EP0161667B1 - Process for the treatment of metal surfaces - Google Patents

Abstract

A composition and process for treating ferrous substrates including black plate container bodies to inhibit in-process corrosion or rusting of the surfaces thereof by contacting the ferrous substrate with aqueous acidic composition containing controlled effective amounts of aluminum, fluoride, optionally a second metal selected from the group consisting of zirconium, titanium, hafnium and mixtures thereof and hydrogen ions to provide a pH on the acid side.

Description

The invention relates to a method for the corrosion-protective treatment of metal surfaces made of iron and steel with fluoride-containing solutions.

In particular, the treatment of containers made from low-carbon steel sheet - usually referred to as black sheet - is considered. They are usually manufactured by pre-forming, deep drawing and smoothing. Such containers have a desirable light gray glossy surface which, after a coating treatment with a clear organic varnish or printing on the outer surface, make them suitable as attractive packaging.

The process sequence in the manufacture of black plate containers usually consists in unwinding the black plate strip provided with a protective oil layer from the coil, in applying drawing lubricants, in a first preliminary deformation into a cup and in deep drawing and smoothing to form the final shape. In addition, cooling lubricants such as water or aqueous emulsions that facilitate the deep-drawing process are usually used in the drawing and smoothing process. The containers are then cut smooth in a trimmer and cleaned in a multi-stage washing process. Lubricants, protective oils, cooling lubricants and other contaminants are removed first in a cleaner with a low concentration and then in a cleaner with a higher concentration. The containers are then passed through one or more water rinse stages, whereupon they are completely dried in a drying oven. This is followed by a one- or multi-stage painting and decorative printing on the outer surface. Usually, the outer surface is first provided with a basecoat or a decorative print and after drying with an outer layer of can lacquer. After it has hardened, it is coated internally with subsequent hardening. If the opacity of the selected paint is low, the container finally obtained must retain the shiny, light gray metallic appearance - as far as visible through the paint.

When manufacturing such containers, however, it is found that rust often occurs during drying if too much water is retained in certain areas. This applies in particular to curved floors of cans, drainage edges or contact points between two adjacent containers. Then the containers have to be discarded - or, which is very expensive - reworked. Inadvertent stoppage of the production line, where the containers remain in the rinsing process stages for a long time, often also causes unsightly rust stains or streaks, which are even visible through the paint. They are also responsible for low paint adhesion and therefore make the product unsellable.

The object of the invention is to provide a method by means of which the aforementioned problems can be remedied. It should be able to be integrated as an integrable component in the multi-stage cleaning or rinsing process and retain the desirable light gray shiny metal surface at least until the final protection by painting or another type of surface treatment is guaranteed.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the metal surface is brought into contact with a solution which contains 10 to 5000 ppm aluminum ions, 10 to 200 ppm fluoride ions and up to 1000 ppm ions of at least one of the Contains metals titanium, zirconium and / or hafnium and which has a pH of 2 to 5.5.

It is known that acidic solutions on metal surfaces which contain fluoride in the form of complex compounds such as BF ₄ , TiF _s ² , SiF ₆ ² , A1F ₆ ³ and heterocyclic nitrogen compound [JP-A-59-74 281 , CA 101 (1984) 59039b] or acidic pastes which contain, inter alia, aluminum compound and fluoride (EP-A-58 257). However, completely different goals are pursued here. In the first case, tin or tin alloys should be removed from copper substrates. In the second case, the intention is to remove oxide or scale layers from metal surfaces.

The aluminum can be introduced into the solution intended for carrying out the process according to the invention in the form of any soluble and compatible aluminum salt. Suitable salts are e.g. Aluminum fluoride, but especially aluminum sulfate hydrate. The aluminum concentration may be above the solubility limit. Concentrations in the range of 25 to 250 ppm are preferred.

The fluoride can be introduced into the aqueous treatment solution as a simple fluorine compound, such as hydrofluoric acid, as a simple or bifluoride salt of alkali or ammonium. Complex fluorides of e.g. As boron, silicon, aluminum, zircon, hafnium and / or titanium can be used. A maximum fluoride concentration of 150 ppm is preferred. The fluoride concentration generally depends on the amount of aluminum present in the solution, the nature of the metal surface to be treated, the temperature of the treatment solution and the duration of the treatment.

The ions of at least one of the metals titanium, zirconium and hafnium can be introduced into the solution in the form of any compound which is soluble in the acidic aqueous medium, unless disturbing bath constituents are added. Suitable compounds are, for example, potassium fluorotitanate, titanium zirconium fluoride, fluozirconic acid, ammonium or alkali fluorozirconates, zirconium fluoride, zirconium sulfate and hafnium oxide and hafnium acids and salts, for example: hafnium nitrate, fluoride or chloride. Alkali fluorozirconate, in particular potassium fluorozirconate (K ₂ ZrF ₆ ) is preferably used, especially since zirconium and fluorine are introduced at the same time.

Although the concentration of ions of at least one of the metals titanium, zirconium and hafnium can range up to 1000 ppm, those in the range from 40 to 320 ppm are preferred. The most common concentration in the working bath is around 80 ppm.

In addition to the aforementioned bath components the solution holds hydrogen ions in such an amount that a pH of 2 to 5.5 results. At pH values above 5.5, no visible treatment or coating formation is obtained and consequently no corrosion protection is created. In general, it can be said that the pH to be set depends on the duration of the treatment, the bath temperature and, for example, the spray pressure, but also on the concentration of the other bath components. As a guideline, with a treatment time of 30 seconds to 1 minute at a bath temperature of around 49 ° C and normal spray pressure, a pH value of 4.0 to 4.5 can achieve particularly favorable results.

The aqueous treatment solution is usually obtained from a concentrate by dilution with water. Both batch and supplement concentrate contain about 1 to 25, preferably 2.5 to 10, g / 1 aluminum, 0.1 to 5 g / l fluoride and up to 10 g / 1 titanium, zirconium and / or hafnium. The pH of the concentrate is usually in the range from 0 to 4.

The aqueous acidic treatment solution is applied to the surface of iron or steel at a temperature of 26.7 to 82 ° C, preferably 32.2 to 54 ° C. The treatment time can be 2 seconds to 5 minutes, preferably 5 seconds to 1 minute. The contact of the metal surface with the solution can take place in dipping, flooding and in particular in spraying. The spray treatment is also advantageous because the conventional washing treatment into which the process according to the invention can be integrated, in particular because of the shape of the containers to be treated and the necessary thorough contact with the solution, is generally designed as a spray process anyway.

The process according to the invention can be carried out in the second stage of a three-stage washing process, in the third stage of a five-stage washing process or in the fourth stage of a six-stage washing process. In the case of the six-stage washing process, the fourth stage is used to rinse with water in the fifth stage and with deionized water in the sixth stage before drying takes place in a convection oven. The duration of the contact between the metal surface and the treatment solution is largely determined by the operation of the existing system. When setting the treatment temperature, too, you will usually follow the treatment temperatures otherwise prevailing in the system.

The invention is illustrated by the following examples, for example and in more detail.

example 1

• 6,5 g/I Fluorborsäure (HBF₄)
• 8 g/1 Kaliumfluorozirkonat (K₂ZrF₆)
• 130 g/1 Aluminiumsulfathydrat (Al₂(SO₄)₃. 14 H₂0 Rest Wasser

It contained a concentrate suitable for dilution with water

• 6.5 g / l fluoroboric acid (HBF ₄ )
• 8 g / 1 potassium fluorozirconate (K ₂ ZrF ₆ )
• 130 g / 1 of aluminum sulfate hydrate (Al ₂ (SO _{4) 3.} 14, H ₂ 0 balance water

produced. The pH was approximately 0.7.

The treatment solution was obtained by adding 3 of the aforementioned concentrate to 140 l of water. The pH was adjusted to 3.8 to 4.5.

The treatment of a black sheet container was carried out in a five-stage washing plant with the stages of alkaline cleaning, water rinsing, treatment with the above-mentioned treatment solution for a period of 1 min, water rinsing and rinsing with demineralized water. The container treated in this way with a remainder of water in the curved bottom was then dried at 163 ° C. No rust was visible on the surface of the container.

A continuous treatment line for black tin cans with the same treatment steps as above was stopped for half an hour. Thereafter, the cans in level 2 showed signs of rust, whereas the cans in levels 1, 3, 4 and 5 showed no visible rust.

Example 2

In a pilot plant of the type mentioned in Example 1, black tin cans were subjected to the alkaline cleaning, water rinsing, and contacting with an aqueous solution which contained 200 ppm aluminum, 75 ppm HBF ₄ , 80 ppm zirconium and had a pH of 4.4, Water rinsing, rinsing treated with deionized water. The individual treatment tanks were of practically the same length, so that the treatment times were practically the same at 40 seconds each. The temperature of the solution applied in the third stage was 49 ° C.

When the system was at a standstill for 35 minutes, the cans treated with the aqueous acid treatment solution showed no rust. In contrast, the cans which had not yet been treated by the process according to the invention, ie only cleaned and rinsed, showed rust almost immediately.

Example 3

Black tin cans were treated according to the procedure according to Example 2, with the exception that the pH of the aqueous acidic solution of stage 3 was set to 3.5. After the final rinsing treatment, the cans were dried in an oven at 193 ° C. for 3 minutes. The cans showed a brown-golden discoloration, which is intolerable if only a final treatment with a clear varnish is provided.

Example 4

Black tin cans were again treated according to the procedure in Example 2, with the exception that the pH of the acidic treatment solution was set to 5.5 in stage 3. After drying in the oven, the cans were shiny and without significant discoloration. Only some of the cans showed local discolouration on the floor curvature, the edges and contact surfaces of two neighboring cans. Some cans that had been removed from the system before the oven dried and therefore remained damp showed rust very quickly.

Example 5

According to the scheme of Example 1, a number of treatment solutions were prepared, which differed in the content of titanium, zirconium, hafnium. Finally, a solution was prepared as a control solution that contained only fluoride and was free of aluminum, zirconium, titanium and hafnium. In the first-mentioned solutions, the zircon was introduced as potassium fluorozirconate, the hafnium as hafnium dioxide (Hf0 ₂ ) and the titanium as hexafluorotitanic acid. In a procedure similar to Example 1, black tin cans were treated in a 19 liter spray tank for 1 min at 49 ° C. All treatment solutions had a pH of 4.3 and a fluoride content of 100 ppm (introduced as HBF ₄ ). The aluminum and titanium, zirconium and / or hafnium content (in ppm) of the solution and the results obtained are shown in the table below. The appearance of the can jacket and the base of the can after drying in the oven were used for evaluation.

Bei einem weiteren Vergleichsversuch wurden wässrige Lösungen eingesetzt, die frei von Fluorid waren und bei einem pH-Wert von 4,3 lediglich Aluminiumionen in einer Menge von 250 ppm enthielten. Bei Anwesenheit von überschüssigem Wasser im gewölbten Dosenboden wurde während der Ofentrocknung ein Rosten des Bodens festgestellt.

In a further comparison experiment, aqueous solutions were used which were free of fluoride and only contained aluminum ions in an amount of 250 ppm at a pH of 4.3. In the presence of excess water in the vaulted can base, rusting of the base was found during oven drying.

Example 6

In this example, the effect of pH on the appearance of the can was evaluated, using two different treatment solutions with identical conditions with regard to contact time of 5 seconds, temperature of 49 ° C. and fluoride concentration of 100 ppm (introduced as HBF ₄ ). One of these solutions contained 200 ppm aluminum ions and none of the metals titanium, zircon and hafnium. The second treatment solution contained 50 ppm zirconium and was free of aluminum. Corrosion and appearance of the cans after a plant shutdown were assessed. The results are summarized in Table 2.

It can be seen from Table 2 that the best results are obtained with treatment solutions with a pH of more than 2 and less than 4.6, which contain aluminum and are free of zircon, with a treatment duration of 5 seconds.

The best results are achieved with a zirconium-containing treatment solution (free of aluminum) at a pH in the range from above 3 to below 5.

The above examples clearly illustrate the influence of fluoride in preventing or greatly reducing rust in black tin cans and the effectiveness of aluminum in preventing discoloration of the treated can.

Claims (8)

1. Process for a corrosion protective treatment of metal surfaces of iron or steel using fluoride containing solutions, characterized in that the surfaces are brought in contact with a solution containing 10 to 5000 ppm aluminum ions, 10 to 200 ppm fluoride ions and up to 1000 ppm ions of at least one of the metals titanium, zirconium and/or hafnium and having a pH-value of 2 to 5,5.

2. Process according to claim 1, characterized in that the surfaces are brought in contact with a solution containing 25 to 250 ppm aluminum ions.

3. Process according to claim 1 or 2, characterized in that the surfaces are brought in contact with a solution containing at the utmost 150 ppm fluoride ions.

4. Process according to claim 1, 2 or 3, characterized in that the surfaces are brought in contact with a solution containing 40 to 320 ppm ions of at least one of the metals titanium, zirconium and/or hafnium.

5. Process according to one or more of the claims 1 to 4, characterized in that the metal surfaces are brought in contact with the solution at a temperature of 26,7 to 82°C.

6. Process according to claim 5, characterized in that the metal surfaces are brought in contact with the solution at a temperature of 32,2 to 54°C.

7. Process according to one or more of the claims 1 to 6, characterized in that the metal surfaces are brought in contact with the solution for a period of time ranging from 2 sec to 5 min.

8. Process according to claim 7, characterized in that the metal surfaces are brought in contact with the solution for a period of time ranging from 5 sec to 1 min.