METHOD FOR DEPOSITING A CHROMIUM-COMPRISING PASSIVATION LAYER ON A ZINC-COMPRISING COATING

The present invention refers to a method for depositing a chromium-comprising passivation layer on a zinc-comprising coating, wherein the zinc-comprising coating additionally comprises Fe, Sn, Mn, or mixtures thereof. The method utilizes a passivation composition comprising 0.001 mg/L to 200 mg/L, based on the total volume of the passivation composition, of at least one corrosion-inhibiting agent selected from the group consisting of unsubstituted azole compounds, substituted azole compounds, unsubstituted aliphatic organic acids with at least one mercapto-group, substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof.

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Description
FIELD OF THE INVENTION

The present invention refers to a method for depositing a chromium-comprising passivation layer on a zinc-comprising coating, wherein the zinc-comprising coating additionally comprises Fe, Sn, Mn, or mixtures thereof. The method utilizes a passivation composition comprising 0.001 mg/L to 200 mg/L, based on the total volume of the passivation composition, of at least one corrosion-inhibiting agent selected from the group consisting of unsubstituted azole compounds, substituted azole compounds, unsubstituted aliphatic organic acids with at least one mercapto-group, substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof.

BACKGROUND OF THE INVENTION

Different methods are known in the prior art to protect metal substrates from corrosive environmental influences. For example, a protective coating of a metal or metal alloy is applied on the metal substrate, which is a widely used and well-established method.

Among such protective coatings, the deposition of a zinc coating or zinc-comprising coating additionally comprising nickel (i.e. a zinc and zinc-nickel galvanization layer, respectively) on metal substrates (in particular iron substrates) appears to be the most prominent approach.

In zinc-comprising coatings additionally comprising nickel, the chemical element nickel is mandatory. However, nickel and nickel ions as utilized in respective galvanization compositions are hazardous to the environment and health. Thus, there is an ongoing demand to provide alternative protective coatings being free from nickel. A promising alternative protective coating is a zinc-comprising coating further comprising for example iron.

Typically, such coatings are additionally protected (post-treated) by so called conversion coatings (often also referred to as passivation layers). Such conversion coatings typically comprise insoluble compounds as a result from reacting the protective coating with a conversion treatment solution (i.e. passivation composition).

In many cases the passivation composition comprises trivalent chromium ions in an acidic solution. During the passivation process, the protective coating is slightly dissolved such that metal ions are released, for example zinc ions. In turn, these metal ions react with compounds in the passivation composition. For example, if a zinc coating or zinc-comprising coating with nickel is contacted with such a composition, some of the zinc and/or nickel will dissolve and form respective ions thereof. Without applying any current, a chromium (III) hydroxide passivation layer or a p-oxo or p-hydroxo-bridged chromium (III) passivation layer is deposited on the surface of the protective coating. As a result, a respective passivation layer is obtained on the protective coating.

As a matter of fact, also alternative protective coatings such as zinc-comprising coatings with iron are typically post-treated by conversion coatings (i.e. with a passivation composition) to further increase corrosion resistance thereof.

However, conversion coatings typically applied to zinc coatings or zinc-comprising coatings additionally comprising nickel are not automatically applicable to such alternative protective coatings due to the different chemical composition of the protective coating and its own corrosive characteristics.

In addition, conversion coatings for alternative protective coatings are also known.

US 2006/237098 A1 refers to compositions and to a process for using said compositions for preparing protective coatings on various metal substrates. US'098 discloses a post-treatment for tin-zinc.

CN 108914106 A relates to the field of metal surface treatment liquids, in particular to a galvanized sheet surface passivation self-filling treatment liquid which is non-toxic and can realize self-filling long-term protection.

EP 2 189 551 A1 relates to a trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. It discloses zinc alloy coatings including zinc-iron and tin-zinc.

JP 2007 239002 A relates to a trivalent chromate liquid for treating of a zinc-galvanized substrate, the liquid comprising a corrosion inhibitor in an amount of 0.001% to 10%.

EP 3 045 564 A1 relates a treatment liquid for a black trivalent chromium conversion coating.

Irrespective of the used protective coating, it is typically used to protect iron and/or steel substrates, which otherwise would undergo dramatic corrosion.

However, sometimes the (alternative) protective coating is slightly damaged such that the metal substrate is at least partly exposed and not any longer completely covered by it. As a result, also the metal substrate dissolves in these areas while in contact with a passivation composition and the iron ion concentration increases over time. It turned out that a comparatively high iron ion concentration in a passivation composition often leads to a negative coloring of the passivated substrate or can even impair the corrosion resistance. Moreover, passivation compositions with a comparatively high iron ion concentration must be replaced more often, which leads to higher costs. Therefore, there is a constant demand to improve existing passivation compositions, in particular if applicable to alternative protective coatings, to increase the lifetime of such alternative passivation compositions without compromising corrosion resistance.

OBJECTIVE OF THE PRESENT INVENTION

It was therefore the objective of the present invention to provide a method for depositing a chromium-comprising passivation layer on such alternative protective coatings, in particular comprising zinc and additionally Fe, Sn, Mn, or mixtures thereof, which show on the one hand an excellent corrosion protection and on the other hand an increased lifetime for the passivation composition and therefore a more sustainable passivation method, even in the presence of contaminating metal ions such as iron ions.

SUMMARY OF THE INVENTION

The objectives mentioned above are solved by a method for depositing a chromium-comprising passivation layer on a zinc-comprising coating, the method comprising the following steps:

    • (a) providing a substrate comprising the zinc-comprising coating,
    • (b) providing a passivation composition for depositing the chromium-comprising passivation layer, the composition comprising
      • (i) trivalent chromium ions;
      • (ii) at least one complexing agent for the trivalent chromium ions, being different from the at least one corrosion-inhibiting agent; and
      • (iii) 0.001 mg/L to 200 mg/L, based on the total volume of the passivation composition, of at least one corrosion-inhibiting agent selected from the group consisting of unsubstituted azole compounds, substituted azole compounds, unsubstituted aliphatic organic acids with at least one mercapto-group, substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof; and
    • (c) contacting said substrate with said passivation composition such that the chromium-comprising passivation layer is deposited on the zinc-comprising coating,
    • wherein the zinc-comprising coating additionally comprises Fe, Sn, Mn, or mixtures thereof.

By utilizing the at least one corrosion-inhibiting agent as defined throughout the present text in the passivation composition an excellent corrosion protection of said zinc-comprising coating is obtained.

Moreover, by utilizing said at least one corrosion-inhibiting agent the release of iron ions from the substrate into the passivation composition is significantly suppressed or even fully prevented. As a result, the passivation composition utilized in the method of the present invention, has a significantly longer lifetime compared to a passivation composition not comprising said at least one corrosion-inhibiting agent but otherwise being identical.

The at least one complexing agent for the trivalent chromium ions is different from the at least one corrosion-inhibiting agent. Thus (ii) and (iii) are not the same compounds but rather different compounds, which are distinct from each other.

Preferred is a method of the present invention, wherein the zinc-comprising coating is a galvanized layer or galvanic layer, respectively. This means that preferably the zinc-comprising coating is electrolytically deposited onto the substrate from a respective coating composition.

Preferred is a method of the present invention, wherein the zinc-comprising coating is preferably a zinc-alloy coating and thus the substrate is preferably a zinc-alloy coated substrate. In the context of the present invention, the zinc-comprising coating is not a purely zinc-comprising coating or an only zinc-comprising coating. It always comprises at least one of said additional metals. Furthermore, said additional metals are willfully added/included in the zinc-comprising coating, i.e. intentionally incorporated.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, the term “at least one” or “one or more” denotes (and is exchangeable with) “one, two, three or more than three”. Furthermore, “trivalent chromium” refers to chromium with the oxidation number +3. The term “trivalent chromium ions” refers to Cr3+-ions in a free or complexed form.

In the context of the present invention, the term “chromium-comprising passivation layer” describes a layer, which comprises preferably trivalent chromium compounds. The chromium-comprising passivation layer preferably comprises trivalent chromium hydroxide. In some cases it is preferred that the passivation layer comprises additional metals, preferably cobalt (i.e. most preferably cobalt compounds). The trivalent chromium compounds are preferably insoluble in water.

The Substrate Comprising the Zinc-Comprising Coating

In step (a) of the method of the present invention, a substrate comprising the zinc-comprising coating is provided, preferably a zinc-comprising coating as defined throughout the present text, more preferably defined as being preferred.

Preferred is a method of the present invention, wherein in step (a) the substrate comprises a metal, preferably is a metal substrate. Most preferably, the substrate is not a plastic substrate, preferably is not an organic substrate.

More preferred is a method of the present invention, wherein the substrate comprises iron (preferably is an iron substrate), preferably as a base-material on which the zinc-comprising coating is deposited. Therefore, preferably iron ions are released from the substrate and base material, respectively, which in particular occurs if the zinc-comprising coating is damaged.

Preferred is a method of the present invention, wherein in step (a) the substrate comprises at least a fastener, preferably screws, nails, nuts, clamps and/or springs. As mentioned above, they preferably comprise or consist of a metal.

In the method of the present invention, the zinc-comprising coating additionally comprises Fe, Sn, Mn, or mixtures thereof. This means, that at least one of Fe, Sn, Mn is present together with (i.e. in addition to) zinc. In some cases it is preferred that even two (or more) of them are present together with zinc. Thus, Fe, Sn, and Mn are alloying elements forming the galvanized layer, respectively the galvanic layer.

Preferred is a method of the present invention, wherein in the zinc-comprising coating zinc together with Fe, Sn, and Mn represent 95 wt.-% or more of all metals (preferably of all elements) in the zinc-comprising coating, based on the total weight of the zinc-comprising coating, preferably 96 wt.-% or more, more preferably 97 wt.-% or more, even more preferably 98 wt.-% or more, yet even more preferably 99 wt.-% or more, most preferably 99.5 wt.-% or more, yet even most preferably 99.9 wt.-% or more.

Thus, besides zinc, Fe, Sn, and Mn no significant further metals are preferably involved in forming the zinc-comprising coating.

Preferred is a method of the present invention, wherein in the zinc-comprising coating zinc is present in a total amount ranging from 1 wt.-% to 99.5 wt.-%, based on the total weight of the zinc-comprising coating, preferably ranging from 2 wt.-% to 99 wt.-%, more preferably from 3 wt.-% to 95 wt.-%, even more preferably from 4 wt.-% to 93 wt.-%, yet even more preferably from 5 wt.-% to 91 wt.-%, most preferably from 9 wt.-% to 85 wt.-%.

Preferred is a method of the present invention, wherein the zinc-comprising coating comprises Fe and Fe is preferably present in a total amount ranging from 0.1 wt.-% to 35 wt.-%, based on the total weight of the zinc-comprising coating, preferably from 0.3 wt.-% to 30 wt.-%, more preferably from 0.5 wt.-% to 28 wt.-%, even more preferably from 0.9 wt.-% to 26 wt.-%, yet even more preferably from 1.3 wt.-% to 25 wt.-%, most preferably from 2 wt.-% to 24 wt.-%.

More preferred is a method of the present invention, wherein the zinc-comprising coating comprises Fe and Fe is preferably present in a total amount ranging from 4 wt.-% to 35 wt.-%, based on the total weight of the zinc-comprising coating, preferably from 5 wt.-% to 30 wt.-%, more preferably from 6 wt.-% to 28 wt.-%, even more preferably from 7 wt.-% to 26 wt.-%, yet even more preferably from 8 wt.-% to 25 wt.-%, most preferably from 10 wt.-% to 24 wt.-%.

Very preferred is a method of the present invention, wherein the zinc-comprising coating comprises Fe and Fe is preferably present in a total amount ranging from 8 wt.-% to 23 wt.-%, based on the total weight of the zinc-comprising coating, preferably from 9 wt.-% to 22 wt.-%, more preferably from 10 wt.-% to 21 wt.-%, even more preferably from 11 wt.-% to 20 wt.-%, yet even more preferably from 12 wt.-% to 19 wt.-%, most preferably from 13 wt.-% to 18 wt.-%. This most preferably applies if the zinc-comprising coating is substantially free of, preferably does not comprise, Sn and/or (preferably and) Mn.

In other cases, preferred is a method of the present invention, wherein the zinc-comprising coating comprises Fe and Fe is preferably present in a total amount ranging from 0.1 wt.-% to 10 wt.-%, based on the total weight of the zinc-comprising coating, preferably from 0.2 wt.-% to 9 wt.-%, more preferably from 0.3 wt.-% to 8 wt.-%, even more preferably from 0.5 wt.-% to 7 wt.-%, yet even more preferably from 0.7 wt.-% to 6 wt.-%, most preferably from 0.9 wt.-% to 5 wt.-%.

Preferred is a method of the present invention, wherein the zinc-comprising coating comprises Sn and Sn is preferably present in a total amount ranging from 40 wt.-% to 95 wt.-%, based on the total weight of the zinc-comprising coating, preferably from 50 wt.-% to 92 wt.-%, more preferably from 57 wt.-% to 90 wt.-%, even more preferably from 58 wt.-% to 88 wt.-%, yet even more preferably from 60 wt.-% to 86 wt.-%, most preferably from 62 wt.-% to 85 wt.-%. This most preferably applies if the zinc-comprising coating is substantially free of, preferably does not comprise, Fe and/or (preferably and) Mn.

Preferred is a method of the present invention, wherein the zinc-comprising coating comprises Mn and Mn is preferably present in a total amount ranging from 1 wt.-% to 60 wt.-%, based on the total weight of the zinc-comprising coating, preferably from 2 wt.-% to 50 wt.-%, more preferably from 5 wt.-% to 48 wt.-%, even more preferably from 10 wt.-% to 47 wt.-%, yet even more preferably from 15 wt.-% to 45 wt.-%, most preferably from 20 wt.-% to 41 wt.-%. This most preferably applies if the zinc-comprising coating is substantially free of, preferably does not comprise, Fe and/or (preferably and) Sn.

Preferred is a method of the present invention, wherein the zinc-comprising coating is substantially free of, preferably does not comprise, nickel. This in particular denotes that the zinc-comprising coating does not comprise intentionally added nickel. In contrast, unavoidable nickel, for example as an impurity and/or contamination is acceptable, preferably as long as it does not show a significant effect on the zinc-comprising coating.

Preferred is a method of the present invention, wherein the zinc-comprising coating comprises Fe, Sn, or mixtures thereof, preferably Fe. This means that in such preferred cases, the zinc-comprising coating does not comprise manganese (but Fe and/or Sn), preferably does not comprise manganese and tin (but preferably additionally only Fe), respectively. Thus, the method of the present invention is most preferably for a zinc-iron coating (i.e. a zinc-comprising coating additionally comprising Fe). In other cases, however, the method of the present invention is very preferably for a zinc-tin coating (often also equally referred to as tin-zinc coating; i.e. a zinc-comprising coating additionally comprising tin).

The Passivation Composition

In step (b) of the method of the present invention, a passivation composition is provided as defined throughout the present text, preferably as defined as being preferred.

Preferred is a method of the present invention, wherein the passivation composition is an aqueous passivation composition, wherein preferably the concentration of water is more than 50 vol.-%, based on the total volume of the passivation composition, more preferably 65 vol.-% or more, even more preferably 80 vol.-% or more, most preferably 90 vol.-% or more.

In the method of the present invention, the passivation composition comprises (i) trivalent chromium ions.

Preferred is a method of the present invention, wherein the passivation composition comprises trivalent chromium ions in a total concentration from 0.1 g/L to 25 g/L, based on the total volume of the passivation composition, preferably from 0.2 g/L to 20 g/L, more preferably from 0.3 g/L to 15 g/L, even more preferably from 0.4 g/L to 10 g/L, most preferably from 0.5 g/L to 9 g/L. If the total concentration is significantly below 0.1 g/L typically an insufficient passivation is obtained. On the other hand, if the total concentration is significantly exceeding 25 g/L, the entire method is not sufficiently ecological.

In some cases, very preferred is a method of the present invention, wherein the passivation composition comprises trivalent chromium ions in a total concentration from 0.5 g/L to 3 g/L, based on the total volume of the passivation composition, preferably from 1 g/L to 2.5 g/L. With such concentrations, very excellent results were obtained.

In the method of the present invention, the passivation composition comprises (ii) at least one complexing agent for the trivalent chromium ions.

Preferred is a method of the present invention, wherein the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of organic complexing agents and inorganic complexing agents. The proviso still applies that the organic complexing agent is different from the at least one corrosion-inhibiting agent as defined throughout the present text.

Preferably, said at least one complexing agent is not only for the trivalent chromium ions but is additionally also a complexing agent for iron ions, most preferably for released iron ions. If they are also complexed, an undesired sludge formation is strongly reduced or even prevented.

Preferred is a method of the present invention, wherein in the passivation composition the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of monocarboxylic acids, dicarboxylic acids, salts thereof (of both monocarboxylic acids and dicarboxylic acids), halogen ions, and mixtures thereof.

Most preferably, the at least one complexing agent comprises at least one dicarboxylic acid and/or salts thereof.

Preferred is a method of the present invention, wherein in the passivation composition the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of unsubstituted monocarboxylic acids, hydroxyl-substituted monocarboxylic acids, amino-substituted monocarboxylic acids, unsubstituted dicarboxylic acids, hydroxyl-substituted dicarboxylic acids, amino-substituted dicarboxylic acids, salts thereof (of all aforementioned acids), halogen ions, and mixtures thereof.

Preferred is a method of the present invention, wherein the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of oxalate/oxalic acid, acetate/acetic acid, tartrate/tartaric acid, malate/malic acid, succinate/succinic acid, gluconate/gluconic acid, glutamate/glutamic acid, glycolate/glycolic acid, diglycolate/diglycolic acid, ascorbate/ascorbic acid, and butyrate/butyric acid.

Preferred is a method of the present invention, wherein the halogen ions comprise fluoride ions, preferably are fluoride ions, most preferably are only fluoride ions out of halogen ions.

Preferred is a method of the present invention, wherein the at least one complexing agent for the trivalent chromium ions does not comprise a mercapto-group.

Preferred is a method of the present invention, wherein the at least one complexing agent for the trivalent chromium ions does not comprise a triazole, preferably not an azole, more preferably not an aromatic organic compound.

In some cases preferred is a method of the present invention, wherein the at least one complexing agent for the trivalent chromium ions has a total concentration in a range from 0.01 mol/L to 2 mol/L, based on one mol/L trivalent chromium ions in the passivation composition, preferably from 0.03 mol/L to 1 mol/L, more preferably from 0.05 mol/L to 0.8 mol/L, even more preferably from 0.1 mol/L to 0.5 mol/L.

Also preferred is a method of the present invention, wherein in the passivation composition the at least one complexing agent for the trivalent chromium ions has a total concentration in a range from 0.05 wt.-% to 15 wt.-%, based on the total weight of the passivation composition, preferably from 0.1 wt.-% to 10 wt.-%, more preferably from 0.2 wt.-% to 9 wt.-%, even more preferably from 0.5 wt.-% to 8 wt.-%, most preferably from 0.8 wt.-% to 7 wt.-%.

Typically, in the above defined (preferred) concentration ranges the trivalent chromium ions are efficiently stabilized in the passivation composition by the complexing agents (preferably complexing agents as defined as being preferred).

In the method of the present invention, the passivation composition comprises (iii) 0.001 mg/L to 200 mg/L, based on the total volume of the passivation composition, of at least one corrosion-inhibiting agent as defined throughout the present text, preferably as defined as being preferred. This concentration range refers to a total concentration and is to be understood as (and preferably exchangeable with) “(iii) 0.001 mg/L to 200 mg/L in total, based on [. . . ]”. This preferably also applies to all preferred concentration ranges defined throughout the present text as being preferred.

If (iii) is present significantly above 200 mg/L in most cases an insufficient corrosion resistance is obtained, which means that in for example a NSS test undesired results are obtained (although an excellent inhibiting effect might be achieved; see examples below). In order to achieve an inhibiting effect at all, a comparatively low concentration is already sufficient (see examples below), at least 0.001 mg/L, preferably at least 0.01 mg/L, more preferably at least 0.1 mg/L, which are preferably individually combined with said 200 mg/L or other upper limits.

Preferred is a method of the present invention (preferably as described as being preferred throughout the present text), wherein in the passivation composition (iii) has a total concentration ranging from 0.005 mg/L to 180 mg/L, based on the total volume of the passivation composition, preferably ranging from 0.01 mg/L to 160 mg/L, more preferably ranging from 0.1 mg/L to 150 mg/L, even more preferably ranging from 1 mg/L to 135 mg/L, yet even more preferably ranging from 2 mg/L to 120 mg/L, most preferably ranging from 3 mg/L to 110 mg/L.

Preferred is a method of the present invention (preferably as described as being preferred throughout the present text), wherein in the passivation composition (iii) has a total concentration ranging from 0.01 mg/L to 150 mg/L, based on the total volume of the passivation composition, preferably ranging from 0.05 mg/L to 120 mg/L, more preferably ranging from 0.1 mg/L to 100 mg/L, even more preferably ranging from 0.5 mg/L to 80 mg/L, yet even more preferably ranging from 1 mg/L to 50 mg/L, most preferably ranging from 2 mg/L to 25 mg/L. In some cases it is very preferred that these preferred concentration ranges apply in particular to corrosion-inhibiting agents selected from the group consisting of unsubstituted aliphatic organic acids with at least one mercapto-group, substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof; in particular as further defined as being preferred throughout the present text.

Preferred is a method of the present invention (preferably as described as being preferred throughout the present text), wherein in the passivation composition (iii) has a total concentration ranging from 0.001 mg/L to 9.9999 mg/L, based on the total volume of the passivation composition, preferably ranging from 0.01 mg/L to 9.9 mg/L, more preferably ranging from 0.1 mg/L to 9.8 mg/L, even more preferably ranging from 0.5 mg/L to 9.7 mg/L, yet even more preferably ranging from 0.8 mg/L to 9.6 mg/L, most preferably ranging from 1 mg/L to 9.5 mg/L, even most preferably ranging from 2 mg/L to 9.4 mg/L. In some cases it is very preferred that these preferred concentration ranges apply in particular to corrosion-inhibiting agents selected from the group consisting of unsubstituted azole compounds, substituted azole compounds, salts, and mixtures thereof; in particular as further defined as being preferred throughout the present text.

Most preferred is a method of the present invention, wherein in the passivation composition (iii) has a total concentration ranging from 0.001 mg/L to 9 mg/L, based on the total volume of the passivation composition, preferably ranging from 0.01 mg/L to 8.8 mg/L, more preferably ranging from 0.1 mg/L to 8.5 mg/L, even more preferably ranging from 0.5 mg/L to 8.3 mg/L, yet even more preferably ranging from 0.8 mg/L to 8 mg/L, most preferably ranging from 1 mg/L to 7.5 mg/L, and even most preferably ranging from 2 mg/L to 7 mg/L. Own experiments have shown that typically such low concentrations result in excellent results and are already sufficient (see examples below) although higher concentrations can be used too. It was surprising that a very significant effect was already obtained with a comparatively low total concentration.

In the passivation composition the at least one corrosion-inhibiting agent is selected from the group consisting of unsubstituted azole compounds, substituted azole compounds (which are preferred azole compounds), unsubstituted aliphatic organic acids with at least one mercapto-group (which are preferred aliphatic organic acids), substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof.

Preferred is a method of the present invention, wherein in the passivation composition the substituted azole compounds, salts, and mixtures thereof independently comprise one or more than one substituent selected from the group consisting of amino, nitro, carboxy, hydroxy, sulfonate, and thiol, wherein preferably the substituent is a thiol group.

Preferred is a method of the present invention, wherein in the passivation composition the unsubstituted, and substituted azole compounds, salts, and mixtures thereof are independently selected from the group consisting of monoazoles, diazoles, triazoles, and tetrazoles, preferably diazoles and triazoles, most preferably triazoles.

Preferred is a method of the present invention, wherein in the passivation composition the unsubstituted and substituted (preferably the substituted) azole compounds, salts, and mixtures thereof are independently selected from the group consisting of 1,2,4-triazoles. This most preferably means 1,2,4-H-triazoles.

Preferred is a method of the present invention, wherein in the passivation composition the substituted azole compounds, salts, and mixtures thereof independently comprise at least a mercaptotriazole, preferably at least 3-mercapto-1,2,4-triazole (most preferably denoting 3-mercapto-1,2,4-H-triazole).

In the method of the present invention, the at least one corrosion-inhibiting agent can also be a substituted aliphatic organic acid with at least one mercapto-group, salts, and mixtures thereof. In this case, the organic acid is substituted by means of one or more than one substituent. Preferably, the acid is a carboxylic acid, a sulfonic acid, salts, or mixtures thereof. The respective acid moiety, i.e. preferably the carboxy group(s) and sulfonic group(s) is (are) not the substituent because it characterizes the compound as acid. In contrast, preferred is a method of the present invention, wherein in the passivation composition the aliphatic organic acid with at least one mercapto-group, salts, and mixtures thereof independently comprise one or more than one substituent (i.e. in addition to the acid moieties) selected from the group consisting of amino, nitro, and hydroxy.

Preferred is a method of the present invention, wherein in the passivation composition the unsubstituted and substituted aliphatic organic acids with at least one mercapto-group and salts thereof, respectively, comprise a carboxylic acid and/or salts thereof.

More preferred is a method of the present invention, wherein in the passivation composition the unsubstituted and substituted aliphatic organic acids with at least one mercapto-group and salts thereof, respectively, comprise a monocarboxylic acid and/or salts thereof.

Preferred is a method of the present invention, wherein in the passivation composition the unsubstituted and substituted aliphatic organic acids with at least one mercapto-group and salts thereof, respectively, comprise 1 to 12 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 3 to 8 carbon atoms, most preferably 3 to 6 carbon atoms.

In some cases, very preferred is a method of the present invention, wherein (iii) comprises at least one unsubstituted aliphatic organic acid with at least one mercapto-group and/or salts thereof.

Preferred is a method of the present invention, wherein in the passivation composition the unsubstituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof comprise at least 3-mercaptopropionic acid and/or salts thereof, most preferably is 3-mercaptopropionic acid.

Generally preferred is a method of the present invention, wherein (iii) comprises at least one of said azole compounds (including compounds defined as being preferred) or at least one of said aliphatic organic acids (including compounds defined as being preferred). However, in some cases preferred is a method of the present invention, wherein there is a mixture of at least one of said azole compounds with at least one of said aliphatic organic acids is present in the passivation composition.

In some cases, preferred is a method of the present invention, wherein the passivation composition further comprises (iv) divalent cobalt ions, preferably in a total concentration from 0.01 g/L to 5 g/L, based on the total volume of the passivation composition, preferably from 0.1 g/L to 4 g/L, more preferably from 0.3 g/L to 3.5 g/L, even more preferably from 0.5 g/L to 3 g/L, most preferably from 0.7 g/L to 2.5 g/L.

In many cases, cobalt ions positively affect an optional heat-treating. In the context of the present invention, heat-treating as a post-treatment is an optional step (for heat-treating see text below).

However, in some cases a method of the present invention is preferred, wherein the passivation composition is essentially free of, preferably does not comprise, divalent cobalt ions; preferably is essentially free of, preferably does not comprise, any cobalt ions; most preferably is essentially free of, preferably does not comprise, cobalt at all. By alternatively excluding cobalt or cobalt ions, respectively, from the passivation composition, a cost-reduction is typically achieved, since the use of expensive cobalt compounds is avoided, and the wastewater treatment is simplified, thereby improving the sustainability of the passivation composition. Furthermore, in many cases cobalt and cobalt ions, respectively, do not result in any benefit and can therefore omitted.

Preferred is a method of the present invention, wherein the passivation composition is essentially free of, preferably does not comprise, intentionally added hexavalent chromium compounds. In other words, if hexavalent chromium compounds are present, they are a result of typically undesired reactions of the trivalent chromium ions. Thus, a method of the present invention is preferred, with the proviso, if hexavalent chromium ions are present, they originate from said trivalent chromium ions.

Preferred is a method of the present invention, wherein the passivation composition has a pH ranging from 0.5 to 6.5, preferably from 0.7 to 6, more preferably from 0.9 to 5, even more preferably from 1.1 to 4, yet even more preferably from 1.4 to 3, most preferably from 1.6 to 2.7, yet even most preferably from 1.8 to 2.3. If the pH is significantly exceeding 6.5, in some cases undesired precipitation is observed. If the pH is significantly below 0.5, in some cases an undesired and too strong dissolution of the substrate is observed; in particular if iron substrates are used. The preferred pH ranges as defined above are in particular beneficial for effectively depositing the chromium-comprising passivation layer and to maintain a comparatively long lifetime of the passivation composition.

Preferred is a method of the present invention, wherein the passivation composition further comprises

    • (v) iron ions in a total concentration ranging from 0 mg/L to 2000 mg/L, based on the total volume of the passivation composition, preferably ranging from 0 mg/L to 500 mg/L, more preferably ranging from 0 mg/L to 300 mg/L, most preferably ranging from 0 mg/L to 250 mg/L, even most preferably ranging from 0 mg/L to 200 mg/L.

Preferred is a method of the present invention, wherein the iron ions, if present, are from the substrate.

In some cases, iron ions are (almost) permanently present, preferably at a very low concentration, most preferably not even reaching the upper concentration limits as defined above. In view of the present invention, in many cases the upper concentration limits as defined above are not critical, in particular if the total concentration is not exceeding 500 mg/L. Typical very low concentrations of the iron ions are preferably 0.001 mg/L or more, based on the total volume of the passivation composition, more preferably 0.01 mg/L, even more preferably 0.1 mg/L, and most preferably 1 mg/L. Preferably, such low concentrations are freely combined with the upper concentration limits defined above. Without wishing to be bound by theory, it appears that the at least one corrosion-inhibiting agent primarily minimizes the release of iron ions but it cannot be ruled out completely that it also positively interacts with released iron ions such that a respective passivation composition has a prolonged lifetime.

Contacting the Substrate With the Passivation Composition

In step (c) of the method of the present invention, the substrate is contacted with the passivation composition such that the chromium-comprising passivation layer is deposited.

Most preferred is a method of the present invention, wherein step (c) is performed without applying an electrical current, i.e. in the absence of an electrical current.

Preferred is a method of the present invention, wherein step (c) is performed at a temperature in a range from 10° C. to 90° C., preferably from 13° C. to 70° C., more preferably from 15° C. to 60° C., even more preferably from 20° C. to 50° C., yet even more preferably from 23° C. to 45° C., most preferably from 26° C. to 40° C. Such temperatures allow an effective and/or sustainable operation of the method of the present invention.

If the temperature is significantly exceeding 90° C., in some cases an undesired evaporation of water is observed along with an undesired consumption of energy. If the temperature is significantly below 10° C., in many cases an insufficient depositing of the chromium-comprising passivation layer is obtained, thereby compromising the quality of the corrosion protection.

Preferred is a method of the present invention, wherein step (c) is performed for a time period from 5 seconds to 600 seconds, preferably from 10 seconds to 400 seconds, more preferably from 15 seconds to 300 seconds, even more preferably from 20 seconds to 200 seconds, most preferably from 25 seconds to 150 seconds. If the time period is significantly below 5 seconds, in many cases an insufficient depositing of the chromium-comprising passivation layer is obtained, thereby compromising the quality of the corrosion protection.

By performing step (c) in the preferred temperature ranges and the preferred time periods, particularly advantageous deposition kinetics are obtained. In many cases, exceeding the specified longest period of time and highest specified temperature, no further benefits are usually obtained.

Preferred is a method of the present invention, wherein after step (c) (preferably after a step (c)) the passivation composition comprises iron ions in a concentration of 200 mg/L or less, based on the total volume of the passivation composition, preferably 100 mg/L or less, most preferably 200 mg/L or less after each step (c), even most preferably 100 mg/L or less after each step (c).

More preferred is a method of the present invention, wherein after step (c) (preferably after a step (c)) the passivation composition comprises iron ions in a concentration of 500 mg/L or less, based on the total volume of the passivation composition, preferably 400 mg/L or less, more preferably 300 mg/L or less, most preferably 250 mg/L or less, even most preferably 200 mg/L or less, each with the proviso that the passivation composition comprises zinc ions with a concentration of 15 g/L or less.

Even more preferred is a method of the present invention, wherein after step (c) (preferably after a step (c)) the passivation composition comprises iron ions in a concentration of 500 mg/L or less, based on the total volume of the passivation composition, preferably 400 mg/L or less, more preferably 300 mg/L or less, most preferably 250 mg/L or less, even most preferably 200 mg/L or less, each with the proviso that the passivation composition comprises zinc ions with a concentration of 10 g/L or less.

Preferred is a method of the present invention, wherein the method comprises after step (c), additionally step

(d) heat-treating the substrate with the chromium-comprising passivation layer.

Typically, the heat-treating minimizes hydrogen embrittlement.

Preferred is a method of the present invention, wherein in step (d) the heat-treating is performed at a temperature in a range from 150° C. to 230° C., preferably from 180° C. to 210° C.

Preferred is a method of the present invention, wherein in step (d) the heat-treating is performed for a time period from 1 hour to 10 hours, preferably from 2 hours to 8 hours, most preferably from 2.5 hours to 5 hours.

Most preferred is a method of the present invention, wherein after step (c) or (d), preferably (c), the substrate with the chromium-comprising passivation layer has a white rust formation of 1% or below if tested according to DIN 9227. A white rust formation of 1% or below according to DIN 9227 serves as a particularly good criteria for proving the excellent corrosion protection obtained with the method of the present invention.

In some cases, preferred is a method of the present invention, wherein the method comprises after step (c) or (d), additionally step

(e) sealing the substrate with the chromium-comprising passivation layer with a sealing layer such that a sealed substrate is obtained.

Preferred is a method of the present invention, wherein the sealing layer comprises a compound (or a reaction product thereof) selected from the group consisting of inorganic silicates (preferably as particles), silanes, organic polymers, and mixtures thereof.

Regarding aforementioned inorganic silicates (preferably as particles), alternatively or in addition, such particles are preferably comprised in the passivation composition utilized in the method of the present invention in order to further increase corrosion resistance.

The Chromium-Comprising Passivation Layer

In step (c) of the method of the present invention, the chromium-comprising passivation layer is deposited.

Preferred is a method of the present invention, wherein the chromium-comprising passivation layer has a layer thickness in a range from 1 nm to 1200 nm, preferably from 10 nm to 1000 nm, more preferably from 15 nm to 800 nm, most preferably from 20 nm to 500 nm.

Preferred is a method of the present invention, wherein the chromium-comprising passivation layer is a bluish, preferably a blue, chromium-comprising passivation layer.

Even more preferred is a method of the present invention, wherein the chromium-comprising passivation layer is a bluish, preferably a blue, chromium-comprising passivation layer and has a layer thickness in a range from 30 nm to 150 nm, preferably from 40 nm to 140 nm, more preferably from 45 nm to 130 nm, most preferably from 50 nm to 120 nm, and even most preferably from 55 nm and 90 nm.

In a few cases a method of the present invention is preferred, wherein the chromium-comprising passivation layer is iridescent and has a layer thickness in a range from 155 nm to 1200 nm, preferably from 170 nm to 1000 nm, more preferably from 190 nm to 800 nm, most preferably from 200 nm to 600 nm.

In a few cases a method of the present invention is preferred, wherein the chromium-comprising passivation layer is transparent or yellow and has a layer thickness in a range from 1 nm to 25 nm, preferably from 3 nm to 22 nm, more preferably from 5 nm to 20 nm, most preferably from 8 nm to 18 nm.

The present invention is described in more detail by the following non-limiting examples.

EXAMPLES 1. First Set of Experiments (Inhibitor Test)

In a first set of experiments it was investigated whether 3-Mercaptotriazole (3-MTA) and 3-Mercaptopropionic acid (3-MPA) are suitable to prevent iron dissolution from iron substrates. For that, aqueous test passivation compositions were prepared, comprising about 2 g/L trivalent chromium ions, optional cobalt ions, a dicarboxylic acid as complexing agent, and either (3-MTA) or (3-MPA), respectively, as corrosion-inhibiting agent. In a control experiment, no corrosion-inhibiting agent was used. The pH in each case was about 2.2 to 2.5.

In each experiment an iron substrate (a 3.5 cm×5.0 cm iron plate) was tested in a beaker for 2 hours to allow the test passivation compositions to dissolve the iron substrate.

The test procedure was as follows: After setting up each test passivation composition the pH was adjusted as mentioned before. During each test, the pH was monitored. A final pH was measured after the 2 hours. It was observed that the pH remained stable if the corrosion-inhibiting agent was successfully minimizing the iron dissolution. However, if the total amount of the corrosion-inhibiting agent was too low or consumed over time, the pH increased, typically exceeding a pH of 3.0, even up to 3.5. A stable pH is considered as “passing” the test, wherein an increased pH up to 3.0 or even more is considered as “failed”. The results are summarized in Table 1 below.

TABLE 1 Exp. 3-MTA [mg/L] 3-MPA [mg/L] result A 0 0 Failed B 7 0 Passed C 100 0 Passed D 1000 0 Passed E 0 7 Passed F 0 100 Passed G 0 1000 Passed

The inhibitor test clearly shows that both 3-MTA and 3-MPA are effectively minimizing the dissolution of iron. In the absence of both, a significant pH increase was observed (see Experiment A). Thus, in the absence of a corrosion-inhibiting agent, an undesired release of iron ions occurred.

In contrast, in the presence of a corrosion-inhibiting agent the pH remained stable over said 2 hours. Thus, no significant iron dissolution occurred. Furthermore, no significant difference was observed between a comparatively low concentration (e.g. 7 mg/L) and a comparatively high concentration (e.g. 100 mg/l and 1000 mg/L, respectively) of a corrosion-inhibiting agent. In each case a stable pH was observed, indicating a sufficiently minimized iron dissolution.

Furthermore, it appears that the presence or absence of cobalt has no significant influence on the iron dissolution. Also, there seems to be no significant advantage or disadvantage of having cobalt in combination with a corrosion-inhibiting agent.

Similar results and conclusions were obtained with 5-Mercapto-1-methyltetrazole and benzotriazole (data not shown) with slightly less good results for benzotriazole.

2. Second Set of Experiments (Zn-Comprising Coating Additionally Comprising Fe; Corrosion Resistance Test)

In this second test (and the following third test below) the effect of the corrosion-inhibiting agent in the passivation composition on the corrosion resistance after passivation was investigated.

For this set of experiments, aqueous test passivation compositions with the numbering as introduced in Table 2 were prepared, being identical to the test passivation compositions as used for the first set of experiments. No iron ions were actively/intentionally added nor expected due to dissolution from the substrates because of the short utilization of the passivation composition.

The method of the present invention was carried out as follows: As substrates a plurality of ZnFe coated iron screws (M8x60; about 12 wt.-% Fe in the ZnFe) were pre-treated and subsequently passivated for 30 seconds in the respective aqueous test passivation compositions (volume each: 2 L) at room temperature (appr. 20° C.). Afterwards the passivated screws were optically inspected and subjected to a NSS test. The optical inspection was in all cases passed.

Further details regarding the passivation compositions and the results obtained after passivating the screws are summarized in Table 2 below.

TABLE 2 Exp. 3-MTA [mg/L] 3-MPA [mg/L] NSST [24/96/168] 1 7 0 +/+/− 2 100 0 0/−/− 3 0 7 +/+/+ 4 0 100 0/−/− C1 0 0 +/+/+ C2 300 0 −/−/− C3 1000 0 −/−/− C4 0 300 −/−/− C5 0 1000 −/−/−

In Table 2, “NSST [24/96/168]” denotes neutral salt spray test according to DIN 9227 with a duration of 24, 96, and 168 hours, wherein “+” denotes that the test was passed with no or (in only a few cases) an acceptable degree of white rust formation; “0” denotes an undesired degree of white rust formation on almost all tested screws but still acceptable (basically no red rust formation); and “−” denotes an inacceptable degree of white rust formation on all screws including in some cases even a significant red rust formation.

Experiments 1 to 4 are experiments according to the invention, wherein experiments C1 to C5 are comparative experiments.

As mentioned above, no intentionally added iron ions were present in the aqueous test passivation compositions. It is generally desired to avoid iron ions in a passivation composition because it is typically observed that they negatively affect the corrosion resistance after passivation, e.g. in a NSS test. In addition, with increasing amounts of iron ions the lifetime of a respective passivation composition is significantly shortened.

Experiments 1 to 4 and C1 to C5 clearly show how the corrosion resistance after passivation is affected in the presence of varying concentrations of 3-MTA and 3-MPA, respectively. It was generally observed that with increasing concentration of 3-MTA and 3-MPA, respectively, the corrosion resistance decreases. As shown, the total concentration of 3-MTA and 3-MPA, respectively, should never reach or even exceed 300 mg/L because a dramatic detrimental effect on corrosion resistance was observed (as well as for 1000 mg/L). In contrast, a comparatively low total concentration of 7 mg/L does not at all negatively affect corrosion resistance and can be easily tolerated such that the positive inhibitory effect can be achieved without compromising the corrosion resistance quality. If the total concentration is 100 mg/L, in many cases a still acceptable corrosion resistance is obtained initially and after 24 hours NSS testing. However, the corrosion resistance is reduced for 96 and 168 hours NSS testing, respectively (see Table 2, Experiments 2 and 4).

Since no iron ions were intentionally added, comparative example C1 showed excellent results even in the absence of 3-MTA and 3-MPA, respectively. This example represents an ideal situation, in which no iron ion contamination was present; thus, no corrosion inhibitor must be utilized. However, such ideal situation does not represent a real day-to-day situation, wherein an increased iron ion contamination is typically observed; mainly because of defective zinc-comprising coatings.

Although 3-MTA and 3-MPA in concentrations of even up to 1000 mg/L well minimize the release of iron ions (see Table 1, Experiments D and G), Table 2 clearly shows that such comparatively high concentrations (i.e. 300 mg/L and 1000 mg/L, respectively) negatively affect the corrosion resistance of respective passivated substrates (see Table 2, Experiments C2 to C5).

As a result, the total concentration of the corrosion-inhibiting agent utilized in the method of the present invention must be well balanced in order to achieve a sufficient minimization of iron dissolution on the one hand and to avoid a reduced corrosion resistance of the passivated substrate on the other hand.

3. Third Set of Experiments (Zn-Comprising Coating Additionally Comprising Sn; Corrosion Resistance Test)

In this third test again the effect of the corrosion-inhibiting agent in the passivation composition on the corrosion resistance after passivation was investigated.

For this set of experiments, aqueous test passivation compositions as defined for the first and second set of experiments were used. Again, no iron ions were intentionally added nor expected due to dissolution from the substrates because of the short utilization of the passivation composition.

The method of the present invention was carried out as defined for the second set of experiments with the difference that the screws were galvanized with a zinc-comprising coating additionally comprising Sn (about 60 wt.-% Sn in the ZnSn) instead of Fe. Further details regarding the passivation compositions and the results obtained after passivating the screws are summarized in Table 3 below.

TABLE 3 Exp. 3-MTA [mg/L] 3-MPA [mg/L] NSST 5 7 0 + 6 100 0 0 7 0 7 + 8 0 100 0 C6  0 0 + C7  300 0 − C8  1000 0 − C9  0 300 − C10 0 1000 −

In Table 2, “NSST” denotes neutral salt spray test according to DIN 9227 with a duration of 24 hours, wherein “+” denotes that the test was passed including a still acceptable white rust formation; “0” denotes a significantly increased degree of white rust formation compared to “+” on almost all tested screws but still acceptable (basically no red rust formation occurred); and “−” denotes an inacceptable degree of white rust formation on all screws including in some cases even a significant red rust formation.

Experiments 5 to 8 are experiments according to the invention, wherein experiments C6 to C10 are comparative experiments.

In a few cases, the passivation quality of ZnFe-coated substrates was better/higher than for ZnSn-coated substrates. However, the same conclusion as for ZnFe-coated substrates applies likewise to ZnSn-coated substrates. Under ideal circumstances, no corrosion-inhibiting agent is needed. However, in real day-to-day situations an iron ion increase is typically observed over time such that a corrosion-inhibiting agent is of great advantage. If such a corrosion-inhibiting agent is utilized, the total concentration thereof must be carefully balanced; too high concentrations (i.e. again 300 mg/L and 1000 mg/L, respectively) show a negative impact on the over-all corrosion resistance after passivation.

Claims

1. A method for depositing a chromium-comprising passivation layer on a zinc-comprising coating, the method comprising the following steps:

(a) providing a substrate comprising the zinc-comprising coating,
(b) providing a passivation composition for depositing the chromium-comprising passivation layer, the composition comprising (i) trivalent chromium ions; (ii) at least one complexing agent for the trivalent chromium ions, being different from the at least one corrosion-inhibiting agent; and (iii) 0.001 mg/L to 200 mg/L, based on the total volume of the passivation composition, of at least one corrosion-inhibiting agent selected from the group consisting of unsubstituted azole compounds, substituted azole compounds, unsubstituted aliphatic organic acids with at least one mercapto-group, substituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof; and
(c) contacting said substrate with said passivation composition such that the chromium-comprising passivation layer is deposited on the zinc-comprising coating,
wherein the zinc-comprising coating additionally comprises Fe, Sn, Mn, or mixtures thereof.

2. The method of claim 1, wherein the zinc-comprising coating is substantially free of nickel.

3. The method of claim 1, wherein the zinc-comprising coating comprises Fe, Sn, or mixtures thereof.

4. The method of claim 1, wherein in the passivation composition (iii) has a total concentration ranging from 0.005 mg/L to 180 mg/L, based on the total volume of the passivation composition.

5. The method of claim 1, wherein in the passivation composition (iii) has a total concentration ranging from 0.01 mg/L to 150 mg/L, based on the total volume of the passivation composition.

6. The method of claim 1, wherein in the passivation composition (iii) has a total concentration ranging from 0.001 mg/L to 9.9999 mg/L, based on the total volume of the passivation composition.

7. The method of claim 1, wherein in the passivation composition the substituted azole compounds, salts, and mixtures thereof independently comprise one or more than one substituent selected from the group consisting of amino, nitro, carboxy, hydroxy, sulfonate, and thiol.

8. The method of claim 1, wherein in the passivation composition the unsubstituted, and substituted azole compounds, salts, and mixtures thereof are independently selected from the group consisting of monoazoles, diazoles, triazoles, and tetrazoles.

9. The method of claim 1, wherein in the passivation composition the unsubstituted and substituted azole compounds, salts, and mixtures thereof are independently selected from the group consisting of 1,2,4-triazoles.

10. The method of claim 1, wherein in the passivation composition the substituted azole compounds, salts, and mixtures thereof independently comprise at least a mercaptotriazole.

11. The method of claim 1, wherein in the passivation composition the unsubstituted aliphatic organic acids with at least one mercapto-group, salts, and mixtures thereof comprise at least 3-mercaptopropionic acid and/or salts thereof.

12. The method of claim 1, wherein in the passivation composition the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of monocarboxylic acids, dicarboxylic acids, salts thereof, halogen ions, and mixtures thereof.

13. The method of claim 1, wherein in the passivation composition the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of unsubstituted monocarboxylic acids, hydroxyl-substituted monocarboxylic acids, amino-substituted monocarboxylic acids, unsubstituted dicarboxylic acids, hydroxyl-substituted dicarboxylic acids, amino-substituted dicarboxylic acids, salts thereof, halogen ions, and mixtures thereof.

14. The method of claim 1, wherein in the passivation composition the at least one complexing agent for the trivalent chromium ions has a total concentration in a range from 0.05 wt.-% to 15 wt.-%, based on the total weight of the passivation composition.

15. The method of claim 1, wherein the passivation composition further comprises

(v) iron ions in a total concentration ranging from 0 mg/L to 2000 mg/L, based on the total volume of the passivation composition.
Patent History
Publication number: 20240035165
Type: Application
Filed: Feb 4, 2022
Publication Date: Feb 1, 2024
Applicant: Atotech Deutschland GmbH & Co. KG (Berlin)
Inventors: Sebastian HAHN (Berlin), Katrin KRÜGER (Berlin)
Application Number: 18/264,288
Classifications
International Classification: C23C 22/53 (20060101);