EP3456863A1 - Method for phosphating parts with incorporation of an optical tracer - Google Patents

Abstract

The invention relates to a method for phosphating mechanical steel parts comprising a step (7) during which a phosphatization layer is deposited on the parts by immersion of the parts in an acid pH bath containing at least phosphate ions and ions another element capable of reacting with the phosphate ions, a partial dissolution of the steel of the parts and a precipitation of a layer of mixed phosphates of iron and the other element taking place on each piece. An optical tracer is added to the acidic pH bath (14) and is incorporated into the mixed phosphate layer of each piece.

Application in the field of control of mechanical parts including motor vehicles.

Description

The invention relates to a method for phosphating iron-based mechanical parts with incorporation of an optical tracer. This process can however be generalized to all mechanical metal parts undergoing a phosphating treatment.

In addition, the invention relates to a method of detecting on a finished mechanical part a post-phosphatation treatment of the part in accordance with a phosphating process according to the present invention. This detection method is mainly used to identify if the mechanical part has not undergone post phosphating treatment that may alter its physical, chemical or mechanical properties.

The present invention is intended primarily for the treatment and control of mechanical parts in the automotive field, for example bodies present in a heat engine, including screws. This is however not limiting and can be applied to any type of mechanical parts.

Taking as a non-limiting example for illustration of the present invention that relating to heat engine screws, these mechanical parts are generally processed via a chemical conversion process called "phosphating manganese" or "iron manganese" or "parkerization". This treatment is carried out by immersion in an acid bath containing phosphate ions and manganese ions.

There is partial dissolution of the steel and precipitation of a layer of mixed phosphates of iron and manganese [(Mn, Fe) ₅ H ₂ (PO _{4) 4,} 4H ₂ O]. Manganese phosphating is followed by lubrication to avoid corrosion of the underlying steel but also and especially to provide the desired lubrication properties to the treated screws.

Indeed, the crystallographic structure of the deposit, the pink type of sands, allows good retention of oils and greases. This treatment is generally recommended for parts that require a friction or sliding capacity under low to high load, a temporary storage protection and the simultaneous presence of painted areas and unpainted functional areas for a friction application. . In addition, a gray-black color is often sought for these pieces.

Some manganese phosphating formulations use nickel salts, the use of which could become regulated by legislation in the near future due to environmental constraints. As an alternative to phosphating in the presence of manganese, it has been proposed to carry out a phosphatation in the presence of zinc called "zinc or iron-zinc phosphating" or "bonding" for several reasons.

Such zinc phosphating is a more stable formulation and more widespread in the industry, especially used for parts for which a good adhesion of the paint is required. Such zinc phosphating respects the environmental constraints in the absence of nickel salts.

Phosphating in the presence of zinc is carried out by immersion in an acid bath containing phosphate ions and zinc ions. It is obtained a partial dissolution of the steel and the precipitation of a layer of mixed phosphates of iron and zinc [Zn ₂ Fe (PO ₄ ) ₄ , 4H ₂ O] and zinc phosphates [Zn ₃ (PO ₄ ) ₄ , 4H ₂ O]. Zinc phosphating is of course also followed, in a very short time, lubrication to avoid corrosion of the underlying steel but also and especially to provide the desired lubrication properties treated screws.

The parts coated with a phosphating layer in the presence of manganese have a dark gray to black appearance while the parts coated with a phosphating layer in the presence of zinc have a light gray to dark appearance.

These two types of phosphating do not pose particular problems when the desired properties are well achieved, including the recovery, the size of the crystals and the weight of phosphating layer. In the automotive field as in other areas phosphating takes place at a supplier of mechanical parts therefore partially escapes the control of the principal, for example the car manufacturer.

In general, the client, because phosphating does not take place in his workshops, needs to follow the manufactured parts in order to control his supplier. A non-limiting but particularly important example of monitoring the phosphating of parts will now be given.

Thus, in the case of parts with specific traces of oxidation and / or corrosion, the supplier may treat them in an acid bath, mainly hydrochloric acid, to remove these traces and then re -phosphate then. This is not always provided for in the specifications imposed by car manufacturers and is therefore sometimes achieved without their consent.

The phosphatization process, even when it follows the specifications of car manufacturers as prime contractors, carried out in an acid medium allows a certain amount of hydrogen to be introduced into the steel. In order to avoid any deferred rupture phenomena related to embrittlement by hydrogen, it is recommended for mechanical friction parts with a high mechanical strength, for example tensile strength above 1,000 MPa, to proceed with stoving at 100 ° C for 8 hours.

Unfortunately, during the practice of an acid attack of parts exhibiting traces of oxidation and / or corrosion, then of a re-phosphatation, practice post phosphating and made in the ignorance of the client by the supplier, the client has no means of detecting this harmful practice. In addition, this practice can cause a delayed break post-processed parts and in the absence of adequate baking.

The parts thus treated have a reduced dimensioning while remaining in a depth of engraving yet acceptable, that is to say a depth of attack of the surface: such parts can not be discarded considering their external appearance and their dimensional tolerances.

Similarly, because of the re-phosphating after the acid attack not provided for by the specifications, the phosphatation layer weights can increase but these increases can be difficult to detect because covered by a weight of phosphatation layer initially lower or having decreased during acid attack.

In addition, the phosphating after acid attack can not be certainly recognized from an initial phosphating according to the specifications. This phosphatation after acid attack is finer, but the difference in crystallography with initial phosphating is difficult to master perfectly and / or reproducible on complex geometry parts such as screws.

The document FR-A-2 918 585 discloses a verification method if a solid substrate, for example glass, is coated with an alkoxysiloxane-based varnish by raising an optical signature, in the form of a mixture comprising at least one photoluminescent ion incorporated in the varnish of coating, the luminescence spectrum of the substrate to be authenticated to compare with that of a reference substrate. This However, the document does not relate to metal mechanical parts having undergone phosphating treatment. Nor does it relate to the presence of a layer of a mixture of a component of the part closely related to the coating of the piece. In this document the coating and the support do not form a chemically bonded common layer.

Therefore, the problem underlying the invention is, for a mechanical steel part subjected to phosphatation with formation of a layer of mixed iron phosphates, on the one hand, to follow the evolution of the layer of mixed phosphates of iron after phosphating and, secondly, to detect whether the mechanical part undergoes post-phosphatation treatment likely to reduce its physical, chemical and mechanical properties.

To achieve this objective, there is provided according to the invention a method of phosphating mechanical steel parts comprising a step during which a phosphating layer is deposited on the parts by immersing the parts in an acidic pH bath containing at least ions phosphates and ions of another element capable of reacting with the phosphate ions, a partial dissolution of the steel of the pieces and a precipitation of a layer of mixed phosphates of iron and the other element taking place on each piece characterized in that an optical tracer is added to the acidic pH bath and is incorporated into the mixed phosphate layer of each part.

The optical tracer according to the present invention can be divided and be in the form of several portions of optical tracers advantageously uniformly distributed in the phosphatization layer and even in the entire room, the latter case occurring after diffusion in the entire room, for example during a degassing of the room.

There may be several different types of optical tracers integrated in each room, for example an optical tracer only signaling its presence or an optical tracer indicating that the part has undergone a post phosphating treatment by dyeing itself according to a temporary variation of a treatment parameter such as pH or temperature.

The technical effect is to obtain a follow-up of the part and in particular of the layer of mixed phosphates of iron by nondestructive optical control. The optical tracer may only signal its presence or the optical tracer may signal that the part has been temporarily subjected to post-phosphatation treatment conditions, for example, but not only to a treatment that has caused an alteration of its properties. The tracer may, for example, for a finished part to indicate that the part has been temporarily in contact with an acid bath or temporarily brought to a temperature having caused a change in the crystallographic structure of the part or a diffusion of the tracer in the part, this even if it was only temporary.

For an optical tracer only signaling its presence, according to the optical tracer used and its concentration in the layer, it is expected that the optical tracer returns a signal of a specific light intensity for a conformal phosphating layer, the optical tracer being advantageously distributed in the phosphating bath. If the signal of the optical tracer present in a room is decreased, it is because the phosphatization layer is not sufficient, for example during a deficient phosphating but also because of a subsequent treatment of the room having destroyed part of the phosphating layer.

There are therefore two essential applications of the present invention. The first application is the monitoring of the phosphatization layer in a normal course of manufacture of the part. For example, the quality of the phosphatization layer can be evaluated according to the optical signal delivered by the sensor and in particular give indications on its completion and its dimensions, in particular on its sufficient thickness or not. This assists in non-destructive testing of phosphated parts.

In addition, the presence of an optical tracer can be used to monitor the diffusion of the optical tracer in the room during a possible degassing step subsequent to the phosphating step and help to better set the parameters of this degassing operation . It is then possible to note that a part that does not require a degassing step during its manufacture has however undergone such treatment, which can make it put off.

Thus, the first application contributes to optimize the process of phosphating parts and also serves as non-destructive testing of parts by serving to discard parts. This is not limiting.

The second application which is the preferred embodiment of the present invention is to detect whether an undesired step in the phosphating process has occurred after phosphatation, this undesired step being capable of altering the properties of the part.

For example, for an optical tracer, in the case of inappropriate reprocessing subsequent to the phosphating by immersing the pieces in a bath of strong acid, for example of pH less than 3, as an acid attack starts the phosphatization layer, this tracer will be partly removed and its optical signature will be different from that of an initial phosphating layer that has not been attacked.

As another example, for an optical tracer as a pH tracer integrated in the phosphating layer, the optical pH tracer remains in the phosphatization layer, especially when there is no degassing operation and, in subsequent contact in the case of phosphatation with a very acidic medium, for example with a pH of less than 3, the optical pH tracer will surface-color the parts exposed in an inappropriate manner to this highly acidic medium.

The optical pH tracer can be thermally diffused into the matrix of the part during the degassing treatment which can be performed within a predetermined time after phosphating. In the case of inappropriate post-phosphatation reprocessing, such as, for example, an attack in acidic medium, this tracer will then be in contact with an acidic medium. The optical pH tracer will surface-color the part exposed improperly to a highly acidic medium. The visual signature of a post phosphating treatment and damaging parts will be identified by an abnormal coloring of the parts, detectable to the naked eye or under bright excitation.

Advantageously, the other element is manganese or zinc with respectively a precipitation of a layer of mixed phosphates of iron and manganese of chemical formula [(Mn, Fe) ₅ H ₂ (PO ₄ ) ₄ , 4H ₂ O] or a precipitation of a layer of mixed phosphates of iron and zinc of chemical formula [Zn ₂ Fe (PO ₄ ) ₄ , 4H ₂ O] and zinc phosphates of chemical formula [Zn ₃ (PO ₄ ) ₄ , 4H ₂ O], the phosphatation with manganese or zinc being followed by a lubrication of the parts after removal of the acid phosphatation pH bath pieces.

The process of the present invention applies equally to all types of phosphatation, in particular to phosphatation in the presence of manganese and phosphatation in the presence of zinc, as well as to all their variants.

Advantageously, the optical tracer provides a recognizable optical signature in the room with the naked eye or under experimental excitation when the optical tracer is present in the room and / or indelibly colors when the room incorporating the tracer, after removing the acid phosphating pH bath pieces by subjecting the pieces to at least one rinse, is in contact with an acid medium having a pH below a predetermined pH or when the piece is brought to a temperature above a predetermined temperature threshold.

The optical tracer is designed not to stain in the phosphating bath but only for post-phosphatation treatment. If not, for example, a pH tracer would indicate that the part has been exposed to an acid bath during phosphatization. which would mask any exposure of the room to an acid bath after phosphatation.

An optical signature visible to the naked eye does not require any specific equipment but is less precise and especially less concealed than an optical signature under light excitation, a recognition of such an optical signature under light excitation can be made in a control laboratory pieces. For parts manufactured at a supplier, it is advantageous that the optical signature is not visible to the naked eye in order to conceal such control to the parts supplier.

Advantageously, the tracer is in the form of integrated nanoparticles in the pH bath of acid phosphating, the nanoparticles integrating into the phosphating layer of each of the parts.

This had never been implemented for the control of steel mechanical parts with a phosphating layer incorporating iron ions and chemically bound phosphates. Nanoparticles had already been used for non-metallic parts to verify the presence of a layer on a substrate with however no chemical interaction between the substrate and the layer, the substrate not being a metal part in this state of being. the technique mentioned above.

Advantageously, the nanoparticles are luminescent nanoparticles and / or nanoparticles with invisible coloration or not when the part integrating them is in contact with an acid medium having a pH below a predetermined pH or when the workpiece is brought to a temperature at above a predetermined temperature threshold.

Unlike solid phosphors, luminescent nanoparticles have the advantage of being able to combine the luminescence properties and the properties related to the nanometric dimensions of objects. Nanoparticles can thus be dispersed in a carrier invisibly, as is a phosphating layer and emit luminescence only under adequate excitation, as a revelation by the technique of measuring photoluminescence.

This can be particularly interesting if a material must remain identical in appearance when it is not excited. This is also of great interest when the marking must remain undetectable to the naked eye for reasons of safety or detection of fraud, for example when the phosphating is done by a subcontractor and the quality control of the parts is done at his own expense. knowledge.

Another advantage of luminescent nanoparticles is tracing applications. Indeed, by measuring the light emitted by these particles, it is possible to study the treatments suffered by the parts in different media, for example partial acid attacks and to quantify the damage to the parts.

Finally, the nanoparticles must be distributed uniformly in the phosphatization layer. A non-uniform distribution of the nanoparticles can be observed during the examination, which suggests that the phosphating layer is not uniformly distributed over the contour of the examined part and may be deficient.

Advantageously, the nanoparticles emit a luminescence invisible to the naked eye and quantifiable by a photoluminescence measurement technique.

Advantageously, the process comprises, after the step of depositing a phosphatization layer on the parts, a degassing step under a predetermined temperature and duration, at least a part of the nanoparticles diffusing, during the degassing step, in the room out of the layer of mixed phosphates of the room.

This takes place for example for metal parts whose tensile strength is less than 1,000 MPa. The degassing step is therefore not necessarily part of the phosphating process according to the present invention. When a degassing step is provided, this step is less than 4 hours after phosphating, the room door up to 150 ° C for 6 or 8 hours.

The invention also relates to a process for the detection on a finished mechanical part of a post-phosphatation treatment of the part in accordance with such a phosphating process, characterized by an examination on the finished part of the optical tracer incorporated in the part, the posterior treatment having been previously identified as leading to at least partial elimination of the optical tracer or to a modification its color as a result of a temporary increase of the room temperature or an acid attack of the room and when such a modification of the optical tracer is detected during the examination, it is concluded that the subsequent treatment has been implemented on the piece.

The main objective of such a method according to the invention is to be able to implement a detection of any treatment of parts after phosphating not provided by the specifications and may damage parts, for example by acid attack. It was previously possible to re-phosphatate again to disguise this acid attack, this subsequent re-phosphating giving parts resembling the parts having undergone the initial phosphating but giving the parts only a reduced protection compared to phosphating initial.

The presence of the optical tracer integrated in the initial phosphating layer but not present in the subsequent re-phosphating bath makes it possible by its absence at least partial in the parts to recognize the parts having undergone a subsequent re-phosphatation, probably after an attack. acid. Alternatively or in addition, the acid attack can also be recognized or confirmed by the presence of an optical pH tracer integrated in the room.

The detection method according to the invention makes it possible, on the one hand, to verify that the phosphating layer present on the parts is authentic, thus obtained during the initial phosphating, and, on the other hand, to detect the pieces coming from a non-recommended phosphating process having for example implemented a step of stripping the parts with the acid after initial phosphating and rephosphated thereafter.

The invention finally relates to a mechanical part obtained according to such a phosphating process or undergoing such a detection method, characterized in that the part incorporates an optical tracer.

Such a mechanical part is recognizable from a mechanical part of the state of the art because it incorporates at least one optical tracer, preferably in the form of nanoparticles.

Given the economic, environmental and safety issues, the mechanical strength of structural parts remains a major issue. Despite this, in many industrial sectors, such as automotive or aeronautics, there are reports of deferred part failure. In particular, this type of damage can be more or less frequently encountered on fasteners. These steel parts, whose mechanical properties are increasingly high, can be sensitive to hydrogen embrittlement phenomena that induce degradation of mechanical properties at the microscopic scale. Mechanical parts according to the present invention may signal non-compliant parts due to the presence of an optical tracer.

Advantageously, the part is a member present in a motor vehicle engine such as a crankshaft or bolt. This is a preferred but non-limiting application of the present invention.

• la figure 1 est une représentation schématique selon la présente invention.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawing given by way of non-limiting example and in which:

• the figure 1 is a schematic representation according to the present invention.

It should be borne in mind that the figure is given by way of example and is not limiting of the invention. All the mentioned steps of the phosphating process are not essential for the practice of the present invention.

The present invention relates to a process for phosphating mechanical steel parts, comprising a step 7 during which a phosphating layer is deposited on the parts by immersing the parts in an acid pH bath containing at least phosphate ions and ion ions. another element capable of reacting with the phosphate ions.

In this bath, a partial dissolution of the steel of the pieces occurs and a precipitation of a layer of mixed phosphates of iron and the other element taking place on each piece.

According to the present invention, at least one optical tracer is added to the acidic pH bath and is incorporated in the mixed phosphate layer of each piece. This optical tracer may be sensitive to a parameter such as temperature or pH during a post-phosphatation treatment then called initial phosphating. This optical tracer may only signal its presence.

There may be a plurality of optical tracers of the same type or a plurality of optical tracers signaling their presence optically and at least a plurality of optical tracers indicating that a treatment parameter judged unfavorable to the final quality of the mechanical part has been reached during the manufacture of the parts, this after the phosphating, the parameter being for example a pH of a bath in which have been immersed parts or exposure to a temperature after phosphatation, the pH or temperature may have consequences on the quality of phosphating previously performed.

The figure 1 shows a possible unfolding of the steps of a phosphatization process in the presence of zinc of mechanical degreasing steel parts, of which only the phosphating step is essential, the rinsing step and the step of lubricating the parts being recommended, the other steps being optional but advantageous. Similar steps can be envisaged for phosphatation in the presence of manganese by replacing zinc with manganese.

The reference 1 indicates one or more stages of degreasing the mechanical parts made of steel. The step referenced 2 is at least one rinsing with water of the mechanical parts. Step 3 is an acid pre-etching of the parts, advantageously for highly oxidized parts. Step 4 is acid etching, ideally with corrosion inhibitors. Step 5 consists of one or more rinses with water.

For phosphatation in the presence of zinc, step 6 is zinc refining. Step 7 is the essential step of the process consisting of zinc phosphating, with according to the present invention, a referenced addition 14 of an optical tracer or in the form of nanoparticles in the bath.

The optical tracer can be a simple optical tracer signaling its presence or a tracer signaling a change of state of the part being sensitive for example to the pH of a solution in which the part is immersed or at a temperature causing a change of structure of the room. The optical tracer may be multiple by being substantially uniformly distributed in the phosphating layer or mixed phosphate layer of each part.

After the phosphating step, one or more rinses with water are carried out on the parts being referenced 8. Step 9 is the addition of a corrosion inhibitor on the parts followed by a drying operation. .

Step 11 is only suitable for a certain type of part and consists of a degassing for example, under setting of less than 150 ° C parts for 6 to 8 hours, this at the latest 4 hours after phosphating. During this degassing occurs a diffusion of the nanoparticles of the optical tracer being carried out in the room, as illustrated in 15.

Degassing is appropriate for mechanical parts of friction with high mechanical strength (for example tensile strength) greater than 1.000 MPa, while it is not suitable for mechanical parts with mechanical strength, for example tensile strength, less than 1,000 MPa.

Step 12 provides lubrication of the phosphated parts and step 13 a steaming of the phosphated parts.

As previously mentioned, there are mainly two types of phosphatation, respectively in the presence of manganese and in the presence of zinc. The other element of the phosphatation can thus be manganese or zinc with respectively a precipitation of a layer of mixed phosphates of iron and manganese of chemical formula [(Mn, Fe) ₅ H ₂ (PO ₄ ) ₄ , 4H ₂ O] or a precipitation of a layer of mixed phosphates of iron and zinc of chemical formula [Zn ₂ Fe (PO ₄ ) ₄ , 4H ₂ O] and zinc phosphates of chemical formula [Zn ₃ (PO ₄ ) ₄ , 4H ₂ O].

Phosphating with manganese or zinc may be followed by lubrication of the parts after removal of the acid phosphatation pH bath parts, this advantageously after rinsing and possible degassing.

The present invention also applies to the phosphating variants mentioned above with, for example, coating the mechanical parts with zinc lamellar.

The tracer may be an optical tracer providing an optical signature recognizable in the room with the naked eye or under experimental excitation when the optical tracer is present in the room, so a tracer only signaling its presence and not a chemical or temperature parameter having impacted the pieces.

The tracer may be a tracer sensitive to pH or temperature. Such a tracer preferably stains indelibly when the part incorporating the tracer, after removal of the acid phosphating pH bath parts by advantageously subjecting the parts to at least one rinse, has been temporarily in contact with an acid medium having a pH below a predetermined pH or when the part has been temporarily raised to a temperature above a predetermined temperature threshold.

The predetermined pH may be a pH equal to 3 or less than 3. The temperature may be a temperature imposing a change of iron structure that may impact the resistance or wear resistance of the part, or any other characteristic of the part. It is thus possible to recognize a stoving or degassing step that took place after phosphating. A pH tracer is preferred in the context of the present invention.

The tracer, whether it be an optical tracer signaling its presence or a tracer identifying a parameter facing the part after its phosphating, for example an acidic pH or a high temperature, may be in the form of integrated nanoparticles advantageously uniformly in the bath pH of acid phosphatation.

A mixing of the phosphatizing bath can be implemented, which is not limiting. The nanoparticles are integrated in the phosphating layer of each of the parts, advantageously in a uniform manner and each serve as portions of optical tracer.

The nanoparticles may be luminescent nanoparticles and / or nanoparticles with invisible coloration or not when the part integrating them is in contact with an acid medium having a pH below a predetermined pH or when the workpiece is brought to a temperature above above a predetermined temperature threshold.

It is possible to mention a pH of less than 3 as damaging to the part and in particular its phosphatizing layer. The temperature can be that causing degassing desired or not or an alteration of the structure of the parts.

In a preferred embodiment of the present invention, the nanoparticles can emit luminescence invisible to the naked eye and quantifiable by a photoluminescence measurement technique. This is particularly relevant when the parts are phosphated by a subcontractor to check the conformity of the parts without the subcontractor's knowledge.

The photoluminescence measurement technique can be performed in the laboratory but can give a quantifiable result, for example a measured loss of luminescence compared to the expected luminescence, this loss corresponding to a decrease in the phosphatation layer compared to the initial phosphating.

The method may comprise, after the step of depositing a phosphating layer on the parts, a degassing step under a temperature and during a predetermined duration, at least a portion of the nanoparticles diffusing, during the degassing step, into the room outside the layer of mixed phosphates of the room.

The degassing can be done at a temperature below 150 ° C for 6 to 8 hours being carried out within a maximum of 4 hours after phosphating.

As preferential application of the phosphating process according to the present invention with insertion of an optical tracer during the phosphatization of the parts, the invention also relates to a method of detection on a finished mechanical part of a post-phosphatation treatment of the piece in accordance with such a phosphating process.

The detection is done by an examination on the finished part of the optical tracer incorporated in the room. The post-phosphatation treatment has been previously identified as leading to at least partial elimination of the optical tracer, for an optical tracer only signaling its presence or a temporary increase in the temperature of the room or an acid attack of the piece, for an optical tracer indicating that the part has been confronted with conditions that can alter its properties.

In these cases, respectively the at least partial removal of the optical tracer corresponds to a decrease in the phosphating layer, the temperature increase or the acid attack having caused a modification of the optical tracer has affected the properties of the parts. Thus, when such a modification of the optical tracer is detected during the examination, it is concluded that the subsequent treatment has been implemented on the part.

For example, for mechanical parts integrating into the phosphatization layer of each mechanical part an optical tracer only signaling its presence without identifying conditions of exposure of the mechanical parts such as a temperature or a pH, in the case of inappropriate reprocessing as an acid attack, this tracer will be eliminated or decreased in the phosphating layer by emitting an optical signature different from that expected for a phosphating layer having not undergone any subsequent reprocessing. This proposal can be applied for mechanical friction parts with high mechanical strength, for example traction, greater than 1,000 MPa.

As other examples, for mechanical parts integrating in the phosphatization layer of each mechanical part a pH sensitive optical tracer, the Optical pH tracer can either remain in the phosphating layer or be diffused partially or totally throughout the room.

In the first case, in contact not provided by the specifications and inappropriate with exposure of the parts to a highly acidic medium having a pH for example less than 3, this subsequent to phosphating, the optical tracer said pH dye surface-colored parts exposed in a manner not provided for this very acidic medium, this ideally with an indelible ink. This applies preferentially for mechanical friction parts with high mechanical strength, for example traction, greater than 1,000 MPa.

In the second case, the pH dye tracer is thermally diffused into the matrix during the degassing treatment being carried out at a temperature below 150 ° C for 6 to 8 hours being carried out within a maximum of 4 hours after phosphating.

In the case of improper reprocessing such as an acid attack, the optical tracer is then in contact with the acidic medium and colored on the surface of the piece inappropriately exposed to a very acidic medium. The optical signature will be visible by abnormal coloring of parts, parts that are to be rejected. This applies preferentially for mechanical friction parts with mechanical strength, for example traction, less than 1,000 MPa.

The invention finally relates to a mechanical steel part obtained in accordance with such a phosphating process or undergoing such a detection method, the part incorporating an optical tracer. Even in its finished state, a part according to the present invention is recognizable from a similar piece according to the state of the art by the presence in its interior of an optical tracer, advantageously in its phosphating layer or having diffused into the room during a degassing.

In an advantageous use, the part is a member incorporated into a motor vehicle engine such as a crankshaft or bolt.

The invention is in no way limited to the described and illustrated embodiments which have been given by way of example only.

Claims (10)

Process for the phosphating of mechanical steel parts comprising a step (7) during which a phosphatization layer is deposited on the parts by immersing the pieces in an acid pH bath containing at least phosphate ions and ions of another suitable element reacting with the phosphate ions, partially dissolving the steel of the pieces and precipitating a layer of mixed iron phosphates and the other element taking place on each piece, characterized in that an optical tracer is added in the acidic pH bath (14) and incorporated in the mixed phosphate layer of each piece. Process according to the preceding claim, in which the other element is manganese or zinc with respectively a precipitation of a layer of mixed phosphates of iron and manganese of chemical formula [(Mn, Fe) ₅ H ₂ (PO ₄ ) ₄ , 4H ₂ O] or a precipitation of a layer of mixed iron and zinc phosphates of chemical formula [Zn ₂ Fe (PO ₄ ) ₄ , 4H ₂ O] and zinc phosphates of chemical formula [Zn ₃ ( PO ₄ ) ₄ , 4H ₂ O], the phosphatation with manganese or zinc being followed by a lubrication of the parts after removal of the acid phosphatation pH bath pieces. The method of claim 1 or 2, wherein said least one optical tracer provides an optical signature recognizable in the workpiece with the naked eye or under experimental excitation when the optical tracer is present in the workpiece and / or indelibly colors when the part incorporating the tracer, after removal of the acid phosphatation pH bath pieces by subjecting the parts to at least one rinsing, is in contact with an acid medium having a pH below a predetermined pH or when the piece is worn at a temperature above a predetermined temperature threshold. Process according to any one of the preceding claims, in which the tracer is in the form of nanoparticles integrated in the acid phosphatation pH bath, the nanoparticles integrating into the phosphating layer of each of the pieces. Process according to the preceding claim, in which the nanoparticles are luminescent nanoparticles and / or nanoparticles with invisible coloring or otherwise when the integrating member is in contact with an acidic medium having a pH below a predetermined pH or when the workpiece is brought to a temperature above a predetermined temperature threshold. Method according to the preceding claim, wherein the nanoparticles emit a luminescence invisible to the naked eye and quantifiable by a photoluminescence measurement technique. Process according to any one of claims 4 to 6, which comprises, after the step of depositing (7) a phosphating layer on the parts, a degassing step (11) under a temperature and for a predetermined duration, at least a part of the diffusing nanoparticles (15), during the degassing step (11), in the piece outside the layer of mixed phosphates of the part. A method of detecting on a finished mechanical part a post-phosphatation treatment of the part according to a method according to any of the claims, characterized by an examination on the finished part of the optical tracer incorporated in the part, the subsequent treatment having been previously identified as leading to at least partial removal of the optical tracer or to a change in its color as a result of a temporary increase in room temperature or acid attack of the room, and when such a modification of the optical tracer is detected during the examination, it is concluded that the subsequent treatment has been implemented on the part. Mechanical steel part obtained according to a phosphating process according to any one of claims 1 to 7 or undergoing a detection method according to claim 8, characterized in that the part incorporates at least one optical tracer. Part according to the preceding claim, which is a member present in a motor vehicle engine as a small-end screw or cylinder head.

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