Patent Application
20180044797
This application claims priority to Japanese Patent Application No. 2016-159221 filed on Aug. 15, 2016, the entire disclosure of which is incorporated by reference herein.
The present disclosure relates to a repair agent and a repair method for a plated base which includes a plating layer disposed on a surface of a metal base, as well as to the plated base.
Zinc-plated steel sheets which are steel sheets with a zinc plating layer disposed on their surface are generally used as automobile parts. Zinc, which is included in the zinc plating layer, has a stronger tendency to ionization than iron, which is included in the steel sheets. Therefore, if the zinc plating layer suffers a scratch such that the steel sheet is exposed, the zinc plating layer offers sacrificial protection for the steel sheet as the zinc is eluted, and has the ability to form a protective film as the zinc eluted forms a film on the surface of the steel sheet exposed. Thus, the zinc plating repairs itself, and features corrosion resistance. It has been found, however, that conventional zinc plated steel sheets do not feature sufficient corrosion resistance.
Japanese Unexamined Patent Publication No. 2010-174273 discloses a corrosion-preventing film which is disposed on the surface of a metal body and which includes an underlayer comprised of electrically conductive microparticles and a surface comprised of electrically conductive macromolecules. De facto, however, even such a technique fails to provide sufficient corrosion resistance.
The inventors of the present disclosure have come across yet another problem: in attempting to add a repair agent to a protective film, and trying to add all kinds of chemical compounds as single components, no sufficient corrosion resistance could be achieved.
In automotive lightweighting, steel sheets made of, e. g., high-tensile material (high-tensile steel sheets) are employed. A plated base which includes such a steel sheet having a surface covered with a plating layer is more prone to hydrogen brittleness and strength degradation caused by corrosion than a plated base including a different kind of steel sheet. This even further exacerbates the above-described problem.
The present disclosure attempts to provide a repair agent which features increased corrosion resistance.
The present disclosure relates to a repair agent for a plated base including a plating layer which is disposed on a surface of a metal base and which includes a metal with a stronger tendency to ionization than metal comprising the metal base, wherein
the repair agent includes a phosphoric acid compound and a phosphonic acid compound.
The repair agent of the present disclosure features increased corrosion resistance. More specifically, even if in an actual use environment the plated base suffers a scratch which reaches the metal base, the repair agent of the present disclosure offers such an advanced sacrificial protection and has such a remarkable ability to form a protective film that it can excellently repair itself and thus features increased corrosion resistance.
The repair agent of the present disclosure is particularly useful when the steel sheet is a high-tensile material (high-tensile steel sheet).
A repair agent according to the present disclosure is intended for a plated base. When the plated base suffers a defect which reaches from a surface of a plating layer to a metal base of the plated base, the repair agent forms a protective film on an internal surface of the defect, in particular on a surface of the metal base which is exposed where the plated base has suffered the defect. Hereinafter, the present disclosure will be described in detail with reference to the drawings. Note that elements shown in the drawings are illustrated in a schematic and exemplary manner, and only serve the purpose of making the present disclosure comprehensible. Actual appearance and dimensions may vary from the drawings. Unless stated otherwise, the same reference characters and symbols refer to the same part or have the same meaning.
As shown in
In the field of automobile parts, it is beneficial if the metal base 1 is a steel sheet, and even more beneficial if the metal base 1 is a so-called carbon steel sheet, particularly a high-tensile steel sheet (high-tensile material).
The plating layer 2 includes as main component a metal with a stronger tendency to ionization than a metal comprising the metal base 1. Hereinafter, the metal included as the main component of the plating layer 2 and having a strong tendency to ionization is referred to as a “metal A with a strong tendency to ionization.” If the metal base 1 is a steel sheet, the metal comprising the metal base 1 is iron. As metal with a stronger tendency to ionization than iron, for example, one or more metals from a group consisting of zinc, aluminum, and magnesium may be employed. Beneficially, zinc is employed. Such a metal A with a strong tendency to ionization included in the plating layer 2 generally contributes in ionic form to a formation of a protective film, as will be described later.
In terms of the formation of the protective film on an exposed surface of the metal base 1, it is beneficial if the plating layer 2 is a zinc plating layer. A zinc plating layer is a plating layer which includes zinc. Beneficially, the zinc plating layer is a zinc alloy layer.
All kinds of processes may be employed for forming the plating layer 2. Possible processes include, for example, so-called wet-plating processes such as electroplating, electroless plating, and hot-dip plating, and so-called dry-plating processes such as vacuum plating (physical vapor deposition (PVD)), chemical vapor deposition (CVD), and mechanical plating. Beneficially, a dry-plating process, in particular mechanical plating, is employed. In the scope of mechanical plating, a plating layer (film) is formed by projecting composite particles onto an object (metal base 1) subjected to the mechanical plating. The composite particles have a core (e.g., an iron core) which has an outer shell including constituent metal particles of the plating layer.
The plating layer 2 is not particularly limited in its thickness, and may be, e.g., 1 μm thick or thicker. The plating layer 2 generally has a thickness of 1 to 50 μm, and beneficially a thickness of 1 to 10 μm.
The repair agent of the present disclosure includes a phosphoric acid compound and a phosphonic acid compound. The repair agent is defined as an agent which forms a protective film on an exposed surface of a metal base.
The phosphoric acid compound is an inorganic phosphoric acid compound of phosphoric acid (H3PO4), or a phosphate, or both. For the formation of the protective film, the phosphoric acid compound beneficially is a phosphate. The phosphate is a salt of phosphoric acid ions such as first phosphoric acid ions (H2PO4−), second phosphoric acid ions (HPO42−), or third phosphoric acid ions (PO43−), and cations. For the formation of the protective film, it is beneficial if the phosphoric acid ions are the first and second phosphoric acid ions, and even more beneficial if the phosphate ions are the first phosphoric acid ions. The cations are one or more ions selected from a group consisting of monovalent metal ions, divalent metal ions, trivalent metal ions, and ammonium ions. Beneficially, the cations are monovalent metal ions and ammonium ions. A metal comprising the monovalent metal ions may be an alkali metal (e.g., sodium, potassium, or lithium), and beneficially is sodium and potassium. A metal comprising the divalent metal ions may be an alkaline earth metal (e.g., magnesium, calcium, strontium, or barium) or manganese, and beneficially is calcium, barium, and manganese. A metal comprising the trivalent metal ions may be, e.g., chrome or aluminum, and beneficially is chrome.
Concrete examples of the phosphoric acid compound beneficial for the formation of the protective film include: phosphoric acid (H3PO4), sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, calcium dihydrogen phosphate, barium dihydrogen phosphate, manganese dihydrogen phosphate, lithium dihydrogen phosphate, ammonium sodium hydrogen phosphate, diammonium hydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, barium hydrogen phosphate, manganese(II) hydrogen phosphate, chromium(III) phosphate, tripotassium phosphate, trisodium phosphate, and a condensed phosphoric acid compound. The condensed phosphoric acid compound may be, e.g., a compound comprised of cations and anions of tripolyphosphate, pyrophosphate, metaphosphate, or phosphorous acid. The cations are selected among alkali metal ions, alkali earth metal ions, or amphoteric metal ions (zinc ions or aluminum ions).
Examples of the phosphoric acid compound beneficial for the formation of the protective film include: sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium sodium hydrogen phosphate, diammonium hydrogen phosphate, dipotassium hydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, and a condensed phosphoric acid compound. Beneficial concrete examples of the condensed phosphoric acid compound include: aluminum dihydrogen tripolyphosphate, calcium tripolyphosphate, zinc tripolyphosphate, sodium tripolyphosphate, calcium metaphosphate, calcium pyrophosphate, aluminum phosphite, and zinc phosphite.
The phosphoric acid compound is commercially available, and thus easy to obtain. As an alternative, two or more compounds may be used as the phosphoric acid compound.
As long as the phosphonic acid compound includes atoms including unshared electron pairs which contribute to adhesion of the protective film to the metal base 1, the phosphonic acid compound is not particularly limited. For example, the phosphonic acid compound may be an organic phosphonic acid compound of a nitrogen-containing phosphonic acid compound, or a salt of the nitrogen-containing phosphonic acid, or both. The organic phosphonic acid compound is a compound including an organic group and a phosphonate group. The organic group may be an alkylene group. In particular, an alkylene group with one to three carbon atoms are beneficial. The phosphonate group is represented with the formula —P(═O)(OH)2, and may as well have the form of a salt. The phosphonate group having the form of a salt means that hydrogen ions of the hydroxyl group of the phosphonate group are liberated, and may be replaced by, e.g., metal ions. The metal ions may be, e.g., sodium ions, potassium ions, or calcium ions.
As long as the nitrogen-containing phosphonic acid compound is an organic compound including nitrogen atoms and a phosphonate group, the nitrogen-containing phosphonic acid compound is not particularly limited. The nitrogen-containing phosphonic acid compound may be an amine containing a phosphonate group of, e.g., amino tris(methylene phosphonate) (ATMP) (chemical formula: N[CH2PO(OH)2]3), aminotris(ethylene phosphonate) (chemical formula: N[CH2CH2PO(OH)2]3), and their metallic salts. If the compound includes two or more hydroxyl groups in one molecule, in the metallic salts the hydrogen ions in a part of the hydroxyl groups may be replaced by metal ions. Alternatively, the hydrogen ions in all of the hydroxyl groups may be replaced by metal ions.
The phosphonic acid compound is commercially available, and thus easy to obtain. As an alternative, two or more compounds may be used as the phosphonic acid compound.
The phosphoric acid compound and the phosphonic acid compound are generally included in a weight ratio ranging from 10/90 to 90/10. For the formation of the protective film, the weight ratio beneficially ranges from 20/80 to 80/20, more beneficially from 40/60 to 80/20, and even more beneficially from 55/45 to 75/25. If two or more compounds are used as the phosphoric acid compound, it is beneficial if the total amount of these compounds falls within the above ranges. If two or more compounds are used as the phosphonic acid compound, it is beneficial if the total amount of these compounds falls within the above ranges.
Since the repair agent of the present disclosure includes a combination of the phosphoric acid compound and the phosphonic acid compound, an advantageously non-conductive and adhesive protective film can be formed on the surface of a metal base which is exposed because of a defect. As a result, it may be assumed that corrosion resistance is sufficiently improved. If a compound such as a nitrate compound, a carbonate compound, a carbonate hydrogen compound, a chromate compound, a silicate compound, a fluoride metal, or a metallic oxide is used instead of the phosphoric acid compound, or if an aromatic or aliphatic carboxylic acid, or an organic amine is used instead of the phosphonic acid compound, the protective film is not formed, or—even if the protective film is formed—the protective film declines in non-conductivity, or in adhesiveness, or in both. As a result, a sufficient corrosion resistance fails to be achieved. In the present description, non-conductivity is defined as an insulating property where volume resistivity is higher than or equal to 1012 Ω·cm.
When the protective film is formed, the phosphoric acid ions in the phosphoric acid compound generate, due to a reaction (see, e.g., schematic chemical equation (I) below) with ions of the metal A with a strong tendency to ionization included in the plating layer, a non-conductive compound which serves as a main component for the structure of the film. On the other hand, while the phosphonic acid compound forms a complex together with ions of the metal A with a strong tendency to ionization (see, e.g., schematic chemical equation (II) below), unshared electron pairs of the nitrogen atoms included in the phosphonic acid compound make the phosphonic acid compound adhere to the surface of the metal base. In addition, this phosphonic acid compound complex promotes an amorphization of the film, and enhances flexibility and adhesiveness of the film with regard to the surface of the metal base. As a result, a protective film of a superb non-conductivity and adhesiveness is formed, and it may be assumed that corrosion resistance is sufficiently improved. Note that in the chemical equations below, formation of products derived from main materials involved in the formation of the protective film is expressed schematically.
Chemical Equation 1
Zn2+NaH2PO4→Zn3(PO4)2.4H2O (I)
Zn2++ATMP→Zn−ATMP(complex) (II)
In this description, corrosion resistance is defined as a feature which allows for resisting corrosion, in particular the ability to resist corrosion sufficiently even in the case of a defect where a metal base is exposed. The concept of corrosion resistance includes the ability of a substance to repair itself. The ability of a substance to repair itself is a behavior where the substance repairs a defect by forming a protective film on a surface of a metal base exposed due to the defect.
To enhance the formation of the protective film, the repair agent of the present disclosure beneficially further includes a compound including a metal with a strong tendency to ionization. Hereinafter, the metal with a strong tendency to ionization which is included in such a compound included in the repair agent is distinguished from the metal A with a strong tendency to ionization included in the plating layer, and is referred to as a “metal B with a strong tendency to ionization.” The metal B with a strong tendency to ionization also contributes in ionic form to the formation of the protective film. The metal B with a strong tendency to ionization may be chosen from the same range of metals as the metal A with a strong tendency to ionization. Beneficially, the metal B with a strong tendency to ionization is the same kind of metal as the metal A with a strong tendency to ionization.
As long as the metal has an ionic form in water, the metal B with a strong tendency to ionization is not particularly limited. The metal B with a strong tendency to ionization may be, e.g., zinc, iron, magnesium, cobalt, nickel, chromium, silver, zirconium, or aluminum. Among these metals, divalent to quadrivalent (beneficially divalent and quadrivalent) metals—in particular zinc, iron, nickel, and zirconium—are beneficial, and zinc is even more beneficial, for the formation of a complex in water, in particular for the formation of a complex with ATMP. As long as the metal has an ionic form in water, the compound including the metal B with a strong tendency to ionization is not particularly limited. Beneficial examples of the compound included in the metal B with a strong tendency to ionization include, for instance, zinc sulfate, iron sulfate, nickel sulfate, zirconium sulfate, zinc nitrate, aluminum sulfate, aluminum nitrate, magnesium sulfate, and magnesium nitrate.
The compound including the metal B with a strong tendency to ionization is beneficially included at 10 to 400 parts by weight per 100 parts by weight of a total amount of the phosphoric acid compound and the phosphonic acid compound. For the formation of the protective film, the compound is particularly beneficially included at 30 to 300 parts by weight, more beneficially at 80 to 200 parts by weight, and most beneficially at 110 to 150 parts by weight. If two or more compounds are used as the compound including the metal B with a strong tendency to ionization, it is beneficial if the total amount of these compounds falls within the above ranges.
The present disclosure further provides a method for repairing a plated base using the repair agent described above.
In the scope of the repair method for a plated base, the repair agent may be already included in the plating layer, or may be applied separately as an aqueous solution.
If the repair agent is already included in the plated layer, for instance in the scope of a mechanical plating process, the repair agent is allowed to adhere to surfaces of the constituent metal particles in the outer shells of the composite particles projected onto the object (metal base 1) subjected to mechanical plating. As a result, as shown in
If, in this case, the plated base 10 suffers a defect 13 reaching to the metal base 1 as the one shown in
Further, in the case where the repair agent is already included in the plating layer, for example a solution of the repair agent may be applied onto the surface of the plating layer 2 and dried such that a layer of the repair agent 30 forms on the surface of the plating layer 2, as shown in
If, in this case, the plated base 10 suffers a defect 13 reaching to the metal base 1 as the one shown in
In the case where the repair agent is used separately from the plated base in the form of an aqueous solution, when the plated base suffers a defect reaching to its metal base, an aqueous solution 31 of the repair agent comes into contact with the defect 13 as shown in
In
In this case, the concentration (total concentration of all components) of the repair agent in the aqueous solution is higher than or equal to 100 ppm, in particular higher than or equal to 500 ppm. Beneficially the concentration ranges from 500 to 10000 ppm (inclusive), and even more beneficially from 800 to 10000 ppm (inclusive). Here, “ppm” is an entity indicating a proportion by weight base.
In the scope of the repair method for the plated base of the present disclosure, the protective film 14 is selectively formed on the exposed surface of the metal base 1. This method is not bound to any specific theory but is employed for the following reasons:
(1) In an initial state the metal base 1 has a corrosion potential (negative), which is why, when the metal with a strong tendency to ionization is electrostatically pulled as cations toward the exposed surface of the metal base 1, other constituent materials of the protective film are electrostatically pulled toward the cations.
(2) After having adhered to the surface of the metal base 1, the ions which have been pulled toward the exposed surface bond together and form a film. While forming a two- or three-dimensional film, the ions are strongly absorbed into or strongly bonded to the surface of the metal base 1, and become a strongly adhesive film.
The fact that the protective film 14 has been formed on the exposed surface of the metal base 1 can be easily verified by taking an SEM photograph of the surface, or by performing an X-ray diffraction analysis (XRDA) of the film on the surface.
In the present description, the defect 13 is a scratch so deep that it reaches from the surface of the plated base 2 to the metal base 1.
In a device 50 which is shown in
Subsequently, while keeping the working electrode 51 immersed in the test fluid 52, the potential of the working electrode 51 has been altered using a potentiostat 55. Simultaneously, cathodic polarization has been performed at the working electrode 51 and cathodic current values have been determined. One example of a current-potential curve obtained this way is shown in
The current reduction rate E has been calculated based on the below equation.
E (%)=(Io−I)/Io×100 Equation (1)
In the above equation, Io is current density when no repair agent has been added to the test fluid. Specifically, Io is the current density when no repair agent has been added in the scope of Comparative Example A1 which will be described later.
In the equation, I is the current density when the repair agent of Example A1 is added to the test fluid. Further, I is the current density measured in this example.
The cathodic current value decreasing due to the addition of the repair agent means that the protective film has been formed on the exposed surface of the metal base, and indicates that the repair agent features corrosion resistance and, in particular, can repair itself.
In Comparative Examples A1 to A34, except for having dissolved a predetermined amount of a predetermined compound in a test fluid, the cathodic current value has been determined and the current reduction rate E has been calculated in the same manner as in Example A1. Kinds and amounts of the compounds of each of the Comparative Examples have been listed in the below table. Potentials for determining the current reduction rate have been −0.8 V and −1.1 V. In
In the scope of Example A1 and Comparative Examples A1, A18, A24, and A28, SEM photographs of surfaces of working electrodes have been taken after determination of cathodic current values. The SEM photographs are shown in
Results of these examples have shown that by forming a protective film with an excellent adhesiveness on a surface of a metal base, the repair agent of the present disclosure allows for forming of a protective film with an excellent cathodic current reduction rate, i.e., a protective film excellent in reducing oxygen and hydrogen reduction reactions.
In Example A2, apart from having set the concentration of sodium chloride in the test fluid to 3.5 wt %, cathodic polarization has been performed in the same manner as in Example A1. After cathodic polarization, the protective film on the surface of the working electrode has been subjected to thermogravimetric analysis (TGA). Analysis results are shown in
In Comparative Example A35, except for having set the concentration of sodium chloride in the test fluid to 3.5 wt %, cathodic polarization has been performed in the same manner as in Example A18. After cathodic polarization, the protective film on the surface of the working electrode has been subjected to thermogravimetric analysis (TGA). Analysis results are shown in
In Examples B1 to B10 and Comparative Examples B1 to B6, except for having dissolved a predetermined amount of a predetermined compound in a test fluid, the cathodic current values have been determined and the current reduction rate E has been calculated in the same manner as in Example A1. Kinds and amounts of the compounds of each of the Examples and each of the Comparative Examples have been listed in the below table.
The current reduction rates E have been ranked in the following manner.
−0.8 V
S: E≧85%;
A: E≧80%;
B: E≧75%;
C: E≧70% (unproblematic in practical use);
D: E<70% (problematic in practical use).
−1.2 V
S: E≧91%;
A: E≧88%;
B: E≧85%;
C: E≧76% (unproblematic in practical use);
D: E<70% (problematic in practical use).
In Comparative Example B4, an SEM photograph of a surface of a working electrode after determination of cathodic current values has been taken, and shown in
In Reference Example C1, except for having saturated the test fluid with nitrogen instead of air, cathodic current values have been determined and a current-potential curve has been obtained in the same manner as in Comparative Example A1. In
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-159221 | Aug 2016 | JP | national |
