JPH0285384A - Method for preventing crevice corrosion of stainless steel - Google Patents

Abstract

PURPOSE:To simply protect the surfaces of stainless steel members brought into contact with a liq. such as water from crevice corrosion by forming Zn or Zn alloy coating films or zinc hydroxide coating films on the surfaces or further forming coating films on the Zn or Zn alloy coating films by chemical treatment. CONSTITUTION:A water passing apparatus or a water storage vessel made of stainless steel undergoes crevice corrosion by chloride ions contained in water. In order to prevent this corrosion, Zn or Zn alloy coating films 4a, 4b are formed on the surfaces 2a, 2b of stainless steel members 1a, 1b brought into contact with water by plating in 0.5g/m<2> thickness or zinc hydroxide coating films 6a, 6b are formed. Chemical coating films 5a, 5b such as chromate coating films or zinc hydroxide coating films 6a, 6b may further be formed on the Zn or Zn alloy coating films 4a, 4b. Corrosion at the crevice 3 between the surfaces 2a, 2b brought into contact with water is effectively prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for preventing crevice corrosion in stainless steel.

[Background of the invention and problems with the prior art]

As is well known, stainless steel is an extremely corrosion-resistant material and is widely used in various types of equipment that require corrosion resistance. However, five-cavity corrosion occurs even in stainless steel. Crevice corrosion of stainless steel occurs not only in a chlorine ion environment where it comes into contact with seawater or sea salt particles, but also by chlorine ions contained in tap water. Generally, when assembling water-conducting or water-storage equipment using stainless steel on the side that comes into contact with water, a gap structure inevitably occurs in some areas at welding, caulking, and gasket areas, while corrosion resistance is maintained in other areas. However, only this gap is preferentially corroded.

Reduces the useful life of equipment. In other words, crevice corrosion of stainless steel is induced by chloride ions contained in water, and the higher the Cl- content.

Also, the higher the temperature, the stronger the crevice corrosion. Therefore, prevention of crevice corrosion is an important matter for durability in stainless steel appliances such as water heaters and heat exchangers.

Conventionally, in order to prevent this crevice corrosion, cathodic protection methods using an external power source or sacrificial anode methods using a sacrificial anode have been most commonly employed.

These anti-corrosion methods are effective when the structure of the equipment is simple, but when the structure becomes complex, installation becomes difficult and costs increase. Another measure is to use highly alloyed stainless steel with a high grade of corrosion resistance, but this increases the cost.

[Purpose of the invention]

The present invention aims to provide a method for effectively preventing crevice corrosion of stainless steel without using cathodic protection or separately providing a sacrificial anode.

[Structure of the invention]

Crevice corrosion of stainless steel occurs when a wetted surface with a gap structure is made of stainless steel.

According to the invention there is a first order method, viz.

(1) The stainless steel surface on the liquid contact side of the gap structure portion is covered with a Zn or Zn alloy plating film having a thickness of 0.5 g/m or more.

(2) The stainless steel surface on the liquid contact side of the gap structure is coated with Zn.
Alternatively, it is covered with a Zn alloy plating film and a chemical conversion film formed on the surface of this plating film.

(3) The stainless steel surface on the wetted side of the gap structure is coated with Zn.
Alternatively, it is covered with a Zn alloy plating film and a zinc hydroxide film formed on the surface of the plating film.

(4) Cover the stainless steel surface on the liquid contact side of the gap structure with a zinc hydroxide film.

It has been found that this can be easily and reliably prevented.

The stainless steel gap corrosion prevention structure according to the present invention is schematically shown in FIGS. 1 to 4. In these figures, la, lb are stainless steel, 2a,
2b indicates the surface of the stainless steel on the liquid side, and 3 indicates the gap on the liquid side. FIG. 1 shows the basic configuration of the present invention corresponding to (1) above, and shows the wetted surfaces 2a and 2b of stainless steel facing the gap 3.

It is covered with plating layers 4a and 4b of Zn or Zn alloy with a film thickness of 0.5 g/@" or more. In other words, the gap 3
The outermost surface (the surface in contact with the liquid) forming the stainless steel 1a, lb consists of plating layers 4a, 4b that are in close contact with each other.6 In the present invention, the plating layers 4a, 4b form the gap 3. It is necessary to be in close contact with the entire surfaces 2a and 2b of the stainless steel. Stainless steel surfaces that do not form the gap 3, and therefore where the gap 1lfi alone is not a problem, are outside the scope of the present invention, but if it is advantageous to cover them with a plating layer for the sake of equipment manufacturing. Just cover it.

In addition, Zn or Z is added to the surface apart from the surface of this gap 3.
If an n-alloy plating layer exists, it may serve as a substitute for the conventional sacrificial anode inserted into the liquid, and the zinc hydroxide film formed here acts as a barrier to oxygen reduction and prevents the increase in self-potential. It has an advantageous effect on preventing crevice corrosion. therefore.

With the stainless steel wetted surface forming a gap.

Forming a similar plating layer on surfaces other than this surface is more advantageous in preventing crevice corrosion. but,
It is necessary to keep the plating layer in close contact with at least the surface of the stainless steel on the liquid contact side forming the gap.

FIG. 2 shows the crevice corrosion prevention structure of the present invention corresponding to (2) above, in which the stainless steel surface 2 facing the gap 3
a, 2b + Zn or Zn alloy plating layer 4a,
4b and chemical conversion coatings 5a and 5b formed on the surface thereof. That is, the stainless steel surface 2a * 2b facing the gap 3 is brought into close contact with the plating layers 4a, 4b of Zn or Zn alloy, and this plating N4a,
A chemical conversion coating 5a such as chromate treatment is applied to the liquid contact side surface of 4b,
5b is formed.

FIG. 3 shows the crevice corrosion prevention structure of the present invention corresponding to (3) above, in which the stainless steel surface 2 facing the gap 3
a * 2 b, Zn or Zn alloy plating layer 4
a, 4b, and the zinc hydroxide coating 6a+ formed on the surface thereof.
6b. That is, the stainless steel surfaces 2a and 2b facing the gap 3 are brought into close contact with the plating layers 4a and 4b of Zn or Zn alloy, and the plating layer 4a is
, 4b, zinc hydroxide coatings 6a, 6b are formed on the liquid contact side surface. The zinc hydroxide coatings 6a, 6b are preferably formed by forming the plating layers 4a, 4b and then converting the surface portion of the plated zinc into zinc hydroxide through a chemical reaction to form a zinc hydroxide coating.

FIG. 4 shows the crevice corrosion prevention structure of the present invention corresponding to (4) above, in which the stainless steel surface 2 facing the gap 3
a and 2b are covered with lead hydroxide coating 1116a and 5b. The zinc hydroxide coatings 6a, 6b are preferably formed by chemically changing the entire thickness of the plating layer in the structure (3) above to zinc hydroxide.

As a result, the stainless steel surfaces 2a and 2b are coated with lead hydroxide II! 6a and 6b can be formed.

In any of these structures (1) to (4), in addition to cases where the gap on the liquid contact side is formed between two stainless steel surfaces, even when only one surface forming the gap is made of stainless steel, the stainless steel By forming a coating having the above-described structure on the liquid-contacting surface of the stainless steel, crevice corrosion of the stainless steel can be prevented. Further, the shape between # is not limited to the illustrated example, and the present invention can be applied to any gap that causes gap corrosion. In this case, the coating must be formed in close contact with the wetted surface of the stainless steel.
There is no particular problem even if the coating is formed on the liquid-contacting surface of the stainless steel other than in the gap, and in fact, it is often preferable.

[Details of the Invention The presenters have carried out extensive testing and research on improving the properties of stainless steel surfaces with the aim of improving the crevice corrosion resistance of stainless steel. It has been found that forming a plating layer is the most effective and simple measure to prevent crevice corrosion. It has been found that the Zn plating layer alone can prevent crevice corrosion, but it also prevents corrosion of stainless steel, especially when Zn in the Zn plating layer is present, but also when Zn becomes zinc hydroxide. It was found that this special effect can be obtained. in this case.

It has been found that since zinc hydroxide is derived from the plating layer, it adheres well and prevents the zinc hydroxide from falling into the liquid.

Therefore, the main structure of the present invention is to form a plating layer of Zn or Zn alloy on the surface of the stainless steel in the gap structure portion of the stainless steel. It is also acceptable to denature the material, or to perform a chemical conversion treatment such as chromate treatment on the outermost surface.

Zn plating is generally used to prevent corrosion of ordinary steel, but corrosion occurs immediately in ordinary steel when Zn disappears. Even when Zn plating is applied to stainless steel, Z
It was generally thought that the anticorrosion effect of Zn was the same as that of ordinary steel, but the inventors investigated various effects of Zn plating on stainless steel, and found that In particular, it has been found that even when Zn corrodes and becomes zinc hydroxide, it has the effect of preventing corrosion of stainless steel. It was also found that stainless steel had less wear on the two openings than ordinary steel. Especially Zn
It has been found that the effect is particularly effective in preventing crevice corrosion and is ideal for preventing corrosion of crevice structures of stainless steel equipment.

The effect of preventing crevice corrosion according to the present invention can be considered as follows. First, Zn in the gap is difficult to elute, and as long as Zn is present, the anticorrosion effect is exerted by galvanic action. On the other hand, Zn outside the gap naturally serves as sacrificial corrosion protection. Even when Zn melts and becomes zinc hydroxide, the hydroxide acts as an oxidation-reduction barrier outside the gap, suppressing the rise in self-potential and preventing 9gI eclipse. Furthermore, zinc hydroxide produced by dissolving zinc in the gap suppresses a decrease in pH in the gap and prevents crevice corrosion from occurring.

〔Example〕

Figure 5 shows the results of examining the crevice corrosion of 5TLS316 stainless steel material and 5LIS316 stainless steel plated with Zn with different plating weights.
It was formed as shown in the figure. Namely.

A hole is made in the center of the plate-shaped test piece 8, and a Ti bolt 9 is inserted into the hole and a Teflon gasket 10 and a Ti washer 11 are inserted.
It is fixed with a Ti nut 12 through. As a result, a gap is formed between the Teflon gasket 10 and the test piece 8. The test pieces were plated after bolt holes were drilled. Therefore, a plating layer is also present on the surface of the hole. The test is 3.5χN
The test structure was immersed in an aCl aqueous solution at 80° C., and the change in natural potential and the state of corrosion within the gap between the test pieces were observed.

From the results shown in FIG. 5, it can be seen that although the potential of the stainless steel material is high, it corrodes in a short time and decreases to the corrosion potential. On the other hand, in the case of Zn-plated stainless steel, while Zn exists outside the gap, the potential is low and an anticorrosion effect is observed, but as Zn is consumed, the potential increases. The consumption rate of Zn in the gap is slow, and it disappears one by one depending on the amount of plating deposited, and the potential gradually increases accordingly.

However, those with a coating weight of 0.5 g/m" or more do not corrode. This is thought to be because the pH 11 buffering effect of zinc hydroxide has been working effectively.

Table 1 shows the state of corrosion after conducting the same test for 6 months. As seen in the results in Table 1, even if it becomes zinc hydroxide, crevice corrosion will not occur if the coating weight of Zn is 0.5g/+I or more.
/m'', crevice corrosion will eventually occur.

Furthermore, when the Zn-plated surface is subjected to chromate treatment, the consumption of the plated Zn is further slowed down, and the durability is significantly improved. Table 1 also shows the crevice corrosion test results for stainless steel that was subjected to Zn plating and then chromate treatment. Even after chromate treatment, the effect of Zn plating is maintained without loss.

Table 2 shows the results when the same test was conducted for 10 days with different steel types, and as seen in these results, the effects of the present invention are fully recognized even when stainless steels of different steel types are used.

Table 3 shows the results of the same tests as those described in Figure 1, except that 5US316 stainless steel was plated with various Zn alloys. Even in alloy plating, crevice corrosion did not occur if the coating weight was 0.5 g/a+' or more.

Figure 7 shows that a test piece was plated with 238/m'' Zn, and the surface of the Zn plating layer was treated with zinc hydroxide.
This figure shows the results of a crevice corrosion test conducted under the same conditions as in FIG. 5 for a Zn-10-ice zinc hydroxide coating and a full zinc hydroxide coating. From the results shown in FIG. 7, it can be seen that the change in potential is different for the Zn plating as it is, the multilayer structure of Zn and zinc hydroxide, and the zinc hydroxide coating. In a multilayer structure in which part of the surface thickness of the Zn plating is hydroxylated, the time for the potential to rise is faster, but the subsequent change in potential is almost the same as with the Zn plating as is. Although corrosion protection due to sacrificial dissolution of Zn is not observed in a coating structure made only of zinc hydroxide, the change in potential shows the same behavior as the change in potential after dissolution of Zn in Zn-plated materials. Table 4 shows the results after this test. Although a corroded state was shown, no crevice corrosion occurred in any of the materials other than the raw material.

Table 4 Next, some examples of successful results obtained by applying the gap 'S corrosion prevention method of the present invention will be described.

When seawater was passed through a known plate heat exchanger with a structure made using 5US316 stainless steel plates, corrosion progressed in a short period of time. Therefore, the present inventors constructed a heat exchanger with the same structure by applying electrolytic Zn plating of approximately Log/+a'' on the wetted surface of a 5O5316 stainless steel plate, and applying approximately 180 g/
M! It was manufactured using molten Zn plating.

Then, artificial seawater at 60'C was circulated for 30 days. After the test, the plates were dismantled and the inner surfaces were examined, but no corrosion was observed on any of the plates, such as on the contact surfaces between the plates or on the contact surfaces of the gaskets. Similarly, Z
When similar tests were conducted on heat exchangers having a film structure of an n-plated ice hydroxide zinc oxide film and a film structure of only zinc hydroxide, no corrosion was observed. The same test was conducted on the known plate heat exchanger made of 5tlS316 stainless steel plate, but crevice corrosion occurred at the above-mentioned locations.

In a domestic water heater whose body was made of 5US304 stainless steel plate, crevice corrosion occurred at the flange joint.

Therefore, the present inventors manufactured the same water heater using a 5IIS304 stainless steel plate plated with approximately 10 g/g'' Zn for the flange part.
, 80"C hot water was circulated for 30 days. After the test, the flange was disassembled and the inner surface was examined.
No corrosion was observed at all on the contact surface of the gasket in the flange section of No. 04. The same test was conducted on a water heater made of 5tlS304 material without Zn plating, but crevice corrosion occurred at the flange.

In this way, according to the present invention, the corrosion resistance of stainless steel devices and joints with gap structures used in a neutral water environment is increased, and as a result, the durability of the nine devices is significantly improved. do. Therefore, it is possible to completely prevent crevice corrosion of stainless steel without using expensive materials containing a large amount of corrosion resistance improving elements such as Cr and Mo, which makes it possible to reduce costs and is extremely beneficial from the perspective of saving resources. . Furthermore, since it is freed from design constraints such as avoiding gap structures, it can advantageously contribute to the development of general-purpose equipment made of stainless steel.

[Brief explanation of the drawing]

1 to 4 are enlarged cross-sectional views of the gap portion showing structural examples of the method for preventing crevice corrosion of stainless steel of the present invention,
FIG. 5 is a diagram showing the change over time in the self-potential in the crevice corrosion test, and FIG. 6 is a side view showing the state of crevice formation in the crevice corrosion test. FIG. 7 is a diagram showing changes in self-potential over time in another crevice corrosion test. la, lb...stainless steel If, 2a, 2b...
The wetted surface of stainless steel, 3 is the gap on the liquid side. 4a, 4b...Zn or Zn alloy plating layer. 5a, 5b - Chemical conversion film. 6a, 6b - Zinc hydroxide coating.

Claims (4)

[Claims] (1) In order to prevent crevice corrosion of stainless steel that occurs when forming a wetted surface with a gap structure with stainless steel, at least the stainless steel surface on the wetted side of the gap structure is coated with a film thickness of 0.5 g. /m^2 or more Zn or Z
A method for preventing crevice corrosion of stainless steel, characterized by covering it with an n-alloy plating film. (2) In order to prevent crevice corrosion of stainless steel that occurs when a liquid contact surface with a gap structure is formed of stainless steel, at least the liquid contact side stainless steel surface of the gap structure is plated with Zn or Zn alloy. A method for preventing crevice corrosion of stainless steel, which is characterized by covering with a film and a chemical conversion film formed on the surface of the plating film. (3) In order to prevent crevice corrosion of stainless steel that occurs when forming a wetted surface with a gap structure with stainless steel, at least the wetted stainless steel surface of the gap structure is plated with Zn or Zn alloy. A method for preventing crevice corrosion of stainless steel, which is characterized by covering with a film and a zinc hydroxide film formed on the surface of the plating film. (4) In order to prevent crevice corrosion of stainless steel that occurs when forming a liquid contact surface with a gap structure with stainless steel, at least the liquid contact side stainless steel surface of the gap structure part should be covered with a zinc hydroxide coating. A method for preventing crevice corrosion of stainless steel.