Multimetal phosphating process comprising an activation pretreatment

Description

[0001] The present invention relates to a phosphating process involving different steps (such as degreasing, pickling, activation, passivation, and various rinsing).

[0002] In particular, the invention relates to an activation bath comprising nitroguanidine or one of its derivatives and its use for the activation of metal parts to be submitted to a pre-painting phosphating treatment.

Prior art

[0003] A phosphating treatment can be used for different purposes:

• Anticorrosion protection prior to oiling or waxing;
• Anticorrosion protection prior to painting (car chassis, household appliances and so on);
• reducing loads in the cold deformation of semi-finished products (tube drawing, wire drawing, extrusion);
• reducing friction between sliding surfaces (manganese phosphating);
• electrical insulation.

[0004] Whatever the purpose for which it is used, the reactions that take place during the phosphating process basically comprise two steps.

[0005] The reaction begins with an acid attack of the metal, which passes in ion form into the solution, the mechanism is electrochemical and is constituted by the anodic reaction of metal corrosion and the cathodic reaction of development of molecular hydrogen. As a result of this attack, the concentration of hydrogen ions falls (the pH rises) in the diffusion boundary layer (few microns) close to the microcathodic zones; as the pH increases, the solubility of phosphates decreases and the less soluble phosphates begin to precipitate in these zones. After a few seconds (less than 10), tiny crystals of zinc phosphate (or zinc-iron, zinc-calcium, manganese or other) nucleate. Throughout the subsequent phosphating time the initial nuclei enlarge, but there is no longer any tendency for their number to increase.

[0006] All modern phosphating baths consist of acid zinc phosphate and accelerators, together with different additives; due to the action of oxidising agents which act as accelerants and the effect of depolarising metals, the molecular hydrogen that forms as a cathodic reaction is immediately reoxidised to ion, thus restoring the local acidity of the bath and guaranteeing the durability of the process. Typical accelerants which have long been known and used in the phosphating field are nitrites, chlorates and nitrates; hydrogen peroxide, hydroxylamine, nitroguanidine and other organic nitro compounds have come into use more recently, especially in the last 20 years, although their activity has been known since the 1950s. The most commonly used depolarising agent is nickel.

[0007] The actual phosphating step is usually preceded by an activation step.

[0008] Before zinc phosphating is performed, activation of the metal surface is always advisable, and very often crucial, especially in the case of pre-painting phosphating, for which it is essential to obtain a thin coating and a fine, compact texture.

[0009] The supersaturation that takes place in the diffusion boundary layer, induced at the cathodic sites by the reduction of pH, creates the thermodynamic conditions which are necessary, but not sufficient, for precipitation of phosphates: their crystallisation is also necessary, the crystal formation mechanism being such that in the first approx. 10 seconds of contact with the solution, the surface is covered by numerous crystalline germs, which do not increase in number, but only in size.

[0010] In the phosphating process, therefore, the first few seconds of contact between surface and solution determine the final result of the process; in other words, if the crystalline germs are numerous and densely packed, the coating will have the desired technological properties, while if they are few and sparse, the resulting coating will have a coarse, rough, crystalline texture, and its technological properties may deteriorate considerably. Nucleation is at the heart of the entire process, and the ways whereby it takes place determine the quality of the final coating.

[0011] "Activation"therefore means a treatment, carried out immediately before phosphating, which deposits on the metal surface a large number of crystalline germs, from each of which a metal phosphate crystal can originate and grow.

[0012] There are basically two known activators of practical importance: one for manganese phosphating, and the other for zinc phosphating.

[0013] The zinc phosphating activator usually consist of sodium titanyl phosphate hydrated - Na₄TiO(PO₄)₂. with 0-7 H₂O - which, to be effective, must be prepared in accordance with a special procedure and ground to an extremely fine powder.

[0014] The activator is generally used mixed with disodium phosphate and/or other phosphates, in liquid or powder form. In water it gives rise to a slightly alkaline colloidal suspension (pH 8-10) which adheres to the metal surface; its efficacy declines with age, and is affected by the presence of alkaline earth metals such as calcium and magnesium.

[0015] WO 03/100130 discloses an aqueous composition comprising polysphosphates, nitroguanidine and titanium useful for the surface conversion process without chromium.

[0016] GB 932987 discloses a process of phosphate-coating of metals by means of a solution containing titanium and condensed phosphate.

DESCRIPTION OF THE INVENTION

[0017] It has now been found that the use of nitroguanidine or its derivatives, even in small amounts, at the activation rather than the phosphating step, produces a finer and more adherent layer than the one obtained with a conventional activator, all other chemical and physical parameters being equal.

[0018] The use of nitroguanidine in the activation bath, rather than the phosphating bath, also significantly reduces the cost both of the chemical phosphating products and the total treatment, and generates fewer environmental problems due to the small amount of nitroguanidine used.

[0019] The process to which the invention relates can be advantageously used for multimetal surface treatments in pre-painting processes, mainly used in the automotive and household appliance industries.

[0020] The process according to the invention comprises the following steps:

a) immersion of the metal part to be treated in an activating bath containing titanium polyphosphates and nitroguanidine;

b) transfer of the metal part from the activation bath to a phosphating bath containing zinc, manganese, P₂O₅, chlorates, fluorosilicates, hydrogen peroxide and, possibly, nickel.

[0021] The activation bath for phosphating processes contains titanium polyphosphates and nitroguanidine.

[0022] Titanium polyphosphates can be present at concentrations ranging from 0.2 g/l to 10 g/l, whereas the concentration of nitroguanidine or its derivative is usually between 0.01 g/l and 10 g/l, depending on the type of application (spray or immersion).

[0023] The components of the phosphating bath are conventional, and are listed in the table below (the values of the bath used in the examples are shown in brackets):

Table 1

Component	Concentration (g/l)
Zinc	0.1-10 (1.3)
Nickel	0.1-5 (0.8)
manganese	0.1-10 (0.8)
P2O5	5-30 (15)
fluosilicates	0.1-10 (1.2)
chlorates	0.1-10 (2.0)
hydrogen peroxide	0.001-0.5 (0.05)

[0024] Nickel and manganese have become essential elements for "tri-cationic" phosphating, which produces a coating with a very fine, compact crystalline texture and limited thickness, and above all not very sensitive to attack by alkaline solutions.

[0025] Fluosilicates improve the corrosive attack of the metal, further refine the crystalline texture, and above all, complex and precipitate aluminium, if that metal is present in the mixing to be treated. From this latter standpoint, the addition of fluorides is essential, because if the aluminium is not removed from the baths, its increasing concentration would poison the solution.

[0026] Chlorates immediately and completely eliminate the divalent iron at medium-low temperatures, while hydrogen peroxide, used here at very low concentrations, has the advantage of not being a pollutant, especially as regards the effluents produced.

[0027] The ideal temperature of the phosphating bath can be between 40°C and 60°C, while the activating bath can reach 40°C.

[0028] Depending on the type of installation, the activation bath can be renewed at every one or two weeks.

[0029] The morphology of the obtained phosphate coating is fine and compact, and, depending on the elements constituting the phosphating bath and the type of application (spray or immersion), the crystals may have a filiform, nodular or pebbly appearance.

[0030] The layer weight of the phosphate coating ranges between 1.5 g/m² and 3 g/m², depending on the substrate and the conduction parameters of the bath. The invention is illustrated in greater detail in the following examples.

EXAMPLES

[0031] The study of the phosphating cycle of the invention was carried out by means of experimental tests simulating the most common types of industrial pre-painting processes and comparing the results obtained with those of the most common phosphating cycles.

[0032] Weakly alkaline multimetal cleaner was used before phosphating in all the performed tests.

[0033] We used a conventional phosphating bath containing at least:

Table 2

Element	Concentration (g/l)
Zinc	1.3
Nickel	0.8
Manganese	0.8
P2O5	15

[0034] We performed the following types of process (spray and immersion), comparing the efficacy of the innovative activating agent (AN) with the one of a conventional activating agent (AT), and finally comparing the results obtained in terms of morphology of the phosphate layer and layer weight.

1 = Conventional process accelerated with nitrite

2 = Process accelerated with nitroguanidine > 100 ppm

3 = Process accelerated with chlorate (2.0 g/l) and hydrogen peroxide (0.05 g/l) with nitroguanidine > 100 ppm

4 = Process accelerated with chlorate (2.0 g/l) and hydrogen peroxide (0.05 g/l) with 80 ppm nitroguanidine

[0035] In particular, process 4, with the innovative activator AN, was conducted as follows:
• spray or immersion degreasing at 50-55°C for 2- 3 minutes, in 20 g/l water solution of an alkaline product based on alkaline poly- and orthophosphates, sequestering and complexing agents and non-ionic surfactants
• two spray or immersion rinsing with tap water at room temperature
• spray or immersion activation at room temperature, for 1 min, in a demineralised water solution containing 5 g/l of the liquid version of AN and 1.5 g/l of the powder version
• phosphating with the bath described in table 2, to which fluosilicates (SiF₆⁼) were added, in the case of treatment on HDG and aluminium, at the amount of 100 and 200 ppm respectively, at 50-55°C, for 2 min by spraying and 3 min by immersion
• spray or immersion rinsing with tap water at room temperature
• spray or immersion rinsing with demineralised water at room temperature
• drying with forced hot air

	Immersion phosphating on LAF						Spray phosphating on LAF
	Layer weight		Morphology				Layer weight		Morphology
Cycles	AT	AN	AT	AN		Cycles	AT	AN	AT	AN
1	2.31 g/m2	2.19 g/m2	++	++		1	1.72 g/m2	1.71 g/m2	++	++
2	2.22 g/m2	2.12 g/m2	++	+++		2	1.86 g/m2	1.86 g/m2	++	+++
3	2.49 g/m2	2.35 g/m2	++	+++		3	1.87 g/m2	1.85 g/m2	++	+++
4	2.71 g/m2	2.30 g/m2	+	+++		4	2.10 g/m2	1.86 g/m2	+	+++

+ = acceptable +++ good ++ = fair - poor

	Immersion phosphating on HDG 100 ppm of fluorides in the form of complex fluorides (fluosilicates) were added						Spray phosphating on HDG 100 ppm of fluorides in the form of complex fluorides (fluosilicates) were added
	Layer weight		Morphology				Layer weight		Morphology
Cycles	AT	AN	AT	AN		Cycles	AT	AN	AT	AN
1	2.56 g/m2	2.52 g/m2	++	++		1	1.82 g/m2	1.81 g/m2	++	++
2	2.41 g/m2	2.32 g/m2	++	++		2	1.84 g/m2	1.81 g/m2	++	++
3	2.91 g/m2	2.25 g/m2	++	+++		3	2.10 g/m2	1.82 g/m2	+	+++
4	3.10 g/m2	2.21 g/m2	+	+++		4	2.51 g/m2	1.81 g/m2	-	+++

	Immersion phosphating on aluminium 200 ppm of fluorides in the form of complex fluorides (fluosilicates) were added						Spray phosphating on aluminium 200 ppm of fluorides in the form of complex fluorides (fluosilicates) were added
	Layer weight		Morphology				Layer weight		Morphology
Cycles	AT	AN	AT	AN		Cycles	AT	AN	AT	AN
1	2.31 g/m2	2.21 g/m2	++	++		1	1.81 g/m2	1.70 g/m2	++	++
2	2.32 g/m2	2.22 g/m2	++	++		2	1.86 g/m2	1.81 g/m2	++	++
3	2.67 g/m2	2.32 g/m2	++	++		3	1.91 g/m2	1.87 g/m2	++	++
4	2.91 g/m2	2.21 g/m2	-	++		4	2.00 g/m2	1.87 g/m2	-	++

[0036] The experimental tests demonstrate the excellent efficacy of the innovative activator both on the morphology of the phosphate layer, which is finer and more compact than the one obtained with the conventional activator in the same process, and on controlling the growth of the crystals, which always form a homogenous, uniform layer.

[0037] The phosphate weight is always within the normal limits for tri-cation phosphating.

Claims

1. Pre-painting multimetal phosphating process comprising the following steps:

a) immersion of the metal part to be treated in an activating bath containing titanium polyphosphates and nitroguanidine;

b) transfer of the metal part from the activation bath to a phosphating bath containing zinc, manganese, P₂O₅, chlorates, fluosilicates and hydrogen peroxide.

2. The process as claimed in claim 1, wherein the activating bath contains 0.01 to 10 g/l of nitroguanidine.

3. The process as claimed in claim 2, wherein the phosphating bath contains 0.1 to 10 g/l of zinc, 0.1 to 10 g/l of manganese, 5 to 30 g/l of P₂O₅, 0.1 to 10 g/l of fluosilicates, 0.001 to 0.5 g/l of hydrogen peroxide, and 0.1 to 10 g/l of chlorates.

4. The process as claimed in claim 3, wherein 0.1 g/l to 3 g/l of nitrates is added to the phosphating bath.

5. The process as claimed in claim 4, wherein 0.05 g/l to 0.5 g/l of free fluorides is added to the phosphating bath.

6. The process as claimed in any one of claims 1 to 5, wherein step a) is carried out at a temperature of between 25 and 40°C.

7. The process as claimed in any one of claims 1 to 5, wherein step b) is carried out at a temperature of between 40 and 60°C.

8. The process as claimed in any one of claims 1 to 5, wherein step b) is carried out by immersion or spraying.

9. The process as claimed in any one of claims 1 to 8, for the treatment of metal parts to be subjected to pre-painting treatments in the automotive or household appliance industries.

10. The process as claimed in any one of claims 1 to 9, wherein the coating weight of the phosphate layer is between 1.5 and 3 g/m².

Ansprüche

1. Multimetall-Phospatierungsverfahren vor dem Lackieren, das die folgenden Schritte umfasst:

(a) Eintauchen des zu behandelnden Metallteils in ein Aktivierungsbad, das Titanpolyphosphate und Nitroguanidin enthält;

(b) Überführen des Metallteils aus dem Aktivierungsbad in ein Phosphatierungsbad, das Zink, Mangan, P₂O₅, Chlorate, Fluorsilicate und Wasserstoffperoxid enthält.

2. Verfahren nach Anspruch 1, wobei das Aktivierungsbad 0,01 bis 10 g/l Nitroguanidin enthält.

3. Verfahren nach Anspruch 2, wobei das Phosphatierungsbad 0,1 bis 10 g/l Zink, 0,1 bis 10 g/l Mangan, 5 bis 30 g/l P₂O₅, 0,1 bis 10 g/l Fluorsilicate, 0,001 bis 0,5 g/l Wasserstoffperoxid und 0,1 bis 10 g/l Chlorate enthält.

4. Verfahren nach Anspruch 3, wobei in das Phosphatierungsbad 0,1 bis 3 g/l Nitrate gegeben werden.

5. Verfahren nach Anspruch 4, wobei in das Phosphatierungsbad 0,05 bis 0,5 g/l freie Fluoride gegeben werden.

6. Verfahren nach einem der Ansprüche 1 bis 5, wobei Schritt a) bei einer Temperatur zwischen 25 und 40 °C durchgeführt wird.

7. Verfahren nach einem der Ansprüche 1 bis 5, wobei Schritt b) bei einer Temperatur zwischen 40 und 60 °C durchgeführt wird.

8. Verfahren nach einem der Ansprüche 1 bis 5, wobei Schritt b) durch Eintauchen oder Sprühen durchgeführt wird.

9. Verfahren nach einem der Ansprüche 1 bis 8 zur Behandlung von Metallteilen in der Automobil- oder Haushaltsgeräteindustrie, die vor dem Lackieren Vorbehandlungen unterzogen werden sollen.

10. Verfahren nach einem der Ansprüche 1 bis 9, wobei das Beschichtungsgewicht der Phosphatschicht zwischen 1,5 und 3 g/m² beträgt.

Revendications

1. Procédé de phosphatage multimétal avant mise en peinture comprenant les étapes suivantes :

a) immersion de la partie métallique à traiter dans un bain d'activation contenant des polyphosphates de titane et une nitroguanidine ;

b) transfert de la partie métallique du bain d'activation à un bain de phosphatage contenant du zinc, du manganèse, du P₂O₅, des chlorates, des fluosilicates et du peroxyde d'hydrogène.

2. Procédé selon la revendication 1, dans lequel le bain d'activation contient de 0,01 à 10 g/L de nitroguanidine.

3. Procédé selon la revendication 2, dans lequel le bain de phosphatage contient de 0,1 à 10 g/L de zinc, de 0,1 à 10 g/L de manganèse, de 5 à 30 g/L de P₂O₅, de 0,1 à 10 g/L de fluosilicates, de 0,001 à 0,5 g/L de peroxyde d'hydrogène, et de 0,1 à 10 g/L de chlorates.

4. Procédé selon la revendication 3, dans lequel de 0,1 à 3 g/L de nitrates sont ajoutés au bain de phosphatage.

5. Procédé selon la revendication 4, dans lequel de 0,05 à 0,5 g/L de fluorures libres sont ajoutés au bain de phosphatage.

6. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel l'étape a) est effectuée à une température comprise entre 25 et 40°C.

7. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel l'étape b) est effectuée à une température comprise entre 40 et 60°C.

8. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel l'étape b) est effectuée par immersion ou pulvérisation.

9. Procédé selon l'une quelconque des revendications 1 à 8 de traitement de parties métalliques qui doivent être soumises à des traitements avant mise en peinture dans les industries de l'automobile ou des appareils électroménagers.

10. Procédé selon l'une quelconque des revendications 1 à 9, dans lequel le poids de revêtement de la couche de phosphate est compris entre 1,5 et 3 g/m².