JPH0251996B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a rust-proof steel sheet used for materials for transportation vehicles such as automobiles, materials for construction, materials for electrical equipment, and the like. [Prior Art] Conventionally, steel sheets plated with zinc or zinc-based alloys have been widely used for rust prevention purposes, and this is based on the sacrificial corrosion prevention effect of zinc. However, in steel sheets plated with zinc or zinc-based alloys, the zinc leaches out quite quickly in corrosive environments, especially in the presence of salt, making it impossible to maintain the rust prevention effect of the steel sheets over a long period of time. . The reasons for this are: firstly, zinc is electrochemically much less base than iron, so the coupling current with iron flows excessively, resulting in a high rate of dissolution of zinc; and secondly, zinc is corroded. It is considered that corrosion current flows through the generated substance relatively easily, and the film of the corrosion generated substance itself is also likely to be eroded and damaged. The main method currently being used to improve this problem is to use a film made by alloying iron or nickel with zinc. This makes the potential of the plating film more noble than that of pure zinc, reduces the potential difference with iron, suppresses the flow of excessive corrosion current, and extends the life of the plating film. [Problems to be solved by the invention] However, with this method, red rust occurs when the iron in the film corrodes in the case of a zinc-iron alloy layer film (Japanese Patent Publication No. 15554/1983), and In the case of a nickel alloy layer coating, the disadvantage is that the nickel component in the coating is difficult to corrode and remains in a metallic state until the end, which promotes pitting corrosion of the base steel. If sacrificial corrosion protection can be maintained in the steel substrate and the rate of erosion of the mating layer in a corrosive environment can be reduced, an ideal rust-proof steel plate can be obtained. The present invention was developed from this point of view, and consists of a eutectoid plating layer of zinc and chromium, Zn-Ti, Zn-Cu, and Sn.
−Mn−Fe, Fe−Mn−Zn, Zn−Cr, or
To provide a rust-preventing steel plate having a multilayer plating layer consisting of a Zn-Fe-P plating layer. [Means for Solving the Problems] As is well known, metallic chromium is an extremely corrosion-resistant material that becomes passivated in the presence of oxygen and is not corroded even in dilute acids. However, when chromium comes into contact with zinc, it becomes electrochemically in a base state similar to zinc, and has a sufficient sacrificial corrosion protection effect on steel substrates. The corrosion product in a humid environment is presumed to be a basic chloride of trivalent chromium, but it is recognized to be a type of extremely poorly soluble polynuclear complex. As an embodiment in which metallic chromium is applied to zinc-based plating, a sufficient amount, for example, 5
In the prior art, it has been virtually impossible to obtain zinc-chromium alloy plating with a chromium content of more than % by weight, either by electroplating or hot-dip plating. As another aspect, a galvanized steel sheet coated with chromium oxide or metallic chromium, or both, has been proposed, but if the surface coating completely dissolves in a corrosive environment, it will not contribute to corrosion resistance. be. By adding zinc ions to a chromium plating bath using trivalent chromium ions, the present inventors have made it possible to eutectoid metallic zinc and metallic chromium by electroplating from this bath. The plating bath to be used may contain, for example, a total of zinc ions and chromium ions of 0.2 to 1.2 mol/liter, anions of one or more of sulfate ions and halogen ions, a complex ion forming agent for trivalent chromium ions, and an antioxidant. As a stabilizer, one or more of formic acid, formate, compounds with amino groups such as various amino acids including glycine, urea, amines, amides, etc. were added in a total amount of 0.2 to 5.0 mol/liter. use something This bath also contains one or two of the following conductivity aids: ammonium sulfate, ammonium chloride, ammonium bromide, other ammonium halides, alkali metal halides, and alkali metal sulfates.
Seeds or more may be added in a total amount not exceeding 4 mol/liter. It is also possible to add one or more types of various acids such as boric acid and phosphoric acid as a PH buffer, and further add one or more types of alkali metal salts or ammonium salts of these acids. It is also possible. Regarding the concentration range of the plating bath, if the sum of zinc ions and chromium ions is less than 0.2 mol/liter, the plating efficiency will be low, and if it exceeds 1.2 mol/liter, the plating bath will become saturated and cannot be applied. Formic acid, formate salts, compounds with amino groups (various amino acids including glycine, urea, amines, amides, etc.)
If the total amount of one or more of these is less than 0.2 mol/liter, the complex ion forming effect and antioxidant effect of trivalent chromium ions will be insufficient;
Above 5.0 mol/liter, the bath reaches saturation. If the conductivity aid concentration exceeds a total of 4 moles/liter, saturation of the bath will occur. The plating current density is preferably 10 A/dm ₂ to 300 A/dm ² . If it is less than 10 A/dm ² , industrial productivity is extremely poor and unrealistic. On the other hand, in a region exceeding 300 A/dm ² , the diffusion of chromium ions to the plating interface becomes impossible to follow, and hydrogen ion discharge is significant at the plating interface, and the resulting PH rise makes the PH buffer no longer effective. It disappears and normal plating becomes impossible. The plating liquid flow rate can be applied from static to 200m/min. When the film thickness decreases as the flow rate increases, the deposition intermediates, such as Cr ²⁺ , lose their ligands.
Although Zn ²⁺ and the like are more likely to be washed away offshore and the plating efficiency is reduced, it is possible to form a preferable plating film by appropriately selecting the concentrations of the various auxiliary agents mentioned above. The plating bath temperature is preferably 20 to 70°C. If it is less than 20°C, the viscosity of the liquid is high, and sufficient diffusion of ions is suppressed, resulting in low plating efficiency, which is not preferable. vice versa
At temperatures higher than 70°C, normal plating becomes impossible due to ligand dissociation of the chromium complex ion. Note that Table 2 shows some examples of manufacturing these plating films. The plating composition of the zinc-chromium eutectoid plating layer or the eutectoid plating layer mainly consisting of zinc and chromium is suitably chromium in a range of more than 5% by weight to 40% by weight. If the content is less than 5% by weight, the positive effect of chromium on corrosion resistance will be small, while if it exceeds 40% by weight, the plated layer will become hard and the workability will be poor. The amount of plating film is preferably 1 g/m ² or more. 1
If it is less than g/m ² , sufficient corrosion resistance cannot be obtained. The upper limit varies depending on the application, but from the viewpoint of manufacturing costs, it is 50
g/m ² , preferably 3 g/m ² . [Operation] According to X-ray diffraction, the zinc-chromium eutectoid plating structure has a lattice spacing d of approximately 2.13 to 2.14 angstroms, 1.50 angstroms, and 1.22 to 1.23 angstroms. This appears to be due to the crystal lattice constant of chromium being shifted by the solid solution of zinc atoms. However, when the zinc composition is high, not only this but also the diffraction peak of the η phase (pure zinc) is accompanied.
Furthermore, when the chromium composition is high, the diffraction peak of normal metallic chromium may occur. The corrosion potential of zinc-chromium eutectoid plating is -1000mV _VS. While it is about SCE, it becomes nobler as the chromium composition increases, and -
850mV _VS. Shift to SCE level. Since this level of potential is clearly less base than that of iron, the zinc-chromium eutectoid plating of the present invention can sufficiently protect the base iron from sacrificial corrosion, and since it is not as base as pure zinc, the corrosion rate is faster. will also become smaller. In addition, the chromium in the film component forms a strong, chemically stable, and highly electrically resistive corrosion product film on the plating surface. This strongly prevents water, oxygen, and various ions from penetrating into the plating layer and base steel remaining on the base, as well as corrosion current, making the present invention ideal for long-term rust prevention of steel plates. As mentioned above, the present invention further improves corrosion resistance in the form of multi-layer plating, and the zinc-chromium eutectoid plating layer is the lowest layer, middle layer, and surface layer of multi-layer plating that is in direct contact with the base steel sheet. That is, it can be used for any of the top layers. When a zinc-chromium eutectoid plating layer is used as the bottom layer or intermediate layer, the top layer plating is Zn-Cu, Sn-Mn-Fe, Fe-Mn-
Examples include those coated with Zn, Zn-Cr, or Zn-Fe-P alloy plating of 1 g/m ² or more. Next, when a zinc-chromium eutectoid plating layer is used as the intermediate layer or the top layer, the bottom layer plating is preferably zinc plating or alloy plating containing 60% or more of zinc. At this time, the metals used to form the zinc alloy include Fe,
Ni, Co, Mn, Al, Mg, Si, Mo, Cu, Sn,
One or more types of Ti, P, etc. are applicable. Thirdly, no matter which layer the zinc-chromium eutectoid plating layer is used for, other plating layers are not limited to zinc-based plating, and may include lead, aluminum, aluminum, etc. depending on the use of the plated product. Tin, chromium, nickel, manganese, etc. and alloy plating based on these can be applied. Furthermore, an embodiment having two or more zinc-chromium eutectoid plating layers having different zinc and chromium content ratios is also effective. [Example] Examples of the present invention will be described below.

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〔Effect of the invention〕

According to the present invention, the corrosion resistance of a steel plate can be significantly improved by plating with a thin coating, and it can be used as a material for transportation vehicles such as automobiles, a material for construction, etc.
By applying it to materials for electrical equipment, etc., excellent industrial effects can be obtained.

Claims (1)

[Claims] 1 A eutectoid plating layer of zinc and chromium with a chromium composition of over 5% to 40% by weight, and Zn-Ti, Zn
−Cu, Sn−Mn−Fe, Fe−Mn−Zn, Zn−Cr,
Or a rust-proof steel sheet with excellent corrosion resistance, which has a multilayer plating layer consisting of a Zn-Fe-P plating layer.