Method for applying phosphate coatings

Abstract

A method for applying phosphate coatings, comprising the following steps: - providing a phosphatizing bath containing zinc Zn; - electrodepositing at least part of the metals contained in the bath on a workpiece to be treated by means of current pulses.

Description

[0001] METHOD FOR APPLYING PHOSPHATE COATINGS

[0002] The present invention relates to a method for applying phosphate coatings.

[0003] Surface treatments based on metal phosphates are typically applied on ferrous materials in order to prevent the formation of rust, in preparation of painting operations, or as a lubricant substrate in cold deformation operations such as drawing steel wire.

[0004] In electrolytic phosphatizing processes, the object to be coated is immersed in an electrolyte containing the precursors of the film of phosphates.

[0005] At the same time, a current is applied by means of a current rectifier, the two poles of which are connected to the object to be coated (cathode) and to a second electrode (anode), which is also immersed in the same electrolyte.

[0006] Since a condition for obtaining the coating is the application of an electric current, workpieces to be treated with this process must necessarily be electric conductors.

[0007] It is therefore possible to coat various metals such as for example carbon steel, stainless steels, copper, galvanized steel, aluminum, titanium, or zinc.

[0008] At the same time, anodes that can be used in this technology do not only have to be good electric conductors, but also be capable of accommodating the oxygen evolution reaction without being damaged during the process.

[0009] The possible materials that can be used therefore include titanium meshes plated with platinum or coated with mixed oxides of rare earth metals, which are already often used in electrodeposition processes.

[0010] Among the conventional baths used for electrodeposition processes, intervals of compositions of electrolytes are known which, in addition to containing the elements that constitute the deposit, i.e. phosphate ions (PO ) and divalent metal cations like Ca2+and Zn2+, feature the use of nitrate ions (NOs’) or chlorate ions (CIOs') to limit the excessive formation of bubbles during the deposition.

[0011] The main advantage of using cathodic phosphatizing over a conventional common phosphatizing process lies in the partial or total elimination of phosphatization sludge, which is the main waste product of deposit-forming reactions.

[0012] In addition, electrolytic phosphatizing baths operate at lower temperatures than conversion phosphatizing baths, sometimes even at ambient temperature.

[0013] Differently from conversion treatment, use of the electrolytic phosphatizing process in drawing steel wire enables the deposition of the layer of phosphates continuously, with consequent increase in the productivity of the plant, with deposition times comprised between 1 and 30 seconds.

[0014] In the majority of cases, the current or voltage applied to facilitate the electrolysis are constant over the full duration of the deposition.

[0015] In fact, the current or voltage values are set on the basis of the desired deposition time and the required coating weight.

[0016] An increase in current or voltage therefore enables an increase in the phosphate deposition rate, with consequent lowering of deposition times.

[0017] At the same time, a high current favors the formation of finer phosphate crystals, which have superior mechanical properties.

[0018] However, it is equally true that using an excessively high current leads to a drastic lowering of the quality of the coating, so the plant will have a maximum deposition rate but with an excessive accumulation of hydrogen bubbles on the surface of the substrate, as a consequence resulting in a coating that, in qualitative and quantitative terms, is poor.

[0019] Although the formation of bubbles is essential to obtain the phosphate, a high current intensity leads to an excessive development of hydrogen, and in these conditions the hydrogen could adhere to the surface of the coating, forming crater-like defects.

[0020] Furthermore, excessive speed of formation of bubbles would obstruct their egress, so decreasing the ion exchange reactions and effectively slowing the phosphatization reaction. The concurrence of both of these phenomena translates to a reduction in the quality and quantity of the final coating.

[0021] The aim of the present invention consists in devising a method for applying phosphate coatings that retains the advantages of processes using high current intensity but without losing out in the quality of the coating.

[0022] This aim and other objects which will become more evident hereinafter are achieved by a method for depositing phosphate coatings according to claim 1 and by a phosphatizing bath according to claim 8.

[0023] Further characteristics and advantages of the invention will become more apparent from the detailed description of a preferred, but not exclusive, embodiment of a method for depositing phosphate coatings, which is illustrated by way of non-limiting examples below and in the attached figures, wherein: figures 1-4 illustrate microscope photos that show a distinct improvement in the surface finish when the pulses used have a short period. figures 5-6 illustrate microscope photos that show that adopting a pulse-reverse current deposition protocol is capable of providing a superior refinement of the grain, with consequent improvement of the mechanical properties of the coating.

[0024] The method for applying phosphate coatings, according to the invention, comprises the following steps:

[0025] - provision of a phosphatizing bath containing zinc Zn;

[0026] - electrodeposition of at least part of the metals contained in said bath on a workpiece to be treated by means of current pulses, selected from the group constituted by cathodic pulses and bipolar pulses, or using pulse-reverse current.

[0027] More specifically, such current pulses have a current intensity comprised between 100 and 2000 amperes.

[0028] In fact, the current to be dispensed varies based on the diameter, speed, and desired coating weight. For example, in order to obtain a coating of 10 g / m2on a wire measuring 11 mm with a speed of 1 m / sec per second, an electric current of intensity equal to 1400 amperes is necessary.

[0029] Furthermore, prior to the step of electrodeposition, there can be preliminary steps including:

[0030] - a step of cleaning the workpiece to be treated, by mechanical abrasion;

[0031] - a step of removing surface oxides by immersion in an acid bath;

[0032] - a step of washing in water;

[0033] - a step of activation.

[0034] Preferably, the above preliminary steps are all envisaged, and in the above sequence.

[0035] In more detail, the step of activation entails a deposition of nanoparticles that activate the surface of the wire in order to increase the quantity, homogeneity and morphology of the phosphate deposited.

[0036] The activator can, for example, consist in aqueous dispersions of sodium titanium phosphate, which is absorbed on the metallic surface during the application of the solution.

[0037] In other words, after the mechanical cleaning, the surface of the wire is rough / coarse; this condition creates the necessary environment to ensure that the activator nanoparticles become “wedged” between the furrows / wrinkling created by the mechanical cleaning on the surface of the wire.

[0038] Subsequently, when the activated substrate comes into contact with the phosphatizing solution, an exchange of ions occurs between the sodium ions present on the surface of the activator particles and the zinc ions originating from the phosphatizing solution.

[0039] Therefore, the activator particles act like nucleating agents for the zinc phosphate crystals (hopeite).

[0040] If the activator were not present, the phenomenon of forming the coating would begin only by virtue of homogenous nucleation, which would result in longer times for the formation of the coating and also a more uneven coating (so for the same immersion time of the wire, there would be less phosphate and also, the phosphate that is deposited would be coarser-grained, and would be more porous).

[0041] Advantageously, the phosphatizing bath used in the method according to the invention has the following chemical formulation:

[0042] - PO43' with a concentration comprised between 10 and 120 g / 1;

[0043] - ZN2+with a concentration comprised between 15 and 150 g / 1;

[0044] - NO3’ with a concentration comprised between 15 and 130 g / 1;

[0045] - NaOH with a concentration comprised between 0 and 50 g / 1.

[0046] And the step of electrodeposition is carried out for a deposition time comprised between 1 and 120 seconds at a deposition temperature comprised between 20 and 90°C.

[0047] Below are some experiments comparing phosphate coating processes using direct current and the method according to the invention using pulsed current and pulse-reverse current.

[0048] EXAMPLE 1

[0049] Samples of steel wire (S235) were coated by means of zinc-based (Zn2+) electrolytic phosphatization using pulsed direct current. Prior to the deposition, all the samples were prepared according to the protocol shown in the table below.

[0050] After the step of activation, the samples were coated using electrolytic phosphatization at 40°C using a titanium anode coated with mixed oxides. The composition of the phosphatizing bath is given in the table below:

[0051] Half of the samples were coated with direct current (-30 A dm-2), while the other half were covered with pulsed current with a rectangular pulse (-60 A dm-2 for 100 ms, 0 A dm-2 for 100 ms). After the deposition the samples were washed, dried and weighed.

[0052] As is shown by the table below, the samples prepared using pulsed current have a greater coating weight, so indicating the advantage of using pulsed current to increase the deposition rate. No differences were observed in adherence and in the appearance of the coatings in the two cases.

[0053] EXAMPLE 2

[0054] Flat samples of steel (S235) were pretreated with the same protocol as Example 1 and coated by means of electrolytic phosphatization with the same bath as described in Example 1. Half of the samples were coated with direct current (-10 A dm-2), while the other half were covered with pulsed current with a rectangular pulse (-20 A dm-2 for 100 ms, 0 A dm-2 for 100 ms). After the deposition the samples were washed, dried and weighed.

[0055] As can be seen from the table below, for flat samples as well, phosphatization with pulsed current brings advantages in terms of coating weight.

[0056] EXAMPLE 3

[0057] Flat samples of steel (S235) were pretreated with the same protocol as Example 1 and coated by means of pulsed electrolytic phosphatization with the same bath as described in Example 1. For depositions, rectangular pulses with currents of -20 A dm-2 were used during the ON time and 0 A dm-2 during the OFF time. The ON and OFF durations were varied for the various different samples, while the total time was set at 30 seconds. After the deposition the samples were washed, dried, weighed and observed using a scanning electron microscope.

[0058] As can be seen from the table below, the data show that changing the period of the pulses does not alter the coating weight of the coating.

[0059] The microscope photos 1-4 show a distinct improvement in the surface finish when the pulses used have a short period

[0060] In other words, using a pulsed current brings an increase in the deposition rate compared to direct current, while the surface properties of the coating depend mainly on the shape of the pulse.

[0061] EXAMPLE 4

[0062] Flat samples of steel (S235) were pretreated with the same protocol as Example 1 and coated by means of electrolytic phosphatization with the same bath as described in Example 1. Half of the samples were coated with direct current (-10 A dm-2), while the other half were covered with pulsed current with a rectangular pulse (-20 A dm-2 for 100 ms, +1 A dm-2 for 100 ms). After the deposition the samples were washed, dried and observed using a scanning electron microscope.

[0063] The microscope photos 5-6 show that adopting a pulse-reverse current deposition protocol is capable of providing a superior refinement of the grain, with consequent improvement of the mechanical properties of the coating.

[0064] In practice it has been found that the method for applying phosphate coatings and the phosphatizing bath according to the invention achieve the set aim.

[0065] In fact, by virtue of the use of pulsed current it is possible to obtain an improvement in the crystalline structure, with increased penetrating power and reduced defects associated with the evolution of hydrogen.

[0066] More specifically, by virtue of the method according to the invention and, more particularly, by virtue of the relaxation period (OFF time) between two pulses, the quantity of hydrogen emitted at the cathode is reduced and a decrease in deformations is obtained.

[0067] Furthermore, by reducing the extent of the formation of bubbles it is possible to use a very high instantaneous current with no formation of defects in the coating and without impoverishing the metallic ions on the electrode surface, favoring the initial phase of the nucleation process and considerably increasing the number of grains per unit of area; this leads to a finer-grained deposit with properties superior to conventional plating using direct current.

[0068] Finally, another advantage which should not be underestimated is the fact that the working conditions can be modulated. While in direct-current deposition it is possible to adjust only the voltage or current parameters, when the plating process with cathodic pulses or pulse-reverse current is used, more independent parameters are available, such as the current intensity of the direct / reverse pulse, the frequency, and the width ratio of the pulse.

[0069] The method for applying phosphate coatings and the phosphatizing bath, thus conceived, are susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.

[0070] Moreover, all the details may be substituted by other, technically equivalent elements.

[0071] In practice the materials employed, provided they are compatible with the specific use, and the contingent dimensions and shapes, may be any according to requirements and to the state of the art.

[0072] The disclosures in Italian Patent Application No. 102024000012322 from which this application claims priority are incorporated herein by reference.

Claims

CLAIMS1. A method for applying phosphate coatings, characterized in that it comprises the following steps:- providing a phosphatizing bath containing zinc Zn;- electrodepositinng at least part of the metals contained in said bath on a workpiece to be treated by means of current pulses.

2. The method according to claim 1, characterized in that, in said electrodepositing step, said current pulses are selected from the group constituted by cathodic pulses and bipolar pulses.

3. The method according to claim 1 or 2, characterized in that, in said electrodepositing step, said current pulses have a current intensity comprised between 100 and 2000 amperes.

4. The method according to one or more of the preceding claims, characterized in that it comprises at least one of the following steps, to be performed prior to said step of electrodeposition, belonging to the group of steps constituted by:- a step of cleaning said workpiece to be treated, by mechanical abrasion;- a step of removing surface oxides by immersion in an acid bath;- a step of washing in water;- a step of activation.

5. The method according to claim 4, characterized in that it comprises, in the following sequence, said step of cleaning, said step of removing surface oxides, said step of washing in water, and said step of activation.

6. The method according to one or more of the preceding claims, characterized in that said phosphatizing bath has the following chemical formulation:- PO ’ with a concentration comprised between 10 and 120 g / 1;- ZN2+with a concentration comprised between 15 and 150 g / 1;- N03' with a concentration comprised between 15 and 130 g / 1;- NaOH with a concentration comprised between 0 and 50 g / 1.

7. The method according to one or more of the preceding claims, characterized in that said electrodepositing step is performed for a deposition time comprised between 1 and 120 seconds.

8. The method according to one or more of the preceding claims, characterized in that said electrodepositing step is performed at a deposition temperature comprised between 20 and 90°C.

9. A phosphatizing bath for carrying out a method according to one or more of the preceding claims, characterized in that it has the following chemical formulation:- PO43' with a concentration comprised between 10 and 120 g / 1;- ZN2+with a concentration comprised between 15 and 150 g / 1;- NO3' with a concentration comprised between 15 and 130 g / 1; - NaOH with a concentration comprised between 0 and 50 g / 1.

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