NO781691L - METHODS OF INHIBITING METAL CORROSION - Google Patents

Abstract

Method of inhibiting metal corrosion

Description

The present invention relates to the inhibition of metallic corrosion of aqueous agricultural preparations containing as an active agricultural ingredient an aminomethylenephosphonic acid, or an agriculturally acceptable salt or ester derivative thereof. More particularly, the invention relates to the inhibition of corrosion of iron or zinc surfaces that come into contact with

aqueous agricultural preparations in which the active agricultural ingredient, an aminomethylenephosphonic acid, or salt or ester derivatives thereof in the presence of water and in the absence of an inhibitor is corrosive to such surfaces and evolves hydrogen gas.

According to the present invention, corrosion of iron or zinc surfaces is inhibited by including in such agricultural preparations a corrosion-inhibiting amount of a thiol compound or salt thereof as described below.

The aminomethylenephosphonic acids used in the preparations according to the invention are covered by the following formula:

where y and z are each 1 or 2, and x is 0 or 1, the sum of x, y and z being 3. Also useful in the preparations according to the invention are the agriculturally acceptable salts and esters of these acids. The term "agricultural preparation" is used here to • include within its scope herbicide and plant growth regulating preparations. Although such preparations are often processed as dry powder preparations and used in this form to dust plant

foliage, they are more commonly processed into solutions, emulsions, suspensions or dispersions for wet application to plant foliage. These liquid preparations usually contain water and more water is added to it at the time of application to dilute the concentration of the active ingredient in the preparation to the level which enables the application of predetermined, controlled amounts to the plant foliage. Generally, the dry powder preparations are non-corrosive to metal surfaces, while depending on the particular active ingredient in an aqueous liquid preparation and the surfactant that may also be present in the preparation, mild to severe corrosion of metal surfaces will occur when these comes into contact with the aqueous agricultural preparations.

The term "active agricultural ingredient" is used herein to include any ingredient that acts as a toxic or growth-regulating agent on plants. The particular function of an active ingredient may be that of a herbicide when applied to the plant in moderate to high application rates, and on the other hand act as a plant growth regulator at low to low application rates. Such double-acting ability is shown by some of the aminomethylenephosphonic acids and their agriculturally acceptable salts as described in US patents 3,455,675 and 3,556,762, the former being aimed at phytotoxic use and the latter at plant growth regulation. The herbicidal activity of N-phosphonomethylglycine and its agriculturally acceptable salt and ester derivatives is described in US patents 2,799,758, 3,868,407, 3,971,649 and 3,977,860. Plant growth regulating use of N-phosphonomethylglycine and its agriculturally acceptable sa.lt - and ester derivatives are described in US patents 3,853,530 and 3,988,142.

The aqueous preparations of the aminomethylenephosphonic acids, such as N-phosphonomethylglycine or derivatives thereof, and more particularly these preparations diluted with water to application quantities are corrosive to iron, steel or galvanized metal surfaces of containers in which the concentrates or mixtures are stored, and to steel or galvanized surfaces of sprayers -equipment. Hydrogen evolution is a feature of the corrosive activity and can cause bursting pressure in closed containers containing the aqueous agricultural preparations as well as constituting a fire and explosion hazard.

It is known that the application of various organic coatings, such as phenolic resins, synthetic rubbers, alkyds, vinyls as well as glass linings on metal surfaces is a practical way to protect or prevent corrosion of metal surfaces, but such coatings increase the price of containers and other equipment" used for application of herbicides or plant growth regulators. Furthermore, the harmlessness of such coatings is exposed to accidental or abrasion stress under working conditions in agriculture, whereby the coating is mechanically worn off, scraped or otherwise detached from the metal surface. When this occurs, the exposed metal surface is then attacked easily by the agricultural preparation, and such corrosion causes frequent loosening or breakdown of the protective coating material adjacent to the exposed metal surface and thus accelerates the overall corrosion of the equipment.

Trabanelli et al published the results of an investigation into the effect of various organic sulfur compounds on the inhibition of corrosion of iron immersed in sulfuric acid, and noted that mercaptans were generally poor inhibitors against iron corrosion and in some cases even acted as corrosion stimulants (Chemical Abstracts , 72, pp. 206, 58134?).

It was thus most surprising to discover that thiol compounds, for example mercaptans, as well as ammonium and alkali metal thio salts of inorganic polyvalent acids and the thio alkali metal salts are effective inhibitors of metal corrosion for aqueous agricultural preparations containing such. an active agricultural component an aminomethylenephosphonic acid, such as N-phosphonomethylglycine and the agriculturally acceptable salt or ester derivatives thereof. Of course, a satisfactory inhibitor against acid corrosion, measured by hydrogen evolution and

- metal corrosion rate, for a herbicidal preparation or plant growth regulating preparation not to modify the agricultural activity of the preparation in a harmful way. It has

proved that both the retention of agricultural activity and sufficient inhibition of acid corrosion was achieved by the addition to aqueous preparations of an aminomethylenephosphonic acid or the agriculturally acceptable derivatives of relatively small amounts of these thio compounds, such as alkanethiols and -dithiols, alkali metal salts of alkanethiols and -dithiols, and ammonium and alkali metal thiosalts of polyvalent inorganic acids, i.e. sulfuric acid and phosphoric acid, Sufficient inhibition of acid corrosion as measured by hydrogen evolution, can be achieved with a minimum of approx. 0.15% by weight of the thio compound calculated on the weight of N-phosphonomethylglycine. In order to ensure long-term corrosion inhibition, it is preferred to use the thio compound in amounts of 0.3 to 3% by weight calculated on the weight of N-phosphonomethylglycine or aminomethylenephosphonic acid or derivatives thereof, although the thio compound can be used in amounts as high as 20% by weight, calculated for N-phosphonomethylglycine. Not infrequently, aqueous concentrates of the herbicides or plant growth regulating preparations are stored in the seller's metal containers by the farmer for many months before use, therefore it is desirable to reduce the corrosion of the container as much as possible in order to prevent possible leakage of the concentrate due to rusting of the container's core number. walls. Amounts in excess of 5% by weight of the thio compound may be used if desired, but no further corresponding benefit with regard to corrosion is usually obtained.

Anionic, cationic or non-ionic surfactants can be used in the agricultural preparations according to the invention. The surfactants which are useful in the preparations according to the invention include those of the cationic, anionic and non-ionic type, and also amine oxides, imidazolines, propoxylated ethoxylated ethylenediamines, quaternary ammonium compounds, betaine derivatives as well as amphoteric surfactants. Examples of the amine oxides are lauryldimethylaminoxyd, cetyldimethylaminoxyd, myristyldimethylaminoxyd, bis-(2-hydroxy-ethyl)-cocoaminoxyd and the like. Examples of quaternary amine surfactants are cocotrimethylammonium chloride, alkyl-amidoethyl-alkyl-imidazolium-methyl-methosulphate. Examples of cationic surfactant compounds are N,N-bis-(2-hydroxy-ethyl)-alkylamines where the alkyl groups are ci^~ ciq derived from tallow, N,N-bis-(a-ethyl-omega-hydroxy)-poly- (oxyethylene)-alkylamines with an average of 3 oxyethylene groups, where alkyl is 14~13 derived from tallow and (1-lauramidopropyl)-trimethyl-ammonium-methylsulphate. Some anionic surfactants are the sulfated fatty alcohols and the alkylaryl sulfonates. Representative of the sulfated fatty alcohols are the sodium or lower alkanolamine salts of the monoesters of sulfuric acid with N-aliphatic alcohols containing from 8 to 18 carbon atoms. The alkylarylsulfonates include the products obtained from the alkylation of an aromatic hydrocarbon, for example benzene, naphthalene, diphenyl, diphenylmethane and phenoxybenzene, sulfonation of the resulting alkylated aromatic hydrocarbon and neutralization of the sulfonation product with sodium or potassium hydroxide, or with a primary or secondary amine.

Some nonionic surfactants are the ethoxylated monoamines with the formula: •

where R is alkyl with from approx. 8 to 16 carbon atoms, and m is an integer from 2 to 25. Preferred anionic surfactants are the aliphatic amine salts of monoalkyl-(C 8 -C 8 -phenoxybenzenedisulfonic acids).

All the alkanethiols and dithiols containing from 2 to 16 carbon atoms in the alkane group that have been investigated for corrosion inhibition proved effective in essentially eliminating hydrogen evolution under the test conditions described here. The alkanethiols, especially those containing 8 or fewer carbon atoms in the alkane group, have a distinctly unpleasant smell. It is therefore preferable to use alkanethiols with more than 8 carbon atoms in the alkane group. The higher alkanethiols are less odorous, and are therefore preferred for reasons of to reduce the discomforts. for the worker and the user. The terms "alkanethiol" and "alkanedithiol" are intended to include the normal, secondary and tertiary isomers of these compounds. Representative thiols and dithiols useful in the practice of the present invention include. 1,2-ethanedithiol, ethane-thiol, 1,3-propanedithiol, 1-propanethiol, 2-propanethiol, 2-raethyl-2-propanethiol, 1,4-butenedithiol, 1-butanethiol, 3-methyl-1-butanethiol, 1-hexanethiol, 2-hexanethiol, 3-hexanethiol, 1-octanethiol, 2-octanethiol, 1-decanethiol, 1-dodecanethiol and 1-hexa-decanethiol. Representative of the alkali metal salts of the alkane thiols and dithiols are the sodium and potassium salts of 1-ethanethiol, 1-butanethiol, 1-hexanethiol and 1-dodecanethiol. Representative of the ammonium and alkali metal thiosalts of organic polyvalent acids are ammonium thiosulphate, ammonium thiophosphate, ammonium thiocarbonate, sodium thiophosphate, potassium thiophosphate, sodium dimethyldithiocarbonate and sodium thiocarbonate, which unlike the alkanethiols do not contain any -SH group in their structure, but nevertheless inhibit hydrogen evolution to a significant extent from steel and zinc surfaces. One can also . add compounds such as sodium sulphite to the thiosulphate-containing preparations. Although no improvement in corrosion inhibition is achieved, this prevents dissociation of the ammonium thiosulphate in this preparation. The corrosion-inhibiting agricultural preparations according to the invention, including concentrates that require dilution with water before application to the plants, contain from 5 to 95 parts by weight of an agriculturally active agent, from approx. 5 to 95% by weight of an excipient comprising from 0.25 to 25 parts by weight of a non-ionic or anionic surfactant, from 0 to 25 parts by weight of a dispersing agent and from approx. 4.5 to approx. 95% by weight of an inert liquid extender, for example water, and from 0.1 to 2% by weight of a suitable thiophene compound. The preparations are made by mixing the active ingredient, thiophor the binder, the surfactant and the liquid extender to obtain liquid preparations in the form of solutions, suspensions, dispersions or emulsions. Just before application to the plants, these liquid preparations are diluted with water as needed to achieve the desired effects (herbicide or plant growth regulation). .To determine hydrogen evolution, concentrated liquid preparations were prepared according to the following recipe, with all parts being by weight:

The liquid concentrates were then diluted with water to the actual spray application concentrations, d<y>s. 5 parts concentrate to 95 parts water and the diluted preparations were examined for hydrogen evolution and corrosion properties. The diluted concentrates were usually more corrosive to metals than the concentrates.

Hydrogen evolution measured according to the following' method.

A metal plate sample (mild steel or zinc) was placed horizontally in the nozzle of an inverted plastic funnel, the supports being toothpicks attached to the edge of the funnel. The dimensions of the steel sample were 32 mm x 14 mm x 6 mm; zinc sample size was 32 mm x 14 mm x 2 mm. The funnel arrangement with the supported metal sample was placed in a 250 ml beaker and sufficient agricultural diluent was poured into the beaker to completely submerge the funnel. A 100 ml conical graduated glass centrifuge tube was filled with the preparation, the end of the tube was then closed with a finger, turned upside down and placed over the stem of the funnel. Hydrogen gas evolved during attack on the metal sample of the preparation rises up inside the conical part of the funnel, then into

1 funnel stem and finally into the sample tube where it is collected

and displaces the liquid preparation. The amount of collected gas is read directly from the graduations on the test tube. The beaker and its contents are kept at room temperature during the trial period

24 hours.

The corrosion rate of the samples was determined by immersing degreased mild steel and zinc plate samples in a given preparation at room temperature for 96 hours and then measuring the weight loss or gain of the sample and extrapolating the result to an annual corrosion rate.

Water-diluted concentrates prepared as described above and containing the monoisopropylamine salt of N-phosphonomethylglycine as the active agricultural ingredient were modified by including various thiol compounds and surfactants as indicated in Table I. Experimental data indicated in Table I were obtained at room temperature.

<x>Surfactant "A" is a surfactant of the nonionic type comprising an ethoxylated tallow amine of the formula:

where m has an average value of between 15 and 20, and R is alkyl with an average number of carbon atoms of approx. 17-18.

Surfactant "B" is of the anionic type comprising a mixture with an average of approx. 80% by weight or more of a monoisopropyl j insalt of a C-4-alkyl-phenoxybenzene-disulfonic acid and up to approx. 20% by weight dialkylated products of phenoxybenzene-disulfonic acid.

Surfactant "C" is also an anionic surfactant and is the triethanolamine salt of a (C8_i0^~alcohol sulfate; (C&_10)-OS02ONH(CH2CH2OH)3. Weight gain.

Consideration of the data given in Table I shows that all the tested alkanethiols and -dithiols (experiments 4-9). and the alkali metal salt of an alkane thiol (Experiment 10) were 100% effective in preventing the evolution of hydrogen on the steel and zinc test plates. Moreover, they showed the lowest corrosion rates for steel and zinc of all the tested inhibitors and were less corrosive than even distilled water alone (experiment 33).

Almost as effective as the thiols and dithiols and the alkali metal salts of the thiols in reducing hydrogen evolution were ammonium thiosulfate (experiments 16, 17 and 18), sodium thiophosphate (experiments 24, 25 and 26), sodium thiocarbonate (experiments 30, 31 and 32) and sodium urndimethyldithiocarbamate (experiment 27 , 28 and 29). However, inhibition of corrosion was only moderate. The inhibition of hydrogen evolution is sufficient for the safe "storage of ammonium thiosulphate, sodium thiophosphate and sodium thiocarbonate-inhibited preparations in metal jugs, metal tanks and other metal equipment.

Experiment 11 involved a herbicidal preparation containing, in addition to the dodecanethiol inhibitor, the simultaneous presence of oxalic acid in an amount equal to the weight of the isopropylamine salt of N-phosphonomethylglycine. The use of oxalic acid in herbicide preparations containing N-phosphonomethylglycine or its derivatives is discussed in "Research Disclosure" publication number RD15334, published January 1977 by Industrial Opportunities Ltd.,

Homewell-Havant-Hampshire P09 1EF, United Kingdom. According

to this publication, when herbicidal preparations containing N-phosphonomethylglycine or its derivatives are diluted for application purposes with hard water, i.e. water containing calcium and magnesium ions in the range from 100 to 2000 or more parts by weight per million parts by weight of water, the diluted preparations reduced herbicidal activity compared to the same preparations diluted with deionized water. The publication teaches application

sen of oxalic acid in herbicidal preparations diluted with hard water to restore the herbicidal activity and recommends

that the amount of oxalic acid should be at least equal to 50% of calcium

or the magnesium ion to up to 200% or more of such ions present in the hard water used for dilution. The weight ratio of the N-phosphonomethylglycine compound to oxalic acid varies from 1 to 10 parts by weight of the glycine compound per 1-10 parts by weight of oxalic acid. Oxalic acid is known to be corrosive to iron surfaces. As shown by the data in Table I for Experiment 11, the normal corrosive action of oxalic acid on iron surfaces is satisfactorily inhibited when a thiol compound is present in the herbicidal composition.

That the use of thio compounds as inhibitors

for metal corrosion in herbicidal preparations containing an amine salt of N-phosphonomethylglycine does not particularly reduce the post-emergence herbicidal activity of the preparation, is clear from

data set out in Table II on post-emergence killing of common quail using preparations described in Table I, in experiments 1 - 6 and 16 - 18. The test preparations were suitably diluted with water and applied to quail plants produced from vegetative cuttings, in an amount of 187 l/ha. Plants treated with the experimental preparations were placed in a greenhouse and observed and the result recorded 12 days after treatment with the herbicidal preparation.

In order to determine the effect on post-emergence herbicide activity there would be when the amount of thiol inhibitor in a herbicide preparation containing the monoisopropylamine salt of N-phosphonomethylglycine as the active ingredient was increased many times above that required for sufficient inhibition of hydrogen evolution and metal corrosion, two control preparations were prepared, one containing the previously described surfactant "A" and the other surfactant "C" according to the following composition, where all parts are by weight:

The preparations were then diluted with water and sufficient additional surface-active agent was added to the diluted preparations so that the surface-active agent in each case would constitute 1 weight of the diluted preparation. The amount of water used to prepare the diluted preparations was adjusted so that each diluted preparation could be sprayed onto the plants in a common amount of 187 l/ha, although the amount of active ingredient in each diluted preparation was kept at different levels. The amount of thiol inhibitor in each diluted preparation was also adjusted to maintain a constant application of 4.48 kg/ha when the diluted preparation was sprayed at a rate of 187 l/ha. The diluted preparations (with and without inhibitor) were sprayed on 3-week-old sorghum halepense and quack grown in greenhouses, and observations with regard to herbicidal activity made 28 days later are given in Table III.

Although the amount of thiol inhibitor to active ingredient in the preparations described in Table III varied from 164:1; at 0.2 8 kg/ha the amount to as high as 1312:1 at 0.035 kg/ha

amount of active ingredient, only a slight reduction in herbicide effectiveness was observed, and this mainly occurred at a ratio between thiol inhibitor and active ingredient of more than 164:1.

Although the inhibitory effectiveness of various thio compounds is exemplified by the monoisopropylamine salt of N-phosphonomethylglycine in Table I, substantially similar corrosion inhibition can be expected when a thio compound listed is mixed with other salts and esters of N-phosphonomethylglycine such as alkyl. the metal salts disclosed in US Patent No. 3,977,860. Such salts and esters include, but are not limited to

the following:

dipotassium salt a<y>N -phosphonomethylglycine

Tripotassium salt " " monosodium salt of ethyl-N-phosphonomethylglycinate monosodium salt of chloroethyl-N-phosphonomethylglycinate methyl-N-phosphonomethylglycinate

dimethyl-N-phosphonicmethylglycinate

ethyl-N-phosphonomethylglycinate 2-chloroethyl-N-phosphonomethylglycinate

n-propyl-N-phosphonomethylglycinate

n-butyl-N-phosphonomethylglycinate

n-hexyl-N-phosphonomethylglycinate

cyclohexyl-N-phosphonomethylglycinate

n-octyl-N-phosphonomethylglycinate

n-dodecyl-N-phosphonomethylglycinate

n-decyl-N-phosphonomethylglycinate

Representative but not inclusive of the aminophosphonates described in US patent no. 2,556,762, and which in aqueous preparations cause corrosion of metal surfaces are the following compounds: nitrido-di-(acetic acid)-(methylphosphonic acid)

tris-(dimethylammonium)-iminoacetate-N-methylphosphonate tri-sodium iminodiacetate-N-rnethylphosphonate

tetra-(dimethylammonium)-aminoacetate-N,N-bis-methylphosphonate 2,2<1->bis-phosphonomethyliminoacetic acid

dipotassium iminodiacetate-N-methyl-0-potassium-O-ethylphosphonate

Data for hydrogen evolution and corrosion inhibition of dodecanethiol for several agricultural preparations containing as active ingredient N-phosphonomethylglycine or its salt or ester derivative or an aminophosphonate compound are given in Table IV. The preparations used in Table IV are of two types, liquids and dry powders. Tests 34 and 35 were carried out on liquid preparations of the following composition, where all parts are by weight.

Attempt 34

Attempt 35

The dry powder preparations, trials 36 to 47 had the following composition, where all parts are by weight:

Attempt 36

Attempt 37 Attempt 38 Attempt 39

Attempt 40

Attempt 41.

Attempt 42 Attempt 43 Attempt 44 Attempt 45

Attempt 46 Attempt 47

Surfactant "C" is described above.

Surfactant "D", an anionic surfactant, is a complex of sodium dioctyl sulfosuccinate.

Trials 34, 36, 33, 40, 42 and 44 were control trials that did not contain any inhibitor. Experiments 37, 39, 41, 4 3 and 4 5 were dry solid preparations containing a complex of urea and dodecanethiol. Alkanethiols such as dodecanethiol are essentially insoluble in water and although a surfactant is an aid in effecting dispersion of the thiol in an aqueous preparation, it has been shown that a solid complex of urea and a straight chain alkanethiol, when mixed with a dry mixture of the active ingredient and surfactant increases the dispersibility of the thiol in aqueous agricultural preparations and reduces separation of the thiol component. The complex dissolves easily in water during the recovery of urea and thiol with the thiol finely dispersed and remaining in suspension. The urea thiol complex is prepared by dissolving the alkane thiol in a solvent such as isooctane and mixing the solution at room temperature with sufficient urea prewetted with methanol until the urea increases in volume; the reaction is slightly exothermic. The molar ratio of urea to alkanethiol used to form the complex is proportional to the thiol's chain length and is at least 6:1 for alkanethiols containing 6 carbon atoms, 10:1 mol for dodecanethiol and approx. 16:1 for thiols with 16 carbon atoms. The solvents used in this reaction are removed by washing and drying. The corrosion tests indicated in Table IV were all carried out with the liquid as well as the solid type of preparations diluted with water in an amount corresponding to normal application dilutions, namely a

• dilution that would apply 2.24 kg/ha of active ingredient (calc net as N-phosphonomethylglycine or 2,2'-bis-phosphonomethylimino-acetic acid) at a spray quantity per 187 l/ha.

Thiols other than the alkane thiols have also been shown to be effective in inhibiting corrosion of aqueous agricultural preparations as considered here. Such thiols include the aromatic and cycloaliphatic thiols. For example, 2% inhibition amounts of p-chlorothiophenol, 2-aminothiophenol, 2-furanomethanethicl, tholeunthiol, 2-benzoxazolethiol and mercapto-benzoethioazole will enable a dry disodium salt of N-phosphonomethylglycine preparation corresponding to experiment 36, which when diluted with water reduces hydrogen evolution and the metal corrosion rate. Similar corrosion inhibition was obtained when fertilizer diluent components other than urea were incubated in the agricultural preparations, for example mono-ammonium phosphate and di-sodium phosphate.

Claims (20)

1. Agricultural <p> repair, characterized in that it contains an active ingredient selected from the aminomethylenephosphonic acids with the formula: wherein y and z are each 1 or 2, and x is 0 or 1, the sum of x, y and z being 3, and agriculturally acceptable salts and esters thereof, and a metal corrosion inhibiting amount of a thio compound selected from alkanethiols and .-dithiols with from 2 to 16 carbon atoms in the alkane group, aromatic thiols, cyclo aliphatic thiols, alkali metal salts of the aforementioned thiols and dithiols, and ammonium and alkali metal thio salts of polyhydric inorganic acids. 2. Agricultural preparation according to claim 1, characterized in that the alkanethiol is 1-dodecanethiol. 3. Agricultural preparation according to claim 1, characterized in that the alkanethiol is 1-octanethiol. 4. Agricultural preparation according to claim 1, characterized in that the alkanethiol is 1-hexadecylthiol. 5. Agricultural preparation according to claim 1, characterized in that the ammonium thiosalt is ammonium thiosulphate. 6. Agricultural preparation according to claim 1, characterized in that the ammonium thiosalt is ammonium thiophosphate. 7. Agricultural preparation according to claim 1, characterized in that the amount of the thio compound is between 0.15 and 3% by weight of the active ingredient. 8. Agricultural preparation according to claim 1, characterized in that it contains 1-10 parts by weight of oxalic acid per 1-10 parts by weight of an amine salt of N-phosphonomethylglycine. 9. Dry agricultural preparation according to claim 1, characterized in that the thiol is a straight-chain alkane thiol and complexed with urea. 10. Agricultural preparation according to claim 1, characterized in that the active ingredient is N-phosphonomethylglycine. 11. Agricultural preparation according to claim 1, characterized in that the active ingredient is 2,2 <1-> bis-phosphono-methyliminoacetic acid. 12. Agricultural preparation according to claim 1, characterized in that the active ingredient is a salt of N-phosphonomethylglycine. 13. Agricultural preparation according to claim 12, characterized in that the salt is the mono-isopropylamine salt of N-phosphonomethylglycine. 14; Process for inhibiting 'corrosion of iron and zinc metal surfaces in contact with an aqueous agricultural preparation containing water, a surfactant and an active ingredient selected from the aminomethylenephosphonic acids of the formula: where y and z are each 1 or 2, and z is 0 or 1, the sum of x, y and z being 3, and agriculturally acceptable salts and esters thereof, characterized in that an inhibitory amount of a thio compound selected from alkanethiols is added to the preparation and -dithiols with 2 to 16 carbon atoms in the alkane group, aromatic thiols, cycloaliphatic thiols, the alkali metal salts of the aforementioned thiols and dithiols, and ammonium and alkali metal thio salts of polybasic inorganic acids. .15. Method according to claim 14, characterized in that the alkanethiol is 1-dodecanethiol. 16. Method according to claim 14, characterized in that the alkanethiol is 1-octanethiol. 17. Process according to claim 14, characterized in that the alkanethiol is 1-hexadecylthiol. 18. Method according to claim 14, characterized in that the thio compound is ammonium thiosulphate. 19. Method according to claim 14, characterized in that "the thio compound constitutes 0.15 - 3% by weight of amino- the methylene phosphoric acid, or the salts or esters thereof. 20. Method according to claim 14, characterized in that the thiol is an alkanethiol complexed with urea and the urea-thiol complex is mixed with a dry mixture of the active ingredient and the surfactant before it is dispersed. perfused in water.