EP0716163B1 - Chromating bath and process for finishing surfaces of zinc, zinc alloys or cadmium - Google Patents

Description

The present invention relates to the finishing of surfaces of zinc, of alloy of zinc, or cadmium in order to give them a high resistance to corrosion.

It has long been known that corrosion resistance can be improved of said surfaces by subjecting them after zinc plating to a passivation treatment using chromating baths. However, for parts used under engine cover and therefore subject to thermal shock and corrosion, the requirements of the automotive industry have recently become more severe since manufacturers are now asking for corrosion resistance conforms after said surfaces have undergone a thermal shock of hour at 120 ° C, even 180 ° C.

Corrosion resistance is assessed in the laboratory by an accelerated test salt spray, executed in accordance with AFNOR NFX41002 / ASTM B117-73 / DIN40046-11. Automotive industry demands that shocked parts have no zinc salt point (or white rust) after 200 hours of salt spray exposure and no red rust spots after 400 hours, even 600 hours. In addition, also after thermal shock, said parts must also successfully pass natural corrosion testing.

On the other hand, parts intended for the automobile industry must satisfy aesthetic requirements of color: yellow, green, and more particularly black. This black color which must be deep, shiny and uniform, is particularly difficult to obtain when high corrosion resistance is required at the same time after one hour thermal shock at 120 ° C.

To improve corrosion resistance, patent RO 94 238 summarized in Chemical Abstracts under number 111: 138906s recommends adding to the bath chromating of metal ions (Cu, Ag, Mo, Co, Fe) or a combination of these (Cu + Ag + Mo or Cu + Fe + Co). All the baths exemplified contain copper.

Patent FR 947 164 describes a chromium-containing chromating bath hexavalent, silver and sulfate and acetate ions. This bath does not contain cobalt and its minimum content of sulfate ions is 34 g / l.

Patent EP 274 543 which describes a chromating bath comprising a acrylic polymer emulsion provides for the optional addition of heavy metals such as Co, Ni, Mn or Zn, but it indicates that sulfate ions are to be avoided.

In patent EP 508 207 relating to the black coloring of galvanized steel sheets, the chromatization step is followed by the application of a composition comprising thermosetting resin and acid black dye.

For the finishing of galvanized surfaces, patent DE 4 005 112 recommends incorporating copper sulphate into the chromating bath and applying a black dye.

After chromation in a solution containing chromic anhydride, cobalt sulfate, copper acetate and sodium formate, patent HU 18,661 summarized in Chemical Abstracts under number 94: 144073q recommends immersion pieces in a bath of sodium acetate, sodium benzoate and nigrosine.

In patent FR 2 522 023 are described chromation baths based on chromium, copper and silver or molybdenum which provide a coating shiny black. However, after an hour's thermal shock at 120 ° C, this type of coating does not last more than 48 hours in the salt spray corrosion test.

The use of a chromating bath comprising an acrylic polymer, a chromate and a strong mineral acid allows according to patent EP 264 472 to obtain a beautiful black coloration resistant to corrosion in salt spray after thermal shock. However, the implementation of such a bath is more delicate than that of a classic chromate. On the other hand, the film obtained turns out to be unstable (formation of salts chromium yellow) and the use of a polymer causes the appearance of "drops" on the parts treated at the fastener and difficulties in cleaning the equipment (assemblies and wringer baskets) used for processing.

It was also proposed to subject the parts to chromating conventional, then treating them in a separate bath containing the acrylic polymer and phosphate. This two-step process does not give reproducible results and, like the previous method, has the disadvantage of leading to training of "drops".

It has now been found that by using a chromating bath comprising cobalt instead of copper and having a content of 5-20 g / l of sulfate ions, and a finishing bath based on colloidal silica, can be coated on zinc, zinc alloy, or cadmium surfaces beautiful black color with excellent corrosion resistance in salt spray after thermal shock.

The present invention therefore relates to a process for finishing surfaces zinc, zinc alloy, or cadmium, characterized in that the parts to to be treated are brought into contact first with an acid chromate bath based on chromium, cobalt and silver salts, then with an aqueous finishing bath comprising colloidal silica, a corrosion inhibitor, a complexing agent, an agent surfactant and a black dye.

• 5 à 40 g (de préférence 7 à 15 g) d'ions chrome hexavalent,
• 5 à 20 g (de préférence 7 à 12 g) d'ions sulfate,
• 0,1 à 0,3 g (de préférence 0,1 à 0,2 g) d'ions cobalt,
• 0,1 à 0,65 g (de préférence 0,15 à 0,3 g) d'ions argent, et
• 50 à 150 ml (de préférence 70 à 120 ml) d'au moins un acide organique faible.

The chromating bath according to the invention is an aqueous solution having a pH of between 1 and 2 and containing per liter:

• 5 to 40 g (preferably 7 to 15 g) of hexavalent chromium ions,
• 5 to 20 g (preferably 7 to 12 g) of sulfate ions,
• 0.1 to 0.3 g (preferably 0.1 to 0.2 g) of cobalt ions,
• 0.1 to 0.65 g (preferably 0.15 to 0.3 g) of silver ions, and
• 50 to 150 ml (preferably 70 to 120 ml) of at least one weak organic acid.

To prepare the chromating bath according to the invention, preferably used sodium dichromate, cobalt sulfate and silver nitrate, but we would not depart from the scope of the present invention by using other water-soluble salts hexavalent chromium, cobalt or silver. As weak organic acid whose role is to reduce hexavalent chromium and regulate the attack of zinc, we preferably uses acetic acid, formic acid or oxalic acid. The pH of chromating bath, advantageously adjusted by means of sulfuric acid, is preferably between 1.2 and 1.8.

The finishing bath must be able to operate at a pH of between 1 and 5, preferably between 2.5 and 4, and have excellent chemical stability without any gelling of the colloidal silica during storage or working. To do this, in the finishing bath to be used according to the invention, the colloidal silica is present at a concentration such that there is from 20 to 40 g / l expressed as SiO ₂ . The weak complexing agent, used in a concentration of 0.1 to 2 g / l, can be for example gluconic acid, oxalic acid, citric acid, maleic acid, phthalic acid, or a salt potassium, sodium or ammonium of such an acid. The corrosion inhibitor is used at a concentration of 0.01 to 1 g / l and can be, for example, hydrazine hydrate or a benzoate such as sodium benzoate. The surfactant used at a concentration of 0.01 to 2 g / l can be of nonionic or anionic nature.

To improve the final appearance and without any negative impact on the resistance to corrosion, a soluble black dye can be added to the finishing bath in water, preferably a dye of the azo metalliferous complex type, at a concentration from 1 to 8 g / l.

When the surface at the end of treatment is to be resistant to high abrasion, PTFE particles can be added to the finishing bath about 0.1 to 0.2 microns. This product must be compatible with the pH of the medium of use; for a 60% aqueous dispersion of PTFE, the concentration can go from 1 to 10 ml / l. This also allows, during the barrel treatment of the pieces of screws, to obtain an adequate coefficient of friction.

When you want an improved gravel resistance, you can incorporate in the finishing bath an acrylic polymer, for example an aqueous emulsion density 1.055 g / ml at a concentration of 10 to 100 ml / l; we can also use a polyethylene glycol.

The contacting of the parts with the chromating bath, then with the finishing bath can be made by spraying, but preferably carried out by immersion. Depending on the nature of the parts to be treated, this operation can be carried out the attachment (dead bath) or barrel (loose), with or without taking back to the basket in the case of barrel treatment.

The treatment can be carried out at a temperature ranging from 15 to 40 ° C, but it is preferably carried out at a temperature between 20 and 30 ° C. The duration of bringing the parts into contact with each of the baths can vary within wide limits. It is generally between 10 seconds and 10 minutes, but is preferably one to two minutes.

The chromation is advantageously carried out with stirring, this being preferably obtained by means of an air ramp. After chromating, the parts are not dried, but only rinsed with water before contacting them with the finishing bath, this treatment preferably being carried out without agitation. Finally, the pieces are dried for 5 to 15 minutes at a temperature ranging from 60 to 100 ° C. Their corrosion resistance is only tested after a storage for at least 48 hours so that the film formed is stabilized.

The following example illustrates the invention without limiting it.

EXAMPLE

To treat pieces of electrolytic galvanized steel, a bath was prepared aqueous chromatography with 30 g / l of sodium dichromate dihydrate, 5 g / l of anhydrous sodium sulfate, 8 g / l sulfuric acid, 0.6 g / l cobalt sulfate heptahydrate, 0.37 g / l of silver nitrate and 85 ml / l of acetic acid.

• silice colloïdale(*)   27 g (exprimé comme SiO₂)
• gluconate de sodium   0,2 g
• benzoate de sodium   0,02 g
• tensio-actif fluoré(**)   0,1 g
• dispersion aqueuse à 60 % de PTFE   5 ml
• colorant noir (***)   4 g

The parts were immersed for 90 seconds in this bath maintained at a temperature of 24 ° C and with air stirring, then rinsed with water and immersed for one minute at 20-22 ° C in an aqueous finishing bath which contains per liter:

• colloidal silica (*) 27 g (expressed as SiO ₂ )
• sodium gluconate 0.2 g
• sodium benzoate 0.02 g
• fluorinated surfactant (**) 0.1 g
• aqueous dispersion at 60% PTFE 5 ml
• black dye (***) 4 g

(*) Silice colloïdale sodique présentant un diamètre particulaire moyen de 12 nm, une surface spécifique de 230 m2/g et une viscosité à 25°C de 9 mPa.s

(**) Perfluorooctanesulfonate de tétraéthyl ammonium

(***) Complexe métallifère azoïque.

After leaving this bath, the shiny black pieces were not rinsed, but directly dried at 80 ° C for 10 minutes.

(*) Sodium colloidal silica with an average particle diameter of 12 nm, a specific surface of 230 m2 / g and a viscosity at 25 ° C of 9 mPa.s

(**) Tetraethyl ammonium perfluorooctanesulfonate

(***) Azo metalliferous complex.

Claims (8)

1. Chromating bath, characterized in that it consists of an aqueous solution having a pH of between 1 and 2 and containing, per litre, 5 to 40 g of hexavalent chromium ions , 5 to 20 g of sulphate ions, 0.1 to 0.3 g of cobalt ions, 0.1 to 0.65 g of silver ions and 50 to 150 ml of at least one weak organic acid.
2. Chromating bath according to claim 1, having a pH of between 1.2 and 1.8 and containing, per litre, 7 to 15 g of hexavalent chromium ions , 7 to 12 g of sulphate ions, 0.1 to 0.2 g of cobalt ions, 0.15 to 0.3 g of silver ions and 70 to 120 ml of weak organic acid.
3. Chromating bath according to claim 1 or 2, in which the organic acid is acetic acid, formic acid or oxalic acid.
4. Chromating bath according to one of claims 1 to 3, prepared from sodium dichromate, cobalt sulphate and silver nitrate.
5. Process for finishing zinc, zinc alloy or cadmium surfaces, characterized in that the components to be treated are brought into contact first with a chromating bath according to one of claims 1 to 4 and then with a top coat bath having a pH of between 1 and 5 and containing, in water and per litre, 20 to 40 g (expressed as SiO₂) of colloidal silica, 0.1 to 2 g of a weak complexing agent, 0.01 to 1 g of a corrosion inhibitor, 0.01 to 2 g of a surfactant and 1 to 8 g of a water-soluble black dye.
6. Process according to claim 5, in which the top coat bath additionally contains PTFE particles and/or an acrylic polymer.
7. Process according to claim 5 or 6, in which the treatment in each bath is carried out at a temperature ranging from 15 to 40°C.
8. Process according to claim 7, in which the temperature is between 20 and 30°C.