Patent Application
20150090369
The present invention relates generally to the field of conversion coating of metals, and more particularly, to conversion coating of magnesium and its alloys.
Particularly by virtue of their light weight, high strength-to-weight ratio, and good recyclability, magnesium-based alloys are promising candidates for many engineering applications, including automotive applications. Indeed, there is substantial interest in replacing steel and aluminum parts in machinery and transportation vehicles with magnesium alloys. However, the corrosion susceptibility of magnesium alloys remains a particular issue of significant concern. Magnesium alloys are susceptible to corrosion because of their high reactivity and low electrode potential (E0=2.37 V), which prevents them from widespread applications. The corrosion mechanisms of magnesium alloys have been well-studied, and can generally include galvanic corrosion, intergranular corrosion, stress corrosion cracking, and/or corrosion fatigue.
Various types of treatments have been used for protecting magnesium surfaces, such as laser and ion beams, PVD, CVD, chemical conversion, anodization, electro- or electro-less plating, organic coating, and others. Among them, chemical conversion coatings are commonly used as paint bases and in some cases as stand-alone protection against atmospheric tarnishing. Traditional conversion coatings are based on hexavalent chromium compounds, but these are associated with severe environmental risks. Some chromate-free conversion coatings have been studied, such as stannate, cerium, aluminum, zirconium, niobium, zinc phosphate, or phosphate permanganate coatings. However, current conversion coating technologies of the art are generally incapable of providing sufficiently passivated magnesium alloys suitable for use in critical applications, such as machinery and automobiles, where mechanical resilience and high wear-resistance are necessary.
In one aspect, the instant invention is directed to an improved method for conversion coating a magnesium-containing surface. The method generally involves contacting the magnesium-containing surface with an ionic liquid compound (i.e., “ionic liquid”) under conditions that result in decomposition of the ionic liquid to produce a conversion coated magnesium-containing surface. Unexpectedly, it has herein been found that exposing a magnesium-containing surface to an ionic liquid under decomposing conditions results in a magnesium-containing surface having a significantly improved corrosion resistance relative to the magnesium-containing surface before the instant conversion coating process. Moreover, the advantages of using an ionic liquid in place of conventional conversion chemicals are numerous. For example, ionic liquids are generally known to possess low volatility, high thermal and electrochemical stability, low flammability, and low toxicity. Compared with conventional conversion chemistry, ionic liquids also provide higher flexibility in molecular design.
The ionic liquid can have any suitable cationic component, particularly a nitrogen-containing cation, such as a nitrogen-containing heterocycle (e.g., imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolium, piperazinium, etc.), or ammonium species. In more specific embodiments, the cationic component of the ionic liquid has the following formula:
In Formula (1), Z is either N or P, and R1, R2, R3, and R4 are each independently selected from hydrogen atom and hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described below for hydrocarbon groups R, provided that at least one of R1, R2, R3, and R4 is a hydrocarbon group R when Z is N, and provided that R1, R2, R3, and R4 are all hydrocarbon groups when Z is P.
In particular embodiments of Formula (1), the cationic component of the ionic liquid has any of the following formulas:
In the above formulas, R1a, R2a, R3a, and R4a are each independently selected from hydrogen atom and hydrocarbon groups having at least one and up to twenty carbon atoms, wherein the hydrocarbon groups are optionally substituted with one or more fluorine atoms, provided that at least one of R1a, R2a, R3a, and R4a is a hydrocarbon group; and R1b, R2b, R3b, and R4b are independently selected from hydrocarbon groups having at least one and up to twenty carbon atoms, wherein the hydrocarbon groups are optionally substituted with one or more fluorine atoms.
The ionic liquid compound can have any suitable anionic component, and particularly, a phosphorus-containing anion. In particular embodiments, the anionic component of the ionic liquid has a phosphorus-containing anion of the formula:
In Formula (2), R5 and R6 are independently selected from hydrocarbon groups having at least one and up to twenty carbon atoms, wherein the hydrocarbon groups are optionally substituted with one or more fluorine atoms, and R5 and R6 may optionally interconnect to form a ring; X1, X2, Y and Z are independently selected from O and S atoms; and subscripts r and s are independently selected from 0 and 1. Thus, Formula (2) includes organophosphates, organophosphonates, organophosphinates, and mono-, di-, tri-, and tetra-thio derivatives of any of the foregoing sub-classes under Formula (2).
In particular embodiments, the ionic liquid compound has the general formula:
In the above formula, Z, R1, R2, R3, and R4 are as described above, and X″ is an anionic component of the ionic liquid. The anionic component X″ can be any anion, and more particularly, a phosphorus-containing anion, such as an anion encompassed by Formula (2) above.
In another aspect, the invention is directed to a magnesium-containing material that has been treated by the above-described conversion coating process. The treated magnesium-containing material surprisingly exhibits a significantly improved corrosion resistance relative to the magnesium-containing surface before the instant conversion coating process, and even a significantly improved corrosion resistance relative to the same magnesium-containing surface coated with the same ionic liquid but under conditions where the ionic liquid has not decomposed. The improved corrosion resistance may be evidenced in a remarkable substantial absence of corrosion even after rigorous corrosion tests, such as being treated in a salt solution of at least 0.1 M concentration for several hours.
In yet another aspect, the invention is directed to a mechanical component or device containing the conversion-coated magnesium-containing surface described above. The mechanical component or device is typically directed to an application in which mechanical resilience, corrosion resistance, and high wear-resistance are all required.
As used herein, the term “about” generally indicates within ±0.5%, 1%, 2%, 5%, or up to ±10% of the indicated value. For example, the term “about 100° C.” generally indicates, in its broadest sense, 100° C. ±10%, which indicates 90-110° C. The term “about” may alternatively indicate a variation or average in a physical characteristic of a group.
The term “hydrocarbon group” or “hydrocarbon linker” (also identified as “R”), as used herein, designates, in a first embodiment, groups or linkers composed solely of carbon and hydrogen. In different embodiments, one or more of the hydrocarbon groups or linkers can contain precisely, or a minimum of, or a maximum of, for example, one, two, three, four, five, six, seven, eight, nine, ten, eleven, or twelve carbon atoms, or a number of carbon atoms within a particular range bounded by any two of the foregoing carbon numbers. In some embodiments, the hydrocarbon group may include up to 20 or more carbon atoms. Hydrocarbon groups or linkers in different compounds described herein, or in different positions of a compound, may possess the same or different number (or preferred range thereof) of carbon atoms in order to independently adjust or optimize the activity or other characteristics of the compound.
The hydrocarbon groups or linkers (R) can be, for example, saturated and straight-chained (i.e., straight-chained alkyl groups or alkylene linkers). Some examples of straight-chained alkyl groups (or alkylene linkers) include methyl (or methylene linker, i.e., —CH2—, or methine linker), ethyl (or ethylene or dimethylene linker, i.e., —CH2CH2— linker), n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, and n-dodecyl groups (or their respective linker analogs).
The hydrocarbon groups or linkers (R) can alternatively be saturated and branched (i.e., branched alkyl groups or alkylene linkers). Some examples of branched alkyl groups include isopropyl(2-propyl), isobutyl(2-methylprop-1-yl), sec-butyl(2-butyl), t-butyl, 2-pentyl, 3-pentyl, 2-methylbut- l -yl, isopentyl(3-methylbut-1-yl), 1,2-dimethylprop-1-yl, 1,1-dimethylprop-1-yl, neopentyl(2,2-dimethylprop-1-yl), 2-hexyl, 3-hexyl, 2-methylpent-1-yl, 3-methylpent-1-yl, isohexyl (4-methylpent-1-yl), 1,1-dimethylbut-1-yl, 1,2-dimethylbut-1-yl, 2,2-dimethylbut-1-yl, 2,3-dimethylbut-1-yl, 3,3-dimethylbut- l -yl, 1,1,2-trimethylprop-1-yl, and 1,2,2-trimethylprop-1-yl groups, wherein the “1-yl” suffix represents the point of attachment of the group. Some examples of branched alkylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary branched alkyl groups (e.g., isopropylene, —CH(CH3)CH2—).
The hydrocarbon groups or linkers (R) can alternatively be saturated and cyclic (i.e., cycloalkyl groups or cycloalkylene linkers). Some examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl groups. The cycloalkyl group can also be a polycyclic (e.g., bicyclic) group by either possessing a bond between two ring groups (e.g., dicyclohexyl) or a shared (i.e., fused) side (e.g., decalin and norbornane). Some examples of cycloalkylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary cycloalkyl groups.
The hydrocarbon groups or linkers (R) can alternatively be unsaturated and straight-chained (i.e., straight-chained olefinic or alkenyl groups or linkers). The unsaturation occurs by the presence of one or more carbon-carbon double bonds and/or one or more carbon-carbon triple bonds. Some examples of straight-chained olefinic groups include vinyl, propen-1-yl (allyl), 3-buten-1-yl (CH2═CH—CH2—CH2—), 2-buten-1-yl (CH2—CH═CH—CH2—), butadienyl, 4-penten-1-yl, 3-penten-1-yl, 2-penten-1-yl, 2,4-pentadien-1-yl, 5-hexen-1-yl, 4-hexen-1-yl, 3-hexen-1-yl, 3,5-hexadien-1-yl, 1,3,5-hexatrien-1-yl, 6-hepten-1-yl, ethynyl, and propargyl (2-propynyl). Some examples of straight-chained olefinic linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary straight-chained olefinic groups (e.g., vinylene, —CH═CH—, or vinylidene).
The hydrocarbon groups or linkers (R) can alternatively be unsaturated and branched (i.e., branched olefinic or alkenyl groups or linkers). Some examples of branched olefinic groups include propen-2-yl (CH2═C.—CH3), 1-buten-2-yl (CH2═C.—CH2—CH3), 1-buten-3-yl(CH2═CH—CH.—CH3), 1-propen-2-methyl-3-yl (CH2═C(CH3)—CH2.), 1-penten-4-yl, 1-penten-3-yl, 1-penten-2-yl, 2-penten-2-yl, 2-penten-3-yl, 2-penten-4-yl, and 1,4-pentadien-3-yl. Some examples of branched olefinic linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary branched olefinic groups.
The hydrocarbon groups or linkers (R) can alternatively be unsaturated and cyclic (i.e., cycloalkenyl groups or cycloalkenylene linkers). The unsaturated and cyclic group can be aromatic or aliphatic. Some examples of unsaturated and cyclic hydrocarbon groups include cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, cyclohexadienyl, phenyl, benzyl, cycloheptenyl, cycloheptadienyl, cyclooctenyl, cyclooctadienyl, and cyclooctatetraenyl groups. The unsaturated cyclic hydrocarbon group can also be a polycyclic group (such as a bicyclic or tricyclic polyaromatic group) by either possessing a bond between two of the ring groups (e.g., biphenyl) or a shared (i.e., fused) side, as in naphthalene, anthracene, phenanthrene, phenalene, or indene. Some examples of unsaturated cycloalkenylene linkers are those derived by removal of a hydrogen atom from one of the foregoing exemplary cycloalkenyl groups (e.g., phenylene and biphenylene).
One or more of the hydrocarbon groups or linkers (R) may (i.e., optionally) be substituted with (i.e., include) one or more heteroatoms, which are non-carbon non-hydrogen atoms. Some examples of heteroatoms include oxygen (O), nitrogen (N), sulfur (S), and halogen (halide) atoms. Some examples of halogen atoms include fluorine, chlorine, bromine, and iodine. In some embodiments, the heteroatom atom inserts between at least two carbon atoms (as in —C—O—C— ether, —C—N(R)—C— tertiary amine, or —C(═NR)C— imine) or between at least one carbon atom and at least one hydrogen atom (as in —C—OH, —C—SH, —C—NH2, —C—NH—C—, or —C(═NH)C—), wherein the shown carbon atom in each case can be considered part of a hydrocarbon group R described above. In other embodiments, the heteroatom replaces one or more hydrogen atoms and/or one or more carbon atoms in the hydrocarbon group, as in halogen-substituted groups (e.g., a —CH2F, —CHF2, and —CF3) and carbonyl-substituted groups, such as ketone and aldehyde groups. In the case of nitrogen or sulfur substitution, the nitrogen or sulfur atom may be bonded to a sufficient number of groups to make it positively charged, as in an ammonium group (e.g., —NR′3+) or sulfonium group (e.g., —SR′2+), in which case the positively charged moiety is necessarily associated with a counteranion (wherein R′ independently represents hydrogen atom or any of the hydrocarbon groups described above). Likewise, a heteroatom may bear a negative charge, as in a deprotonated alkoxide or thio group, in which case the negatively charged moiety is necessarily associated with a countercation.
When two or more same or different heteroatoms are bound to each other or located on the same carbon atom, the resulting group containing the heteroatoms is herein referred to as a “heteroatom-containing group”. Thus, substitution with one or more heteroatoms also includes heteroatom-containing groups, unless otherwise specified. Some examples of heteroatom-containing groups and linkers include carboxy (—C(O)OR′ or —OC(O)R′), carboxamide (—C(O)NR′2, —C(O)NR′—, or —N(R′)C(O)—), urea (—NR′—C(O)—NR′2 or —NR′—C(O)—NR′—), carbamate (—NR′—C(O)—OR′, —OC(O)—NR′2, or —NR′—C(O)-O-), nitro (NO2), nitrile (CN), sulfonyl (—S(O)2R′or —S(O)2—), sulfinyl (i.e., sulfoxide, —S(O)R′ or —S(O)—), disulfide (—C—S—S—C—), sulfonate (—S(O)2R′), and amine oxide (as typically found in a nitrogen-containing ring), wherein R′ independently represents hydrogen atom or any of the hydrocarbon groups (R) described above. For example, —C(O)OR′ includes carboxylic acid (—C(O)OH) and carboxylic ester (—C(O)OR), where R is any of the hydrocarbon groups described above. The heteroatom-containing group may also either insert between carbon atoms or between a carbon atom and hydrogen atom, if applicable, or replace one or more hydrogen and/or carbon atoms.
In some embodiments, the hydrocarbon group or linker (R) is substituted with one or more halogen atoms to result in a partially halogenated or perhalogenated hydrocarbon group. Some examples of partially halogenated hydrocarbon groups include —CHY2, —CH2Y, —CH2CY3, —CH(CY3)2, or a halo-, dihalo-, trihalo-, or tetrahalo-substituted phenyl group, wherein Y represents any of F, Cl, Br, or I, and more commonly, F or Cl. Some examples of perhalogenated hydrocarbon groups include —CY3, —CY2CY3, —CY2CY2CY3, —CY(CY3)2, or perhalophenyl, —C6Y5.
When the hydrocarbon group is or includes a cyclic or polycyclic (i.e., bicyclic, tricyclic, or higher cyclic) group, the cyclic hydrocarbon group can be carbocyclic or heterocyclic. A carbocyclic group is a saturated or unsaturated cyclic or polycyclic hydrocarbon group that does not contain ring heteroatoms (i.e., it contains only ring carbon atoms). A heterocyclic group is a cyclic or polycyclic hydrocarbon group that includes at least one ring heteroatom (for example, one, two, three, four, or higher number of heteroatoms). As used herein, a “ring heteroatom” is an atom other than carbon and hydrogen (typically, selected from nitrogen, oxygen, and sulfur) that is inserted into, or replaces a ring carbon atom in, a hydrocarbon ring structure. In some embodiments, the heterocyclic group is saturated, while in other embodiments, the heterocyclic group is unsaturated (i.e., aliphatic or aromatic heterocyclic groups, wherein the aromatic heterocyclic group is also referred to herein as a “heteroaromatic ring”, or a “heteroaromatic fused-ring system” in the case of at least two fused rings, at least one of which contains at least one ring heteroatom). In some embodiments, the heterocyclic group is bound via one of its ring carbon atoms to another group (i.e., other than hydrogen atom and adjacent ring atoms), while the one or more ring heteroatoms are not bound to another group. In other embodiments, the heterocyclic group is bound via one of its heteroatoms to another group, while ring carbon atoms may or may not be bound to another group.
Some examples of saturated heterocyclic groups containing at least one oxygen atom include oxetane, tetrahydrofuran, tetrahydropyran, 1,4-dioxane, 1,3-dioxane, and 1,3-dioxepane rings. Some examples of saturated heterocyclic groups containing at least one nitrogen atom include pyrrolidine, piperidine, piperazine, imidazolidine, azepane, and decahydroquinoline rings. Some examples of saturated heterocyclic groups containing at least one sulfur atom include tetrahydrothiophene, tetrahydrothiopyran, 1,4-dithiane, 1,3-dithiane, and 1,3-dithiolane rings. Some examples of saturated heterocyclic groups containing at least one oxygen atom and at least one nitrogen atom include morpholine and oxazolidine rings. An example of a saturated heterocyclic group containing at least one oxygen atom and at least one sulfur atom includes 1,4-thioxane. Some examples of saturated heterocyclic groups containing at least one nitrogen atom and at least one sulfur atom include thiazolidine and thiamorpholine rings.
Some examples of unsaturated heterocyclic groups containing at least one oxygen atom include furan, pyran, 1,4-dioxin, benzofuran, dibenzofuran, and dibenzodioxin rings. Some examples of unsaturated heterocyclic groups containing at least one nitrogen atom include pyrrole, imidazole, pyrazole, pyridine, pyrazine, pyrimidine, 1,3,5-triazine, azepine, diazepine, indole, purine, benzimidazole, indazole, 2,2′-bipyridine, quinoline, isoquinoline, phenanthroline, 1,4,5,6-tetrahydropyrimidine, 1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, quinoxaline, quinazoline, pyridazine, cinnoline, 5,6,7,8-tetrahydroquinoxaline, 1,8-naphthyridine, and 4-azabenzimidazole rings. Some examples of unsaturated heterocyclic groups containing at least one sulfur atom include thiophene, thianaphthene, and benzothiophene rings. Some examples of unsaturated heterocyclic groups containing at least one oxygen atom and at least one nitrogen atom include oxazole, isoxazole, benzoxazole, benzisoxazole, oxazoline, 1,2,5-oxadiazole (furazan), and 1,3,4-oxadiazole rings. Some examples of unsaturated heterocyclic groups containing at least one nitrogen atom and at least one sulfur atom include thiazole, isothiazole, benzothiazole, benzoisothiazole, thiazoline, and 1,3,4-thiadiazole rings.
In some embodiments, any of the generic substituents (e.g., R, R1, R2, R3, R4, R5, and R6, and the like) described below may independently exclude any one or more of the classes, subclasses, or particular hydrocarbon groups described above, or may independently include only specific hydrocarbon groups selected from the hydrocarbon groups (R) described above. Similarly, any of the generic substituents described below may independently exclude any one or more heteroatoms or heteroatom-containing groups.
In a particular aspect, the invention is directed to a novel method for producing a conversion-coated (i.e., passivated and corrosion-resistant) magnesium-containing surface (substrate). The corrosion resistance of the treated substrate is significantly improved over the corrosion resistance of the substrate before the instantly described treatment and significantly improved over the corrosion resistance of the substrate treated with an ionic liquid but where the ionic liquid has not decomposed at the surface of the substrate. The improved corrosion resistance may be evidenced by a substantial absence of corrosion even after rigorous corrosion tests, such as being treated in a salt solution of at least 0.1 M, 0.2 M, 0.5 M, 1.0 M, 1.5 M, or 2.0 M concentration for at least 0.5, 1, 2, 3, 4, 5, 6, 12, 18, 24, 36, or 48 hours.
The term “magnesium-containing”, with respect to the magnesium-containing substrate before undergoing the conversion treatment described herein, encompasses any metallic substrate that contains metallic (i.e., elemental) magnesium. The amount of magnesium in the substrate is generally at least 20%, 30%, 40%, 50%, 60%, or 70%, and more typically, at least 80%, 85%, 90%, 95%, 96%, 97%, or 98%. In some embodiments, the magnesium-containing substrate is substantially all magnesium, which is generally at least 99%, 99.5%, 99.9%, or even 100% magnesium. In other embodiments, the magnesium-containing substrate is a magnesium alloy. The magnesium alloy can be any of the magnesium alloys known in the art, including a cast alloy or a wrought alloy. Some examples of magnesium alloys include the AZ31, AZ61, AZ63, AZ80, AZ81, AZ91, AM60, ZE41, ZC63, ZC71, ZK51, ZK60, ZK61, Elektron 21, Elektron 675, HK31, HM21, HZ32, QE22, QH21, WE54, and WE43 types of magnesium alloys. The alloying metal is typically selected from at least one, and typically, at least two or more of aluminum (Al), zinc (Zn), manganese (Mn), silicon (Si), copper (Cu), calcium (Ca), nickel (Ni), and iron (Fe).
In the method, the magnesium-containing substrate (i.e., “magnesium-containing surface”) is contacted with the ionic liquid compound by any suitable means. For example, the magnesium-containing substrate may be immersed in the ionic liquid compound, or a coating (film) of the ionic liquid compound may be applied to the magnesium-containing substrate, e.g., by dipping, spraying, painting, or spin-coating.
During contact of the magnesium-containing substrate with the ionic liquid compound, the ionic liquid in contact with the substrate is subjected to conditions that cause the ionic liquid compound to decompose (i.e., the ionic liquid is subjected to “decomposition conditions”). In decomposing, a structural change involving the breakage and/or creation of one or more covalent bonds occurs in either the cationic or anionic component of the ionic liquid to produce decomposition products of the ionic liquid. Any conditions that result in the decomposition of the ionic liquid compound are considered herein. Without being bound by any theory, it is believed that reaction between the substrate surface and decomposition products of the ionic liquid are primarily responsible for the substantially improved corrosion resistance of the treated substrate surface.
100451 In a first set of embodiments, the ionic liquid compound in contact with the magnesium-containing substrate is heated to a decomposing temperature to effect decomposition of the ionic liquid compound. The decomposing temperature is any temperature at which the ionic liquid decomposes, wherein it is understood that each ionic liquid possesses a distinct minimum temperature at which it decomposes. Typically, the decomposing temperature is at least 80° C., 90° C., or 100° C. In different embodiments, the decomposing temperature is precisely, about, at least, above, up to, or less than, for example, 80° C., 90° C., 100° C., 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., 260° C., 270° C., 280° C., 290° C., 300° C., 310° C., 320° C., 330° C., 340° C., 350° C., 360° C., 370° C., 380° C., 390° C., 400° C., 410° C., 420° C., 430° C., 440° C., 450° C., 460° C., 470° C., 480° C., 490° C., or 500° C., or a temperature within a range bounded by any two of the foregoing temperatures. Moreover, the decomposing temperature can be selected to be the minimum decomposing temperature of the ionic liquid, or alternatively, a temperature above the minimum decomposing temperature, e.g., a temperature of precisely, about, or at least 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, or 200° C. above the minimum decomposing temperature (e.g., to ensure reaction of the IL decomposition products with the substrate surface). For purposes of the invention, the ionic liquid is required to decompose at the magnesium-containing surface (and not, for example, only or substantially in the bulk of ionic liquid not in contact with the substrate surface); thus, when a decomposing temperature is used, the magnesium-containing surface (i.e., object) is generally heated to the decomposing temperature during contact with the ionic liquid compound to effect decomposition of the ionic liquid compound specifically at the substrate surface. When the ionic liquid decomposes at the surface of the substrate, the substrate is believed to react with decomposition products at the surface. These reactions at the surface of the substrate are believed to be responsible for the significantly improved corrosion resistance observed in the treated substrate. In some embodiments, the temperature at which the process is conducted is sufficient to not only decompose the IL, but to cause or promote sufficient reaction of the IL decomposition products with the substrate surface.
The ionic liquid in contact with the substrate can be subjected to a suitable decomposing temperature, such as any of those above, for any amount of time (i.e., processing time). The amount of time that the ionic liquid in contact with the substrate is subjected to the decomposing temperature is highly dependent on the temperature employed. As would be appreciated, higher temperatures generally require smaller processing times to achieve the same or similar effect achieved for lower temperatures at longer processing times. Depending on the decomposing temperature, the processing time may be, for example, 1 minute, 2 minutes, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 30 minutes, 40 minutes, 50 minutes, 1 hour, 2 hours, 3 hours, 6 hours, 12 hours, 18 hours, or 24 hours. In particular embodiments, a decomposing temperature of 150° C. to 300° C. is applied for 1-10 minutes, or a temperature of 100° C. to 150° C. is applied for 10-30 minutes, or a temperature of 300° C. to 400° C. is applied for 0.5 to 5 minutes. Moreover, the processing time may be the minimum time required to effect decomposition and reaction at the substrate surface, or a time beyond the minimum time required to effect decomposition and reaction at the substrate surface.
In a second set of embodiments, the magnesium-containing substrate is tribologically treated during contact with the ionic liquid compound to effect decomposition of the ionic liquid compound. The tribological treatment can be any of the means known in the art for tribologically treating a surface. By tribologically treating the surface is meant that the substrate surface is rubbed with another solid surface to the extent that the cation and/or anion component of the ionic liquid is decomposed by frictional heating or mechanical (shear) stresses provided by the tribological treatment. The rubbing action may or may not include transfer of a portion of the substrate material to the rubbing solid surface or vice-versa. For example, the substrate in contact with the ionic liquid may be subjected to a polishing process, or a non-polishing yet frictional process. The tribological process can be conducted at any suitable temperature, such as room temperature (generally, 18-30° C. or 20-25° C.) or an elevated temperature (e.g., precisely, about, at least, above, up to, or less than 40, 50, 60, 70, 80, 90, or 100° C., or a higher temperature as provided above), which may or may not be a decomposition temperature.
Other means for inducing decomposition of the ionic liquid may be employed. For example, the ionic liquid in contact with the substrate may undergo electrochemical decomposition by subjecting the ionic liquid to a suitable voltage. As another example, the ionic liquid in contact with the substrate may undergo photochemical decomposition by subjecting the ionic liquid to electromagnetic radiation (e.g., ultraviolet, x-ray, or gamma ray) at sufficient power. The decomposition process is suitable as long as it permits the ionic liquid to decompose at and react with the substrate surface.
In some embodiments, any of the means for decomposing the ionic liquid can be combined. For example, a substrate in contact with the ionic liquid may undergo tribological treatment while the ionic liquid is also heated to a decomposition temperature, such as any of the decomposition temperatures provided above. As another example, a substrate in contact with the ionic liquid may undergo heat treatment in addition to electrochemical or photochemical decomposition of the ionic liquid.
In some embodiments, the magnesium-containing surface is subjected to an acid pickling step prior to contacting the surface with an ionic liquid under conditions that result in decomposition of the ionic liquid. In the acid pickling step, the magnesium-containing surface is contacted with one or more acids, which can be an inorganic or organic acid. Typically, the acid is in the form of an aqueous solution, which may have a concentration of, for example, 0.1 to 10 M. Some examples of inorganic pickling acids include hydrohalogenic acids (e.g., HF, HCl, HBr, HI), nitric acid, sulfuric acid, and phosphoric acid. Some examples of organic pickling acids include carboxylic acids (e.g., acetic or propionic acids, and fluorinated versions, e.g., trifluoroacetic acid) and sulfonic acids (e.g., methylsulfonic or triflic acids). In some embodiments, any one or more of the foregoing acids are excluded from the acid pickling step.
In further embodiments, an alkaline conditioning step may be employed after the acid pickling step and prior to contacting the surface with an ionic liquid under conditions that result in decomposition of the ionic liquid. In the alkaline conditioning step, the magnesium-containing surface is contacted with one or more alkaline substances, typically in the form of an alkaline aqueous solution. Some examples of alkaline compounds that may be used in the alkaline conditioning step include the metal hydroxides (e.g., sodium hydroxide), carboxylate salts, metal carbonates or bicarbonates (e.g., sodium carbonate or bicarbonate), and amines (e.g., dimethyl or trimethyl amine).
The ionic liquid compound in contact with the surface of the magnesium-containing substrate can have any compositions of the art. As understand in the art, the term “ionic liquid compound” or “ionic liquid” is an ionic compound that is, itself, a liquid, i.e., without being dissolved in or solvated with a solvent. For purposes of the invention, both the cationic and anionic components of the ionic liquid contain at least one hydrocarbon group (R) in each.
The ionic liquid compound can be conveniently expressed by the formula Y+X−, wherein Y+ is a cationic component of the ionic liquid and X− is an anionic component of the ionic liquid. The formula (Y+)(X−) is meant to encompass a cationic component (Y+) having any valency of positive charge, and an anionic component (X−) having any valency of negative charge, provided that the charge contributions from the cationic portion and anionic portion are counterbalanced in order for charge neutrality to be preserved in the ionic liquid molecule. More specifically, the formula (Y+)(X−) is meant to encompass the more generic formula (Y+a)y(X−b)x, wherein the variables a and b are, independently, non-zero integers, and the subscript variables x and y are, independently, non-zero integers, such that a.y=b.x (wherein the period placed between variables indicates multiplication of the variables). The foregoing generic formula encompasses numerous possible sub-formulas, such as, for example, (Y+)(X−), (Y+2)(X−)2, (Y+)2(X−2), (Y+2)2(X−2)2, (Y+3)(X−)3, (Y+)3(X−3), (Y+3)2(X−2)3, and (Y+2)3(X−3)2. For simplicity, numerous embodiments of ionic liquids, described below, designate the anion as X−, which in its strict sense indicates a monovalent anion. However, the anion designated as X− is meant to encompass an anion of any valency, such as any of the valencies described above and further below, unless otherwise specified. In some embodiments, Y− can be a metal cation (e.g., an alkali metal, such as Li+), while in other embodiments Y− is not a metal cation. In some embodiments, Y+ can be an inorganic species, while in other embodiments, r is an organic species.
The ionic liquid is typically a liquid at room temperature (e.g., 15, 18, 20, 22, 25, or 30° C.) or lower. However, in some embodiments, the ionic liquid may become a liquid at a higher temperature than 30° C. if it is used at an elevated temperature that melts the ionic liquid. Thus, in some embodiments, the ionic liquid may have a melting point of up to or less than 100, 90, 80, 70, 60, 50, 40, or 30° C. In other embodiments, the ionic liquid is a liquid at or below 10, 5, 0,-10,-20,-30, or -40° C.
The density of the ionic liquid is typically in the range of 0.6-1.6 g/mL at an operating temperature of interest, and particularly at a temperature within 20-30° C. The viscosity of the ionic liquid is typically no more than 50,000 centipoise (50,000 cP) at an operating temperature of interest, and particularly at a temperature within 20-30° C. More typically, the viscosity of the ionic liquid is no more than about 25,000 cP, 10,000 cP, 5,000 cP, 2,000 cP, 1,000 cP, 800 cP, 700 cP, 600 cP, 500 cP, 400 cP, 300 cP, 200 cP, 100 cP, or 50 cP.
In particular embodiments, the cationic group Y+ of the ionic liquid has the generic formula:
In Formula (1), Z is either N or P, and R1, R2, R3, and R4 are each independently selected from hydrogen atom and hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described above for hydrocarbon groups R, provided that at least one of R1, R2, R3, and R4 is a hydrocarbon group R when Z is N, and provided that R1, R2, R3, and R4 are all hydrocarbon groups when Z is P. In particular embodiments, one, two, three, or all of R1, R2, R3, and R4 are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In other embodiments, one, two, three, or all of R1, R2, R3, and R4 are selected from saturated or unsaturated cyclic hydrocarbon groups, which may be carbocyclic (e.g., cycloalkyl or aryl) or heterocyclic (e.g., heterocycloalkyl or heteroaryl).
In particular embodiments of Formula (1), Z is N, which corresponds to ammonium species having the following formula:
In Formula (1a), R1a, R2a, R3a, and R4a are each in dependently selected from hydrogen atom and hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described above for hydrocarbon groups R, provided that at least one of R1a,R2a,R1a, and R4a is a hydrocarbon group R.
In particular embodiments, one, two, three, or all of R1a, R2a, R3a, and R4a of Formula (1a) are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. Some examples of such ammonium species include methylammonium, ethylammonium, vinylammonium, n-propylammonium, isopropylammonium, allylammonium, n-butylammonium, isobutylammonium, n-pentylammonium, n-hexylammonium, n-heptylammonium, n-octylammonium, 2-ethylhexylammonium, n-nonylammonium, n-decylammonium, n-undecylammonium, n-dodecylammonium, dimethylammonium, diethylammonium, divinylammonium, ethylmethylammonium, dipropylammonium, methylpropylammonium, diisopropylammonium, diallylammonium, dibutylammonium, methylbutylammonium, diisobutylammonium, dipentylammonium, methylpentylammonium, dihexylammonium, diheptylammonium, dioctylammonium, di(2-ethylhexyl)ammonium, dinonylammonium, didecylammonium, didodecylammonium, trimethylammonium, dimethylethylammonium, triethylammonium, trivinylammonium, tripropylammonium, triisopropylammonium, dimethylisopropylammonium, triallylammonium, tributylammonium, triisobutylammonium, diethylisobutylammonium, ethyldiisobutylammonium, tripentylammonium, trihexylammonium, triisohexylammonium, ethyldioctylammonium, trioctylammonium, tris(isooctyl)ammonium, methylbis(2-ethylhexyl)ammonium, tris(2-ethylhexyl)ammonium, trinonylammonium, tridecylammonium, tridodecylammonium, tetramethylammonium, tetraethylammonium, tetravinylammonium, tetrapropylammonium, tetraisopropylammonium, dimethyldiisopropylammonium, tetraallylammonium, tetrabutylammonium, tetraisobutylammonium, dimethyldiisobutylammonium, diethyldiisobutylammonium, methyltriisobutylammonium, ethyltriisobutylammonium, tetrapentylammonium, tetrahexylammonium, tetraisohexylammonium, ethyltrioctylammonium, tetraoctylammonium, tetrakis(isooctyl)ammonium, methyltris(2-ethylhexyl)ammonium, ethyltris(2-ethylhexyl)ammonium, tetrakis(2-ethylhexyl)ammonium, tetranonylammonium, tetradecylammonium, and tetradodecylammonium.
In other embodiments, one, two, three, or all of R1a, R2a, R3a, and R4a of Formula (1a) are selected from saturated or unsaturated cyclic hydrocarbon groups, which may be carbocyclic (e.g., cycloalkyl or aryl) or heterocyclic (e.g., heterocycloalkyl or heteroaryl) and may or may not include a hydrocarbon linker and/or one more hydrocarbon substituents. Some examples of such ammonium species include trimethylcyclopentylammonium, trimethylcyclohexylammonium, trimethylphenylammonium, trimethylbenzylammonium, trimethylnaphthylammonium, triethylcyclopentylammonium, triethylcyclohexylammonium, triethylphenylammonium, triethylbenzylammonium, triisopropylcyclopentylammonium, triisopropylcyclohexylammonium, triisopropylphenylammonium, triisopropylbenzylammonium, dimethylcyclopentylammonium, dimethylcyclohexylammonium, dimethylphenylammonium, dimethylbenzylammonium, diethylcyclopentylammonium, diethylcyclohexylammonium, diethylphenylammonium, diethylbenzylammonium, diisopropylcyclopentylammonium, diisopropylcyclohexylammonium, diisopropylphenylammonium, diisopropylbenzylammonium, dimethyldicyclopentylammonium, dimethyldicyclohexylammonium, dimethyldiphenylammonium, dimethyldibenzylammonium, diethyldicyclopentylammonium, diethyldicyclohexylammonium, diethyldiphenylammonium, diethyldibenzylammonium, diisopropyldicyclopentylammonium, diisopropyldicyclohexylammonium, diisopropyldiphenylammonium, diisopropyldibenzylammonium, dihexyldiphenylammonium, dioctyldiphenylammonium, dihexyldibenzylammonium, dioctyldibenzylammonium, methyltricyclopentylammonium, methyltricyclohexylammonium, methyltriphenylammonium, methyltribenzylammonium, ethyltricyclopentylammonium, ethyltricyclohexylammonium, ethyltriphenylammonium, ethyltribenzylammonium, isopropyltricyclopentylammonium, isopropyltricyclohexylammonium, isopropyltriphenylammonium, hexyltriphenylammonium, octyltriphenylammonium, isopropyltribenzylammonium, hexyltribenzylammonium, octyltribenzylammonium, tetracyclopentylammonium, tetracyclohexylammonium, tetraphenylammonium, and tetrabenzylammonium.
In particular embodiments of Formula (1), Z is P, which corresponds to phosphonium species having the following formula:
In Formula (1b), R1b, R2b, R3b, and R4b are each independently selected from hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described above for hydrocarbon groups R. In particular embodiments, one, two, three, or all of R1b, R2b, R3b, and R4b are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In other embodiments, one, two, three, or all of R1b, R2b, R3b, and R4b are selected from saturated or unsaturated cyclic hydrocarbon groups, which may be carbocyclic (e.g., cycloalkyl or aryl) or heterocyclic (e.g., heterocycloalkyl or heteroaryl).
In particular embodiments of Formula (1b), R1b, R2b, R3b, and R4b of Formula (1b) are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. Some examples of such phosphonium species include tetramethylphosphonium, tetraethylphosphonium, tetravinylphosphonium, tetrapropylphosphonium, tetraisopropylphosphonium, dimethyldiisopropylphosphonium, tetraallylphosphonium, tetrabutylphosphonium, tetraisobutylphosphonium, dimethyldiisobutylphosphonium, diethyldiisobutylphosphonium, methyltriisobutylphosphonium, ethyltriisobutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetrakis(isohexyl)phosphonium, ethyltrioctyiphosphonium, tetraoctylphosphonium, tetrakis(isooctyl)phosphonium, methyltris(2-ethylhexyl)phosphonium, ethyltris(2-ethylhexyl)phosphinium, tetrakis(2-ethylhexyl)phosphonium, trihexyldodecylphosphonium, tetranonylphosphonium, tetradecylphosphonium, and tetradodecylphosphonium.
In other embodiments, one, two, three, or all of R1b, R2b, R3b, and R4b of Formula (1 b) are selected from saturated or unsaturated cyclic hydrocarbon groups, which may be carbocyclic (e.g., cycloalkyl or aryl) or heterocyclic (e.g., heterocycloalkyl or heteroaryl) and may or may not include a hydrocarbon linker and/or one more hydrocarbon substituents. Some examples of such phosphonium species include trimethylcyclopentylphosphonium, trimethylcyclohexylphosphonium, trimethylphenylphosphonium, trimethylbenzylphosphonium, trimethylnaphthylphosphonium, triethylcyclopentylphosphonium, triethylcyclohexylphosphonium, triethylphenylphosphonium, triethylbenzylphosphonium, triisopropylcyclopentylphosphonium, triisopropylcyclohexylphosphonium, triisopropylphenylphosphonium, triisopropylbenzylphosphonium, dimethyldicyclopentylphosphonium, dimethyldicyclohexylphosphonium, dimethyldiphenylphosphonium, dimethyldibenzylphosphonium, diethyldicyclopentylphosphonium, diethyldicyclohexylphosphonium, diethyldiphenylphosphonium, diethyldibenzylphosphonium, diisopropyldicyclopentylphosphonium, diisopropyldicyclohexylphosphonium, diisopropyldiphenylphosphonium, diisopropyldibenzylphosphonium, dihexyldiphenylphosphonium, dioctyldiphenylphosphonium, dihexyldibenzylphosphonium, dioctyldibenzylphosphonium, methyltricyclopentylphosphonium, methyltricyclohexylphosphonium, methyltriphenylphosphonium, methyltribenzylphosphonium, ethyltricyclopentylphosphonium, ethyltricyclohexylphosphonium, ethyltriphenylphosphonium, ethyltribenzylphosphonium, isopropyltricyclopentylphosphonium, isopropyltricyclohexylphosphonium, isopropyltriphenylphosphonium, hexyltriphenylphosphonium, octyltriphenylphosphonium, dodecyltriphenylphosphonium, isopropyltribenzylphosphonium, hexyltribenzylphosphonium, octyltribenzylphosphonium, tetracyclopentylphosphonium, tetracyclohexylphosphonium, tetraphenylphosphonium, and tetrabenzylphosphonium.
In the cationic species of Formula (1), two or more of R1, R2, R3, and R4 may (i.e., optionally) be combined to form one or more cyclic groups that includes Z as a ring heteroatom. By analogy, two or more of R1a, R2a, R3a, and R4a of Formula (1a), or two or more of R1b, R2b, R3b, and R4b of Formula (1b), may (i.e., optionally) be combined to form one or more cyclic groups that includes Z as a ring heteroatom. Thus, if R1 and R2 are taken as ethyl groups, R1 and R2 may interconnect to form a five-membered ring that includes Z. The interconnected moiety may also contain one or more heteroatoms as a ring heteroatom, in addition to Z. Alternatively, or in addition to two or more of R1, R2, R3, and R4 interconnecting to form a ring, two of R1, R2, R3, and R4 may (i.e., optionally) be combined to form a group linked to Z by a double bond.
For example, R1a and R2a in Formula (1a) or R1b and R2b in Formula (1b) can be interconnected to result in cationic species having any of the following formulas:
In the above formulas, R7, R8, R9, R10, R11, R12, R13, and R14 are each independently selected from hydrogen atom and hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described above for hydrocarbon groups R, provided that R9, R10, R13, and R14 of the cyclic phosphonium species are all hydrocarbon groups R. Moreover, any of the groups, above, that are not shown as interconnected, may either interconnect with each other to make a second ring that includes N or P, or may interconnect with the existing ring to form a bicyclic structure. Some examples of such structures include:
The interconnection of R1a and R2a in Formula (1a) or R1b and R2b in Formula (1b) can also include one or more heteroatoms. Some examples of such cationic species include:
In the above structures, R7, R8, R11, and R12 are as defined above. The group R′ bound to the additional one or more hydrogen atoms is independently selected from hydrogen atom and any of the hydrocarbon groups (R) described above, particularly straight-chained or branched alkyl and/or alkenyl groups having 1, 2, 3, 4, 5, or 6 carbon atoms.
The interconnection of R1a and R2a and R2a in Formula (1a) or R1b and R2b in Formula (1b) can also be accompanied by one or more double bonds in the ring containing N or P. If one of the double bonds is connected to N or P in the ring, then one of R3a and R4a in Formula (1a) or one of R3b and R4b in Formula (1b) participates to make a double bond in the ring. If none of the double bonds are connected to N or P in the ring, then R3a and R4a in Formula (1a) or R3b and R4b in Formula (1b) are not required to participate in making a double bond in the ring, i.e., the double bond originated in this case from one of R1a and R2a in Formula (1a) or one of R1b and R2b in Formula (1b). As above, the interconnection may or may not also include one or more heteroatoms. Some examples of such cationic species include:
In yet other embodiments, the cationic component Y+ of the ionic liquid compound may be a sulfonium species of the formula:
In the above formula, R15, R16, and R17 are each independently selected from hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described above for hydrocarbon groups R. In particular embodiments, one, two, or all of R15, R16, and R17 are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In other embodiments, one, two, or all of R15, R16, and R17 are selected from saturated or unsaturated cyclic hydrocarbon groups. Moreover, as provided for the ammonium and phosphonium species in Formulas (1), (1a), and (1b), two or three of R15, R16, and R17 can be interconnected to form a cyclic or bicyclic structure.
In still other embodiments, the cationic component Y+ of the ionic liquid compound may be a guanidinium species of the formula:
In the above formula, R18, R19, R20, R21, R22, and R23 are each independently selected from hydrocarbon groups (R) having at least one and up to twenty carbon atoms and optionally substituted with one or more heteroatoms selected from fluorine, nitrogen, oxygen, and sulfur, as described above for hydrocarbon groups R. In particular embodiments, one, two, three, four, five, or all of R18, R19, R20, R21, R22, and R23 are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In other embodiments, one, two, three, four, five, or all of R18, R19, R20, R21, R22, and R23 are selected from saturated or unsaturated cyclic hydrocarbon groups. Moreover, as provided for the ammonium and phosphonium species in Formulas (1), (1a), and (1b), any one or more of R18,R19, R20, R21,R22, and R23 can be interconnected to form a cyclic or bicyclic structure.
The anionic component X− of the ionic liquid compound is any counteranion which, when associated with the cationic component, permits the resulting ionic compound to behave as an ionic liquid. As known in the art, the composition and structure of the counteranion strongly affects the properties (e.g., melting point, volatility, stability, viscosity, hydrophobicity, and so on) of the ionic liquid. In some embodiments, the counteranion is structurally symmetrical, while in other embodiments, the counteranion is structurally asymmetrical.
In one embodiment, the counteranion (X−) of the ionic liquid is inorganic. The inorganic counteranion may, in one embodiment, lack fluorine atoms. Some examples of such counteranions include chloride, bromide, iodide, hexachlorophosphate (PCl6−), perchlorate, chlorate, chlorite, cyanate, isocyanate, thiocyanate, isothiocyanate, perbromate, bromate, bromite, periodiate, iodate, dicyanamide (i.e., N(CN)2−), tricyanamide (i.e., N(CN)3−), aluminum chlorides (e.g., Al2Cl7− and AlCl4−), aluminum bromides (e.g., AlBr4−), nitrate, nitrite, sulfate, sulfite, hydrogensulfate, hydrogensulfite, phosphate, hydrogenphosphate (HPO42−), dihydrogenphosphate (H2PO4−), phosphite, arsenate, antimonate, selenate, tellurate, tungstate, molybdate, chromate, silicate, the borates (e.g., borate, diborate, triborate, tetraborate), anionic borane and carborane clusters (e.g., B10H102− and B12H122−), perrhenate, permanganate, ruthenate, perruthenate, and the polyoxometallates. The inorganic counteranion may, in another embodiment, include fluorine atoms. Some examples of such counteranions include fluoride, bifluoride (HF2−), hexafluorophosphate (PF6−), fluorophosphate (PO3F2−), tetrafluoroborate (BF4−), aluminum fluorides (e.g., AlF4−), hexafluoroarsenate (AsF6−), and hexafluoroantimonate (SbF6−).
In another embodiment, the counteranion (X−) of the ionic liquid is organic. The organic counteranion may, in one embodiment, lack fluorine atoms. Some examples of such counteranions include carbonate, bicarbonate, the carboxylates (e.g., formate, acetate, propionate, butyrate, valerate, lactate, pyruvate, oxalate, malonate, glutarate, adipate, decanoate, salicylate, ibuprofenate, and the like), the sulfonates (e.g., CH3SO3−, CH3CH2SO3−, CH3(CH2)2SO3−, benzenesulfonate, toluenesulfonate, dodecylbenzenesulfonate, docusate, and the like), the alkoxides (e.g., methoxide, ethoxide, isopropoxide, phenoxide, and glycolate), the amides (e.g., dimethylamide and diisopropylamide), diketonates (e.g., acetylacetonate), the organoborates (e.g., BR1R2R3R4−, wherein R1, R2, R3, R4 are typically hydrocarbon groups containing 1 to 6 carbon atoms), the alkylsulfates (e.g., diethylsulfate), hydrocarbylphosphates (e.g., ethylphosphate or diethylphosphate), hydrocarbylphosphonates (e.g., diethylphosphonate, diisopropylphosphonate, dibutylphosphonate, or bis-(2,4,4-trimethylpentyl)phosphonate), phosphinates (e.g., bis-(2,4,4-trimethylpentyl)phosphinate), and the analogous thio, dithio, and trithio versions of the phosphates, phosphonates, and phosphinates. The organic counteranion may, in another embodiment, include fluorine atoms. Some examples of such counteranions include the fluorosulfonates (e.g., CF3SO3−, CF3CF2SO3−, CF3(CF2)2S03−, CHF2CF2SO3−, and the like), the fluoroalkoxides (e.g., CF3O−, CF3CH2O−, CF3CF2O−, and pentafluorophenolate), the fluorocarboxylates (e.g., trifluoroacetate and pentafluoropropionate), and the fluorosulfonylimides (e.g., (CF3SO2)2N).
In particular embodiments, the counteranion (X−) of the ionic liquid has a structure according to the following general formula:
In above formula, subscripts m and n are independently 0 or an integer of 1 or above. Subscript p is 0 or 1, provided that when p is 0, the group —N—SO2—(CF2)nCF3 subtended by p is replaced with an oxide atom connected to the sulfur atom (S). The generic structures corresponding to when p is 1 or 0 are, respectively:
In other particular embodiments, the counteranion (X−) of the ionic liquid is a phosphorus-containing anion, including any of the phosphorus-containing anions identified above. In further specific embodiments, the phosphorus-containing anion has the generic formula:
In Formula (2), R5 and R6 are independently selected from hydrocarbon groups having at least one and up to twenty carbon atoms, wherein the hydrocarbon groups are optionally substituted with one or more fluorine atoms. The groups X1, X2, W, and Y are independently selected from O and S atoms, and the subscripts r and s are independently selected from 0 and 1. In particular embodiments, one or both of R5 and R6 are selected from straight-chained or branched alkyl and/or alkenyl groups having at least 1, 2, 3, or 4 and up to 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms, or at least 5, 6, 7, or 8 and up to 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms. In other embodiments, one or both of R5 and R6 are selected from saturated or unsaturated cyclic hydrocarbon groups. Moreover, as provided for the ammonium and phosphonium species in Formulas (1), (1a), and (1b), R5 and R6 can optionally be interconnected to form a cyclic structure.
In one set of embodiments of Formula (2), all of X1, X2, W, and Y are oxygen atoms, which corresponds to the following sub-formula:
In a separate set of embodiments of Formula (2), subscripts r and s are both 1, which corresponds to the following sub-formula:
In one set of embodiments of Formula (2a), all of X1, X2, W, and Y are oxygen atoms, which corresponds to the following sub-formula (i.e., phosphate diester):
In another set of embodiments of Formula (2a), one of X1, X2, W, and Y is a sulfur atom. Generally, the single sulfur atom is at group W, which corresponds to the following sub-formula (i.e., thiophosphate diester):
In another set of embodiments of Formula (2a), two of X1, X2, W, and Y are sulfur atoms. Generally, the two sulfur atoms are at groups W and Y, which corresponds to the following sub-formula (i.e., dithiophosphate diester):
In the above formula, one or two of the remaining oxygen atoms may be replaced with sulfur atoms to result in a trithiophosphate or tetrathiophosphate species, respectively.
In a separate set of embodiments of Formula (2), one of subscripts r and s is 0 (e.g., r is 1 and s is 0), which corresponds to the following sub-formula:
In one set of embodiments of Formula (2b), all of X1, W, and Y are oxygen atoms, which corresponds to the following sub-formula (i.e., phosphonate ester):
In another set of embodiments of Formula (2b), one of X1, W, and Y is a sulfur atom. Generally, the single sulfur atom is at group W, which corresponds to the following sub-formula (i.e., thiophosphonate ester):
In another set of embodiments of Formula (2a), two of X1, W, and Y are sulfur atoms. Generally, the two sulfur atoms are at groups W and Y, which corresponds to the following sub-formula (i.e., dithiophosphonate ester):
In the above formula, the remaining oxygen atom may be replaced with a sulfur atom to result in a trithiophosphonate species.
In a separate set of embodiments of Formula (2), both subscripts r and s are 0, which corresponds to the following sub-formula:
In one set of embodiments of Formula (2c), both of W and Y are oxygen atoms, which corresponds to the following sub-formula (i.e., phosphinate):
In another set of embodiments of Formula (2c), one of W and Y is a sulfur atom. Generally, the single sulfur atom is at group W, which corresponds to the following sub-formula (i.e., thiophosphinate):
In another set of embodiments of Formula (2c), both W and Y are sulfur atoms, which corresponds to the following sub-formula (i.e., dithiophosphinate):
In yet other embodiments of Formula (2) or any of its sub-formulas, r and s are both 1 (i.e., X1 and X2 are both present), but one of R5 or R6 may be absent, which results in a divalent anion. The divalent anion can be depicted, for example, as follows:
or in exemplary sub-embodiments thereof:
The ionic liquid compound includes any of the above cationic species Y+ and any of the above anionic species X−. Thus, in particular embodiments, the ionic liquid compound can be expressed according to the generic formula:
or according to any of the sub-generic formulas:
In some embodiments, a single ionic liquid compound is used, while in other embodiments, a combination of two or more ionic liquid compounds may be used. The combination of ionic liquid compounds may correspond to the inclusion of two or more cationic species (such as those described above) in the presence of one, two, or more anionic species (such as those described above). Alternatively, the combination of liquid compounds may correspond to the inclusion of a single cationic species in the presence of two or more anionic species.
In one embodiment, the ionic liquid is not dissolved or otherwise mixed or in contact with a non-ionic liquid solvent (i.e., “solvent”) during contact of the ionic liquid with the substrate surface. In other embodiments, the ionic liquid is dissolved or otherwise mixed or in contact with one or more solvents during contact of the ionic liquid with the substrate surface. The solvent can be, for example, protic or non-protic and either polar or non-polar. Some examples of protic solvents include water and the alcohols (e.g., methanol, ethanol, n-propanol, isopropanol, n-butanol, and isobutanol). Some examples of polar aprotic solvents include ether, ester, nitrile, sulfoxide, and amide solvents. Some examples of ether solvents include diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, diglyme, triglyme, 1,3-dioxolane, and the fluorinated ethers (e.g., mono-, di-, tri-, tetra-, penta-, hexa- and per-fluoro derivatives of any of the foregoing ethers). Some examples of ester solvents include 1,4-butyrolactone, ethylacetate, methylpropionate, ethylpropionate, propylpropionate, methylbutyrate, ethylbutyrate, the formates (e.g., methyl formate, ethyl formate, or propyl formate), and the fluorinated esters (e.g., mono-, di-, tri-, tetra-, penta-, hexa- and per-fluoro derivatives of any of the foregoing esters). Some examples of nitrile solvents include acetonitrile, propionitrile, and butyronitrile. Some examples of sulfoxide solvents include dimethyl sulfoxide, ethyl methyl sulfoxide, diethyl sulfoxide, methyl propyl sulfoxide, and ethyl propyl sulfoxide. Some examples of amide solvents include formamide, N,N-dimethylformamide, N,N-diethylformamide, acetamide, dimethylacetamide, diethylacetamide, gamma-butyrolactam, and N-methylpyrrolidone. The polar aprotic solvent may also be, for example, an organochloride (e.g., methylene chloride, chloroform, 1,1,-trichloroethane), ketone (e.g., acetone, 2-butanone), organoethers (e.g., diethyl ether, tetrahydrofuran, and dioxane), hexamethylphosphoramide (HMPA), N-methylpyrrolidinone (NMP), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), and propylene glycol monomethyl ether acetate (PGMEA). Some examples of non-polar solvents include the liquid hydrocarbons, such as the pentanes, hexanes, heptanes, octanes, pentenes, hexenes, heptenes, octenes, benzene, toluenes, or xylenes. In some embodiments, one or more classes or specific types of any of the above solvents are not included with the ionic liquid during the treatment process.
In another aspect, the invention is directed to a mechanical component or device (and more generally, an object) containing the passivated magnesium-containing surface described above. The mechanical component can be, for example, one which is intended to be used in operations that subject the component or device to moderate to severe wear and physical or tribological stress. An example of such a mechanical component is a bearing, such as a slide bearing. Other types of mechanical components considered herein include engine parts, rotary blades, turbine fans, compressor blades, pistons, and structural components (e.g., framework of an automobile or airplane). The mechanical component or device can also have any suitable shape or form. For example, in different embodiments, the passivated magnesium-containing substrate may have a generally cylindrical, tubular, ring, spherical, ingot, disk (e.g., circular blade or saw), sheet, wire, mesh, grooved, particulate, or jagged type of shape, depending on the particular application.
Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein.
Tetraoctylammonium bis(2-ethylhexyl)phosphate ([N8888][DEHP]) was synthesized by the following general scheme:
Specifically, tetraoctylammonium bromide ([N8888]Br, 11.05 g, 20.2 mmol) and di(2-ethylhexyl)phosphoric acid (HDEHP, 6.53 g, 20.2 mmol) were mixed in 50 mL of deionized water (DI H2O, 18.2 Me-cm) and 30 mL of hexanes. To this stirred suspension was added a solution of sodium hydroxide (NaOH, 0.808 g, 20.2 mmol) in 25 mL of DI water dropwise at room temperature. The white suspension became clear after the addition of NaOH was completed. The mixture continued to be stirred at room temperature overnight. The upper organic phase was separated and washed with DI water four times to ensure removal of NaBr. Solvents were distilled off by rotary evaporator and the product was dried at 70° C. under vacuum for 4 hours to yield [N8888][DEHP] as a viscous liquid (14.93 g, 18.9 mmol, yield: 93.8%). The water content of [N8888][DEHP] was found to be 0.3%. TGA: Tonset=236° C.
Trioctylammonium di(2-ethylhexyl)phosphate ([N888H][DEHP]) was synthesized by the following general scheme:
Specifically, the protic IL [N888H][DEHP] was synthesized by neutralization of an equal molar ratio of trioctylamine (2.37 g, 6.69 mmol) and di(2-ethylhexyl) phosphoric acid (HDEHP, 2.16 g, 6.69 mmol) at room temperature for two hours. The mixture became more viscous upon stirring. The water content of [N888H] [DEHP] was 980 ppm. TGA: Tonset=239° C.
Numerous other ionic liquids can be synthesized by analogous or similar methods to those provided above. For example, ionic liquids having a phosphonium cation can similarly be prepared. For example, trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P66614][DEHP]) may be prepared by reaction of trihexyltetradecylphosphonium bromide ([P66614]Br) with di(2-ethylhexyl)phosphoric acid (HDEHP). Numerous other ionic liquids within the scope of ionic liquids described herein can be obtained commercially, or synthesized, if necessary, by methods analogous to those herein described and as well known in the art.
The Mg AZ31B material used in these experiments is a wrought alloy believed to have the following approximate composition, as provided in Table 1 below:
All surfaces were polished using SiC abrasive papers up to grit P1200. The first block was untreated. The second block was covered by a thin film of the IL tetraoctylammonium bis(2-ethylhexyl)phosphate ([N8888][DEHP]) and was kept at ambient (room temperature, i.e., “RT”) environment overnight. The third block was wetted by the same IL and then baked at 250° C. (i.e., high temperature or “HT”) for 5 minutes followed by an air cool. Both coupons after the treatment were rinsed with ethanol and then distilled water to remove any residual IL. All three coupons were then immersed in a 0.1 M NaCl solution. As shown by the photograph in
One side of a Mg AM60B disc was polished with SiC abrasive papers up to grit P4000 while in contact with distilled water. The foregoing treated side of the disc is herein referred to as “untreated”. The other side of the disc was subjected to the same procedure, except that in the last step of polishing, the IL trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate ([P66614] [DEHP]) was used instead of water. The foregoing treated side of the disc is herein referred to as “IL tribo-treated”. After the treatments, the entire disc was cleaned by organic solvents and dried before immersion in 1.0 M NaCl solution for 4 hours (higher CF concentration was used because of the better corrosion resistance of AM60B compared to AZ31B alloy).
Square (25.4 mm×19.05 mm×6.35 mm) and disk (15 mm diameter×3 mm thickness) AZ31B samples were cut and the test surfaces were ground using SiC abrasive paper up to P2400. The samples were then cleaned with acetone and deionized water (DI) and dried by air stream. The conversion coating was formed by applying a layer of the ionic liquid [N888H] [DEHP] to the sample surface and then heat treating the sample at 300° C. in a furnace for 7 minutes. After the heat treatment, the sample surface was cleaned with ethyl alcohol in an ultrasonic cleaner and dried by air stream. For comparison, another sample was treated at room temperature (RT) using the same IL for 24 hours.
Potentiodynamic polarization measurements were conducted at potentials from −300 to +600 mV vs. OCP (open circuit potential) at a scanning rate of 0.166 mV/s under aerated conditions at room temperature. The test solution was 1 wt. % NaCl saturated with Mg(OH)2 (0.25 g/L). A three-electrode electrochemical cell was used. A Pt sheet (25 mm×25 mm×0.2 mm) was used as a counter electrode and Ag/AgCl (4M KCl electrolyte saturated with AgCl) as a reference electrode and a AZ31B disk sample with 1 cm2 exposed area as a working electrode. The sample to be tested was immersed for 30 minutes in the test solution before starting the experiment.
Square (25.4 mm×19.05 mm×6.35 mm) and disk (15 mm diameter×3 mm thickness) samples of Mg AZ31B alloy were cut and the test surfaces were ground using SiC abrasive paper up to P2400. The samples were then cleansed with acetone and deionized water, and dried by an air stream. The conversion coating was formed by applying a layer of [N8888][DEHP] to the sample surface and then heat treating the sample at 300° C. in a furnace for 7 minutes. After the heat treatment, the sample surface was cleansed with ethanol in an ultrasonic cleaner and dried with a stream of air. For comparing the corrosion protection with the literature-reported IL treatment at room temperature (RT), another sample was treated at room temperature (RT) using the same IL for 24 hours. The name of those latter samples is joined by the suffix “RT” to distinguish them from the other IL-treated samples treated at 300° C.
This study also investigated the synergy between pretreatments and the IL coating process. Magnesium generally forms a stable protective film in a high pH solution but often is severely attacked by acid solutions with some exceptions like chromic acid (H2CrO4), hydrofluoric acid (HF), and phosphoric acid (H3PO4). Selected AZ31B samples were pickled in phosphoric (H3PO4) or nitric acid (HNO3), and some were followed by conditioning in sodium hydroxide (NaOH) solution, as detailed in Table 2. The concentration of the acid or base used was 40 g/L and the application time was 20 seconds at room temperature (25±2° C.). Solutions were prepared using chemical reagents and DI water.
The methods used for surface characterization and electrochemical evaluation were similar to those in the Example 4. To investigate the effect of corrosive solution on the IL conversion film, the best-performing surface (P+IL—300C) was exposed to 1 wt. % NaCl saturated with Mg(OH)2 for 4 hours. XPS analysis was conducted on this sample before and after the exposure to the corrosive solution.
To further elucidate the synergy between the IL treatment at 300° C. and pretreatment, electrochemical results were benchmarked against those of the samples treated with the IL at RT, as shown in
Important corrosion parameters deduced from polarization curves shown in
To establish a more fundamental understanding of the mechanism of IL/alloy interactions that provided the corrosion protection, XPS analyses were conducted on selected samples after the IL treatment but before corrosion testing. XPS was also conducted on the best performing P+IL—300C after a four-hour exposure to the chloride-containing solution (1 wt. % NaCl sat. with Mg(OH)2). This latter sample is referred to as “P+IL—300C corr”.
Table 4 shows XPS survey data for the IL-coated samples at 300° C. taken on the surface immediately after a 10 second Ar-ion sputter etching. This short etching was performed to remove any contaminations adsorbed on the surface. The data in Table 4 indicates high oxygen content in the surface layer on all investigated samples, and this suggests substantial oxidation during the coating process. The phosphorus content on IL—300C and B+IL— 300C appears to originate solely from the IL treatment, while the phosphorus content on P+B+IL—300C and P+IL—300C appears to be in part contributed by the phosphoric acid pickling as well. The decreased phosphorus content on the P+IL—300C surface after the corrosion test implies that the conversion coating actively interacted with the salt solution and part of the coating was dissolved in the corrosion process. The unexpected Si content on the P+IL—300C_corr may be introduced by impurities in the salt (NaCl).
The XPS core level spectra of Mg, P, O, and C are shown in
Aided by ion-sputtering, XPS composition-depth profiles were generated for the IL—300C, P+IL—300C, and P+IL—300C_corr surfaces.
The XPS composition-depth profile of the P+IL—300C_corr surface showed slightly higher oxygen concentration and longer sputtering time to remove the non-metallic surface layer imply growing surface oxides when attacked by CF ions in the salt solution. The phosphorus content (˜2 at. %) appeared lower at the top layer than that (˜6 at. %) of the surface before corrosion. This was caused by the interactions, chemically reacting and physically dissolving, between the IL conversion coating and the salt solution. It is significant that the bulk of the surface film maintained >2 at. % phosphorus even after the four-hour aggressive corrosion test, which suggests good corrosion inhibition for the IL conversion coating.
Results of electrochemical corrosion evaluations and XPS surface chemical analyses show strong anti-corrosion characteristics for the P+IL—300C conversion coating. The combination of phosphoric acid pickling and elevated temperature (300° C.) IL treatment showed an unexpected favorable synergy.
While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.
This invention was made with government support under Prime Contract No. DE-AC05-000R22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
