Naturally derived phosphoric acids as acidic corrosion inhibitors

Description

BACKGROUND OF THE INVENTION

[0001] Cooling lubricants used in metalworking and related manufacturing processes, in the form of complex oil- or water-based emulsions or solutions, typically contain numerous additives. Their classification (e.g., as anti-wear, anti-foaming, antimicrobial, and corrosion inhibitors) is regulated by DIN 51385. Phosphonic acids, in particular, are used as "acidic corrosion inhibitors," either as free acids (at low pH values) or (neutralized) as salts. Phosphonic acids have a high affinity for various metals and are capable of modifying surfaces in self-assembled monolayers.

[0002] The application of state-of-the-art phosphonic acids as acidic corrosion inhibitors is largely limited to the processing of stainless steel or iron, and, in some cases, aluminum. The processing of magnesium, titanium, beryllium, and zirconium, the formulation of suitable cooling lubricants, and the provision of effective corrosion-inhibiting phosphonic acids present particular challenges. For example, magnesium exhibits significantly higher reactivity in aqueous media compared to steel and aluminum, and especially at low pH values, it reacts with the release of hydrogen. Finally, even small changes in the alloy composition during machining can have a dramatic effect on its sensitivity and reactivity.

[0003] The most frequently described acidic corrosion inhibitorsThe phosphonic acids used are divided into two classes: alkylphosphonic acids and aminophosphonic acids.

[0004] Alkylphosphonic acids (left) and aminophosphonic acids (right) which are used industrially as acidic corrosion inhibitors.

[0005] Alkylphosphonic acids can be synthesized from haloalkanes, for example, via the Michaels-Arbuzov reaction. The starting materials required for this synthesis are derived from petroleum. Aminophosphonic acids are typically synthesized via a Phospha-Mannich reaction using the amine, formaldehyde, and a phosphorylating reagent. However, in addition to the necessity of using formaldehyde in the synthesis, this class of substances also presents other disadvantages. α-Aminophosphonic acids are highly susceptible to oxidative degradation. Specifically, when combined with oxidative, halogen-containing biocides, this leads to the degradation of the inhibitor, forming phosphates that can, in turn, form sparingly soluble calcium complexes, as well as the release of formaldehyde.

[0006] Each of the documents WO 2008 / 089857 A1, WO 01 / 42532 A2 and DE 10 2012 204683 A1 discloses a corrosion-inhibiting composition which is water-based and / or oil-based and contains an alkylphosphonic acid with 4-14 C atoms in the alkyl group, in particular alkylphosphonic acids with 6 to 12 C atoms, as well as an amine.

[0007] This creates a need for cost-effective yet effective corrosion inhibitors that can be produced from renewable raw materials using a simple, toxicologically safe, and industrially applicable process. Particularly in the processing and machining of the commonly used metals iron and aluminum—but also for the more demanding magnesium—and their alloys, these corrosion inhibitors must be effective even at low concentrations. Finally, the acidic corrosion inhibitors themselves should also be environmentally safe.

[0008] These problems are solved by the phosphonic acids according to the invention, which can be used as highly effective corrosion inhibitors – particularly for steel, for aluminum alloys (especially Al 5083 and Al 2024), as well as for magnesium (e.g., Mg AZ31), titanium, beryllium, and zirconium. The synthesis of the phosphonic acids according to the invention is carried out using sustainable processes starting from readily available, renewable raw materials. Detailed description of the invention

[0009] The present application relates to a corrosion-inhibiting composition according to claim 1. Furthermore, the present application relates to the use of a phosphonic acid of formula I according to claim 5.

[0010] The phosphonic acids according to the invention contain as an organic function a terpene residue (in particular a residue derived from a mono- or sesquiterpene) or a fatty acid residue (such as 10-undecylenic acid obtainable from castor oil via pyrolysis and vacuum distillation), and they are obtained by reaction of corresponding terpenes or fatty acids which have functionalities suitable for the synthesis of phosphonic acids (e.g. carbon-carbon double bonds or hydroxy groups). where R = terpene residue or fatty acid residue.

[0011] The present invention relates in particular to the use of the phosphonic acids of the formulas as acidic corrosion inhibitors and as a component of corrosion-inhibiting compositions.

[0012] In addition to these phosphonic acids, all such phosphonic acids that are obtainable from terpenes or fatty acids are suitable according to the invention. For the preparation of the phosphonic acids of the present invention, terpenes (in particular mono- or sesquiterpenes) and terpenoids, such as citronellol (stereoisomerically pure and as a stereoisomeric mixture), valencene (stereoisomerically pure and as a stereoisomeric mixture), limonene (stereoisomerically pure and as a stereoisomeric mixture), geraniol, farnesol, α-pinene (stereoisomerically pure and as a stereoisomeric mixture), β-pinene (stereoisomerically pure and as a stereoisomeric mixture), myrcene, linalool (stereoisomerically pure and as a stereoisomeric mixture) and camphene (stereoisomerically pure and as a stereoisomeric mixture), which are readily and extensively available from renewable sources, can be used, wherein β-pinene (stereoisomerically pure and as a stereoisomeric mixture), citronellol (stereoisomerically pure and as a stereoisomeric mixture),Valencene (stereoisomerically pure and as a stereoisomer mixture), limonene (stereoisomerically pure and as a stereoisomer mixture), and geraniol, preferably geraniol, 1-menthene, citronellol, pinene, or limonene, are used. Suitable fatty acids include monounsaturated fatty acids such as oleic acid, but especially undecylenic acid, palmitoleic acid, elaidic acid, and erucic acid, as well as polyunsaturated fatty acids such as linoleic acid, linolenic acid, calendic acid, stearidonic acid, and arachidonic acid, and substituted derivatives of unsaturated fatty acids such as ricinoleic acid or pristanic acid, wherein undecylenic, linoleic, and ricinoleic acids are preferred according to the invention. The phosphonic acids of the structures , PA1-PA8 are preferred according to the invention.

[0013] Within the scope of the present inventions, chemically stable and low-volatility phosphonic acids, which exhibit high surface activity due to their amphiphilic character, could be obtained by using such renewable and cost-effective starting compounds (terpenes, terpenoids and fatty acids) through a simple one- or two-step synthesis.

[0014] Terpenes, terpenoids, and fatty acids possessing a terminal carbon-carbon double bond or an allylic hydroxyl group can be converted to the phosphonic acids according to the invention via a metal-catalyzed hydrophosphorylation (K. Bravo-Altamirano, JL Montchamp, Tetrahedron Lett 2007, 48, 5755-5759). The Pd-catalyzed hydrophosphinylation initially yields a phosphinic acid, which can then be oxidized to the phosphonic acid. Suitable oxidizing agents include atmospheric oxygen, iodine, or a hydrogen peroxide solution. Further details of this strategy are exemplified in Synthesis Examples a and b.

[0015] Alternatively, radical variants can also be used to synthesize phosphonic acids (DM Bartley, JK Coward, The Journal of Organic Chemistry 2005, 70, 6757-6774 and H. Francois, R. Lalande, Comptes rendus de l'Académie des sciences 1974, 279, 117-119). Under these conditions, depending on the structure of the starting material, ring openings, eliminations, and rearrangements can occur—such as the ring opening of β-pinene. 1, which lead to a phosphonic acid PA3 This leads to the formation of products whose terpene residue has the structure of 1-menthene. Such products are included in the present invention. Further details of this strategy are exemplified in synthesis example c).

[0016] Even functionalized fatty acids, such as ricinoleic acid, can produce products that seem unexpected at first glance, such as the product mixture. PA8 as shown in example d). The intermediate phosphonic acid ester and the final compound PA8These are obtained as a mixture of 8 different regio- and stereoisomers, of which only one is shown here and which is referred to here in its entirety as "PA8". The phosphonic acid PA8 The compound is obtained as a product of an elimination and radical addition reaction, which may also include a rearrangement, and can therefore alternatively be prepared from linoleic acid and conjugated linoleic acid derivatives using the same protocol. PA8 This thus refers to the mixture of regio- and stereoisomers obtainable from the reaction of, for example, linoleic acid or ricinoleic acid with an activator (such as ditert-butyl peroxide) in the presence of a short-chain dialkyl phosphite (e.g., diethyl phosphite).

[0017] This is known as "phosphonic acid" PA8"The product mixture described above already constitutes an acidic corrosion inhibitor according to the invention, i.e., without separation or isolation of the components. Of course, any component of this product mixture can also be used as a corrosion inhibitor.

[0018] Aliphatic alcohols, such as citronellol, can ultimately be converted into the corresponding phosphonate via bromination and subsequent Michaelis-Arbuzov reaction. 5 be converted. The unsaturated phosphonate 5 can also be a saturated derivative 6 be hydrogenated, and both phosphonates - both the unsaturated phosphonate 5 as well as the saturated derivative 6 - can be obtained by reaction with bromotrimethylsilane to form the free acid phosphonic acid ( PA5 or PA4 ) implement. Further details of this strategy are exemplified in synthesis example d).

[0019] The acidic corrosion inhibitors according to the invention can be formulated in the form of water- or oil-based lubricant compositions, wherein they are neutralized for the formulation of water-based compositions by a polar base, e.g. an amine with short and polar alkyl groups, in particular triethanolamine, and for the formulation of oil-based compositions by a nonpolar base, e.g. an amine with larger alkyl groups, e.g. oleylamine.

[0020] The acidic corrosion inhibitors and the lubricant compositions derived therefrom according to the present invention can be used successfully and extremely advantageously as lubricants, particularly as cooling lubricants, in the processing and machining of metals, passivating the metal surfaces and protecting them from thermal and oxidative attack. The phosphonic acids according to the invention, comprising a terpene residue (in particular one derived from a mono- or sesquiterpene) or a fatty acid residue, are not only capable of protecting the surfaces of iron and steel alloys from corrosion. Surprisingly, they are also suitable for use in the processing and machining of light metals, such as aluminum and magnesium and their alloys (such as Al 2024, Al 5084, and Mg AZ31). Examples: Synthesis of the required phosphonic acids Processes A and B: Palladium-catalyzed hydrophosphonylation Procedure A:

[0021] Procedure B:

[0022]

[0023] Pd₂DBA₃ (75%, 0.01 eq., 0.1 mmol, 122 mg) and Xantphos (0.022 eq., 0.22 mmol, 127 mg) were dissolved in 10 mL of dry DMF under argon as a protective gas. Hypophosphoric acid (50% in H₂O, 2 eq., 20 mmol, 2.35 mL) and the olefin, or the allyl alcohol (1 eq., 10 mmol), were added. The reaction solution was heated to 80°C under argon for 16 h and then to 110°C in air for 24 to 48 h with vigorous stirring. The solution was filtered and the solvent removed under reduced pressure. The residue was dissolved in 50 mL of 2 M aqueous HCl and 50 mL of EtOAc. The phases were separated, and the aqueous phase was extracted twice with 50 mL of EtOAc each time. The combined organic phases were washed with 50 mL of saturated NaCl solution and dried over Na₂SO₄. Removal of the solvent yielded the crude product, which was optionally oxidized or purified by reversed-phase chromatography. Example a): (E)-(3,7-Dimethylocta-2,6-dien-1-yl)phosphonic acid

[0024]

[0025] 1 eq., 10 mmol, 1.75 mL) was reacted according to method B. (chromatographic separation: H₂O:MeCN 95:5 v / v → 5:95 v / v, 0.1% formic acid). After freeze-drying, 1.29 g (6 mmol, 60%) of the product was obtained as a colorless solid. Example b): 10-Phosphonoundecylenic acid (PUDA)

[0026]

[0027] 10-Undecylenic acid (3.69 g, 20.0 mmol, 1.0 eq) was reacted according to process A. After recrystallization from acetone and freeze-drying, 5.26 g (19.7 mmol, 99%) of the product were obtained as a colorless solid. Method C: Radical variant for the synthesis of phosphonic acids.

[0028] Example c): ((4-Isopropylcyclohex-1-en-1-yl)methyl)phosphonic acid

[0029]

[0030] β-Pinene (1 eq., 20 mmol, 3.13 mL) was dissolved in 140 mL of dry methanol. NH₄H₂PO₂ (2.5 eq., 50 mmol, 4.2 g) and triethylborane (1 M in THF, 2.1 eq., 21 mL) were added. The reaction solution was stirred vigorously in air for 4 h. All volatile components were removed under reduced pressure, and the residue was dissolved in 50 mL of EtOAc and 50 mL of 2 M HCl. The aqueous phase was extracted twice with 50 mL of EtOAc each time. The combined organic phases were dried over Na₂SO₄, filtered, and concentrated to dryness. The residue was dissolved in 50 mL of THF, treated with DMSO (1.1 eq., 23 mmol, 1.6 mL) and I₂ (0.01 eq., 0.2 mmol, 48 mg), and heated to 60 °C for 3 h. (H₂O:MeCN 95:5 v / v → 5:95 v / v, 0.1% formic acid). After freeze-drying, 2.92 g (13.4 mmol, 60%) of the product were obtained as a colorless solid. Example d): 9-Phosphonooctadec-9-enoic acid

[0031]

[0032] The reaction was carried out under argon as a protective gas. 3.01 g (10.08 mmol, 1 eq.) of ricinoleic acid was dissolved in 6.50 mL (50.39 mmol, 5 eq.) of diethyl phosphite. The reaction solution was degassed with argon for 15 min. 190 µL (1 mmol, 0.1 eq.) of ditert-butyl peroxide was added. The reaction solution was heated to 125 °C for 24 h. After 2 h and 17 h of reaction time, further 190 µL of ditert-butyl peroxide was added. The solvent was removed under reduced pressure, the residue was dissolved in 150 mL of diethyl ether, and washed three times with 100 mL of saturated sodium bicarbonate solution. 3.95 g (9.4 mmol, 94%) of the intermediate ester were obtained as a yellow oil. The ester is obtained as a mixture of eight different regio- and stereoisomers. 1< H NMR (400 MHz, CDCl 3 ) δ = 4.32 - 3.95 (m, 5H, 10-H, 19-H), 2.19 (t, J = 7.5 Hz, 2 H, 2-H), 2.06 - 1.12 (m, 34 H, 20-H, CH 2 ), 0.80 (t, 3< J= 6.9 Hz, 3H, 18-H). 31<P NMR (150 MHz, CDCl 3 ) δ = 50.5 (s), 48.8 (s), 47.9 (s), 46.9 (s), 29.8 (s), 28.8 (s), 27.9 (s), 27.4 (s).

[0033] 1.00 g (2.39 mmol) of the phosphonic acid ester thus obtained were dissolved in 75 mL of dioxane. 75 mL of 12 M HCl (aq.) were added. The reaction solution was heated to 100 °C for 48 h. The solvent was removed under reduced pressure, and the residue was dissolved in 100 mL of 2 M aqueous sodium hydroxide solution. The aqueous phase was washed three times with 100 mL of ethyl acetate each time and then adjusted to pH 1 with hydrochloric acid. The aqueous phase was extracted three times with 100 mL of diethyl ether each time. The combined organic phases were dried over Na₂SO₄, filtered, and the solvent was removed under reduced pressure. 650 mg (1.79 mmol, 75%) of the product were obtained. PA8 It is obtained as a yellow resin. Phosphonic acid is obtained as a mixture of eight different regio- and stereoisomers.

[0034] 1< H NMR (400 MHz, CDCl 3 ) δ = 9.42 (s br 3 H, -OH), 4.42 - 3.76 (m, 1 H, 10-H), 2.20 - 1.18 (m, 26 H, CH 2 ), 0.88 (t, 3< J = 6.0 Hz, 3 H, 18 H).

[0035] 31< P NMR (150 MHz, CDCl 3 ) δ = 52.1 (s br ), 50.8 (s br ), 32.4 (s br ), 32.1 (s br ). Procedure D: Michaelis-Arbuzov Variant for the synthesis of phosphonic acids.

[0036] Beispiel e): 8-Bromo-2,6-dimethyloct-2-ene (4)

[0037]

[0038] Citronellol 3 (1 eq., 2.0 mL, 11 mmol) and tetrabromomethane (1.1 eq., 4.0 g, 12 mmol) were dissolved in 2 mL of CH₂Cl₂. Triphenylphosphine (1.2 eq., 3.5 g, 13 mmol) dissolved in 2 mL of CH₂Cl₂ was added at 0 °C. The reaction solution was stirred for 5 h at room temperature. The solvent was removed under reduced pressure. After column chromatographic purification on silica gel (pentane), 2.03 g (9.3 mmol, 85%) of the product were obtained as a colorless oil. Diethyl (3,7-dimethyloct-6-en-1-yl)phosphonate (5)

[0039]

[0040] bromide 4 (1 eq., 2.03 g, 9.3 mmol) was dissolved in triethyl phosphite (6.2 eq, 10 mL, 58 mmol). The reaction solution was heated to 150 °C for 12 h. The solvent was removed under reduced pressure. After column chromatographic purification on silica gel (pentane:EtOAc 1:1 V / V → EtOAc), 1.06 g (3.8 mmol, 41%) of the product was obtained as a colorless oil. Diethyl (3,7-dimethyloctyl)phosphonate (6)

[0041]

[0042] Phosphonate 5 (1 eq., 400 mg, 1.45 mmol) was dissolved in 5 mL of methanol and treated with Pd / C (10%, 0.1 eq, 555 mg, 145 µmol). The reaction solution was stirred for 16 h under a hydrogen atmosphere and filtered through Celite®. After removal of the solvent under reduced pressure, 406 mg (1.45 mmol, quant.) of the product were obtained as a colorless oil. (3,7-Dimethyloctyl)phosphonic acid PA5

[0043]

[0044] Aliphatic phosphonate 6(1 eq., 406 mg, 1.45 mmol) was dissolved in 5 mL of CH₂Cl₂ under argon as a protective gas. Bromotrimethylsilane (4 eq., 465 µL, 5.8 mmol) was added dropwise. The reaction solution was stirred for 30 min at room temperature. All volatile components were removed under reduced pressure. 20 mL of a mixture of water and THF (1:1 v / v) was added. The reaction solution was stirred for 2 h at room temperature. The organic solvent was removed under reduced pressure. After freeze-drying, 301 mg (1.36 mmol, 94%) of the product was obtained as a colorless solid. (3,7-Dimethyloct-6-en-1-yl)phosphonic acid PA4

[0045]

[0046] Phosphonate 5(1 eq., 400 mg, 1.45 mmol) was dissolved in 5 mL of CH₂Cl₂ under argon as a protective gas. Bromotrimethylsilane (4 eq., 465 µL, 5.8 mmol) was added dropwise. The reaction solution was stirred for 30 min at room temperature. All volatile components were removed under reduced pressure. 20 mL of a mixture of water and THF (1:1 v / v) were added. The reaction solution was stirred for 2 h at room temperature. The organic solvent was removed under reduced pressure. The crude product was freeze-dried and dissolved (H₂O:MeCN 95:5 v / v → 5:95 v / v, 0.1% formic acid). After further freeze-drying, 296 mg (1.34 mmol, 92%) of the product were obtained as a colorless solid. Gray cast iron chip tests

[0047] The corrosion-inhibiting properties of the substances for iron were evaluated according to DIN 51360-02-A using aqueous, base-neutralized solutions of the acidic corrosion inhibitors. For this purpose, 2 mL of the solutions at the specified concentration (based on the acid) were incubated with 2.0 g of grey cast iron chips for 2 h.

[0048] A defined quantity of gray cast iron chips was placed on a circular filter and mixed with an aqueous test solution of acidic corrosion inhibitors neutralized with bases. The corrosion patterns on the filter paper were then evaluated and assigned a corrosion grade. Corrosion grades ranged from 0 for complete corrosion inhibition (no discoloration of the filter paper) to 4 for no inhibition (strong discoloration of the filter paper). The established coolant additive system TC®< 85 ("TC") in combination with triethanolamine ("TEA") was evaluated as a comparison system. The results of the corrosion tests are summarized in Table 1. Table 1: Results of the grey cast iron chip tests according to DIN 51360-2. Evaluation after 2 hours of incubation at room temperature (base = TEA) acid Acid [% wt.] 1< Base Eq. 2< Corrosion score TC 3 4.5 0 TC 2 4.5 0 PA1 3 3.6 0 PA1 3 4.5 0 PA1 3 3 0 PA4 3 2.4 0 PA4 2 2.4 0 PA4 1 2.4 0 PA5 3 2.4 0 PA5 2 2.4 0 PA5 1 2.4 0 PA6 3 2.4 0 PA6 2 2.4 0 PA6 1 2.4 0 PA8 3 4.5 0 PA8 2 4.5 0 PA8 1 4.5 0 1. Concentration of the acid in weight percent based on solvent. 2. Equivalents based on acid.

[0049] All tested derivatives consistently demonstrate convincing efficacy as acidic corrosion inhibitors for use on iron. The citronellol derivatives, in particular, show... PA5 and PA4 as well as the pine derivative PA6 Even at a mass fraction of 1% and only 2.4 equivalents of base, the pinene derivative exhibits almost complete or complete inhibition. Even at a mass concentration of only 0.5%, the pinene derivative shows [the same effect]. PA6 additional corrosion protection. Light Metal Staining Tests

[0050] The evaluation of the corrosion-inhibiting properties of the phosphonic acids according to the invention for light metals was carried out using aqueous, base-neutralized solutions of the acidic corrosion inhibitors prepared in hard water, following the standard used for gray cast iron chip tests. Aluminum or magnesium plates measuring 1 x 3 cm were mechanically cleaned with pentane and air-dried, then immediately placed in a closed container with 2.5 mL of the solution at the specified concentration (based on the acid), so that the plate was approximately half-submerged in the liquid. The mixture was incubated for 24 h at 40 °C and subsequently rinsed with water and acetone. The optical assessment of the corrosion was carried out following a method developed by Watanabe. et al.The scale used for corrosion grades (S. Watanabe, J. Oleo Sci. 2008, 57, 1-10) ranges from 0 for complete corrosion inhibition (no discoloration of the metal plate) to 4 for no inhibition (strong discoloration of the metal plate).

[0051] The tests were performed using the aluminum alloys Al 2024 (4% Cu) and Al 5083 (4% Mg), as well as magnesium alloy AZ31 (3% Al, 1% Zn). The results of the staining tests are shown in Table 2. Table 2a: Results of the light metal staining tests on Al 2024. Evaluation after 24 h incubation at 40 °C (Base = TEA). acid Acid [% wt.] 1< Base Eq. 2< degree TC 4 4.5 2 TC 3 4.5 3 TC 2 4.5 3 TC 1 4.5 4 GRANDA 4 3.0 0 GRANDA 3 3.0 0 GRANDA 2 3.0 0 GRANDA 1 3.0 4 PA1 4 4.5 0 PA1 3 4.5 0 PA1 2 4.5 0 PA3 3 3.0 0 PA3 2 3.0 0 PA3 1 3.0 0 PA8 3 4.5 0 PA8 2 4.5 0 PA8 1 4.5 0 Tab. 2b: Results of the light metal staining tests on Al 5083. Evaluation after 24 h incubation at 40 °C (Base = TEA). acid Acid [% wt.] 1< Base Eq. 2< degree TC 4 4.5 3 TC 3 4.5 3 TC 2 4.5 3 TC 1 4.5 3 GRANDA 4 3.0 0 GRANDA 3 3.0 0 GRANDA 2 3.0 0 GRANDA 1 3.0 2 PA1 4 4.5 0 PA1 3 4.5 0 PA3 3 3.0 0 PA3 2 3.0 0 PA3 1 3.0 0 PA8 3 4.5 0 PA8 2 4.5 0 PA8 1 4.5 0 Tab. 2c: Results of the light metal staining tests on AZ 31. Evaluation after 24 h incubation at 40 °C (Base = TEA). acid Acid [% wt.] 1< Base Eq. 2< degree TC 4 4.5 4 TC 3 4.5 4 TC 2 4.5 4 TC 1 4.5 4 GRANDA 4 3.0 2 GRANDA 3 3.0 2 GRANDA 2 3.0 2 PA1 4 4.5 3 PA1 3 4.5 3 PA3 3 3.0 3 PA3 2 3.0 3 PA2 2 3.0 2 1. Concentration of the acid in weight percent based on solvent. 2. Equivalents based on acid.

[0052] In addition to the synthesized phosphonic acids, comparative tests were carried out with the commercially available octylphosphonic acid (OPA) and TC®< 85, which is also used in the gray cast iron chip test. All tested phosphonic acids yielded surprisingly good results even at low concentrations. For aluminum Al 5083, generally good results were achieved at a concentration of 3%. The phosphonic acid according to the invention PA3 Remarkably, it shows complete inhibition for both aluminum alloys at all tested concentrations.

[0053] Even with magnesium, AZ31, a reactive and very corrosion-sensitive alloy, a surprisingly large effect is observed when using phosphonic acids according to the invention.

Claims

1. A corrosion-inhibiting composition comprising a phosphonic acid of formula I where R = a terpene residue or a fatty acid residue and an amine, which is preferably used in an equimolar amount relative to the phosphonic acid; as the terpene radical, radicals derived from a monoterpene or sesquiterpene are preferred.

2. A corrosion-inhibiting composition according to claim 1, wherein the phosphonic acid of formula I has the structure of formulae PA1 (fatty acid residue = 10-undecylenic acid), PA2 (terpene residue = geraniol), PA3 (terpene residue = 1-menthene), PA4 (terpene residue = citronellol), PA5 (terpene residue = aliphatic citronellol), PA6 (terpene residue = pinene), PA7 (terpene residue = limonene) or PA8 (fatty acid residue = ricinoleic or linoleic acid), wherein the compound PA8 denotes the mixture of regio- and stereoisomers obtainable from the reaction of ricinoleic acid with ditert-butyl peroxide in the presence of diethyl phosphite.

3. A corrosion-inhibiting composition according to claim 1 or 2, wherein the composition is water-based.

4. A corrosion-inhibiting composition according to claim 1-3, wherein the composition is oil-based.

5. Use of a phosphonic acid of formula I where R = a terpene or fatty acid residue as an acidic corrosion inhibitor for metals, wherein the terpene residue is preferably derived from a mono- or sesquiterpene.

6. Use according to claim 5, wherein the phosphonic acid of formula I has the structure of formulae PA1 (fatty acid residue = 10-undecylene acid), PA2 (terpene residue = geraniol), PA3 (terpene residue = 1-menthene), PA4 (terpene residue = citronellol), PA5 (terpene residue = aliphatic citronellol), PA6 (terpene residue = pinene), PA7 (terpene residue = limonene) or PA8 (fatty acid residue = ricinoleic or linoleic acid).

7. Use according to claim 5 or 6, wherein the acidic corrosion inhibitor is used within a water- or oil-based cooling lubricant or coolant during the machining or processing of metals.

8. Use according to claims 5-7, wherein the metal comprises iron, aluminium, magnesium, titanium, beryllium and / or zirconium.

9. Use according to claim 8, wherein the metal is in the form of an alloy; preferably in the form of the alloy Al 5083, Al 2024 or Mg AZ31.