FR2704560A1 - A method of electroplating a surface of a steel substrate with a layer of a zinc-based alloy coating and a steel material coated with a zinc-based alloy coating layer. - Google Patents

Abstract

The subject of the invention is a process for electroplating a layer of a coating of a zinc-based metal alloy, of the ZnX type, X being the second element of this alloy, onto a surface of a steel substrate. The process consists in depositing, between the surface of the substrate and the layer of coating of the said alloy, an undercoat layer of the said alloy with a percentage of the second element X such that the reduction potential of the said alloy of the said undercoat layer with respect to a saturated-calomel electrode is greater than or equal to or substantially less than the potential for hydrogen evolution on the steel of the substrate, in order to obtain the desired percentage of the second element X. The subject of the invention is also a coated steel material comprising a steel substrate and a coating layer of a zinc-based metal alloy.

Description

The subject of the present invention is an electroplating process on a

  surface of a steel substrate with a coating of a zinc-based metal alloy. The present invention also relates to a coated steel material comprising a steel substrate and a metal alloy coating layer

based on zinc.

  It is known that a layer of an alloy based on

  zinc such as zinc / nickel, zinc / a-

  luminium, or zinc / cobalt deposited on a steel substrate has excellent protective activity for said

substrate against corrosion.

  Thus, steel materials coated with a zinc / nickel alloy are widely useful as corrosion-resistant materials in the field of motor vehicles, household appliances and appliances.

  building materials, particularly for buildings

  is lying. By way of example in the automotive field, steel sheets coated with a layer of zinc on one side and a coating layer of a zinc alloy

  nickel on the other side are commonly used.

  The interest of this type of differential coated sheet is particularly visible in the manufacture of automobile body parts, as

  for example the doors or the wings.

  Indeed, these elements are subject to two

types of corrosion.

  The first so-called cosmetic corrosion initiated mainly by mechanical aggression of the type of scratching or graveling can expose the steel in

  contact with the outside atmosphere.

  The second so-called perforating corrosion appears most often in hollow bodies and progresses

  from the inside to the outside of the body.

  Whereas cosmetic corrosion preferentially uses the sacrificial power of the coating with respect to steel, the resistance to perforation is

  related to the barrier effect of the coating.

  The use of a zinc-coated sheet on one side and zinc / nickel on the other side makes it possible to use the sacrificial power of the zinc coating on the outer face of the element.

  bodywork and the barrier effect of aluminum coating

  zinc / nickel bonding on the inside of this element

of bodywork.

  Generally, to achieve the deposition of a coating layer of a zinc-based alloy on the surface of a steel substrate, an electrodeposition process is used consisting of passing said substrate in an electrolyte bath containing zinc ions and nickel ions in a chloride medium or in a sulphate medium. By way of example, the bath may have the following composition: ZnCl 2: 2 to 3 moles per liter NiCl 2: 0.2 to 1 mole per liter The remainder being deionized water or not, bath temperature 50 to 70 ° C. ,

pH: 4 to 5.

  The current density is adjusted to obtain

  the desired percentage of nickel in the zinc / nickel alloy

  kel (usually around 12% nickel).

  However, it is found that the alloy coating

  zinc / nickel thus electrodeposited has a property of ad-

relatively deplorable hero.

  Indeed, in a simple adhesion test consisting of using a coated steel pellet covered with a standard adhesive tape, which is folded to 900 before removing the adhesive tape, it is found that the entire coating s' is detached from the steel substrate. In order to improve the adhesion of the zinc / nickel alloy coating to the surface of the steel substrate on which said coating has been deposited, one solution consists in depositing a first coating layer of

  nickel by chemical quenching without electrical current

  and then electroplating a layer of zinc / nickel alloy on the first layer of nickel

previously filed.

  In this case, the adhesion of the coating is

  better than in the case of an electrodeposit

  directly, but it is not totally satisfactory.

  health. The so-called "adhesive tape" test shows that the coating is partially torn from

the coated test pellet.

  The object of the invention is therefore to avoid the disadvantages mentioned above by proposing a method of electroplating a surface of a steel substrate with a layer of a coating of a zinc-based metal alloy, of the type ZnX, of which X is the second element of this alloy, which ensures a good adhesion of the coating on said steel substrate, without producing a coating layer by chemical deposition by dipping. The subject of the invention is therefore a process for electrodeposition on a surface of a steel substrate of a zinc-based metal alloy coating layer of the ZnX type, X being the second element of this alloy, characterized in that between the surface of the substrate and the coating layer of said alloy is deposited a layer of undercoating of said alloy with a percentage of said element X such that the potential of

  reducing said alloy of said undercoat layer

  relative to a saturated calomel electrode is greater than or equal to or substantially less than the potential for hydrogen evolution on the substrate steel,

  to obtain the percentage of the second element X desired.

  According to other features of the invention,

tion:

  - is deposited between the undercoat layer

  and the coating layer of said alloy, at least one undercoating intermediate layer with a percentage of the second element X such that the reduction potential of said alloy of the undercoating intermediate layer n is greater than or equal to or substantially less than the disengaging hydrogen on said alloy from the n-1 underlay previously deposited,

  - is deposited between the undercoat layer

  and the coating layer of said alloy, a

  intermediate layer of undercoating with a

  percent of the second element X such that the reduction potential of said alloy of the undercoat interlayer is greater than or equal to or substantially less than the hydrogen evolution potential of

  said alloy of the underlayment layer previously

deposited.

  The invention also relates to a coated steel material comprising a steel substrate and a coating layer of a zinc-based metal alloy of ZnX type, X being the second element of this alloy, said undercoat layer being deposited by electroplating on a surface of said substrate, characterized in that it comprises between the surface of the substrate and the coating layer, a

  undercoating layer of said alloy with a percentage

  the second element X such that the reduction potential

  said alloy of the undercoat layer relative to a saturated calomel electrode is greater than or equal to or substantially less than the

  release of hydrogen on the substrate steel.

  According to other features of the invention,

  the material comprises, between the undercoat layer and the coating layer, at least one

  intermediate layer of undercoating with a percentage

  the second element X such that the reduction potential

  said alloy of the intermediate layer n of sub

  coating is greater than or equal to or substantially less than

  the potential for hydrogen evolution on said

  alloy of the n-1 intermediate layer of undercoat

  previously deposited, - the material comprises, between the undercoat layer and the coating layer of said alloy, an intermediate layer of undercoating with a percentage of the second element X such that the reduction potential of said alloy of the intermediate layer of sub coating is greater than or equal to or substantially less than the hydrogen evolution potential on

  said alloy of the undercoating layer previously

duly deposited.

  The invention further relates to a coated steel material comprising a steel substrate and a coating layer of a zinc / nickel alloy with 12% nickel, said coating layer being electroplated on a surface of said substrate, characterized in that it comprises between the surface of the substrate and the coating layer, a layer of zinc / nickel alloy undercoat with 24% nickel and, on

  said undercoat layer, an intermediate layer

  zinc / nickel alloy undercoat with 18% nickel.

  The characteristics and advantages of the

  will appear in the description that goes

  follow, given only as an example. In what follows, the invention will be described for a zinc-based metal alloy of the ZnX type, the second element X of this alloy being nickel, but may also be for example iron, cobalt or

chromium.

  To deposit a metal alloy by electrodeposition on a surface of a steel substrate, for example a zinc / nickel alloy with 12% nickel,

  polarizes the metal substrate to bring it to

  reduction of the alloy to be deposited, that is to say that one polarizes said substrate to the potential of -0.95 V / Ecs in the case of the alloy zinc / nickel 12% nickel. According to the results of the investigations carried out by the Applicant, it is supposed that the adhesion of the coating thus electrodeposited depends directly on the

release of hydrogen.

  Indeed, the electrodeposition is done in aqueous acid medium for example in a bath containing ZnCl 2, KCl, NiCl 2 and H 2 O, the pH of the solution is very acidic and there is a large number of H + ions in solution.

  During the electroless deposition reaction

  The hydrogen degasses in the form of bubbles and often these hydrogen bubbles form below the coating layer preventing it from adhering to the surface.

of the metal substrate.

  It has been found that more hydrogen is released

  during the electroplating reaction, the higher the adhesion

  coating on the surface of the metal substrate

is less good.

  A series of tests have been carried out to

  try to understand this phenomenon of degassing of the hy-

  during the electrodeposition reaction to find a solution to minimize this outgassing and to avoid as much as possible the formation of hydrogen in the solution. Test specimens were coated with a coating layer of zinc / nickel alloy with 12% nickel

and a thickness of 6pm.

  Some specimens were electroplated between the surface of the substrate and the zinc / nickel alloy coating layer with a zinc / nickel alloy undercoating layer 0.04 μm thick with a different percentage of nickel depending

the test pieces.

  Other test specimens were electroplated between the undercoat layer

  of zinc / nickel alloy and the coating layer of

  zinc / nickel bonding, an undercoating layer of a zinc / nickel alloy with a thickness of

  0.04pm, with a different nickel percentage.

  These specimens then underwent the test

  adhesion called "tape test".

  Depending on the degree of tearing of the coating, the adhesion was classified in a range of 1 to 5, the coefficient 1 being reserved for the total tearing of the coating, the coefficient 5 has no tearing and the

  coefficients 2 to 4 depending on the degree of tearing.

  The table below gives the results of

these different tests.

  N tests% Ni% layer or adhesion layer under intermediate coating

underwear

ment

1 1

2 15 2

3 18 2

4 21 4

18 15 2

6 21 15 3

7 21 18 4

8 21 21 5

9 24 18 5

  Examining the results of these tests, it can be seen that the higher the percentage of nickel in the underlayment layer, the better the adhesion

coating.

  The explanation of this phenomenon is related to

  potential for hydrogen evolution.

  In the first test, we directly

  trodéposé a layer of zinc / nickel alloy coating

  at 12% nickel on the surface of the steel substrate.

  For this, the substrate was polarized to bring it to - 0.95V / Ecs, the reduction potential of the zinc / nickel alloy at 12% nickel compared to a

saturated calomel electrode.

  However, to reach -0.95V / Ecs, the steel substrate was first brought to -0.78V / Ecs, potential for

  release of hydrogen on the steel.

  Thus, as soon as the substrate was raised to -0.78V / Ecs, the hydrogen evolution reaction started and continued until the potential

substrate reaches -0.95V / Ecs.

  At this point, the deposition of the zinc / nickel alloy on the surface of the steel substrate begins and as far as the release of hydrogen is concerned it is no longer the potential of its release on the steel that is needed. take into account but its release potential

on the alloy that has been deposited.

  Indeed, as soon as the deposit begins and a thin layer of alloy has been deposited on the surface of the steel substrate, the subsequent deposition of this alloy takes place on the layer of said alloy previously

filed.

  In Test No. 2, a 15% zinc / nickel alloy undercoat layer was first deposited prior to depositing the 12% nickel coating layer. Since the reduction potential of the zinc / nickel alloy layer at 15% of nickel is equal to -0.90 V / Ecs, the hydrogen evolution reaction was shorter in the first test before the beginning of the deposition of the nickel. alloy, which explains why adhesion is

slightly better.

  Test 4, for example, gives a good result.

  state in adhesion due to the reduction potential of the zinc / nickel alloy at 21% of nickel which is equal to

  -0,85V / Ecs, ie quite close to -0,78V / Ecs.

  These tests already show that in the case where a coating layer is carried out with a zinc / nickel alloy whose percentage of nickel is such that the potential of said alloy is close to the potential of release of hydrogen on the substrate. steel, the adhesion of the coating is significantly increased. Tests 5 to 9 show that it is possible

  to further improve the quality of the adhesion of the coating-

  depositing an intermediate layer of undercoating between the undercoat layer and the

coating layer.

  The zinc / nickel alloy undercoat interlayer has a percentage of nickel such that the reduction potential of said alloy is close to the hydrogen evolution potential on the zinc / nickel alloy previously deposited in the undercoat layer . The best result is also obtained in test n 9, in which a layer of undercoating of 0.04 μm of zinc / nickel alloy with 24% of nickel and an intermediate layer of nickel was first deposited. coating of 0.04pm of 18% nickel alloy before

  deposit the final coating layer of zinc / nickel alloy

kel at 12% nickel.

  Such successive deposits are made of

  known manner for example on a line of electrodes

  CAROSEL-type position, in which the diffusion current in the bath, a first conductive roll to ensure a nickel / zinc alloy deposit 24% nickel and in the bath, a second conductive roll for ensure an 18% nickel alloy deposit

of nickel.

  The speed of travel of the steel strip forming the substrate is calculated to obtain the thickness

  desired coating, ie 0.04 μm.

  In order to be able to regulate differently the current density at the level of each roll, it suffices, for example, to equip them at least the first two, with anode bridges in order to be able to clamp these bridges to obtain very low current densities and therefore deposits. zinc / nickel alloy with high nickel percentage. During the passage of the steel substrate on the first roll of the electrodeposition line, it is polarized to the reduction potential of the alloy

  zinc / nickel at 24% nickel, ie -0.80V / Ecs.

  The release of hydrogen begins as soon as the

  steel substrate reaches -0.78V / Ecs and the deposit of

  zinc / nickel bonding begins when the substrate reaches -

  0,80V / Ecs, almost immediately after the

  release of hydrogen which is very limited.

  Then, when the steel substrate coated with the zinc / nickel alloy undercoating layer with

  24% nickel passes to the second conductive roll

  its polarity is brought to the reduction potential of zinc / nickel to 18% nickel, or -0.90V / Ecs. Hydrogen evolution becomes active again when the steel substrate reaches the hydrogen evolution potential on the 24% nickel / nickel alloy, and the 18% zinc / nickel nickel deposit is deposited when the potential of said substrate reaches -0.90V / Ecs. At the level of the following conductive rolls, the steel substrate is polarized at -0.95V / Ecs, the reduction potential of the zinc / nickel alloy at 12% nickel

  allowing its deposit on the steel substrate.

  Thus, in order to limit the release of hy-

  and to ensure a good adhesion of the coating, the method according to the invention consists in depositing, between

  the surface of the steel substrate and the coating layer

  zinc / nickel alloy layer, a zinc / nickel alloy undercoating layer with a percentage of nickel such that the reduction potential of said alloy of said undercoat layer relative to a saturated calomel electrode is greater than or equal to or substantially less than the hydrogen evolution potential on the steel of the substrate, and then depositing, between the undercoat layer and the zinc / nickel alloy coating layer, at least one undercoating intermediate layer with a percentage of nickel such that the reduction potential of the zinc / nickel alloy of the intermediate layer n undercoating is greater

  or equal to or substantially less than the release potential

  hydrogen on said alloy of the intermediate layer.

  n-1 undercoat previously deposited,

  until depositing a layer of zinc / nickel alloy.

  In a satisfactory manner, it is also possible to directly deposit the intermediate layer of undercoat of zinc / nickel alloy with the percentage of

  desired nickel on the undercoat layer previously

deposited.

  The reduction potential of zinc alloy

  nickel of the undercoat layer relative to a saturated calomel electrode may be between the hydrogen evolution potential on the substrate steel and said hydrogen evolution potential

  less 15%, or less 10%, or less 5% or even less 2%.

  The reduction potential of the zinc / nickel alloy of the undercoating intermediate layer may be between the hydrogen evolution potential on the previously deposited undercoating layer and the hydrogen evolution potential.

  less 15%, or less 10%, or less 5% or even less 2%.

  The undercoat layer has a thickness

  greater than 0.01pm and preferably greater than 0.02pm.

  The intermediate layer or layers

  coating have a thickness greater than 0.01pm.

  The process according to the invention is not limited to the deposition of a zinc / nickel alloy, but can very well be applied to the deposition of other zinc-based alloys,

  zinc / iron, zinc / chromium or zinc / cobalt.

Claims (38)

  1. Electroplating process on a   surface of a steel substrate a layer of a coating   ZnX is the second element of this alloy, characterized in that between the surface of the substrate and the coating layer of said alloy is deposited a   undercoating layer of said alloy with a   centage of the second element X such that the potential of   reducing said alloy of said undercoat layer   relative to a saturated calomel electrode is greater than or equal to or substantially less than the hydrogen evolution potential on the substrate steel, for   obtain the percentage of the second element X desired.   2. Method according to claim 1, characterized in that is deposited between the undercoating layer and the coating layer of said alloy, at least one intermediate layer of the undercoat with a percentage of the second element X such that the potential for reducing said alloy of the undercoating intermediate layer n is greater than or equal to or substantially less than the hydrogen evolution potential on said alloy of the intermediate layer   n-1 under previously deposited coating.   3. The process according to claim 1, wherein   characterized in that between the undercoating layer and the coating layer of said alloy is deposited a   intermediate layer of undercoating with a percentage   second element stage X such that the reduction potential of said alloy of the undercoat interlayer is greater than or equal to or substantially less than the hydrogen evolution potential of   said alloy of the underlayment layer previously deposited.   4. The process according to claim 1, wherein   characterized in that the reduction potential of the alloy of the undercoat layer with respect to a   saturated calomel electrode is between the   hydrogen evolution on the substrate steel and said hydrogen evolution potential minus 15%.   5. The method of claim 1, wherein   characterized in that the reduction potential of the alloy of the undercoat layer with respect to a   saturated calomel electrode is between the   release of hydrogen on the substrate steel   and said hydrogen evolution potential minus 10%.   6. The method of claim 1, wherein   characterized in that the reduction potential of the alloy of the undercoat layer with respect to a   saturated calomel electrode is between the   release of hydrogen on the substrate steel   and said hydrogen evolution potential minus 5%.   7. The method of claim 1, wherein   characterized in that the reduction potential of the alloy of the undercoat layer with respect to a   saturated calomel electrode is between the   release of hydrogen on the substrate steel   and said hydrogen evolution potential minus 2%.   8. Method according to claim 2 or 3, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited underlayment layer and   said hydrogen evolution potential minus 15%.   9. Method according to claim 2 or 3, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited underlayment layer and   said hydrogen evolution potential minus 10%.   10. Method according to claim 2 or 3, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited underlayment layer and   said hydrogen evolution potential minus 5%.   11. The method of claim 2 or 3, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited undercoat layer and   said hydrogen evolution potential minus 2%.   12. Method according to claim 1, characterized in that the undercoating layer has a thickness greater than O, Olpm.   13. The method of claim 12, characterized in that the undercoat layer has a thickness greater than 0, 02pm.   14. Process according to claim 2 or 3, characterized in that the intermediate layer or layers   undercoating have a thickness greater than 0.01 μm.   15. Method according to one of the claims   preceding, characterized in that the second element X   the zinc-based alloy is nickel.   16. Method according to one of claims 1   14, characterized in that the second element X of the Zinc binding is iron.   17. Method according to one of claims 1   14, characterized in that the second element X of the   Zinc binding is cobalt.   18. Method according to one of claims 1   at 14, characterized in that the second element X of   the zinc-based alloy is chromium.   19. A coated steel material comprising a steel substrate and a zinc-based metal alloy coating layer of the ZnX type, where X is the second element of said alloy, said coating layer being deposited by electrodeposition on a surface of said substrate, characterized in that it comprises between the surface of the substrate and the coating layer, a   undercoating layer of said alloy with a percentage   the second element X such that the potential of said alloy of said undercoat layer relative to a saturated calomel electrode is greater than or equal to or substantially less than the release potential   of hydrogen on the substrate steel.   20. Material according to claim 19, characterized in that it comprises between the undercoat layer and the coating layer, at least one   intermediate layer of undercoating with a percentage   the second element X such that the reduction potential   said alloy of the intermediate layer n of sub   coating is greater than or equal to or substantially less than   the potential for hydrogen evolution on said alloy of the intermediate layer n-1 undercoating previously filed.   21. Material according to claim 19, characterized in that it comprises between the undercoat layer and the coating layer of said alloy, a   intermediate layer of undercoating with a percentage   the second element X such that the reduction potential   said alloy of the intermediate layer of sub   coating is greater than or equal to or substantially less than   the potential for hydrogen evolution on said alloy of the previously deposited underlayment layer. 22. Material according to claim 19, characterized in that the reduction potential of the alloy of the undercoating layer relative to a saturated calomel electrode is between the hydrogen evolution potential on the substrate steel. and said hydrogen evolution potential minus 15%.   23. Material according to claim 19, characterized in that the reduction potential of the alloy of the undercoat layer relative to a saturated calomel electrode is between the hydrogen evolution potential on the substrate steel. and said hydrogen evolution potential less 10%.   24. Material according to claim 19, characterized in that the reduction potential of the alloy of the undercoat layer relative to a saturated calomel electrode is between the hydrogen evolution potential on the steel of the   substrate and said hydrogen potential minus 5%.   25. Material according to claim 19, characterized in that the reduction potential of said alloy of the undercoat layer relative to a   saturated calomel electrode is between the   release of hydrogen on the substrate steel   and said hydrogen evolution potential minus 2% -.   26. Material according to claim 20 or 21, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited underlayment layer and   said hydrogen evolution potential minus 15%.   27. Material according to claim 20 or 21, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited underlayment layer and   said hydrogen evolution potential minus 10%.   28. Material according to claim 20 or 21, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited undercoating layer and said potential of hydrogen evolution minus 5%. 29. Material according to claim 20 or 21, characterized in that the reduction potential of the alloy of the undercoat intermediate layer is between the hydrogen evolution potential on the previously deposited underlayment layer and   said hydrogen evolution potential minus 2%.   30. Material according to claim 19, characterized in that the undercoating layer has a thickness greater than 0.01pm.   31. Material according to claim 30, characterized in that the undercoating layer has a thickness greater than 0.02pm.   32. Material according to claim 20 or 21, characterized in that the intermediate layer or layers   undercoated have a thickness greater than 0, Olpm.   33. Material according to one of the claims   19 to 32, characterized in that the second element X of   the zinc-based alloy is nickel.   34. Material according to one of the claims   19 to 32, characterized in that the second element X of   the zinc-based alloy is iron.   35. Material according to one of the claims   19 to 32, characterized in that the second element X of   the zinc-based alloy is cobalt.   36. Material according to one of the claims   19 to 32, characterized in that the second element X of   the alloy based on zinc and chromium.   37. Coated steel material comprising a steel substrate and a coating layer of a zinc / nickel alloy with 12% nickel, said coating layer being electroplated on a surface of said substrate, characterized in that comprises, between the surface of the substrate and the coating layer, a first zinc-nickel alloy undercoat layer of 24% nickel and, on said undercoating layer, an undercoat interlayer   zinc / nickel alloy with 18% nickel.   38. Material according to claim 37, characterized in that the undercoating layers have a thickness equal to 0.04pm.