CA2162459A1

CA2162459A1 - Corrosion inhibitors for silver (ii)

Info

Publication number: CA2162459A1
Application number: CA002162459A
Authority: CA
Inventors: Juergen Haerer; Helmut Blum; Birgit Burg; Thomas Holderbaum; Willi Buchmeier; Peter Jeschke; Horst-Dieter Speckmann; Frank Wiechmann; Christian Nitsch
Original assignee: Individual
Current assignee: Henkel AG and Co KGaA
Priority date: 1993-05-08
Filing date: 1994-05-02
Publication date: 1994-11-24
Also published as: EP0697036A1; DK0697036T3; JPH08509778A; EP0697036B1; PL311624A1; ES2134944T3; CZ287850B6; WO1994026860A1; PL177935B1; CZ262595A3; HU218021B; HUT73025A; HU9503200D0; HU9503201D0; HU218008B; HUT73028A; ATE182615T1; DE59408548D1

Abstract

The invention concerns the use of organic redox compounds, in particular ascorbic acid, indole, methionine N-(C1-C4 alkyl) glycines, 2-phenylglycine or coupler and/or development compounds selected from the group comprising diaminopyridines, aminohydroxypyridines, dihydroxypyridines, heterocyclic hydrazones, aminohydroxypyrimidines, dihydroxypyrimidines, tetraaminopyrimidines, triaminohydrox-ypyrimidines, diaminodihydroxypyrimidines, dihydroxynaphthalines. naphthols, pyrazolones, hydroxyquinolines, aminoquinolines, primary aromatic amines which have, in addition, a free hydroxy or amino group, or a hydroxy or amino group substituted with C1-C4 alkyl or C2-C4 hydroxyalkyl groups, at the ortho, meta or para position, and di- or trihydroxybenzenes as silver-corrosion protection agents in dishwasher washing agents, in particular low-alkali dishwaster washing agents.

Description

Corrosion inhibitors for silver (II) 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 are introduced into the wash liquor.
The much higher temperatures prevailing in dishwashing machines and the longer contact 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-les, lipstick residues and the like.

Wo 94/26860 2 PCT/EP94/01387 These active oxygen compounds are used in conjunc-tion with bleach activators above all in modern low-alkali machine dishwashing detergents of the new genera-tion. These modern detergents generally consist of the following 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-gents containing benzotriazoles as corrosion inhibitors 2162~5~

Wo 94/26860 3 PCT/EP94/01387 for silver. US 3,549,539 describes highly alkalinemachine 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 benzotriazoles and 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 organic redox-active substances, more particularly the primary and/or secondary intermediates typically used in oxida-tion dyes which have not hitherto been described as corrosion inhibitors for silver, effectively prevent the corrosion of silver in dishwashing machines.
The present invention relates to the use of organic 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.
"0rganic redox-active substances" are organic sub-stances which are accessible to readily occurring, reversible oxidation and/or reduction. For example, typical complexing agents, for example EDTA or hydroxy-ethane diphosphonic acid and related compounds, do not 2162~5~

fall under this definition.
Typical "organic redox-active substances" are, for example, ascorbic acid (vitamin C), indole, methionine (~-amino-~-methylmercaptobutyric acid).
N-Mono-(C14-alkyl)-glycines, for example N-monomethyl glycine, and N,N-di-(C14-alkyl)-glycines, for example N,N-dimethyl glycine and 2-phenyl glycine, are also suitable.
Certain primary and/or secondary intermediates known from oxidation dyeing are also particularly suitable.
The organic substances preferably used to prevent the corrosion of silver are primary and/or secondary intermediates selected from the group of diaminopyri-dines, aminohydroxypyridines, dihydroxypyridines, hetero-cyclic hydrazones, aminohydroxypyrimidines, dihydroxypyr-imidines, tetraaminopyrimidines, triaminohydroxypyrimi-dines, diaminodihydroxypyrimidines, dihydroxynaphtha-lenes, naphthols, pyrazolones, hydroxyquinolines, amino-quinolines, primary aromatic amines with another free or C14-alkyl- or C24-hydroxyalkyl-substituted hydroxy or amino group in the ortho, meta or para position and di-or trihydroxybenzenes.
The primary and secondary intermediates used in accordance with the invention to prevent the corrosion of silver are the substances from the above-mentioned groups normally used in oxidation dyes. Examples of such pri-mary and secondary intermediates can be found, for exam-ple, in Venkataraman, "The Chemistry of Synthetic Dyes", Vol. V, Academic Press, New York/London, 1971, pages 478-495 and in the literature cited therein. Primary and secondary intermediates selected from the group consist-ing of p-hydroxyphenylglycine, 2,4-diaminophenol, 5-chloro-2,3-pyridinediol, l-(p-aminophenyl)-morpholine, hydroquinone, pyrocatechol, hydroxyhydroquinone, gallic acid, phloroglucinol, pyrogallol are particularly suit-able for preventing the corrosion of silver.

2162~59 The organic redox-active substances 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 coatingmaterials, 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 applied 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 spray mist zone of the molten coating material. The melting point has to be selected so that the coating 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 organic redox-active substances de-scribed 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 2162~S~

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, 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 organic redox-active substance being present as the silver corro-sion inhibitor. Particularly suitable silver corrosion inhibitors are ascorbic acid, indole, methionine, N,N'-di-(C14-alkyl)-glycine and 2-phenyl glycine, but above all primary and/or secondary intermediates selected from the group of diaminopyridines, aminohydroxypyridines, dihydroxypyridines, heterocyclic hydrazones, aminohy-droxypyrimidines, dihydroxypyrimidines, tetraaminopyri-midines,triaminohydroxypyrimidines,diaminodihydroxypyr-imidines, dihydroxynaphthalenes, naphthols, pyrazolones, hydroxyquinolines, aminoquinolines, primary aromatic amines with another free or C14-alkyl- or C24-hydroxy-alkyl-substituted hydroxy or amino group in the ortho, meta or para position and di- or trihydroxybenzenes.
Preferred dishwashing detergents contain primary and/or secondary intermediates selected from the group consisting of p-hydroxyphenylglycine, 2,4-diaminophenol, 5-chloro-2,3-pyridinediol, 1-(p-aminophenyl)-morpholine, hydroquinone, pyrocatechol, hydroxyhydroquinone, gallic acid, phloroglucinol, pyrogallol and mixtures thereof.
The organic redox-active substances are preferably present in the detergents according to the invention in a total quantity of 0.05 to 6% by weight and preferably 0.2 to 2.5% by weight, based on the detergent as a whole.
Basically, suitable water-soluble builder components are any of the builders typically used in machine dish-washing detergents, for example polymeric alkali metal phosphates, which may be present in the form of their alkaline, neutral or acidic sodium or potassium salts.

2162g~

Examples include tetrasodium diphosphate, disodium dihydrogen diphosphate, pentasodium triphosphate, so-called sodium hexametaphosphate and the corresponding potassium 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 30~6 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-acrylic acids and copolymers of maleic anhydride and acrylic acid and also the sodium salts of 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 SPO2 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 Wo 94/26860 8 PCT/EP94/01387 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, for example PAG (pentaacetyl glucose), DADHT (1,5-diace-tyl-2,4-dioxohexahydro-1,3,5-triazine) and ISA (isatoic anhydride), N,N,N',N'-tetraacetyl ethylenediamine (TAED) being preferred. In addition, it can also be useful to add small quantities of known bleach stabilizers, for example phosphonates, borates or metaborates and metasil-icates 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 proteases, 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);
Amylase-LT~ (Solvay Enzymes) or Maxamyl~ P 5000, CXT 5000 or CXT 2900 (Ibis); lipases such as Lipolase~ 30 T
(Novo); cellulases, such as Celluzym~ 0.7 T (Novo Nordisk). The dishwashing detergents preferably contain proteases and/or amylases.
In a preferred embodiment, the detergents according to the invention additionally contain the alkali carriers present in typical low-alkali machine dishwashing deter-2162~59 gents, for example alkali metal silicates, alkali metalcarbonates 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 SiOz 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 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 C1218 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 2I 62~ 5~

C12l8 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 C814 alkyl polyglucosides with a degree ofpolymerization of about 1 to 4 (for example APG~ 225 and APG~ 600, Henkel KGaA) and/or C12l4 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 = C6l4 fatty alcohol) are also suitable. In some cases, it is of advantage to use the 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 mm.
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 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 organic 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 2162~59 Wo 94/26860 12 PCT/EP94/01387 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 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 1000 g/l.
The granules are then mixed with the missing com-ponents of the dishwashing detergent, including organic redox-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 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 by means of suitable dispensers. The in-use concentration in the dishwashing liquor is about 2 to 8 g/l and prefer-2162~9 ably 2 to 5 g/l.
The dishwashing program is generally extended andterminated 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 1 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 the cutlery basket of a domestic dishwashing machine (DDWM) of the Bosch S 712 type. The wash program (65C, 16dH) was then started and 50 g of a soil~l) 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 10 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 Tables 1 to 3).

Composition of the soil:
Ketchup: 25 g Mustard (extra sharp)25 g Gravy: 25 g Potato starch: 5 g Benzoic acid: 1 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 o to 10 where 0 = no removal of tea stains and 10 =
complete removal of tea stains; values in the lower right-hand part of Tables 1 to 3.

Preparation of the tea stain 16 Liters of cold local water (16dH) 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 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 1-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 (C810 alkyl oligoglucoside) and Dehydol~ LS2 (C1zl4 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 Tables 1 to 3.

2162~59 Wo 94/26860 15 PCT/EP94/01387 81 - 86% by weight basic product 12 % by weight sodium percarbonate 0 - 10% by weight TAED
0 - 3% by weight pyrocatechol, gallic acid or hydro-5quinone 1 % by weight protease 1 % by weight amylase Table 1 Removal of tea stains/protection of silver aqainst corrosion Machine: Bosch S 712 Tea: 1 = no removal Dosage: 30 g 10 = optimal removal Program: 65C universal Silver: 0 = no tarnishing Water 4 = heavy tarnishing hardness: 16H

Redox-active substance Scores: tarnishing/tea Pyrocatechol 3 0% ~ / ~ /

l 0% / 3 / / / ~ /
//////

0.0% ~ /5 0.0 % 1.0 % 2.0% 3.0 % 4.0 % 10.0 %
TAED

2162~S9 Table 2 Removal of tea stains/protection of silver against corrosion Machine: Bosch S 712 Tea: 1 = no removal Dosage: 30 g 10 = optimal removal Program: 65C universal Silver: 0 = no tarnishing Water 4 = heavy tarnishing hardness: 16H

Redox-active substance Scores: tarnishing/tea Gallic acid 5~ ~ / / /

0.0 % 1.0 % 2.0% 3.0 % 4.0 % 10.0 %
TAED

Table 3 Removal of tea stains/protection of silver against corroslon Machine: Bosch S 712 Tea: 1 = no removal Dosage: 30 g 10 = optimal removal Program: 65C universal Silver: 0 = no tarnishing Water 4 = heavy tarnishing hardness: 16H

Redox-active substance Scores: tarnishing/tea Hydroquinone /////
2-0% ~2 1.5% 0 / 1 / 1 / 2 / 3 1.0% ~ ~ ~ ~ ~ /

~ ~ ~ ~3 ~ ~ 10 /
0.2% ~ ~ ~ ~ ~ /

0.0% 4 / 3 / 4 / 4 / 4 / 4 0.0 % 1.0 % 2.0% 3.0 % 4.0 % 10.0 %
TAED

2162~59 In addition, machine dishwashing detergents with the following compositions were prepared (see Table 4). Com-pounds A to M were used as silver corrosion inhibitors:
A: p-hydroxyphenylglycine B: 2,4-diaminophenol C: 5-chloro-2,3-pyridinediol D: l-(p-aminophenyl)-morpholine E: ascorbic acid F: monomethylglycine G: N,N-dimethylglycine H: 2-hydroxy-4-aminopyrimidine I: 2,4-dihydroxy-5-methylpyrimidine K: indole L: methionine M: 2-phenylglycine 2162~9 C~
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The silver spoons were all awarded a score of 0 to 1, i.e. "very slight tarnishing, if any". Identical compositions, but without silver corrosion inhibitors A
to D, 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 a Haber-Luggin capillary. The measurements were carried out with 5 g/l of detergent in tap water having a hard-ness of 16dH and a salt concentration of around 600 mg 2I62i5~

(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 the potential) and the slope of the curve at the zero-axis crossing (reciprocal polarization resistance), E. Heitz, R. Henkhaus, A. Rahmel, ~Rorrosionskunde im Experiment~
Verlag Chemie (1983), pages 31 et seq.; H. Kaesche, "Die Rorrosion der Metalle", 2nd Edition, Springer Verlag (1979), pages 117 et seg.. 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 2I 624 ~9 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 (86%) 319 0.5 + 12% Percarbonate + 2% Hydroquinone

Claims

1. The use of organic redox-active substances in dishwashing detergents as corrosion inhibitors for silver.

2. The use claimed in claim 1, characterized in that the organic redox-active substance is ascorbic acid, indole or methionine.

3. The use claimed in claim 2, characterized in that the organic redox-active substance is an N-mono-(C1-4-alkyl)-glycine, an N,N-di-(C1-4-alkyl)-glycine or 2-phenylglycine.

4. The use claimed in claim 1, characterized in that the organic redox-active substance is a primary and/or secondary intermediate selected from the group of di-aminopyridines, aminohydroxypyridines, dihydroxypyri-dines, heterocyclic hydrazones, aminohydroxypyrimidines, dihydroxypyrimidines, tetraaminopyrimidines, triaminohy-droxypyrimidines,diaminodihydroxypyrimidines,dihydroxy-naphthalenes, naphthols, pyrazolones, hydroxyquinolines, aminoquinolines, primary aromatic amines with another free or C1-4-alkyl- or C2-4-hydroxyalkyl-substituted hydroxy or amino group in the ortho, meta or para position and di- or trihydroxybenzenes.

5. The use claimed in claim 4, characterized in that the primary or secondary intermediate is selected from the group consisting of p-hydroxyphenylglycine, 2,4-diaminophenol,5-chloro-2,3-pyridinediol,1-(p-aminophen-yl)-morpholine, hydroquinone, pyrocatechol, hydroxyhydro-quinone, gallic acid, phloroglucinol, pyrogallol.

6. A low-alkali machine dishwashing detergent of which 1% by weight solutions have a pH value of 8 to 11.5 and preferably 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 prefer-ably 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, 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, characterized in that an organic redox-active substance is present as the silver corrosion inhibitor.

7. A low-alkali machine dishwashing detergent as claimed in claim 6, characterized in that ascorbic acid, indole or methionine is present as the silver corrosion inhibitor.

8. A low-alkali machine dishwashing detergent as claimed in claim 6, characterized in that an N-mono-(C1 4-alkyl)-glycine, an N,N-di-(C1-4-alkyl)-glycine or 2-phenylglycine is present as the silver corrosion inhib-itor.

9. A low-alkali machine dishwashing detergent as claimed in claim 6, characterized in that a primary and/or secondary intermediate selected from the group of diaminopyridines, aminohydroxypyridines, dihydroxypyri-dines, heterocyclic hydrazones, aminohydroxypyrimidines, dihydroxypyrimidines, tetraaminopyrimidines, triaminohy-droxypyrimidines,diaminodihydroxypyrimidines,dihydroxy-naphthalenes, naphthols, pyrazolones, hydroxyquinolines, aminoquinolines, primary aromatic amines with another free or C1-4-alkyl- or C2-4-hydroxyalkyl-substituted hydroxy or amino group in the ortho, meta or para position and di- or trihydroxybenzenes is present as the silver corrosion inhibitor.

10. A detergent as claimed in claim 9, characterized in that the primary or secondary intermediate is selected from the group consisting of p-hydroxyphenylglycine, 2,4-diaminophenol,5-chloro-2,3-pyridinediol,1-(p-aminophen-yl)-morpholine, hydroquinone, pyrocatechol, hydroxyhydro-quinone, gallic acid, phloroglucinol, pyrogallol.

11. A detergent as claimed in claims 6 to 10, charac-terized in that the organic redox-active substances are present in a quantity of 0.05 to 6% by weight and prefer-ably in a quantity of 0.2 to 2.5% by weight, based on the detergent as a whole.

12. A detergent as claimed in claims 6 to 11, charac-terized in that the water-soluble builder component is a salt of citric acid, preferably sodium citrate.

13. A detergent as claimed in claims 6 to 12, charac-terized in that the oxygen-based bleaching agent is a percarbonate salt, preferably sodium percarbonate.

14. A detergent as claimed in claims 6 to 13, charac-terized in that the organic bleach activator is N,N,N', N'-tetraacetyl ethylenediamine (TAED).

15. A detergent as claimed in claims 6 to 14, charac-terized in that the enzyme is an amylase and/or protease.

16. A detergent as claimed in claims 6 to 15, charac-terized in that it additionally contains up to 60% by weight and preferably 10 to 40% by weight, based on the detergent as a whole, of an alkali carrier system con-sisting essentially of carbonate and hydrogen carbonate, preferably sodium carbonate and sodium hydrogen car-bonate.

17. A detergent as claimed in claims 6 to 16, charac-terized in that it additionally contains up to 5% by weight and preferably up to 2% by weight, based on the detergent as a whole, of surfactants, preferably low-foaming nonionic surfactants.

18. A detergent as claimed in claims 6 to 17, charac-terized in that it is present in tablet form and is obtainable by mixing all its ingredients in a mixer and tabletting the resulting mixture in a tabletting press under pressures of 2107 Pa to 1.5108 Pa.

19. A detergent as claimed in claims 6 to 17, charac-terized in that it is present in the form of a powder or granules and has an apparent density of 750 g/l to 1000 g/l.