DE2253697A1 - PROCESS FOR INCREASING THE HARDNESS OF A POROUS COATING ON CORROSION-RESISTANT CHROME-IRON ALLOYS - Google Patents

Description

Dipl.-Ing. H. Sauerland · Dr.-lng. R. König ■ Dipl.-lngj. K. Bergen Patent Attorneys ■ 4000 Düsseldorf so · Cecilienallee 7s · Telephone 43273a

October 31, 1972 28 031 K

International Nickel Limited, Thames House, Millbank,

London, S ₀ W. Ί England

"Method for increasing the hardness of a porous coating on corrosion-resistant chromium-iron alloys"

The invention relates to a method of increasing the hardness of a porous coating produced by immersion in an aqueous solution of chromic and sulfuric acid corrosion-resistant chromium-iron alloys,

It is already a method of improving corrosion resistance and wear resistance of stainless steel and other chromium-iron alloys with a porous, coating produced by dipping and optionally electrolytic treatment has been proposed. In detail the metal provided with the porous coating can undergo cathodic electrolysis in a chromium containing Bath for as long as necessary for the coating to cure, but not so long that it becomes shows visible chromium as a white precipitate on the surface. The curing of the coating was based on the deposition of metallic chromium returned in the pores of the coating, which is why the use of an electrolyte is essential, from which chromium can be deposited.

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A disadvantage of the aforementioned method is that it allows only relatively low current densities because in practical application the increasing tendency of chromium shows itself with increasing current densities to be deposited on the surface of the coating.

Surprisingly, it has now been found that the coating is hardened less by metallic chromium than by an oxidic deposit in the pores, so that hardening is also possible with other electrolytes. Accordingly, the hardness of a porous, on the surface of a corrosion-resistant chromium-iron alloy by treatment in an aqueous one Solution of chromic and sulfuric acid as well as possibly other solution components applied coating thereby increases that the alloy is galvanically treated as a cathode in an electrolyte with which an oxidic Precipitation can be generated in the pores of the coating. The oxidic precipitate arises in the Ways of the cathodic reduction of soluble components of the solution and / or an increase in the pH value in the pores as a result of electrolysis. The cathodic electrolysis and consequently also the deposition of the oxidic Precipitation is continued until the coating has assumed sufficient hardness and the The composition of the electrolyte and the current density have changed so much that if the Electrolysis would deposit metallic chromium. The oxidic precipitate can consist of insoluble oxides, hydroxides or hydrated oxides. The electrolyte can also contain chromium; in this case it was determined

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shows that the acidity of the electrolyte plays an extremely important role in curing the coating and that no chromium is deposited above certain pH values, _etc.

In a variant of the method according to the invention, the pH of the chromium-containing electrolyte is adjusted so that that no metallic chromium is deposited. This can be done, for example, by adding a suitable Base happen.

Aqueous electrolytes with 25 to 750 g / l hexavalent chromium as CrO ^ and a base are suitable as a bath in an amount sufficient to at least partially neutralize the chromium-containing anions and the Maintain the pH of the electrolyte essentially at 3 to 7.5. Cathodic treatment can be done with a Cathode current density of 2 to 40 A / dm · take place and up to 40 minutes at a bath temperature of 20 to 60 ° C. Suitable bases are sodium, potassium or Ammonium hydroxide or carbonate.

Advantageously ^ - ^ f ¹ VaIt the electrolyte at least 100 g / l chromic acid, measured as CrO ^ ₁ However, an electrolyte with about 2.5 mol or 250 g CrO, per liter and sufficient NaOH or NH ^ OH, ie about 3.5 to 5.2 moles of NaOH to keep the pH of the electrolyte at 6.3 to 7.5. The cathode current density during curing is advantageously up to 12 A / dm ² .

Which is initially on the stainless steel in a chrome

3 U 9 8 1 9/1 (J B 3

and sulfuric acid solution forming coating likely contains hydrated iron and chromium-rich spinel-like oxides. The hardening of the coating in a chromium-containing electrolyte is likely to be due to CrO, deposits in the pores of the coating. A decrease in the acidity of the electrolyte appears to improve the electrolytic conditions in the coating that favor the deposition of CrpO in the pores. In addition, this is likely to be the reason why no metallic chromium is formed in the pores. For example, from an electrolyte with 2.5 M CrO and 0.01 M (ΝΗλJpSpOg in a 15-minute electrolysis with a current density of about 10.8 A / dm metallic chromium is deposited on the surface and there is only a low hardness If more and more sodium hydroxide solution is added to the solution, metallic chromium will continue to be deposited on the surface during longer electrolysis until the molar ratio NaOH / CrO is above 0.8: 1. With a molar ratio of 1.4 to 2, The hardness of the coating increases remarkably 1: 1. At a molar ratio of 2.1: 1, a precipitate of Cr (OH) _x forms on the surface. The precipitation of chromium hydroxide prevents the Cr ₂ O from penetrating into the coating and therefore does not allow hardening. In the case of an electrolyte with 2.5 M CrO, the molar ratio is therefore increased by adding 3.5 to 5.2 M NaOH preferably adjusted to 1.4: 1 to 2.1: 1 in order to reduce the p Maintain the H value at 6.3 to 7.5.

The invention is explained in more detail below with reference to several exemplary embodiments:

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example 1

The hardnesses achieved in a series of tests are shown in Table I below, in which stainless steels of type 304 with 18 to 20% chromium and 8 to 12% nickel after a blue dyeing by dipping in a chromium and sulfuric acid solution in a 2, Electrolytes containing 5 M CrO ^ and 0.01 M (NH ^) ₂ S ₂ Og were treated cathodically at 40 ° C., to which various amounts of sodium hydroxide solution were added in order to change the pH value. The hardness of the coatings was determined by rubbing tests in which the surface was rubbed with an ink eraser and the number of strokes until the coating was destroyed was used as a measure of the hardness. In contrast to the other experiments, experiments A to D do not fall under the scope of the invention.

NaOH supply
sentence
(M) Table I. pH Current density
(A / dm ² ) Time
(min) hardness metal "chrome attempt - 9.7 15th 35 ti A. 1 11.6 15th 50 II ¹ B. 2 -0.025 11.2 15th ^ C. 3 6.06 15.9 15th 60 1A 4th 6.65 11.6 15th 65 1B 5 7.22 14.3 15th 80 CrO / H), 1C 6th 13.60 12.0 15th 25th D.

* indeterminate negative pH value.

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The data in Table I above show that the cathodic treatment was most effective in an electrolyte with 2.5 M CrO, and 0.01 M (NH ^) ₂ S ₂ Og at a bath temperature of 40 ^° C. and with a NaOH -Addition of 3 to 5 M or at a pH of 6.06 to 7.22 and a current density of 11.6 to 15.9 A / dm after 15 minutes of treatment resulted in good hardness. If the pH value is too low, metallic chromium is deposited on the coating, while if the pH value is too high, the chromium-containing ions tend to hydrolyze and lead to the deposition of Cr (OH) ^ on the coating.

The process according to the invention can also be carried out using electrolytes with 0.25 to 7.5 M CrO, and Sodium, potassium or ammonium hydroxide individually or side by side Perform in an amount that ensures a pH of 3 to 7.5. Such electrolytes can by adding chromic acid to a solution of sodium, potassium or ammonium chromate or by adding of sodium hydroxide or carbonate, potassium hydroxide or carbonate, or ammonium hydroxide or carbonate to one Solution of chromic acid.

Experiments have also shown that not only chromium oxide but also other oxides are found suitable for hardening the coating. Accordingly, the inventive Process can also be carried out in such a way that that with a porous coating or a paint coating metal provided as cathode is placed in an aqueous solution of the salt of an element which is contained in Water forms insoluble oxides. In the following ca

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_ 7 -

static electrolysis forms in the pores of the Coating a hardening oxide deposit.

Suitable electrolysis contain the salt of an egg. mentes, which forms water-insoluble oxides and, when the pH value increases, an oxide precipitate and / or forms oxides by cathodic reduction of a component of the solution. The aforementioned Oxides form, for example, the elements aluminum, titanium, vanadium, molybdenum, tungsten, manganese, Iron, cobalt, nickel, copper, zinc and silicon. The color of the oxidic precipitate only affects has little or no effect on the hue of the coating, although the color usually varies in hue something changes, namely in the color series brown, blue, gold, fuchsin, peacock blue and green.

Some exemplary embodiments are described below, in which a blue coating is initially applied to a Mirror-like stainless steel plate of the type obtained by immersing the plate in a solution of chromic and sulfuric acid generated and this then a cathodic electrolysis in a chromium-free electrolyte after Invention was subjected. In these exemplary embodiments, apart from the solution, the current density counts, the treatment time, the pH value and the bath temperature to the variables and was the electrolysis used to to harden the coating and to darken it a little in color. The undesirable consequences of electrolysis include metallic or oxidic precipitation on the coating and an excessive or uneven change in the

0 53

Hue. The hardness of the coating was determined in each case in a friction test in which the surface was marked with a Pencil eraser, preferably treated with a "Remington" eraser under a load of 400 g became.

In such an attempt, uncured coatings fall out after just one or two strokes a coating hardened by the process according to the invention withstands 100 to 150 lines and is particularly suitable Coatings withstand 400 to 600 strokes.

Example 2

An aqueous solution of potassium permanganate served as the electrolyte. The hardening is based on the fact that the Mn as MnO ^ anion in the neutral-alkaline range the pH value is stable. It can be assumed that during the reduction of the MnO ^ at the cathode, i.e. within

IV

of the pores of the paint coating, Mn accumulates and precipitates as insoluble MnO ₂ , possibly in hydrated form. The electrolysis was carried out with an electrolyte with 0.4 M or 63 g / l KMnO ^ and a pH of 8.5 at room temperature, in which the manganese was present as a seven-valent ion.

The results of the tests are compiled in Table II below.

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Table II

Current density 'time hardness
(A / dm ² ) (see.)

0.05 30 30 slight color change to

Light Blue

0.05 60 30 moderate color change to

light gold

0.2 20 20 moderate color change to

golden red

0.2 60 30 strong color change to

blue green

The data in Table II above show that the electrolysis in potassium permanganate solutions to a moderate increase in hardness, an improvement in resistance fingerprints and a significant change in shade leads.

Example 3

The electrolyte consisted of an aqueous solution of nickel

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- ίο -

nitrate. It can be assumed that apart from KMn ^ or CrO ^ hardening, no electrochemical reduction of metal ions takes place. The main reaction at the cathode is the hydrogen ion discharge H ⁺ + e ~ ^ 1/2 H ₂ . This leads to an increase in the pH value in the pores of the colored coating, so that the solubility product of the metal hydroxide is exceeded and nickel hydroxide is precipitated in the pores. This was found with an electrolyte containing 0.5 M or 91 g / l Ni (NO ^) ₂ , the Nikkei of which was present as a divalent nickel ion.

The test results are compiled in Table III below.

Table III

Current density time pH hardness
(A / dm ² ) (see.)

0.5 60 6.7 - Ni (OH) ₂ precipitation in the

Pores, visible as a change in color; Ni (OH) ₂ precipitation on the paint coating

0.2 20 6.7 40 Ni (0H) ₂ precipitation only in the pores of the paint coating; slight change in color.

3 ü 9 8 1 9 / H) S 3

Continuation of table III:

0.4 15 6.7 80 Ni (0H) ₂ precipitate exclusively in the pores of the paint coating; slight change in color.

0.8 5 6.7 40 Ni (0H) ₂ precipitate exclusively in the pores of the paint coating; slight change in color

The last three experiments in Table III were _{repeated using a Ni (NO ^) 2} solution with a pH value of 5.7 and 4.7, with similar results being shown. A temperature increase to 40 and 60 ^° C. resulted no improvement.

The tests show that a remarkable increase in hardness is possible with a Ni (NO ^) ₂ solution if the current density is 0.2 to 0.8 A / dm and the treatment time is 5 to 20 seconds. The data in Table III show that curing is not associated with any impermissible change in color tone. In contrast, the first experiment in Table III shows that excessively high current densities and excessively long treatment times result in an oxidic deposit on the coating.

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Example 4

The electrolyte consisted of an aqueous solution of zinc nitrate. It is to be assumed that the hardening occurs in the same In the same way as in a Ni (NO *) p electrolyte according to Example 3. The electrolyte contained 0.5 M. or 95 g / l Cn (NO,) p and zinc as a divalent one Zinc ion; it was kept at room temperature. The test results are shown in Table IV below compiled.

Table IV

Current density time pH hardness (see.)

(A / dm ² )

15 3.7

15 3.7 40

Zn (OH ^ precipitate in the pores, recognizable as a slight change in color; some Zn (OH) ρ -Deprecipitation on the paint coating.

Zn (OH) ₂ precipitate exclusively in the pores of the paint coating; slight change in color.

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Table IV continued:

1.0 30 1.5 20 Zn (OH) ^ precipitate

exclusively, in the pores of the paint coating; 1 slight change in color.

2.0 30 1.5 - Zn (0H) ₂ precipitate in

the pores and on the paint coating.

The table above shows that even with a ^ p Electrolytes a certain increase in hardness is possible, although the results are not as good as in the case of the Ni (NO,), -, - electrolytes of example 3. The current density must be low enough and the treatment time short enough to allow the formation of a loose oxide / hydroxide precipitate to avoid on the paint coating, thereby increasing the hardness a certain limit is set. A reduction in the pH value to 1.5 shifted the permissible current densities and treatment times to higher values without losing resulted in an improvement in hardness.

example 5

The electrolyte consisted of an aqueous solution of

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Aluminum sulphate, in which the hardening takes place in the same way as according to example 4 and 3 should proceed. The electrolyte was kept at room temperature and contained 0.5 M or 171 g / l AIp (SO ^) - ,; in it was the aluminum as a trivalent aluminum ion. The test results are compiled in Table V below.

Table V

Current density time pH hardness
(A / dm ² ) (see.)

0.4 60 2 20 slight change in color shade;

4.0 15 2 100 moderate change in color, especially at the edges;

4.0 15 1 80 moderate color changes, especially on the edges.

With Al ₂ (SO ^), electrolytes, a considerable increase in hardness can be achieved without too great a change in color and without oxide deposits on the color coating.

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Claims (1)

International Nickel Limited, Thames House, Millbank, London , SW 1 England Claims; 1. Method of increasing the hardness of a porous, by Immersion in an aqueous solution of chromic and sulfuric acid creates a coating on corrosion-resistant Chromium-iron alloys, characterized in that the one provided with the porous coating Metal in an oxidic deposit in the pores of the coating due to cathodic reduction a solution component of the electrolyte forming and / or an increase in the pH value in the Electrolytes causing pores in the coating during the electrolysis while avoiding a chromium deposit. is treated cathodically. 2. The method according to claim 1, characterized in that that the electrolyte in aqueous solution ions of the elements aluminum, titanium, vanadium, chromium, Contains molybdenum, tungsten, manganese, iron, cobalt, nickel, copper, zinc, silicon and tin individually or side by side. 3. The method according to claim 2, characterized in that that the electrolyte contains hexavalent chromium ions and the pH is 3 to 7.5. 813/105 4. The method according to claim 2, characterized in that that the electrolyte contains trivalent aluminum ions. 5. The method according to claim 2, characterized in that the electrolyte is seven-valent manganese ions contains. 6. The method according to claim 2, characterized in that that the electrolyte contains divalent nickel ions. 7. The method according to claim 2, characterized in that that the electrolyte contains divalent zinc ions. 8. The method according to claim 1, characterized in that that the electrolyte consists of an aqueous solution with 0.25 to 7.5 M CrO ^ per liter and a basic one There is an additive and its pH is 3 to 7 »5. 9. The method according to claim 8, characterized in that that the pH is 6.3 to 7 »5. 10. The method according to claim 9, characterized in that that the aqueous solution per liter 2.5 M CrO, and 3.5 to 5.2 M sodium, potassium and ammonium hydroxide or carbonate individually or side by side. 11c method according to one or more of claims 1 to 10, 3 Ü 9 U i i) j \ {\ S 3 characterized in that a rustproof ar provided with a porous coating Steel is treated. 12. Electrolyte for cathodic hardening of a porous, on the surface of a corrosion-resistant chromium-iron alloy by immersion in an aqueous solution of Chromic and sulfuric acid generated coating, marked by 0.25 to 7.5 M CrO ^ ge liters as well as a basic addition and a pH from 3 to 7.5. 13. Electrolyte according to claim 13, but its pH value is 6.3 to 7.5. 14. Electrolyte according to claim 13, but which is at least 2.5 M / l contains one of the bases sodium, potassium and ammonium hydroxide or carbonate.