WO1994024333A1

WO1994024333A1 - Method of increasing the corrosion resistance of stainless steel

Info

Publication number: WO1994024333A1
Application number: PCT/EP1994/001090
Authority: WO
Inventors: Wolfgang Preibsch
Original assignee: Henkel-Ecolab Gmbh & Co. Ohg
Priority date: 1993-04-16
Filing date: 1994-04-08
Publication date: 1994-10-27
Also published as: DE4312417A1

Abstract

The invention concerns a method which does not use chromium for the production of a coloured corrosion-protection layer on stainless steel with a molybdenum content of less than 1 % by wt. The method is characterized in that the surface of the steel is treated with an aqueous alkali solution at temperatures between 50 °C and 240 °C, the solution containing at least one complexing agent and at least one oxidizing agent in completely demineralized water and having a pH between 12 and 14.

Description

"Process for increasing the corrosion resistance of stainless steel"

The invention relates to a chemical process for increasing the corrosion resistance of stainless steel.

In general, steels are referred to as non-rusting or rust-free, in which rust formation is prevented under usual environmental conditions such as atmospheric oxygen, moisture and in aqueous solutions. The mostly higher-alloyed so-called corrosion-resistant or acid-resistant steels resist harsh corrosion conditions such as acids and salt solutions. In summary, these steels are called stainless steels. In Ullmann's Encyclopedia of Technical Chemistry, 4th edition, volume 22, pp. 106-112 and in the German industrial standard DIN 17440, July 1985, there is a list of the technically most important stainless steels with their material numbers, designations and alloy components as well as mechanical and chemical Features included.

The higher proportion of alloy components, in particular the addition of molybdenum, leads to a higher price of the corrosion-resistant compared to the stainless steel. For the handling of corrosive media such as acids, however, it is necessary to use high-alloy steels. An alternative to this would be a chemical surface treatment which gives low-alloy steels such as V2A steel (material number 1.4301) a corrosion resistance which is characteristic of high-alloy steels such as V4A (material number 1.4571). Steels according to DIN 17440 which contain less than 1% by weight of molybdenum are referred to as low-alloyed in the sense of the invention. Steels according to DIN 17440 that contain at least 1% by weight of molybdenum are referred to as high alloys. However, chemical surface treatment of stainless steels to increase the corrosion resistance is not common in technology, rather the corrosion protection requirements are taken into account by choosing a suitable alloy. In contrast, the conversion treatment of stainless steel surfaces with oxalic acid is known in order to deposit an oxalate layer which acts as a lubricating film in non-cutting forming processes. This treatment does not have the effect of increasing corrosion resistance. A dyeing with subsequent hardening, in addition to the desired color effect, also increases the corrosion resistance ("stainless steels", publisher: Stainless Steel Association with the Association of German Ironworkers, Verlag Stahl Eisen, 2nd edition 1989, p. 227). For this purpose, the object is first immersed in a strongly sulfuric acid solution of chromium (III) oxide at 65 to 95 ° C. and then switched cathodically in a bath containing chromic acid and phosphoric acid.

Annealing colors are also produced on stainless steels by annealing in air, the color scale of which depends on the respective annealing temperature and their intensity as well as on the annealing duration. They occur at temperatures from around 250 ° C. The coloring begins with a yellow tone. When the temperature increases, a yellow-brown color occurs. Further temperature increases cause blue, red and violet tints. However, the oxidation of the surface carried out in this form greatly reduces the corrosion resistance of the steel. Products colored in this way can therefore not be used either in the outside atmosphere or when exposed to aggressive media ("stainless steels", loc. Cit. P. 226).

Furthermore, a passivating aftertreatment of pickled stainless steels is known ("stainless steels", loc. Cit. P. 204). When pickling, the surface of the stainless steels is activated and an attack occurs on the material surface underneath the scale. After pickling, the passive layer is formed (oxygen adhering to the surface), through which the material obtains its corrosion resistance. This process also takes place under the influence of oxygen in the air. If the oxygen supply is increased due to an oxidizing acid, the passive layer is formed faster and more densely. For this reason, it is common practice to immerse stainless steel after a pickling treatment for passivation in an approx. 20% nitric acid solution at room temperature and then rinse with water.

Treatment with warm, alkaline, complexing agent-containing solutions is known for the passivation of normal steel, in particular of heat exchanger components. For example, DE-B-1521732 describes a process for cleaning and passivating heat exchangers, in which, after removing scale and copper deposits, the bright steel at a temperature between 37.5 and 90.5 ° C. and an alkaline pH Value, preferably between 7.5 and 10, is treated with a solution of a complexing agent, for example an EDTA salt, under oxidizing conditions (atmospheric oxygen). A similar process is described in DD-A3-24 270, where passivation of the cleaned heat exchanger surface takes place by treatment with an alkaline oxidizing complexing agent solution. Further similar methods for the same purpose are given in DE-A-1546116. Based on mechanistic studies (WW Frenier and WC Kennedy: "Passivation of Steel in Ammonium Ethylenediamine Tetraacetic Acid Solutions", Corrosion 42 (1986), 613-622), the use of strong oxidizing agents such as hydrogen peroxide or air / sodium nitrite is recommended . In all these cases, the formation of a passive layer can be attributed to the production of iron oxides.

Related to this are the "browning" processes for browning and blackening steel (see Ull ann's Encyclopedia of Industrial Chemistry, Volume 16, 4th Edition 1978, p. 549). They are essentially based on the treatment of steel parts in strongly alkaline and oxidizing solutions at temperatures of around 150 ° C, whereby layers of up to 2 μm thick are mainly made of Fe3Ü4. The same reference mentions the blackening of stainless steel by treatment with a solution of sodium dichromate in sulfuric acid.

Under certain conditions, colored deposits on machine internals and washware made of stainless steel may appear to be disruptive in machine dishwashing. Analytical studies on this (J. Küpper, B. Egert and H.-J. Grabke, "Origin and structure of colored layers on Stainless steel: II. Discoloration under the conditions of a dishwasher ", materials and corrosion 34 (1983), 84-88) make it probable that the optically disturbing color effects in this special case are due to thin layers of titanium oxides. For their formation it is assumed that under the oxidizing conditions of a rinse cycle and especially in the presence of citric acid all alloy components except titanium are removed from the stainless steel surface, according to DIN 17440, p , according to the results of Küpper et al, loc. cit. p. 85, however, about 0.03% Ti is found.

The object of the invention is to achieve the corrosion resistance of high-alloy steels by means of a chrome-free chemical surface treatment of low-alloy stainless steels.

The invention accordingly relates to a chromium-free process for producing a colored corrosion-protective layer on stainless steel, which has a molybdenum content of less than 1% by weight, characterized in that the steel surfaces are treated with an aqueous alkaline solution at temperatures between 50 ° C and 240 ° C treated, which contains at least one complexing agent and at least one oxidizing agent in deionized water and has a pH between 12 and 14.

The object is therefore achieved by treating the surface of low-alloy steels with a solution containing complexing agents and oxidizing agents in demineralized water with a pH of at least 12 at temperatures between 50 ° C. and 240 ° C. The treatment time at a given treatment temperature depends on the complexing agent used and on the desired layer thickness of the corrosion-protective surface layer to be built up and is between about one and about 48 hours. Depending on the thickness of the corrosion-protective layer, the surface takes on a color from golden yellow to slightly brownish, violet, deep brown to black. Because of the lower apparatus requirements, an unpressurized process control at temperatures between 50 and 100 ° C. is preferred. When the process is carried out in a pressure reactor at temperatures between 100 and 240 ° C. under the water however, treatment times can be shortened due to pressure. The method is applicable to ferritic, martensitic and austenitic steels.

The alkaline pH of the treatment solution can be adjusted with alkali metal hydroxide, with ammonia or with organic amines. For odor and work physiological reasons, alkali metal hydroxides and in particular sodium hydroxide are preferred.

A large number of complexing agents suitable according to the invention are known. They can belong to different chemical groups. The following are suitable individually or in a mixture:

a) hydroxypolycarboxylic acids such as tartaric acid and citric acid, b) nitrogen-containing mono- or polycarboxylic acids such as ethylenediaminetetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid, diethy- lentria inpentaacetic acid, hydroxyethyliminodiacetic acid, nitrilodiacetic acid, 3-n-propanoic acid, n-propanoic acid, (ß-hydroxyethyl) glycine, N- (1,2-dicarboxy-2-hydroxyethyl) glycine, N- (1,2-dicarboxy-2-hydroxyethyl) aspartic acid or nitrilotriacetic acid (NTA), c ) geminal diphosphonic acids such as l-hydroxyethane-ll-diphosphonic acid (HEDP), their higher homologues with up to 8 carbon atoms and derivatives thereof containing hydroxyl or amino groups, and 1-aminoethane-1,1-diphosphonic acid, their higher homologues with up to 8 Carbon atoms and derivatives thereof containing hydroxyl or amino groups, d) aminophosphonic acids such as ethylenediaminetetra (methylenephosphonic acid), diethylenetriapenta (ethylenephosphonic acid) or nitrilotri (methylenephosphonic acid) s owie e) phosphonopolycarboxylic acids such as 2-phosphonobutane-l, 2,4-tricarboxylic acid.

Complexing agents from the group of nitrogen-containing polycarboxylic acids, in particular EDTA, are preferred. At the alkaline pH values of the treatment solutions required according to the invention, these Komolex formers are at least partially present as anions. It is immaterial whether they are introduced in the form of acids or in the form of water-soluble salts. In the case of use as salts, alkali, ammonium or alkylammonium salts, in particular sodium salts, are preferred. Suitable oxidizing agents are peroxides such as hydrogen peroxide and alkali or alkaline earth peroxides, salts forming hydrogen peroxide and compounds such as percarbonates, in particular sodium percarbonate, perborates and salts of peroxo acids such as peroxomonosulfuric acid (Caro's acid) or peroxodisulfuric acid. Salts of oxidizing acids without a peroxo group, such as chlorates or bromates, are also suitable. The oxidizing agents can be used individually or in a mixture with one another.

The effective concentrations of the complexing agents in the treatment baths are in the range of 0.01 to 0.075 mol / 1. They depend on the "denticity" of the chelating molecules. For hexadentate complexing agent molecules such as EDTA, the effective concentrations are in the range from 0.01 to 0.05 mol / 1, preferably 0.02 to 0.04 mol / 1. Correspondingly higher concentrations should be selected for complexing agent molecules with lower denticity. For example, for the preferably tidentate coordinating nitrilotriacetic acid, the effective concentrations are in the range 0.015 to 0.075 mol / 1, preferably 0.03 to 0.06 mol / 1. Tetrasodium EDTA as a preferred complexing agent is therefore particularly effective in concentrations between 0.7 and 1.3% by weight.

The concentrations of the H2O2-releasing oxidizing agents are selected such that the calculated total concentration of H2O2 is between 0.05 and 0.5 mol / 1, preferably between 0.08 and 0.15 mol / 1. Sodium perborate is preferably used, for example as sodium perborate monohydrate ("NaBθ3.H2θ"). Concentrations of about 1 to 5% by weight are particularly effective. Salts of oxidizing acids without a peroxo group, in particular chlorates or bromates, are used in concentrations between 0.05 and 0.5 mol / l, preferably 0.08-0.15 mol / 1. Higher concentrations are possible, but have no further advantage.

The amount of alkali required to adjust the pH depends on whether the complexing agents and / or the oxidizing agents are introduced as salts or as free acids. It is to be chosen so that the pH range according to the invention is adjusted from 12 to 14, preferably 12.5 to 13.5. For example, if you use a solution in deionized water that contains 1% by weight of tetrasodium EDTA and 1% by weight of sodium perborate monohydrate. holds, the required amount of NaOH is at least 2% by weight. Higher pH values accelerate the layer formation.

Examples 1-5, Comparative Examples 1-3

Test sheets made of V2A steel (material no. 1.4301) were immersed at a temperature of 80 ° C for one, ten and 24 hours in sample and comparison solutions of the composition given in Table 1. To prepare the solutions, complexing agents and oxidizing agents were dissolved in demineralized water and the specified pH was adjusted by adding 50% by weight sodium hydroxide solution. At the end of the treatment period, the sheets were rinsed with deionized water and dried with compressed air. The color of the sheets, which indicates the formation of the corrosion protection coating, was assessed optically. The results are shown in Table 1.

To test the corrosion resistance of the coatings and the comparative sheets according to comparative examples VI to V3, the sheets were dissolved in an aqueous solution of 1% by weight (NH4) HF2 and 3% by weight of citric acid, pH = 2.5 Suspended room temperature and the surface examined after 6 hours. The results are shown in Table 2.

Example 6

Test sheets made of V2A steel (material no. 1.4301) were placed in a nickel autoclave together with 1 1 solution of the composition according to Example 2. The autoclave was heated to 190 ° C within one hour (autogenous pressure 14-15 bar), held at this temperature for different times and then cooled to approx. 90 ° C within 45 minutes. The sheets were removed, cooled and rinsed by immersion in demineralized water, dried with compressed air and the color was assessed. Result :

Holding time at T = 190 ° C color

0 minutes brown 30 minutes dark brown 90 minutes black

Table 1

Table 2

Sheet from example treatment time Appearance after corrosion test

1 lh rough, pitting

10h matt, attacked

24h bright yellow blank

2 lh bright yellow blank

10h unchanged

24h unchanged

3 lh rough, pitting

10h bright yellow blank

24h unchanged

4 lh unchanged

10h unchanged

24h unchanged

5 lh light yellow matt

10h unchanged

24h unchanged

VI 24h rough, pitting

V2 24h rough, pitting

V3 24h rough, pitting

Claims

1. Chromium-free process for producing a colored corrosion protection layer on stainless steel, which has a molybdenum content of less than 1 wt .-%, characterized in that the steel surfaces with an aqueous alkaline solution at temperatures between 50 ° C and Treated 240 ° C, which contains at least one complexing agent and at least one oxidizing agent in deionized water and has a pH between 12 and 14.

2. The method according to claim 1, characterized in that to adjust the pH from 12 to 14 alkali metal hydroxides and / or ammonia and / or organic amines, preferably sodium hydroxide, are used.

3. The method according to one or both of claims 1 and 2, characterized ge indicates that the pH of the solution is between 12.5 and 13.5.

4. The method according to one or more of claims 1 to 3, characterized ge indicates that the complexing agents are selected from at least one of the groups a) hydroxypolycarboxylic acids, b) nitrogenous mono- or polycarboxylic acids, c) geminal diphosphonic acids, d) aminophosphonic acids , e) phosphonopolycarboxylic acids, and water-soluble salts of the acids of groups a) to e).

5. The method according to claim 4, characterized in that ethylenedia intetraacetic acid or its water-soluble salts are used as complexing agents.

6. The method according to one or more of claims 1 to 5, characterized ge indicates that the concentration of the complexing agent is altogether between 0.01 and 0.075 mol / 1 and for hexidentate complexing agent concentrations between 0.01 and 0.05 mol / l, in particular 0.02-0.04 mol / 1, for tidentate complexing agents concentrations between 0.015 and 0.075, in particular between 0.03 and 0.06 mol / 1, are preferred. 7. The method according to claim 6, characterized in that as a complexing agent tetrasodium EDTA in concentrations between 0,

7 and 1.3 wt .-% is used.

8. The method according to one or more of claims 1 to 7, characterized ge indicates that the oxidizing agent is selected from one or more of the groups: a) peroxides, in particular hydrogen peroxide and alkali or alkaline earth metal peroxides, b) hydrogen peroxide-forming salts and compounds , in particular percarbonates, perborates and salts of peroxo acids, in particular peroxomonosulfuric acid (Caro's acid) or peroxodisulfuric acid, c) salts of oxidizing acids without peroxo group, in particular chlorates or bromates.

9. The method according to claim 8, characterized in that Oxidations¬ agents from group a) or b) are used in a concentration that a calculated total concentration of H2O2 between 0.05 and 0.5 mol / 1, preferably between 0, 08 and 0.15 mol / 1 corresponds and that when using one or more oxidizing agents from group c) their total concentration is between 0.05 and 0.5 mol / 1, preferably between 0.08 and 0.15 mol / 1 lies.

10. The method according to one or both of claims 8 and 9, characterized ge indicates that sodium perborate is used as the oxidizing agent.

11. The method according to any one of the claims 1 to 10, characterized ge indicates that the temperature of the solution is between 50 and 100 ° C.