FR2730506A1 - Forming protective coating on zinc (alloy) component - Google Patents

Abstract

A protective coating comprising Zn3(PO4)2 and ZnCr2O7, is formed on top of a layer of Zn (alloy), which itself coats an Fe or steel component (P), by submerging the component and a stainless steel electrode (3) in an aq. bath (2) contg. 5-100 g/l of chromic acid and 5-100 g/l of phosphoric acid, and applying a pulsed electric potential of 3-10 V between component (P) and electrode (3).

Description

Method for passivating a part coated with a layer
zinc or zinc alloy
The present invention relates to a method for passivating a part coated with a layer of zinc or alloyed zinc.

 It is well known that, in all fields of industry such as, for example, the automobile, household appliances, the canning industry, etc., it is necessary to protect the iron, cast iron or steel against corrosion.

 Among the processes most commonly used to perform this protection, mention may be made of galvanization which has been known for many decades.

 This operation, which consists in covering the parts to be protected with a protective layer based on zinc, can be carried out either by dipping in a bath of molten zinc (or coating with a roller) or by electrozincing. The deposit, which must be of controlled thickness, may or may not be carried out on all the faces of the part. It consists of pure zinc or added with elements such as, for example, nickel, aluminum, tin, copper, lead, iron or even cobalt.

 Galvanization allows the parts to be coated with a layer of zinc or alloyed zinc having a shiny appearance and a uniform thickness.

 Specialists have, however, found that the parts having undergone the galvanizing treatment often remain still too sensitive to corrosion and for this reason have proposed to coat them with a complementary inhibiting film, essentially based on metallic oxides which is called passivation film.

 The conventional process for passivating zinc or alloyed zinc parts consists of immersing them in a chromic acid solution so as to coat them by chemical reaction with a corrosion-inhibiting film based on chromate and zinc dichromate.

 This chemical reaction is, however, very difficult to control and the passivating film thus obtained is very often fragile and thin, and therefore does not provide the desired protection.

 The object of the present invention is to remedy this drawback by proposing a new process for passivation of zinc or of alloyed zinc allowing better control of the reaction of formation of the passivating film and thus of improving its performance with respect to the corrosion resistance.

The invention applies very particularly to parts which have undergone prior galvanization but is, of course, not limited to such parts and can also be applied to the treatment of articles made of zinc or alloyed zinc
The method according to the invention is characterized by the implementation of the following steps - the part to be treated is immersed in an aqueous solution
containing between 5 and 100 g / l of chromic acid and between
5 and 100 g / l of phosphoric acid, - this part is connected to a pole of a current generator
pulsed associated with a DC generator and whose
the other pole is connected to a counter-electrode in a mate
neutral line also immersed in the aqueous solution,
and - between the workpiece and the counter-electrode, a
pulsed current at a voltage of the order of 3 to
10 volts.

 We have been able to verify that the implementation of this process makes it possible to deposit, electrolytically, on zinc or alloyed zinc, a passivation film essentially based on phosphate and zinc dichromate having very good corrosion resistance and allowing a significant increase in the life of the part thus coated.

 The essential characteristic of the process according to the invention is linked to the use of pulsed current: it has indeed been found that in the absence of pulsed current, it is impossible to obtain a passivation of zinc at the pH of the solution aqueous chromic / phosphoric, the surface layer of chromate which forms immediately blocking the continuation of the reaction.

On the contrary, the use of a pulsed current makes it possible to activate the anodic oxidation of zinc to 2+ ions
Zn and its dissolution as well as the formation of dichromate and phosphate.

It has been possible to verify that, during the implementation of the process according to the invention, the reactions which occur within the electrolytic solution are essentially the following:

 Consequently, at the level of the counterelectrode which can be produced in any inert material in the solution, in particular in graphite or more advantageously in stainless steel, a release of hydrogen is observed while the desired passivating film is applied to the part. .

 According to another characteristic of the invention, the frequency of the pulsed current is of the order of 0.1 to 1 kHz for the anode and cathode frequencies and more particularly about 1 kHz in anode frequency and about 0.2 kHz in cathode frequency .

 According to another characteristic of the invention, the duration of each current pulse is of the order of 1 to 10 ms.

 According to another characteristic of the invention, the average current density is between 1 and 5 A / dm.

 It has indeed been possible to verify that the aforementioned choice of the various parameters of the pulsed current makes it possible to obtain a particularly satisfactory passivating film.

 It was also possible to verify that the treatment time of the part should preferably be of the order of 1 to 5 min.

The characteristics of the process which is the subject of the invention will be described in more detail with reference to the accompanying drawings in which - Figure 1 is a diagram showing an installation
allowing the implementation of the method, - Figure 2 is a diagram (t / V) corresponding to the neck
rant applied.

According to FIG. 1, the method is implemented in an installation essentially constituted by an electrolytic tank 1 filled with an aqueous solution 2 containing between 5 and 100 g / l of chromic acid and between 5 and 100 g / l of phosphoric acid as well as by a pulsed current generator G2 associated with a direct current generator G1
The part to be treated P plunges into solution 2 and is connected to the generator G2. This generator is also connected to a counter electrode 3, preferably made of stainless steel, which plunges into solution 2 and surrounds the part P.

 According to FIG. 2, the generator G1 delivers a DC voltage VO to which the generator G2 cyclically superimposes a voltage in the form of a train of pulses.

 Consequently, the generator G2 applies, between the part P and the counter-electrode 3, a pulsed current which can be represented by the following characteristics - Voltage 3 at 10 volts, - Anode frequency 1 kHz, - Cathode frequency 0.2 kHz - Duration of each pulse. 1 to 10 ms.

2 - Average current density: 1 to 5 A / dm
The electrolytic solution 2 is partially dissociated and contains, in particular, H ions
2- 3 of Cr207 and P04 ions
Under the action of the above-mentioned pulsed current, the H + ions as well as the Cr2O72- and PO43- anions migrate towards the part P to form the desired passivating film and give off hydrogen.

 It should be noted that the part P passes alternately from + to - according to the train of pulses generated by the pulsed current generator G2.

Claims (1)

CLAIMS 1 ") Process for passivation of a part coated with a layer of zinc or alloyed zinc, characterized in that - the part to be treated is immersed in an aqueous solution containing between 5 and 100 g / l of chromic acid and between  5 and 100 g / l of phosphoric acid, - this part is connected to a pole of a current generator  pulsed associated with a DC generator and whose  the other pole is connected to a counter-electrode in a mate  neutral line also immersed in the aqueous solution,  and - between the workpiece and the counter-electrode, a  pulsed current at a voltage of the order of 3 to 10 volts  allowing to deposit electrolytically, on the  zinc or zinc alloy, an essential passivation film  based on phosphate and zinc dichromate per  putting protect the workpiece against corrosion and  to increase its lifespan.  20) Method according to claim 1, characterized in that the frequency of the pulsed current is of the order of 0.1 to 1 kHz for the anode and cathode frequencies.  30) Method according to any one of claims l and 2, characterized in that the duration of each current pulse is of the order of 1 to 10 ms.  40) Method according to any one of claims 1 to 3, characterized in that the average current density is between 1 and 5 A / dm.  50) Method according to any one of claims 1 to 4, characterized in that the part is treated for 1 to 5 minutes.