NL2017398B1 - Method of treating metal surfaces with an aqueous composition and aqueous composition - Google Patents

Abstract

The invention relates to a method of treating a metal surface of iron or iron alloy using an aqueous composition for imparting corrosion resistance and bonding properties, as well as to the aqueous composition. The aqueous composition consists of: trivalent chromium (Cr3+): 1,16-7,0 g/l total fluoride (F-): 1,3-7,7 g/l organic corrosion inhibitor: 0-2,0 g/l water soluble polymers: 0-4,0 g/l water soluble surfactant 0-1,0 g/l organofunctional silane and/or oligomer 0-4,0 g/l pH adjusting agent 0-1,0 g/l fluoride adjusting agent 0-1,0 g/l wherein the molar ratio of Cr3+ to :F- ranges from 0.25-0.4, and wherein the pH ranges from 2.0-4.4.

Description

Patent center

The Netherlands

Θ 2017398

BI PATENT (51) Int. CL:

C23C 22/34 (2017.01) C23C 22/83 (2017.01) (21) Application number: 2017398 © Application submitted: 31/08/2016

Application registered: (73) Patent holder (s): 08/03/2018 AD Productions B.V. in HEIJNINGEN. (43) Application published: - (72) Inventor (s): Richard Johannes van der Net Patent granted: in DORDRECHT. 08/03/2018 Robin Arthur Langelaar in BERGEN OP ZOOM. (45) Patent issued: Roland Lambertus Maria van Meer 19/03/2018 to KLUNDERT. θ Authorized representative: ir. H.V. Mertens et al. In Rijswijk.

(54) METHOD OF TREATING METAL SURFACES WITH AN AQUEOUS COMPOSITION AND AQUEOUS COMPOSITION © The invention relates to a method of treating a metal surface or iron or iron alloy using an aqueous composition for imparting corrosion resistance and bonding properties, as well as to the aqueous composition. The aqueous composition consists of:

trivalent chromium (Cr ³⁺ ): 1.16-7.0 g / l total fluoride (F): 1.3-7.7 g / l organic corrosion inhibitor: 0-2.0 g / l water soluble polymers: 0-4.0 g / l water soluble surfactant 0-1.0 g / l organofunctional silane and / or oligomer 0-4.0 g / l pH adjusting agent 0-1.0 g / l fluoride adjusting agent 0-1 , 0 g / l in the molar ratio of Cr ³⁺ to ^: F 'ranges from 0.25-0.4, and in the pH ranges from 2.0-4.4.

NL BI 2017398

This patent has been granted regardless of the attached result of the research into the state of the art and written opinion. The patent differs from the documents originally submitted. All submitted documents can be viewed at the Netherlands Patent Office.

-τ P32818NL00 / JV

METHOD OF TREATING METAL SURFACES WITH AN AQUEOUS COMPOSITION AND AQUEOUS COMPOSITION

The invention relates to a method of treating metal surfaces with an aqueous composition and to an aqueous composition for treatment of metal surfaces.

Mechanical and chemical treatment of metal surfaces for enhancing corrosion resistance and / or improving bonding to a subsequently applied coating such as an adhesive layer, paint layer, lacquer layer or other finishing layer is well known in the art. E.g. mechanical treatment for example grid blasting has been used to improve adhesion, when chemical treatment steps were not practical to apply. Only cleaning or mechanical treatment of metal parts made or especially iron and iron alloys without any suitable chemical treatment will result in flash rusting, if the parts are not painted within a few hours. Chemical treatment of metal surfaces or zinc (alloy) coated steel, mild steel, or aluminum and their alloys with aqueous chromate (chromium VI) solutions results in a so called “chromate conversion layer”, which offers corrosion resistance and improved adhesion, and avoids the occurrence of flash rusting before painting ..

St is also recognized that these chromate-based aqueous solutions suffer from the toxicity of the

Cr® ⁺ component ther.Cr ³ * is classified as carcinogenic and will be banned from most industrial applications with high exposure risks for the co-workers. has been converted into the comparatively innocuous trivalent chromium. However, such a conversion brings about additional costs and expenses.

Also have phosphate coatings used to improve adhesion of coatings, such as paint, and corrosion resistance of steel. Some major disadvantages or phosphate coatings are: several rinsing steps, sludge disposal and power consumption. Additional these coatings are often sealed with a hexavalent chromium solution for optimum adhesion and corrosion. Therefore these phosphate coatings suffer from several environmental, health and safety drawbacks. Today the use of chromate-free (paint) primers has become more common. It has appeared that mechanical and chemical pre-treatments of metals become more important to guarantee corrosion protection performance of metal paint systems ..

Ongoing legislation is also developed and comes gradually into force in order to reduce and ultimately abandon hexavalent chromium based metal treatment compositions.

Therefore, in the art there is a need for treatments that are substantially free of hexavalent chromium compounds, that offer corrosion resistance and bonding performance to the metal

-2surfaces treated similar to those obtained by treating these metal surfaces with conventional solutions including hexavalent chromium.

Various propositions to satisfy this need have been disclosed in the patent literature. E.g. WO 2006/088518 has disclosed a process for preparing zirconium-chromium conversion coatings on iron and iron alloys to improve corrosion resistance and adhesive bonding strength. This known method comprises treating iron and iron ailoys with an acidic aqueous solution having a pH ranging from about 2.5 to 5.5, preferably 3.7-4.0 for steel surfaces. The acidic aqueous solution comprises, per iitre or solution, from about 0.01 to 22 grams of a trivalent chromium compound, about 0.01 to 12 grams of a hexafiuorozirconate, about 0.0 to 12 grams or at least one fluorocompound selected from the group consisting of tetrafluoroborate, hexafluorosilicate and mixtures of, from about 0.0 to 10 grams or at least one divalent zinc compound, from 0.0 to about 10 grams or at least one water soluble thickener and from 0.0 to about 10 grams or at least one water soluble surfactant. Above known treatment contains at least one zirconium and the preferred soluble trivalent chromium species is a sulphate anion. These extraneous cations and anions affect the formation of insolubie species negatively. Therefore the remaining unreacted solution should be from the substrate with tap or demineralised water resulting in an additional waste stream that requires disposal Or other processing.

WO 2006/088519 A1 discloses a similar treatment, according to the solution also comprises a stabilizing compound selected from polyhydroxy and carboxylic compounds.ln practice, these preparations are used at iow concentrations of the effective species to avoid over-etching and flash rest during drying. However, iow concentrations result in iess dense protective layers on the metal surface and therefore might affect the protective and / or bonding performance.

An object is to provide a method of protecting an organic coated surface or iron or iron against corrosion and / or improving durable adhesion properties or such an organic coating or an adhesive using a metal surface treatment solution.

Another object of the invention is the provision of an alternative metal treatment method and solution based on trivalent chromium for protection of an organic coated surface or iron or iron alloy against corrosion and / or for improvement of the adhesion properties of such an organic coating or an adhesive using a metal surface treatment solution ..

Yet another object is to provide a dry-in-place treatment method and solution requiring no rinse step after application.

Yet another object is to provide a dry-in-place metal treatment solution for use at room temperature on iron and iron alloys without flash rusting.

-3Stili another object is to provide a metal treatment method and solution for an application to an already phosphated surface or iron and iron alloy replacing a conventional chromate seal on such surface.

Always, in a first aspect the invention provides a method of treating a metal surface or 5 iron or iron alloy for providing corrosion protection, adhesion or coating and / or adhesive, which method comprises the application of an aqueous composition on the meta! surface of iron or iron alloy, which composition consists of:

trivaient chromium (Cr ³ *): 1.16-7.0 g / S 10 total fluoride (F): 1.3-7.7 g / i organic corrosion inhibitor: 0-2.0 g / i water soluble polymers: 0-4.0 g / l water soluble surfactant 0-1.0 g / i organo functional silane and / or oligomer 0-4.0 g / i PH adjusting agent 0-1.0 g / i fluoride adjusting agent 0-1.0 g / i

if the moiar ratio or Cr ³⁺ to F ranges from 0.25-0.4, and including the pH ranges from 2.0-4.4.

In a second aspect the invention provides an aqueous composition as defined above for treating a metal surface or iron or iron alloy.

Below the invention is explained first or all in terms of composition and components thereof. The aqueous composition according to the invention is free of hexavalent chromium and contains as main constituents trivaient chromium ions and fluoride ions in a moiar ratio or Cr ³⁺ to F ranging from 0.25-0.4. Surprisingly it has found a solution having a relatively simple composition regarding its components without the need or specific more complex (fluorometalate) compounds as a source of fluoride ions offers good results regarding corrosion resistance and adhesion. These fluorometalate compounds used in the prior art will introduce extraneous polyvalent metal ions and these are believed not to play a significant role regarding corrosion resistance and adhesion and could complicate the preparation of the composition regarding desired ratios of components. Contrary thereto the composition according to the invention can be easily prepared .. Metal surfaces that can be suitably treated with the composition according to the invention include iron, iron alloys such as cold rolled steel, mild steel and carbon steels .. The metal surface may have a conventional phosphate conversion coating applied to it before it is exposed to the composition according to the invention. The composition can be easily applied, even in repair and maintenance conditions such as outdoor pipeline field applications, ship building, road work, offshore, industrial equipment and other (non-mobile) steel structures. Typically

-4the composition can be applied directly to the meta! surface, after conventional mechanical and / or chemical pre-treatment, such as grit blasting and degreasing / pickling respectively. Rinsing after application or the composition is not necessary. The treatment solution can be based on air under prevailing conditions and does not require any special measures or apparatuses. However the treated metal substrate should be dry before subsequent painting or adhesive bonding processes are carried out. Forced to reduce process time forced drying methods can be used for example oven drying, infrared drying and forced air drying. After the composition has been applied and sufficiently dried, the thus treated surface of iron or iron can be coated with an organic paint, optionally including the pre-application of a paint primer, and / or with an adhesive bonding system.

The layer formed from the composition enhances the adhesion or the subsequently applied coating such as a paint layer system or adhesive bonding system. The enhanced adhesion offers good corrosion resistance when the thus coated metal surface is exposed to corrosive conditions. Furthermore, it has appeared that the layer formed may allow a conventional paint primer. Additionally, the layer thickness can be less than that or a conventional primer.

Trivalent chromium is present in amount or 1.18-1.7.0 g / l. A preferred range is 3.0-8.0 g / l. Total fluoride is in the range of 1.3-7.7 g / l. The molar ratio of Cr ³⁺ to F 'is 0.30-0.38, more preferably 0.32-0.34, such as 1: 3. it has been found that the stoichiometric ratio of CrF ₃ or slightly above offer good results regarding corrosion resistance and / or bonding characteristics.

The trivalent chromium can be obtained by reducing chromic acid (H ₂ CrO ₄ ) with chemical agents that can be oxidized by chromic acid iike methanol or hydrogen peroxide leaving no residual products in the starting solution after heating, Another attractive source is using

CrF ₃ .4H ₂ O as a starting material. This compound is hardi-soluble in water, but accompanied by acidic components iike HF and acidic homopoiymers and copolymers. HF is preferably used as it does not introduce extraneous anions.

The pH ranges from 2.0-4.4, preferably 2.7-3.4. In order to set the acidity at the required level the composition may contain pH adjusting agents, such as alkali meta! hydroxide like sodium hydroxide, potassium hydroxide, and ammonia, in an amount of 0-1.0 g / l. It is believed that alkali metal ions do not - or to a substantially lesser extent - contribute to the formation of the protective layer and thus its protection and / or bonding properties.

As said, the molar ratio of fluoride to trivalent chromium is preferably equal to or slightly above the stoichiometric ratio of CrF ₃ . Adjustment may be carried out by incorporating fluoride adjusting agents that offer an additional source of fluoride anions, if present, these fluoride adjusting agents are present in an amount up to 1.0 g / l. Preferred examples include

-5 fluoric acid, fluorides or alkali metals and ammonium, in particular sodium fluoride and ammonium in fluoride.

The composition according to the invention may additionally include an organic corrosion inhibitor in amount up to 2.0 g / l, preferably 0.0001-2.0, more preferably 0.1-1.0 g / l. The organic corrosion inhibitor can act as a flash corrosion inhibitor, which inhibits so called “flash rusting” during drying of the applied treatment composition on iron and iron alloys surfaces, in addition the organic corrosion inhibitor is thought to contribute to the final corrosion resistance after application of a coating like paint. The organic corrosion inhibitor should be slightly soluble in water or miscible therewith. Examples include: N, N-dimethyl propylene urea, tolytriazole, zinc phthalate, imidazolinemaleate, caprylic acid, phtalic acid, phosphonic acid alkyl esters, n-butyric acid, benzotriazole, tolytriazole, phthalate divalent salts, nitrobenzoate, 1-octanol, tannic acid, nitro maleate divalent salts, 2mercaptobenzimidazole, propargyl alcohol, propargyl alcohol ethoxylates, iso nitro phtalate zinc salt, 2-butyn 1,4 diol, 2-butyn 1,4 diol alkoxylates, alkanolamine salt or a nitrogenous organic acid, quaternary amines and combinations of .

Concentration and the nature of the organic corrosion inhibitor or a mixture of corrosion inhibitors should be chosen in a way that it will not block the trivalent chromium deposition on the metal during treatment ..

Other optional compounds include water soluble homopolymers and copolymers that are preferably based on the following monomers: acrylic acid, methacrylic acid, vinyl alcohol, vinyl ether, maleic acid, vinylphosphonic acid, vinylsulphonic acid, methyl vinyl ether and combinations thereof, up to 4.0 g / l, preferably 0.01-4.0 g / l, more preferably 0.1-1 g / l. These polymers improve wetting behavior of the treatment composition, as well as adhesion or subsequently applied organic coatings. Too high concentrations will reduce wet adhesion or an organic coating.

Another optional compound is a water soluble surfactant, which may be present in an amount up to 1.0 g / l. A preferred concentration range is 0.001-0.5 g / l, while a more preferred concentration range is 0.01-0.1 g / l. Surfactant that can be used in the composition according to the invention include acid stable Sow foaming anionic and non-ionic surfactants like alkaryl sulfonates and poly ethylene glycol fatty amines. The surfactant provides uniform wetting of the substrate. If the amount of surfactant is too high, it can cause excessive foaming in the process.

Yet another component that may be present is an organo functional silane and / or a hydrolysed oligomer. If present, the concentration ranges up to 4.0 g / l. The reactive functional group is at least one selected from a mercapto group, an amino group, a vinyl group, an epoxy group and a methacryloxy group, advantageously in an amount or 1 to 40 mg / l based on Si.

-6The method of treating a metal surface or iron or iron for providing corrosion resistance and adhesion or a coating and or adhesive, comprising a step or applying the aqueous composition according to the invention and outlined ahead of the metal surface. Typically the metal surface to be treated with the composition according to the invention is pre-treated using known mechanical and / or chemical pre-treatment processes or combination to obtain a legible surface, which typically requires the surface to be roughened and to be substantial free of rest, fat, oil and the like. Mechanical pretreatment processes include grit blasting and abrading. Chemical pre-treatment includes (acidic / alkaline) degreasing and pickling. Typically a chemical pre-treatment is followed by a rinsing step using tap water or demineralised water. Combinations of mechanical pretreatment and chemical pre-treatment in any order is also possible.

The way of applying the composition according to the invention to the metal surface is not limited. However, homogeneity and uniformity of the applied law film on the substrate before drying will be advantageous. Suitable application methods include spraying, dipping, wiping, brushing, roll coating and the like. Excess of treatment fluid on parts with intricate geometries can be removed with compressed air before drying. After application it is not necessary to perform a step to remove unreactive and / or unreacted species from the formed layer. Instead of the metal surface to which the composition according to the invention is applied can be allowed to dry immediately, e.g. in air optionally at elevated temperature like an oven having conditioned air.

Advantageously the coating weight (after drying measured by XRF (X ray fluorescence)) ranges from 20 to 200 mg of chromium / m ² . Higher coating weights want reduce adhesion properties or further applied organic coating iayers. At Sower coating weights no beneficial effect in corrosion protection has been measured.

A applied applied paint system and / or adhesive bonding system can be applied using conventional methods and equipment, such as spraying, brushing and roli coating.

The various features of the treatment composition as discussed above are equally applicable to the method according to the invention.

The invention is illustrated by the following examples according to the invention and comparative examples.

The trivalent chromium compound “Cr (lll) Fluoride” as indicated in the below Tables was obtained by reducing a chromic acid solution in a stoichiometric ratio chromium to fluoride 1 to 3. The fluoride source was an aqueous solution of hydrogen fluoride. Methanol in water was used as a reducing agent. After 4 hours of reduction at 80 ° C no hexavaient chromium couid detected by using a diphenyl carbide test method (detection limit for Cr (VI) is narrower than 0.03 ppm). Methanol and oxidation products of methanol like formalin and formic acid could not be detected by TOC (total organic carbon) measurements.

-7Aqueous metal surface treatment liquids having a composition as indicated in Tabie 1 were prepared and applied to metal surfaces as indicated in Tabie 2 ..

The thus pre-treated, treated and organic coated surfaces were subject to adhesion tests and corrosion tests according to quality regulations prescribed GSB and Quaiitysteeicoat:

Adhesion Cross cut adhesion pull-off EN ISO 16276-2; ASTM D3359 Adhesion Reversed impact ASTM D2794 or EN ISO 6272-1 and Corrosion tests: Accelerated corrosion testing according to ASTM B117 Neutrai Salt Spray for iron, iron alloys and zinc coated steels. Tables 2 and 3 summarize the test resuits.

St appears that the examples according to the invention passed the adhesion test and had a significantly better performance regarding corrosion than the comparative exampies.

Table 1. Examples composition

£ a | 3.0 | [3.1 | 3.4 |  3.0 | 3.0 3.0 | 3.0 years IZ9J 1¾ © § © 3 © S © 5 o yes 1 200 I 2500 350 I 002 200 I O £ M 250 I Additive PAA | PAA j caprylic acid zinc phthalate | © O Έ 8 c 2 propargyl alcohol PAA, PVA I © 1 you © z § Ü co co co co co CO co v- CM - v- X- T " T " © © © ' 1 ® x © a * co t Q a -53- •X CO t O HF + ABF hf; H2ZrF6 LHF [HF HF - I HF | © © 3 S is ® ™ © © 5 i.q. ! 0.28 Ό 3e. Μ ^- O χφ v Xt - < Ammonium bi Fluoride: Chromium trifluoride O < Is | l 3 rd Dissolved CrF3 * 4aq dissolved CrF3 * 4aq I Cr (III) Fluoride I None! i Crilil) Fluoride i I Di Chromium i © S JZ a n co • c S ™ I Crilil) Fluoride i Q Έ O 33 X O I Cr (III) Fluoride i Nonet i I Crdll) Fluoride i None Hydrogen fluoride b n c g N 2 O O £ SX ffi X © X O G cs O © ¢ 8 >> © O X: 8 >. c > >> O X © a s2 co | Comp 3 xt Ω. £ © O to tn a E O O Comp 6 Comp 7 £ 33 so a E O HF X m < O CM X X5J « t O CO s CM X $ X $ X co

σ>

Table 2. Examples method ©

Η © & y

I *

E ©

2 ^s ©

* 6 533 ° S

Η © i, § ° o g a ϋ 5 S e

©

E αδ a

<

a.

J ® 3 SS © ©

©, £

© £

co co

Q, £

δ

Ω.

E o

O

CO. 2 i

a ©

• s

531 ©

© • s a

o s

X o

E © © 3 ©

C ©

©

O

CO o

Tj © 3s £ © © 5

CM ©

© • s

E

CO in

Q.

E δ

co e

E

CM> 1

Q.

© ©

© ©

X5

E

S3 co

Qj

E ©

O

CM

Ω.

© ©

©

J5

-Ω

E

S3

-s ©

© ©

J3

E © Ϊ h »© 4

E δ

COs

© · # * iii ©

X3

Έ © 3

O

Q ω

c o

e «S o

CM

CO ** ¢ 8 ¢ 8 ©

• S c

E co

S3

E

Es

O ¥

X

Ω.

£ 533

E ©

_ ©

Ü ©

E ©

©>

O

E ©

i—

E ©

533

E

E ©

.E

S3

O

O

M

CM · *** ©

?

S 'ω' ©

E §

a

E ©

· * - *

E o

© □

o co boq © <© 2 53) E

Έ ¢ 3 ©?

S3

E e

S3

E ©

ö * = E

Ω,

E δ

ω tz o

co ©

E ©

© ©

©

E "©

X ©

© ©

«W>» ω

k »α

ω € 8 &

©

Ε ©

©

E

E o

o co k ©

E

E

Q.

s?

O

Ω.

©

O

CO

S3

E u, ©

a ©

© ©

>>

O

G.

5T

CZ3 ω

S3 ©

©

X3

Έ

S3

C7 o

Q.

© ©

a ©

br «jbrf ε:

ω

S

Table 3: Test results S3 8 © O g © a ra © ws s O © © 8 YOU .2 5 g -1 ι- Ι? - * 5 I 8 © o g * § «3 © II § S g 8 © © ferf © © E you process time is not feasible in practice 1 process time is not feasible in practice 1 process time is not feasible in practice j process time is not feasible in practice j X ©, ra d σ> c \ i 0 c \ i CO co co CO CO CO X co co co y 1 1 «S E , 2 31 1 8 8 | s | <© © 1 1 1 © © © Q. © © © a © © © Q. © © © Q. M ffi ex © © ffl ex YOU) "5 CX 1 SHE IS £ ® S3 2 s ffi 3 IS 11 "S s © © «8 gP © , 2 © e 1 8 ® e © g e e © £ S3 © co 0 co 0 00 CO 0 co O co CO 0 co 0 03 co 0 co 0 TO © © 33 s K. © Έ 0 Ü 0H, IDE in m in 10 in TO CM TO CM TO cm 1 8ΖΌ | 1 82Ό | CO CM O

Claims (18)

CONCLUSIONS A method for treating a metal surface of iron or iron alloy, for corrosion protection, adhesion of a coating and / or glue, which comprises applying an aqueous composition to the metal surface of iron or iron alloy, which composition comprises: trivalent chromium (Cr ³⁺ ): 1.16-7.0 g / i total fluoride (F): 1.3-7.7 g / i organic corrosion inhibitor: up to 2.0 g / i water soluble polymers: 0-4.0 g / i water-soluble surfactant 0-1.0 g / i organofunctional silane and / or oligomers: 0-4.0 g / i pH adjusters 0-1.0 g / i fluoride adjusters 0-1.0 g / i wherein the mole ratio Cr ³⁺ : F 'is in the range of 0.25-0.4, and wherein the pH is in the range of 2.0-4.4. The method of claim 1, wherein the concentration of trivalent chromium (Cr ³⁺ ) in the aqueous composition is in the range of 3.0-6.0 g / l. The method according to claim 1 or 2, wherein the mole ratio Cr ³⁺ : F 'in the aqueous composition is in the range of 0.30-0.36, preferably 0.32-0.34 and more preferably 1: 3. 4. A method according to any one of the preceding claims, wherein the composition contains dissociated CrF ₃ .4H ₂ O. A method according to any one of the preceding claims, wherein the concentration of organic corrosion inhibitor in the composition is 0.0001-2.0, preferably 0.1-1.0 g / l. A method according to any one of the preceding claims, wherein the concentration of water-soluble polymers in the composition is in the range of 0.01-4.0, preferably of 0.1-1 g / l. A method according to any one of the preceding claims, wherein the concentration of water-soluble surfactant in the composition is in the range of 0.001-0.5, preferably 0.01-0.1 g / l. The method according to any of the preceding claims, wherein the pH is in the range of 2.7-3.4. The method of claim any of the preceding claims, wherein the coating weight is in the range of 20-200 mg of chromium / m ² measured with XRF after drying. The method according to any of the preceding claims, wherein the metal surface is a metal surface of iron or iron alloy that is already provided with a phosphate conversion coating. An aqueous composition for treating a metal surface of iron or iron alloy, in particular for use in the method according to any one of the preceding claims, which composition comprises: trivalent chromium (Cr ³⁺ ): 1.16-7.0 g / i total fluoride (F): 1.3-7.7 g / i organic corrosion inhibitor: up to 2.0 g / i water soluble polymers: 0-4.0 g / i water-soluble surfactant 0-1.0 g / i organofunctional silane and / or oligomers: 0-4.0 g / i pH adjusters 0-1.0 g / i fluoride adjusters 0-1.0 g / i wherein the mole ratio Cr ³⁺ : F 'is in the range of 0.25-0.4, and wherein the pH is in the range of 2.0-4.4. The composition of claim 11, wherein the concentration of trivalent chromium (Cr ³⁺ ) is in the range of 3.0-6.0 g / l. The composition according to claim 11 or 12, wherein the mole ratio Cr 3+: F- is in the range of 0.30-0.36, preferably 0.32-0.34 and more preferably 1: 3. The composition according to any of the preceding claims 11-13, which contains dissociated CrF3.4H2O. A composition according to any one of the preceding claims 11-14, wherein the concentration of organic corrosion inhibitor is 0.0001-2.0, preferably 0.1-1.0 g / l. A composition according to any one of the preceding claims 11-15, wherein the concentration of water-soluble polymers is in the range of 0.01-4.0, preferably of 0.1-1 g / l. 5 A composition according to any one of the preceding claims 11-16, wherein the concentration of water-soluble surfactant is in the range of 0.001-0.5, preferably 0.01-0.1 g / l. The composition of any one of the preceding claims 11-17, wherein the pH is in 10 the range of 2.7-3.4.