WO1988000252A1

WO1988000252A1 - Pickling bath for steel parts and pickling method using said bath

Info

Publication number: WO1988000252A1
Application number: PCT/FR1987/000267
Authority: WO
Inventors: Michel Poupart; Guy Turluer; Alain Le Duigou; Georges Plante
Original assignee: Commissariat A L'energie Atomique
Priority date: 1986-07-09
Filing date: 1987-07-07
Publication date: 1988-01-14
Also published as: FR2601379A1; EP0276241A1; JPH01500281A

Abstract

In the method for pickling steel parts, by using a pickling bath, the part is brought in contact with a pickling bath containing only from 0.1 M to 0.3 M of citric acid, from 0.2 M to 0.5 M of maleic acid and from 0 to 0.1 M of hydrofluoric acid, thereby producing efficient pickling without intergranular corrosion. Application to the pickling of components of nuclear reactors.

Description

STRIPPING BATH FOR STEEL PARTS AND STRIPPING METHOD

USING THIS BATH DESCRIPTION The present invention relates to a pickling bath which can be used in particular for the decontamination by pickling of steel parts used as components of nuclear reactors.

The present invention is especially applicable for the decontamination of large parts, such as for example certain stainless steel components used in fast neutron reactors, and which have been in contact with liquid sodium at temperatures of the order of 300 to 50C ^* C approximately, possibly higher, for a significant period

(a few hundred or a few thousand hours). In fact, sodium deposits radionuclides in certain areas (notably chromium 51, manganese 54, cobalt 58, cobalt 60 and iron 59), part of which diffuses into the metal. It is therefore necessary, after washing, to carry out a pickling to obtain sufficient decontamination: it is generally considered that it is sufficient to remove a thickness of material of approximately 10 micrometers to remove all the contamination.

It is known, to decontaminate such parts, to pickle them with a bath based on sulfamic acid added with an inhibitor, for example 2-mercaptobenzimidazole. However, if it is possible to pickle and decontaminate the stainless steel parts, the use of this bath has the drawback of giving rise to non-negligible intergranular corrosion in the treated products. This intergranular corrosion can be explained by the hydrolysis of sulfamic acid, which leads to the formation of sulfates, which is linked to the appearance of intergranular corrosion.

The object of the present invention is to remedy these drawbacks by proposing an effective pickling bath which avoids any localized attack on the material, in particular corrosion. intergranular.

According to the main characteristic of the pickling bath object of the present invention, it contains:

- from 0 1 M to 0.3 M citric acid, - from 0.2 M to 0.5 M maleic acid,

- from 0 to 0.1 W of hydrofluoric acid, and accidental impurities.

In the above mixture, citric acid plays the role of a reducing agent and maleic acid plays the role of an acidifier. As for hydrofluoric acid, it plays the role of overpowering.

The subject of the invention is also a method of pickling a steel part in which the part is dipped in a pickling bath, characterized in that this pickling bath is the bath described above »Other characteristics and advantages of the invention will emerge from the description which follows of some examples of implementation, this description being given by way of illustration and not limitation.

Tests were carried out on a stainless steel type Z6 CND 18-12 coming from an exchanger head shell of a nuclear reactor. The part in question had remained 35,000 hours at 525 ° C in contact with liquid sodium, which had clearly sensitized it to intergranular corrosion. The samples were all taken from the same piece of sheet metal 460x160x15 mm. The composition of this steel was as follows (in percentage by weight):

- chrome: 16.2 X

The treated samples were cylinders with rounded ends, 8 mm in diameter and 15 or 19 mm in length, taken from the core of the sheet.

The tests were carried out in a pyrex reactor of 1 liter in volume (internal diameter 110 mm), thermostatically controlled by a reactor heater to within + 2 ° C. The cover of this reactor had several passages used for the following functions:

- passage for a reflux condenser, - passage for a dip tube used for bubbling and argon sweeping,

- passage for an electrolytic bridge used to connect the samples to an external reference electrode (calomel electrode in saturated potassium chloride solution, called ECS ⁾ ,

- passage for the sample holders, and

- passage for an auxiliary platinum electrode.

The metal surface / volume ratio of this container -1 was about 0.2 dm. Several samples were used in each test (three to six). These were connected in turn to a high input impedance illivoltmeter allowing to know their potential compared to the external reference electrode.

These values were plotted on a diagram by an analog recorder.

On the other hand, a device for measuring the polarization resistance made it possible to determine and record a quantity proportional to 1 / Rp, Rp being the polarization resistance: this quantity is therefore proportional to the instantaneous corrosion rate. The weighings were carried out on a 1/10 mg balance and micrographic sections were taken and observed using an optical microscope.

The progress of each test was as follows: the solutions were first deaerated by bubbling argon or nitrogen, then added with hydrazine, and finally heated to the desired temperature _* The role of hydrazine is to remove the last traces of oxygen. The samples were previously degreased and weighed, then immersed in the pickling bath when it had reached the desired temperature. By monitoring the polarization resistance Rp during the test, it was possible to determine approximately the rate of erosion at each instant and, consequently, the thickness of metal lost by the sample III. For each test, the experiment was extended until the pickling reached about 10 micrometers. The 0 samples were then rinsed, dried and weighed, which made it possible to determine the exact thickness of metal removed by pickling. It was thus possible to deduce therefrom for each bath the exact cofficient which it was appropriate to apply to the measurements of 1 / Rp to translate them into thickness of metal lost per unit of time. For each batch, one of the samples was then cut, coated and polished for micrographic observation on a longitudinal section.

A second observation was made after metallographic attack (electrolytic attack in oxalic acid at Q 10% ⁾ ”

The examples below relate to tests which have been carried out with pickling baths in accordance with the invention r EXEWPLE _, 1

The mixture used contained 0.3 tt of citric acid and 0.5 ft of maleic acid.

A corrosion rate of about 1 micrometer per day was observed and an absence of intergranular corrosion. EXAMPLE 2

A product containing 0.3 MQ of citric acid, 0.5 M of maleic acid, and 0.1 W of hydrofluoric acid was used for this test. There has been a constant pickling speed of the order of 5 micrometers per day. On the other hand, after a pickling of 2 days, which made it possible to remove a thickness of the order of 10 micrometers, no intergranular corrosion was observed, which means that there is no no attacks of this type or, if there are any, they are limited and less than 10 micrometers.

EXAMPLE 3

In this example, tests were carried out on Z15 CN 22-12 steel samples containing less than 1500 ppm of carbon having been in contact with liquid sodium for 350 hours at 450 ° C. The pickling bath used was a mixture containing 0.3 M citric acid, 0.5 M maleic acid and 0.05 M; hydrofluoric acid. Also on this steel, the use of a pickling bath _. according to the invention did not give intergranular corrosion.

It should be noted that, in the pickling bath object of the invention, citric acid plays the role of a reducing agent (it has a mp of 3.08 at 20 ° C) and that maleic acid plays the role an acidifier (it has a mp of 1.83 at 25 ° C). As for hydrofluoric acid, it plays the role of a protractor of the surface of the treated steels. Indeed, we prefer to work in the field of steel activity rather than in its area of passivity. The reason is the following: if, starting from the field of activity, the conditions change, one passes into the field of passivity and the attack is much slower, even completely stopped, which is not dangerous. If, on the contrary, one works in the area of passivity, a change in the working conditions risks passing the metal into its area of activity and there is a risk of runaway reaction and significant intergranular corrosion.

Thus, in the case of highly sensitized steels, that is to say which have been irradiated for 50,000 to 80,000 hours or even more, a pickling bath containing only maleic acid and citric acid is sufficient to give satisfactory pickling results. It is in the case where sensitization is less important that the presence of hydrofluoric acid becomes necessary. The temperature of treatment of the steels with the pickling bath object of the invention is generally between 70 and 80 ° C, and preferably close to 75 ° C.

On the other hand, it is preferred to work in a reducing medium, hence the use of citric acid because, if one worked in an oxidizing medium, the material to be treated would be passive and there would be too slow pickling. EXAMPLE_4

In this example, we treated a series of samples all coming from the same piece of Z6-CND 17-11 steel with 6.55 p.p.m. of carbon remained 53000 hours at 510 ° C in sodium. Samples 1 to 4 were pickled with a bath containing 0.3 K

2M citric acid and 0.5M maleic acid to which 10 hydrazine was added. Samples 5 and 6 were pickled with a bath containing 0.3 M citric acid, 0.5 M maleic acid and

* 2 0.1 M hydrofluoric acid to which 10 M hydrazine has been added. The pickling was carried out in a deaerated medium at 75 ° C. for a period of 24 hours. The results are collated in the table below.

It has been observed that decontamination is faster with a bath containing hydrofluoric acid ⁽ samples 5 and 6) but, as, in this example, the material is very sensitized, there is intergranular corrosion. With the baths not containing hydrofluoric acid (samples 1 to 4), there was no intergranular corrosion.

We will now give some examples of tests which have been carried out with pickling baths different from the pickling bath which is the subject of the present invention. In this series of examples, the samples used were those used in Examples 1 and 2 above. EXAMPLE ^

In this example, a series of tests was carried out with sulfamic acid in accordance with the prior art, the sulfamic acid being or not containing an inhibitor.

With sulfamic acid alone at 0.32 M, an irregular and high pickling speed was observed, and significant intergranular corrosion (approximately 200 micrometers for a pickling of 10 micrometers).

If 2-mercaptobenzimidazole is added to the sulfamic acid, the corrosion rate is practically constant and there is practically no intergranular attack. However, a major localized bite attack appeared. 2Q J.XJÏ.P_ _S E_6

In this test, a mixture containing 0.5 M maleic acid and 0.32 M sulfamic acid was used. A stable corrosion rate of about 5 micrometers per day has been observed, but significant intergranular corrosion of up to about 100 micrometers for pickling on the order of 10 micrometers. EXAMPLE_7

In this example, a pickling bath containing 0.5 M citric acid and 0.5 M oxalic acid was used. The tests were carried out at a temperature of 85 ° C instead of 75 ° C. Despite a higher temperature than in the previous examples, a very low pickling speed was observed which,

Q moreover, decreases over time, which practically prohibits the use of this product for pickling. EXAMPLE_8

The same samples as in the previous example were treated at 85 ° C. in a bath containing 0.5 M of citric acid, 0.2 M of oxalic acid, and 1 g per liter of diamine tetracetic acid (EDTA ). This product is capable of complexing ferrous ions and gives a corrosion rate of one micrometer per day. However, micrographic observations reveal significant intergranular corrosion, the depth of which can reach 60 micrometers for a pickling of 7 micrometers.

Thus the pickling bath object of the invention has particularly interesting advantages since it is the only one which allows to have a constant pickling speed and sufficiently high to be able to be used on an industrial scale and which, on the other hand, does not give intergranular corrosion, or pitting or other attack. In addition, this process is adaptable, by the flexibility of the concentration ranges of the products used, to a very wide range of steel grades and degrees of sensitization.

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Claims

1. Stripping bath, usable in particular for pickling steel parts, characterized in that it contains:

- from 0.1 M to 0.3 M of citric acid, - from 0.2 M to 0.5 M of maleic acid,

- from 0 to 0.1 M hydrofluoric acid, and accidental impurities.

2. Process for pickling a steel part, in which the part is brought into contact with a pickling bath, characterized in that the pickling bath is the bath according to claim 1.