MXPA98010683A - Passivation of stainless steels in an acid medium organosulfon - Google Patents

Abstract

To prevent corrosion of stainless steels in an organosulfonic acid medium, at least one oxidizing agent selected from the group is added enoxides or salts, nitrites and persulfates of cerium (IV), iron (III), molybdenum (VI) or vanadium (V). ) in a sufficient quantity to place the spontaneous potential between passivation and transpasivation potentials

Description

PASSIVATION OF STAINLESS STEELS IN AN ORGANOSULPHONIC ACID MEDIUM Field of the Invention The present invention relates to the field of stainless steels and also to that of organosulfonic acids. The invention relates more particularly to the protection of stainless steels against corrosion by organosulfonic acids such as methanesulfonic acid. STATE OF THE ART Methanesulfonic acid (MSA) is a strong acid that has found many applications, in particular in catalysts and in the treatment of surfaces (electroplating, pickling, descaling, etc.). However, aqueous solutions of MSA attack stainless steels; the corrosion rates depend, simultaneously, on the MSA concentration, the temperature and the nature of the stainless steel. Thus, at room temperature, type 304L stainless steel can be corroded with concentrations of MSA greater than 10"2 mol / liter, which clearly limits the fields of use of the MSA, in order to protect stainless steels. against corrosion by sulphonic acids (in particular p-toluenesulfonic acid and polystyrenesulfonic acid), it has been proposed in patent application JP 07-278 854 to add a copper salt to said acids, said document being directed more particularly towards the protection of the installation made of stainless steel (types 304 and 316) used in plants for the synthesis of alcohols from olefins and water in the presence of an organosulphonic acid as a catalyst The temperature range illustrated in this document ranges from the temperature environment at approximately 100 ° C. In the article entitled "Corrosion of stainless steel during acétate production" published in July 1996 in the magazine Co rrosion Engineering Vol. 2, No. 7, page 558, J.S. Qi and J.C. Lester indicates that the use of copper sulfate during esterification in the presence of sulfuric acid or p-toluenesulfonic acid allows a considerable reduction in the corrosion of stainless steels 304L and 316L. However, static tests carried out on MSA and copper (II) salt compositions at temperatures between 100 and 150 ° C, show that a thin layer of relatively non-adherent copper metal is formed on the surface of the tested materials (AISI 304L and 316L) During the industrial use of this method, sedimentation of copper metal particles was actually observed at the bottom of the reactor, whose particles are prone to cause serious damage to the recycling pumps or are prone to damage the quality of the In this way, it is necessary to perform an additional filtration step in order to separate these copper particles from the film deposited on the walls of the reactor, in fact, during the changes of operating conditions (for example, temperature, pressure, stirring speed), this protective film comes off very easily.It has now been found that stainless steels can be protected in a effective, over a wide range of temperatures, against corrosion by organosulfonic acids and in particular by MSA, by adding to the medium an oxidizing agent chosen from oxides or salts, nitrites and persulfates of cerium (IV), iron (III), molybdenum (VI) ) or vanadium (V). SUMMARY OF THE INVENTION Therefore, the object of the present invention is a process for protecting stainless steels against corrosion by an organosulfonic acid, characterized in that at least one oxidizing agent chosen from oxides or salts is added to the aqueous solution of organosulfonic acid, nitrites and persulfates of cerium (IV), iron (III), molybdenum (VI) or vanadium (V). The object of the invention is also an aqueous solution of organosulfonic acid containing at least one oxidizing agent selected from among oxides or salts, nitrites and persulfates of cerium (IV), iron (III), molybdenum (VI) or vanadium (V), in an amount that is sufficient for its spontaneous potential, measured using a stainless steel electrode, is within the passivation zone determined under the same conditions in the absence of the oxidizing agent. DETAILED DESCRIPTION OF THE INVENTION Stainless steels are passivable materials. Physically, passivation is due to the formation of an oxide layer on the metal surface. The passivation is finally imparted to the alloy by the development of an adhesive layer which is relatively thin but of very low ionic permeability. The transfer of cations from the metal to the solution can be considered as being very considerably slow and, in certain cases, practically negligible. Actually, the phenomenon of passivation must be considered as a state of dynamic equilibrium. The rate of dissolution (v) of a stainless steel immersed in a medium such as an aqueous solution IM of MSA depends on the established electrochemical potential E. The curve v = f (E) has a typical shape which, as shown in the single attached figure, essentially comprises three parts, namely: an area of "activity" 1 corresponding to the anodic dissolution of the metal (oxidation); - a zone of "passivation" 2 located between a passivation potential (Ep) and a transpassivation potential (Etp); - a "transpassivation" zone 3 where the metal again becomes active by oxidation of the passive film in a soluble substance (Cr203 solution as Cr042 '). In the passivation potential Ep, the corrosion rate falls sharply to a very low value. In Zone 2, the very low dissolution rate corresponds to a region of corrosion resistance. The measurement of the spontaneous potential and its comparison with Ep and Etp makes it possible to determine instantaneously whether the stainless steel is corroding or not. Whenever it is soluble in organosulfonic acid or in the aqueous solution of organosulfonic acid, the nature of the oxidizing agent chosen is not critical and thus any oxide or salt of cerium (IV), iron (III), molybdenum ( VI) or vanadium (V) as well as any soluble nitrite or persulfate. The following are more particularly preferred: - nitrites of alkali metals, ammonium or copper and more especially sodium nitrite; - double ammonium-cerium (IV) salts such as nitrate or ammonium-cerium sulfate. As non-limiting examples of other oxidizing agents according to the invention, iron (III) sulfate, ferric chloride, ferric nitrate, ferric perchlorate, ferric oxide, sodium molybdate, ammonium molybdate tetrahydrate, molybdenum oxide, sodium metavanadate, oxytrichloride may also be mentioned. of vanadium, vanadium pentoxide, sodium persulfate and ammonium persulfate. The amount of oxidizing agent to be used according to the invention can vary within a wide range; it depends, inter alia, on the nature of the oxidizing agent and on the concentration of organosulfonic acid. When a salt is used, the concentration of Ce4 + ions is generally between 1 x 10"5 and 1 x 10" 1 moles / liter; preferably between 1 x 10"4 and 5 x 10" 2 moles / liter. When a nitrite or other oxidizing agent is used, the amount used is generally between 1 x 10"4 and 1 mol / liter, preferably between 0.001 and 0.5 mol / liter, a particularly advantageous way of carrying out the process according to the invention. The invention consists in associating a salt of molybdenum (VI), preferably sodium molybdate, with a salt of cerium (IV), preferably a double salt of ammonium-cerium (IV) .The amount to be used of each of the salts can vary Within a wide range, but preferably it is between 1 x 10"3 and 2 x 10" 2 moles / liter and more particularly between 5 x 10"3 and 1 x 10" 2 moles / liter, although the process according to the invention is directed more especially to the protection of common stainless steels (such as AISI 304L and 316L), it can be applied generally to any stainless steel as defined in the standard NF EN 10088-1 The invention relates more particularly to methanesulfonic acid ( MSA) .The process of pro However, the invention according to the invention can be applied, however, to other alkanesulfonic acids, for example, ethanesulfonic acid, or to aromatic sulfonic acids such as p-toluenesulfonic acid (PTSA). In the following examples, which illustrate the invention without thereby limiting it, electrochemical and static tests were carried out by operating as follows. 1. Electrochemical tests The test consists of immersing an electrode constituted by the test material in the test solution and verifying that its spontaneous potential, under stable conditions, is actually in the passivation region. Before the test, a polarization is carried out in the cathode region for 30 seconds. The electrolysis cell consists of a container that can contain 80 ml of the test solution and that allows the installation of 3 electrodes: a reference electrode (Ag / Ag Cl of the Thermag-Tacussel type), an auxiliary electrode (platinum) and a working electrode (stainless steel test). 2. Static tests These tests make it possible, on the one hand, to verify the passivation of the materials and, on the other hand, to calculate the corrosion rate. The study of the corrosion by loss of mass is made from metal plates that are cut using a lubricated disc saw. The surface area of these cut lengths is accurately calculated, with approximate dimensions of 25 x 50 x 2 mm. Said lengths of cut metal are drilled with a hole of 6.5 mm in diameter, so that they can be joined to a Teflon sample holder. Before immersing them in the MSA test solution, the cut lengths are degreased with acetone, pickled in an aqueous solution containing 15% nitric acid and 4.2% sodium fluoride, rinsed with demineralised water and then with acetone, dried with oil-free and heavy compressed air. After immersion for 8 or 30 days in the MSA test solution, the cut lengths are washed with demineralized water and then with acetone, weighed, freed from any deposit (corrosion products) by mechanical cleaning, and weighed again. The loss of mass, expressed in g / m2. day, allows to calculate the corrosion rate, expressed in mm / year.

EXAMPLE 1 Since the electrochemical tool is particularly suitable for checking the passive states of stainless steels, electrochemical tests were carried out at 45 and 90 ° C for an MSA concentration of 2.08 M and for two grades of stainless steel ( AISI 304 L and 316L) previously submitted to an over-hardening treatment according to NF A35-574. The corrosive baths consisted of MSA aqueous solutions at 2.08 mol / liter containing varying amounts of sodium nitrite or ammonium-cerium (IV) nitrate. The obtained results are collected in the following Tables I and II, which indicate, in mV, the passivation, spontaneous and transpasivation potentials (E).

TABLE I Electrochemical Tests in MSA 2.08 M for 316L Stainless Steel TABLE II Electrochemical tests in MSA 2.08 M for 304L stainless steel The spontaneous potential is always between the potentials of passivation and transpasivation. In this way, the risks of generalized corrosion are negligible. EXAMPLE 2 In order to extend the results of Example 1, static tests were carried out at 150 ° C. The results are shown in the following Table III.

TABLE III Static tests at 150 ° C in MSA 2.08 M EXAMPLE 3 Working as in Example 1, the protective effect of other species for 316L stainless steel was studied. These tests and their results are shown in the following Table IV.

TABLE IV EXAMPLE 4 Using a 70% aqueous solution of MSA and a 65% aqueous solution of PTSA, three aqueous solutions S ^ S2 and S3 were prepared having the following composition by weight: Two oxidizing agents were used together: - Ox.l = ammonium nitrate-cerium (IV) - Ox.2 = sodium molybdate in variable proportions (5 to 10 mmol / l) to pass stainless steels 304L and 316L at different temperatures (45 , 90 and 150 ° C) in the Sx, S2 and S3 solutions. Operating as in the previous examples, passivation, spontaneous and transpasivation potentials were measured. The results obtained are collected in the following Tables V and VI. It can be seen that the spontaneous potential is always between the potentials of passivation and transpasivation. The risks of widespread corrosion are thus negligible.

TABLE V Stainless steel 304L TABLE VI Stainless steel 316L EXAMPLE 5 Static corrosion tests at 45 ° C (duration: 8 days) were carried out in more or less dilute aqueous solutions of MSA. These solutions were prepared by adding water to a 70% solution of MSA containing 5 mmol / liter of ammonium cerium (IV) nitrate and 5 mmol / liter of sodium molybdate. For comparative purposes, static tests were carried out simultaneously with aqueous MSA solutions without oxidizing agents. In the following Tables VII and VIII, which summarize the results obtained, the number shown in the column "DILUTION" indicates the proportion (% by volume) of 70% MSA in the aqueous solution of the test.

TABLE VII 304L Stainless Steel TABLE VIII 316L Stainless Steel

Claims (12)

1. CLAIMS Having described the present invention, it is considered as novelty and, therefore, claimed as property contained in the following claims: 1. - Process for protecting stainless steels against corrosion by an organosulfonic acid, characterized in that the aqueous solution of said acid is added a sufficient amount of at least one oxidizing agent chosen from among oxides or salts, nitrites and persulfates of cerium (IV), iron (III), molybdenum (VI) or vanadium (V).
2. 2. Process according to claim 1, characterized in that an alkali metal nitrite, preferably sodium nitrite, is used.
3. 3. Process according to claim 2, characterized in that the amount of nitrite is between 1 x 10"4 and 1 mol / liter, preferably between 0.001 and 0.5 mol / liter. - Process according to claim 1, characterized because the cerium (IV) is used in the form of a double salt of ammonium-cerium (IV), preferably nitrate or ammonium-cerium sulfate 5. Process according to claim 4, characterized in that the concentration of Ce + ions is between 1 x 10"5 and 1 x 10" 1 moles / liter, preferably between 1 x 10"4 and 5 x 10" 2 moles / liter. 6. Process according to claim 1, characterized in that a salt of molybdenum (VI), preferably sodium molybdate, is associated with a salt of cerium (IV), preferably a double salt of ammonium-cerium (IV). 7. Process according to claim 6, characterized in that the amount of each salt is between 1 x 10'3 and 2 x 10"2 moles / liter, more particularly between 5 x 10" 3 and 1 x 10'2 moles / liter. 8. Process according to any of claims 1 to 7, characterized in that the organosulfonic acid is methanesulfonic acid. 9. An aqueous solution of organosulfonic acid, characterized in that it contains at least one oxidizing agent selected from among oxides or salts, nitrites and persulfates of cerium (IV), iron (III), molybdenum (VI) or vanadium (V), in a quantity that is sufficient for its spontaneous potential, measured using a stainless steel electrode, to be within the zone of passivation determined under the same conditions in the absence of the oxidizing agent. 10. An aqueous solution according to claim 9, characterized in that the oxidizing agent is an alkali metal nitrite, preferably sodium nitrite, or a double salt of ammonium-cerium (IV), preferably nitrate or ammonium-cerium sulfate. 11. An aqueous solution according to claim 9, characterized in that it contains a salt of molybdenum (VI), preferably sodium molybdate, and a salt of cerium (IV), preferably a double salt of ammonium-cerium (IV). 12. An aqueous solution according to any of claims 9 to 11, characterized in that the organosulfonic acid is methanesulfonic acid.