generation detergents free from active chlorine with
their reduced pH values and activated oxygen bleaching.
During the machine dishwashing process, these detergents
release the actual bleaching agent, hydrogen peroxide or
active oxygen, in the wash cycle. The bleaching effect
of detergents containing active oxygen is enhanced by
bleach activators so that a good bleaching effect is
obtained even at low temperatures. In the presence of
these bleach activators, peracetic acid is formed as a
reactive intermediate compound. Under the modified wash-
ing conditions, not only are sulfidic coatings formed in
the presence of silver, oxidic coatings are also formed
on the silver surfaces through the oxidizing effect of
the intermediately formed peroxides or the active oxygen.
Chloride coatings can also be formed in the presence of
high salt concentrations. In addition, tarnishing of the
silver is intensified by relatively high residual water
hardness values during the wash cycle.
Avoiding the corrosion of silver, i.e. the formation
of sulfidic, oxidic or chloridic coatings on silver, is
the subject of numerous publications. In these publica-
tions, the corrosion of silver is prevented above all by
so-called silver protectives.
GB 1,131,738 describes alkaline dishwashing deter-
2162960
WO 94/26859 3 PCT/EP94/01386
gents containing benzotriazoles as corrosion inhibitors
for silver. US 3,549,539 describes highly alkaline
machine dishwashing detergents which may contain inter
alia perborate as oxidizing agent in conjunction with an
organic bleach activator. Additions of inter alia benzo-
triazole and iron(III) chloride are recommended to pre-
vent tarnishing. pH values of, preferably, 7 to 11.5 are
mentioned. EP 135 226 and EP 135 227 describe low-alkali
machine dishwashing detergents containing peroxy com-
pounds and activators in which inter alia benzotriazolesand fatty acids may be present as silver protectives.
Finally, it is known from DE-OS 41 28 672 that peroxy
compounds activated by addition of known organic bleach
activators prevent the tarnishing of silver in highly
alkaline detergents.
It has now surprisingly been found that inorganic
redox-active substances, more particularly the salts or
complex compounds of certain metals not hitherto de-
scribed as corrosion inhibitors for silver, effectively0 prevent the corrosion of silver in dishwashing machines.
The present invention relates to the use of inor-
ganic redox-active substances as corrosion inhibitors for
silver in dishwashing detergents.
The word "corrosion" is to be interpreted in its
broadest chemical sense. More particularly, "corrosion"
in the context of the present invention is intended to
stand for any visually just discernible change in a metal
surface, in the present case silver, whether for example
in the form of discolored spots or, for example, in the
form of stains covering a relatively large area.
"Inorganic redox-active substances" are inorganic
substances which are accessible to readily occurring,
reversible oxidation and/or reduction. For example, the
oxides, hydroxides or halides of ammonium salts or of
alkali or alkaline earth metals do not fall under this
2162~60
definition.
"Inorganic redox-active substances" are, for exam-
ple, the substances Na2S203 (sodium thiosulfate), Na2S04
(sodium dithionite) or N2S205 (sodium disulfite) which are
based on various oxidation stages of sulfur.
However, the salts or complex compounds of certain
metals are particularly suitable. Metal salts and/or
metal complexes selected from the group consisting of
manganese, titanium, zirconium, hafnium, vanadium, cobalt
and cerium salts and/or complexes are preferably used to
prevent the corrosion of silver, the metals being present
in one of the oxidation stages II, III, IV, V or VI.
The definition commonly used in chemistry for
"oxidation stage" can be found, for example, in Rompp'~
Chemie Lexikon, Georg Thieme Verlag Stuttgart/New York,
9th Edition, 1991, page 3168.
The metal salts or metal complexes used should be at
least partly soluble in water. The counterions suitable
for salt formation include any inorganic anions with one,
two or three negative charges, for example oxide, sul-
fate, nitrate, fluoride, and also organic anions, such as
stearate for example.
Metal complexes in the context of the invention are
compounds which consist of a central atom and one or more
ligands. The central atom is one of the metals mentioned
above in one of the oxidation stages mentioned above.
The ligands are neutral monodentate or polydentate
molecules or anions. The term ~'ligand" in the context
of the present invention is defined, for example, in
Rompp's Chemie Lexikon, Georg Thieme Verlag Stuttgart/New
York, 9th Edition, 1990, page 2507. If the charge of the
central atom and the charge of the ligand(s) in a metal
complex do not add up to zero, either one or more of the
above-mentioned anions or one or more cations, for
example sodium, potassium, ammonium ions, provide for
2162460
charge equalization, depending on whether an excess
cationic or excess anionic charae is present. Suitable
complexing agents are, for example, citrate, acetyl
acetonate or 1-hydroxyethane-l,1-diphosphonate.
Particularly preferred metal salts and/or metal
complexes are selected from the group consisting of MnSO4,
Mn(II) citrate, Mn(II) stearate, Mn(II) acetyl acetonate,
Mn(II) [l-hydroxyethane-1,1-diphosphonate], V205, V204,
V02~ TiOSo4~ K2TiF6~ K2ZrF6~ cos04, Co(NO3)2, Ce(NO3)3 and
mixtures thereof. MnSO4 is particularly preferred.
These metal salts or metal complexes are generally
commercially available substances which may be used
without preliminary purification for the protection of
silver against corrosion in accordance with the present
invention. For example, the mixture of pentavalent and
tetravalent vanadium (V205, VO2, V204) known from the
production of S03 (contact process) is suitable as is the
titanyl sulfate (TioSo4) formed by dilution of a Ti (S4) 2
solution.
The inorganic redox-active substances, more par-
ticularly metal salts or metal complexes, are preferably
coated, i.e. completely surrounded by a material which is
water-resistant, but readily soluble at the dishwashing
temperatures in order to prevent their premature decom-
position or oxidation during storage. Preferred coating
materials, which are applied by known methods, for
example by the Sandwik melt coating process used in the
- food industry, are paraffins, microwaxes, waxes of
natural origin, such as carnauba wax, candellila wax,
beeswax, relatively high-melting alcohols, such as
hexadecanol for example, soaps or fatty acids. The
coating material, which is solid at room temperature, is
app-lied in molten form to the material to be coated, for
example by projecting fine-particle material to be coated
in a continuous stream through a continuously produced
21629 60
spray mist zone of the molten coating material. The
melting point has to be selected so that the coatir.g
material readily dissolves or rapidly melts during the
subsequent use of the silver corrosion inhibitor in a
dishwashing machine. For most applications, therefore,
the melting point should ideally be between 45C and 65C
and is preferably between 50C and 60C.
However, the inorganic redox-active substances
described above are particularly suitable for preventing
the corrosion of silver when used in low-alkali machine
dishwashing detergents. This is all the more surprising
insofar as these silver corrosion inhibitors are not
affected in their performance by the presence of oxygen-
based bleaching agents typically present in low-alkali
detergents.
Accordingly, the present invention also relates to
low-alkali machine dishwashing detergents of which 1% by
weight solutions have a pH value of 8 to 11.5 and prefer-
ably 9 to 10.5 and which contain 15 to 60~ by weight and
preferably 30 to 50% by weight of a water-soluble builder
component, 5 to 25~ by weight and preferably 10 to 15% by
weight of an oxygen-based bleaching agent, 1 to 10% by
weight and preferably 2 to 6% by weight of an organic
bleach activator containing 0- or N-(C1l2)-acyl groups,
0.1 to 5% by weight and preferably 0.5 to 2.5% by weight
of an enzyme, based on the detergent as a whole, and
silver corrosion inhibitors, an inorganic redox-active
substance being present as the silver corrosion inhibi-
tor. Metal salts and/or metal complexes selected from
the group of manganese, titanium, zirconium, hafnium,
vanadium, cobalt, cerium salts and/or complexes, the
metals being present in one of the oxidation stages II,
III, IV, V or VI, are particularly suitable.
Preferred dishwashing detergents contain metal salts
or metal complexes selected from the group consisting of
2162~ 60
MnSO4, Mn(II) citrate, Mn(II) stearate, Mn(II) acetyl
aretonate, Mn(II) ~l-hydroxyethane-l,l-diphosphonate],
V2o5/ V24~ V2~ TioSo4, K2TiF6, K2ZrF6, CoS04, Co(N03)2,
Ce(NO3)3 and mixtures thereof. MnSO4 is particularly
preferred.
The inorganic redox-active substances, more par-
ticularly metal salts and/or metal complexes, are prefer-
ably present in the detergents according to the invention
in a total quantity of 0.05 to 6% by weight and preferab-
ly 0.2 to 2.5% by weight, based on the detergent as awhole.
Organic bleach activators containing O- or N-(C~l2)-
acyl groups are substances in which at least one Cllz acyl
group, preferably the acetyl group, is attached to an O
atom or an N atom present in the substance and of which
the perhydrolysis gives Cll2 alkane peracids, preferably
peracetic acid.
Basically, suitable water-soluble builder components
are any of the builders typically used in machine dish-
washing detergents, for example polymeric alkali metalphosphates, which may be present in the form of their
alkaline, neutral or acidic sodium or potassium salts.
Examples include tetrasodium diphosphate, disodium
dihydrogen diphosphate, pentasodium triphosphate, so-
called sodium hexametaphosphate and the correspondingpotassium salts or mixtures of sodium hexametaphosphate
and the corresponding potassium salts or mixtures of
sodium and potassium salts. The quantities of phosphate
are up to about 30% by weight, based on the detergent as
a whole. However, the detergents according to the inven-
tion are preferably free from such phosphates. Other
possible water-soluble builder components are, for exam-
ple, organic polymers of native or synthetic origin,
above all polycarboxylates, which may act as cobuilders,
particularly in hard water systems. For example, poly-
2162~6(1
acrylic acids and copolymers of maleic anhydride andacrylic acid and also the sodium salts cf these polymer
acids may be used. Commercial products are, for example,
Sokalan~ CP 5 and PA 30 (BASF), Alcosperse~ 175 or 177
(Alco), LMW~ 45 N and SP02 N (Norsohaas). Native poly-
mers include, for example, oxidized starch (for example
German patent application P 42 28 786.3) and polyamino-
acids, such as polyglutamic acid or polyaspartic acid,
for example the products of Cygnus or SRCHEM.
Other possible builder components are naturally
occurring hydroxycarboxylic acids such as, for example,
monohydroxysuccinic acid, dihydroxysuccinic acid, ~-
hydroxypropionic acid and gluconic acid. Preferred
builder components are the salts of citric acid, more
particularly sodium citrate. The sodium citrate used may
be anhydrous trisodium citrate and is preferably dihydra-
ted trisodium citrate. Dihydrated trisodium citrate may
be used in the form of a fine or coarse crystalline
powder. Depending on the pH value ultimately established
in the detergents according to the invention, the acids
corresponding to citrate may also be present.
Suitable oxygen-based bleaching agents are, above
all, sodium perborate monohydrate and tetrahydrate or
sodium percarbonate. The use of sodium percarbonate has
advantages because sodium percarbonate has a particularly
favorable effect on the corrosion behavior of glasses.
Accordingly, the oxygen-based bleaching agent is prefer-
ably a percarbonate salt, more particularly sodium per-
carbonate. Since active oxygen only develops its full
effect at elevated temperatures, so-called bleach activa-
tors are used to activate it in the dishwashing machine.
Suitable bleach activators are organic bleach activators
containing O- or N-(C112)-acyl groups, for example PAG
(pentaacetyl glucose), DADHT (1,5-diacetyl-2,4-dioxohexa-
hydro-1,3,5-triazine) and ISA (isatoic anhydride), N,N,-
21624 6D
N',N'-tetraacetyl ethylenediamine (TAED) being preferred.
In addition, it can also be useful to add small quanti-
ties of known bleach stabilizers, for example phospho-
nates, borates or metaborates and metasilicates and also
magnesium salts, such as magnesium sulfate.
To improve the removal of protein-, fat- or starch-
containing food remains, the dishwashing detergents
according to the invention contain enzymes, such as
proteases, amylases, lipases and cellulases, for example
10proteases, such as BLAP~ 140 (Henkel); Optimase~ -M-440,
Optimase~ -M-330, Opticlean~ -M-375, Opticlean~ -M-250
(Solvay Enzymes); Maxacal~ CX 450.000, Maxapem~ (Ibis);
Savinase~ 4.0 T, 6.0 T, 8.0 T (Novo); Esperase~ T (Ibis),
and amylases, such as Termamyl~ 60 T, 90 T (Novo);
15Amylase-LT~ (Solvay Enzymes) or Maxamyl~ P 5000, CXT 5000
or CXT 2900 (Ibis); lipases such as Lipolase~ 30 T
(Novo); cellulases, such as Celluzym9 0.7 T (Novo
Nordisk). The dishwashing detergents preferably contain
proteases and/or amylases.
20In a preferred embodiment, the detergents according
to the invention additionally contain the alkali carriers
present in typical low-alkali machine dishwashing deter-
gents, for example alkali metal silicates, alkali metal
carbonates and/or alkali metal hydrogen carbonates. The
alkali carriers normally used include carbonates, hydro-
gen carbonates and alkali metal silicates with a molar
ratio of sio2 to M2O (M = alkali metal atom) of 1.5:1 to
2.5:1. Alkali metal silicates may be present in quan-
tities of up to 30% by weight, based on the detergent as
a whole. The highly alkaline metasilicates are preferab-
ly not used as the alkali carrier. The alkali carrier
system preferably used in the detergents according to the
invention is a mixture of - essentially - carbonate and
hydrogen carbonate, preferably sodium carbonate and
hydrogen carbonate, which is present in a quantity of up
- 216246~
to 60% by weight and preferably 10 to 40% by weight,
based on the detergent as a whole. The ratio of car-
bonate used to hydrogen carbonate used varies according
to the pH value ultimately required or established.
However, an excess of sodium hydrogen carbonate is
normally used, so that the ratio by weight of hydrogen
carbonate to carbonate is generally from 1:1 to 15:1.
Surfactants, more particularly low-foaming nonionic
surfactants, may optionally be added to the detergents
according to the invention to improve the removal of fat-
containing food remains. They also serve as wetting
agents, as granulation aids or as dispersion aids to
improve and homogenize the distribution of the silver
corrosion inhibitors in the wash liquor and on the silver
surfaces. The surfactants are used in quantities of up
to 5% by weight and preferably in quantities of up to 2~
by weight. Extremely low-foaming compounds are normally
used and preferably include C12lg alkyl polyethylene
glycol polypropylene glycol ethers with up to 8 moles of
ethylene oxide and propylene oxide units in the molecule.
However, it is also possible to use other nonionic
surfactants known as low foamers, including for example
Cl218 alkyl polyethylene glycol polybutylene glycol ethers
containing up to 8 moles of ethylene oxide and butylene
oxide units in the molecule, end-capped alkyl polyalky-
lene glycol mixed ethers and the foaming, but ecological-
ly attractive Cg14 alkyl polyglucosides with a degree of
polymerization of about 1 to 4 (for example APG~ 225 and
APG~ 600, Henkel KGaA) and/or Cl2 14 alkyl polyethylene
glycols containing 3 to 8 ethylene oxide units in the
molecule. A bleached quality should be used because
otherwise brown granules are formed. Surfactants from
the family of glucamides such as, for example, alkyl-N-
methyl glucamides (alkyl = C614 fatty alcohol) are also
suitable. In some cases, it is of advantage to use the
2162~60
described surfactants in the form of mixtures, for
example a mixture of alkyl polyglycoside with fatty
alcohol ethoxylates or a mixture of glucamides with alkyl
polyglycosides, etc.
If the detergents foam excessively in use, a foam-
suppressing compound, preferably from the group of
silicone oils, mixtures of silicone oil and hydrophobi-
cized silica, paraffin oil/Guerbet alcohols, paraffins,
hydrophobicized silica, bis-stearic acid amides and other
known commercially available defoamers, may be added to
them in quantities of up to 6% by weight and preferably
in quantities of about 0.5 to 4% by weight. Other
optional additives are, for example, perfume oils.
The dishwashing detergents according to the inven-
tion are preferably present as powders, granules or
tablets which may be produced in known manner, for
example by mixing, granulation, roll compacting and/or by
spray drying.
To produce detergents according to the invention in
tablet form, all the constituents are preferably mixed
together in a mixer and the mixture obtained is tabletted
in a conventional tabletting press, for example an
eccentric or rotary press, under pressures of 200-105 Pa
to 1500-105 Pa. Breaking-resistant tablets with a
flexural strength normally in excess of 150 N, which
still dissolve sufficiently rapidly under in-use condi-
tions, are readily obtained in this way. A correspond-
ingly produced tablet weighs 15 g to 40 g and, more
particularly, 20 g to 30 g for a diameter of 35 mm to 40
The production of machine dishwashing detergents in
the form of non-dust-emitting, storable free-flowing
powders and/or granules with high apparent densities of
750 to 1000 g/l is characterized in that, in a first
process step, the builder components are mixed with at
2162~60
least part of the liquid components with an increase in
the apparent density of this premix, after which the
other components of the machine dishwashing detergent,
including the inorganic redox-active substances, are
combined with the premix obtained, if desired after
drying.
Since the possible presence of alkali metal car-
bonate can have a considerable effect on the alkalinity
of the product, the intermediate drying step must be
carried out in such a way that the decomposition of
sodium bicarbonate to sodium carbonate is minimal (or at
least constant). Any additional sodium carbonate formed
as a result of drying would of course have to be taken
into consideration in the formulation for the granules.
Low drying temperatures not only counteract the decompos-
ition of sodium bicarbonate, they also increase the solu-
bility of the granulated detergent in use. Accordingly,
drying is advantageously carried out at an inflowing air
temperature which, on the one hand, should be as low as
possible to avoid the decomposition of bicarbonate and
which, on the other hand, must be as high as necessary to
obtain a product having good storage properties. An
inflowing air temperature of around 80C is preferable
for drying. The granules themselves should not be heated
to temperatures above about 60C. In the first stage of
the mixing process, the liquid components are applied to
the builder generally after it has been mixed with at
least one other component of the dishwashing detergent.
For example, the liquid nonionic surfactants and/or the
solution of perfumes may be applied to and thoroughly
mixed with the builder component in the form of a mixture
with perborate. The remaining components are then added
and the mixture as a whole is compounded and homogenized
in the mixer. There is generally no need to use addi-
tional quantities of liquid, i.e. additional water. The
216246~
WO 94/26859 13 PCT/EP94/01386
mixture obtained is present in the form of a free-flow-
ing, dust-free powder with the required high apparent
density of around 750 to lO00 g/l.
The granules are then mixed with the missing com-
ponents of the dishwashing detergent, including inorganicredox-active substances, to form the end product. In all
the cases illustrated here, the mixing time both in the
preliminary stage of compacting mixing in the presence of
liquid components and in the following final mixing stage
where the other components are incorporated is a few
minutes, for example from 1 to 5 minutes.
In one particular embodiment, it can be useful in
the production of fine granules to ensure further stabi-
lization and equalization by dusting the surface of the
lS granules formed with powder. Small amounts of waterglass
powder or powder-form alkali metal carbonate are par-
ticularly suitable for this purpose.
The detergents to be used may be used both in
domestic dishwashing machines and in institutional dish-
washing machines. They are added either by hand or bymeans of suitable dispensers. The in-use concentration
in the dishwashing liquor is about 2 to 8 g/l and prefer-
ably 2 to 5 g/l.
The dishwashing program is generally extended and
terminated by a few intermediate rinses with clear water
after the main wash cycle and by a final wash cycle using
a commercial rinse aid. Not only completely clean and
hygienically satisfactory crockery but also and above all
shining silverware is obtained after drying.
E x a m p l e s
- Silver spoons (type WMF, hotel cutlery, style
Berlin) were cleaned with a silver cleaner, degreased
with naphtha and dried. Three spoons were then placed in
2162460
14
the cutlery basket of a domestic dishwashing machine
(DDWM) of the Bosch S 712 t~pe. The wash program (65C,
16dH) was then started and 50 g of a soil(1~ and 30 g of
the detergent were directly introduced into the machine
during the main wash cycle. After rinsing and drying,
the DDWM was opened for lO minutes, then closed again and
operated in the same way. After the tenth wash cycle,
the spoons were removed and evaluated. Tarnishing was
evaluated on a scale of 0 to 4 where 0 = no tarnishing,
1 = very slight yellowing, 2 = stronger yellowing, 3 =
spoons completely gold to brown in color, 4 = spoons
violet to black in color; values in the upper left-hand
part of Table 1.
~l) Composition of the soil:
Ketchup: 25 g
Mustard (extra sharp)25 g
Gravy: 25 g
Potato starch: 5 g
Benzoic acid: l g
Egg yolk: 3 eggs
Milk: 1/2 l
Margarine: 92 g
Local water: 608 ml
At the same time, china was evaluated for the
removal of tea stains. Evaluation was based on a scale
of 0 to 10 where 0 = no removal of tea stains and 10 =
complete removal of tea stains; values in the lower
right-hand part of Table 1.
Preparation of the tea stain
16 Liters of cold local water (16~dH) are heated
briefly to boiling point in a tank. 96 g of black tea
are allowed to draw for 5 minutes in a nylon net with the
2162~ 60
cover on, after which the tea is transferred to an
immersion apparatus with a heating system and stirrer.
60 Tea cups were immersed in the tea thus prepared 25
times at l-minute intervals at a temperature of 70C.
The cups are then removed and placed on a metal plate to
dry with the opening facing downwards.
Detergent composition
The following low-alkali basic product was first
prepared (a 1% by weight solution in distilled water
having a pH value of 9.5):
56.0% trisodium citrate dihydrate
36.1% sodium hydrogen carbonate
6.1% sodium carbonate, anhydrous
1.8% mixture of nonionic surfactants of APG 225 (C8l0
alkyl oligoglucoside) and Dehydol~ LS2 (Cl2l4 fatty
alcohol 2E0 ethoxylate) (1:1)
Test variations corresponding to the following
formulation were then carried out with this basic prod-
uct. The results are set out in Table 1.
81 - 86% by weight basic product
12 % by weight sodium percarbonate
0 - 10% by weight TAED
0 - 2% by weight paraffin-coated manganese sulfate
monohydrate
1 % by weight protease
1 % by weight amylase
2162460
16
Table 1
Removal of tea stains/protection of silve- aqainst
corroslon
Machine: Bosch S 712 Tea: 1 = no removal
Dosage: 30 g 10 = optimal removal
Program: 65~C universal Silver: 0 = no tarnishing
Water 4 = heavy tarnishing
hardness: 16H
Redox-active substance Scores: tarnishing/tea
MnS04
/ o / O / / /
2-0% / 3.0 / 3.2 / - / 3.5 / - / 3.3
O / O / o /o / O / O
1.0% / 5.0 / 4.0 / 6.6 / 7.8 / - / 4.8
O /0 / o /0 / O / O
0.5% / 7.3 / 7.8 / 8.0 / 8.5 / 8.5 / 9.8
O / o / o /o / o / o
0.4% / - / 8.2 / - / 8.5 / 9.5 / 9.3
O /1 /0 /0 / o / 1
~ 0.3% / 6.1 / 9-7 / 9.2 / 8.7 / 9.0 / 8.7
0.2% ~ - ~ /9-7 /8.5 " '~
0.0% ~ , /5.2 /7.0 ,' ~.~ ~8
O.o % l.o % 1.5% 2.0 % 2.5 % 3.0 %
TAED
In addition, machine dishwashing detergents with the
following compositions were prepared (see Table 2). Com-
pounds A to E were used as silver corrosion inhibitors:
A: V204/v20s D: Ce(N03) 3
B: Tioso4 E: Na2S2O3 5H20
c: CoSO4
2162~60
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2162~60
19
The silver spoons were all awarded a score of 0 to
1, i.e. "very slight ta~nishing, if any"O In addition,
compositions 1 to 20 performed excellently against
bleachable stains, such as tea for example.
Although identical compositions, but without silver
corrosion inhibitors A to D, also performed very well
against bleachable stains, they also turned silver spoons
yellow to violet in color (score: 2 to 4).
Electrochemical measurements
Sample preparation: -
Instead of silver cutlery, silver wire (D = 2 mm,99.99%) was used as sample material for the tests. The
silver wire was cut into approximately 10 cm long pieces,
that part of the sample dipping into the measuring
solution being rubbed with SiC abrasive paper (600
grain). The samples were then thoroughly rinsed with
twice-distilled water and any abrasion residues adhering
to the samples were wiped off with a fluff-free cloth.
If desired, this procedure was repeated several times
until the sample left a visually satisfactory impression.
After rubbing with the abrasive paper, the samples were
immediately used for the measurement to forestall any
reaction of the metallic silver with the laboratory air.
The effective surface area of the sample immersed in the
solution amounted to 0.70 cm2.
Electrolytes and electrodes:
The experiments were conducted in a Duran glass
cell. The above-mentioned silver wires (A = 0.70 cm2)
were used as the measuring electrodes. The counterelec-
trode consisted of a gold foil (99.99~) with a surface
area of 1 cm2. In view of the alkaline electrolyte
solutions, the reference electrode was an Hg/HgO/0.1 M
NaOH electrode which was connected to the electrolyte by
2162460
a Haber-Luggin capillary. The measurements were carried
out with 5 g/l of detergent in tap wzter having a hard-
ness of 16dH and a salt concentration of around 600 mg
(dry residue).
To prepare the detergent solutions, the low-alkali
basic product (see above) was first dissolved and the
resulting solution was heated to 65C. The bleaching
agent and the bleach activator and/or the silver corro-
sion inhibitor were added immediately before the measure-
ment. The electrochemical measurement was then carried
out. During the electrochemical experiments, the elec-
trolyte solutions were kept at 65C and purged with air.
Apparatus and recording of the measuring curves:
To record the current/voltage curves, the electrode
potential was increased at a constant rate from -0.62 V,
based on a standard hydrogen electrode (SHE). After a
total increase of 1.1 V, the potential was reduced at the
same rate. A standard potentiostat consisting of a
regenerative amplifier, differential amplifier, adder and
impedance transformer and a function generator (Prodis 16
of Intelligent Controls CLZ GmbH) were used for this
purpose.
Results:
The corrosion behavior was characterized on the
basis of current/voltage curves. Essential information
comes from the zero-axis crossing of the current/voltage
curve (quiescent potential which is spontaneously estab-
lished even without any external influencing of thepotential) and the slope of the cur~e at the zero-axis
crossing (reciprocal polarization resistance), E. Heitz,
R. Henkhaus, A. Rahmel, ~Korrosionskunde im Experiment"
Verlag Chemie (1983), pages 31 et seq.; H. Kaesche, "Die
Rorrosion der Metalle~, 2nd Edition, Springer Verlag
21 62~ 6~
WO 94/26859 21 PCT/EP94/01386
(1979), pages 117 et seq.. The addition of the silver
corrosion inhibitor produces a shift in the potential of
the zero-axis crossing to lower values and a reduction in
the slope. Accordingly, electrochemical measurements
also show that the corrosion of silver is considerably
reduced by addition of the silver corrosion inhibitors.
Composition of Position of zero- Slope at zero
Detergent axis crossing axis crossing
E (mV) (SHE) di/dE (mA/V)
- Basic product (87%) 435 25
+ 12% Percarbonate
+ 1% TAED
Basic product (87%) 360 0.3
+ 12% Percarbonate
+ 1% Silver corrosion
inhibitor*)
Basic product (86.5%) 405 7
+ 12% Percarbonate
- + 1% TAED
+ 0.5% Silver corrosion
inhibitor*)
Basic product (86%) 375 0.6
+ 12% Percarbonate
+ 1% TAED
+ 1% Silver corrosion
inhibitor*)
*) Silver corrosion inhibitor: manganese sulfate
monohydrate
