DE4312417A1 - Method for increasing the corrosion resistance of stainless steel - Google Patents

Description

The invention relates to a chemical method for increasing the corro Resistance to corrosion of stainless steel.

As stainless or stainless steels in general language steels in which under normal environmental conditions such as atmospheric oxygen, Moisture and in aqueous solutions, the rust formation is prevented. Harsher corrosion conditions such as acids and salt solutions resist the usually higher alloyed so-called corrosion resistant or acid-resistant steels. In summary, these steels are called Stainless steels referred. In Ullmanns Encyklopadie der technischen Chemie, 4. Edition, Volume 22, pp. 106-112 and in the German Industrial Standard DIN 17440, July 1985, is a listing of the most technically important noble steels with their material numbers, designations and alloy components as well as mechanical and chemical properties.

The higher proportion of alloy constituents, in particular addition of Mo lybdenum, leads to a higher price of corrosion-resistant the stainless steel. For handling corrosive media such as For example, acids, it is necessary to use high-alloyed steels Zen. An alternative would be a chemical surface treatment in Question, the low-alloyed steels such as V2A steel (Werk Substance number 1.4301) gives a corrosion resistance, as for high-alloy steels such as V4A (material number 1.4571) cha is characteristic. As low alloy within the meaning of the invention are steels according to DIN 17440, containing less than 1 wt .-% molybdenum. As high-alloy steels are designated according to DIN 17440, the at least 1 wt .-% molybdenum.  

A chemical surface treatment of stainless steels to increase the Kor rosionswiderstandes is not common in the art, but rather contributes To meet the corrosion protection requirements by choosing a suitable Legie invoice. In contrast, the conversion treatment of stainless steel Surfaces known with oxalic acid, one in non-cutting forming processes to deposit as oxalate layer acting as a lubricating film. This treatment has not the effect of increasing corrosion resistance. By a coloring In addition to the desired color effect, the treatment with subsequent hardening also becomes achieved an increase in corrosion resistance ("stainless steels", Publisher: Edelstahl-Vereinigung with Verein Deutscher Eisenhüttenleute, Verlag Stahl Eisen, 2nd edition 1989, p. 227). This is the subject first in a strong sulfuric acid solution of chromium (III) oxide at 65 to 95 ° C. dipped and then in a chromic acid and phosphoric acid halti bath cathodically switched.

Annealing in the air also causes rusting on stainless steels colors whose color scale depends on the respective annealing temperature and their intensities depends on the annealing time. They come off at temperatures about 250 ° C. The color begins with a yellow tone. At tempera increase in color, a yellow-brown coloration occurs. Further temperature increase conditions cause blue, red and purple tints. The in this form made oxidation of the surface, however, sets the corrosion resistance severity of the steel. Such colored products can be of neither in the outside atmosphere nor when exposed to aggressive Media are used ("stainless steels", loc cit, p 226).

Furthermore, a passivating after-treatment is pickled stainless Steels known ("Stainless steels", loc cit, p. When pickling the surface of the stainless steels is activated and it enters Attack on the surface of the material under the scale. After this Pickling is the formation of the passive layer (adhesive to the surface accumulated oxygen), through which the material its corrosion resistance receives. This process also takes place under the influence the oxygen of the air. With increased oxygen supply by a oxidizing acid takes place the formation of the passive layer fast ler and denser. For this reason, it is generally common to produce  of stainless steels after a pickling treatment for passivation in to dip an approximately 20% nitric acid solution at room temperature and then rinse with water.

For passivation of mild steel, in particular heat exchanger installation The treatment involves warm, alkaline, complexing agent-containing Known solutions. For example, DE-B-15 21 732 describes a method ren for cleaning and passivation of heat exchangers, in which after Entfer of scale and copper deposits of bare steel in one Temperature between 37.5 and 90.5 ° C and an alkaline pH, before preferably between 7.5 and 10, with a solution of a complexing agent, for example, an EDTA salt under oxidizing conditions (Luftsau erstoff) is treated. A similar process is described in DD-A3-2 44 270, where the passivation of the cleaned heat exchanger surface by Treatment with an alkaline oxidizing complexing agent solution takes place. Other similar procedures for the same purpose are described in the DE-A-15 46 116 indicated. Due to mechanistic studies (W.W. Frenier and W.C. Kennedy: "Passivation of Steel in Ammonium Ethylenediami netetraacetic Acid Solutions ", Corrosion 42 (1986), 613-622), Ver strong oxidizing agents such as hydrogen peroxide or air / sodium nitrite recommended. In all these cases, the training of a pas be due to the production of iron oxides.

Related to this are the procedures called "blacking" Brown and black color of steel (see Ullmanns Encyklopadie der Technical Chemistry, Volume 16, 4th edition 1978, p 549). They are based in essential to the treatment of steel parts in strongly alkaline and oxidizing solutions at temperatures of about 150 ° C, with up to 2 microns thick layers of mainly Fe₃O₄ be generated. The same reference Renz mentions the blackening of stainless steel by treatment with a Lö solution of sodium dichromate in sulfuric acid.

Under certain conditions, machine dishwashing may be considered Disruptive colored coatings on machine installations and items to be washed out Stainless steel occur. Analytical investigations (J. Küpper, B. Egert and H.-J. Grabke, "Origin and structure of colored layers  stainless steel: II. discoloration under the conditions of a harness Dishwasher ", Materials and Corrosion 34 (1983), 84-88) make it true Apparently, that the visually disturbing color effects in this particular Fall due to thin layers of titanium oxides. For your Formation is believed to occur under the oxidizing conditions of a Rinse cycle and especially in the presence of citric acid all Legie except for titanium removed from the stainless steel surface become. According to DIN 17440, p. 8, the investigated material no. 1.4301 Although not containing titanium, according to the results of Küpper et. al., loc. cit. P. 85, however, about 0.03% Ti is found.

The invention is based on the object by a chromium-free chemical Surface treatment of low-alloy stainless steels the Kor rosion resistance of high-alloyed steels.

The invention accordingly relates to a chromium-free process for the production a colored anti-corrosive layer on stainless steel, the has a molybdenum content of less than 1 wt .-%, characterized gekenn records that the steel surfaces with an aqueous alkaline Lö treated at temperatures between 50 ° C and 240 ° C, the at least a complexing agent and at least one oxidizing agent in deionized Contains water and has a pH between 12 and 14.

The object is thus achieved by lowering the surface gated steels with a complexing agent and oxidizing agent-containing Solution in deionized water with a pH of at least 12 at Temperatures between 50 ° C and 240 ° C treated. The treatment duration depends on the used at a given treatment temperature Complexing agent and according to the desired layer thickness of the to be built anticorrosive surface layer and is between about one and about 48 hours. Depending on the thickness of the anti-corrosive layer takes the Surface a color of golden yellow over slightly brownish, purple, deep brown to black on. Because of the lower equipment claims is a pressureless process control at temperatures between 50 and 100 ° C before Trains t. When carrying out the process in a pressure reactor at Temperatu between 100 and 240 ° C below the respectively adjusting water  Vapor pressure, however, can shorten the treatment times. The procedure It is applicable to ferritic, martensitic and austenitic steels bar.

The adjustment of the alkaline pH of the treatment solution can with Alkali metal hydroxide, carried out with ammonia or with organic amines. For odor and work-physiological reasons Alkalimetallhy are hydroxides and in particular sodium hydroxide preferred.

Complexing agents which are suitable according to the invention are known in large numbers. They can belong to different chemical groups. Are suitable individually or mixed with each other:

• a) hydroxypolycarboxylic acids such as tartaric acid and citric acid,
• b) nitrogen-containing mono- or polycarboxylic acids such as ethylenediamine tetra acetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid, Diethy lentriaminepentaacetic acid, hydroxyethyliminodiacetic acid, nitrilodies acetic acid 3-propionic acid, isoserine diacetic acid, N, N-di- (β-hydroxy ethyl) glycine, N- (1,2-dicarboxy-2-hydroxyethyl) glycine, N- (1,2-Dicar boxy-2-hydroxyethyl) aspartic acid or nitrilotriacetic acid (NTA),
• c) geminal diphosphonic acids such as 1-hydroxyethane-1,1-diphosphonic acid (HEDP), whose higher homologues with up to 8 carbon atoms as well Hydroxy- or amino-containing derivatives thereof and 1-aminoethane 1,1-diphosphonic acid, whose higher homologues contain up to 8 carbon atoms and hydroxyl- or amino-containing derivatives thereof,
• d) aminophosphonic acids such as ethylenediaminetetra (methylenephosphonic acid), di ethylenetriaminepenta (methylenephosphonic acid) or nitrilotri (methylene phosphonic acid) as well
• e) phosphonopolycarboxylic acids such as 2-phosphonobutane-1,2,4-tricarboxylic acid.

Complexing agent from the group of nitrogen-containing polycarboxylic acids, especially EDTA, are preferred. In the invention required alkaline pH values of the treatment solutions are these complexing agents at least partially as anions. It does not matter if they are in shape the acids or in the form of water-soluble salts. in the Traps of use as salts are alkali metal, ammonium or alkyl ammonium salts, in particular sodium salts, are preferred.  

Suitable oxidizing agents are peroxides such as hydrogen peroxide and Alka alkali or alkaline earth peroxides, hydrogen peroxide-forming salts and verbin such as percarbonates, especially sodium percarbonate, perborates and Salts of peroxo acids such as Peroxomonosschwefelsäure (Caro's acid) or Peroxodisulfuric. Also suitable are salts of oxidizing acids without peroxo group such as chlorates or bromates. The oxidizing agents can used singly or in admixture with each other.

Effective concentrations of complexing agents in treatment baths are in the range 0.01 to 0.075 mol / l. They depend on the "toothy for chelating molecules, for hexadentate chelating agents Like EDTA, the effective concentrations range from 0.01 to 0.05 mol / l, preferably 0.02 to 0.04 mol / l. For complexing agent molecules with lower denticity correspondingly higher concentrations are too choose. For example, for the preferred vierzähnig co-ordinate nitrilotriacetic acid the effective concentrations in the range 0.015 to 0.075 mol / l, preferably 0.03 to 0.06 mol / l. Tetrasodium EDTA as Accordingly, preferred complexing agent is in concentrations between 0.7 and 1.3 wt .-% particularly effective.

The concentrations of H₂O₂-releasing oxidizing agents are so ge chooses that the calculated total concentration of H₂O₂ between 0.05 and 0.5 mol / l, preferably between 0.08 and 0.15 mol / l. Preferably Sodium perborate is used, for example as sodium perborate monohy drat ("NaBO₃ · H₂O"). Here are concentrations of about 1 to 5 wt .-% especially effective. Salts of oxidizing acids without peroxo group, esp especially chlorates or bromates are used in concentrations between 0.05 and 0.5 mol / l, preferably 0.08-0.15 mol / l used. Higher concentration NEN are possible, but bring no further advantage.

The amount of alkali required for the pH adjustment depends on whether the Complexing agents and / or the oxidizing agents as salts or as free Acids are introduced. It should be chosen so that the inventive pH range from 12 to 14, preferably 12.5 to 13.5. For example, using a solution in demineralized water, the 1 Wt .-% tetrasodium EDTA and 1 wt .-% sodium perborate monohydrate ent  holds, then the required amount of NaOH is at least 2 wt .-%. higher pH values accelerate layer formation.

Examples 1-5, Comparative Examples 1-3

Sample sheets of V2A steel (material number 1.4301) were tempered at a tempera temperature of 80 ° C for each one, ten and 24 hours in sample and Ver equal solutions of the composition given in Table 1 immersed. to Preparation of the solutions were complexing agents and oxidizing agents in dissolved demineralized water and by addition of 50 wt .-% sodium hydroxide solution ge set the specified pH. After the end of the treatment period the plates were rinsed with demineralized water and compressed air dried. The coloring of the sheets, which indicates the formation of the anti-corrosion coating indicates was optically assessed. The results are in Table 1 included.

To test the corrosion resistance of the coatings and the comparison sheets according to Comparative Examples V1 to V3, the sheets were in an aqueous solution of 1% by weight of (NH₄) HF₂ and 3% by weight of citric acid, pH = 2.5, from room temperature and the surface after 6 hours examined tet. The results are included in Table 2.

Example 6

Sample sheets made of V2A steel (material number 1.4301) were together with each That is, 1 liter solution of the composition of Example 2 in a nickel car given a slave. The autoclave was heated to 190 ° C in one hour heated (autogenous pressure 14-15 bar), for different times at held at this temperature and then within 45 minutes cooled to about 90 ° C. The sheets were removed by immersion in demineralized water cooled and rinsed, dried with compressed air and assessed by color.  

Result:

Holding time at T = 190 ° C colour 0 minutes brown 30 minutes dark brown 90 minutes black

Table 2

Claims (11)

1. Chrome-free process for producing a colored anticorrosive layer on stainless steel, which has a molybdenum content of less than 1 wt .-%, characterized in that ren the steel surfaces with an aqueous alkaline solution at Temperatu between 50 ° C and 240 ° C, which contains at least one complexing agent and at least one oxidizing agent in demineralized water and has a pH between 12 and 14. 2. The method according to claim 1, characterized in that for adjustment the pH of 12 to 14 alkali metal hydroxides and / or ammonia and / or organic amines, preferably sodium hydroxide become. 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) nitrogen-containing Mono- or polycarboxylic acids, c) geminal diphosphonic acids, d) amino phosphonic acids, e) phosphonopolycarboxylic acids, as well as water-soluble Salts of the acids of groups a) to e). 5. The method according to claim 4, characterized in that as a complex bildend ethylenediaminetetraacetic acid or its water-soluble salts be used. 6. The method according to one or more of claims 1 to 5, characterized ge indicates that the concentration of the complexing agent total between is 0.01 and 0.075 mol / l and for hexadentate complexing agents Concentrations between 0.01 and 0.05 mol / l, in particular 0.02-0.04 mol / l, for tetradentate complexing agents concentrations between 0.015 and 0.075, especially between 0.03 and 0.06 mol / l are preferred.   7. The method according to claim 6, characterized in that as a complex tetrasodium EDTA in concentrations between 0.7 and 1.3 % By weight 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 several of the groups: a) peroxides, in particular hydrogen peroxide and Alkali or alkaline earth peroxides, b) hydrogen peroxide-forming salts and compounds, in particular percarbonates, perborates and salts of Peroxo acids, in particular peroxomonosulphuric 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 oxidation agent from group a) or b) used in a concentration be that of a calculated total concentration of H₂O₂ between 0.05 and 0.5 mol / l, preferably between 0.08 and 0.15 mol / l ent speaks and that when using one or more oxidizing agents from the group c) whose total concentration is between 0.05 and 0.5 mol / l, preferably between 0.08 and 0.15 mol / l. 10. The method according to one or both of claims 8 and 9, characterized ge indicates that used as the oxidant sodium perborate becomes. 11. The method according to any one of claims 1 to 10, characterized ge indicates that the temperature of the solution is between 50 and 100 ° C is.