JPS595661B2 - Al alloy with excellent pitting corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an A1 alloy that has excellent extrudability and moderate strength, and particularly has excellent pitting corrosion resistance.

BACKGROUND ART Conventionally, as an A1 alloy having excellent extrusion processability and moderate strength, for example, an A1 alloy in the 6000 series of AA standard is known.

This conventional A1 alloy of the AA standard 6000 series has moderate strength and excellent extrusion processability due to the presence of fine Mg2Si compounds precipitated in the base material by aging treatment, and is used in various applications such as construction. It is widely used as a structural material.

In addition, this conventional A1 alloy has excellent surface treatment properties such as anodization, so when it is used for construction, it is often subjected to surface treatments to impart corrosion resistance and decorative properties. There are many.

Furthermore, this conventional A1 alloy is Al-
It has relatively good corrosion resistance next to A1 alloys such as Mg-based and AI-Mn-based alloys, so when the usage environment is, for example, a neutral aqueous solution, it is possible to use the material without surface treatment as described above. Even when used as is, it exhibits sufficient corrosion resistance due to the stable passive film formed on its surface. Combined with its excellent extrusion processability, it is used, for example, as a piping material for water supply and drainage. ing.

However, the conventional A1 alloy of the AA standard 6000 series,
If the usage environment contains ionic species that destroy passive films, such as Cl ions, pitting corrosion may occur on the surface, and if heavy metal ions, especially Cu ions, are included. Under such usage environments, the pitting corrosion caused by the Cl ions and the like grows further due to the CU ions and develops into through holes, often causing serious accidents.

This kind of pitting corrosion is particularly noticeable in usage environments such as freshwater where the Cl ion concentration is low (and the specific resistance of the solution is high) rather than in environments where the Cl ion concentration is high (and the specific resistance of the solution is low, such as seawater). It appeared in

That is, for example, tap water contains Cl ions of several tens of ppm or less because sterilization powder is added to the tap water. When the manufactured parts are exposed,
Pitting corrosion occurs in the member relatively early.

Therefore, 1. From the above-mentioned viewpoint, the present inventors focused on a conventional AI alloy of the AA standard 6000 series, which has good extrusion processability, surface treatment properties, and moderate strength, and developed a method that does not impair these properties. Excellent pitting corrosion resistance in environments where tap water containing Cl ions, aqueous solutions containing Cu ions in addition to Cl ions, and other environments where pitting corrosion is extremely likely to occur. As a result of research to obtain a material exhibiting ~1.5% (weight%, below % is weight%)
), and the relative content of Mg and Si is such that Si is somewhat larger than the atomic ratio (2:1) of the Mg2Si compound precipitated in the matrix, The structure after aging is a two-phase structure in which Mg2Si compounds are finely precipitated in the Al-8i solid solution matrix, and this Mg2Si compound is electrochemically less noble than the matrix. One reason for pitting corrosion is that micro galvanic corrosion is more likely to occur due to the potential difference between the two.

(b) Therefore, if we reduce the potential difference between the substrate and the Mg2Si compound by making the Mg2Si compound more noble or by making the substrate more base, the smaller the potential difference, the less galvanic corrosion will occur. The occurrence of pitting corrosion will be suppressed.

(e) Mg2Si in the base of the above Al-8i solid solution
When an A1 alloy having a two-phase structure in which a compound is precipitated contains a small amount of Zn within the solid solubility limit, the base material becomes more base and the potential difference with the Mg2Si compound becomes extremely small.

The findings shown in a) to (e) above were obtained.

This invention was made based on the above findings, and contains Si: 0.2 to 1.2%, Mg: 0.45 to 1.5%, and Zn: 0.3 to 1%. The A1 alloy has a composition with the remainder consisting of AI and unavoidable impurities, and has excellent pitting corrosion resistance, as well as moderate strength and good extrusion processability.

Next, the reason why the component composition of the A1 alloy of the present invention is limited as described above will be explained.

(a) Si and Mg As mentioned above, these components have the effect of forming Mg2Si that is finely dispersed and precipitated in the matrix to improve the extrusion workability and strength of the alloy. Si: less than 0.2% and Mg: less than 0.45%,
The desired effect was not obtained in the above action, while Si: 1.2%
, Mg: If the content exceeds 1.5%, the toughness of the alloy will decrease rapidly. ~1.
It was set at 5%.

(b) Zn The Zn component is dissolved in the base material to make it electrochemically base, so that there is no potential difference between the base material and the Mg2Si compound, and galvanic corrosion is suppressed as much as possible. It has the effect of moderately weakening the natural oxide film that forms on the alloy surface and slightly promoting general corrosion, thereby significantly suppressing the occurrence of pitting corrosion, but its content is 0.3
If the content is less than 1%, pitting corrosion resistance cannot be obtained, while if the content exceeds 1%, overall corrosion will increase significantly and uneven corrosion will also occur.
Its content was determined to be 0.3 to 1%.

In addition, in the A1 alloy of this invention, as mentioned above, Zn
Despite containing Zn, it has surface treatment properties equivalent to those not containing Zn, so surface treatments such as anodic oxidation can be performed satisfactorily.

Next, the A1 alloy of the present invention will be specifically explained using examples.

EXAMPLE After preparing A1 alloy molten metals having the compositions shown in Table 1 by the usual melting method, they were cast into ingots, and then subjected to soaking treatment, hot rolling, and cooling under normal conditions. The plate material is rolled to a thickness of 1 mm, and then this plate material is subjected to solution treatment by holding at a temperature of 550°C for 1 hour and then air cooling, and aging treatment by holding at a temperature of 160°C for 24 hours. According to the present invention A1 alloys 1 to 3, and Zn
Comparative A1 alloy 1°2 whose content is outside the range of this invention
Each plate material was manufactured.

Next, CuCl2・2H
Temperature at which Cu ion concentration was set at lppm by adding 20:
C by adding tap water at 30℃ and sodium hypochlorite.
Using tap water with an I ion concentration of 30 ppm, a test was conducted by immersing it in each of these tap waters for one month. After the test, corrosion products were removed with a chromic acid solution, and corrosion weight loss, pitting number, and The maximum pitting depth was measured for each.

The results of these measurements are shown in Table 1.

As shown in Table 1, in the Cu ion-containing tap water test, the A1 alloys 1 to 3 of the present invention took the form of general corrosion compared to the comparison A1 alloy 1 that does not contain Zn.
As the Zn content increases, the corrosion loss tends to increase, but the number of pitting corrosion and the maximum pitting depth are less than half, and when the Zn content is higher than the range of the present invention. Comparative A1 Alloy 2, which is out of the range, has a large corrosion loss due to the high Zn content (and the number of pitting corrosion and maximum pitting depth also have values almost equivalent to Comparative A1 Alloy 1, which does not contain Zn). It showed poor pitting corrosion resistance.

In addition, the CI ion-containing tap water test also showed the same tendency as the Cu ion-containing tap water test, and the present invention A
It is clear that no pitting corrosion is observed in Alloys 1 to 3.

As mentioned above, the A1 alloy of the present invention has excellent extrusion processability and moderate strength, and has particularly excellent pitting corrosion resistance. It exhibits excellent pitting corrosion resistance even when used in harsh environments where it is exposed to water.

Claims (1)

[Claims] A composition containing lsi: 0.2 to 1.2%, Mg: 0.45 to 1.5%, Zn: 0.3 to 1%, with the remainder consisting of Al and inevitable impurities. An A1 alloy with excellent pitting corrosion resistance, characterized by having (the above weight %).