Patent Application
20130299355
The application relates generally to electrodeposition and more specifically to methods, processes, and chemical solutions for preparing and activating active metal substrate surfaces to accept electrodeposited metallic coatings in ionic liquids.
Thorough surface cleaning and activation is required to insure adequate adhesion and coverage of the electrodeposited coatings to a substrate. Most active metals and alloys naturally form an oxide layer immediately on exposure to air. This oxide layer forms a physical barrier to bonding metallic coatings such as electrodeposited coatings and must be removed or prevented from forming. There are processes established for preventing oxide formation when using a water based electrodeposition process. However, traditional surface pretreatment used with aqueous deposition solutions is incompatible with certain ionic liquid based electrolytes and processes. Many ionic liquid based electrolytes and techniques are sensitive to residual water. Neither the ionic liquid solvent nor the electrolyte can effectively remove the surface layer formed during the aqueous pretreatment. The ionic liquid electrodeposition solution also cannot prevent regeneration of surface oxides and contamination.
A surface preparation solution comprises an ionic liquid solvent and a water and oxygen scavenging species.
A method for making a surface preparation and activation solution for ionic liquid electrodeposition comprises providing an ionic liquid solvent compatible with the substrate surface and a species to be deposited from the ionic liquid. A first water and oxygen scavenging species is added to the ionic liquid solvent.
A method for preparing and activating a substrate surface for electrodeposition comprises removing contamination and oxides from the substrate surface. The surface is dried in a substantially oxygen and water free environment. The substrate surface is immersed into a surface preparation and activation solution. The solution comprises an ionic liquid solvent and a water and oxygen scavenging species.
Aqueous bulk electrodeposition is a well established process for depositing a variety of common metal coatings. However, aqueous electrodeposition does not readily work for a wide variety of metals and alloys. For example, bulk electrodeposition of pure aluminum or titanium from aqueous solutions onto active substrates (such as aluminum or magnesium alloys) is essentially impossible. This is due to the chemistry of the required plating solution, as well as the tendency toward formation of surface oxides and other contamination of the substrate. Further, water from the aqueous solution dissociates into hydrogen and oxygen ions at an applied voltage lower than what is necessary for many metal cations to dissociate and reduce to a metallic state onto the substrate. The end result is that effective bulk electrodeposition of many active metals must take place from other forms (i.e. non-aqueous solutions such as molten salts), each of which have their difficulties.
Ionic liquid solutions have shown promise for bulk electrodeposition of many different metals, including those not well-suited for electrodeposition from aqueous solutions. For example, aluminum and titanium can be deposited from ionic liquid solutions onto various Al or Mg alloys. The exact definition of an ionic liquid solvent varies based on the particular use but ionic liquids are typically but not exclusively large ionic complexes existing in a liquid state at or near ambient conditions. Specific examples are described below. Traditional depositing species include metal halide salts, such as aluminum chloride (AlCl3). Traditional depositing species like AlCl3 are soluble in many ionic liquids, making the combination useful for bulk electrodeposition. However, these metal halide salts alone are not considered ionic liquids because their molten state is not reached until heated up to and beyond 2000° F. In contrast, ionic liquid solvents exist in a molten state at much lower temperatures than metal halide salts, in many cases below about 212° F. (100° C.).
Prior to bulk electrodeposition, the substrate surface to be coated is cleaned and activated. This has two primary functions: 1) creating a contaminant- and oxide-free interface between a coating and a substrate; and 2) enhancing chemical and/or metallic bond between a coating and a substrate via control of the interface chemistry of the substrate.
In a well established cleaning and surface activation process for aqueous bulk electrodeposition, a protective zinc layer is immersion coated onto the substrate after removal of the outer surface oxides and foreign debris. Aluminum oxide dissolution and zinc deposition occur spontaneously as described by the reactions shown in Equations 1a and 1b:
Al2O3+2OH−→AlO2+H2O [1a]
ZnO22−+2Al+2H2O→3Zn+4OH−+2AlO2− [1b]
The zinc coating tends to be thin (<500 nm) and it protects the deoxidized surface from reacting with atmospheric oxygen and other substances present during transfer to an aqueous plating bath. The strongly acidic aqueous electrodeposition solution dissolves the zinc layer, resulting in an activated deposition surface. One of the important steps of a traditional pretreatment process is keeping the substrate surface wet to maintain surface activity.
Because there are substantial differences between aqueous and ionic liquid electrodeposition solutions, the traditional surface preparation methods and surface activation solution cannot be used with ionic liquids. First, unlike aqueous plating solutions, ionic liquids do not have the same acidity, or affinity for donating protons. Thus they cannot be used to achieve desirable coating properties without additional preparation of the receiving surface. Specifically, most ionic liquid electrolytes are inactive and thus do not help to remove the zinc or similar protective layer formed from surface pretreatments established for aqueous plating. Another difference is stronger sensitivity and reactivity of ionic liquid electrolytes and the nature of active substrates like aluminum and titanium upon exposure to water and oxygen containing environments. Surface oxides form extremely rapidly when those active substrates are exposed to ambient atmospheres. In addition, residual environmental or surface moisture tends to react with the ionic liquid to produce species detrimental to electrodeposition. Specifically, chlorooxoaluminate (AlOCl) can be formed on the substrate surface from residual water reacting with chloroaluminate-based ionic liquids. Oxides and chlorooxoaluminate, and other similar contaminants on the active substrate surface can disrupt or completely prevent bonding between the substrate and the electrodeposited coating. These or similar contaminants can hamper electrodeposition or even terminate the process completely.
Chlorooxoaluminate precipitates onto aluminum alloy surfaces in the presence of water as shown in equation [2]. The reaction should therefore be avoided or inhibited so as to maintain a clean and activated surface suitable for quality ionic liquid bulk electrodeposition.
AlCl4−+H2O→AlOCl+2HCl+Cl+Cl− [2]
To minimize formation of chlorooxoaluminate on the substrate after mechanical and/or chemical surface cleaning, the substrate surface can be further prepared and activated by a surface preparation solution. The ionic liquid based solution can contain one or more water and oxygen scavengers. The solution contains at least one chemical species operating as a scavenging additive. The additive may remain in substantially the same form in solution as when added to the ionic liquid. Alternatively, an additive may decompose in the ionic liquid or react with other constituents in the solution to produce one or more scavenging species as explained below.
A surface cleaning and activation solution can optionally also include a depositing species added to the mixture during or after addition of the scavenging additive(s), such as a metal halide salt that is compatible with the ionic liquid solvent and the scavenging additive(s). The ionic liquid solvent may be any one of a number of commercially available ionic liquids. In certain embodiments where aluminum is the depositing species, the ionic liquid solvent is a form of methylimidazolium chloride. In certain of those embodiments, the ionic liquid solvent comprises at least one of: 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) amide, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) amide, trihexyl-tetraadecyl phosphonium bis(trifluoromethylsulfonyl) amide, and mixtures thereof. As for the depositing species, in certain embodiments the metal halide salt is at least one aluminum halide. In certain of those embodiments the aluminum halide is aluminum chloride (AlCl3).
In certain embodiments where titanium is the depositing species, the ionic liquid solvent can include at least one of: 1-butyl-3-methyl-imidazolium, bis-(trifluoromethylsulfonyl) amide, or a form of 1-dialkylpyrrolidinium. As for the depositing species, in certain embodiments the metal halide salt comprises a titanium halide. In certain of those embodiments the titanium halide is titanium chloride (TiCl4).
In certain embodiments, the water and oxygen scavenging species or additive comprises an acid halide. Such compounds are characterized by the inclusion of an acyl group on one end of the additive molecule(s), where the acyl group includes a halogen atom bonded to the carbon atom of a carbonyl (C═O) group. The additive can additionally or alternatively include longer acyl halide molecules. These larger scavenging molecules can additionally or alternatively include halogen saturated carbon atoms, e.g., CCl3(CCl2)x(C═O)Cl, where x>0. In certain of these embodiments, all of the carbon atoms comprising the remainder of the molecular chain, apart from the acyl group, are saturated with halogen atoms, resulting in a scavenging species absent any hydrogen atoms. The halogen atom(s) of the acid halide can be the same species as the halide anion of the depositing species described above. For example, the depositing species is AlCl3 and all of the halogen atoms in the acid halide are chlorine atoms.
In certain embodiments, the water and oxygen scavenging species additionally or alternatively comprises at least one member of the phosgene family. Phosgenes, related to acyl chlorides, are based on a single carbonyl (C═O) group with two remaining bonding positions on the carbon atom. In phosgenes, each of the two available bonding positions has either a single chlorine atom or a chlorine-saturated carbon atom. Phosgenes lack the carbon chain of longer acid halides, thus phosgene chemicals can be characterized as stand-alone acyl groups.
Phosgene is also known by its IUPAC name of carbonyl dichloride , Cl(C═O)Cl, with the two bonding positions of the carbonyl group both having chlorine atoms. Other members of the family include trichloromethyl chloroformate, CCl3(C═O)Cl, and bis-trichloromethyl carbonate, CCl3(C═O)CCl3, known respectively as diphosgene and trisphosgene. In diphosgene and triphosgene, one or both of the chlorine atoms of phosgene are respectively replaced with a trichloromethyl (CCl3) group. Upon being added to the ionic liquid, liquid diphosgene and/or solid triphosgene can react with free chloride anions in solution. A substantial portion of the molecules will decompose, leaving behind a solution where the scavenging species exists primarily as phosgene, Cl(C═O)Cl.
The example scavenging molecules from the additive(s) described above include bonds with extremely strong dipole moments. Both the oxygen of the carbonyl group and the halogen atoms have partially balanced dipole moments and provide stability for the molecules until added to the ionic liquid. The halogen atom(s) are highly electronegative, leaving highly polar bonds between the halogen (or halogen saturated carbon) atoms on one side, and the carbonyl group on the scavenging molecule. Given the polarity of the various bonds, along with the charged ions of the ionic liquid solvent, in an equilibrium state, a substantial number of halogen (or halogen saturated carbon) atoms dissociate in solution from the respective scavenging molecules. When the halogen atoms dissociate, the resulting halide (e.g., chloride Cl−) anions exist in solution. This leaves the carbon of the carbonyl group strongly positive.
In yet another set of alternative embodiments, the carbonyl group in the example acyl based scavengers can be replaced by a thionyl (S═O) group. The thionyl bond will have similar polarity to a carbonyl group when combined with a halogen atom such as chlorine, and thus can react in a similar manner as carbonyl-based scavengers.
An example with respect to aluminum deposition onto an aluminum alloy will illustrate the general mechanism of the simultaneous scavenging reactions. The reaction utilizes an ionic liquid electrodeposition solution containing an ionic liquid solvent, a scavenging species, and a metal halide salt. The scavenging species here will be phosgene, which may come directly from phosgene added to the ionic liquid or from one or more of the above members of the phosgene family (i.e., diphosgene and/or triphosgene). The metal halide salt is AlCl3 and the ionic liquid solvent is one of the example aluminum-compatible solvents listed above.
In part because of the highly polar carbonyl or thionyl bonds, the example additive molecules operate as both water scavengers and oxygen scavengers. As noted in equation [2] above, the aluminum halide or one of its anion complexes is reactive with water, which leaves behind hydrolysis products such as chlorooxoaluminate, AlOCl, which accumulate on the substrate surface. Equation [3] shows how an additive like phosgene prevents AlOCl contamination by removing adsorbed water from a cleaned substrate surface.
COCl2(scavenger)+H2O→HCl+CO2 [3]
As noted above, aluminum oxide (Al2O3) rapidly forms in the presence of air and in aqueous electrolytic solutions. It can form and accumulate on the previously cleaned substrate. In an aqueous solution, a zinc displacement layer prevents reformation of surface oxides, and is removed upon contact with the acidic solution. Since ionic liquids cannot dissolve the zinc layer spontaneously, the oxide must be removed completely prior to electrodeposition. As shown in Equation [4], phosgene reacts with the oxide to form aluminum chloride (AlCl3).
3 COCl2(scavenger)+Al2O3→2AlCl3+3CO2 [4]
The reaction forms additional AlCl3, some of which can be optionally added to the solution during or after addition of the scavenging additive. AlCl3 then combines with free chloride (Cl−) or tetracholoroaluminate (AlCl4−) from the ionic liquid electrodeposition solution. This forms anionic complexes shown below in equations [5a]-[5b].
AlCl3+Cl−→AlCl4− [5a]
AlCl3+AlCl4−→Al2Cl7− [5b]
With regard to the example of phosgene, it is believed that a proportion of the highly electronegative chlorine atoms from the scavenging species dissociate into solution as chloride anions as noted above. This leaves at least one highly reactive free bonding location on many of the carbon atoms as part of the respective carbonyl (C═O) groups. On contact, oxygen atoms (either from water, air, or previously formed surface oxides) are then believed to preferentially form a second C═O bond on the positively charged carbon atom. This displaces the remaining chlorine atom, which also ends up as a free chloride (Cl−) anion in the solution. The second C═O bond results in CO2 gas, some of which remains dissolved, with the remainder bubbling out of the solution based on relative vapor pressures.
Acid halides, thionyl halides, and other highly polar scavenging species are believed to work according to similar mechanisms, where a proportion of highly electronegative atoms freely dissociate from the scavenger molecule when dispersed in an ionic liquid solvent. This is more likely to occur in an ionic liquid solvent due to the high number of dissociated ions and molecules having highly polar bonds. The presence of contaminant species containing polarized or ionic species of oxygen (e.g. water and metal oxides) is therefore highly reactive with similarly polarized scavenging molecules.
Suitable additive concentration is dependent in part on individual reactivity of the species and quantity of residual water and oxygen. Phosgenes are effective as scavengers at concentrations as low as 0.1 mol/L. Higher concentrations will certainly improve the reaction rate, however, many instances of these example species, particularly members of the phosgene family, have well-known environmental, health and safety effects. Since they are often used in many other industrial processes, handling procedures of phosgenes are well established, and are needed in only the smallest of quantities. Since these additives are highly reactive with surface oxides and water, by keeping concentrations to a minimum, the additive is fully and quickly consumed by the above example reaction(s). The spent solution will thus pose a substantially reduced risk. Handling, storage, and disposal risks can be further mitigated by utilizing more stable versions such as diphosgene or triphosgene, which are respectively a liquid and a solid at ambient conditions. When added to the ionic liquid solution, substantially all of the larger diphosgene and/or triphosgene molecules will dissociate in solution into phosgene.
Those skilled in the art will recognize that the other example water and oxygen scavengers can operate in a similar manner to the example provided above. These and other scavenging molecules will have easily removed chlorine or other halogen atoms, leaving behind a highly reactive positive carbon or sulfur dipole ready to accept and preferentially react with residual oxygen atoms.
Pretreatment and activation employing an additized ionic liquid, such as the solution described above, will ensure clean and activated aluminum and aluminum alloy surfaces characterized by oxide-free surfaces protected by a moisture free ionic liquid until the substrate is ready for electrodeposition. The solution protects the surface, enabling good metallic coating adhesion.
To summarize,
Example method 30 is shown in
Method 30 includes step 32 which is a conventional polishing, degreasing, and/or deoxidation step. This may be done by any suitable mechanical and/or chemical means as described above. After the initial preparation at step 32, it is likely that the substrate will have some surface water. Thus at step 34, the active alloy substrate is dried in a substantially oxygen and water free environment. This environment can comprise substantially pure nitrogen, or the nitrogen can also include quantities of other inert gases such as argon. As noted above, the active alloy substrate may be but is not limited to an Al or Mg alloy.
At step 36, the cleaned surface of the substrate is immersed in a compatible surface preparation and activation solution. During this step, water and oxygen are scavenged from the solution and the substrate. This can be done by subjecting the substrate to the additized ionic liquid, which can include one or more of the example solutions described above. This step eliminates residual water and surface oxides by reacting the one or more scavenging species in solution with the impurities so as to provide and maintain a clean, active surface. Step 36 can be done in a dry nitrogen environment and the substrate can be kept in solution or in the dry nitrogen environment until the substrate is ready for electroplating. A dehumidified nitrogen atmosphere limits the presence of water and oxygen, minimizing the chance of recontamination of the solution or the substrate. Step 36 can result in a thin layer comprising ionic liquid solvent cations, which adhere to the cleaned substrate surface. This protective layer can remain in place until further coating steps are taken so long as the solution and/or the surrounding environment is kept substantially free of water and oxygen.
Method 30 also includes optional step 38. During pretreatment step 36 above, a reverse current (5-50 mA/cm2) may be applied to the surface of the substrate, thereby facilitating or expediting the cleaning and activation process. The reverse current increases equilibrium dissociation of halogen atoms from the scavenging species as well as dissociation of the ionic liquid solvent and electrolyte into constituent cations and anions. This results in more available reactants to facilitate the above described scavenging reactions. The exact current will depend on surface reactivity and relative quantities of impurities present in solution or the surrounding environment.
The cleaning and activating method can include up to three key elements for a suitable coating. The method can 1) remove the oxide layer formed during and after the mechanical and/or chemical cleaning steps; 2) further remove any residual water from the previous steps; and 3) protect the surface with a thin layer of ionic liquid compatible with the electrolyte (e.g., AlCl3) for electrodeposition from an ionic liquid.
The method for cleaning and activating the surface can be performed in conjunction with other steps of a more general method of bulk electrodeposition from an ionic liquid. At step 42, the bulk coating is deposited from a standard electrodeposition solution. This solution may be similar or identical to the cleaning and activation solution described above, except with a sufficient concentration of metal halide electrolyte in addition to the ionic liquid solvent and scavenging additive(s). The scavenging additive(s) may be retained in the bulk solution at step 42 to prevent contamination of the surface during this step. As noted above, bulk electrodeposition methods are known in the art, but generally speaking, electric current is applied to the solution where the substrate surface operates as a cathode. Reduced metal cations from the electrolyte (e.g. aluminum Al3+ cations from aluminum chloride) form a substantially pure bulk coating layer.
The electrodeposition method also includes optional step 40 which further enhances bulk coating adherence and quality. In one example, prior to bulk deposition step 42, a very thin film of compatible coating atoms is disposed onto the cleaned and activated substrate. The thin film layer or the monolayer provides more control of the microstructure between the substrate surface and the bulk coating layer and reduces the need to interdiffuse or otherwise process the substrate and coating layer. As a result, strength and repairability of the bulk coating can be further enhanced.
In certain embodiments the thin film is deposited by conventional thin film processes, including but not limited to chemical vapor deposition (CVD), physical vapor deposition (PVD), flame spray, and laser deposition. In certain other embodiments, the thin film is a monolayer, which is substantially a single atomic coating layer deposited onto the substrate whereby the lattice parameters of the substrate and the coating are closely aligned. The monolayer can be deposited via the above thin film techniques or by underpotential deposition (UPD). An example process for forming a monolayer onto a substrate via UPD is described in commonly assigned United States Patent Application entitled “Underpotential Deposition of Metal Monolayers from Ionic Liquids”, filed on even date herewith, the entirety of which is incorporated by reference. The substrate can optionally be electrified with a similar current during transfer between the surface activation bath, the monolayer bath, and/or the bulk electrodeposition bath.
And while the illustrative examples above were described with reference to aluminum coatings onto aluminum or magnesium alloys, it will be appreciated that coating such alloys with other pure metals like titanium also can present similar difficulties as they relate to the presence of water and surface oxides. Scavenging additives, including but not limited to those examples described above, can similarly be added to an ionic liquid solvent as part of the above described methods to facilitate surface preparation and activation of a substrate surface prior to bulk deposition from ionic liquid solutions.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
The following are non-exclusive descriptions of possible embodiments of the present invention.
A surface preparation solution comprises an ionic liquid solvent and a water and oxygen scavenging species.
The solution of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
the water and oxygen scavenging species comprises an acid halide including a halogen atom bonded to the carbon atom of a carbonyl group;
the solution further comprises a metal halide salt, wherein the halide of the salt is a halide anion form of the halogen atom bonded to the carbon atom of the carbonyl group;
the acid halide is an acid chloride;
the water and oxygen scavenging species comprises a member of the phosgene family;
the water and oxygen scavenging species exists in solution as phosgene (carbonyl dichloride, COCl2);
the water and oxygen scavenging species comprises a thionyl halide having a halogen atom bonded to the sulfur atom of a thionyl group;
the thionyl halide is thionyl chloride; and
the ionic liquid solvent comprises at least one of: 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) amide, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) amide, trihexyl-tetraadecyl phosphonium bis(trifluoromethylsulfonyl) amide, and mixtures thereof.
A method for making a surface preparation and activation solution for ionic liquid electrodeposition comprises providing an ionic liquid solvent compatible with the substrate surface and a species to be deposited from the ionic liquid. A water and oxygen scavenging species is added to the ionic liquid solvent.
The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following steps, features, configurations and/or additional components:
the water and oxygen scavenging species is an acid halide including a halogen atom bonded to the carbon atom of a carbonyl group;
the water and oxygen scavenging species is selected from the phosgene family;
the water and oxygen scavenging species is added to the ionic liquid in at least one of a diphosgene form and a triphosgene form; and
the water and oxygen scavenging species is thionyl chloride.
A method for preparing and activating a substrate surface for electrodeposition comprises removing contamination and oxides from the substrate surface. The surface is dried in a substantially oxygen and water free environment. The substrate surface is immersed into a surface preparation and activation solution. The solution comprises an ionic liquid solvent and a water and oxygen scavenging species.
The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following steps, features, configurations and/or additional components:
the removing step comprises both mechanical and chemical removal of the contamination and oxides;
the drying step utilizes substantially pure dehumidified nitrogen;
the water and oxygen scavenging species comprises an acid chloride;
the water and oxygen scavenging species comprises at least one member of the phosgene family; and
the method further comprises applying a reverse current during the immersing step to expedite scavenging of water and oxygen.
A method for electrodepositing a bulk coating onto a substrate surface comprises preparing and activating the substrate surface by removing contamination and oxides from the substrate surface. The surface is dried in a substantially oxygen and water free environment. The substrate surface is immersed into a surface preparation and activation solution. The solution comprises an ionic liquid solvent and a water and oxygen scavenging species. The substrate surface is submerged into a coating solution, which comprises an ionic liquid solvent and a metal halide salt dissociated in the ionic liquid solvent. An electrical current is applied through the coating solution between an anode and the substrate surface operating as a cathode to deposit a bulk coating thereon.
The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following steps, features, configurations and/or additional components:
the metal halide salt comprises aluminum.
the method further comprises depositing a thin film layer onto the prepared and activated substrate surface;
the thin film layer is a monolayer deposited via underpotential deposition.
