FI101234B - Process for pickling and passivation of stainless steel without using nitric acid - Google Patents

Abstract

A process for pickling and passivating stainless steel using a bath of the following initial composition: H2SO4 at least 150 g/l Fe<3><+> at least 15 g/l HF at least 40 g/l stabilized H2O2 1-20 g/l non-ionic surfactants and acid attack inhibitors 1 g/l, into which an air flow and an H2O2 quantity of between 0.3 and 1 g/l per hour, controlled to maintain the bath Redox potential at >/= 350 mV, are fed in continuously.

Description

5 Method for pickling and passivation of stainless steel without the use of nitric acid.

1 101234

Technical Problem 10 It is well known that during the manufacture of hot-rolled steel products or heat-treated, for example hardened, semi-finished products, the surface of the material is covered with an oxidizing layer of varying thickness. Since the finished product needs to be brightened to a finished surface, these oxide layers must be completely removed. This is achieved by well-known metal pickling methods using inorganic mineral acids, such as hydrochloric, sulfuric, nitric and hydrofluoric acids, either alone or in mixtures with varying mixing ratios.

In the field of stainless steels, the most common or indeed almost exclusively used metal pickling process is a mixture of nitric acid and hydrofluoric acid, the relative concentrations of which vary according to the type of plant, the quality of the steel to be pickled, its surface properties and the shape of the product to be treated. The method is undoubtedly economical and makes it possible to achieve excellent results. Its very downside, however, is that it causes significant environmental problems that are difficult to solve precisely because of the use of nitric acid. This * · results in the release of nitrogen oxide fumes of the general formula '35, NOx, which are highly polluting and corrosive to metals and non-metals in contact with them, as well as high nitrate concentrations in washing water and spent baths, resulting in a problem for disposal.

40 2 101234 ΝΟχ the removal of vapors from the air and the removal of nitrates from used baths cause significant plant problems (for example, there is currently no NOx removal method with no technical problems), 5 high operating costs, and no assurance that the results meet current regulations. Therefore, in most industrial plants, it is difficult to accept the cost in the final calculations for inclusion in the capital invested. A metal pickling system that does not involve the use of 10 nitric acids is therefore of considerable industrial importance, and various plans have been developed in this regard throughout the world, in particular, especially over the last ten years.

15 Methods as alternatives to the use of nitric acid: state of the art

An alternative study to traditional HNO3 + HF-based stainless steel pickling methods has been the publication of 20 proposed patents on nitric acid-free step series and the main literature on the subject disclosed the following: A) C. and a processing time of 20 seconds.

• B) The method disclosed in two Japanese Patent Publications JP 55018552, published February 8, 80, and JP 55050468, published April 12, 80, comprises three steps, namely 1) initial removal of dandruff in sulfuric acid or hydrochloric acid, 2) subsequent immersion first in potassium permanganate and \ to a solution of inorganic acids (not HF) and secondly to a solution of ferric nitrate, ferric sulphate and peroxydisulphuric acid 35, and 3) final washing with pressurized water or ultrasound.

40 3 101234 C) Swedish patent publication SE 8001911, published on 12.10.81, describes the continuous treatment of sulfuric acid at a temperature between 10 and 90 ° C (preferably 30-60 ° C) for a time between 1 and 120 minutes (preferably 1 to 20 minutes). 5 and in a solution of hydrogen peroxide.

D) German patent publication DD 244362, published 1.4.87, uses a solution of chromic acid, sulfuric acid, hydrofluoric acid and an inhibitor (hexamethylenetetramine) at a temperature of 15 to 30 ° C; then the bath is neutralized with calcium and barium salts.

E) German patent publication DE 3937438, published on 30.8.90, is intended mainly for the yarn manufacturing industry and uses a solution of hydrofluoric acid containing Fe 3+ in the form of a fluoride complex added; the oxygen-containing gaseous or liquid substance is then added to the solution, which is electrolyzed to provide oxygen in the nascent state, which is capable of oxidizing divalent oxygen to trivalent oxygen.

F) German patent publication DE 3222532, published on 22.12.83, describes the metal pickling of pipes and vessels made of austenitic steel, the inner surfaces 25 of which are treated at a temperature of 15 to 30 ° C with a solution made of hydrofluoric acid and peroxides (either stabilized hydrogen peroxide or sodium perborate or organic peroxides (not specified), while the outer surfaces are covered with pastes made of 4 101234 hydrofluoric acid, peroxides and fillers (carboxymethylcellulose); pastes must be disposed of by neutralization with calcium salts, peroxides destroyed either by catalysts or by heating »5 G) Tokai Denka Kogyo's English patent publication 2,000,196 uses a metal pickling bath containing ferrous sulphate and hydrofluoric acid. SO 2 and hydrogen peroxide are fed in a molar ratio of 1: 1 and consistently to maintain a sufficient Ferri ion concentration during the treatment. The claims also include a method of controlling the treatment by continuously measuring the oxidation-reduction potential of the mixture, which should be kept at · £. 500 mV by adjusting the supply of H 2 SO 4 + I 2 Og; H) In two very similar European patents EP 188975 and EP 236354 (= WO 87/01739) with priority dates of 22.1.85. and 19.9.35. accordingly, a metal pickling solution containing hydrofluoric acid (5-50 g / l) and trivalent Ferri-Ion is used, which is added in the form of fluorinated complexes and to which air or oxygen is continuously blown; the treatment time is between 30 seconds and 5 minutes and the temperature varies from 10 ° C to 70 ° C,. 25 it is also recommended to adjust the oxidation-reduction potential, it should be kept between -200 and +800 mV according to the first patent publication and between +100 mV and +300 mV according to the second patent publication, by adding an oxidant such as potassium permanganate or hydrogen peroxide to increase the potential, if necessary. All the tests performed only concern the pickling of the steel plate without the use of oxidizing compounds other than atmospheric air.

There are two other patents relating to the possibility of preventing or reducing to an extreme 5 101234 ap0χ the formation of oxides of nitrogen in nitric acid baths by feeding suitable oxidants to a metal pickling bath. Among them, in the earlier Japanese patent publication JP 58110682 dated 5 July 1, 2003. hydrogen peroxide is used; another Swedish patent publication (SE 8305648 from 15.4.85. »priority date 14.10.83., SE 835648) again uses hydrogen peroxide and / or alternatively urea.

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Despite this increase in patents, the traditional method based on nitric acid and hydrofluoric acid has so far been widely used throughout the world, and none of the above-mentioned proposed alternatives have been adopted for industrial use.

Method according to the invention The method according to the present patent application combines the interesting aspects of certain prior art methods 25, which are rationalized into a comprehensive and exhaustive program, to which various aspects of a completely new quality have been incorporated; The method is based on the use of a metal pickling bath containing Ferri ions, H 2 SO 4, HF, H 2 O 2 and conventional additives, emulsifiers, brighteners and corrosion inhibitors, to which a strong air stream is continuously blown. It is usually operated at a temperature between 30 and 70 ° C and preferably between 45 and 55 ° C. The basic features of the method are as follows:

Inorganic mineral acids contained in the bath: in the preparation of the metal pickling bath, a solution containing at least 150 g / l and preferably about 170 g / l of 1 SO 2, and at least 40 g / l and preferably 50 g / l of HF is made.

These acids have various functions, the most important of which is to keep the process p ^ less than 1 and preferably between 0 and 0.5 to dissolve the oxides from the heat treatment, and in the case of hydrofluoric acid the aim is to form complexes with Fe 10 and Cr ^ + ions as much as possible.

As the concentrations of these two acids, and in particular hydrofluoric acid, tend to decrease during the pickling treatment of the metal, they must be increased from time to time on the basis of an analysis performed from the bath (free acid and fluoride ion values) o

Fe 2+ ion content of the bath: During the preparation of the bath, an amount of Fe 2+ ion not less than 15 g / l in the form of ferric sulphate is added to the metal pickling solution. The purpose of this Joni is to replace nitric acid as a scavenging agent in the reaction 2Fe ^ + Fe - ^ 3Fe, which is promoted by the p ^. Conditions of the bath. During the treatment, the right conditions must be continuously obtained to obtain the amount of Ferri form. as much as possible at the cost of the Ferro form * in the iron dissolved in the bath.

O 4- ^ +

The oxidation of Fe ions to Fe ions during the treatment to keep the concentration of the latter above a predetermined minimum value is achieved by the combined action of the air blown into the bath and the ^ O 2 continuously added to the bath in small batches.

Continuous addition of stabilized hydrogen peroxide

For the process to be economical, the amount of 7 101234 hydrogen peroxide consumed must be as small as possible. For this reason, it is important to use hydrogen peroxide containing a known stabilizer which is effective in preventing or at least significantly slowing down the decomposition of per-5 oxide under operating conditions (temperature up to 70 ° C, pH of a very acid bath, iron up to 100 g / l, Presence of Ni and Cr ions) »A particularly suitable stabilizer is the one marketed by Interox (Laporte-Solvay) called Interox S 333 or Interox S 333C» The use of suitably stabilized 1 ^ 02 ^ in combination with the use of air blown into the bath as a complementary oxidizing agent has produced resulting in a method in which the use of H 2 O 2 is economically feasible, this was never possible with known methods. The pickling bath of metal 15 is prepared so that the concentration of H 2 O 2 is between 1 and 20 g / l, preferably 2-5 g / l.

During the pickling of the metal, the continuous supply of H 2 O 2 is adjusted according to the quality of the steel to be pickled and the surface properties of the material 20 (or semi-finished product) and the amount and quality of dandruff from rolling or heat treatment. Usually H 2 O 2 is fed in an amount between 0.3 and 1 g / l per bath and the duration of the treatment per hour.

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Continuous air blowing β

During the pickling of the metal, a continuous flow of air is introduced into the bath at a rate of at least 3 mVm per bath and per hour of treatment. This air flow, which is fed at a suitable speed, contributes to good mixing in the bath, which is an important prerequisite for efficient metal pickling, as it mixes the laminar layer in the vicinity of the surface to be treated, thus ensuring that this surface is always in direct contact with fresh H 101234 metal pickling with solution. In order to ensure the best possible mechanical mixing and homogenization of the treatment solution, it is advantageous to blow air through perforated pipes 5 perforated at the bottom of the basin or by using a suitable blowing device.

As already stated, the blown air also oxidizes Ferro ions together with hydrogen peroxide, thus resulting in a considerable reduction in the latter consumption.

Oxidation-Reduction Potential Adjustment: It is well known that the behavior of stainless steel in an acid alloy is described by polarization curves showing the activity, passivity and transpassivity steps at different potentials, the bath only needs to be kept under non-corrosive conditions, i.e. the treatment must be carried out at a potential within a range of passivity which can be determined in advance-20 in hand on the basis of the quality of the steel.

During the treatment, while the concentration of divalent Ferro jön in the bath increases, the oxidation-reduction potential of the bath tends to decrease, the addition of hydrogen peroxide 25 'together with the oxidizing effect of the blown air returns it to the most suitable values, clearly above 300 mV.

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By constantly adjusting the potential, it is therefore possible not only to ensure good pickling of the material, but also to ensure that a passive film is formed on its surface. In this respect, industrial experiments have in all cases resulted in bright, shiny and perfectly planar surfaces in which no corrosion was visible, for example due to pitting-35 corrosion or excessive metal pickling treatment.

9 101234 In this regard, it should be noted that often the traditional method of pickling metal based on nitric acid and hydrofluoric acid results in defects similar to those described above, and it is not at all uncommon for the conventional system to result in "burning" (i.e., crystalline corrosion) of the material.

At times when the metal pickling bath 10 is not in operation (at the end of the week, at night), it is necessary to blow only the minimum amount of air to keep the oxidation-reduction potential within the most suitable values, thus making it possible to leave the material immersed in the solution for several hours without corrosion.

Various Additives in a Metal Pickling Bath The metal pickling bath of the present invention is formed using the usual additives of this type of process, optionally non-ionic pmta-active agents which act as wetting agents, emulsifiers, brighteners and acid inhibitors. These additives enhance and enhance the effect of the metal scaffolding through a moderate synergistic effect. They are used in a total amount of about 1 g / l bath.

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Advantages of the method 30, ί Sludge-free The method according to the present invention reduces the formation of sludge to an extremely low level or even completely, which results in further cost savings. This advantage is due, inter alia, to the appropriate HF content during treatment and the appropriate adjustment of the Ferro ion content,

Claims (5)

A method of pickling and passivating stainless steel, comprising bringing the material to be treated into contact with a bath, which is poured at a temperature between 30 and 70 ° C and preferably between 45 and 55 ° C and whose beneficial composition are the following: a) H2 SO4 at least 150 g / l b) Fe at least 15 g / l c) HF at least 40 g / l d) H2 O2 (containing known stabilizers) 1-20 and preferably 2-5 g / l e ) nonionic surfactant additives (emulsifiers, wetting agents, clarifiers) and acid attack inhibitor type; a total of about 1 g / l; in said bath is continuously fed: 20. an air stream of at least 3 m / h per m of bath, using a suitable diffuser for diffusing the stream into the float; - a quantity of stabilized H2 O2 between 0.3 and 1 g / l per hour, controlled on the basis of the redox potential of the bath, which should be maintained at> - 350 mV; - and possibly sufficient quantities of ingredients of a), c) and e) to pour their concentration into the bath at optimum levels and the bath at pH below 1 and preferably between 30 and 0.5. Process according to claim 1, characterized in that the Fe3 + ions are introduced into the initial bath in the form of; ferric sulfate. 35 Method according to claim 1, characterized in that H 2 O 2 stabilized with the stabilizer named Interox S 333 by Interox (Laporte-Solvay) was used. 1K 101234 4. A process according to claim 1, characterized in that a bath having the following initial aeration is used: 172 g / l H 2 SO 4 48 g / l HF 10 g / l Fe 3 + 5 g / l H 2 O 2 1 g / l nonionic surfactant and acid attack inhibitors such as additives 2 g / l stabilizer for H2O2 in the form of Interox S 333. Process according to Claim 1, characterized in that for the material to be treated (stainless steel), preliminary partial removal of oxides by a known method was carried out.