EP1022357B1 - Surface treatment process for stainless steels - Google Patents

Abstract

Process for treating rust-free steels comprises oxidizing the steel surface and then removing the oxide layer.

Description

The present invention is in the field of steels and Machining or treatment of steels, preferably for the production of Devices e.g. are intended for home use. In particular, the present invention such devices and parts or their treatment that the Exposed to heat.

More specifically, the present invention provides a method of treatment of stainless steel available for these applications are intended, the treatment of the invention Steels prevent their yellowing from occurring during daily use.

The use of stainless steel in the production of a variety of Products, especially household appliances such as kitchen stoves, Refrigerators, kettles, deep fryers or dishwashers known.

It is also known that the stainless steel parts that the regular Exposed to the effects of heat, generally one Yellowing experience what the aesthetic appearance of the device strongly impaired. This is e.g. the case with the inside of the doors from Dishwashers, the steel worktops in kitchens, the rings that make up the Lock hot plates, deep fryers and other devices.

To eliminate this yellowing (yellowing), which is mainly due to the Formation of oxidation products on the surface of the steel is caused by the effect of the heat commonly used abrasion techniques. For example, the use of lubricating gel, metal rags etc. often. Also be also used abrasives. These products may contain a Mixture of phosphoric acid and certain surfactants (such as FINOX® DH ISO 9002, manufactured by Chimiderouil) normally applied to the already damaged (yellowed) steel in order to to eliminate the yellowing.

These aggressive treatments eventually scratch the steel and are useful it also, aside from the risks of using products as FINOX® represents to the user, making it still desirable is to find a better solution to this problem.

Furthermore, coating processes for stainless steels are in the prior art Technology already known. This includes the 5-step process described in JP-A 52 64 045, where the surface of the steel is first sandblasted, then in an etchant such as HF, salt, saltpetre or Phosphoric acid immersed, then coated with a resin film and heated is so that the film adheres to the surface in cured form. The JP-A 61 06 693 describes another method which is a sandblast treatment of the steel to make its surface more corrosion resistant do. This process is also followed by an immersion of the steel in a mineral acid before applying a resin layer. Another one Process for the treatment of stainless steel, a so-called Chromating process is described in JP-A 52 145 346. Doing so The surface is film-coated by immersing the steel in a hot one aqueous solution containing sulfuric acid, oxidizing agents e.g. based on chrome (such as potassium dichromate) and phosphoric acid.

According to the method described in JP-A 80 58 015 heat-resistant stainless steel produced by the steel with a Composition is coated, the phosphoric acid and silicon dioxide contains. This wording only serves as an adhesion promoter for the Resins that are then applied to the surface.

Vollmer et al. (Materials and Corrosion, 46 , 92-97 (1995)) describes a surface treatment process for stainless steels, in which samples are stored in an oven at 600 ° C in order to obtain a uniform oxide layer covering the entire surface. To remove the oxide layer, the samples are treated with non-liquid pickling pastes, which after an exposure time under running water are brushed off together with the oxide layer. Since the thickness of the oxide layer formed on the steel surface depends on the operating temperature during the oxidation, a relatively thick passive layer with a high chromium content protecting against corrosion is formed at the described operating temperature of 600 ° C. after removal of the oxide layer. However, this passive layer changes the original surface structure of the stainless steel, so that the steel surface is usefully not given a special surface structure (for example a brush cut) before the surface treatment, since this changes optically after the surface treatment has been carried out. In addition, brushing off the oxide layer by means of slightly affects the surface structure of the steel.

US 5,269,957 describes a rust removing agent for stainless surfaces Steel made from phosphoric acid, a polyhydroxymonocarboxylic acid (or one of its salts) and a surfactant.

JP-A 62 124 019 discloses a bath for immersing steels at elevated heights Temperatures. This bath is based on an aqueous solution of phosphoric acid, Nitric acid (or nitrate) and an amine group inhibitor.

US 4,078,949 describes a method of machining stainless steel surfaces, to e.g. Remove oxides. This is done by heating the steel to a specific one Oxygen potential.

Finally, JP-A 07 286 215 discloses a method to improve the corrosion resistance of To improve ferrite steels. Two process steps involve exposing the steel (i) compared to a hydrogen or (ii) a hydrogen / nitrogen atmosphere.

However, these and other known methods can either address the problem the yellowing of stainless steel, which is regularly the effect of heat exposed, prevent, unsatisfactory, or require it a considerable effort and the use of aggressive Chemicals. The object of the present invention is accordingly to provide a simple method by means of which Stainless steel surface resistant to dirt and Discoloration (yellowing) can be made.

After numerous trials and tests, the inventors managed to get one Process for developing stainless steels to develop the above solves the described problem better, i.e. the yellowing of the steel avoid thermal stress (largely). This procedure includes treatment of the steel with mineral acids, preferably phosphoric acid, especially with aqueous phosphoric acid, the presence of one or of several surfactants is preferred. Particularly suitable as phosphoric acid for the process according to the invention FINOX® DH ISO 9002. The concentration of the aqueous phosphoric acid is preferably included 50% (v / v) or less, concentrations in the range are particularly preferred of 25, 30, 35, 40 and 45% (v / v).

This steel treatment process according to the invention should generally be carried out beforehand the use of steel in the manufacturing chain of equipment or parts respectively. In any case, this procedure achieves that later Discoloration (yellowing) of the steel can be prevented.

An advantage over the prior art is that The method according to the invention does not require a large outlay on equipment and without sandblasting and coatings of any kind (e.g. with Resins and other polymers).

The present invention thus relates to a method for Surface treatment of stainless steels, which is particularly suitable To prevent yellowing after the steel has been exposed to heat.

a) Oxidation der Stahloberfläche durch Erhitzen in einem Ofen und

b) Entfernen der Oxidschicht,

dadurch gekennzeichnet, daß die Oxidation durch Erhitzen des Stahls auf eine Temperatur von 300-400°C erfolgt und das Entfernen der Oxidschicht durch Eintauchen des Stahls in ein Flüssigbad auf Grundlage von Mineralsäure erfolgt. Bevorzugt ist das Eintauchen des erhitzten Stahls in ein Flüssig-Bad auf der Grundlage von Phosphorsäure, vorzugsweise in Gegenwart von einem oder mehreren grenzflächenaktiven Stoffen.The method for treating stainless steel according to the present invention comprises the following steps:

a) Oxidation of the steel surface by heating in an oven and

b) removing the oxide layer,

characterized in that the oxidation takes place by heating the steel to a temperature of 300-400 ° C and the removal of the oxide layer by immersing the steel in a liquid bath based on mineral acid. It is preferred to immerse the heated steel in a liquid bath based on phosphoric acid, preferably in the presence of one or more surfactants.

According to a preferred embodiment, the heated steel becomes immediate immersed in the liquid bath after step (a) (step (b)), i.e. without previous Cooling of the steel. However, this is not absolutely necessary, since that too Step (a) steel cooled to room temperature or slightly above (30-55 ° C) after immersing it in a hot mineral, preferably phosphoric acid (Temperature greater than approx. 50 ° C, preferably greater than 80 ° C), resistant to Yellowing is or will be.

A particularly preferred phosphoric acid in step (b) is an aqueous one Phosphoric acid mixed with surfactants such as FINOX® DH ISO 9002. The use of a 33% (v / v) aqueous is particularly preferred Solution of FINOX® DH ISO 9002 (Chimiderouil Productos Tamosa, S.A.).

According to the manufacturer, FINOX® DH eliminates ISO 9002 by immersion, circulation, Sprinkling or other application of greasy contaminants from Surfaces. It also enables the deoxidation of numerous metals (i.e. the Removal of oxide layers or derusting) such as the carbon and Cast steels, stainless ferritic and martensitic steels, des Aluminum and copper and their two alloys.

ZUSAMMENSETZUNG

Chemische Beschaffenheit: wässrige Lösung von Phosphorsäure und grenzflächenaktivem Stoff (Surfactant)

Gefährliche Bestandteile: Phosphorsäure Cas Nr. 7664-38-2 Konz. (Gew.%) >25, C; R 34 (Richtlinie 88/379/CEE)

Verunreinigungen (gefährlich): keine

PHYSIKALISCHE/CHEMISCHE EIGENSCHAFTEN

Aussehen: saure, leicht viskose Flüssigkeit

Farbe: farblos

Dichte bei 20 °C: 1,30 ± 0,01 g/ml

pH: unter 1

Entflammbarkeit: unentflammbar

FINOX® DH ISO 9002 has the following specifications:

COMPOSITION

Chemical nature: aqueous solution of phosphoric acid and surfactant (surfactant)

Hazardous components: phosphoric acid Cas No. 7664-38-2 conc. (Wt.%)> 25, C; R 34 (directive 88/379 / CEE)

Impurities (dangerous): none

PHYSICAL / CHEMICAL PROPERTIES

Appearance: acidic, slightly viscous liquid

Color: colorless

Density at 20 ° C: 1.30 ± 0.01 g / ml

pH: below 1

Flammability: non-flammable

The treatment in step (b) is preferably carried out over a period of 1-10 min, preferably 2-8 min, particularly preferably 3-6 min. Another a particularly preferred embodiment provides for a duration of about 5 minutes Step (b) above.

Other preferred mineral acids required for the liquid bath in step (b) Can be used are hydrochloric acid, nitric acid, sulfuric acid, chloric acid, Perchloric acid, bromic acid, perbromic acid, iodic acid, periodic acid, Silica, tungstic acid, 12-tungstosilicic acid, 12-tungstophosphoric acid or a mixture of two or more of these or other mineral acids.

c) Spülen des behandelten Stahls bzw. der behandelten Platten mit Wasser, beispielsweise durch Beregnung im Inneren eines Spülbottichs;

d) Trocknen des gespülten Stahls (der Platten), insbesondere mit Hilfe eines Luftstroms, insbesondere eines heißen Luftstroms, z.B. im Inneren einer Trockenkammer.

Steps (a) and (b) are preferably followed by further steps (c) and / or (d):

c) rinsing the treated steel or the treated plates with water, for example by sprinkling inside a rinsing tub;

d) drying the rinsed steel (the plates), in particular with the aid of an air stream, in particular a hot air stream, for example inside a drying chamber.

The task of steps (c) and (d) is to remove any rest of the liquid bath Remove step (b) completely. Otherwise there is the, albeit minor, Possibility of streaked or grained during finishing Form surfaces, making the end product less aesthetically acceptable would.

The temperature of the air flow in step (d) is usually 50-150 ° C, in particular at 70-130 ° C or at 80-120 ° C, particularly preferably at 90-110 ° C or at 95-105 ° C, with a temperature of 100 ° C especially suitable is.

The treatment of e.g. Steel plates is except through that Operating characteristics shown above characterized in that they are the insertion of e.g. Plates run in the manufacturing chain of the devices can be.

In this way, the device and especially its steel parts, which the Effects of heat are regularly exposed, no longer over the course of Time.

This treatment prevents the user from using his device subject to mechanical or chemical abrasion to eliminate yellowed surfaces. The result of this is damage of the steel parts and the risk to the user when using them resulting from such aggressive chemical products can be avoided.

• einen Ofen 1 zur Durchführung von Schritt (a);
• ein Bad 2 zur Durchführung von Schritt (b);
• einen Behälter 3 zur Durchführung von Schritt (c);
• einen Bereich bzw. eine Trockenkammer 4 zur Durchführung von Schritt (d),

die mit Hilfe der entsprechenden Verbindungen und Transportmittel in einem Hauptkreislauf (dargestellt in Fig. 1 durch eine durchgehende Linie) zur Behandlung z.B. der Stahlplatten miteinander verbunden sind, wobei der Hauptkreislauf in einer Zone 8 des Eintrittes des Stahls in den Ofen 1 beginnt.The device designed to carry out the method of the invention is shown schematically in Figure 1. It is characterized in that it essentially comprises the following elements:

• an oven 1 for performing step (a);
• a bath 2 for performing step (b);
• a container 3 for performing step (c);
• an area or a drying chamber 4 for performing step (d),

which are connected to one another by means of the corresponding connections and means of transport in a main circuit (represented by a solid line in FIG. 1) for the treatment of, for example, the steel plates, the main circuit starting in a zone 8 of the entry of the steel into the furnace 1.

In addition, the device comprises a zone 5 of the entry of water into a cooling and rinsing circuit (shown in Fig. 1 by a line "-----"), a zone of separation 7 between the water and the liquid bath which reprocesses the water from zone 6 for the cooling and rinsing cycle and the separated phosphoric acid 9 in the steam cycle (shown in Fig. 1 by a line "----") for recovery and reprocessing it is fed. This steam cycle also includes a zone for cooling 11, which is common to the cooling and rinsing circuit, and a zone 10 for collecting vapors, which allows the Phosphoric acid attributed to the treatment 2 bath.

In accordance with a preferred embodiment of FIG Invention to be treated stainless steel plates through the zone 8 inserted in the oven 1, where they are heated to 300-400 ° C before be led into bath 2 for a period of 4.5 minutes, the one Contains 33% (v / v) solution of FINOX® DH ISO 9002 in water. The plates are then guided into the rinsing tub 3, where they are one Be rinsed with water by sprinkling. From there they will into a drying chamber with hot air at 105 ± 5 ° C. In connection the plates are then sent into the assembly chain to continue with Continue normal procedures to manufacture the appropriate device.

From the point of view of environmental protection, the device has how already stated above, via a circuit for reprocessing the Water and phosphoric acid and their return to the system. In addition to saving water and treatment product Avoid waste products that are harmful to the environment.

1: Ofen

2: Bad der Behandlung

3: Spülbottich

4: Trockenkammer

5: Eingangszone des Wassers

6: Wiedergewonnenes Wasser

7: Zone der Trennung

8: Zone des Einganges des zu behandelnden Stahls

9: Wiedergewonnenes FINOX DH

10: Sammler der Dämpfe

11: Zone der Kühlung

12: Zur Montagekette

Figure 1 is a schematic representation of the device that can be used to carry out the method of the present invention. The numerals have the following meanings:

1: oven

2: bath of treatment

3: Rinsing tub

4: drying chamber

5: Entry zone of the water

6: Recovered water

7: Zone of separation

8: Zone of entry of the steel to be treated

9: Recovered FINOX DH

10: Vapor collector

11: Zone of cooling

12: To the assembly chain

Example:

The present invention is further illustrated by the following example shown that in no way intends to limit their reach.

Steel plates of 890 x 480 mm were used, which were previously at 375 ° C were heated. A liquid bath with FINOX® DH ISO 9002 showed Capacity of 4000 liters, in the 1333 liter of the Finox product were introduced, the total volume being completed with water to obtain a 33% (v / v) solution.

The panels to be treated were introduced through zone 8 into oven 1 where they were heated to 375 ° C. After this step, they were in the liquid bath led, which contained the 33% (v / v) solution of FINOX® DH ISO 9002, where they remained for 4.5 minutes. Then they were put in rinsing tub 3, where they have been irrigated with water. Of there they were led into a drying chamber with hot air at 105 ± 5 ° C, from where they were sent to the assembly chain to match the normal Proceed with the processing of the plates to manufacture the equipment.

Claims (12)

1. Method of treating stainless steels, which comprises the following steps:

   a) oxidation of the steel surface by heating in an oven and

   b) removal of the oxide layer,

   characterised in that the oxidation is carried out by heating the steel to a temperature of 300 to 400° C and removal of the oxide layer is carried out by immersion of the steel in a liquid bath on the basis of a mineral acid.
2. Method according to claim 1, characterised in that the mineral acid is a phosphoric acid.
3. Method according to claim 2, characterised in that the mineral acid is an aqueous solution of phosphoric acid.
4. Method according to claim 1, characterised in that the mineral acid is hydrochloric acid, nitric acid, sulfuric acid, chloric acid, perchloric acid, bromic acid, perbromic acid, iodic acid, periodic acid, silicic acid, tungstic acid, 12-tungstosilicic acid, 12-tungstophosphoric acid or a mixture of two or more of these or other mineral acids.
5. Method according to one of the preceding claims, characterised in that the treatment in step (b) is carried out over a time period of 1 to 10 minutes.
6. Method according to claim 5, characterised in that the treatment in step (b) is carried out over a time period of approximately 5 minutes.
7. Method according to one of the preceding claims, characterised in that the liquid bath contains, apart from a mineral acid, also one or more surface-active substances (surfactants).
8. Method according to claim 7, characterised in that the liquid bath of step (b) contains FINOX® DH ISO 9002, in particular a 33 percentile (v/v) aqueous solution of FINOX® DH ISO 9002.
9. Method according to one of the preceding claims, characterised in that the heated steel is immersed in the liquid bath directly after step (a), i.e. without previous cooling of the steel, or the heated steel is cooled down to 20 to 55° C and then is dipped in a heated liquid bath of > 50° C.
10. Method according to one of the preceding claims, characterised in that the further step or steps (c) and/or (d) is or are consecutive to the steps (a) and (b):

    c) washing of the treated steel with water; and

    d) drying of the washed steel, particularly by means of an air flow.
11. Method according to one of the preceding claims, characterised in that the temperature of the air flow in step (d) lies at 50 to 150° C.
12. Method according to claim 11, characterised in that the temperature of the air flow lies at 90 to 110° C or 95 to 105° C.

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