JPS63130785A - Method for preventing intergranular stress corrosion cracking of ni-based alloy - Google Patents

Abstract

PURPOSE:To effectively prevent the intergranular stress corrosion cracking of the Ni-based alloy which is brought into contact with a deaerated alkaline soln. at high temp. by adding a specified amt. of a cyclic amine to the deaerated alkaline soln. CONSTITUTION:From 0.3ppm to 1%, based on the weight of a soln., cyclic amine such as morpholine, cyclohexylamine, and piperazine is added to the deaerated alkaline soln. to be used in a heat exchanger, etc., and exposed to a high temp. An increase in the corrosion potential of an Ni-based alloy due to the impurities such as Fe3O4, Cu, Cu2O, and CuO which are eluted into the soln. in the form of sludge or scales and deposited in the high-temp. deaerated alkaline soln. is controlled by the cyclic amine. Furthermore, the entrance of the potential into the corrosion cracking generating potential region is also prevented. As a result, even when a member consisting of an Ni-based alloy is incorporated into an existing plant, the corrosion cracking of the member can be prevented without disassembling the plant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention is directed to intergranular stress corrosion cracking (IGSCC), which occurs when a Ni-based alloy comes into contact with a high-temperature degassed alkaline solution.
).

[Conventional technology]

Nickel-based alloys, such as AAtoy 600 (16Cr
-”/5Ni-8Fe) is SUS 304 or 3
Conventionally, it has better resistance to stress corrosion cracking (SCC) than low Ni alloys such as 16+7).
In particular, it is used as a material for members exposed to environments where stress corrosion cracking is likely to occur, such as heat exchanger tubes in steam generators.

In such a heat exchanger tube, the inside (1
A high-temperature, high-pressure degassed boric acid aqueous solution having a temperature of 315 to 325°C and a pressure of 125 to 160 kg/cm2 is passed through the outside (secondary side), for example, at a temperature of 260 to 300°C. ℃ and pressure crab 50-60kg
/crIL2, so if the hot water on the outside is simply degassed, even if the heat exchanger tube is made of a nickel-based alloy such as Attoy 600, the outer surface of the tube will Because of its tendency to cause intergranular stress corrosion cracking, current
= e treatment ) processing is being performed.

This AVT treatment adds ammonia to the water on the secondary side to adjust its pH to 8.5 to 9.2, thereby fixing components other than the AAAoy 600 tube, such as the tube. Fe3O suppresses corrosion of assembled carbon steel parts and melts into alkaline solution.
4 (magnetite) to a minimum, and further added hydrazine (N2H4) to reduce the dissolved oxygen in the solution to 10 I
) I) By degassing to below b and controlling the residual amount of hydrazine to a maximum of 5 ppb,
This prevents intergranular stress corrosion cracking from occurring.

[Problem that the invention seeks to solve]

However, even with this more advanced AVT treatment, grain boundaries may occur in nickel-based alloys such as Ajtoy 600, especially when the degassed alkaline solution prepared by the treatment is concentrated at high temperatures and its alkaline concentration is locally high. There was a problem that stress corrosion cracking (Gsec) occurred.

[Findings based on research] Therefore, the present inventors have conducted various studies to solve the above-mentioned problems, and as a result, (1) If the high-temperature alkaline solution is degassed by the above-mentioned AVT treatment, AtAoy 600 The corrosion potential of is expected to fall and move out of the I()SCC generation potential region, but the Fe3O4, Cu, and Cu20 eluted from the cupronickel of the carbon steel member and the condenser tube
, CuO is present in the alkaline solution, for example,
Degassed alkaline solution at 325°C containing 40% NaOH
Fe3O4: 467 g (o), and Fe3O4: 416. ? +Cu:0.8.9+Cu
O: 4 (AtAoy when 1 (/→ is melted)
As seen in Figure 9, which shows the change in corrosion potential of A.600 over time, even if the high temperature alkaline solution is degassed,
The corrosion potential of AAoy600 is higher than that of the above-mentioned single alkali containing no impurities (A), and is in the IGSCC generation potential region (lowest potential: -1350 mV vs. 5
(2) Adding a cyclic amine such as morpholine or cyclohexylamine to the above hot degassed alkaline solution by 0.3% based on the weight of the solution. It has been found that when more than ppm is added, even if the above-mentioned impurities are present in the alkaline solution, the corrosion potential of the nickel-based alloy does not increase, so it is possible to prevent IGSCC from occurring in the nickel-based alloy. .

[Means for solving problems]

This invention was invented based on the above findings, and an object of the present invention is to provide a method for preventing intergranular stress corrosion cracking of a Ni-based alloy that comes into contact with a high-temperature degassed alkaline solution. Based on the weight of the solution,
It is characterized by adding 0.3 ppm to 1 volume of cyclic amine.

[Detailed description of the invention]

1. High-temperature degassed alkaline solution The above-mentioned AVT-treated solution generally has a pH adjusted to 8.5 to 9.2 with ammonia and further contains 5 ppb of hydrazine, while containing substantially no oxygen. It is an aqueous solution that exhibits alkalinity with a pH of 8.5 to 9.2 without containing it, and is held on the secondary side of the heat exchanger (outside the tube) of the steam generator and is usually exposed to a temperature of 315 to 325°C. However, the high-temperature degassed alkaline solution in this invention is not limited to the high-temperature degassed alkaline solution used in such a heat exchanger, but is a degassed alkaline solution that is exposed to high temperatures, and This refers to any high temperature degassed alkaline solution that can cause GSCC on nickel-based alloys.

2. Cyclic amine As the cyclic amine in this invention, any cyclic amine such as morpholine, cyclohexylamine, benzotriazole, ethyleneimine, sulfonazetidide, pyrazolidine, piperazine, trimethylenetriamine, and quinoline can be used, among which morpholine , cyclohexylamine, benzotriazole, pyrazolidine, and pyradizine are preferably used.

As mentioned above, these cyclic amines elute in a high-temperature degassed alkaline solution and precipitate in the form of sludge or scale.
, has the effect of suppressing the increase in the corrosion potential of nickel-based alloys due to impurities such as CuO and preventing the corrosion potential from entering the IGSCC generation potential region, but based on the weight of the degassed alkaline solution, The added amount was 0.3 pp.
If it is less than m, a sufficient effect will not be obtained for the above action, while if it exceeds 1 m, no particular improvement effect will be seen in the action, and the cyclic amine will be wasted. Therefore, the amount added was determined to be 0.3 ppm −1 based on the weight of the degassed alkaline solution.

〔Example〕

Next, the invention will be explained by way of examples.

Example 1 At7oy 600 as a test material for intergranular stress corrosion cracking
AA7oy600 with an internal volume of 2t was selected as the alkaline solution for I()SCC accelerated test for this.
In a manufactured autoclave, a NaOH aqueous solution with a concentration of 40% by weight was added as an annual product, Fe3O4:416,! i'
, Cu: 0.8.9 and CuO: 40 f/ were added.

On the other hand, as shown in FIGS. 7 and 8, both A
Test piece for corrosion potential measurement made by TToy 600 and C-
Each ring test piece was prepared, a strain gauge was attached to the C-ring test piece, and the Young's modulus of AAAoy600 was set to 20,000, and a strain was applied so that the stress was 20 kgf 7wn2. Apply a special remover (manufactured by Shinto Paint) to the strain gauge and dissolve it, then wash the remover thoroughly with pure water, degrease with acetone, dry, and then use this C,-IJ ring test piece IG.
This was used as a test piece for the SCC test.

Figure 7 is a side view of a test piece for corrosion potential measurement, and this test piece 1 has an outer diameter of 22.2MX and a wall thickness of 1.27.
Length cut in the axial direction from a mm tube: 2
00mm x Width: 2M elongated arc-shaped part 1a and lOwnXIOM plate-shaped part 1b connected integrally at one end.
(A) in Fig. 8 is the outer diameter:
22.2wn x Width: 15M x Wall thickness: 1.2'i'm is a front view of a C-ring test piece 2, in which stress is applied to the through holes 2a, 2a facing each other in the diametrical direction. The bolt 3 for
Figure 8 (b) shows only the C-ring test piece 2 with these bolts 3 and nuts 4 removed. FIG.

Next, after setting the potential measurement test piece and C-ring test piece in an alkaline solution in an autoclave,
The solution was degassed with Ar, and a predetermined amount of morpholine was injected into the autoclave using a microsyringe.

Next, the autoclave was heated, the degassed alkaline solution therein was heated to 325°C, the internal pressure of the autoclave was set to 65 kg/crrL2, and an immersion test was conducted in which each test piece was exposed to this temperature and pressure for 200 hours. The corrosion potential generated during that time was measured by connecting a potentiostat (potential measurement instrument) to the corrosion potential measurement test piece. An Ag/AgC4 electrode was used as a standard electrode, and the measured value was converted into a SHE (hydrogen electrode) value.

After this immersion test, the center part of the C-ring test scissors taken out from the autoclave in the width direction was cut with a micro cutter, the cut surface was embedded in epoxy resin, and after polishing, the maximum crack depth of IGSCC formed on the cut surface was measured. was measured using an optical microscope.

The corrosion potential measurement test and the intergranular stress corrosion cracking test consisting of the above series of operations were carried out with the addition amount of morpholine being 0.3 ppm, based on the weight of the degassed alkaline solution.
3 ppm, 30 ppm, and for comparison, the added amount was changed to Olo, 1 ppm, 0.2 p.
pm and repeat (all shown as black circles),
The resulting corrosion potential (mV, vs, SHE
) and the maximum crack depth (μm) of IGSCC are shown in Figures 1 and 2, respectively.

Example 2 The type and amount of cyclic amine added to the same type of degassed alkaline solution as that used in Example 1 were changed to morpholine: 0.2 ppm + cyclohexylamine: 0.1 ppm (Figs. 3 and 4). (Indicated by ◇ in the figure)
, Morpholine: 0.2 ppm + cyclohexylamine-1〇-: 0.3 ppm (indicated by ■ mark), Morpholine: 2 ppm + cyclohexylamine: 1 ppm (indicated by φ mark), Morpholine: 15 ppm + cyclohexylamine :
15 ppm (same, indicated by ◆), morpholine:
0.1 ppm + cyclohexylamine 20, 2 p
pm (indicated by C mark), benzotriazole: 0
．． 1 ppm, 0.3 ppm, 3 ppm, 30
ppm (indicated by △ in Figures 5 and 6), trimethylenetriamine: 0.3 ppm, 30
ppm (indicated by mark A), piperazine: 0.3 ppm (indicated by mark M),
Benzotriazole: 1 ppm +) Rimethylenetriamine: 2 ppm (indicated by the symbol), Benzotriazole: 1 ppm + Pyrazine: 2 ppm (
Benzotriazole: 200 ppm +) Rimethylenetriamine: 100 ppm (Same as shown, marked with a star), Benzotriazole: 100 ppm + Pizrazine: 200
ppm (indicated by ★), and for comparison, the amount of cyclic amine added was O (indicated by e in Figures 1, 3, and 5), and cyclohexylamine: 0.
．． 1 ppm (indicated by 0 mark in Figures 3 and 4)
I()SCC is the same as in Example 1, except that
The test was repeated, and the resulting corrosion potential and maximum crack depth are shown in Figures 3 and 5, and Figures 4 and 6, respectively.

〔Effect of the invention〕

From the results shown in Figures 1, 3 and 5, Fe3O4
-4-Cu-) A for 40% NaOH aqueous solution with neither CuO sludge nor cyclic amine added
The corrosion potential of tAoy600 is -1450mV,
When this alkaline aqueous solution contains the sludge, the corrosion potential of AtAoy 600 increases to -1250 mV. When varazine or two or more of these is added, the corrosion potential gradually decreases as the amount added increases, and when this amount reaches 0.2 ppm, the above-mentioned minimum potential for IGSCC occurs - 1350 mV vs, approaching SHE, the evil shadow of Jingji Su-77 is 0.2 of the amine.
It can be seen that it is completely eliminated at addition amounts of ppm or more.

Furthermore, from the results shown in FIGS. 2, 4, and 6, if no cyclic amine is added to the alkaline aqueous solution containing the sludge, the maximum depth based on IGSCC is 12'.
A crack of i'Oμm is generated in the test piece, but when morpholine, cyclohexylamine, benzotriazole, trimethylenetriamine, piperazine, or two or more of these are added to this alkaline aqueous solution, the IGSCC depth changes depending on the amount of cyclic amine added. IC)S gradually becomes smaller as it increases, and when the amount added reaches 0.3 ppm,
The CC depth is 30 μm or less, and these results show that the addition of 0.3 ppm or more of cyclic amine is AAAoy 6 in contact with the hot degassed alkaline solution containing the sludge.
It can be seen that this method is very effective in preventing the occurrence of engineering GSCC in 00.

In the above explanation, 5Atto is one of the Ni-based alloys currently widely used in high-temperature degassed alkaline environments.
y600 as an example, it goes without saying that the present invention can be applied to any Ni-based alloy that is likely to cause I48CC under the above environment.

Thus, the present invention can effectively prevent intergranular stress corrosion cracking that occurs in a Ni-based alloy that comes into contact with a high-temperature degassed alkaline solution by the simple means of adding a trace amount of cyclic amine to the solution. Therefore, according to this invention, even if a member made of the above-mentioned Ni-based alloy is incorporated into an existing plant, intergranular stress corrosion cracking of the Ni-based alloy member can be prevented without dismantling the plant. Benefits can be obtained.

[Brief explanation of the drawing]

Figures 1 and 2 are graphs showing the corrosion potential and GSCC depth, respectively, generated in Examples and Comparative Examples of the present invention, Figures 3 and 5, and Figures 4 and 6 are graphs of the invention. Graphs showing the corrosion potential and IGSCC depth generated in another example and comparative example, respectively,
Figure 7 is a side view showing the corrosion potential measurement test piece used in each of the above Examples and Comparative Examples, and Figure 8 (a) and 1) are IGSCC measurement test pieces used in the same manner as above.
- A front view and a side view of a ring test piece, and FIG. 9 is a graph showing how the corrosion potential of AAtoy 600 changes with respect to a degassed alkaline solution due to the introduction of impurities into the solution. In the figure, 1... Test piece for corrosion potential measurement, 2... C-ring test piece, 3... Bolt, 4... Nut. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person Figure 1 Amount of morneline/l ora (ρpm) 0 0, Io, 20.3 3 30 iV,
t riff/)7”ka1] f (oom) (EHS
°s8 °Δuu),! I//kt4/W (LLJ
rl) A 3g DDSE) 1 nest 5m 2J1/) Lacquer amount ζppm) 6th year + 600□ 7th R 8m (0) (In□β- Power call and l: Q (radio °/4 Rijiθ1o!chi) Figure 9 (h)

Claims (1)

[Claims] A method for preventing intergranular stress corrosion cracking of a Ni-based alloy that comes into contact with a high-temperature degassed alkaline solution, the method comprising: adding 0.3 ppm to 0.3 ppm to the degassed alkaline solution based on the weight of the solution;
The said prevention method, characterized in that 1% of cyclic amine is added.