Metals such as magnesium (Mg), aluminum (Al), titanium (Ti) and copper (Cu), alloys containing those metals as main components, and steel containing iron (Fe) as a main component, are utilized as structural material components appropriate for engineering applications.
For example, Mg among these is an element that is abundant in the earth's surface, and is known to be a metal that can be used for engineering applications, is light, has high toughness per unit mass, has high vibration absorbability, is non-toxic, and has satisfactory castability. Therefore, Mg is an important metal that is utilized in industrial application and daily goods. In fact, Mg is used in automotive wheels, aircraft parts, mobile telephone components, and the like.
However, metal elements such as Mg described above are reaction-active substances, and readily react with water molecules, halide ions including chloride ions, or the like in air or in water to be oxidized. Through this oxidation reaction, these materials have a problem that rust containing the reaction active substances as main components is generated and deteriorates durability. For example, when Mg is immersed in an acidic solution, an alkaline solution, or saline, Mg immediately undergoes a chemical reaction represented by the following chemical formula (1), and is corroded along with the generation of hydrogen.
                    [                  Chemical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]                                                                                                                Mg                →                                                      Mg                                          n                      +                                                        +                                      ne                    -                                                                                                                                                                2                    ⁢                    n                    ⁢                                                                                  ⁢                                          H                      +                                                        +                                      2                    ⁢                                                                                  ⁢                                          ne                      -                                                                      →                                  n                  ⁢                                                                          ⁢                                                            H                      2                                        ↑                                                                                      }                            (        1        )            
Such an oxidative deterioration reaction becomes particularly problematic when a non-noble metal such as Mg is used as a structural material. In the case of an Al alloy or steel, since the oxide film produced on the alloy surface works as a passivation film, oxidation of the alloy matrix itself can be prevented. On the other hand, since a Mg alloy or the like has high corrosion activity, it is difficult to produce a stable passivation film.
Therefore, in order to suppress corrosion reactions, anti-rust coating materials that protect the surface of metal substrates such as Mg and Mg alloys, and the like have been hitherto developed to enhance anti-rust properties.
For example, there has been suggested a method for producing a corrosion resistant iron material having high antirust properties and durability thereof, without using or including any chromium-based compounds that have antirust properties but are hazardous (Patent Literature 1). In this case, the corrosion resistant iron material is coated with an antirust coating composition that includes a compound having at least one phenolic hydroxyl group in the molecule and a silane compound as essential components. However, in Patent Literature 1 of the prior art, the composition was not applied to magnesium alloys, which have high corrosion tendency and are not very effective to anti-corrosion technologies.
In the case of copper alloys, it has been traditionally known that nitrogen-containing aromatic compounds such as benzotriazole exhibit superior performance as corrosion inhibitors (Non-Patent Literature 1). It is considered that when such a corrosion inhibitor is used, non-covalent bonds are formed between the atoms at the metal surface and the ligand present in corrosion inhibitor, and finally the nitrogen-containing aromatic compound molecules form a two-dimensional polymer network in atomic level, so that an antirust effect can be obtained thereby. That is, it is disclosed that, as a result of the metal-ligand interaction in atomic level and the corrosion inhibitor compactly and completely covering the surface of the metal substrate, hydrogen, chlorine, water and the like are effectively removed, and consequently, the nitrogen-containing aromatic compounds exhibit high anti-corrosion properties for copper and copper alloys.
However, the antirust effect of such a nitrogen-containing aromatic compound is a phenomenon limited to copper and copper alloys, and an organic coating agent which exhibits an antirust effect irrespective of the kind of metal has not yet been developed.
Furthermore, a surface-treated metal plate that does not contain any hexavalent chromium, which imposes high environmental toxicity, and exhibits excellent corrosion resistance, solvent resistance, alkali resistance and adhesiveness to top coat materials, and a method for producing the surface-treated metal plate have been suggested (Patent Literature 2). In the surface-treated metal plate of this case, a surface treating agent containing an organic resin having an anionic functional group is applied on the surface of the metal plate or the like, and after heating and drying, a surface treatment coating film is formed through contact with an aqueous solution containing a metal cation such as Mg2+.
However, the antirust effect of these conventional antirust coating materials and the like was not sufficient in any of the cases. Furthermore, in the case of using these antirust coating materials, a coating agent must be developed and produced for each kind of metal or alloy or each composition, and the operation was complicated. Moreover, in a case in which such an antirust coating material was used, if a thick film having a thickness of about several dozen μm was not formed, it was difficult to secure the adhesive power for a metal, or to prevent penetration of water or halide ions through cracks of the coating film. Therefore, in a case in which antirust coating is applied to a finely processed product that uses an alloy, using these antirust coating materials, there occurs a problem that the delicate processing is impaired. For this reason, there is an increasing demand for a “nano-coating material” which exhibits a high antirust effect even if a film having a small thickness in the order of nanometers.
Furthermore, a nano-coating material having excellent adhesive power for a metal is expected to be applicable to an antirust coating material, as well as various applications such as, for example, a coating agent for an electrode of a battery.