Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
  • C23G1/086—Iron or steel solutions containing HF

Definitions

• the field of the present invention is that of surface treatments and more precisely the acid pickling of stainless steel products.
• the acid pickling of stainless steels is usually carried out with fluonitric baths, in which the use of nitric acid has the disadvantage of causing the formation of nitrous vapors polluting the atmosphere and soluble nitrates polluting the liquid effluents.
• J.H.G. MONYPENNY indicates pp. 183-184 that, to minimize the problem of vapors from fluonitric pickling baths, baths containing 6 to 12% of 90% solution of ferric sulfate and 1.5 to 3% were used for pickling stainless steel sheets hydrofluoric acid and this for example at 70-80 ° C for descaling a hot rolled sheet.
• the initial concentration of ferric iron in previous baths is therefore about 16.5 to 33 g / l.
• the Applicant's tests have shown that, when successive samples of stainless steel sheet are stripped in such baths, the speed and quality of the stripping deteriorate rapidly. These acid pickling baths are therefore not satisfactory as such for series or continuous pickling of stainless steel products.
• the subject of the invention is a process for pickling stainless steel products in which, as is known, a pickling bath of initial composition is used:
• the ferric iron content of the bath is maintained at at least 15 g / l by oxidation of the bath comprising at least one or more air injections with a total flow rate greater than or equal to 1 Nm 3 per m 2 of pickled stainless steel and per hour of pickling of each pickled surface element, or else equivalent ventilation by circulation in the open air.
• one or more pickling baths are used, initially containing HF 10 at 35 g / l and Fe 3+ ⁇ 20 g / l, and during the pickling operation (s) the Fe 3+ content of this bath or these baths is kept at least 20 g / l thanks to oxidation of the bath or baths comprising one or more air injections total flow between 1 and 8 Nm 3 per m 2 of pickled stainless steel and per hour of pickling of each pickled surface element. Air injections with a higher total flow rate have proved to be of no interest, the saturation of the oxygen bath of the air being undoubtedly reached and the additional air flow rates no longer apparently serving only to agitate. the bath, and this in a possibly excessive way.
• the oxygen of the introduced air seems to intervene in the process of the invention as oxidizing agent regenerating Fe 2+ in Fe 3+ , whereas Fe 3+ constitutes an oxidizing agent acting on the base metal to dissolve it.
• the main reactions could be:
• Fe + 2 HF H 2 + Fe F 2 (B) also possible in an oxidizing medium, which is the case;
• the ferric iron concentration in the bath can be calculated as the difference between the total iron concentration, determined for example by atomic absorption, and the concentration of Fe 2+ measured by its oxidation to Fe 3+ in the presence of KMnO 4 permanganate. Adequate aeration of the pickling bath, typically by air injection, allows the quality of pickling to be maintained during successive pickling or continuous pickling of stainless steel products by regenerating Fe 3+ .
• the total volume of air injected into the pickling bath essentially depends on the amount of pickled stainless steel, which quantity itself is proportional to the pickled surface and the duration of pickling of this surface.
• the total flow rate of air injected into the pickling bath of the invention is typically between 2 and 5 Nm 3 per m 2 d '' pickled stainless steel and per hour of pickling of each pickled surface element. So that the pickling bath is adequately aerated, it is then advisable to inject a good part of this volume of air, typically typically at least half of this volume, with nozzles directed towards the bottom of the bath at the lower half. of this bath.
• the injected air is preferably preheated to a temperature close to that of the bath, ie typically between 35 and 60 ° C.
• HF recharges are carried out as usual, and, rather than determining the concentration of Fe 3+ in the bath, it is practical to determine the REDOX potential of the bath and to adjust it between 0 and +800 mV and preferably between +100 and +300 mV by acting if necessary on the oxidation of the bath.
• the reference REDOX potential is chosen according to the shade and surface condition of the strip and readjusted, if necessary, based on surface condition observations after pickling.
• the REDOX potential is measured between a platinum electrode and an Ag / AgCl reference electrode or with fixed, reproducible potential and with zero irreversibility power.
• a device for measuring this REDOX potential can be suitably sealed so as to allow continuous measurements in the bath.
• an oxidation means temporarily and / or locally supplementing the action of the air to return more quickly to the desired Fe 3+ concentration or at the set REDOX potential, so as to find a good pickling.
• strong oxidant for example hydrogen peroxide or potassium permanganate, is then used as a complementary means of oxidizing the bath. It is still possible in certain cases to introduce an oxygen injection or to increase the air flow.
• the Applicant has found that it was then possible to modify the solubility of the sludge, or precipitated from the spent bath, by adjusting the REDOX potential of the bath during pickling.
• the "sludge" is not very soluble when the bath has been adjusted below +100 mV or above +300 to 350 mV, and their solubility is greatly improved between +100 mV and +300 mV, and more particularly between +190 mV and +260 mV, an optimal bath control being 220 + 20 mV.
• ferric fluoride or ferric sulphate or ferric chloride is generally used, with a ferric iron concentration of between 20 and 40 g / l, with a preference for ferric fluoride, so as to have only one acid radical in the bath.
• the pickling process of the invention is applied to stainless steel sheets or strips with typically the initial HF concentrations and the following pickling temperatures:
• - ferritic stainless steels HF 10 to 25 g / l, 35 to 50 ° C.
• - austenitic stainless steels HF 20 to 35 g / l, 40 to 60 ° C.
• the pickling process of the invention provides industrial advantages with significant advantages:
• the adjustment of the quality of the bath is all the more convenient and precise that the major part of the oxidation is produced by the air injection (s);
• the pickling tests were carried out on samples of ferritic stainless steel 17% Cr type AISI 430 hot rolled, shot and electroplated, having the shape of rectangular test pieces 50 x 25 x 3 mm.
• the pickling conditions for these samples were as follows:
• initial concentration of dissolved iron (ferric fluoride) varying from 0 to 60 g / l
• This air injection here was of the order of 1 1 / min, that is to say very in excess relative to the useful flow rate.
• Consistent pickling tests were carried out in the laboratory of several hundred samples similar to the samples in test series No. 1, always in the same pickling solution of initial composition HF 20 g / l, with periodic recharging on the one hand in HF to keep HF 20 g / l and on the other hand in H 2 O 2 to the minimum necessary taking into account the concentration of iron in the solution, this with injection of air into the pickling bath.
• the total dissolved iron concentration, the cumulative consumption of HF and the cumulative consumption of hydrogen peroxide H 2 O 2 were monitored respectively according to the number of pickled samples, each for 2 min. It has been observed that up to 275 to 300 pickled samples, corresponding to 25 to 27 g / l of dissolved iron, the consumption of HF and H 2 O 2 are quite high and roughly proportional to the number of pickled samples, and that beyond that consumption of HF and H 2 O 2 becomes very low. Thus, when the dissolved iron concentration becomes greater than 25 g / l, the consumption of concentrated HF at 70% surprisingly goes from 7 ml per 100 pickled samples to 0.3 ml per 100 pickled samples.
• the oxygen in the air injected into the bath acts as an ion regenerator (Fe 3+ ) according to the equilibrium reaction (C) already indicated, by moving this equilibrium in the direction 3 of the formation of Fe 3+ , the pH of the solution being favorable and of the order of 2 as a result of the HF concentration.
• this reaction (C) is regulated so that it allows a regeneration of Fe 2+ to Fe 3+ fast enough to always have Fe 3+ > 20 to 25 g / l, there is almost no need for H 2 O 2 .
• the consumption of HF is surprisingly much lower than for the lower concentrations of iron and therefore Fe 3+ .
• the bath contained 20 g / l of HF and initially 25 g / l of Fe 3+ , coming from ferric fluoride dissolved in the bath. Air was injected into the bath mainly with nozzles spaced 2 to 3 m apart and directed downwards with an inclination of 15 ° with respect to the vertical, the air being released at the end of these nozzles towards the bottom of the tray and 15 cm from this bottom.
• the total flow rate of air injected into the bath was 100 Nm 3 / h, 2/3 of which towards the bottom and in the vicinity of this bottom with the nozzles which have just been described.
• the bath temperature was 40 to 45 ° C.
• the bath was controlled by measuring and adjusting its REDOX potential above +150 mV.
• the baths contained 25 g / l of HF and initially 20 g / l of Fe 3+ . Air was injected with nozzles with a layout similar to that of Example 1 with a total flow rate for each tank of 80 m 3 / h and a pressure of 0.2 MPa, ie a flow rate of approximately 160 Nm 3 / h.
• the bath temperature was 50 to 55 ° C.
• the bath was controlled by measuring and adjusting its REDOX potential above +200 mV. Additions of hydrogen peroxide were planned as an additional means of oxidation to readjust the REDOX potential when it had become too low. We were able to operate for periods of several days without using this additional oxidation means and while retaining a REDOX potential of +200 to +300 mV with good pickling quality.
• the injected air flow here is 4 Nm 3 per m 2 of pickled stainless steel and per hour of pickling of each pickled surface element.
• the complex formed is of the FeF3, 3H 2 O type. It has been found that this compound was soluble neither in water at 20 ° C. nor in an aqueous solution of 20 g of HF per liter at 20 ° C. hydrolyzes). On the other hand, at 50 ° C, it is moderately soluble: at 31 g / l in water and at 38 g / l in HF 20 g / l. This dissolution, unstable on cooling, is not satisfactory.
• Fe 2+ represents approximately 80% of the dissolved iron, and the complex formed is of the FeF 2 , nH 2 O type.
• the same dissolution tests as in Example 3 were carried out. This compound is sparingly soluble, the only dissolution noted is 13 g / l in the case of HF 20 g / l at 50 ° C.
• the pickling conditions correspond to those of Example 2, with the exception of the REDOX potential set to +220 mV + -20 mV (measured between a platinum electrode and an Ag / AgCl reference electrode).
• Fe 3+ represents 70 to 80% of the dissolved iron, and the majority compound formed seems to be of the type: Fe 2 F 5 , 7 H 2 O.
• the dissolution tests provided the following results, in dissolved g per liter:
• Solubility at 20 ° C Solubility at 50 ° C in water in solution in water in solution HF 20 g / l HF 20 g / l
• This type of "mud” can be recycled in a new bath, according to the method described above.

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• Chemical & Material Sciences (AREA)
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• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
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• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
• Detergent Compositions (AREA)
• ing And Chemical Polishing (AREA)

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• 1985
  • 1985-09-19 FR FR8514220A patent/FR2587369B1/fr not_active Expired - Lifetime
• 1986
  • 1986-07-25 CA CA000514703A patent/CA1272980A/fr not_active Expired - Lifetime
  • 1986-07-28 JP JP61504183A patent/JPS62501981A/ja active Granted
  • 1986-07-28 DE DE8686904835T patent/DE3664340D1/de not_active Expired
  • 1986-07-28 BR BR8606873A patent/BR8606873A/pt not_active IP Right Cessation
  • 1986-07-28 EP EP86904835A patent/EP0236354B1/fr not_active Expired
  • 1986-07-28 WO PCT/FR1986/000267 patent/WO1987001739A1/fr active IP Right Grant
  • 1986-07-29 MX MX003290A patent/MX168028B/es unknown
  • 1986-07-29 ES ES8600701A patent/ES2000222A6/es not_active Expired
• 1987
  • 1987-05-18 FI FI872187A patent/FI81126C/fi not_active IP Right Cessation

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