JP3018970B2 - Method for preventing stress corrosion of aluminum alloy castings - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing stress corrosion in aluminum alloy castings such as mold castings, sand castings, shell castings, and the like. The present invention relates to a method for preventing stress corrosion of this aluminum alloy casting in a structure in contact with the aluminum alloy.

[0002]

2. Description of the Related Art Aluminum or aluminum alloys are used in many fields because of their features such as light weight and no pollution. However, they are extremely low-grade metals as industrially used metal materials. Is easily corroded in the presence of
In order to use these aluminums or aluminum alloys as industrial materials, how to prevent corrosion becomes an important issue.

[0003] Therefore, wrought aluminum or the like obtained by plastic working of aluminum or aluminum alloy has been conventionally subjected to an anodizing treatment (alumite treatment) for forming an anodized film on the surface thereof. It is widely used in the field of building materials and the like as an industrial material having excellent corrosion resistance.

However, in a casting obtained by casting an aluminum alloy using a mold, a sand mold, a shell mold, or the like, that is, an aluminum alloy casting, a sound anodic oxide film is formed on the surface of the casting more than an aluminum wrought product. It is difficult to form, and even if an anodic oxide film is formed on the surface, it is not always enough. In particular, how to prevent corrosion is important for use in applications that may come into contact with water. It is.

[0005] In particular, in a structure in which such an aluminum alloy casting comes in contact with a steel member of a dissimilar metal under a stress load, it is caused by rainwater, dew condensation water, moisture and corrosive components in soil, snow and road snow melting. When used in a so-called corrosive environment that comes into contact with chemicals, seawater, etc., the effects of this stress and the environment overlap, so-called stress corrosion occurs in the aluminum alloy casting, and this stress corrosion is immediately triggered. Alternatively, stress corrosion cracking occurs after a certain time.

For this reason, it is difficult to use a structure in which an aluminum alloy casting comes in contact with a steel member of a dissimilar metal under a stress load in a corrosive environment where water or the like comes into contact with the structure. The use of many industrial materials such as parts, marine parts, building parts, and railing parts is restricted, and there has been a problem that the properties of aluminum alloy castings such as light weight and toughness cannot be effectively utilized.

[0007] In the present invention, "the stress corrosion susceptibility aluminum alloy casting and the steel member of dissimilar metal come into contact with each other under stress" means that the aluminum alloy casting having stress corrosion susceptibility is (1) made of the same material or (2) When it is integrated with a steel fastening member for bolt and nut fastening, rivet fastening, screw fitting, etc. for the purpose of assembling with other material members, etc. (3) If a stress is applied during use, a state with residual distortion due to processing, for example, distortion due to strong processing such as threading or drawing is left When the processed part is used in a state where the distortion is not relieved, it means a state that appears in the case where it is used. The “steel member” here refers to a member made of steel such as steel, stainless steel, and anticorrosion-treated steel.

[0008]

Therefore, the present inventors have proposed that an aluminum alloy casting having a stress corrosion susceptibility comes into contact with rainwater, dew condensation water, moisture and corrosive components in soil, snow and road snow melting agents, seawater and the like. Intensive research is being conducted on methods for preventing stress corrosion of aluminum alloy castings so that stress corrosion does not occur in these aluminum alloy castings even when used under conditions of stress loading with steel members of dissimilar metals in a corrosive environment. As a result of the superposition, by interposing a metal member or a metal layer having a specific property in at least a part of the contact region between the casting and the steel member, it is possible to effectively prevent stress corrosion of the aluminum alloy casting. Headings,
The present invention has been completed.

Accordingly, an object of the present invention is to effectively prevent stress corrosion of an aluminum alloy casting in a structure in which an aluminum alloy casting having stress corrosion sensitivity and a steel member of a dissimilar metal are in contact with each other under a stress load. It is to provide a method that can be performed.

[0010]

That is, the present invention is to prevent stress corrosion of the aluminum alloy casting in a structure where an aluminum alloy casting having stress corrosion susceptibility and a steel member of a dissimilar metal come into contact under a stress load. At least a part of a contact area between the casting and the steel member is set at 100 mVvs SC from the spontaneous potential of the casting.
Above E or -1500m at mixed potential
This is a method for preventing stress corrosion of an aluminum alloy casting in which a metal member or a metal layer which is more noble than VvsSCE is interposed.

In the present invention, an aluminum alloy casting having susceptibility to stress corrosion is, for example, a conductive member for overhead wire,
Al-Cu alloys (AC1A, AC1) used as materials for automotive parts, bicycle parts, aircraft hydraulic parts, etc.
1B), Al-Si-Mg alloys (AC4A, AC4C, AC4CH) used as materials for engine parts, vehicle parts, marine parts, railing panels, etc .;
Al-Mg-based alloys (AC7A, AC4B) used as materials for marine parts, construction fittings, rail supports, etc., and Al-Mg-Zn-based alloys used as materials for train receiving racks, plastic molds, fishing boat parts, etc. (CX2A), Al-M used as a material for fishing equipment, bicycle parts, etc.
Sand casting material such as n-based alloy (DX24) or Al
-A casting made of a bare material cast using a die-cast material such as a Mg-based alloy (ADC5, ADC6) or an anodized material having an anodized film on its surface after being anodized. As the casting material, any casting method such as sand casting, shell casting, gravity die casting, low pressure casting, and molten metal forging may be used.

Here, when the aluminum alloy casting is an anodized material, the method of the anodizing treatment at that time is not particularly limited. For example, the anodizing treatment in a sulfuric acid bath, an oxalic acid bath, a chromic acid bath, There is a hard anodic oxidation treatment performed at a low temperature, usually 1 to 100 μm, preferably 5 to 80 μm.
An anodic oxide film having a thickness of m is formed. The anodized film formed on the surface of the aluminum alloy casting by anodizing in this way usually has fine defects,
When the film defect is exposed to a corrosive environment, stress corrosion occurs in the film defect, so that there is no difference in corrosion potential between the bare material and the bare material due to the thickness of the anodic oxide film.
However, the required anticorrosion current density is reduced, so that the intervening metal can act over a long period of time.

[0013] The sand casting material and the die casting material have grain boundaries.
Al-Cu-based as an intermetallic compound that affects corrosion
The alloy has a noble tendency of electric potential with respect to the matrix.
Al _TwoAnd θ'CuAl_TwoBecomes an Al-Si-Mg alloy
Is Si or Mg, which has a noble potential tendency with respect to the matrix._Two
Si has an electric charge against the matrix for Al-Mg based alloys.
ΒAlMg (Mg_TwoAl_Three) Is Al-
Mg-Zn based alloy has a tendency of electric potential to matrix
Is MgZn_TwoHowever, the Al-Mn alloy is
Al with a noble potential tendency₆Mn_xFe_1-xBut
Each exists (crystallized / precipitated), and rainwater, dew condensation, and soil
Contact with water and corrosive components, snow and road snow melting agents, seawater, etc.
In a corrosive environment, a local battery will
From which intergranular corrosion occurs. Al-
JIS AC4C alloy, which is a Si alloy,
The matrix near the recon crystallized material dissolves and
Pseudo-intergranular corrosion with erosion occurs, and
It also causes stress corrosion.

The dissimilar metal steel member is a metal that is nobler than the aluminum alloy from which it is cast,
The potential difference is 30 mV vs SCE or more at the natural potential as compared with the above-mentioned aluminum alloy.

Further, the structure in which the aluminum alloy casting and the dissimilar metal steel member come into contact with each other under a stress load is, for example, assembled and integrated with a split casting such as an automobile part by a steel bolt nut or rivet. In the structure that is being fixed, the structure where the anchor bolt made of steel material is screwed into the screw hole drilled in the casting post like the railing, and fixed to the base surface, or in the through hole drilled in the casting There are various structures such as a structure in which a fixing member made of steel is screwed and fixed.

According to the present invention, in a structure in which such an aluminum alloy casting and a steel member of a dissimilar metal come into contact under a stress load, at least a part of a contact area between the casting and the steel member includes a casting material. 100 from natural potential
mV vs SCE or more low, preferably 120 mV vs SC
Above E or -1500m at mixed potential
More than VvsSCE, preferably -1200mVvsS
A metal member or metal layer that is nobler than CE is interposed. With a base metal member or metal layer having a potential less than 100 mV vs. SCE from the natural potential of the casting, the effect of preventing stress corrosion is insufficient depending on the use environment (corrosion environment) of the structure, and a reliable and stable prevention of stress corrosion is achieved. Becomes difficult. In addition, the composite potential of the casting and the steel member is -1500 mV vs SC
With a noble metal member or metal layer having a value less than E, the corrosion of the casting is accelerated by the alkali generated by the corrosion.

As a metal material used for such a purpose, which is lower than the aluminum alloy casting forming the metal layer or metal layer, specifically, pure aluminum (1100
Alloys), aluminum such as 5052 alloy, 7072 alloy and the like for wrought working, and aluminum alloys for casting such as Al-Zn-In based alloys and its alloys, pure zinc (purity 99.9 wt% or more) And its alloys, such as metallic zinc, pure magnesium (purity of 99.9% by weight or more), and magnesium alloy such as AZ91.

Here, the following Table 1 shows typical examples of the metal material which is lower than the aluminum alloy casting forming the metal member or the metal layer. In addition, the measurement of the natural potential of each simple metal material and the mixed potential of a system in which different metals come into contact with each other, even in the same corrosive environment, the corrosion potential and pitting potential change over time. Since it is necessary to perform measurement in a close state, the potential after immersing in a 3.5% by weight aqueous NaCl solution for 3,000 hours was measured and evaluated.

[0019]

[Table 1]

The shape of the metal member is not particularly limited as long as it is a shape that can be attached to at least a part of the contact area between the aluminum alloy casting and the steel member so as to straddle the casting and the steel member. For example, it is formed into a casting, such as a ring, a washer, a plate, or a slab, or a molded product. The sizes of these metal members can be selected in consideration of the institution used, the use environment, the use method, the degree of safety, and the like of the target product.

As a method of attaching such a metal member to a part of the contact area between the aluminum alloy casting and the steel member, for example, a method in which the steel member is a bolt nut, a rivet or the like fastened to the casting is used. In addition, a metal member is formed in a washer shape and fixed by being sandwiched between these castings and a steel member, or fixed with a conductive adhesive (for example, Dotite or the like manufactured by Fujikura Kasei Co., Ltd.), or a lead wire. And an appropriate method such as bonding by soldering (for example, flux and solder (95 wt% Zn-5 wt% Al)).

Examples of the metal material that is more base than the aluminum alloy casting forming the metal layer include a coating material composed of zinc powder, chromic acid and glycol, and a zinc powder and a volatile material such as ethanol, xylene and toluene. Metal powder coating agent such as a coating agent composed of a conductive dispersant, preferably, after being applied to the surface of a casting, only a metal powder adheres without forming an insulating resin or the like to form a metal layer. Good to be.

Here, as the metal powder, pure zinc powder, Z
alloy powder of n (70 to 97% by weight) / Al (3 to 30% by weight); and mixed powder of Zn (70 to 97% by weight) -Al (3 to 30% by weight). The particle size is preferably smaller than 325 mesh, and if it is too coarse, unevenness is generated on the surface, which is not preferable.

As a specific example of the metal powder coating agent, there are typically zinc paint robal (trade name, manufactured by Robal) and zinc paint amel zinc (trade name, manufactured by Amel Chemical Co.) as a pure zinc powder spreader. In addition, as a zinc-containing powder coating agent, there is a dacro treating agent (trade name, manufactured by Nippon Dacro Shamrock Co., Ltd.).

In order to form a metal layer using such a metal powder coating agent, for example, a method such as brush coating, spray coating, dip coating or the like is preferably used to apply the coating agent to the surface of the aluminum alloy casting at 1 to 500 g / g. It is applied at a rate of m ² and dried and fixed at a temperature of from room temperature to 350 ° C. for 3 to 300 minutes to form a film having a thickness of 0.2 to 140 μm. The metal layer
If no work such as disassembly and maintenance is performed after assembly,
It is assumed that the coating is formed once, but if there is a disassembly and maintenance, the coating may be formed a plurality of times over time.

The method for preventing stress corrosion according to the present invention is particularly useful for building materials such as railing panels, railing columns, building hardware, transportation parts such as vehicle parts, automobile parts, bicycle parts, engine parts, brake calibres, and overhead wire fittings. The present invention can be suitably applied to railway parts such as train receiving racks, conductive members for overhead lines, marine parts such as fishing boat parts, marine parts, fishing tools, aircraft parts such as hydraulic parts, and various electric parts. Among these, for parts having a use environment of normal temperature or higher, for example, higher than 60 to 80 ° C., those which have been solid-dissolved at the crystal grain boundaries due to low-temperature aging are precipitated as intermetallic compounds, and are more susceptible to stress corrosion. In particular, the present invention is suitably applied because the above-mentioned state is easily caused.

[0027]

[Example] JIS with high stress corrosion sensitivity in the seaside atmosphere
AC1B alloy (Al-4wt% Cu-based), JIS AC
7A alloy (Al-5wt% Mg-based), Al-3wt% Z
70 mm × 150 mm using n-3.5 wt% Mg alloy, Al-1.3 wt% Mn alloy, and JIS AC4C alloy (Al-7 wt% Si-0.3 wt% Mg alloy) having high susceptibility to pseudo-intergranular corrosion. A test piece having a size of × 4 mm and having a bolt through-hole having a diameter of 6 mm at the center was cast, and this test piece was subjected to 3.5 wt% NaC kept at room temperature.
Polarization was measured in an aqueous solution, and pitting potential, spontaneous potential, and the like were determined as shown in Tables 2 to 5.

Further, as a steel member, a diagonal length of 10 mm
Galvanized steel (SS400) bolt (thickness: 8 μm, surface chromate treatment), untreated steel bolt (SM400B)
The pitting potential, spontaneous potential, and the like of those described in Tables 2 to 5 were determined in the same manner as described above.

Further, (A) Al-2.
5 wt% Mg alloy, (B) Al-1 wt% Zn alloy,
(C) Al-3 wt% Zn-0.02 wt% In alloy,
(D) 99.9 wt% Zn plate, (E) 99.9 wt% M
g plate, (F) 9 wt% Al-1 wt% Zn-90 wt%
A washer having an outer diameter of 12 mm, an inner diameter of 6 mm, and a thickness of 1 mm was prepared from a metal material of an Mg plate and a (G) 99.0 wt% Al plate, and the pitting potential of these washers was determined in the same manner as described above. The natural potential and the like were determined. The symbols in parentheses correspond to the symbols in Tables 2 to 5.

(H) A commercially available high-concentration zinc powder paint (zinc content: 96 wt%) is applied on the surface of the test piece by a spray method to form a zinc layer having a thickness of 10 μm as a metal layer. And (I) dispersing a flat zinc powder having a particle size of 20 to 100 μm prepared using atomized powder in a mixture of chromic anhydride and glycols to prepare a zinc powder suspension, Immerse the test piece in the suspension,
Baking at 300 ° C. for 15 minutes to form a zinc layer with a thickness of 6 μm as a metal layer, and (J) a coating with only a 50 μm coating thickness of a sulfuric acid anodic oxide coating, respectively, in the same manner as described above. The pitting potential, spontaneous potential and the like were determined. The symbols in parentheses correspond to the symbols in Tables 2 to 5.

Next, 3.5% by weight of NaC kept at room temperature
1. Using a test piece, steel bolt, washer and steel nut (of the same material as the steel bolt) after immersing in the aqueous solution for 3,000 hours, the head of the steel bolt and the bolt through hole of the test piece A washer is interposed between the peripheral portion and a steel nut, which is screwed into the screw portion of the steel bolt, and a tightening pressure of 15 mm is applied.
A test piece having a zinc layer on its surface was tightened at a strength of 0 kg / cm ² with a steel bolt and a steel nut at a tightening pressure of 150 kg / cm ^2. The structures were constructed by tightening, and these structures were further placed in a 3.5% by weight aqueous NaCl solution kept at room temperature.
The test piece was immersed for 000 hours, and the corrosion state of the test piece in the steel contact portion and the steel member was examined by visual observation, and a comprehensive judgment of the corrosion state was made.

The results are shown in Tables 2 to 5, and the evaluation of the corrosion state of the steel contact portion and the steel member of the test piece in the structure was as follows.
◎: no intergranular corrosion and pseudo-intergranular corrosion occurred, ：: white corrosion product was observed but no red rust was observed, ▲: red rust was observed, and ×: intergranular corrosion and pseudo-intergranular corrosion were observed. There was occurrence, and the evaluation was performed in four steps.

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

[Table 5]

As is clear from the above results, samples Nos. 1 to 70 corresponding to the examples of the present invention were all corroded by a washer which is a metal member for corrosion protection and a zinc layer which is a metal layer. The prevention effect is remarkably exhibited,
It turns out that the effect is large. Therefore, the anticorrosion effect can be exhibited by applying as a member having a specific shape or a coating layer formed on the surface according to the shape of the product to be anticorrosion or the use conditions of the product.

In addition, a bolt (film thickness 8) made of electrogalvanized steel (SS400 material) that rusts after a long time by itself.
μm, surface chromate treatment), the anticorrosion treatment of the present invention dramatically extends the time until rusting,
It was also found that contact corrosion between the aluminum alloy casting and the steel member was prevented. Further, it has been found that it is also effective to apply an anodic oxide film treatment to the aluminum alloy casting to perform the anticorrosion treatment of the present invention.

In addition, it was confirmed that a sufficient corrosion preventing effect was obtained by interposing a metal member of Al-2.5 wt% Mg alloy (A) between the cast of AC1B alloy and the steel member. Al-2.5w between casting and steel member
When a metal member of t% Mg alloy (A) is interposed, the corrosion prevention effect is poor, and the same phenomenon is observed in Al-3 wt%.
Also observed for Zn-3.5wt% Mg alloy castings,
Further, even with a 99.0 wt% Al aluminum material (G) having a small potential difference, intergranular corrosion which causes stress corrosion could not be prevented.

[0040]

According to the present invention, stress corrosion (intergranular corrosion) can be effectively prevented in a structure in which an aluminum alloy casting having stress corrosion susceptibility and a steel member of a dissimilar metal are in contact with each other under a stress load. This solves the problem of stress corrosion cracking in aluminum alloy castings having stress corrosion susceptibility, and enables the use of aluminum alloy castings in harsh corrosive environments. It becomes. Further, according to the present invention, not only stress corrosion of the aluminum alloy casting can be prevented, but also rusting on the steel member side can be prevented.

Further, by subjecting the aluminum alloy casting to anodizing treatment in advance, the consumption of the metal member or metal layer serving as a sacrificial anode occurs only at the defective portion of the anodic oxide film, and the required anticorrosion current density is reduced. Thus, the effect of preventing stress corrosion can be exerted for a longer period of time.

Claims (4)

(57) [Claims] 1. A method for preventing stress corrosion of an aluminum alloy casting in a structure where an aluminum alloy casting having stress corrosion susceptibility and a steel member of a dissimilar metal come into contact under a stress load. At least in part of the contact area between
A method for preventing stress corrosion of an aluminum alloy casting, comprising interposing a metal member or a metal layer which is no less than 0 mV vs SCE or noble at a mixed potential of -1500 mV vs SCE or more. 2. The aluminum alloy casting according to claim 1, wherein the metal material lower than the casting forming the metal member or the metal layer is a metal material selected from aluminum and its alloys, metal zinc, magnesium and its alloys. Stress corrosion prevention method. 3. The method for preventing stress corrosion of an aluminum alloy casting according to claim 1, wherein the metal member is fixed to the casting side. 4. The method for preventing stress corrosion of an aluminum alloy casting according to claim 1, wherein the metal layer is formed by applying a metal powder.