JPS62192592A - Method for preventing crevice corrosion of aluminum or aluminum alloy - Google Patents

Abstract

PURPOSE:To prevent the crevice corrosion of Al (alloy) by bringing the Al (alloy) into contact with a sacrificial anode of an Al alloy contg. In or Sn and by keeping the corrosion potential of the contact body at a specified value. CONSTITUTION:When Al (alloy) is preset with a crevice in an environment in which halogen ions such as Cl ions are present, the Al (alloy) undergoes crevice corrosion. In order to prevent the corrosion, the Al (alloy) is brought into contact with a sacrificial anode of an Al alloy contg. 0.03-0.5wt% In and/or 0.01-0.03wt% Sn, and the corrosion potential E of the contact body is kept at -1.3--1.0 VvsSCE. The crevice corrosion of the Al (alloy) in the corrosive environment is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for preventing corrosion of aluminum or its alloy, particularly crevice corrosion caused by the presence of gaps.

<Prior Art> As is well known, aluminum or its alloys can be said to have high corrosion resistance in the shell, and therefore are currently extremely widely used as various structural materials, parts, etc.

<Problems to be Solved by the Invention> However, even with such highly corrosion-resistant aluminum or the same alloy, if there are gaps between these parts and other parts used in contact with them, It is known that due to this gap, corrosion progresses more significantly inside the gap than in the open area outside. This is what is called crevice corrosion.

This crevice corrosion progresses rapidly, especially in environments where halogen ions such as chlorine ions are present (for example, salt-affected areas, chemical plants, etc.), causing great damage and potentially causing serious accidents. In addition, in the case of crevice corrosion, the extent of the corrosion is often not visible from the outside, which is extremely dangerous and can suddenly lead to a major accident.

Currently, there are no effective means to prevent this crevice corrosion, and the conventional method has been to fill the gaps with fillers such as compounds, but this filling process is difficult and can be prevented. The effects could not be said to be sufficient.

Therefore, the present inventors focused on the electrical corrosion protection method and studied a cathode corrosion protection method using a sacrificial anode material.

However, according to research conducted by the present inventors, research on crevice corrosion and examples of countermeasures based on them are concentrated on stainless steel, and there are very few examples targeting aluminum or its alloys. Therefore, it was found that the electrochemical conditions and appropriate sacrificial anode materials to prevent such crevice corrosion are largely unknown.

In addition, when applying cathodic corrosion protection to aluminum or its alloys, especially when there are gaps as in this case,
As described below, it is necessary to be careful about damage caused by excessive corrosion protection. In other words, when the sacrificial anode material (anode side) is brought into contact with the material to be corrosion-protected, a reduction reaction of hydrogen ions occurs on the material-to-be-corrosion-protected side (cathode side), and as a result, 0H- ions are generated near the surface of the material to be corrosion-protected. Concentration is relatively high. At this time, if there is no gap, the OH- ion concentration will hardly increase because the movement with the surrounding area is easy due to diffusion, but if there is a gap, there will be no increase in the OH- ion concentration. Since movement is restricted, OH- ions gradually increase and the pH increases. Such an increase in pH is actually beneficial for corrosion protection in stainless steel materials, etc., but in the case of aluminum or its alloys, the protective oxide film on the surface dissolves at high pH (>8) because it is an amphoteric metal. Moreover, there was a fear that the t-th solution (corrosion) within the gap would be accelerated by contact with the sacrificial anode material.

In order to overcome this situation, the present inventors investigated in detail the conditions for the occurrence and growth of gap g1 corrosion in aluminum or the same alloy, and found the following.

In other words, if aluminum or the same alloy is brought into contact with a sacrificial anode material made of a special alloy material, and the corrosion potential E of this contact body is set in the range of -1,3≦E≦-1,0 (VvsSCE), crevice corrosion will occur. It has been found that corrosion does not progress, and in practical use, there are no harmful effects due to excessive corrosion.

The present invention was made based on this result.

Means for Solving the Problems and Their Effects> The characteristics of the present invention are that aluminum or its alloy is brought into contact with a sacrificial anode material, and that the corrosion potential E of this contact body is -1.3.
The present invention provides a method for preventing crevice corrosion of aluminum or its alloy in a range of ≦E≦-1,0 (V vs SCE).

The reason why the corrosion potential E of the contact body between aluminum or its alloy and the sacrificial anode material is set in the range -1,3≦E≦-1,0 (VvsSCE) in the present invention is based on the following experimental results.

(IIAl-, I Result of constant potential holding test with l gap specimen Regarding the gap formed between Al specimen and A1 specimen, after various combinations of potential and time, 5% NaC
A constant potential holding test was conducted in an aqueous solution (30° C.) to investigate the occurrence and growth of crevice corrosion, and the results shown in FIG. 1 were obtained.

In this table, ×: Area where crevice corrosion occurs and grows;
Δ: indicates a part where crevice corrosion occurs but does not grow; ○: indicates a part where crevice corrosion does not occur.

From FIG. 1, it can be seen that if the potential E can be maintained at E≦-1, 0, crevice corrosion will not grow and corrosion can be prevented.

(2) Result of repassivation potential measurement of crevice corrosion in A I - A l gap specimen The gap formed between the Al specimens was treated with 5% NaCl aqueous solution at 30°C, depassivated. The repassivation potential of crevice corrosion was measured under atmospheric conditions. The results are shown in Figure 2.

From this FIG. 2, it can be seen that the potential E is E=-0,78 (Vvssc
In E), crevice corrosion grows, but this growth is E=-1,
It can be seen that it stops near 0, which is consistent with the result of section (1) above.

The critical potential for growth and repassivation potential in crevice corrosion mentioned above depend on the geometric shape of the crevice, the corrosion environment, etc., but C is the most common corrosion environment.
In a neutral aqueous solution containing X ions, gaps of various sizes,
When the repassivation potentials E and I were measured for NaCl aqueous solutions with various concentrations, they were found to be approximately ET under all conditions.
1 to -1, and it was also found that corrosion does not progress when El≦-1,0.

On the other hand, when an anode electrode material is brought into contact with aluminum or the same alloy, the aluminum or the same alloy on the corrosion-protected side becomes a cathode, but if the potential E is too low (over-corrosion protection),
Due to the following reaction, H ions near H'+ e --> 1/2 Hz are rapidly consumed, and as a result, the surrounding p
H increases. Since aluminum or its alloys are amphoteric metals, they begin to dissolve when the pH is >8.

Therefore, it would be bad if the potential E to be controlled is too low (there is naturally a lower limit).

This lower limit potential E was determined by tests conducted by the inventors, etc. (see Table 1 and Figure 3), which will be described later.
SCE).

In this way, in the present invention, the potential E to be controlled is set in the range of -1,3; E≦-1,0 (VvsSCE).

From this conclusion, as a sacrificial anode material made of aluminum or the same alloy, the corrosion potential E of the contact body obtained when it comes into contact with aluminum or the same alloy should be set within the above setting range (-1,
It can be seen that any material that holds within the range (3≦E≦−1,0) may be used.

Therefore, after considering various materials, commercial line Aj!
(99.8 wt% purity) with 0.0 indium (In)
3≦In≦0.5wt% or tin (S n) 0.0
It has been found that an alloy obtained by adding L5Sn≦0.03 wt% alone or in combination is preferable.

Here, the addition amount of each element (In, Sn) was first limited to the lower limit, that is, 0.03wt%≦In,
The reason for setting the range of 0.01wt%≦Sn is that if the addition amount is less than this, the corrosion potential E of the contact body is not within the range of E≦−1,0.On the other hand, in the case of the upper limit, i.e. I
The reason for setting n≦0.5wt% and 5n50.03wt% is that if the amount added is too large than this range, the corrosion potential E of the contact body will not fall within the range of -1,3≦E, and the sacrificial This is because the rate of consumption of the anode itself increases, and based on the results of salt water immersion tests and salt water spray tests, if the amount added is larger than this, practical electrode durability cannot be obtained.

Incidentally, when AJ-In-based and Al-3n-based sacrificial anode materials are processed into a washer shape and this processed product is fixed to a pure Al plate with acrylic bolts and nuts, the overall corrosion potential E is When measured, the results shown in Figure 3 were obtained.

From this result, it can be seen that the above addition amounts (contents) of Sn are within the corrosion potential E range set in the present invention.

<Example> Sacrificial anode materials made of various alloy compositions shown in Table 1 were prepared, processed into a washer shape as shown in Fig. 4, and washer 1 (outer diameter 20 mmφ,
Inner diameter 11mmφ, thickness 1゜5mm) was made of pure/l plate 2 (J
I 5I100, length 50mm, width 5Qmm, thickness 2m
m) with acrylic bolts 3 and nuts 4, and the sample was subjected to a salt spray test specified in JIS Z2371 to observe conditions such as the occurrence of crevice corrosion.

The results are also listed in Table 1.

In addition, in the evaluation in Table 1, O: degree of surface discoloration;
Δ: There is pitting corrosion. XX: There is extensive corrosion. In particular, 1 indicates that the corrosion is due to crevice corrosion, and 1 indicates that the pH within the crevice is ~11.

From this Table 1, it can be seen that in the case of the comparison pure Al washer and the case without washer, crevice corrosion occurred over a wide range in the gaps formed between each constituent material, whereas the sacrifice that satisfies the conditions of the present invention It can be seen that when the anode material was a washer, only the degree of discoloration on the surface was observed, and no corrosion due to crevice corrosion or excessive corrosion protection was observed. In other words, it can be seen that the effect of preventing crevice corrosion is sufficiently achieved.

<Application Examples> The method for preventing crevice corrosion of the present invention can be applied to all uses as long as the conditions of the present invention are satisfied. It is.

FIG. 5 shows that an alloy that satisfies the conditions of the present invention is provided as a thermal spraying material 5, and this is sprayed as a sacrificial anode material onto the joint of aluminum plates 7 and 7 that are joined together using a fixture 6, etc., and the gap is This is a method to prevent corrosion.

In FIGS. 6 and 7, the filament 8 is made of an alloy that satisfies the conditions of the present invention, and this is used, for example, as a sacrificial anode wire of AC3R9 in FIG. 6, and as an A! In this method, each aluminum wire is used as a sacrificial anode wire of the distribution line 10 to prevent crevice corrosion in other aluminum wires 11. In addition, 12 is Aj
2 is a sheath of the distribution line 10.

<Effects of the Invention> As is clear from the above description, according to the present invention, as a sacrificial anode material that is brought into contact with aluminum or the same alloy, an alloy of a special composition (Int-0゜03≦In≦
0.5 wt% or 0.5 wt% of Sn. O1≦Sn≦o, 03
wt% added singly or in combination), the corrosion potential E of the body in contact with aluminum or the same alloy is -1,
Since it is set in the range of 3≦E≦−1°9 (V vs SCE), it is possible to provide a method for preventing crevice corrosion of aluminum or the same alloy, which is extremely effective in preventing corrosion.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between time and holding potential in a constant potential holding test on an Al-A/gap specimen, and Figure 2 is a graph showing the relationship between time and holding potential in a constant potential holding test with an Al-A/gap specimen.
- A graph showing the repassivation potential E of crevice corrosion in an AIl crevice specimen, and the polarization curve in the measurement. A graph showing the relationship between alloy content and potential,
Fig. 4 is a side view showing a test device for a salt water immersion test using the sacrificial anode material alloy of the present invention as a washer, and Fig. 5 is a method for preventing crevice corrosion using the present invention alloy as a thermal spray material for sacrificial anode materials. FIGS. 6 and 7 are longitudinal sectional views showing a method for preventing crevice corrosion using the alloy of the present invention as a sacrificial anode wire of an aluminum wire. In the figure, 1... washer for sacrificial anode material, 2... pure An plate 4, 5... sprayed material for sacrificial anode material, 7... aluminum plate, 8... strand of sacrificial anode material, 9 ...AC3R. 10...Al distribution line, 11...Aluminum wire, Kametane (, UA) Maintenance position (VvsSCE)

Claims (2)

[Claims] (1) Aluminum or its alloy is brought into contact with the sacrificial anode material, and the corrosion potential E of this contact body is -1.3≦E≦-1.
．． 1. A method for preventing crevice corrosion of aluminum or its alloy, characterized by keeping the VvsSCE in the range of 0 (V vs SCE). (2) The sacrificial anode material is commercially pure Al with 0.03 In
≦In≦0.5wt%, or Sn 0.01≦Sn≦
The method for preventing crevice corrosion of aluminum or its alloy according to claim 1, characterized in that the alloy is added alone or in combination in an amount of 0.03 wt%.