Process for stainless steel pickling and passivation without using nitric acid

Abstract: Process for stainless steel pickling consisting in placing the material to be treated in a bath kept at a temperature ranging from 30°C to 70°C having the following initial composition: a) H₂SO₄ 150 g/l at least b) Fe³⁺ 15 g/l at least c) HF 40 g/l at least d) H₂O₂ (added with known stabilizers) 1-20 g/l e) emulsifiers, wetting agents, polishing agents, acid attack inhibitors; in the bath being fed continuously: an air flow equal at least to 3 m³/h per m³ bath min. and a stabilized H₂O₂ quantity adjusted to the bath redox potential to be kept at ≧250 mV.

Description: Technical field As is known when, during the manufacturing process, iron and steel industry products undergo hot-rolling or intermediates undergo heat treatment, such as for instance annealing, the material is coated with a thinner or thicker oxidation layer. In consideration of the final products having to exhibit a polished and glossy finish, the oxidation layer is to be removed entirely. This is done through the well-known pickling process generally using mineral inorganic acids, such as hydrochloric acid, sulphuric acid, nitric acid, and hydrofluoric acid, either individually or as mixtures. According to the industrial processes currently applied, stainless steel pickling is normally almost exclusively based on the use of a nitric-hydrofluoric acid mixture, the respective acid concentrations depending on the type of plant, on the type of steel to be pickled, on the steel surface properties and on the shape of the manufactured article to be treated. Although the process is undoubtedly economic and leads to excellent results, it involves extremely serious ecological problems hard to solve, brought about by the use of nitric acid. Actually, while on the one hand highly polluting nitrogen oxide vapours having general formula NO x , aggressive toward metallic and non-metallic materials with which they come into contact, are vented to the atmosphere, on the other hand high nitrate concentrations are reached in wash water and spent baths, both types of pollutants requiring treatment prior to disposal. The removal of NO x from air and of nitrates from baths involves huge plant operation problems and high operating costs, with no certainty about the obtainment of targets complying with the regulations in force. This means that the resulting industrial plant investment costs can be hardly borne in most cases. A pickling method not requiring the use of nitric acid is therefore demanded by industry and various proposals in this sense have been made worldwide mainly in these last ten years. Methods alternative to those using nitric acid: state of the art A critical examination both of patents covering methods alternative to the traditional stainless steel pickling process based on the use of HNO 3 + HF, no longer containing nitric acid, and of the main relating technical literature has demonstrated the following: A) Japanese patent JP 50071524 published on 13th June, 1975 (see Derwent Abstract No. 76-78076X/42) provides for the use of hydrochloric acid and ferric chloride at 70°C, for a treatment time of 20″. B) Japanese patents JP 55018552 published on 8th February, 1980 (see Derwent Abstract No. 80-21157C/12) and JP 55050468 published on 12th April, 1980 (see Derwent Abstract No. 80-37402C/21) provide for three steps: (1) initial descaling in sulphuric or hydrochloric acid, (2) immersion, in the former case, in a potassium permanganate and inorganic acids ( non HF) solution, in the latter case, in a ferric nitrate, ferric sulphate and peroxydisulphuric acid solution, (3) pressure water jet or ultrasonic final washing. C) Swedish patent SE 8001911 published on 12th October, 1981 (see Derwent Abstract No. 81-94307D/51) relates to a treatment in a sulphuric acid and hydrogen peroxide solution; treatment time range: from 1 to 120 minutes (preferred range: 1-20′); temperature range: from 10°C to 90°C (preferred range: 30-60°C). D) German patent DD 244362 (see Derwent Abstract No. 87-228825/33) published on 1st April, 1987 provides for the use, at 15-30°C, of a solution formed by chromic acid, sulphuric acid, hydrofluoric acid and an inhibitor (hexamethylenetetramine); the bath is later neutralized with calcium and barium salts. E) German patent DE 3937438 published on 30th August, 1990 (see Derwent Abstract No. 90-268965/36) mainly applies to the wire treatment industry and provides for the use of a hydrofluoric acid solution containing Fe 3+ fed as additive in the form of complex fluoride. Then, the solution is fed with a gas and/or an oxygenated fluid means, subjected to electrolysis to obtain nascent oxygen capable of oxidizing iron from bivalent to trivalent. F) German patent DE 3222532 published on 22nd December, 1983 (see Derwent Abstract No. 84-000662/01) relates to the pickling of austenitic steel pipes or vessels, the inner surfaces whereof are treated at 15-30°C with a solution consisting of hydrofluoric acid and peroxides (hydrogen peroxide, or stabilized sodium perborate, or organic peroxides in general), while the outer surfaces are pickled with pastes consisting of hydrofluoric acid, peroxides and filler (carboxymethylcellulose); pastes must be removed by neutralization with calcium salts, while peroxides are destroyed either by catalysis or by heating. G) TOKAI Denka Kogyo's English patent 2,000,196 provides for the use of a pickling bath consisting of ferric sulphate and hydrofluoric acid. Sulphuric acid and hydrogen peroxide are added continuously in a 1:1 molar ratio, for the purpose of keeping an adequate ferric ion concentration. The patent claims the pickling treatment control method by continuous checking of the redox potential to be kept at ≥300 mV by controlled H 2 SO 4 + H 2 O 2 feeding. H) FR-A-2.587.369 provides for the use of a pickling solution consisting of hydrofluoric acid (5-50 g/l) and trivalent ferric ion added as fluorinated complexes, continuously blown into with air and oxygen; treatment time range: 0,8 to 2 mm, temperature range: 15°C to 70°C; continuous checking is recommended for redox potential, which should he kept at 0 to +800 mV, preferably at +100 to +300 mV,; if the potential requires to be raised, an oxidizer such as Potassium permanganate or hydrogen peroxide should be added. All pickling tests were conducted on sheets only. Jap. 61 235 581 discloses a pickling process based on the use of solution of HF, H 2 O 2 and H 2 SO 4 but the teaching is relevant to the treatment of steels based on Fe and Ni or Fe/Ni/Co alloys. This teaching is not obviously applicable to the stainless steels which are substantially based on Cr/Fe alloys and have a different behaviour in the pickling process. There are two further patents regarding the possibility of avoiding or minimizing the formation of nitrogen oxides NO x in baths using nitric acid, by the direct addition of suitable oxidizers to the pickling bath: the former, Japanese patent JP 58110682 dated 1st July, 1983 (see Derwent Abstract No. 83-731743/32), provides for the use of hydrogen Peroxide; the latter, Swedish patent SE 8305648 dated 15th April, 1985 - priority date 14th October, 1983, (see Derwent Abstract No. 85-176174/29) - provides for the use of hydrogen peroxide and/or, as an alternative, of urea. Finally EP 505.606 discloses a pickling process for stainless steel wherein a solution containing HF, H 2 SO 4 and Fe 3+ ions is used and the Redox potential of the bath is controlled through the addition of H 2 O 2 in order to maintain a value higher than 350 mV the pH of the solution being less than 1. This disclosure excludes some working conditions ranges which are advantageously adopted in the process of the present invention. Description of the invention The process which is the subject or the present patent application can be affirmed - after the brilliant results of months of treatment in commercial-scale plants - to constitute an unobjectionably surpassing of any of the aforementioned methods. When compared with such methods, the invention deepens some of their interesting principles, which are harmonized and rationalized according to an exhaustive scheme, integrated with a great number of elements of an absolutely innovative character. The process is based on the use of a pickling bath containing iron ions, H 2 SO 4 , HF, H 2 O 2 and conventional additives - such as wetting agents, emulsifiers, polishing agents, inhibitors - continuously blown into with a strong air flow. The operating temperature normally ranges from 30°C to 70°C, its value depending to a large extent on the type of steel and on the type of plant, in which connection it is of basic importance that the possibility of performing mechanical descaling upstream of chemical pickling be secured. The basic process features are described hereinafter. Content of inorganic mineral acids in the bath: a solution containing the following is prepared for the pickling bath: at least 150 g/l H 2 SO 4 , preferably 170 g/l, and at least 40 g/l HF, preferably 50 g/l. Several are the functions of both acids: among the most important, those of maintaining process pH below 1.5, preferably from 0 to 1, and of removing the oxides due to heat treatment from the steel surface. Hydrofluoric acid is meant to complex Fe 3+ and Cr 3+ ions as much as possible and depassivate the oxidized material, bringing the electrode potential to an active and/or active/passive dissolution area (see hereinafter). In the absence of hydrofluoric acid, the operating potential rises to the material steady passivity field and descaling practically does not take place. Besides contributing to the total and free acidity of the solution, sulphuric acid exerts a passivating effect similar to the one exerted by nitric acid. During picking the H 2 SO 4 concentration is kept at at least 80 g/l and the HF at at least 25 g/l. Since, in the course of pickling, the contents of the two acids - mainly of hydrofluoric acid - tend to reduce, periodical feeding of same has to be performed as a function of the results of bath analysis (determination of free acidity and fluoride ions), as illustrated in the examples hereinafter. Fe 3+ ion content in the bath : still at the time of bath preparation, the pickling solution contains an amount of Fe 3+ ions not below 15 g/l, added as ferric sulphate: the function of such ions is of replacing - as oxidizer - nitric acid, according to the reaction 2Fe 3+ + Fe -- 3Fe 2+ , favoured by the bath pH conditions. During the process cycle, proper conditions must continuously be secured to allow at least 55% of the total iron dissolved in the bath to be present as Fe 3+ . The oxidation of Fe 2+ to Fe 3+ ions during the process to keep the latter concentration above the minimun preset value is secured by a combined mechanical-chemical action due to the air blown into the bath as well as to H 2 O 2 added continuously to the bath in small quantities. Continuous addition of stabilized hydrogen peroxide: needless to say that to secure process economics it is necessary to use as little hydrogen peroxide as possible. This is why it is very important to use hydrogen peroxide containing a known stabilizer capable of preventing, or at least of reducing significantly, the peroxide decomposition process under the following conditions: temperature up to 70°C, strongly acid bath pH, iron content even exceeding 100 g/l, presence of ions of transition metals such as Ni and Cr - known to be destabilizers. Stabilizers for H 2 O 2 effective in acid medium are for instance: 8-hydroxy-quinoline, sodium stannate, phosphoric acids, salycylic acid, pyridincarboxylic acid. As a particularly suitable stabilizer came out phenacetin (i.e. acetyl-p-phenetidine) used in amount corresponding to 5÷20 ppm to the pickling bath. As this stabilizator undergo a slow decomposition in the pickling bath, a continous or periodical addition of stabilizer to the bath is necessary. The use of duly stabilized H 2 O 2 , combined with the use of air blown into the bath, has made it possible to develop a process based on the use of H 2 O 2 , which has resulted to be economic, an advantage that no known process has ever been capable of offering. The pickling bath is prepared with an initial H 2 O 2 quantity (as 35% by w. (or 130 vol/vol) commercial product) ranging from 1 to 20 g/l, preferably from 2 to 5 g/l. During pickling, continuous H 2 O 2 feeding is adjusted to the type of steel to be pickled, to the surface properties of the manufacture (or semimanufactured product), as well as to the quantity and quality of hot-rolling or annealing scales. The addition of H 2 O 2 during the process cycle is substantially adjusted to the pre-set bath oxidation potential of at least 250 mV and less than 350 mV, which is kept at the pre-set value by the combined action of H 2 O 2 and air blown into. Continuous air blowing: during pickling, a continuous air flow is kept in the bath, in the order of at least 3 m 3 /m 3 bath per pickling hour. The air flow, admitted at a proper rate, favours bath agitation, a major condition for good pickling. Actually, agitation continuously perturbs the liminal layer of the bath, near the surface to be treated, which is thus continuously kept in direct contact with fresh pickling solution. Air blowing into from the vessel bottom, through drilled pipes or proper blowing items, secures excellent mechanical agitation and pickling liquid homogenization. Redox potential control : as is known, stainless steel behaviour in acid mixtures is characterized by polarization curves, which exhibit activity, passivity and transpassivity phases depending on the potential value. The Redox potential is kept at at least 250 mV and less than 350 mV To make sure that descaling proper and a thorough removal of the dechromized alloy take place during pickling, with the restoration of max. surface passivability, the bath has to be placed under potentiostatic control conditions. This means that the operating redox potential has to be adjusted so that during the very pickling step it may remain in the range where the dechromized alloy anodic dissolution rate is the highest when compared with that of the basic alloy. It is possible to pre-set the said range as a function of the steel type, while guaranteeing basic metallic material passivation, after dechromized alloy removal. During pickling, as the bivalent iron ion concentration in the bath rises, the bath redox potential tends to lower, but the addition of hydrogen peroxide and air restores said potential to optimal values, normally higher than 300 mV. In case of any particular upstream steel treatment and if a subsequent passivation stage in a separate bath is envisaged, the pickling bath potential may be kept at lower values, anyway not below 250 mV. A constant potential control, therefore, secures not only good steel pickling, but also the formation of a passivity film on steel. Commercial-scale tests have, in fact, demonstrated the possibility of obtaining polished, bright, and perfectly even surfaces, free from any corrosion phenomenon due, for instance, to pitting, material burning or an excessive pickling action. During pickling bath operation or in case of accidental shutdowns, it is sufficient to guarantee a minimum air blowing to keep the redox potential at optimal values, which makes it possible to leave steel immersed in the solution even for hours with no risk of chemical attack. Additive content in the bath : when preparing the pickling bath according to the present invention, the normal additives used - in a total amount of approx. 1 g/l bath - are non-ionic surfactants acting as wetting agents, emulsifiers, polishing agents, and acid attack inhibitors. Thanks to a synergic action, these additives improve pickling by favouring it. Particularly advantageous additives are perfluorinated anionic surfactants as well as non-ionic surfactants belonging to the polyethoxylated alkanol derivatives class containing 10 or more C atoms. An efficient inhibitor guarantees basic metal protection, reduces losses drastically, and results highly effective mainly in the case of batch processes requiring long pickling time (rods, pipes, bars). The additives present in the bath, particularly acid attack inhibitors, do not inhibit the attack against oxides caused by heat treatment, hence they do not absolutely limit pickling kinetics, as shown e.g. by the results of tests conducted on AISI 304 shot-peened sheet steel, indicated in Table 1. Tests for the evaluation of the inhibiting effect of acid attack inhibitors Tests Conditions Test A) T°C = 50°C; treatment time = 3'; inhibitor quantity: 0.5 g/l Test B) as per A), but in the absence of wetting agents and inhibitors. Results obtained - AISI 304 affected by oxides due to annealing and, therefore, by oxides attack: Test A) = 26.0 g/m2 Test B) = 25.5 g/m2 - AISI 304 with minimum traces of oxides due to annealing, therefore affected by attack of the bare metal: Test A) = 4.0 g/m2 Test B) = 6.0 g/m2 Advantages of the Process Absence of mud: the invention process minimizes or even prevents the formation of mud and sludge, with a consequent clear further saving. Such an advantage is also due to an appropriate HF concentration during the process cycle, as well as to a control of the concentration of ferrous ions, readily and suitably oxidized to ferric ions. Differently from the mud and sludge produced by traditional baths using nitric and hydrofluoric acids, the mud and sludge produced to a greatly smaller extent by the invention process bath are a greenish slush, friable and incoherent in the dry state, with no tendency to packing and lumping into large crystals, consequently easy to remove. Automatic control possibility: the invention process can always be kept under control by automatic means, which - through analytical determinations (free and total acidity, free fluoride ion content, iron ion content, redox potential) - continuously meter the amounts of pickling materials and of stabilized hydrogen peroxide necessary to secure correct operating parameters. The use of said means offers the following advantages: safety and environment: more timely and quicker process parameter adjustment, no risk of pollution, no risk of loss or test sample transfer, smaller amount of products to be eliminated; steady pickling quality thanks to idling absence, close control, and sampling frequency; decrease in costs due to out of standard material reduction and no need for laboratory tests. Process versatility: the invention process suits any existing commercial plant working stainless steel as the required adjustments are quite modest. Furthermore, it is appropriate for the treatment of manufactures and semimanufactured products of any type whatever, from wire to rod, from belts to sheets and pipes, thanks to operating parameters (temperature, times, concentrations) being changeable without detriment of results. A typical example of such a versatility is represented by the continuous application of the invention process to steel rolling units: by merely changing the working potential, the process can, in fact, be used both during the sole pickling stage (in the case of hot-rolled steel), when only descaling and dechromized surface layer removal are required, and during the stages when steel is to be given final passivation too (in the case of cold-rolled steel). The following examples are reported for the sole purpose of illustrating the possible applications of the invention process. Example 1 - Commercial Plants for Continuous Sheet Production Continuous treatment has been carried for four months in commercial plants producing continuous sheets. Example 1.1 In a plant, pickling concerns two hot-rolling lines handling austenitic, martensitic, and ferritic stainless steel. Pickling process conditions are, therefore, a function of the type of steel to be treated and of its physical state, namely of whether steel has undergone mechanical descaling. Moreover, since the lines are meant for hot-rolling, the primary object of pickling is descaling and dechromized alloy removal, rather than final steel passivation. Thus, pickling process conditions are as per the following tables: Austenitic steel, series 300 -shot-peened 1st vessel 2nd vessel Temperature, °C ≤ 55 ≤ 55 H 2 SO 4 , g/l 80-100 80-100 Fe 3+ , g/l 30-50 30-50 Free F - , g/l 25-35 25-35 E redox, mV 300-320 300-320 There are two 25 m 3 pickling vessels and pickling time ranges, on an average, from 60″ to 90″ per vessel. Air is forced continuously into the two vessels, at a rate of 50 m 3 /m 3 /h, along with a continuous feeding of hydrogen peroxide stabilized with Interox S 333 C registered trademark made by Interox. Acid formulations are fed continuously with H 2 SO 4 , HF and the other various additives referred to in on page 12. The amount of steel already treated by the invention process exceeds 350,000 tons, the material to be recycled being below 1% of the total treated material. H 2 O 2 (130 vol.) consumption is 2.3 kg/t treated steel. Example 1.2 In a second plant, this time meant for cold-rolling, over 100,000 t continuous sheets of steel series 300 and series 400 has already been treated as follows: 1st vessel: electrolytic pickling with H 2 SO 4 for 1′ at a temperature from 60°C to 70°C; 2nd vessel: 1′ treatment time, at a temperature from 55°C to 60°C, with the following initial bath: 150 g/l H 2 SO 4 48 g/l HF 15 g/l Fe 3+ 5 g/l H 2 O 2 (130 vol.) 2 g/l H 2 O 2 stabilizer (Interox S 333 C®) 1 g/l various additives (of the type already indicated) 3rd vessel: 1′ treatment time, at a temperature from 55°C to 60°C, bath composition as for 2nd vessel. The working capacity of the 2nd and 3rd vessels is 17 m 3 each. During treatment, air is forced continuously into the 2nd and 3rd vessels, at a rate of 40 m 3 /m 3 /h, along with a continuous feeding of H 2 O 2 (stabilized as indicated above) and of the other ingredients (H 2 SO 4 and HF), so as to keep the following parameters constant: Austenitic steel, series 300 - shot-peened 2nd vessel 3rd vessel Temperature, °C 60-65 60-65 H 2 SO 4 , g/l 100-150 100-150 Fe 3+ , g/l 20-60 15-50 Free F - , g/l 20-30 20-30 E redox, mV 280 ≥ 350 Austenitic steel, series 300 -non-shot-peened 2nd vessel 3rd vessel Temperature, °C 60-65 55-60 H 2 SO 4 , g/l 100-150 100-150 Fe 3+ , g/l 20-60 15-50 Free F - , g/l 30-40 20-30 E redox, mV 280 ≥ 450 Ferritic or martensitic steel, series 400 - shot-peened 2nd vessel 3rd vessel Temperature, °C 50-60 35-50 H 2 SO 4 , g/l 100-150 60 -100 Fe 3+ , g/l 30-80 ≥15 Free F - , g/l 20-30 8 - 15 E redox, mV 250-280 ≥580 The superficial aspect of sheets at the end of the pickling process cycle has always resulted to be polished and bright, even better than secured by the traditional process (HF + HNO 3 ). In this case too, no overpickling or superficial corrosion phenomenon has been recorded. H 2 O 2 (130 vol.) consumption is 2.2 kg/t treated steel. CONCLUSIONS about commercial-scale tests. From the foregoing description and examples it appears evident that the new stainless steel pickling and passivation process, characterized by a bath having a specific composition, by bath control - mainly redox potential control - throughout the pickling cycle, and by continuous air blowing into, represents an optimal solution - from the viewpoint of technical results and process economics (mainly connected with low H 2 O 2 consumption) - of the pollution problem brought about by traditional processes using nitric acid.

Claims: Stainless steel pickling process Consisting in placing the material to be treated in a bath kept at a temperature ranging from 30°C to 70°C, having the following initial composition: a) H 2 SO 4 at least 150 g/l b) Fe 3+ at least 15 g/l c) HF at least 40 g/l d) H 2 O 2 35% by w. added with known stabilizers, 1-20 g/l e) additives of the non-ionic surfactant class (emulsifiers, wetting agents, polishing agents) as well as of the acid attack inhibitor class: approx. in a whole amount of 1 g/l, in the bath being continuously fed : an air flow equal to at least 3 m 3 /h per m 3 bath, through a diffuser distributing the flow in the liquid mass, and if required, quantities of ingredients a) and c) securing a concentration of at least 80 g/l of H 2 SO 4 and at least 25 g/l of HF and a bath pH below 1.5, and of additives e) in order to secure the optimal concentration of 1 g/l (as whole amount), and characterized in that a stabilized H 2 O 2 (35% by w.) is fed continuously in the bath in quantity adjusted to keep the redox potential of the bath at a value of at least 250 mV and less than 350 mV. Process according to claim 1 wherein the amount of H 2 SO 4 and HF fed continously into the bath secure a bath pH value from 0 to 1. Process for pickling stainless steel according to claim 1 wherein the material to be treated is shot-peened austenitic stainless steel, the pickling bath is kept at a temperature lower than or equal to 55°C and wherein an air flow of 50 m 3 /h per m 3 bath is introduced through a diffuser distributing the flow in the liquid mass whereby the bath is maintained during the process in the following ranges: H 2 SO 4 between 80 and 100 g/l Fe 3+ between 30 and 50 g/l free F - (added as HF) between 25 and 35 g/l additives of the non-ionic surfactant class and of acid attack inhibitors approximately in a whole amount of 1 g/l, and furthermore feeding H 2 O 2 into the bath in a controlled amount in order to keep the redox potential at a value of 300-320 mV. Process for pickling stainless steel according to claim 1 wherein the pickling bath is kept at temperature between 50° and 60°C, an air flow of 40 m 3 /h per m 3 bath is introduced through a diffuser distributing the flow in the liquid mass and whereby the bath is maintained during the process in the following ranges: H 2 SO 4 between 100 and 150 g/l Fe 3+ between 30 and 80 g/l free F - (added as HF) between 20 and 30 g/l and furthermore feeding H 2 O 2 into the bath in a controlled amount in order to keep the redox potential at a value of 250-280 mV.

Inventors: BIANCHI MARCO, BIANCHI, MARCO
Applicants: HENKEL KGAA, HENKEL KOMMANDITGESELLSCHAFT AUF AKTIEN
Application year: 1993
Main classification: C23G1
IPC codes: C23G1, C23C22