JP2715758B2 - Joint structure with excellent corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a case where a magnesium-based member such as magnesium or a magnesium alloy is incorporated into a member made of a dissimilar metal material such as steel, due to a potential difference between the dissimilar metal material and the magnesium-based member. The present invention relates to a joint structure that suppresses contact corrosion.

[0002]

2. Description of the Related Art Aluminum-based materials are used as components for automobiles and various components for mounting, taking advantage of their light weight and excellent corrosion resistance. Especially recently,
The field of use of aluminum-based materials as light-weight materials that can respond to the trend of low fuel consumption is expanding. Also, lighter magnesium alloys tend to be used as alternatives to aluminum-based materials. For example, wheels, crankcases, gearboxes, inlet housings,
Magnesium alloys such as alloy castings MC1-8 specified in JIS H5203 are used for engine parts and the like. Since the magnesium-based member is insufficient in strength by itself, it is attached to a steel strength member by a fastener such as a bolt. Also, not limited to cars,
Magnesium-based components are also incorporated in optical devices and home appliances.

[0003] Magnesium-based parts are rarely used in a naked state, and in order to impart aesthetic appearance and decorativeness, etc.
Some surface treatment is applied to the entire surface. As the surface treatment, a coating method is generally used. However, the coating is not perfect, and has defects such as gas pins, uneven coating, and poor adhesion. If these minute defects exist in the contact portion with the dissimilar metal, corrosion occurs. At this time, the form of corrosion proceeds under the coating film through the defective portion, and significant ones destroy the coating film. Most of this part is pitted.

For example, in a structure in which a magnesium alloy component is assembled to a mating member using steel bolts, washers, and the like, the difference in ionization tendency is caused by the presence of an electrolyte solution such as water or salt water in the atmosphere around the connection. From this, a local battery is formed. As a result, the magnesium alloy component is preferentially eluted into the electrolyte solution, causing so-called contact corrosion.
In order to prevent this contact corrosion, it is disclosed in Japanese Utility Model Publication No. 63-13248 that a high-silicon aluminum alloy insert having excellent set resistance is interposed at and around a portion where a magnesium alloy part contacts a fastening member. Proposed. High silicon aluminum alloy inserts
It is said that even if an electrolyte solution adheres to a contact portion with a steel part, contact corrosion does not occur due to ionization tendency.

[0005]

However, A390, which is a typical material of a high silicon aluminum alloy, is a material having lower corrosion resistance than other wrought aluminum alloys. Moreover, since it shows a potential of about -0.67 V in seawater, there is no contact corrosion. That is, although the high silicon aluminum alloy is interposed in the connection part with an emphasis on the suppression of contact corrosion, the problem of the occurrence of corrosion is still completely solved because the self-corrosion of the high silicon aluminum alloy itself is large. Absent. The present invention has been devised in order to solve such a problem, and an intermediate for reducing corrosion current is interposed between a magnesium-based member and a steel component to prevent corrosion of the magnesium-based component. The purpose is to suppress it more reliably.

[0006]

According to the present invention, as an intermediate for reducing corrosion current, a plurality of washers, bushes, and the like having a potential row inclined by changing the material thereof are combined with a steel strength member and a steel bolt. It is interposed between the magnesium-based members.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an intermediate for reducing the corrosion current,
It is also conceivable to use an aluminum-based alloy provided with a hard anodic oxide film, and to form an aluminum layer or a magnesium layer on the working surface of a steel strength member and a steel bolt. For example, when an aluminum alloy having a hard anodized film is used as an intermediate, the joint structure shown in FIGS. 1 and 2 is adopted. When attaching a magnesium-based member 1 such as a wheel to a strength member 2 such as steel with a steel bolt 3, an aluminum alloy bush 4 and a washer 5 are interposed to avoid direct contact of the magnesium-based member with the strength member 2. . At this time, bush 4
A washer 5 having a hard anodic oxide film 6 formed on its surface is used. In addition, as the bush 4,
There are various shapes including those having an inclined flange portion (FIG. 1) and those having a right-angled flange (FIG. 2).

As the aluminum alloy of the bush 4 and the washer 5, a material having a high mechanical strength per se and capable of forming a uniform hard anodic oxide film 6 is used. Aluminum alloys of this type include Al-Mg alloys, Al-
Si-based, Al-Mg-Si-based, Al-Mg-Zn-based and the like are known. The hard anodic oxide film 6 has a coating thickness of 10 to 10.
100 μm (preferably 30 to 60 μm) and film hardness 200 to 600 Hv (preferably 350 to 550 Hv)
It is desirable to have Further, it is also effective to form a hard anodic oxide film 6 on the aluminum alloy bush 4 and the washer 5 and then perform a sealing treatment to enhance the film insulation.

When a magnesium layer is used as an intermediate, for example, a joint structure shown in FIG. 3 is employed. A magnesium layer 7 is formed on the surface of the strength member 2 and the steel bolt 3, and the magnesium-based member 1 contacts the strength member 2 and the steel bolt 3 via the magnesium layer 7. The magnesium layer 7 is provided at least on the surface in contact with the strength member 2 and the steel bolt 3 and in the vicinity thereof. Materials for forming the magnesium layer 7 include pure Mg and A
There are magnesium alloys such as Z91, AM60 and AM100. In addition, pure Al, Al
The strength member 2 and the steel bolt 3 may be coated with an aluminum-based material such as -Zn or Al-Zn-In. Alternatively, an aluminum washer 5 is used in combination.

However, when an aluminum alloy having a hard anodic oxide film is used as an intermediate or when a magnesium layer is used as an intermediate, a troublesome treatment is particularly required. In this regard, in the present invention, since a washer, a bush, or the like, whose material is selected so as to give a gradient to the potential train, a corrosion current can be suppressed without requiring special treatment. In a joint structure using a bush, washer, etc. with a slope in the potential row, dissimilar metal materials are laminated so that the absolute value of the electrode potential decreases gradually from the magnesium-based member to the steel strength member and steel bolt. Bush, washer, etc. are used. For example, the bush 4 has a two-layer structure of an Mg layer 4b and an Al layer 4a as shown in FIG. 4 (b). It is sandwiched between steel bolts 3. The washer 5 is made of Mg as shown in FIG.
It has a three-layer structure of a layer 5a, an Al layer 5b exhibiting a noble potential, and an Al layer 5c exhibiting a noble potential. In a state where the Mg layer 5a side faces the magnesium-based member 1, the magnesium-based member 1 and the strength member 2 between. Mg
As the layer 5a, pure Mg, AZ91, AM60 or the like is used. As the Al layer 5b exhibiting a low potential, Al-
Aluminum alloys such as Zn-In and Al-Zn are used. Further, as the Al layer 5c exhibiting a noble potential, an aluminum alloy such as pure Al or Al-Mg is used.

[0011]

[Action] Contact corrosion in an atmosphere in which an electrolyte solution is present is affected by the strength of corrosion current generated between dissimilar metal materials. The corrosion current (I) is generally represented by the following equation. By the way, in seawater, a normal magnesium alloy shows an electrode potential of -1.50 V, 5000 series, 6000 series.
An aluminum alloy such as a system shows an electrode potential of -0.75 V, and a normal steel, iron alloy or the like shows an electrode potential of -0.65 V. Substituting these values into the above-mentioned equation of corrosion current (I) (M
The contact system of g-Fe substitutes the electrode potential value of Mg for the anode potential, and the contact system of Al-Fe substitutes the electrode potential value of Al. Δ
V (= Vc−Va) increases. That is, a larger corrosion current flows through the magnesium alloy that comes into contact with steel, an iron alloy, or the like than with an aluminum alloy that comes into contact with steel, an iron alloy, or the like, and the corrosion of the magnesium alloy is accelerated.

When a bush 4, a washer 5 or the like having an inclined potential row is sandwiched between the magnesium-based member 1 exposed to the corrosion system and the steel strength member 2, the magnesium-based member 1 and the steel The potential difference between the strength member 2 and the magnesium member 1, the strength member 2 made of steel, the bush 4, and the washer 5 are less likely to flow. As a result, corrosion prevention of the magnesium-based member 1 is achieved. Washer 5 etc. with inclined potential train (Fig. 4)
The material used for the Al layer 5b exhibiting a lower potential of
Al-Zn-In alloy (electrode potential -1.10V), A
1-Zn (electrode potential -0.9 V). Materials used for the Al layer 5c exhibiting a noble potential include pure Al (electrode potential -0.70 V) and Al-Mg based alloy (electrode potential-
0.75 V). These aluminum-based alloys
It can also be used as the Al layer 4a of the bush 4.
On the other hand, as materials for forming the Mg layer 5a, AZ91 (electrode potential -1.50 V) and AM60 (electrode potential -1.50 V).
V), pure Mg (electrode potential -1.60 V) and the like. Each of the layers 4a, 4b and 5a-
When the material combination 5c is selected, the potential difference ΔV (Vc−
Va) is reduced, and the corrosion current (I) flowing between different kinds of materials is suppressed.

[0013]

Embodiment 1 As a magnesium-based member 1, an electrode potential of -1.5V M
Using a g-alloy AZ91, an epoxy resin cationic electrodeposition coating was applied to the entire surface with a coating thickness of 20 μm. Electrode potential-0.
Using 65V steel bolt 3, electrode potential -0.65V
Was attached directly to the strength member 2 made of mild steel. As the strength member 2, a bare material made of SS41 and a material obtained by applying a coating to SS41 parts were used. Two types of steel bolts 3 were used: bare SS41 material and SS41 plated with Cr.

The AZ91 magnesium alloy painted member 1 was directly attached to the strength member 2 using a steel bolt 3. 3 between the magnesium-based member 1 and the strength member 2
The magnesium alloy member 1 was attached to the strength member 2 with the two-layer bush 4 interposed between the magnesium-based member 1 and the steel bolt 3 with the layered washer 5 interposed therebetween.
A bush 4 having a two-layer structure is formed by laminating 5052 on pure Mg, and a washer 5 having a three-layer structure is formed by adding pure Mg to Al
508% -3% Zn-0.02% In alloy as an intermediate layer
3 was used. Magnesium alloy member 1
The strength member 2 fitted with was exposed to a 5% NaCl sprayed salt spray test tank maintained at 30 ° C. 4
After a lapse of 8 hours, the container was pulled out of the tank, washed with water, and the state of corrosion of each part was observed. Table 1 shows the observation results.

[0015] As is clear from Table 1, when the aluminum alloy member is attached to the strength member using the two-layer bush or the three-layer washer, it is found that the corrosion generated near the contact portion is largely suppressed. This is presumed to be the result of reducing the corrosion current (I) by giving an inclination to the potential train, as described above.

[0016]

As described above, in the present invention, the magnesium-based member is prevented from directly contacting with a dissimilar metal material such as steel, and an intermediate obtained by laminating dissimilar materials with an inclined potential row. Interposed between the two. This intermediate has a small potential difference between the magnesium-based member and the steel member, and exhibits an action of suppressing a corrosion current. as a result,
Corrosion of the magnesium-based member is suppressed, and a structure utilizing the inherent lightness of the magnesium alloy can be obtained. For example, in an automobile or the like, improvement in fuel efficiency and the like can be achieved.

[Brief description of the drawings]

[Fig. 1] Joint structure using a bush and washer on which a hard anodic oxide film is formed

[Fig. 2] Joint structure

[Fig. 3] Joint structure using bush and wire with magnesium layer

FIG. 4 shows a joint structure according to the present invention.

[Explanation of symbols]

1: Magnesium-based member 2: Strength member 3:
Steel bolt 4: Bush 4a: Al layer 4
b: Mg layer 5: Washer 5a: Mg layer 5b: Al layer with low potential 5c: Al layer with noble potential 6: Hard anodic oxide film 7: Magnesium layer

Claims (1)

(57) [Claims] 1. A strength member made of steel and said strength member
Magnesium parts attached to steel bolts
Material, the strength member and the steel bolt and the mag
Aluminum-based block interposed between nesium-based members
A bush and / or a washer.
Or, the washer is used for the strength from the magnesium-based member.
Electrode potential towards the member and / or the steel bolt
Dissimilar metal materials are laminated so that the absolute value decreases sequentially
The joint structure is excellent in corrosion resistance.