Description:
This invention relates to the inhibition of the corrosion of steel by acids, and more particularly to the use of certain organic nitrogen compounds as corrosion inhibitors.
Steel is widely used as material of construction, particularly for chemical plant, and is consequently subjected to attack by a wide variety of corrosive substances, including organic and inorganic acids. Such corrosion presents serious problems, for not only is the structure itself attacked by contact with corrosive substances, thereby making necessary more frequent maintenance and replacement of expensive equipment than would otherwise be the case, but the products made in, for example the chemical plant are contaminated by material resulting from such corrosive attack.
We have found that the corrosion of steel by organic and inorganic acids is inhibited by the addition to the acid of an effective amount of an organic nitrogen compound containing at least one unsaturated carbon-nitrogen bond, or by pre-treatment of the steel with the said organic nitrogen compound, or by a combination of both methods, provided that the organic nitrogen compound is other than a cyclic vicinal dinitrile.
As examples of organic compounds containing at least one unsaturated carbon-nitrogen bond which may be used as corrosion inhibitors according to the present invention there may be mentioned those classes of compounds containing one or more nitrile, isonitrile, oxime, hydrazone, amidine, amidoxime, semicarbazone, hydrazone or imine group, the latter class including groups which form part of a heterocyclic ring system as in, for example, oxazoles, oxadiazoles, furazans, furoxans, iminazoles, pyrazoles, pyrazolones, pyrazolines and triazoles.
Suitable compounds include acrylonitrile and substituted acrylonitriles, for example, crotononitrile, cinnamonitrile, p-methoxycinnamonitrile and 1-cyano-4-phenylbuta-1,3-diene; benzonitriles and naphthonitriles carrying non-ionised electron-donating groups, for example, hydroxy, alkoxy, alkyl, halogen, trihalogenomethyl, thiol and thioether groups, specific examples of which include p-chloro-benzonitrile, p-methoxybenzonitrile, p-dodecylbenzonitrile, p-trichloromethylbenzonitrile, m-tolunitrile and 1-cyano-4-methoxy-naphthalene; other substituted benzonitriles, including p-nitro-benzonitrile, p-dimethylaminobenzonitrile, and p-aminobenzontrile; N,N,N',N'-tetrakis-(cyanomethyl)hexamethylenediamine, N,N'-bis(α-cyano-benzyl)ethylenediamine, diphenylmethane nitriles, for example, 3,3'-dicyanodiphenylmethane, diphenylnitriles, for example, 4,4'-dicyanodiphenyl, naphthylnitriles, for example, 1-cyanophthalene; o- and p-cyanobenzaldehyde, 2-pentenenitrile, cyanoacetic acid, cyanovaleric acid, cyanocaproic acid, 2-, 3- and 4-cyanopyridines, alkyl nitriles, for example, octadecanonitrile, succinonitrile, adiponitrile azelaonitrile, sebaconitrile, decane-1,10 -dicarbonitrile, dodecane dinitrile, 1,4-dicyanobutene-2, 1,4-bis(-cyanoethoxy)-butyne-2, fumaronitrile, maleonitrile, phenylmaleonitrile, acetonitrile and substituted acetonitriles, for example, benzyl cyanide, diphenylacetonitrile and triphenylacetonitrile; 2-cyanocyclopentylideneimine, dimethyl glyoxime, cyclohexanoneoxime, 1,2- and 1,4-cyclohexane diketoxime, acetoxime, benzalacetoxime; dibenzalacetoxime, benzamideoxime, o- and p-benzoquinone monoximes and dioximes, acetophenoneoxime, benzophenoneoxime, benzaldoxime, cinnamaldoxime, guanidine salts, for example, guanidine carbonate, pyridine, 2,2'-bipyridyl, 4,4-bipyridyl, quinoline, isoquinoline, 1-cyanoisoquinoline, benzofurazan, cyclohexylfurazan, benzofuroxan, cyclohexylfuroxan, pyridine-N-oxide and substituted pyridine-N-oxides, benzoxazone, benziminazole and metal-free phthalocyanines.
Polymers and iron-complexing agents such as bipyridyl, ethlyene diamine diacetic acid, phenanthrolines, quinolines and isoquinolines containing unsaturated carbon-nitrogen groupings as defined above are also satisfactory. Mixtures of inhibitors according to the present invention may also be used for wider effects, as synergists or as compositions having a wide vapour pressure range. The compounds are also effective when used together with copper cyanide.
A preferred class of compounds, particularly effective in protecting mild steels as well as stainless steels against acid attack, consists of conjugated unsaturated nitriles. As examples of such compounds there may be mentioned 1-cyano-4-pyrimidin-2'-ylbuta-1,3-diene, 1-cyanobut-1-ene-3-yne, 1-cyanohexa-1,5-diene-3-yne and 6-cyanohexa-1,3,5-triene-1-yl dimethyl phenyl ammonium salts, for example, the chloride, bromide, iodide, methosulphate or ethosulphate, but preferred compounds in this class are cinnamonitrile, p-methoxy-cinnamonitrile, crotononitrile and 1-cyano-4-phenylbuta-1,3-diene.
A second preferred class of compounds, also particularly effective in inhibiting acid attack on mild steel, consists of conjugated unsaturated oximes, of which preferred compounds are cinnamaldoxime, benzalacetoxime and dibenzalacetoxime.
As examples of organic acids against the corrosive attack of which the above compounds are effective there may be mentioned monocarboxylic acids such as formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, stearic acid, monochloro-, dichloro- and trichloroacetic acids and the corresponding bromoacetic acids, lactic acid, dicarboxylic acids such as oxalic acid, succinic acid, gluratic acid, adipic acid, pimelic acid, terephthalic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic and dodecanedioic acid, tartaric acid and tricarboxylic acids such as citric acid and trimesic acid.
The method of the present invention is also effective against attack by organic sulphonic acids.
As exaples of inorganic acids, the corrosive effects on steel of which are minimised according to the invention, there may be mentioned aqueous sulphurous acid, sulphuric acid, aqueous solutions of hydrogen halides, e.g., hydrochloric acid, sulphamic acid and nitric acid.
The method is also effective against the corrosion of steels by derivatives of acids which may undergo chemical reaction with consequent liberation of the free acids, for example, anhydrides, esters and amides in the case of carboxlic acids and is applicable both to the anhydrous acids and aqueous solutions thereof.
The corrosion-inhibiting compounds used according to the present invention are effective at temperatures from ambient temperature or below up to 240°C or even higher.
By `an effective amount` we mean an amount of the organic nitrogen compound which is effective in inhibiting corrosion under the prevailing conditions.
In the case where the inhibitor is added to the organic or inorganic acid an amount of inhibitor from a trace up to 10 percent by weight, more usually up to 5 percent by weight, based on the weight of acid, is generally effective.
Pre-treatment of steel with the inhibitor compound may be carried out using the compound neat, as vapour or as a solution in a suitable solvent, for example water, alcohols, acids, bases such as pyridine and quinoline, and nitriles and at temperatures up to 300°C.
The inhibitor compounds may also be incorporated in paints, primers, and wax or polish compositions intended for application to steel and in impregnated interlining and packing materials such as paper intended for wrapping and protecting steel. The inhibitor compounds may also be used as corrosion inhibitors in acid pickling baths which are employed in removing oxide scale from ferrous metals, and these uses form further features of the invention.
A wide variety of steels may be protected against corrosion using the methods described above, for example, stainless steels of American Iron and Steel Institute Types 304, 316 and 321 (the compositions of which are defined in, for example, British Standard No. 1449, part 4, page 20) and mild steels. The corrosion-inhibiting organic nitrogen compounds hereinbefore described are also effective in protective steels from attack by high performance (jet) lubricants. The invention is illustrated but not limited by the following Examples.
EXAMPLE 1- 27
A weighed rectangular prism of steel, 25- 35 g. in weight, stamped and drilled with an 0.375 inch hole, was placed in a 8 × 1 inch glass tube (fitted with air and water reflux condensers) together with adipic acid (30 g), inhibitor (1g. of a solid or 1 ml. of a liquid) and distilled water (1 ml.). The mixture was heated in a Wood's Metal bath at 240° ± 10°C for 50 hours and the sample was removed, washed, dried, weighed and its appearance noted. Some samples were given a preliminary treatment by heating for 1 hour with a 2 percent w/v solution of inhibitor in an appropriate solvent at the boil and then allowed to cool during 24 hours before testing.
Results are given in the following table.
Example Type of Steel INHIBITOR Weight REMARKS loss ppm __________________________________________________________________________ Stainless Type 321 None (Control) 10,165 Severe general corrosion 1. do. Adiponitrile 38 Undrilled sample. Bright metal surface unchanged on exposure. Slight pitting on edges 2. do. succinonitrile 160 surface slightly dulled on exposure 3. do. 2-cyano cyclopentyl- 32 bright surface unchanged ideneimine 4. do. cyclohexane 1,2-dione 64 Bright surface unchanged dioxime 5. do. cyclohexanone oxime 11 Bright surface unchanged 6. do. o benzoquinone dioxime 15 do. 7. do. p benzoquinone dioxime 24 do. 8. do. benzaldoxime 234 Surface dulled and slight corrosion 9. do. Benzofuroxan 15 Bright surface unchanged 10. do. 22' Bipyridyl 0 Sample pretreated prior to testing 11. do. Dimethyl glyoxime 1,050 Corrosion round hole and stamping- some pitting on sides 12 do. Guanidine carbonate 43 Bright surface retained. Slight evidence of pitting. 13. do. Fumaronitrile 440 Some dulling on edges. Slight pitting 14. do. Acetonitrile 1,975 General corrosion - less than control 15. do. Cyano acetic acid 2,267 do. 16. do. phthalonitrile 4,135 Severe general corrosion 17. do. metal free phthalo- 49 Sample pretreated in inhibitor in cyanine quinoline solvent in addition to that added in normal test. Slight tarnish- no pitting 18. do. Cuprous cyanide 2,604 General corrosion 19. do. Cuprous cyanide + 26.5 Sample pretreated with inhibitors adiponitrile Inhibitors present during test- slight bloom on sample 20. do. Cyclohexane-1,2-dione 646 Sample pretreated with inhibitor in dioxime ethyl alcohol. No inhibitor in test. - Surfade of sample after test generally bright - some areas of corrosion 21. Mild steel -- 57,500 Severe general corrosion 22. do. Cyclohexane 1,2-dione 25,400 General corrosion. Sample surface dioxime smoother than control. Corrosion less severe than control 23. Stainless Type 321 Pyridine N-oxide 10 Bright surface unchanged 24. do. 2-cyano pyridine 26 do. 25. do. Acetoxime 19 do. 26 do. Acetophenoneoxime 0 do. 27. do. Benzamideoxime 7.5 do. __________________________________________________________________________
EXAMPLES 28- 44
A rectangular steel strip (Stainless Type 321), 8- 9g. in weight, approximately, 1.5 × 0.75 × 0.0625 inch, drilled with a 0.375 inch diameter hole, is placed in a 8 × 1 inch glass tube fitted with air-cooled and water-cooled reflux condensers, together with the acid (50 ml) under test and is then heated at the temperatures and for the times indicated in the following table. The inhibitor was added to the acid solution at a concentration of 2 percent w/v. The metal samples were weighed before and after exposure to the acid. Example Acid INHIBITOR Time Temp. Weight Loss (hours) (°C) __________________________________________________________________________ 28 98 % acetic acid None (Control) 100 120 2600 p.p.m. 29 do. Nioxime 100 120 33 do. 30 do. Benzimidazole 100 120 65 do. 31 do. Cyclohexanone oxime 100 120 0 do. 32 do. Benzofuroxan 100 120 23 do. 33 do. None (Control) 100 120 3400 do. 34 90 % Formic acid None (Control) 100 100 5460 do. 35 do. Cyclohexanone oxime 100 100 63 do. 36 do. None (Control) 100 100 6500 do. 37 20 % Sulphuric acid None (Control) 72 80 10.7% 38 do. Cyclohexanone oxime 72 80 2.1% 39 do. 2-cyanopyridine 72 80 1.2% 40 do. Acetoxime 72 80 1.3% 41 do. None (Control) 72 80 8.3% 42 14 % w/v Hydro- None (Control) 24 86 29.8% chloric acid 43 do. 2-cyanopyridine 24 80 5.1% 44 do. None (Control) 24 80 29.6% 45 do. 1-cyano-4-phenyl butadiene 24 80 0.013% 46 do. cinnamaldoxime 24 80 0.04% 47 do. cinnamonitrile 24 80 0.013% __________________________________________________________________________
EXAMPLES 48- 55
A rectangular mild steel strip, - 3.5 g. in weight, approximately 2 × 5/8 × 1/32 inch is placed in an 8 × 1 inch test-tube fitted with air and water-cooled condensers in series, together with 16 w/w hydrochloric acid (50 ml) to which the inhibitor is added and is then heated at 80° ± 1°C for the times indicated in the following table. The metal samples were weighed before and after exposure to the acid. Example INHIBITOR EXPOSURE AT 80°C Compound Conc. in 4 hours 24 hours acid (% w/v) weight loss % weight loss % __________________________________________________________________________ 48 Benzonitrile 0.2 24.5 Dissolved 49 Dodecane dinitrile 0.2 3.2 9.2 50 Fumaronitrile 0.2 6.0 43.0 51 p-methoxybenzonitrile 0.2 0.67 1.9 52 1-cyano-4-methoxy naphthalene 0.2 0.34 0.72 53 p-dodecylbenzonitrile 0.2 2.6 5.9 54 cinnamonitrile 0.1 0.15 0.47 55 Control NIL Dissolved Dissolved __________________________________________________________________________
EXAMPLES 56- 82
A bright metal steel rectangular ticket, 2 × 5/8 × 1/32 inch, was immersed in 25 ml 16 w/w percent hydrochloric acid containing approximately 2 percent w/v of inhibitor in a 6 × 1 inch boiling tube and boiled for 10- 20 seconds. Gas evolution was initiated in the boiling acid by contact of the mild steel ticket with an aluminum rod and the tube and contents were then allowed to cool during 15 minutes. The hydrogen evolution rate and the final appearance of the metal sample were noted. Example INHIBITOR Hydrogen Evolution Appearance of metal Rate after exposure. __________________________________________________________________________ 56 benzyl cyanide practically nil no apparent change 57 N,N,N',N',-tetrakis-(cyanomethyl)hexa- do. do. methylenediamine 58 octadecyl cyanide do. do. 59 1-cyanoisoquinoline do. do. 60 1-cyanonaphthalene do. do. 61 4,4'-dicyanodiphenyl do. do. 62 3,3'-dicyanodiphenylmethane do. do. 63 p-chlorobenzonitrile do. do. 64 acrylonitrile do. do. 65 2-pentenenitrile do. do. 66 phenylmaleonitrile do. do. 67 p-trichloromethyl benzonitrile do. do. 68 m-toluonitrile do. do. 69 p-nitrobenzonitrile fast & continuous dull & corroded 70 p-dimethylaminobenzonitrile do. do. 71 p-aminobenzonitrile Fairly slow but dulled continuous 72 p-cyanobenzaldehyde slow but continuous dulled 73 o-cyanobenzaldehyde fairly slow but dulled continuous 74 N,N'-bis(α-cyanobenzyl)ethylenediamine very slow bright 75 cinnamonitrile practically nil no apparent change 76 p-methoxy cinnamonitrile do. do. 77 1-phenyl-4-cyano butadiene do. do. 78 crotononitrile do. do. 79 cinnamaldoxime do. do. 80 benzalacetoxime do. do. 81 bibenzal acetoxime do. do. 82 none (Control) very fast and dull and severely continuous corroded __________________________________________________________________________