JPS62127484A - Method for removing scale - Google Patents

Abstract

PURPOSE:To easily and perfectly remove Fe3O4 scale formed on the surface of an Ni alloy by using a chemical cleaning soln. contg. specified amounts of ethylenediaminetetraacetic acid and citric acid and adjusted to a specified pH with ammonia. CONSTITUTION:A chemical cleaning soln. contg. 2X10<-3>-1mol/l ethylenediaminetetraacetic acid and 1X10<-3>-1X10<-1>mol/l citric acid and adjusted to 5-6.5 pH with ammonia is prepd. About 0.05-0.5vol% aliphatic carboxylic acid, primary or secondary amine may be added to the cleaning soln. as required. When the cleaning soln. is used, Fe3O4 scale formed on the surface of an Ni alloy in environment at high temp. and pressure is easily and perfectly removed without roughening the surface of the Ni alloy by local corrosion.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to the formation of grain boundary defects such as intergranular cracks in Ni-based alloys that are generated on the surface of Ni-based alloys used for heat exchanger tubes etc. in a high-temperature, high-pressure water environment. The present invention relates to a method for removing scale made of iron-based oxides that causes damage.

Conventional technology Conventionally, NiJλ alloy containing, for example, 75% Ni, 15% Cr, and 8% Fe has been used for materials such as heat exchanger tubes in steam generators because it exhibits excellent corrosion resistance in environments where high-pressure water is used. ing. However, due to alkali concentration and the formation of Fe-based oxide scale such as magnetite (Fe, Oa), grain boundary damage such as grain boundary cracking may occur in the gap between the support plates of the heat exchanger tube.

FIG. 4 is an explanatory diagram showing the shape and holding method of a test piece in a test for investigating the influence of Fe, 04 scale on the above-mentioned grain boundary damage. In the same figure, Ni
Ring-shaped test piece (1) with Fe5O4 formed on the surface of a Ni-based alloy containing 75% Cr, 15% Cr, and 8% Fe.
, bolt (3) and nut (2) (
2), the ring-shaped test piece (1) was tightened from the outside and immersed in a degassed 40% NaOH solution in a lying position with tensile stress applied to the outside of the shaded area near the center of the test piece. After holding at 280° C. for 200 hours, the portion to which the tensile stress was applied was embedded in resin, and the depth of the crack was measured by microscopic observation at four locations a, b, c, and d. For comparison, a similar test was conducted using a test piece without Fe5O1 on the surface. The results are shown in Figure 5. In the same figure, (
A) The figure shows the case where Fe, O, exists on the surface of the test piece, and (B) the case where they do not exist. The horizontal axis is the a, b, c of Fig. 4,
The measurement point corresponding to d, the vertical axis is the grain boundary n flaw depth,
From the same figure, FC on the surface! It can be seen that when Fe5Oa is present, cracks with a depth of 1000 to 1200 microns occur at grain boundaries, whereas when Fe5Oa is not present, no cracks occur.

Among the formation of alkali 113m and Fe5O4 scale that cause grain boundary damage as mentioned above, alkali concentration is caused by the Na+ flowing out due to a defect in the ion exchange resin that treats the water used in the steam generator. The formation of Fe, O, and scale is thought to be due to the Fe5O4 produced when the carbon steel used as the structural material of the steam generator reacts with high-temperature, high-pressure water and adheres to the surface of the Ni-based alloy. It depends on what you do.

In order to prevent the occurrence of grain boundary damage in Ni-based alloys as described above, conventional measures have been taken such as applying special heat treatment or using high Cr materials.

In addition, conventional methods for removing iron-based oxide scale include (■) using a sulfuric acid solution containing a corrosion inhibitor for descaling carbon steel; (2) using a sulfuric acid solution containing a corrosion inhibitor for descaling carbon steel; A method using a hydrochloric acid solution containing a corrosion inhibitor for descaling steel, copper alloys, etc.; (3) a method using a nitric-hydrofluoric acid solution for descaling corrosion-resistant alloys such as stainless steel; A method using citric acid with a corrosion inhibitor added for descaling aluminum alloys; ■ A method using a mixed aqueous solution of ammonium citrate, oxalic acid, ferric nitrate, and diethylthiourea for descaling carbon steel; ( 6) Chemical cleaning methods using a mixed aqueous solution of sodium oxalate, oxalic acid, hydrogen peroxide, peracetic acid, and 8-hydroxyquinoline for descaling carbon steel, stainless steel, Inconel, Zircaloy-2, aluminum, etc. Furthermore, in JP-A No. 58-147570, a method using an acid corrosion inhibitor such as hydroxyethylethylenediami/triacetic acid (HE D T A ) and an organic amine, and containing a non-oxidizing inorganic acid. A method has been proposed in which iron oxide scale is removed from the surface of metal iron using an aqueous solution, and then the surface from which the scale has been removed is brought into contact with an alkaline liquid.

Purpose of the Invention However, in the conventional method described above, applying special heat treatment to the metal material or using high Cr material increases material cost, and the method for removing iron-based oxide scale does not meet the requirements set forth in (1) above. The conventional methods of 1 to 3 are applicable to scales such as carbon steel, low alloy steel, stainless steel, etc., and the method proposed in Japanese Patent Application Laid-Open No. 147570/1983 is also applicable to iron. On the other hand, method (6) uses N
Although it can be applied to the scale of i-based alloys, the liquid composition, temperature,
There is a problem in that it is difficult to control conditions such as processing time, and the surface of the material tends to become rough during descaling.

An object of the present invention is to provide a method for removing iron-based oxide scale, which causes grain boundary damage to Ni-based alloys, using an appropriate chemical cleaning solution in a high-temperature, high-pressure water environment.

Structure of the Invention The present invention has been made for the above-mentioned purpose, and its gist is that ethylenediaminetetraacetic acid x 10-3
moI 171 or more and 1 mon/4 or less and citric acid IX10
-3mall/J2 or more lXl0-' mail/It
Contains the following, adding aliphatic carbo/a or primary or secondary amine as necessary, and adjusting the pH to 5 with ammonia.
Ni! using a chemical cleaning solution adjusted to below A5!
A method for removing iron oxide scale that forms on alloy surfaces.

The reason why the above chemical cleaning solution is used and limited to the above range of degrees a is explained below.

Ethylenediaminetetravinegar # (hereinafter referred to as EDTA) is a Fe5 compound that forms on the surface of a Ni-based alloy in a high-temperature, high-pressure water environment.
It reacts with O4 scale and dissolves Fe as salt, but
Under conditions of PH5 to B-5, when the concentration of EDTA is x 10-' mou/j2, Fc504
The dissolution rate of 1:t[
f range 2 x 10-' moIl/4 or more 1 m
It was determined to be olt/Il or less.

Citric acid penetrates into the gap between the Fe3O4 scale and the Ni-based alloy surface and dissolves a very large amount of the Ni-based alloy surface, thereby removing Fe5O4 from the alloy surface. However, when the 0 degree is less than lXl0-' moIl/1, no significant effect is observed, and I
If added in excess of fl/1, the elution on the alloy surface will increase, causing roughness of the steel, so the concentration range should be set to IXlo-3mail/11 or more lXlo-' mol
/42 or less. Citric acid has a small amount of Fe
It also has the function of dissolving 5o4 and preventing the rapid drop in pH due to 1+ (generated in the reaction of EDTA and Fe3O4).

If the pH of the chemical cleaning solution is in an acidic range of less than 5, the surface of the Ni-based alloy will be roughened due to over-pickling, resulting in a thinner wall thickness and shortened material life; Since the dissolution of e304 does not progress, the range was set as 5 to a5.
This is because it combines with H+ generated by the reaction with Fe and oa, and has the effect of preventing the pH of the cleaning solution from shifting to the acidic side.

Additionally, if necessary, aliphatic carboxylic acids or primary or secondary amines may be added because they act as corrosion inhibitors and prevent the alloy surface from being locally attacked. It is from. The addition amount is not particularly specified, but is preferably α05% by volume to 0.5% by volume.

At 0.05% by volume, the effect of inhibiting corrosion is small;
If it exceeds 5% by volume, the cost will be high.

The above description has been made with reference to Fe1g4 scale formed on the surface of a Ni-based alloy, but as is clear from the examples described below, it is also applicable to Fe3O4 scale formed on the surface of carbon steel.

Example A detailed explanation will be given below based on examples.

Example 1 The 5Ia chemical cleaning solution shown in Table 1 was used as the test liquid, and each IIl was placed in a test container. Fe and 04 powders were added as Fe to give a5X1cm' mofl/It, and the mixture was heated to 70°C while stirring. The material was kept heated and a Fe dissolution test was conducted at 0°. Evaluation of solubility of Fe504 is as follows:
After immersing 04 for a predetermined time and dissolving it, undissolved Fe5
O4 was subjected to i-filtration and multiplication using a Millipore filter, and Fe dissolved in the test liquid was quantitatively determined by atomic absorption spectrophotometry.

The results are shown in Figure 1. In the figure, the horizontal axis is the immersion time,
The vertical axis shows the amount of dissolved Fe and O.

-1,1te, vertical axis f) a5X 10-” moj2/
The broken line drawn parallel to the horizontal axis from 4 indicates the Fe5 added to the test solution.
The total amount of Oa is shown. The pH of the test solution is ammonia.
Adjusted to 5. From the same figure, song II that corresponds to the example of the present invention
Among (1) to (4), 0 degree of E DTA is lXl0-
” In the case of curves (1) to (3) with mofl/4 or more, the entire amount of Fe3O4 is dissolved in 9 to 12 hours after being immersed in the test solution, and the concentration is 2 × 10-' mail/A.
The dissolution of I(41) is also proceeding at a relatively high rate, but in the case of the comparative example, curve (5) with a concentration of IXIO-3mou/4, the amount of dissolution is small even if the immersion time is long.From the same figure, 0 degree of EDTA is × 10-' moIl
/4 or higher is required, and txto-' ma
It can be seen that even when 5X 10-' mo +/, N is added in excess of il/Il, the dissolution rate does not increase significantly.

Table 1 Example 2 Using the same test method as in Example 1, the influence of pH on the dissolution of Fe5O4 was investigated. The exam time is 8 hours, ED
TAQC degree (a) IXIF' mail/11.

(o) I
The test was also carried out in the case where TA was not included. Citric acid was added in an amount of IXl 0-' mo fl/J2 in all cases. On the other hand, 1) H was adjusted using sulfuric acid or sodium hydroxide.

The results are shown in Figure 2. In the same figure, the horizontal axis is f) H1, the vertical axis is the amount of dissolved Fe, and the broken line in the figure shows the total amount of Fe3O4 added to the test solution, as in FIG. 1. The figure shows that in the strongly acidic f1 region below pH 2, dissolution of Fe504 proceeds even if EDTA is not included, but in the pi-12 to a5 range it does not dissolve unless EDTA is present, and at pH 7 or above, EDTA does not dissolve. It can be seen that even if it exists, dissolution does not proceed. In a dark area where the pH is less than 5, roughness occurs on the surface of the material during scale removal, so the practical pH range is 5 to a5, indicated by A in the figure.

Example 3 Three types of chemical cleaning solutions shown in Table 2 were used as test liquids, and Example 1
The influence of amines on the dissolution of Fe5O4 was investigated using the same test method as described above.

The results are shown in Figure 3. As in Figure 1, the horizontal axis is the immersion time,
The vertical axis is the amount of dissolved Fe5 (L), and the broken line shows the total amount of Fe5O4 added to the test solution. , the song shown in FIG. 1 corresponding to Example 1!!a (31.

Hail (4) and (5). From the same figure, the solubility of Fe5O4 shown by curves (61, (71) and comparative example (8) corresponding to the present invention example in which diethylamine was added is 12F31.(ψ and (9), respectively, where diethylamine was not added. It can be seen that there is almost no difference, and it can be used as a corrosion inhibitor to prevent local corrosion of the metal surface during cleaning.

Table 2 Example 4 Carbon steel and NiI alloy containing 75% Ni, 15% Cr, and 8% Fe were used as test materials, and degassed 5% Na
After immersing it in an OH solution for 200 hours at 250°C and 40 atm to form Fe30a scale on the material surface, it was immersed in the chemical cleaning solution shown in Invention Example (6) in Table 2 and heated at 80°C. Chemical cleaning was performed for 4 hours.

As a result, the Fe3O4 scale formed on the surface of both the carbon steel and the Ni-based alloy was completely removed, and a clean surface without local erosion was obtained.

As described in the detailed description of the invention, EDTA 2xlO-'
mou/1 or more and 1 m o II/fl or less and citric acid IXIO-3 mail/4 or more lXl0-' ma
The scale removal method of the present invention uses a chemical cleaning solution containing il/1 or less and adjusted to plI of 5 or more and 5 or less with 77 monia. The scale made of Fe5Oa is
It can be easily and completely removed without causing local erosion such as surface roughness on the base alloy surface, and provides an extremely effective means for preventing grain boundary damage in Ni-based alloys caused by the presence of Fe and Oa. It is something to do.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the dissolution of Fe5O4 and the 'eJ degree of EDTA in the chemical cleaning solution, Figure 2 is a diagram showing the influence of the pH of the chemical cleaning solution on the dissolution of Fc, 04, and Figure 3 is A diagram showing the effect of amine in a chemical cleaning solution on the dissolution of Fe504, Figure 4 shows the influence of Fe50. FIG. 5 is an explanatory diagram showing the shape and holding method of a test piece in a test to investigate the influence of scale, and FIG. 5 is a diagram showing the influence of Fc5O4 scale on grain boundary damage in a Ni-based alloy. 1... Ring-shaped test piece 2... Nut 3... Bolt Fig. 1 1 gin/ll (&) Fig. 2 H Fig. 3 1st e% the (6 Fig. 4

Claims (1)

[Claims] Ethylenediaminetetraacetic acid 2 x 10^-^3 mol/l or more and 1 mol/l or less and citric acid 1 x 10^-^3 mo
l/l or more and 1 x 10^-^1 mol/l or less, and if necessary, an aliphatic carboxylic acid or a primary or secondary amine is added, and the pH is adjusted to 5 or more and 6.5 or less with ammonia. A method of removing iron oxide scale that forms on Ni-based alloy surfaces using a chemical cleaning solution.