NO813368L - PROVISION OF COATING ON METAL SUBSTRATE. - Google Patents

Description

The present invention relates to the deposition of coatings on metal substrates, and in particular a method for applying non-metallic transition coatings containing hydrated metal oxides.

Chromium-containing transition coatings have been deposited under acidic conditions from a

Cr^' - solution containing sulfuric or nitric acid. Coatings deposited from solutions containing sulfuric acid are golden-yellow, while coatings deposited from solutions containing nitric acid tend to have a faint blue colour. These coatings contain Cr^' and are also known as "chromate" coatings.

As discussed in British patent document 1531056 and DE-off.-document P 28 22 463, it is possible by electroplating to produce very satisfactory non-metallic (coatings containing hydrated trivalent chromium oxides from a Cr'" electrolyte.

It has surprisingly been established that by using a non-electrolytic method, non-metallic transition coatings containing Cr'", Fe", Fe'" or Ni" can be produced from a solution containing the corresponding metal ions.

The present invention specifies a method for depositing a coating on a metal substrate, whereby the metal substrate to be coated is brought into contact with an aqueous solution comprising one or more of the metal ion groups Cr'", Fe", Fe"' or Ni" in a concentration of up to 0.1 molar and an oxidizing agent which should depolarize the reaction that takes place on the surface of the substrate to be coated, so that a coating is thereby deposited on it.

Furthermore, according to the invention, a method for depositing a coating on a metal substrate is specified, whereby the metal substrate to be coated is brought into contact with an aqueous solution comprising one or more of the metal ion groups Cr'", Fe", Fe'" or Ni" , a weak complexing agent for the metal ions and an oxidizing agent which should depolarize the surface of the substrate to be coated, so that a coating is thereby deposited on it.

The invention includes in particular a method for depositing a Cr'"-containing coating on a metal substrate whereby the metal substrate to be coated is brought into contact with an aqueous solution comprising Cr(II) ions in a concentration of up to 0.1 molar and an oxidizing agent which must depolarize the reaction that takes place on the surface of the substrate to be coated. More specifically, the invention includes a method for depositing a Cr"'-containing coating on a metal substrate, whereby this is brought into contact with an aqueous solution comprising Cr"' ions, a weak complexing agent for the Cr"' ions and an oxidizing agent which should depolarize the surface of the substrate to be coated, so that a coating is thereby deposited on it.

The theoretical basis for the method according to the present invention differs from the basis for the electrolytic method of chromite deposition, which is known from British patent document 1531056 and DE-off.-skrift 28 22 463. According to the electrolytic method, the substrate to be coated is thus brought to act as a cathode (ie it becomes negatively charged) and various substances present in the electrolyte will react in the immediate vicinity of the cathode and thereby cause an increase in pH near the cathode. This increase in pH at the cathode causes the precipitation of chromite in the cathode film with the consequent deposition of chromite on the cathode surface. In contrast, in the method according to the present invention, the substrate will function as an anode, where a reaction takes place on the surface of the substrate

where M is the metal in the substrate and n+ is the oxidation state of the M ion that fits into the aqueous solution in contact with the substrate. This oxidation reaction will, however, release electrons which polarize the substrate. The aqueous solution contains metal ions and an oxidizing agent, the latter of which causes depolarization of the reaction that takes place at the surface of the substrate, thereby causing an increase in the pH value of the aqueous solution closest to the substrate with consequent precipitation and deposition of a transition coating on the substrate .

If no weak complexing agent is present in the aqueous solution, the practical minimum concentration of the metal ions used in the present invention will generally be 0.002 molar (about 0.1 gl"<*>as the metal ion). Below this concentration the reaction is too slow to to be usable in practice. Moreover, the solution becomes increasingly unstable when the concentration of metal ions drops below 0.002 molar. For example, in connection with Cr'"-containing solutions, there will be a serious risk of spontaneous precipitation of chromium-containing substances from the solution as a whole when Cr The concentration drops below this value. The maximum concentration of metal ions is 0.1 molar (approx. 5 gl~<*>as the metal ion). Above this concentration level, the solution tends to become powdery and non-adherent. Typically, the method will is carried out using a solution with a metal ion concentration of 0.03 to 0.08 molar (1.5 to 4 gl"<*>as the metal ion) and preferably 0.04 to 0.06 (2 to 3 gl-<1>as the metal ion), although the optimum concentration depends on the particular operating conditions.

Much higher concentrations of metal ions can be used in connection with the present invention if the solution contains a weak complexing agent for the metal ions, as the concentration of metal ions in such a case will be between 0.002 and 0.8 molar (0.1 to 40 gl~<*>as the metal ion ). At concentrations below 0.002 molar, the reaction is slow, in the same way as when no weak complexing agent is used. At metal ion concentrations above 0.8 molar, the reaction at the substrate surface will take place so quickly that it is almost non-selective and therefore produces uneven coatings. However, there is no advantage in using high metal ion concentrations, as satisfactory coating will be achieved by using lower concentrations. The use of high metal ion concentrations is also associated with increased capital costs and losses, e.g. ex trahe ring loss. When a weak complexing agent is used, the metal concentration will typically lie between 0.01 and 0.4 molar (0.5 to 20 gl-<*>as the metal ion) and preferably between 0.04 and 0.1 molar (2 to 5 gl~<*>as the metal ion), even whether the optimal concentration depends on the particular operating conditions.

The term "weak complexing agent" denotes a complexing agent which, in association with Cr"', Fe", Fe"' or Ni" forms a coordination complex of sufficient strength to retain a salt in aqueous solution at a concentration of 0.1 molar at a molar ratio of 2:1 between the metal ion and the complexing agent at a pH value of 6.0, but not at a pH value of 9.0.

In the definition, the particular nature of the weakly complexing agent is not particularly critical. Of materials which are suitable for use as a weak complexing agent, mention can be made of hypophosphite ions (only for Cr"' ions), acet ions, formate ions, citrate ions, glycine and glycol cations, of which hypophosphite ions (only for Cr"' ions), acetate ions and formate ions preferred. Although glycine, glycolations and citrate ions can be used, these are not preferred due to their tendency to complex with metal ions to a greater extent than is usually desired. If used, the weak complexing agent will usually be present in such an amount that the molar ratio between the complexing agent and the rnetalions is between 1:10 and 10:1, preferably between 0.3:1 and 2:1 and optimally between 0.5:1 and 2:1.

According to the present invention, the oxidizing agent is used to depolarize the reaction at the substrate surface. In the absence of an oxidizing agent, the low acidity of the solution will cause the metal dissolution reaction that results in the deposition of a protective film to be interrupted, thereby preventing the formation of continuous, protective films. When the oxidizing agent is used, an alternative reaction course is created that does not require the presence of high hydrogen ion concentrations to promote the dissolution and deposition reactions. By its mode of action, the oxidizing agent causes an increase in the pH value of the solution film at the metal substrate surface, which results in the precipitation of hydrated metal oxide on the substrate surface, which forms a continuous transition coating. To depolarize the substrate surface, the red oxide potential of the oxidizing agent must be more positive than the corresponding one for M/M<n+>, i.e. the oxidizing agent must be able to oxidize M to M<n+>, where M is the metal in the substrate surface and n+ is the oxidation state of the M ion from the surface to the substrate. In order to be utilized in the present invention, the oxidizing agent must also be stable to acid, and must be able to oxidize under alkaline conditions. Furthermore, the oxidizing agent, if present in the solution, must not attack the weakly complexing agent. Suitable oxidizing agents include H2O2, inorganic peroxide anions which, on dissociation in aqueous solution, give hydrogen peroxide, and nitrate ions. Examples of peroxide anions include persulfates and perborates. Examples of nitrate ion sources include sodium nitrate and potassium nitrate. The oxidizing agent will generally be present in the solution in an amount of 0.25 to 20

gl-<*>, as the reaction becomes rather slow at concentrations below approx. 0.25 gl"<*>and so quickly at concentrations above 20 gl<-*>that the metal substrate dissolves quickly so that the deposition of chromite does not take place uniformly. The concentration of the oxidizing agent will typically be 2 to 12 gl-<*>and most preferably 5 to 8 gl-<*>, although the optimum concentration depends on the nature of the substrate and the metal ion used and on the reaction conditions, such as pH value, temperature and metal ion concentration. When using a peroxide, it may be advantageous to add one or several additives to stabilize the peroxide anion. Such materials are known in themselves and include, for example, acet ions. If a stabilizer is used, however, this must satisfy the criterion that it does not have a disturbing effect on the other components of the solution.

Another oxidizing agent which is suitable for use in connection with the present invention is the ferricyanide ion, although this must not be used in the presence of metal ions which form insoluble complexes in association with ferricyanide ions. For this reason, ferricyanide ions are unsuitable for use together with Fe", Fe'" and Ni" ions. However, ferricyanide has the advantage of being renewable. During the oxidation reaction, ferricyanide is reduced to ferrocyanide. This ferrocyanide can then be oxidized by adding a suitable oxidizing agent, for example hydrogen peroxide, for regeneration of the ferricyanide. When used, the ferricyanide ion is generally present in the solution at a concentration of 1 to 30 gl"<*>. It can suitably be added in the form of an alkali metal or as an ammonium salt.

According to the invention, the used source for the metal ions (Cr"1, Fe", Fe'" and Ni") is not critical, provided that the anion in the salt used does not interfere with the reaction. Suitable salts include chromium sulfate, chromium chloride, ferroammonium

sulfate and nickel chloride

The presence of chloride ion in the solution has a beneficial effect which provides a more uniform transition coating, and chloride ions, e.g. from NaCL, can be added to the solution if desired. The concentration of chloride ions should generally lie between 0.2 molar and the saturation point of the chloride salt used, but usually not above approx. 0.3 molar (10 gl-<*>as CL-).

Transition coatings produced using the method according to the invention may also contain other ions in addition to one or more Cr"', Fe"-, Fe"'- and Ni"-ions which are already present. These additional ions must of course not have an adverse effect on the transition coatings, or have a disruptive effect on the other components of the system. For that reason, Cr^'-, Ni''- and Mn-ions in a high oxidation state should be excluded from the solutions used in the practice of the present invention. The solutions may, however, contain other ions which do not interfere with the deposition of the transition coatings, but which in reality can change or favor the properties of the transition coatings produced, and of these ions can be mentioned one or more of the ion groups Mg, Al, Zn, Mn", Ti'" and Ti'^ which can be present in the solution in a concentration of 0.1 to 5.0 gl-<*> (as the special ion) and preferably 1 to 3 gl"<*>. The produced transition coatings consist of a mixture of hydrated oxides.

In connection with the present invention, the term hydrated metal oxides is used as a designation for one or more oxides, hydroxides and hydrous oxides of the specific metal in question.

Typical substrates that can be coated according to the invention include zinc (including zinc galvanized steel), tin (including tinned steel), cadmium, iron, steel and especially stainless steel, magnesium, copper, nickel and alloys of these materials. The method according to the invention can also be used for depositing a protective coating on aluminium. In connection with aluminium, aluminum alloys are included. Aluminum is generally resistant to corrosion due to a thin but continuous oxide film that forms naturally on the outer surface. It occurs to an increasingly large extent when aluminum is used that the surface of the metal must later be painted, lacquered or laminated to plastic materials. In such cases, it has been shown that the oxide film on the aluminum surface generally does not allow strong adhesion of paint or varnish. On the other hand, transition coatings and especially chromite coatings produced in accordance with the method according to the invention will favor the adhesion of paint films or varnish films. The application of a transition coating on aluminum is also a generally faster and cheaper process than the usual anodizing process. In order to apply a transition coating of hydrated metal oxide to aluminium, it is necessary to remove the natural oxide film from the aluminum surface before the hydrated metal oxides can be deposited. The method according to the present invention can be used for depositing a transition coating of hydrated metal oxide on an aluminum urn surface, the reaction solution also containing a material for dissolving and removing the natural aluminum oxide film. Such materials must not interfere with the other materials in the solution. A preferred material is fluoride ion which, by affecting the aluminum oxide film, produces water-soluble fluoroaluminate. If fluoride ions are not already present in the solution, F~ ions can be provided by adding a fluoride salt to the solution, e.g. sodium fluoride (or a material which emits fluoride ions in solution, such as fluoroborates or fluorosilicates). The concentration of fluoride in the solution will generally lie between 1-20 gl-<*>, possibly between 3 and 8 gl-<*> (expressed as NaF) when it is added as a single salt, and between 3 and 15 gl-<* > (as fluorosilicate) when added as a compound salt.

According to an alternative method for removing any aluminum oxide film from the surface of an aluminum or aluminum alloy substrate, the surface to be coated is, preferably by immersion, brought into contact with a pretreatment bath before it is brought into contact with the coating solution. This can be carried out appropriately by immersing the aluminum substrate in a pre-treatment bath containing the material, preferably fluoride ions, to dissolve the aluminum film, after which the pre-treated substrate, preferably without rinsing, is transferred to the coating solution. It may of course be desirable for an aluminum urn surface to be pre-treated for some time before it is brought into contact with the coating solution, and in such a case it must be ensured that the "clean" aluminum urn surface is prevented from oxidizing again. Techniques for carrying out this are commonly known, including storage under N2.

During the process according to the invention, the solution will normally be at ambient temperature, as a satisfactory coating can be produced at this temperature. However, if desired, higher solution temperatures can be used. In the absence of a weak complexing agent for metal ions, the temperature should generally not exceed approx. 50°C, as the reaction at even higher temperatures becomes rapid and the deposition uneven. If the solution contains a weak complexing agent, a somewhat higher temperature, for example up to approx. 80°C, is accepted. At these higher temperatures, however, there will be an increased risk of a catastrophic reaction which entails increased dissolution of the metal substrate and the deposition of uneven films of poor quality. If, however, a weak complexing agent is present, the temperature will not, as a rule, be allowed to exceed approx. 60°C

The pH value of the solution is moderate and lies between 1 and 7. The choice of pH in each particular case will depend on the metal in the solution and, to a lesser extent, on the nature of the substrate. Typical and optimal pH values are indicated in the tables A and B below for solutions that do not contain and that contain a weak complexing agent, respectively.

It has surprisingly been found that transition coatings with unexpectedly improved corrosion resistance can be deposited by solutions containing boric acid in addition to the above-mentioned components. In certain cases of appeal, e.g. if the coated substrate is intended for use in corrosion-forming environments, for example in a seawater environment, the increased corrosion resistance achieved by the use of boric acid can be of great importance. Since coatings made from solutions containing boric acid are often opaque or at least unclear, the use of boric acid will not be appropriate if transparent and colorless coatings are desired, e.g. on decorated silverware.

The presence of boric acid in the coating solution increases the coating deposition rate, so that a thicker coating can be formed in a given treatment period. Although it is not completely clear why the use of boric acid leads to this result, the main reason for this effect has to do with the fact that the boric acid in the solution has the ability to function as a pH buffer. By controlling the pH flow at the substrate surface, the boric acid will contribute to the formation and clumping of the metal oxide/hydroxide precipitate in this zone. Boric acid's buffering power in aqueous solutions reaches a maximum at pH values of 4 to 6, and the best results can be expected to be achieved within this pH range. It is clear from this that the typical and the optimal pH values stated in the above tables A and B are in most cases not valid for solutions containing boric acid.

The increase in corrosion resistance of transition coatings deposited from boric acid solutions is believed to be partly due to the fact that certain borate-containing sub-

punches are absorbed into the coatings during deposition. In a solution containing aluminum or titanium ions, hard and resistant aluminum borates or titanium borates can form and be absorbed into the deposited transition coating. The absorption of these borate-containing substances is of significant importance in that, when the transition coating is brought into contact with a corrosion-forming, aqueous solution during use, the borates, due to their buffering effect, will "suffocate" any galvanic reaction that would normally cause corrosion on the surface of a coated substrate .

When used in connection with the present invention, boric acid will usually be added to the solution to a concentration of 1 to 40 gl<-*>, preferably 5 to 25 gl-<1>.

If aluminum or aluminum alloy surfaces are to be coated using treatment solutions containing boric acid, care must be taken when using the free fluoride ions. As previously discussed, fluoride ions are particularly useful for dissolving and removing oxide films that have formed on aluminum surfaces. Unfortunately, free fluoride ions will react with the boric acid and the borate ions derived therefrom to give various fluoroborate substances and ultimately boron tetrate fluoride ions. This difficulty can be avoided to some extent by adding boric acid and sufficient fluoroborate to ensure satisfactory concentrations of fluoride ions. Alternatively, instead of adding free fluoride ions directly to the solution containing boric acid, a material can be added, e.g. a fluorosilicate or a fluoroborate, which releases fluoride ions in a slow and controlled manner. Preferably, the risk of a "mop-up" reaction between fluoride ion and boric acid is eliminated by pre-treating the aluminum

surface to be coated, in a fluoride-containing bath to dissolve and remove the oxide film, after which the treated aluminum urn surface is transferred to the coating solution. Usually, the pre-treated aluminum urn surface will be transferred immediately, and preferably without rinsing, to the coating solution.

Under the previously described conditions, contact between the substrate and the reaction solution for a period of just a few seconds will be sufficient for the deposition of a film on the substrate. In general, the contact period will be determined by the desired coating thickness, which in turn depends on how the coated substrate is to be used. The time during which the substrate is in contact with the reaction solution will typically amount to 5 seconds to 20 minutes and in most cases 30 seconds to 5 minutes.

The method according to the invention can be carried out in portions or continuously without difficulty. Many existing commercial methods for depositing transition coatings have been developed to increase throughput during the process, and the present invention is particularly suitable for a "no rinse" system, where the substrate to be coated, for example metal strip or metal plate, is immersed in the treatment bath for a period of 3 to 15 seconds. After the substrate is retrieved from the treatment bath, the remaining treatment solution on the substrate surface is not washed off, but is allowed to continue the reaction with the metal surface until drying is complete. In this way, transition coatings can be produced with thicknesses that would indicate a longer immersion time.

Within the framework of the invention, a substrate to be coated, instead of being immersed in the treatment bath, can be sprayed with the coating solution which then reacts with the metal substrate surface.

Newly applied films are soft and can be removed from the substrate by careful sanding. However, the films can be hardened and made more resistant to mechanical wear by air drying, usually for at least 24 hours. Preferably, however, the coated substrates are dried in an oven at temperatures above 40°C for at least half an hour, but preferably at 100 - 110° for approx. 1 hour. However, it is important that drying takes place under such conditions that the coating does not break.

According to an alternative method for drying newly applied coatings, the coated substrate is passed through a dewatering liquid. The use of dewatering liquids is well known in and of itself. The solutions according to the present invention will generally not react with the organic compounds in dewatering liquids, and a dewatering step can therefore be used as an alternative to rinsing after immersion in transition coating solutions. The use of dewatering liquids instead of rinsing has the advantage that it does not involve the extraction of transition coating solution, and that the problems in connection with waste water treatment are eliminated or reduced to a significant extent. Dewatering liquids with added corrosion inhibitors or wax can be used to increase the coating's corrosion resistance, as the corrosion inhibitor or wax remains on the object's surface after the dewatering liquid has evaporated. Dewatering liquids generally contain a solvent which is not miscible with water, and preferably also a surfactant and a transfer catalyst solution for the surfactant. The surfactant displaces water from the surface of the coated metal substrate. For this reason, the surfactant must be poorly soluble in water and may include long-chain aliphatic groups. The solvent is usually "white spirit", petroleum or a light mineral oil. Corrosion inhibitors for zinc can consist of 2,5 disulfhydrylthia-diazole, dithiooxamide and several other compounds known in the field.

The typically short period of time during which the substrate is in contact with the reaction solution enables the practice of the present invention both on a continuous and on a portionwise basis. A continuous metal strip or strip can, for example, be drawn through a vessel containing a reaction solution according to the present invention at such a speed that the band or strip has the desired film thickness at the exit of the vessel.

The method according to the invention is carried out under such conditions that a transition coating with a thickness of 0.01 to 5 pm can be generally produced. The coating thickness will of course be determined by the intended purpose and application of the coated substrate. In most industrial applications, where the transition coating is to protect the substrate surface against corrosion, a thickness of 1 to 5 pm will usually be desired. Decorative silver articles can be advantageously applied with a transition coating that will protect the silver surface against corrosion during storage of the articles. Silver articles can be treated in accordance with the invention, preferably using a Cr"'-containing solution together with peroxide as an oxidizing agent to apply a protective chromite coating of appropriate thickness to the objects, usually about 0.05 p. If a transition coating is to be deposited as a "key" -layer on a substrate to promote the adhesion of subsequently applied paint or varnish films, a coating thickness of 0.1 to 1.0 pm will be acceptable in most cases.

It will usually be preferred to prepare a concentrate of the reaction solution containing all components except the oxidizing agent. Such a concentrate can be stored and, prior to use, diluted to the desired degree with subsequent addition of the oxidizing agent. This method is particularly preferred if the oxidizing agent consists of peroxides, especially hydrogen peroxide, as peroxides tend to become unstable when stored in the presence of heavy metal ions, for example Cr'" ions. A Cr'"-containing concentrate will typically contain Cr<1 >" in a concentration of 40 to 50 gl"<*>as Cr1".

The transition coatings obtained by the method according to the invention can serve as a primer coating for later applied paint or varnish layers. In particular, a chromite film will ensure improved adhesion of the paint or varnish coating. Furthermore, the transition coating will provide additional protection against corrosion by preventing substrate metal corrosion under the film. The coatings can also be used to connect layers of plastic material to the metal substrates when manufacturing laminates.

The invention will be illustrated by the following examples. Chromethane is a commercially available, basic chromium sulfate having the approximately stoichiometric formula

3Na2S04.2Cr2(S04)3.Cr2<0>3.nH20 when n<l and which gives 1 gl"<1>chromium ion per 6.25 gl~<*>. ASTM Test No. B - 117 was used in the neutral salt spray test.

Example 1

A steel sheet was galvanized with zinc from a bright plating solution to a thickness of 10 µm. After plating, the zinc plate was blanked by immersion in 0.1% nitric acid and then dried at 60°C for 1 hour. The plate was then sprayed with 5% neutral salu Degradation was evident after 1 hour, and massive zinc corrosion products (white rust) were visible after 4 hours.

A second steel plate was galvanized and blanked in the same manner and then immersed in a solution containing 25 g of chromate for two minutes at a temperature of 25°C and a pH value of 3.0. The plate was rinsed and dried at 60°C for 1 hour and then the salt spray test as before. Corrosion attack occurred quickly.

A third plate was treated in the same manner, except that, after plating and blanking, it was immersed for two minutes in a solution containing 25 gl~<*>chromethane and 12 gl"<*>sodium hypophosphite at a temperature of 25°C and pH value 3.0 The salt spray test resulted in rapid corrosion of the tin.

A fourth plate was treated in the same manner, except that, after plating and blanking, it was immersed for two minutes in a solution containing 25 gl"<*>chromethane, 12 gl~<*>sodium hypophosphite, and 8 gl"<*> sodium nitrate at a temperature of 25°C and pH value 3.0. No corrosion was visible after 24 hours of salt spray testing.

Example 2

A solution was prepared containing 240 gl"<*>chromethane and 120 gl"<*>sodium hypophosphite. A passivation solution was prepared by diluting 1 part of the concentrate with 9 parts of water and adding 8 gl~<*>sodium nitrate.

Steel sheets that were zinc galvanized and blanked as in Example 1 were immersed in this solution for periods of time varying from 10 seconds to 20 minutes at temperatures between 15 and 75°C and pH values between 1.0 and 4.5. After drying, the passivated plates were subjected to the salt spray test. All plates passed 4 hours of testing, but plates passivated at pH values below 1.7, temperatures above 55°C and immersion periods below 20 seconds showed signs of corrosion after 24 hours of testing. All the other boards passed at least 30 hours of testing.

Example 3

A passivation solution was prepared in the same manner as in Example 2, except that the sodium nitrate was omitted and 12 ml of 1" hydrogen peroxide (3096 v/v) was added. A steel plate was zinc coated and blanked as in Example 1, and then immersed for 2 minutes in the passivation solution at a temperature of 25°C and a pH value of 3.0 No zinc corrosion was visible after 24 hours of salt spray testing.

Example 4

A passivation solution was prepared by dissolving 12 gL of chromate (2.0 gL of chromium ion) in water and adding 8 gL of sodium nitrate. A steel plate that was zinc coated and blanked as in Example 1 was immersed in the passivation solution for 4 minutes at temperature 25°C and pH value 3.0 No corrosion was visible after 24 hours of salt spray testing.

Example 5

A steel plate was coated with 10 µm zinc from a dull acid zinc plating solution (a solution commonly used in electrogalvanizing). The zinc plate was passivated by immersing for 2 minutes in a solution containing 24 gl"' of chromate, 12 gl"' of sodium hypophosphite and 6 gl"' of sodium nitrate, at a temperature of 25°C and a pH value of 3.0. No corrosion was detectable after 24 hours of salt spray testing .

Example 6

A steel plate that was zinc-coated and blanked as in example 1 was passivated by immersing for 2 minutes in a solution containing 24 gl"' of chromate, 10 gl"' of sodium formate and 8 gl"' of sodium nitrate at 25°C and a pH value of 3.0 .No corrosion was detectable after 24 hours of salt spray testing.

Example 7

The experiment of Example 6 was repeated, except that instead of sodium formate, the passivation solution contained 10 g of glycine. No corrosion was detectable after 4 hours of salt spray testing, but the plate was unacceptably corroded after 24 hours of salt spray testing.

Example 8

A steel plate was cadmium plated and then passivated by immersing for 2 minutes in a passivation solution containing 24 gl"' of chromate, 12 gl"' of sodium hypophosphite and 8 gl"' of sodium nitrate at a temperature of 25°C and a pH value of 3.0. This plate showed no corrosion after 24 hours of salt spray testing A similar unpassivated cadmium plated steel plate was corroded after only 8 hours of salt spray testing.

Example 9

A brass plate was copper plated, rinsed and then passivated by immersing for 2 minutes in a solution containing 24 gl"' of chromate, 12 gl"' of sodium hypophosphite and 5 ml of 1"' of hydrogen peroxide (30% v/v), at a temperature of 25°C and pH value 3.0. After drying, the plate was immersed in a polysulfide solution, and no blackening of the copper was detectable after 30 seconds of immersion. A corresponding unpassivated copper-coated brass plate was blackened immediately by immersion in the polysulfide solution.

Example 10

Aluminum plates were degreased and immersed for 30 seconds in a solution containing 24 gl"' of chromate, 8 gl"' of sodium nitrate and 5 gl"' of sodium fluoride at a temperature of 30°C and a pH value of 3.5. The plates were rinsed and air-dried at 100°C in 1 hour. These samples underwent a 5% neutral salt spray test together with plates of the same aluminum alloy that had not been treated by immersion. After 96 hours of testing, the untreated samples showed widespread corrosion. The treated samples were in the same condition as at the start of the test. The passivation film was colorful.

Example 11

Aluminum sheets were degreased and immersed for 30 seconds in a solution of the same composition as in Example 10, but additionally containing 12 g/L of sodium hypophosphite. The immersion conditions were as in Example 10. A color playing passivation film was applied which provided excellent corrosion protection for the aluminum alloy during salt spray testing. No deterioration in appearance was detectable after 200 hours of salt spray testing.

Example 12

Degreased aluminum plates were protected by immersing in a solution containing 1 gl" of chromium III cation, 5 gl" of sodium hypophosphite and 5 gl" of sodium nitrate and 5 gl" of sodium fluoride. The solution's pH value was a) 4, b) 3, c) 2 at a temperature of 30°C. All test pieces were rinsed and dried in air at 100°C. All plates showed excellent corrosion resistance to neutral salt spraying.

Example 13

A solution containing 4 g/l of nickel (as nickel chloride) and 8 g/l of sodium nitrate was prepared. The pH value was adjusted to 5.5 and the temperature maintained at 50°C. A steel plate that was zinc coated and blanked as in example 1 was immersed in the solution for 10 minutes. No corrosion was detectable after 24 hours of salt spray testing.

Example 14

The experiment according to example 13 was repeated using a solution containing 4 gl"' of iron (as ferroammonium sulfate) and 8 gl"' of sodium nitrate at a temperature of 25°C and a pH value of 3.0. The plate was immersed in this solution for 2 minutes. No corrosion was detectable after the 2-hour salt spray test. Slight corrosion was visible after 4 hours of testing, and widespread corrosion after 24 hours of testing.

Example 15

A solution was prepared containing 2 gl"' Cr'" (as chromium sulfate), 2 gl-<1>Al"' (as aluminum sulfate), 8 gl"' NaNC>3 and 4 gl"' hypophosphite (as sodium hypophosphite). The solution was adjusted to pH 3.5 at a maintained temperature of 25°C A zinc-coated steel plate was immersed in the solution for 2 minutes and dried.The plate showed a salt spray resistance of 100 hours by neutral salt spray during corrosion testing.

Example 16.

A solution containing 24 gl"' of chromate, 12 gl"' of sodium hypophosphite and 8 gl"' of sodium nitrate was prepared and the pH adjusted to 3.0. Zinc-coated steel plates were immersed in the solution for 1 minute at ambient temperature. The plates were rinsed and dried. A pale blue coating could be seen on the zinc surface. 10 gl"' of boric acid was added to the solution, and a second group of identical plates were treated in the same manner. The plates in the second group had the same appearance as the plates in the first group. The pH value of the solution containing boric acid was increased by the addition of 10% NaOH solution until precipitation of chromium hydroxide could be detected, and the precipitate did not redissolve on standing. A third plate group was treated in this solution. After rinsing and drying, the plates showed a green color.

The three plate groups were tested for corrosion in a salt spray cabinet. Zinc corrosion on the test pieces in the first two groups was detectable after 48 hours of salt spray testing. The plates in the third group showed no zinc corrosion after 300 hours of salt spray testing.

Example 17

The same experiment as in Example 16 was carried out with the exception that the passivation solution did not contain sodium hypophosphite. The first two plate groups showed zinc corrosion after 48 hours of salt spray testing, but the third group showed no corrosion after 200 hours of salt spray testing.

Example 18

A solution containing 18 g of chromate, 7.5 g of sodium fluoride and 10 g of sodium nitrate was prepared and the pH of the solution was adjusted to 2.5. Aluminum plates were degreased, etched, cleaned and rinsed and then immersed in 30 seconds in the transition coating solution at 25°C After rinsing and drying, the panels underwent salt spray testing for more than 1000 hours. only 100 hours.A third panel group was similarly treated except that after being immersed for 2 seconds in the solution, the panels were dried without rinsing, and these panels withstood more than 100 hours of salt spray testing.

Example 19

A solution containing 24 gl-' of chromate, 12 gl-' of sodium hypophosphite and 8 gl-' of sodium nitrate was prepared and the pH value of the solution was adjusted to 3.0. Zinc-coated steel plates were immersed for 1 minute in this solution at 25°C. After being collected, the plates were immersed in Dewater Fluid IL 968 from Esso Ltd. for 2 minutes. After this treatment, the transition coating solution was completely removed from the surface of the test pieces, which had been coated with a film of dewatering liquid.

Example 20

A solution containing 24 gl-' of chromate, 12 gl-' of sodium hypophosphite and 10 gl-' of potassium ferricyanide was prepared, the pH value of the solution was adjusted to 3.0 and the process temperature to 25°C. Zinc-coated steel plates were immersed in the solution for 60 seconds and were , after rinsing and drying, the salt spray test. The specimens withstood 24 hours of salt spray testing. Corresponding plates treated in the solution that did not contain ferricyanide showed extensive corrosion after only 4 hours of salt spray testing.

Example 21

A steel sheet was electroplated with 5 µm of tin, dried and immersed for 3 minutes in a transition coating solution containing

pH value adjusted to 2.1 at temperature 25°C.

After air drying, the plate underwent neutral salt spray testing, and no tin corrosion products could be traced. 24 hour testing. For comparison, a similar tin-plated steel plate without a transition coating was subjected to the salt spray test, and after 8 hours of testing, the tin surface was severely discolored by tin corrosion products.

Example 22

Four aluminum sheets and four electrogalvanized steel sheets (10 pm Zn) were cleaned in the usual way and pretreated for the application of transition coatings. Two transition coating solutions were prepared, each of which contained:

Two of the aluminum sheets and two of the galvanized steel sheets were immersed in each of the solutions for 1 minute to apply a transition coating. The eight plates were rinsed in water, air-dried for 48 hours at ambient temperature and electrostatically powder-ground. Each plate was scored in a criss-cross pattern through the coating and into the substrate metal. All panels underwent neutral salt spray testing, during which all withstood 500 hours and then underwent 300 hours of humidity testing (also in accordance with ASTM Test No. B-117). Examination of the plates revealed that they all met the standard requirements, as they showed no signs of underfilm corrosion or loss of paint adhesion.

Claims (17)

1. Method for depositing a transition coating on a metal substrate, characterized in that the metal substrate is brought into contact with an aqueous solution containing one or more of the metal ion groups Cr'", Fe", Fe'" or Ni", in a weak complexing agent for the metal ions and an oxidizing agent for depolarizing the reaction that takes place on the surface of the substrate to be coated, whereby a transition coating is deposited on the substrate. 2. Method for depositing a transition coating on a metal substrate, characterized in that the metal substrate is brought into contact with an aqueous solution containing one or more of the metal ion groups Cr'", Fe", Fe'" or Ni" in a concentration that does not exceed 0.1 molar, and an oxidizing agent for depolarizing the reaction that takes place on the surface of the substrate to be coated, whereby a transition coating is deposited on the surface of the substrate. 3. Method in accordance with claim 1 or 2, characterized in that the solution contains one or more of the substances Mg", Al'", Zn", Mn", Ti"' or Ti" in addition to the mentioned metal ions. 4. Method in accordance with claim 1, characterized in that the concentration of the metal ions is between 0.002 and 0.8 molar. 5. Method in accordance with claim 2, characterized in that the concentration of the metal ions is between 0.04 and 0.06 molar. 6. Method in accordance with claim 1 or 2, characterized in that the metal ions consist of Cr"' ions. 7. Method in accordance with claim 1, characterized in that the weak complexing agent includes one or more hypophosphite, acetate and formate ions. 8. Method in accordance with claim 1, characterized in that the molar concentration ratio between the weak complexing agent and the metal ions is between 0.3:1 and 2:1. 9. Method in accordance with claim 1, characterized in that the oxidizing agent includes one or more of nitrate ions, hydrogen peroxide, peroxide ions and ferricyanide ions. 10. Method in accordance with claim 9, characterized in that the poison is nitrate ions in a concentration of 2 to 12 gl"'. 11. Method in accordance with claim 1 or 2, characterized in that the substrate consists of zinc, tin, cadmium, iron, steel, magnesium, copper, nickel, silver or aluminum or alloys thereof. 12. Method in accordance with claim 11, characterized in that the substrate consists of aluminum or an aluminum alloy, and that the substrate is treated to remove the oxide film from the surface by being brought into contact with fluoride ions in aqueous solution either before or during the deposition of the transition coating. 13. Method in accordance with claim 1 or 2, characterized in that the solution contains boric acid or borate ions in a concentration of 1 to 40 gl" 1- 14. Method in accordance with claim i or 2, characterized in that the substrate is in contact with the solution for a period of 5 seconds to 20 minutes, during which a transition coating with a thickness of 0.01 to 5 pm is applied. 15. Method in accordance with claim 1 or 2, characterized in that the substrate, after immersion, is dried, optionally with prior rinsing, or dewatered. 16. Method in accordance with claim 1 or 2, characterized in that the transition coating on the substrate undergoes subsequent storage or firing in order to impart increased hardness and resistance to mechanical wear. 17. Method in accordance with claim 1 or 2, characterized in that the substrate is later painted, lacquered or laminated into a layer of plastic material.