Inorganic mixed inhibitor compounds and organic coatings

Abstract

A pigment composition, a corrosion-inhibiting coating composition including the pigment composition, and a method of synthesizing the pigment composition. The pigment composition includes an inorganic salt containing a cation of a rare earth element and an inorganic anion having corrosion inhibitive properties. The cation exists in a +3 oxidation state and the inorganic anion is in an oxidation state ranging from -1 to -4.

Description

INORGANIC MIXED INHIBITOR COMPOUNDS AND ORGANIC COATINGSFIELD OF INVENTION

[0001] The present invention relates to a pigment composition, a corrosion-inhibiting coating composition including the pigment composition, and a method of synthesizing the pigment composition.BACKGROUND

[0002] Aluminum alloys are quite popular in aerospace applications due to their high strength-to-weight ratio. One of the most common alloys used in this field is 2024 aluminum. This alloy, in addition to aluminum, is composed of copper (4.3-4.5%), manganese (0.5-0.6%), magnesium (1 .3-1 .5%) and trace amounts of silicon, zinc, nickel, chromium, lead, and bismuth. While having exceptional physical properties, compounding this aluminum with such high amounts of copper leads to susceptibility of corrosion compared to plain aluminum or other aluminum alloys due to the galvanic corrosion that can occur.

[0003] Traditionally, hexavalent chromium has been used to slow down and mitigate the corrosion of copper-rich aluminum alloys. Hexavalent chromium compounds, either used as a chemical pretreatment or as a corrosion-inhibiting pigment in aerospace primers or both at the same time, provide a stable, long-term protection platform. However, hexavalent chromates are widely known to be carcinogenic, which is dangerous for workers as well as for the environment, and there has been a heavy push over the past 30 years to reduce its usage.

[0004] Many ideas have been developed utilizing other more environmentally friendly alternatives with limited success in replicating the corrosion protecting behavior that hexavalent chromium displays. The reason hexavalent chromium compounds are so effective is that hexavalent chrome acts like a mixed inhibitor, providing both cathodic and anodic corrosion-inhibiting mechanisms. However, among currently available hexavalent chromium-free formulations, none appear to be as effective as hexavalentchromium compounds. Thus, no technology available today can offer the performance of a hexavalent chrome-based conversion coating alongside a chromated epoxy primer.

[0005] Current chrome-free coating offerings still fall short of hexavalent chromium performance, especially when it comes to aluminum alloys. Extensive work has been performed on the use of cerium and praseodymium as corrosion inhibitors, but these compositions never get real traction due to limited success. Such work has focused entirely on the cation or in the form of a rare earth metal oxide passivating the surface, without looking at combinations with an anodic inhibiting species as well.

[0006] When adding color to a corrosion-inhibiting composition, it is important to use a pigment composition that is non-toxic as well. In order to create a pigmented corrosioninhibiting composition, it is also important to use a pigment composition that exhibits corrosion-inhibiting properties.

[0007] It is an object of the present invention to provide a pigment composition that can be incorporated into a corrosion-inhibiting coating composition that works effectively on a variety of aluminum alloys as well as steel and magnesium alloys. It is a further object of the invention to provide a chrome-free, pigmented, corrosion-inhibiting coating composition.

[0008] It is yet another object of the invention to provide a method of synthesizing such a pigment composition.SUMMARY

[0009] These objects are achieved by the compositions according to the present invention, which are derived from the synthesis of novel pigments comprising a rare earth cation along with inorganic anions that are known corrosion inhibitors. These final compounds result in chrome-free pigments that exhibit both anodic and cathodic corrosion-inhibiting mechanisms, thus providing a more robust corrosion-inhibiting package compared to conventional corrosion inhibitors being used today to replace hexavalent chromium compounds. These materials are milled or otherwise dispersed into an organic coating that is used to protect metallic substrates such as aluminum, iron, and magnesium alloys.

[0010] The corrosion inhibiting pigment and the resulting coating composition according to the present invention helps eliminate carcinogenic hexavalent chromium from being used in a variety of areas within the coating industry. These pigments can be used on a variety of metal alloys to offer a potential universal corrosion-inhibiting system. In addition, this corrosion-inhibiting system allows the use of chrome-free pretreatments that, when used in conjunction with the coating system, gives extremely similar corrosion resistance to that of Chrome VI pretreated alloys. This coating system can replace current Chrome VI containing primers on a variety of metallic substrates to improve the corrosion performance while lowering the toxicity. This coating system shows similar performance of corrosion-inhibition on metal alloys when pretreated with both Chrome VI and nonchrome pretreatments, thus allowing the removal of Chrome VI even further.

[0011] The pigment composition according to the present invention suitably includes an inorganic salt containing a cation of a rare earth element and an inorganic anion having corrosion-inhibitive properties. Suitable cations include praseodymium, lanthanum, neodymium, yttrium, scandium, ytterbium, samarium, cerium, gadolinium, dysprosium, and erbium. The inorganic anion may be a vanadate species, a molybdate, a phosphate, a tungstate, a borate, a permanganate, and / or a selenite. The pigment composition may include more than one type of cation and / or more than one type of inorganic anion.

[0012] The cation exists in a +3 oxidation state, and the inorganic anion is in an oxidation state ranging from -1 to -4. An atomic ratio of the anion to the cation may be in a range from 1 :3 to 3:4, depending on the oxidation state of the inorganic anion.

[0013] The pigment composition can be incorporated into a corrosion-inhibiting coating composition that includes the pigment composition and a remainder, which may be an organic film-forming resin, an inorganic film-forming resin, additives, catalysts, and combinations thereof. In a final cured coating, the corrosion-inhibiting compound may include up to 40% by volume of the pigment composition and at least 60% by volume of the remainder.

[0014] In order to synthesize the pigment composition, the method is begun by providing a first solution of an inorganic salt containing a cation of a rare earth element, and providing a second solution of an inorganic anion having corrosion inhibitiveproperties. For example, the first solution may include a rare earth chloride or nitrate dissolved in water. This can also be achieved by dissolving rare earth oxides in an acid, such as hydrochloric acid or nitric acid. The second solution may include a salt containing sodium or potassium and the inorganic ion. The inorganic anion may be a vanadate species, a molybdate, a phosphate, a tungstate, a borate, a permanganate, and / or a selenite.

[0015] The second solution can be pumped into the first solution under agitation to form a combined solution. The combined solution can be filtered to obtain a rare earth salt, which can then be purified to obtain pigments. After drying the pigments, the pigments can be pulverized into pigment particles to form a powder pigment composition. The pigment composition can then be milled or dispersed into an organic or inorganic coating to form a corrosion-inhibiting compound that includes 1 -40% pigment composition by total solids weight. The corrosion-inhibiting compound can be applied to a metallic substrate to form a corrosion-inhibiting system.

[0016] A method of improving corrosion-resistance of a metallic substrate can be achieved by applying the corrosion-inhibiting coating composition to a metallic substrate.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Fig. 1 is a schematic representation of pigment synthesis.

[0018] Fig. 2 shows SEM pictographs of various rare earth corrosion-inhibiting pigments.

[0019] Figs. 3a and 3b are graphical representations of the corrosion creep area of comparative examples of corrosion-inhibiting coating compositions.

[0020] Figs. 4a and 4b are graphical representations of the corrosion creep area of additional comparative examples of corrosion-inhibiting coating compositions.

[0021] Fig. 5 is a series of photographs showing substrate degradation of the comparative examples presented in Fig. 3a.

[0022] Fig. 6 is a series of photographs showing substrate degradation of the comparative examples presented in Fig. 3b.

[0023] Fig. 7 is a series of photographs showing substrate degradation of additional comparative examples of corrosion-inhibiting coating compositions.

[0024] Fig. 8 is a series of photographs showing substrate degradation of more comparative examples of corrosion-inhibiting coating compositions.DETAILED DESCRIPTION

[0025] Compositions and methods described herein solve the problem of providing a pigment composition that can be incorporated into a chrome-free, corrosion-inhibiting coating composition that works effectively on a variety of aluminum alloys as well as steel and magnesium alloys. More particularly, the present invention blends various cations of rare earth elements that have shown promise in the past with corrosion inhibition by cathodically depositing on the copper sites, coupled with known anodic corrosioninhibiting anions. The resulting pigments show sparing solubility compared to traditionally researched rare earth salts, such as nitrates and chlorides. This sparing solubility is similar to zinc and strontium chromate pigments, which means enough should leech out to inhibit corrosion without leeching rapidly and leading to osmotic blistering.

[0026] The pigment composition includes an inorganic salt containing a cation of a rare earth element and an inorganic anion having corrosion-inhibitive properties. Suitable cations include praseodymium, lanthanum, neodymium, yttrium, scandium, ytterbium, samarium, cerium, gadolinium, dysprosium, and erbium. The inorganic anion may be a vanadate species such as metavanadate or orthovanadate, a molybdate, a phosphate, a tungstate, a borate, a permanganate, and / or a selenite. The pigment composition may include more than one type of cation and / or more than one type of inorganic anion. For example, the pigment composition may include a blend of praseodymium molybdate and lanthanum metavanadate, or various other combinations of cations and inorganic anions.

[0027] The cation exists in a +3 oxidation state, and the inorganic anion is in an oxidation state ranging from -1 to -4. An atomic ratio of the anion to the cation may be in a range from 1 :3 to 3:4, depending on the oxidation state of the inorganic anion.

[0028] The inorganic salt and the inorganic anion are each dissolved in a solvent when synthesizing the pigment composition. Suitable solvents that can serve as a carrierfor the inorganic salt include water, methanol, ethanol, or any combination thereof. Suitable solvents that can serve as a carrier for a salt containing the inorganic anion include water, methanol, ethanol, or any combination thereof.

[0029] The pigment composition can be incorporated into a corrosion-inhibiting coating composition. In addition to the pigment composition, the remainder of the corrosion-inhibiting coating composition may include an organic film-forming resin, an inorganic film-forming resin, additives, catalysts, and combinations thereof. In a final cured coating, the corrosion-inhibiting compound may include up to 40% by volume or up to 20% by volume or up to 1 % by volume of the pigment composition, and at least 60% by volume or at least 80% by volume or at least 99% by volume of the remainder, based on the total volume of the corrosion-inhibiting coating composition.

[0030] As used herein, the term "film-forming resin" includes monomers or oligomers which form a polymeric system during curing of the corrosion-inhibiting coating composition. Examples of such monomers or oligomers include siloxanes and silicates (e.g. tetraalkoxysilane or silanes functionalized with epoxy, amine, mercaptan, or olefinic functionalities), alone or in combination with organic monomers or oligomers. The polymeric system can be formed from the monomers or oligomers by various technologies, including sol-gel technology.

[0031] The film-forming resin can be selected from, e.g., epoxy resins, hydroxyfunctional resins (such as polyesters and (meth)acrylates), resins with 1 or more blocked hydroxyl groups (such as acetals, oxazolidine resins, carboxylic-acid-functional resins), polyacrylates, polyurethanes, polyethers, polyaspartic esters, (blocked) isocyanates, mercapto-functional resins, thiol-functional resins, amine-functional resins, amide- functional resins, imide-functional resins (e.g. maleimide), alkyd resins, resins containing at least one olefinically unsaturated bond, silane-containing resins, polysiloxane resins, polysilazane resins, acetoacetate resins, functional (=curable) fluorinated resins, and mixtures and hybrids thereof. The film-forming resins, collectively, may account for 60% to 99% by volume, or 65% to 90% by volume, or 70% to 75% by volume of the remainder.

[0032] Additives that may be included in the corrosion-inhibiting coating composition are, for example, rheology modifiers, surface active additives, defoamers, antisettlingagents, adhesion promotors, and pigment dispersants. Combinations of additives may also be used. These additives, collectively, may account for 1 % to 10% by volume, or 2% to 8% by volume, or 3% to 5% by volume of the remainder.

[0033] Catalysts that may be included in the corrosion-inhibiting coating composition are, for example, organometallic compounds, quaternary ammonium hydroxides, quaternary ammonium fluorides, quaternary phosphonium hydroxides, ammonium carboxylate compounds, and tertiary amines. Combinations of catalysts may also be used. These catalysts, collectively, may account for 0.1 % to 5% by volume, or 0.2% to 1 % by volume, or 0.3% to 0.5% by volume of the remainder.

[0034] The corrosion-inhibiting coating compositions may or may not require the use of solvents to act as a vehicle to aid in the deposition of the coating compositions onto a substrate. Suitable solvents that may be included in the coating compositions are ketones, acetates, glycol ethers, alcohols, alkanes, mineral spirits, aromatic solvents, carbonates, and water. The solvents, collectively, may account for 0% to 75% by volume, or 30% to 60% by volume, or 40% to 50% by volume of the remainder

[0035] The beginning of the synthesis of the pigment composition is illustrated in Fig. 1 , which shows a first solution 20 of an inorganic salt containing a cation of a rare earth element, and a second solution 22 of an inorganic anion having corrosion inhibitive properties. For example, the first solution 20 may be a rare earth oxide dissolved in a strong acid such as hydrochloric acid, nitric acid, sulfuric acid, hydrobromic acid, hydroiodic acid, or perchloric acid. As another example, the first solution 20 may include a rare earth chloride or nitrate dissolved in water. The second solution 22 may include a salt containing sodium or potassium and the inorganic ion. The inorganic anion may be a vanadate species, a molybdate, a phosphate, a tungstate, a borate, a permanganate, and / or a selenite. This synthesis can be a facile synthesis performed at room temperature, for example.

[0036] The second solution 22 can be pumped into the first solution slowly, such as over the course of an hour, under agitation to form a combined solution. The reaction is done in this manner to avoid potential rare earth hydroxides from being formed at high enough pHs. After the appropriate amount of anion salt solution is added, the combinedsolution is left to mix for a minimum of one hour. The combined solution suitably includes 30% to 40% by volume the first solution 20 and 60% to 70% by volume the second solution 22.

[0037] The rare earth salt that is formed collapses out and can then be filtered, such as by using vacuum filtration, and rinsed with excess water to remove impurities. An example chemical reaction synthesizing Praseodymium Metavanadate is shown in the following equation:

[0038] After the filtration and purification processes, the resulting pigments can be dried in conventional ovens to remove water. After drying the pigments, the dried pigments can be pulverized into pigment particles to form a powder pigment composition. The pigment composition can then be incorporated into various coating chemistries depending on the application needed. For example, the pigment composition can be combined with an organic film-forming resin, an inorganic film-forming resin, an additive, and / or a catalyst. The concentration of the pigment composition can be varied in order to achieve desired corrosion performance. For example, the pigment composition can be milled or dispersed into an organic coating to form a corrosion-inhibiting compound or corrosion-inhibiting coating composition that includes 1 % to 40% or 5% to 20% or 10% to 15% pigment composition by total solids weight. The milling or dispersion may be carried out using a mini motor mill or by using a paint shaker, for example.

[0039] The corrosion-inhibiting coating compositions described herein can be applied to a metallic substrate in order to improve corrosion-resistance of the metallic substrate. These coating compositions can be applied to a variety of metallic substrates, including both ferrous and non-ferrous substrates, as a replacement to chrome pigments. For example, the coating compositions can be applied to aluminum, aluminum alloys, magnesium, magnesium alloys, titanium, lithium-aluminum alloys, as well as other nonferrous substrates. Additionally, the coating compositions can be applied to a variety of steel substrates. Substrates for which the coating compositions are particularly beneficialinclude aluminum alloys composed of at least 1 wt% copper, such as aluminum 2024 as well as aluminum 7075.

[0040] Furthermore, these coating compositions can be used in conjunction with non-chrome pretreatments applied to the substrates prior to applying the coating compositions to achieve accelerated results similar to full chrome-based systems. As used herein, the term “pretreatment” refers to a substance applied to the surface of a metallic substrate in order to clean, coat, and / or protect the surface in preparation for subsequent coating, painting, or other finishing techniques. Suitable non-chrome pretreatments include, for example, PreKote® Surface Pretreatment, available from Pantheon Enterprises of Phoenix, Arizona, as well as Boegel™ coating technology, available from Advanced Chemistry & Technology of Garden Grove, California, as AC- 131.EXAMPLES

[0041] These examples include pigments synthesized from inorganic salts containing a cation of a rare earth element and an inorganic anion. The coating composition of each of the admixed paint samples used in these examples is presented in Table 1 .

[0042] As illustrated in Fig. 1 , for each sample, a solution of an inorganic anion having corrosion inhibitive properties was pumped into a solution of an inorganic salt containing a cation of a rare earth element under agitation to form a combined solution. A rare earth salt was filtered from the combined solution, and the rare earth salt was purified in order to obtain the pigments. SEM pictographs of the resulting rare earth corrosion-inhibiting pigments used in these examples are shown in Fig. 2. Specifically,the pigments used were praseodymium vanadate (PrV), praseodymium molybdate (PrM), lanthanum vanadate (LaV), lanthanum molybdate (LaM), neodymium vanadate (NdV), and neodymium molybdate (NdM). Elemental analysis confirmed that with proper washing, all contaminants (sodium and / or chloride ions) were removed, yielding high purity grade material. These pigments were then ground up and dispersed in a chrome- free primer that meets MIL-PRF-23377, available from Hentzen Coatings, Inc. of Milwaukee, Wisconsin, at a 5% loading to form a corrosion-inhibiting coating composition. The corrosion-inhibiting coating compositions were each applied to pretreated aluminum 2024T3 substrates that were 10 cm x 15 cm and 0.8 mm thick, and then subjected to a corrosion-resistance test. Corrosion-resistance was measured using the standard salt spray (fog) test according to ASTM B117 for 2000 hours. After the salt spray test, the accelerated testing panels were removed and evaluated for pitting and corrosion.

[0043] Before subjecting the panels to the corrosion test a scribe was made by mechanically milling. Corrosion around the scribe was evaluated by measuring the corrosion creep or size of the corrosion blister from the scribe.

[0044] Fig. 3a shows the maximum creep area of each sample applied to an aluminum 2024T3 substrate coated with Boegel EP-ll / AC-131 adhesion promoter after the standard salt spray (fog) test according to ASTM B117 for 2000 hours. It is clear that the control paint sample with no corrosion inhibitor experienced considerably more creep than each of the substrates that were treated with the corrosion-inhibiting coating compositions.

[0045] Fig. 3b shows the maximum creep area of each sample applied to an aluminum 2024T3 substrate coated with PreKote® surface pretreatment after the standard salt spray (fog) test according to ASTM B117 for 2000 hours. It is clear that the control paint sample with no corrosion inhibitor experienced considerably more creep than each of the substrates that were treated with the corrosion-inhibiting coating compositions.

[0046] Fig. 4a shows the maximum creep area of each sample applied to an aluminum 2024T3 substrate pretreated with Alodine 5900 after the standard salt spray (fog) test according to ASTM B117 for 2000 hours. As in the previous examples, thecontrol paint sample with no corrosion inhibitor experienced more creep than each of the substrates that were treated with the corrosion-inhibiting coating compositions.

[0047] Fig. 4b shows the maximum creep area of each sample applied to an aluminum 2024T3 substrate pretreated with Alodine 1200 after the standard salt spray (fog) test according to ASTM B117 for 2000 hours. In this example, the control paint sample with no corrosion inhibitor experienced less creep than two of the substrates that were treated with the corrosion-inhibiting coating compositions, and more creep than four of the substrates that were treated with the corrosion-inhibiting coating compositions.

[0048] There is a statistically noticeable improvement of pitting and corrosion resistance when incorporating these rare earth inhibitors into paint going on non-chrome pretreated substrates. This is critical as the goal for many applications is to get away from traditional hex-chrome-based pretreatments.

[0049] For a visual comparison of the effects of the corrosion-inhibiting coating compositions, Fig. 5 shows the samples from Fig. 3a in which the panels were stripped of paint to survey the damage under the primer and the corrosion-inhibiting coating compositions containing the rare earth mixed inhibiting salts. Similarly, Fig. 6 shows the samples from Fig. 3b in which the panels were stripped of paint to survey the damage under the primer and the corrosion-inhibiting coating compositions containing the rare earth mixed inhibiting salts. These photographs, in both Fig. 5 and Fig. 6, show much less degradation in and around the scribe on non-chrome pretreated substrates having the corrosion-inhibiting coating compositions versus the control 16708TEP formulation. With the relatively small addition of the pigments at a 5% by weight ratio, the result is a substantial increase in corrosion protection, which indicates that these pigments are actively protecting the substrate.

[0050] For a further visual comparison of the effects of the corrosion-inhibiting coating compositions, Fig. 7 shows samples of the following corrosion-inhibiting coating compositions containing 15% by weight of the corrosion inhibiting pigment: praseodymium vanadate (PrV), lanthanum metavanadate (LaV), praseodymium molybdate (PrM), a blend of 7.5% by weight of praseodymium vanadate (PrV) and 7.5% by weight of praseodymium molybdate (PrM), a blend of 7.5% by weight of lanthanumvanadate (LaV) and 7.5% by weight praseodymium molybdate (PrM), and a chromium control sample composed of hexavalent chromate paint manufactured by Hentzen Coatings each applied to an aluminum 7075 substrate coated with BOEGEL EP-II / AC- 131 adhesion promoter. The coating compositions of these coatings are listed in Table 1 . The photographs in Fig. 7 show the most degradation in the control sample having no corrosion-inhibiting coating composition, and comparatively less degradation in each of the samples having the corrosion-inhibiting coating compositions. The chromium control sample shows virtually no degradation, and the sample treated with lanthanum metavanadate (LaV), in particular, shows improved corrosion-inhibiting properties that rival the chromium control sample.

[0051] Fig. 8 shows a visual comparison of the effects of different levels of corrosioninhibiting coating compositions. More particularly, Fig. 8 shows samples of a non-chrome primer, a hex-chrome primer, and two different levels of lanthanum metavanadate (LaV). Once again, corrosion-resistance was measured using the standard salt spray (fog) test according to ASTM B117 for 2000 hours. As shown in Fig. 8, the non-chrome primer provided the least corrosion protection, while the two LaV coating compositions provided the most corrosion protection. The LaV coating composition containing 25% by weight LaV provided even better corrosion protection than the LaV coating composition containing 15% by weight LaV.

[0052] The descriptions and figures included herein depict specific implementations to teach those skilled in the art how to make and use the best option. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these implementations that fall within the scope of the invention. Those skilled in the art will also appreciate that the features described above can be combined in various ways to form multiple implementations. As a result, the invention is not limited to the specific implementations described above, but only by the claims and their equivalents.

Claims

CLAIMS1 . A pigment composition, comprising: an inorganic salt containing a cation of a rare earth element and an inorganic anion having corrosion inhibitive properties, wherein the inorganic anion is selected from the group consisting of a vanadate species, a molybdate, a phosphate, a tungstate, a borate, a permanganate, a selenite, and combinations thereof; wherein the cation exists in a +3 oxidation state, and the inorganic anion is in an oxidation state ranging from -1 to -4.

2. The pigment composition according to claim 1 , wherein the cation is selected from the group consisting of: praseodymium, lanthanum, neodymium, yttrium, scandium, ytterbium, samarium, cerium, gadolinium, dysprosium, erbium, and combinations thereof.

3. The pigment composition according to claim 1 , wherein the inorganic anion is selected from the group consisting of a vanadate species, a molybdate, a phosphate, a tungstate, and combinations thereof.

4. The pigment composition according to claim 1 , wherein an atomic ratio of the anion to the cation is in a range from 1 :3 to 3:4, depending on the oxidation state of the inorganic anion.

5. A corrosion-inhibiting coating composition, comprising: the pigment composition according to claim 1 ; and a remainder that includes at least one of the group consisting of an organic filmforming resin, an inorganic film-forming resins, an additive, and a catalyst.

6. The corrosion-inhibiting compound according to claim 5, comprising up to 40% by volume of the pigment composition and at least 60% by volume of the remainder in a final cured coating.

7. A method of synthesizing a pigment composition, comprising: providing a first solution of an inorganic salt containing a cation of a rare earth element; providing a second solution of an inorganic anion having corrosion inhibitive properties, wherein the inorganic anion is selected from the group consisting of a vanadate species, a molybdate, a phosphate, a tungstate, a borate, a permanganate, a selenite, and combinations thereof; pumping the second solution into the first solution under agitation to form a combined solution; filtering a rare earth salt from the combined solution; purifying the rare earth salt to obtain pigments; drying the pigments; and pulverizing the pigments into pigment particles to form a powder pigment composition.

8. The method according to claim 7, wherein the first solution includes a rare earth chloride or nitrate dissolved in water.

9. The method according to claim 7, wherein the second solution includes a salt containing sodium or potassium and the inorganic anion.

10. The method according to claim 7, wherein the method is a facile synthesis performed at room temperature.11 . The method according to claim 7, comprising using vacuum filtration to filter the rare earth salt from the combined solution.

12. The method according to claim 7, comprising purifying the rare earth salt by rinsing the rare earth salt with excess water to remove impurities.

13. The method according to claim 7, further comprising milling or dispersing the pigment composition into an organic coating to form a corrosion-inhibiting compound.

14. The method according to claim 13, wherein the organic coating comprises at least one of the group consisting of an organic film-forming resin, an inorganic filmforming resin, an additive, and a catalyst.

15. The method according to claim 13, wherein the corrosion-inhibiting compound includes 1 -30% pigment composition by total solids weight.

16. The method according to claim 13, further comprising applying the corrosioninhibiting compound to a metallic substrate to form a corrosion inhibiting system.

17. The method according to claim 16, wherein the metallic substrate has been pretreated.

18. A method of improving corrosion-resistance of a metallic substrate, comprising: applying the corrosion-inhibiting coating composition according to claim 5 to a metallic substrate.

19. The method according to claim 18, wherein the metallic substrate is selected from the group consisting of aluminum, aluminum alloys, magnesium, magnesium alloys, titanium, lithium-aluminum alloys, and steel.

20. The method according to claim 18, wherein the metallic substrate has been pretreated.

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