ITMI20071244A1 - PHOSPHATE PROCESS MULTIMETAL PRE-PAINTING AT LOW ENVIRONMENTAL IMPACT - Google Patents

Description

Description of the patent for industrial invention entitled:

"LOW ENVIRONMENTAL IMPACT MULTIMETAL PRE-PAINTING PHOSPHATING PROCESS"

The present invention relates to a phosphating process comprising various phases (such as, for example, degreasing, possible pickling, activation, passivation, various washes).

In particular, the invention relates to an activation bath comprising nitroguanidine or a derivative thereof and its use for the activation of metal parts to be subjected to pre-painting phosphating treatments.

State of the art

A phosphating treatment can serve several purposes:

- anticorrosive protection for subsequent oiling or waxing;

- anticorrosive protection for subsequent painting (car bodies, household appliances and the like);

- lightening of the stresses in the cold deformation of semi-finished products (drawing of tubes, wires, extrusions);

- reduction of friction between sliding surfaces (manganese phosphating);

- electrical insulation.

Whatever the purpose for which it is used, the reactions that occur during the phosphating process essentially comprise two moments.

The reaction begins with the acid attack of the metal, which passes into solution in the form of an ion, through an electrochemical mechanism including the anodic corrosion reaction of the metal itself and a cathodic reaction of simultaneous development of molecular hydrogen. As a consequence of this attack, the concentration of hydrogen ions decreases (the pH rises) in the diffusion boundary layer (a few microns) near the microcathodic zones; since the solubility of phosphates becomes lower as the pH value increases, in these areas the less soluble phosphates present begin to precipitate and already after a few seconds (less than 10) minute crystals of zinc phosphate (or zinc-iron , zinc-calcium, manganese or others) are nucleated. Throughout the subsequent phosphating time, the initial nuclei enlarge, but there is no longer any tendency to increase in number.

All modern phosphating baths are made up of acidic zinc phosphate and accelerators, plus various additives; by the action of oxidizing agents that act as accelerating additives and by the effect of depolarizing metals, the molecular hydrogen that forms as a cathodic reaction is immediately reoxidized to an ion, restoring the local acidity of the bath and thus ensuring the continuation of the process. Typical accelerators known and always used in the world of phosphating are nitrites, chlorates, nitrates; hydrogen peroxide, hydroxylamine, nitroguanidine and other nitrogenous organic compounds are of more recent use, particularly in the last twenty years, although their activity has been known since the 1950s. The most typically used depolarizing agent is nickel.

The actual phosphating phase is normally preceded by an activation phase.

In fact, before zinc phosphating, it is always advisable, and very often absolutely necessary, to activate the metal surface, especially in cases of pre-painting phosphating, for which it is of fundamental importance Γ obtaining a thin, fine-grained and compact coating .

The supersaturation that takes place in the diffusion boundary layer, induced at the cathode sites by the lowering of the pH, creates the thermodynamic conditions necessary, but not sufficient, for the precipitation of phosphates: their crystallization must also take place, whose formation mechanism is such that, in the first 10 ”or so of contact with the solution, the surface is covered with numerous crystalline germs, which then no longer grow in number, but only in size.

In the phosphating process, therefore, it is the very first seconds of contact between the surface and the solution that predetermine the final outcome of the process; in other words, if the crystalline germs are numerous and dense, the coating has the desired technological properties, if vice versa they are few and sparse, then the resulting coating is a coarse and rough crystalline grain and its technological properties can significantly degrade. Nucleation is effectively the heart of the entire process: the ways in which it takes place determine the quality of the final coating.

The term activation therefore means a treatment, to be carried out immediately before phosphating, which deposits a large number of crystalline germs on the metal surface, from each of which a metal phosphate crystal can originate and grow.

There are essentially two known activators of practical importance: one for manganese phosphating, the other for zinc.

The activator for zinc phosphating is normally made up of hydrated titanyl and sodium phosphate - Na4TiO (PO4) 2. with 0 ÷ 7 H2O - which, to be effective, must be prepared following a particular procedure and must be ground into an extremely fine powder.

The activator is generally used in a mixture 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 and whose effectiveness declines with aging and is affected by the presence of alkaline earth metals such as calcium and magnesium.

DESCRIPTION OF THE INVENTION

It has now been found that the use of nitroguanidine or derivatives, even in small quantities, in the activation phase rather than in the phosphating phase, allows to obtain a finer and more adherent layer than that obtained with a traditional activator, for the same other chemical-physical parameter.

The use of nitroguanidine in the activation bath instead of in the phosphating bath also significantly reduces the cost of both the phosphating chemical product and the overall treatment, moreover with fewer environmental problems thanks to the reduced amount of nitroguanidine.

The process of the invention is advantageously used for multi-metal surface treatments intended for pre-painting processes, mainly used in the automotive and appliance industries.

The process of the invention includes the phases of:

a) immerse the metal part to be treated in an activating bath containing titanium polyphosphates and organic nitro-nitrogenous compounds deriving from nitroguanidine;

b) transferring the metal part from the activation bath to a phosphating bath comprising zinc, manganese, P2O5, chlorates, fluosilicates, hydrogen peroxide and possibly nickel.

The invention also provides an activation bath for phosphating processes, said activation bath comprising titanium polyphosphates and nitro-nitrogenous organic compounds deriving from nitroguanidine, preferably nitroguanidine.

Titanium polyphosphates can be present in concentrations ranging from 0.2 g / 1 to 10 g / 1, while the concentration of nitroguanidine or derivative is usually between 0.01 g / 1 to 10 g / 1 depending on the type of application (spraying or dipping).

The components of the phosphating bath are instead conventional and are listed in the table below (in brackets, the values of a bath used in the examples):

Table 1

Nickel and manganese are elements that have become fundamental for the so-called tricationic phosphating, which allows to obtain a coating characterized by a very fine and compact crystalline grain, with limited thicknesses, and, above all, very little sensitive to attack by alkaline solutions.

The fluosilicates improve the corrosive attack of the metal, further refine the crystalline grain and, above all, complex and precipitate the aluminum in all cases where this metal is present in the mixing to be treated. From the latter point of view, the addition of fluorides is essential since aluminum, if not removed from the baths, would tend to poison the solution for increasing concentrations.

Chlorates are able to eliminate divalent iron immediately and completely already at medium-low temperatures while hydrogen peroxide, which is used here in very low concentrations, has the advantage of not being polluting, especially in relation to the effluents produced.

The optimum temperature of the phosphating bath can be between 40 ° C and 60 ° C, while that of the activation bath can reach 40 ° C.

Depending on the type of system, the activation bath can be renewed on a weekly or bi-weekly basis.

The morphology of the phosphate coating obtained is fine and compact and, depending on the constituent elements of the phosphating bath and the type of application (spray or immersion), the crystals can take on a filiform, nodular or pebble appearance.

The layer weight of the phosphate coating can vary from 1.5 g / m to 3 g / m depending on the substrate and the conduction parameters of the bath.

The invention is illustrated in greater detail in the following Examples.

EXAMPLES

The phase of study of the phosphating cycle object of this invention was carried out through experimental tests that tried to simulate the most common types of industrial processes dedicated to pre-painting and comparing the results obtained with those of the most common phosphating cycles.

For all the tests carried out, a weakly alkaline multi-metal degreaser was used before phosphating.

We used a traditional phosphating bath that contained at least:

Table 2

We performed the following types of processes (spraying and dipping), comparing the effectiveness of the innovative activator (AN) with that of a traditional activator (AT) and finally comparing the results obtained from the point of view of the morphology of the phosphate layer and of the layer weight.

1 = Traditional Nitrite accelerated process

2 = Process accelerated with Nitroguanidine> 100 ppm

3 = Accelerated process with chlorate (2.0 g / 1) and hydrogen peroxide (0.05 g / 1) with Nitroguanidine> 100 ppm

4 = Accelerated process with chlorate (2.0 g / 1) and hydrogen peroxide (0.05 g / 1) with Nitroguanidine at 80 ppm

In particular, process 4 with the innovative activator AN was conducted as a whole as follows:

• degreasing by spray or immersion, at 50 ÷ 55 ° C, for 2 ÷ 3 ', in an aqueous solution at 20 g / 1 of an alkaline product based on alkaline poly and orthophosphates, sequestering and complexing agents, non-ionic surfactants

• double rinse by spraying or dipping, with mains water, at room temperature

• activation by spray or immersion, at room temperature, for Γ, in solution in demineralized water at 5 g / 1 of the liquid version of the AN and at 1.5 g / 1 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 Aluminum, in a ratio of 100 and 200 ppm respectively, at 50 ÷ 55 ° C, for 2 'by spray, 3' by immersion

• spray or immersion rinsing with mains water at room temperature

• spray or immersion rinsing with demineralized water at room temperature

• drying with forced hot air

= sufficient ++ good

+ = fair - coarse

The experimental tests show an excellent efficacy of the innovative activator both on the morphology of the phosphate layer, which is finer and more compact than that obtained, with the same process, with the traditional activator, and in the control of the growth of the crystals, which constitute always a homogeneous and uniform layer.

The phosphate weight is always within the normal limits for a tricationic phosphating.

Claims (12)

CLAIMS 1. "Multimetal" pre-painting phosphating process comprising the phases of: a) immerse the metal part to be treated in an activating bath containing titanium polyphosphates and nitro-nitrogenous organic compounds deriving from nitroguanidine; b) transferring the metal part from the activation bath to a phosphating bath comprising zinc, manganese, P2O5, chlorates, fluosilicates and hydrogen peroxide. 2. Process according to claim 1 wherein the activating bath contains 0.01 to 10 g / l of nitroguanidine. 3. Process according to claim 2 wherein the phosphating bath comprises from 0.1 to 10 g / l of zinc, from 0.1 to 10 g / l of manganese, from 5 to 30 g / l of P205, from 0 , 1 to 10 g / 1 of fluosilicates, 0.001 to 0.5 g / 1 of hydrogen peroxide and 0.1 to 10 g / 1 of chlorates. 4. Process according to claim 3 wherein 0.1 g / 1 to 3 g / 1 nitrates are added to the phosphating bath. 5. Process according to claim 4 wherein 0.05 g / 1 to 0.5 g / 1 of free fluorides are added to the phosphating bath. 6. Process according to any one of claims 1 to 5 in which step a) is carried out at a temperature between 25 and 40 ° C. 7. Process according to any one of claims 1 to 5 wherein step b) is carried out at a temperature between 40 and 60 ° C. Process according to any one of claims 1 to 5 in which step b) is carried out by dipping or spraying. 9. Process according to any of claims 1 to 8 for the treatment of metal parts to be subjected to pre-painting treatments in the automotive and household appliances industry. Process according to any one of claims 1 to 9 wherein the weight of the coating on the phosphate layer varies from 1.5 to 3 g / m. 11. Activation bath for “multimetal” phosphating process containing titanium polyphosphates and nitro-nitrogenous organic compounds deriving from nitroguanidine. 12. Use of the bath of claim 11 for the activation of metal parts to be subjected to pre-painting phosphating treatments. Milan, 21 June 2006