DE1771280A1 - Process for the surface treatment of steel - Google Patents

Description

H. LEINWEBER dipl-ing. H. ZIMMERMANN

• MUndMit 2, Rosental 7, 2. Auf «.

Tel.-Adr. UtopatMfeciMII TeMM

Posl «ch« ck account; Munich 22045

* "October 7, 1970

To "r ZeIdMn XIII / C

KOKAd KABUSHIKI KAISEA,
Tokyo / Japan

Processor: for surface treatment of steel

The invention relates to a method for surface treatment of steel to produce a coating with excellent adhesive properties and / or corrosion resistance. To produce this coating, a circulatory cell ".frwenoen" can be used, in which the electrolytic reduction occurs during a Circulation speed of the electrolyte of 70 m / Hin at the overriding the steels to be clad is listed. Further lets sic: apply an ordinary stationary cell in which '; ι ο eioctrolytic reduction with a stationary electrolyte is performed. The electrolyte used is mainly ghromiiiuro etitnultencie solution applied with thiosulfate, sulfite, Ldthionuifit, ßisuifit, thioculphite, pyrosulfuric acid, sulphide

1098A8 / 150 ¹ / - 2 -

Jö L /. Köuagen (Art. 7 | l.At) ». 2Nr.1SaU3tto8Anütrungsge3. ν. 4.196.

BAD ORiGiNAL

and the like is added as an aid.

Until now, corrosion-resistant films were deposited on the surface of steel with hydrated chromium oxide as ^essential: manufactured Iiehern ingredient, as Hauptbestand- "; partly chromic acid or chromate with a relatively low concentration and various auxiliaries were used as additives; to produce this coating, it was baked or electrolytically reduced.

As an auxiliary agent, besides sulfuric acid, the following compounds are known to be effective: tartaric acid, maleic acid, oxalic acid, aromatic sulfonic acids having a hydroxyl group, sucrose, glycol, etc. Film available metallic chrome only 70 to 80 # of the total ^! amount of chromium precipitated; the remainder, $ 20 to $ 30, are present as hydrated chromium oxide. A coating film electrolytically produced in a conventional manner therefore generally has a coating layer of metallic chromium and hydrated chromium oxide layers on the surface of the steel, regardless of the various electrolytic conditions used or the various auxiliaries. Although such a double layer contributes to the corrosion resistance, difficulties arise in terms of the adhesiveness or the color tone of the coating. According to the method according to the invention, a coating of hydrated chromium oxide or metallic chromium as the main component can be produced in a simple manner, with a chromic acid solution having a comparatively lower amount

- 3 -1098A8 / 1507

BAD OR! GJM _Al

Concentration used with that described above! new auxiliary materials are added.

The formation of a double layer in the conventional electrolytic process is due to the following reaction mechanism In addition to the metallic chromium, the conventional process also produces a hydrated one Chromium oxide precipitated because it was not possible to reduce the pH value increase at the cathode under the ί in the commonly used plating cells.

According to the method according to the invention can be a; Coating of great quality with seven kinds of auxiliaries can be achieved, either alone or in a mixture! be admitted. In the known method, an electrolyte with sulfuric acid as an aid is inserted into a cell! out, in which previously trivalent chromium was formed by electrolytic ί reduction. In contrast, in the process according to the invention, trivalent chromium is formed in the cell by reaction with the auxiliaries used according to the invention, so that electrolytic reduction is not necessary. The coating according to the invention obtained has a greater bonding strength and corrosion resistance ί than the known coatings, as is apparent from the following \ description in detail. ι

According to the invention it was possible to produce a film consisting almost exclusively of metallic chromium, a plating cell having an ideal flow state is applied using this means; the ι

109848/1507

electrolytic conditions and the bath temperature fjov / ie the chromic acid solution correspond to conventional conditions (application of the known electrolytic conditions, which: the solution contains the chromic acid in such a low concentration as in the formation of a film of hydrated ^: chromium oxide as the main component); a film made of hydrated chromium oxide was also made using the same tool and a stationary plating cell.

The o _r-making is now erläuert with reference to the reasoned description "and the drawing.

In the drawing means:

Fig. 1 Photographs of electron diffraction at the

electrolytically deposited according to the invention

. coatings.
At Soiel 1

In Fig. 1 are photographs of electron diffraction shown at 100 KV, with the following requirements applied became:

a)

ί ^Crö 3 50 g / l Ka ₂ S ₂ O ₃ .5 K ₂ O 0.75 g / l ι
^; Flow rate 70 m / min. b) Fig. 1-2: CrO, -
j 50 g / l
0.5 g / l FIi tight speed 70 m / min.

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BATH

It was found that these diagrams are almost identical to the diffraction diagram of metallic chromium ASTK-Stondard p. According to Table 1 below.

Table 1

CrO ₅ H-Na ₂ S ₂ 0 ₅ '5 H ₂ O C _r 0 ₈₊ H ₂ S0 ₄ I. ASTM (Cr ⁰ ) dÄ I. dÄ V.S. d £ I. 2.08 V. S. 2.04 S. 2.0390 100 1.45 V.S. 1.45 V.S. 1.4419 16 1.19 V.S. 1.18 M. 1.1774. 29 1.03 M. 1.02 S. 1.0195 17th 0.92 S. 0.91 V.W. 0.9120 21st 0.84 M. 0.83 S. 0.8325 6th 0.78 S. 0.78 V.W. (0.774) S. 0.72 V.W. 0.78 W. (0.722) V.W. 0.6 y H. 0.69 (0.682) M.

Note: JJ refer to values given in brackets

on a Cr-film applied in a vacuum.

10 9 8 4 8/1507

BATH

Example 2

CrO ₅ 50 g / l

Ka ₂ S ₂ 0 ₅ '5 H ₂ O 0.75 g / l

ρ current density at the cathode 30 A / dm

Temperature 5O ⁰ C

Time "3 sec.

Example 3_

CrO ₅ 50 g / l

Na ₂ SO ₅ 0.5 g / l

Current density at the cathode. 30 A / dm

Temperature 5O ⁰ C

Time 3 sec.

In both Example 2 and Example 3, a film consisting of metallic chromium formed underneath Use of a circulation cell. A film consisting essentially of hydrated chromium oxide "formed on the other hand, if a stationary cell is used.

Through further experiments it was found that one metallic chromium film can easily be produced using a flow cell, even if one uses the known Help reducing additives and a bath concentration used to produce a hydrated oxide film.

- 7 109848/1507

BATH ORIGINAL

According to the invention one can essentially made of metallic chromium with a chromic acid content usually used for the production of a coating made from hydrated chromium oxide. The auxiliary additives used here were not previously LiIs additives to predominantly chromic acid Plating solutions known; in detail are here Thiosulphate, dithiosulphite, bisulphite, sulphite, thiosulphite, pyrosulphite, sulphide, etc. as well as a circulation cell jewandt.

na ftf it ti gke i t s-_und_iio s t test

The following table compares the results of cost tests with coatings on two steel panels:

Table_2

Comparative results regarding the adhesive strength of

Coatings

A *
B ** Bending test Cross cut
Adhesive
ability test Erichsen
test Sctilag-
consolidate
ability
test Total-
beurtei
lung 20th
20th 10
2 8th
1 6th
1 50
24

-δ-

109848/1507

BATH ORIGINAL

Remarks:
A *:

The solution contained 50 g / l CrO ₇ and 0.75 g / l

B **: The solution contained 50/1 CrO ₅ and 0.5 g / l H ₂ SO ₄ .

In both cases A and B, the coatings were produced with a stationary solution.

The evaluation of the adhesive strength of the coating according to the values listed in Table 2 was carried out according to the following Rating scale:

no change in the surface

of the coating 20

somewhat wrinkled at the bent points 16

very wrinkled at the bent points τ .... 12

occasional occurrence of breaks in

the coating δ

Peel off the coating 2

Cross cut bond strength test:

100 tests without peeling 20

99 to 90 16

89 to 80 12

79 to 50 8

less than 50 exams 2

Erichsen test:

7.01 mm and more 10

7.00 to 6.01 mm S.

6.00 to 5.01 me, 6

5.00 to 3.01 mm 4

109848/1507

BATH ORIGINAL

■ '"If 1!

less than 3.01 mm

Impact test:

no peculiar swelling of the coating occurring cracks in a part of the coating.

cracks occurring all over the coating ...

Peel off the entire coating

Furthermore, the host test was carried out at a temperature of 50 C, a relative humidity of 100 $ and a _ A duration of 8 hours was carried out for the electrolytically treated steel plates A and B. With the one described first Only a few rusty points were found on the test

observed during surface of the plate, / at the latter

Test countless' host points were observed.

From the following Table 3 is a numerical comparison of the host test procedures for the two samples evident. The evaluation in each test was carried out by subtracting minus points from the full evaluation number 10.

Table 3
comparative evaluation between two rust testers

deducted points deducted points

as a result of rust formation due to the

Food density

kB 6

h 1 4

- 10 -

109848/1507

BATH ORIGINAL

- ίο -

The ftost test described above was continued for 24 hours. In the case of plate A, only occurred an increase in the rust spots of about 30 ^ and it took place practically no decrease in the gloss of the surface. The plate ß, on the other hand, rusted heavily on the entire surface and the surface gloss was completely lost.

Although various types of circulation cells can be used, it is sufficient if the increase of the pli value at the interface of the strip in a such cell can be suppressed; a further improvement in efficiency does not occur if the Flow velocity is also increased. The amount of the chromium oxide film increases with a flow rate less than 5 m / min. The chromic acid content is preferably of the order of about 50 g / l; the procedure ict but also with a weight of 20 g / l or more « manageable. j

Due to the inventive method can be ι so a as such corrosion resistant hvdratisierter ■ '■ chromium oxide film or a substantially bestehenaer of metallic chromium film having excellent color tone and ■ excellent adhesiveness prepared in a simple V / else: wherein a is provided with the additives described above chromic acid solution applied with a relatively low concentration and the flow state of the bath is regulated.

- 11 -

10 9 8 4 8/1507

BAD OP. '. G'.NAt

Claims (2)

Patent claims: 1. A method for surface treatment of steel, characterized in that one has a low ! ■! Close one or more of the compounds thiosulfate, iiithiosulfite, bisulfite, sulfite, thiosulfite, pyrosulite or adding sulfide as an auxiliary to chromic acid and performing an electrolytic reduction with this solution at the cathode. 2. The method according to claim 1, characterized in that the electrolytic reduction in a circulation cell. üe omit - ^ ¹ ^ bS .: .-. l sentence 3 of the amendment. v. 4. 9. Ij '. 109848 / 1B07 BATH L ^ ·, blank page