WO2014035225A2

WO2014035225A2 - Corrosion inhibitor for deicing fluids

Info

Publication number: WO2014035225A2
Application number: PCT/LT2013/050001
Authority: WO
Inventors: Arvydas STONCIUS
Original assignee: Uab Baltic Ground Services
Priority date: 2012-08-27
Filing date: 2013-08-22
Publication date: 2014-03-06
Also published as: WO2014035225A3; LT6060B; LT2012080A

Abstract

This invention is related to a stable and non-toxic anti-corrosive additive, specifically a corrosion inhibitor, comprising tris(hydroxymethyl)aminomethane salts with organic and inorganic acids. This anti-corrosive additive can be used for protecting aluminum and steel surfaces against corrosion in water solutions of glycols and glycerol such as aircraft deicing and anti-icing fluids or heat transfer and cooling liquids. This anti-corrosive additive also performs acidity stabilizing, bacteriostatic and antioxidative functions.

Description

CORROSION INHIBITOR FOR DEICING FLUIDS Field of the Invention

The fuselage and control surfaces of an aircraft along with their equipment must be clean and free of any pollutants during their use. Snow, ice, frost or any other form of frozen water that form on aircrafts during the cold season can be extremely dangerous. It may not only deteriorate the flight performance, but also cause an aircraft to crash. In winter, the snow and ice that has formed on a parked aircraft is removed from its surfaces by using aircraft deicing fluids (abbreviated as ADF). These fluids are specified by the SAE Aircraft Material Specification (AMS) 1424 standard and by the international ISO 11075:2007 standard and are called Type I fluids. These fluids usually consist of substances that lower the freezing temperature (freezing buffers), water and other various additives. Before being sprayed on the surfaces of an aircraft, they are usually diluted with water according to the ambient temperature, heated up to 60-80 °C and are then sprayed by using pressure. However, Type I fluids provide only limited protection against ice formation (usually up to 10 minutes). Frequently, a certain period of time passes after the deicing procedure until takeoff of the aircraft. Therefore, when snowing or under freezing conditions, there is a need for a longer lasting protection for surfaces of the aircraft against forming snow or ice. Anti-icing fluids (AAF) are used for this purpose. These fluids are specified by the SAE AMS 1428 and ISO 11078:2007 standards and are called Type II, Type III and Type IV fluids. Besides freezing temperature lowering substances (freezing buffers), water and other additives, similar to the ones used in Type I fluids, anti-icing fluids also contain shear thinning thickeners. Specifically these thickeners lead to a gel-like coating, which forms on the surfaces of the aircraft and prevents ice and snow sticking to them. In a still state this shear -thinning coating remains stable for 20 minutes to 2 hours, depending on the concentration and environmental conditions. However, once the aircraft starts moving and as its velocity increases, the viscosity of this coating significantly decreases and it is blown off from the surfaces of the aircraft. In other words, as the force acting on these shear thinning fluids increases, their viscosity decreases and fluidity increases. Besides serving their main purpose of protecting the aircraft surfaces from ice formations, these deicing and anti-icing fluids must also be safe to all of the structural materials used in aircrafts, such as: alloys of aluminum, steel and titanium, painted surfaces, plastics, etc. Furthermore, the environmental impact of these fluids and their usage should be as low as possible, as an example, they should have the lowest toxicity to water organisms as possible.

Background art

The major part of deicing fluids' composition consists of water and freezing temperature lowering substances (freezing buffers). The freezing temperature reducing substances as known in the prior art and that have been used for a considerable period of time now, as described in the patent GB1026150A, published on 1966-04-14, are ethylene glycol, propylene glycol, glycerol and /or their mixtures. In most cases, deicing fluids contain 40 to 88% of such glycols and/or glycerol.

Besides substances lowering the freezing temperature (freezing buffers) and water, deicing fluids also contain various additives. These additives are:

a. Substances that regulate and/or decrease the surface tension of fluids, improve wetting of the aircraft surfaces, reduce the surface tension of melt water and/or form a hydrophobic monolayer on the surfaces of an aircraft, and as a result partially inhibit the repeated ice formation on these surfaces. These substances are, for example, polyalcoholamines, preferably diethanolamine or triethanolamine, as described in the patents GB1202697A, published on 1970-08-09, or EP194899B1, published on 1991-01-02. Also used are non- ionic surfactants, such as dioctyl phenol and ethylene glycol ethers, as described in the patent GB1272464A, published on 1972-04-26; C₁₃-C₁₅ alcohols which have been alkoxylated from 11 to 20 alkoxide molecules, N-methyl-N-oleoylglycine, as described in the patent US5759436A, published on 1998-06-02; organic phosphates: phosphoric and/or phosphinic acid organic esters salts as described in the patent US20050087720A1, published on 2005-04-28, and many other surface active substances, that are described in the patent US5817252, published on 1998-10-06. b. Substances, regulating acidity of solution and/or stabilizing it at 7-10 pH, preferably at 8-9 pH. The glycols used in deicing fluids oxidize with oxygen from air and the oxidation products increase acidity of the solution. This not only decreases the stability of the solution but also causes an increased risk of corrosion of the metal surfaces being treated, such as aluminum and carbon steel. Such acidity increase must be compensated by substances which stabilize it, called buffers. These usually are phosphates of sodium, potassium and ammonium, borates, salts of organic carboxylic acids; basic components such as sodium and potassium hydroxides or their carbonates, butyl, nonylamines, mono-, di-, and triethanolamines; acidic components such as phosphoric, boric, organic mono- and di- acids (4-8 C atoms), benzoates, as described in the patent US6921495B2, published on 2005-07- 26, and other substances.

Patent US5708068A, published on 1998-01-13, also states that some amines, including triethanolamine, decrease the thermal stability of these solutions.

c. Antioxidants, which inhibit the oxidation of organic substances with the oxygen of air, especially of glycols and/or glycerol. These can be sodium, potassium or ammonium sorbates, or benzoates as described in the patent US7105105B2, published on 2006-09-12. d. Antimicrobial, bacteriostatic substances, which inhibit the reproduction of bacteria, yeast, fungi and molds in these fluids. Most often these are sodium azide, quaternary ammonium salts, such as N-alkyldimethylbenzylammonium, N-alkyltrimethylammonium (where alkyl - is C₁₂-C₁₈), sodium, potassium or ammonium benzoates and others, as described in the patent CA2503859 Al , published on 2004-05-13.

e. Corrosion inhibitors, which protect from corrosion the metals used in aircraft manufacture: aluminum and magnesium alloys, carbon steel and titanium. Many various corrosion inhibitors known in the prior art are used in the deicing fluids, such as the those described in the patent US7875203B2, published on 2011-01-25: alkali metal phosphates, phosphoric acid esters, such as ethyl, dimethyl, isopropyl phosphates salts, also thiourea, sodium nitrate, butyne-l,4-diol, imidazoles, such as lH-imidazole, methylimidazole, benzimidazole. Preferably - triazoles as they not only provide metal surfaces with good protection against corrosion but are also flame retardant materials that inhibit ignition. As anti-corrosion agents are also used other substances known in the prior art: alkali salts of boric, silicic, fatty, succinic, sebacic and polyaspartic acids,„Sandorocin 8132" (Sandoz AG), usually from 0,1 to 1%.

Besides these additives, dye is usually also added to the deicing fluids. When there is a need, antifoaming agents and other additives are also added.

Since the deicing and anti-icing fluids were started being used, environmental problems related to them emerged. After the use, these fluids drain to sewers and sometimes even to natural waters usually without any further processing. As a result, these ecological problems arise:

a. Some of the substances that lower the freezing temperature, for example ethylene glycol, are toxic to mammals, including human. This problem is solved by gradually eliminating the use of ethylene glycol in the production of deicing and anti-icing fluids.

b.After getting into water bodies, many of the substances that lower the freezing temperature degrade rapidly by biological oxidation. Oxygen that is dissolved in water is used by this process, and due to the lack of it aquatic fauna is perished (refer to "Formulations for Aircraft and Airfield Deicing and Anti-Icing: Aquatic Toxicity and Biochemical Oxygen Demand", Airport Cooperative Research Program, Web-Only Document 3, 2008). Various attempts are made to avoid this. One of them is an improved use of the deicing fluids, as described in patent US4032090A, published on 1977-06-28, where the fluid being sprayed is heated to 90 °C, allowing using these fluids diluted with a higher amount of water. Attempts are also made to regulate the biodegradation rate of these fluids (BOD - biochemical oxygen demand) in such a way that deicing fluids, made of both, the rapidly biodegrading substances, such as propylene glycol or glycerol and the slowly biodegrading triethyleneglycol, are mixed in a way to achieve suitable biodegradation rate according to whether there are biological treatment plants installed at the particular airport, as described in the patent application WO2001029146A1 , published on 2001-04-26. Efforts are also being made to switch to the use of freezing temperature lowering substances that are derived from natural renewable sources, such as 1 ,3 -propanediol made by a method of carbohydrates fermentation, as described in the patent US7972530B2, published on 2011- 07-05, and other similar substances, as described in the patents US201 10024673A1 , published on 2011-02-03, and US20120104305A1 , published on 2012-05-03. Another attempt is made by searching for completely different freezing temperature lowering substances than glycols. One such example is described in the patent US20030034478A, published on 2003-02-20. It describes such a deicing fluid in which the temperature lowering substance is a salt of triethanolamine and formic acid - formate. Another suggestion, described by the patent US8187489B1 , published on 2012-05-29, is to use ionic liquids - choline salts.

c. The degradation products of some of the usually used surface active substances are toxic. Such substances are alkylphenols, the degradation products of alkylphenol ethoxylates, as described in the patent application WO2011103295A1 , published on 201 1-08-25. They are being replaced by the less hazardous alcohol ethoxylates.

d. Some components are toxic and hardly biodegradable, for example, the often used corrosion inhibitors benzotriazole and tolyltriazole. They degrade slowly and such accumulate in live organisms and poison them (refer to D.A. Pillard et al., Wat. Res., 35 (2001), pages 557 - 560). Currently these corrosion inhibitors are hardly used anymore, but are only partially replaced by alkali metal salts of polyaspartic or polyglutamic acids, as described in the patent application WO2002099004A1, published on 2002-12-12.

The triethanolamine found frequently in the deicing and anti-icing fluids is also not completely environment friendly. It degrades quite slowly and its admixtures and degradation products, di- and mono-ethanolamine are quite toxic to water organisms (refer to G. Libralato et al., Journal of Hazardous Materials, 176 (2010), pages 535-539). Besides the environmental problems, other drawbacks of these fluids emerge. For example, the alkali and alkaline earth metal salts, used in the deicing and anti-icing fluids may cause a stress corrosion of the hot titanium aircraft engine parts, as described in the patent CA2387923A1 , published on 2001- 04-26. The same salts are to a large extent responsible for the faults of aircraft carbon fibre composite brakes, these faults arising from catalytic oxidation. In such a way, increasingly strict requirements of ecology, anti-icing efficiency and safety force to search for new substances for deicing fluids and their compositions.

Purpose of the Invention

The purpose of this invention is to present a system of anti-corrosion additives, suitable for alkyleneglycol and/or glycerol based deicing and anti-icing fluids. Such a system of anti- corrosion additives must provide sufficient protection against corrosion for the alloys used in the aircraft manufacture, while also being non-toxic and thermally stable.

Description of the preferred embodiment

The anti-corrosion system suggested by this invention comprises tris(hydroxymethyl)aminomethane (also called TRIS, THAM, or 2-Amino-2- hydroxymethylpropane-l ,3-diol, further referred to as TRIS) salts with organic and inorganic acids. By its chemical structure, TRIS is an amino polyalcohol, what essentially makes it closely related to the amino alcohols often used in anti-corrosion systems, such as triethanolamine. However, TRIS has not been previously used in deicing, anti-icing or other anti-corrosion compositions, excluding one instance, where it has been used in complexes with Zn ions for protection of steel in aqueous environment containing CI^" ions (refer to S.Rejandran et al., Chemistry: An Indian Journal, 4, 2009).

The results of conducted tests unexpectedly showed that TRIS in composition with other anti- corrosion components, such as borates and phosphates, protects an aluminum alloy plate against corrosion better than the industrially used triethanolamine (TEA), imidazole (IMDZ), 2-amino-2methyl- 1 -propanol (AMP) or diethanolglycide (diethanolamine and glycine amide, abbreviated as DE.GLA), and even better than some of the commercial compositions of deicing fluids.

The studied solutions of amino alcohols and other components are prepared as follows:

1. 42 g of propylene glycol and glycerol mixture (ratio by weight 6:4) and 50 g of deionized water are mixed together. 2. To the solution, resulting from 1. is then added 1 g sodium tetraborate decahydrate ( a₂B₄0₇ · 10 H₂0, also known as borax) 10 wt% (% by weight - wt%) solution, previously prepared as follows: to a mixture of 50 g of propylene glycol and glycerol (ratio by weight 6:4) and 40 g of deionized water, 10 g of sodium tetraborate decahydrate is added by portions and stirred until it dissolves.

3. To the solution, resulting from 2. is then added 0.15 g of trisodium phosphate dodecahydrate ( a₃P0₄ ^■ 12 H₂0);

4. To the solution, resulting from 3. is then added 0.2 or 0.1 g of the amino alcohol being studied;

5. To the solution, resulting from 4. is then added deionized water to a total solution weight of 100 g.

This method yields mixtures, which comprise of:

1. 42.5 wt% of propylene glycol and glycerol mixture (ratio by weight 6:4);

2. 0.1 wt% of sodium tetraborate decahydrate ( a₂B₄0₇ · 10 H₂0);

3. 0.15 wt% of trisodium phosphate dodecahydrate (Na₃PC>4 · 12 H₂0);

4. 0.2 or 0.1 wt% of the amino alcohol being studied (as given in table 1);

5. The remainder percentage is deionized water.

Such mixtures, according to glycol/glycerol concentration and their freezing temperature correspond to partially dilute (usually to 50:50 ratio by weight) commercial deicing Type I fluid. For a comparison, Kilfrost DF Plus, diluted with deionized water to 50 wt% and solution "0", which included all the components except amino alcohols, were also studied.

Table 1 reflects the amount of amino alcohols in the studied solutions and their effect on 60x40x0.5 mm 2024 "alclad" aluminum alloy plates fully submerged into the solutions for 1 week (7 days - 168 hours) at a constant 40 °C temperature (similarly to a total immersion test described in ASTM F 483 standard, used for deicing fluids testing), expressed as weight change (WGT Change) units of mg/cm /24 hrs. Table 1

This weight change due to corrosion is also depicted in Figure 1.

TRIS, a widely known substance that usually serves as an acidity stabilizer, is a pH buffer component. This substance is particularly intensively used in biotechnology and biochemistry or even medicine, in principle because of its low toxicity and ecological safety, and also because of its stability and its convenience of use. Therefore, it is rational to use TRIS not only as an anti-corrosion system component, but also as an acidity regulating substance. TRIS has a pKa of 8.07, meaning that it can act as a basic component in buffers, whose pH ranges from 7 to 9, which almost ideally meets the requirements of the deicing fluids. Two components, basic and acidic, are usually mixed together when designing pH stabilizing buffers. In this invention the acidic component was chosen such as to maintain low solution toxicity, be thermally stable and have other positive properties, such as bacteriostatic and/or antioxidative. It is also preferable if they were known not to cause corrosion, or even better, have anticorrosive properties. In this case they are:

a. Benzoic acid - known as a bacteriostatic and antioxidative substance, sometimes used as a food preservative, and also known as an element of anticorrosive compositions.

b. 2-hydroxybenzoic (salicylic) acid - known as a bactericidal and antiseptic substance and for this reason intensively used in the medicine. c. 4-hydroxybenzoic acid (4HyBA) known as a popular antioxidant because of its low toxicity. It is also known as an element of some of the anticorrosive compositions.

d. Boric acid - a form of either boric acid or sodium tetraborate decahydrate (borax). Boric acid is known for its bactericidal, insecticidal, fungicidal and preservative properties. It is also a flame retardant substance that inhibits ignition. Sodium tetraborate is also known as a water softener.

Solutions that include TRIS and the acids listed above are mixed as follows:

1. 0.32 g of TRIS is dissolved in 50 g of deionized water.

2. To the solution, resulting from 1. is then added by portions and while stirring:

- 0.18 g of benzoic acid or

0.2 g of 4-hydroxybenzoic or 2-hydrobenzoic (salicylic) acid

3. To the solution, resulting from 2. are then added 42.5 g of propylene glycol and glycerol mixture (ratio by weight 6:4) and deionized water to a total solution weight of 100 g, all is well stirred. The solutions produced by this method have a pH of 8.1±0,05.

Solution, which besides TRIS and 4-hydroxybenzoic acid also contains 0.05 g of sodium tetraborate decahydrate, is produced as follows:

3. To the solution, resulting from 2. is then added 0.5 g of sodium tetraborate decahydrate (borax) 10 wt% solution, previously prepared as follows: to a mixture of 50 g of propylene glycol and glycerol (ratio by weight 6:4) and 40 g of deionized water, 10 g of sodium tetraborate decahydrate is added by portions and stirred until it dissolves.

4. To the solution, resulting from 3. are then added 42.25 g of propylene glycol and glycerol mixture (ratio by weight 6:4) and deionized water to a total solution weight of 100 g, all is well stirred.

Kilfrost DF Plus, diluted with deionized water to 50 wt% and solution "0", which consisted only of propylene glycol and glycerol (ratio by weight 6:4), overall 42.5 wt% with the remainder percentage of deionized water, were also studied for a comparison. Table 2 reflects the amount of additives in the studied samples and their effect on 60x40x0.5 mm 2024 "alclad" aluminum alloy plates fully submerged into the samples for 1 week (7 days - 168 hours) at a constant 40 °C temperature (same as described previously), expressed as weight change (WGT Change) units of mg/cm²/24 hrs. This is also depicted in Figure 2.

Table 2

Similar studies were also carried out with plates made of titanium and anodized aluminum. Essentially no corrosion signs were observed after the experiments.

The effects of these solutions on carbon steel were also studied.

Table 3 reflects the amount of additives in the studied samples and their effect on 50x28x1.5 mm carbon steel plates fully submerged into the samples for 1 week (7 days - 168 hours) at a constant 40 °C temperature, expressed as weight change (WGT Change) units of mg/cm²/24 hrs. This is also depicted in Figure 3. Table 3

The thermal stability of all the solutions was determined as follows: samples of solutions were kept in hermetically closed glass bottles and subjected to a constant 90 °C temperature. The solution used for comparison consisted of 42.5 g of propylene glycol and glycerol mixture (ratio by weight 6:4), 0.2 g of triethanolamine and deionized water to a total solution weight of 100 g. The study has revealed that the mixture including triethanolamine starts turning yellow and later, approximately after a week, starts turning brown, whereas the solutions including TRIS salts stay essentially unaffected for a longer period of time.

According to these results, mixtures of TRIS salts with previously listed acids, preferably with benzoic acid or 4-hydroxybenzoic acid, provide sufficient protection against corrosion for the aluminum alloys and especially for the carbon steel. These solutions consist of nontoxic components, have sufficient oxidative, thermal stability and bacteriostatic protection, therefore they serve well as anti-corrosive additives in deicing, anti-icing, cooling and heat transfer fluids based on glycol and/or glycerol and water.

Claims

1. Anti-corrosive additive, characterized in that it comprises tris(hydroxymethyl)aminomethane salts with organic and non-organic acids.

2. Anti-corrosive additive according to claim 1, characterized in that it comprises tris(hydroxymethyl)aminomethane salts with organic aromatic acids, such as benzoic, or 2-hydroxybenzoic (salicylic), or 4-hydroxybenzoic and/or inorganic acids, such as boric or phosphoric.

3. Anti-corrosive additive according to any of 1 to 2 claims, characterized in that when it constitutes 0.1 to 1 wt% in mixtures also comprising propylene glycol and/or glycerol or their mixture (20-85 wt%) and water (remaining percentage), protects aluminum alloys and carbon steel from corrosion.

4. Anti-corrosive additive according to any of 1 to 3 claims, characterized in that when it constitutes 0.1 to 1 wt% in mixtures also comprising propylene glycol and/or glycerol or their mixture (20 to 85 wt%), alkali metal borates (0,01 to 0,05 wt%) and/or alkali metal phosphates (0.01 to 0.05 wt%) and water (remaining percentage), protects aluminum alloys and carbon steel from corrosion.

5. Anti-corrosive additive according to any of 1 to 4 claims, characterized in that it can be used in producing fluids for: ice removal from aircraft surfaces, their protection against ice formation, cooling and heat transfer; where any of these fluids are based on glycols and/or glycerol or their mixtures.

6. Anti-corrosive additive according to any of 1 to 5 claims, characterized in that it comprises organic compounds, such as tris(hydroxymethyl)aminomethane benzoate, 2- hydroxybenzoate (salicylate) or 4-hydroxybenzoate, solutions of these compounds having low-toxicity.

7. Anti-corrosive additive according to any of 1 to 6 claims, characterized in that it comprises compounds, such as tris(hydroxymethyl)aminomethane benzoate, 2- hydroxybenzoate (salicylate) or 4-hydroxybenzoate, solutions of these compounds being thermally stable.

8. Anti-corrosive additive according to any of 1 to 7 claims, characterized in that it is also a pH of 7 to 9 H, preferably a pH of 8 to 8.5 stabilizing buffer.

9. Anti-corrosive additive according to any of 1 to 8 claims, characterized in that it is also a means of stabilizing and increasing oxidation resistance of mixtures comprising propylene glycol and/or glycerol and water.

10. Anti-corrosive additive according to any of 1 to 9 claims, characterized in that it is also a means of stabilizing and inhibiting bacteria growth in mixtures comprising propylene glycol and/or glycerol and water.