SE530078C2 - Method, combined agent and use to obtain a corrosion protection surface layer on metals - Google Patents

Description

530 078 Pure aluminum has very good corrosion properties due to the fact that the surface oxidizes in the atmosphere. However, pure aluminum is rarely used as a construction material due to its poor strength properties.

It is common to reinforce the naturally formed oxide layer on metals such as those mentioned above. One way to do this is by anodizing aluminum, which provides excellent corrosion protection. However, anodizing is an electrolytic method and it is therefore expensive and in addition the anodizing layer is electrically insulating. The method is therefore unsuitable for electrical applications where good electrical conductivity is required.

Due to the reasons described above, other surface treatment methods, especially for aluminum and its alloys, have been sought. Said methods should be able to exhibit as good or better corrosion protection as chromating without leading to any negative environmental effects.

US 4,755,224 discloses a metal surface treatment with alkali permanganate, for example potassium permanganate, to obtain a corrosion protective surface of manganese dioxide and alumina. The anti-corrosion properties obtained by this method are said to correspond to those obtained by chromating. The manganese-based method is much more advantageous from an environmental point of view, as manganese does not exhibit the negative properties discussed OVaIl.

A disadvantage of this treatment method is that it requires treatment in most baths at a temperature around 100 ° C, which makes the method expensive and disadvantageous from a work environment point of view.

WO Ol / 31084 describes a method for surface treatment of metals, such as aluminum and zinc, in order to obtain a corrosion-protective surface layer of manganese dioxide on said metals. The surface treatment is carried out in two separate steps. The first step comprises contacting the object to be surface treated with an aqueous solution containing sulfuric acid and an oxidizing agent for the acid, such as an alkali metal permanganate or ammonium persulfate. The second step comprises contacting the object with an aqueous solution containing an alkali metal carbonate and an alkali metal permanganate, thereby passivating the surface of the object.

Thus, one of the objects of the invention is to provide a method by which it is possible to achieve an increased oxidation layer on said metals which is thick enough to achieve a good corrosion protection, but thin enough to achieve sufficient conductivity even at very high high frequencies, for example up to 4O GHz.

Another object of the invention is to provide, like chromating, a immersion method which is not expensive and which is easy to perform.

Another object of the invention is to provide a method which is environmentally friendly.

A further object of the invention is to provide a means for use in the method according to the invention.

A further object of the invention relates to the use of an alkali metal salt of gallic acid to achieve a corrosion protective layer on a metal object of aluminum, magnesium, zinc and alloys thereof.

SUMMARY OF THE INVENTION According to one of the objects of the invention, these objects are achieved by a method of surface treatment of metal objects of aluminum, magnesium, zinc and alloys thereof to obtain a corrosion protective layer, which comprises the following steps: - immersing the object in a solution. 530 078 - immersion of the object in a deoxidizing solution comprising an alkali metal salt of gallic acid in a concentration of 0.1-20 grams / liter, and - immersion of the object in a treatment solution with a temperature in the range 15-60 ° C and a pH in the range of 10.5-11.5, comprising permanganate at a concentration of 2-10 grams / liter, carbonate and persulfate to obtain a corrosion protective layer consisting of oxides of the treated metal and manganese dioxide, the proportion permanganate to carbonate to persulphate is 1: 1: 2, and the above solutions are aqueous solutions.

According to another aspect of the invention, these objects are achieved by a combined agent to be used in conjunction with the method according to the invention in the form of a deoxidation solution comprising an alkali metal salt of gallic acid in a concentration of 0.1-20 grams / liter and a treatment solution with a pH in the range of 10.5-11.5 of alkali metal salts of permanganate in a concentration of 2-10 grams / liter, carbonate and persulfate, the ratio of permanganate to carbonate to persulfate being 1: 1: 2, and the above solutions are aqueous solutions.

According to further aspects of the invention, these objects are achieved by the use of an alkali metal salt of gallic acid in a concentration of 0.1-20 grams / liter in the form of a deoxidation solution to obtain a corrosion protective layer on a metal object made of aluminum, magnesium, zinc or alloys thereof, by immersing the object in a beet solution, immersing the object in the deoxidation solution, and immersing the object in a treatment solution with a temperature in the range 15-60 ° C and a pH in the range 10.5-11, 5, comprising permanganate in a concentration of 2-10 grams / liter, carbonate and persulfate to obtain a corrosion-protective layer consisting of oxides of the treated metals and manganese dioxide, the proportion of permanganate 530 Ü78 to carbonate to persulfate being 1 : l: 2, and the above solutions are aqueous solutions.

BRIEF DESCRIPTION OF THE FIGURES The invention is described below with reference to the accompanying figures, in which Fig. 1 is a diagram of a GDS analysis of a corrosion protective layer which according to the invention has been made on an aluminum alloy and the figure shows the thickness and composition of the corrosion protective layer, and Fig. 2 is an SEM image of the corrosion-protective surface layer obtained by the method of the invention consisting of alumina and with manganese oxide particles as white dots.

DETAILED DESCRIPTION OF THE INVENTION The following description of the invention is based on the treatment of aluminum, but it should be noted that also magnesium, zinc and alloys thereof and of aluminum can be treated in the same way in accordance with the invention.

According to the invention, the metal surface to be provided with a corrosion-protective surface layer, after suitable washing, must be immersed in an alkaline pickling bath, in order to remove contaminants and so-called roller skin. It should be noted that an acidic pickling can also be used, which usually contains fluorides.

The metal surface reacts in the environment of said oxidizing agent and forms a passivating oxide layer with a material thickness of up to 200 nm, which is at least ten times as thick as the naturally formed oxide layer on aluminum.

The chemical reaction can be expressed with aluminum as an example by the following formula: 530 078 4A1 + 2KMI1O4 + Kgsgøg = 2Al2Û3 + 2MI102 + 2K2SO3 Which can be read from the formula, alumina and manganese dioxide are formed on the aluminum surface and thus provide the intended corrosion protection layer. In addition, the manganese dioxide gives the layer a yellow-brown tone, which is important from a quality perspective, as it is easy to see that the surface has received the intended surface treatment.

It should be noted that the anti-corrosion layer also consists of sulfur originating in the persulphate.

The composition of the layer can be analyzed by glow discharge spectroscopy (GDS), which includes an analysis of the atomic composition of the layer at different levels. The analysis provides a diagram in which composition and thickness can be read. Such a diagram is shown in Fig. 1.

The manganese dioxide formed during the reaction appears as white dots on a photo taken with a scanning electron microscope (SEM). See Fig. 2.

A suitable bath for surface treatment as above includes: Potassium permanganate 5 grams / liter Potassium carbonate 5 grams / liter Potassium persulfate 10 grams / liter The bath temperature should be kept in the temperature range from about 15 to about 60 ° C. The pH is adjusted with potassium hydroxide to about 10.5-11.5. A freshly mixed bath works best if a small amount of sodium aluminate is added to the bath. A suitable concentration is 0.1-0.25 grams Al / liter. A suitable process time at 20 - 25 ° C is 2 minutes.

From the above it can be deduced that the ratio of permanganate to carbonate to persulphate should be 1: 1: 2. However, the proportion according to the invention may be within the following range; 1: 1: 0.8 to 1: 1: 1, 2. 530 078 Concentration of potassium permanganate should suitably be between 2 to 10 grams / liter.

In a preferred form of the invention, the mixture of potassium permanganate, potassium carbonate and potassium persulphate is supplied as a dry powder which is dissolved in water.

When the surface treatment is performed on the above-mentioned metals and their alloys, the passivation must be preceded by a washing of the surface to degrease the object's surface and remove residues of cutting oil and fingerprints and thick oxide layers from the manufacture of the metal such as so-called roll skin. Roll skin is removed in a process step called pickling. When picking, the choice of picking method must be adapted to the metal used and its alloys.

Aluminum alloys often contain silicon, magnesium, metal and copper. These substances oxidize during pickling and because of this an additional treatment step called deoxidation is needed.

For surface treatment of aluminum in accordance with the invention, the following pretreatment steps are normally used: 1. Degreasing in an alkaline treatment solution. 2. Rinse in clean water. Pickling in a sodium hydroxide solution, 50 grams / liter: At a temperature of about 20-50 ° C. Process time l min. 4. Rinse in clean water. 5. Deoxidation in dilute nitric acid, 280 grams / liter, at a temperature of about 20-25 ° C. Process time 1 min. 6. Rinse in clean water.

However, the deoxidation step described above may give unsatisfactory results for some alloys due to pre-passivation of the metal surface. This pre-passivation is undesirable before the intended passivation. 530 078 Due to this, there is a need for a better deoxidation method for the metals identified above and their alloys. 3,4,5-Trihydroxybenzoic acid, commonly referred to as gallic acid, is a strong reducing agent in alkaline conditions. The inventor has now surprisingly found that a dilute or concentrated solution of an alkali metal salt of gallic acid, for example potassium trihydroxybenzoate, either alone or in combination with other types of deoxidizing agents gives a satisfactory result. According to the invention, the concentration of gallic acid in the aqueous treatment solution should be around 0.1-20 grams / liter, preferably 0.5 grams / liter.

Without the desire to be bound by a specific theory, it is believed that bile acids form complex compounds with the metals in question.

A concentrated treatment solution of potassium trihydroxybenzoate can be prepared as follows: A suitable container is filled with deionized water or distilled water to% of the intended volume. The water is heated to about 50 ° C. 100 g of potassium carbonate per liter are dissolved in the hot water. When potassium carbonate has dissolved, 100 g of gallic acid per liter are added with stirring. When the reaction and gas formation have ceased, water is added to the desired volume.

The solution formed can be used for deoxidation of the metals identified above and their alloys either in concentrated or dilute form.

The following surface treatment sequence is suitable for surface treatment in accordance with the method according to the invention for aluminum alloys with a low content of alloying elements, for example for the alloy EN 6082 according to the European standard: 1. Degreasing in an alkaline treatment solution. 2. Rinse in clean water. 53Û G78 3. Pickling in a sodium hydroxide solution, 50 grams / liter, at a temperature of about 20-50 ° C. Process time 1 min. 4. Rinse in clean water. Deoxidation in a solution of potassium trihydroxybenzoate, 5 ml / liter, 20-25 ° C, 2 minutes. Immersion without rinsing in the above-mentioned permanganate-containing treatment solution, 2 minutes. 7. Rinse in clean water. 8. Rinse in warm deionized water. 9. Drying in hot air.

If a surface treatment method in accordance with the invention is to be useful for ordinary industrial applications, an approval is required in accordance with a standardized accelerated corrosion test.

Tests have shown that treatment by deoxidation in potassium tri-hydroxybenzoate in connection with surface treatment in the treatment solution containing permanganate significantly increases the corrosion protection of the surface layer. The increased corrosion protection is probably due to a more even formation of the alumina and manganese dioxide layers on the surface of the metal or metal alloy being treated. 10 plates (100 * 100 * 2mm) of alloy EN 6082 (named objects) were tested with and without treatment with gallic acid in accordance with the invention. The evaluation of the results took place after l68h in NSS (Neutral Salt Spray) and the results are shown in Table 1 below Table ll 1 Object: Nr. With gallic acid Without gallic acid 530 078 10 1 <1% corr. 2 <1% corr. 3 <1% corr. 4 <1% corr. 5 <1% corr. 6 <3% corr. 7 <3% corr. 8 <4% corr. 9 <3% corr. 10 <2% corr.

As can be seen from Table 1, plates Nos. 1-5 treated with gallic acid have a corrosion rate of less than 1%, while plates Nos. 6-10 which are not treated with gallic acid have a degree of corrosion of less than 2 to 4%. .

Claims (9)

538 078 ll KRAV Method for surface treatment of metal objects comprising aluminum, magnesium, zinc and their alloys to provide a corrosion-protective layer thereon, characterized by the following steps: - immersion of the object in a pickling solution, - immersion of the object in a deoxidation solution comprising an alkali metal salt of gallic acid in a concentration of 0.1-20 grams / liter, and - immersion of the object in a treatment solution with a temperature in the range 15-60 ° C and a pH in the range 10,5-11, 5, comprising permanganate in a concentration of 2-10 grams / liter, carbonate and persulfate to obtain a corrosion-protective layer consisting of oxides of the treated metal surface and manganese dioxide, the proportion of permanganate to carbonate to persulfate being 1: 1 : 2, and the above solutions are aqueous solutions. Method according to Claim 1, characterized in that the beet solution is an alkaline beet solution. Method according to claim 1, characterized in that the beet solution is an acid beet solution. Method according to claim 1, characterized in that the treatment solution comprises alkali metal salts of carbonate, persulphate and permanganate. Method according to one of Claims 1 to 4, characterized in that the corrosion-protective layer also contains sulfur. Method according to claim 1, characterized in that the concentration of gallic acid in the deoxidation solution is 0.5 grams / liter. 530 078 12 Method according to claim 1, characterized in that no rinsing step is performed between immersion of the object in the deoxidation solution in the form of a reducing agent comprising an alkali metal salt of gallic acid and immersion of the object in the treatment solution. Combined agent for use with the method according to any one of claims 1-7, characterized in that, in order to obtain a corrosion-protective layer on the treated objects, it comprises a deoxidizing solution comprising an alkali metal salt of gallic acid in a concentration of 0.1-20 grams / liter and a treatment solution with a pH in the range 10.5-11.5 of alkali metal salts of permanganate in a concentration of 2-10 grams / liter, carbonate and persulfate, wherein the proportion of permanganate to carbonate to persulphate is 1: 1: 2, in addition to the above solutions are aqueous solutions. The use of an alkali metal salt of gallic acid in a concentration of 0.1-20 grams / liter in the form of a deoxidation solution in the method according to claim 1 to obtain a corrosion protective layer on a metal object consisting of aluminum, magnesium , zinc and its alloys by - immersing the object in a beet solution, - immersing the object in the deoxidation solution, and - immersing the object in a treatment solution with a temperature in the range 15-60 ° C and a pH in the range 10,5 -11.5, comprising permanganate at a concentration of 2-10 grams / liter, carbonate and persulphate to obtain a corrosion-protective layer consisting of oxides of the treated metal surface and manganese dioxide, the proportion of permanganate to carbonate to persulphate being 1: 1: 2, and the above solutions are aqueous solutions.