DE1771207B2 - PROCESS FOR THE CATHODIC AFTER-TREATMENT OF AN IRON OR STEEL PRODUCT WITH A METALLIC CHROME LAYER OR A MULTI-LAYER OF METALLIC CHROME AND CHROMATE - Google Patents

Description

The invention relates to a method for the cathodic post-treatment of an iron or steel product, the one with a layer of metallic chrome or a multiple layer of metallic chrome and chromate is coated.

In a known method of this type, the cathodic treatment is carried out in a first method stage chromium coating formed in an aqueous solution containing chromic acid in the second process stage a second cathodic treatment in an alkali dichromate or alkali chromate containing subjected to aqueous solution to increase the corrosion resistance of the product.

For coatings in which the outer layer consists of chromate and the inner layer of metallic Chromium, the chromate layer shows an interference color which, depending on the thickness, creates the appealing, highly light-reflecting appearance of the metallic chrome surface is somewhat covered and looks light brown overall. Will a steel with So this discoloration painted with a sensitive paint, e.g. B. white, yellow or red, then the paint dull. The invention avoids this phenomenon. Overall, it allows a lower one Thickness of both layers, which shortens the handling time.

The invention achieves this with a method of the type mentioned at the outset in that the coated Iron or steel product in an aqueous solution of a sulfate, sulfite, pyrosulfate, halogen salt, Halogenoxy acid salt or nitrate of an alkali metal !!, Alkaline earth metal or ammonium is aftertreated.

The aqueous solution preferably has a temperature of more than about 20 ^° C. and a pH value of more than 2, the cathodic aftertreatment being carried out for more than 0.5 seconds. Preferably the aqueous solution contains less than 207 2

100 g / l de? dissolved substance and is worked with a current density of less than 50 A / dm ⁸ .

A method for improving the corrosion resistance of cathodic has already been proposed chromated metal surfaces, in which this before drying with an aqueous solution that

1 100 g / liter ammonium hydroxide, alkali or

Contains alkaline earth metal hydroxide or the alkali or ammonium salt of a weak acid, or mixtures thereof, at 30 -7O ⁰ C either 1-30 Seconds or / dm at a current density of up to 10 A * are no longer cathodically treated than 2 seconds. The method according to the invention differs from this proposed method primarily in the different composition of the aqueous solution.

If the steel product has only a single coating of metallic chromium with a thickness of 0.01 μ, then after the aftertreatment according to the invention it has a corrosion resistance which is equal to that of a steel sheet with a chrome coating of 0.06 μ thickness. Above all, the method of the invention is suitable for the post-treatment of steel products with a chromate film has been formed simultaneously with the metallic coating cathodically by the so-called S-treatment, namely one in an aqueous solution of from 15 to 6O ⁰ C with Gehs't of 10 to 60 g of chromic anhydride per liter, which contains 0.2 to 5% by weight of a sulfuric acid radical based on chromic anhydride and a small amount of a water-soluble fluoride, such as fluoroboric acid, the ratio Cr ^e * / Cr ³ ^ being about 20 to 800 is held. The aftertreatment according to the invention makes it possible to produce a material of high corrosion resistance for the production of cans in a short time and with little consumption of electrical energy.

With regard to the chromate coating, it is believed that an increase in pH in the vicinity of Cathode, as a result of the active generation of hydrogen, transforms the shape of the chromate coating and specifically accelerates the introduction of OH radicals into the chromate coating, which leads to the modification of the chromate coating, the elimination of the interference color and would favor the improvement of light reflection and corrosion resistance.

The inventive method is with a lattice conversion in the electroplated Chromium linked as confirmed by experiments, the conditions and results of which are given below are:

A) Deposition conditions for the chromium: Bath composition:

CrO ₃ 250 g / l

H ₂ SO ₄ 2.5 g / l

Electrolysis temperature 5O ⁰ C

Current density 30 A / dm ²

Electrolysis takes 30 minutes

Basic material pure

Copper plate

B) Conditions of the cathodic aftertreatment: bath composition aqueous

Solution of 1 N-Na ₂ SO ₄

Temperature of the electrolysis 50 ⁰ C

Current density 5 A / dm ²

Electrolysis duration 60 minutes

i7 71207

X-ray diffraction conditions:

Anticathode CuK *

Filter Ni

Tube voltage 35 KV

Tube current 15 mA

Slot width 2 ° - 0.1 mm -

Scanning speed 1 ° / minute

Time constant 4 seconds

Table 1
conditions

2Θ »)

Crystal structure

Chromium deposition 44.26 b. - c. (110) under the conditions 81.72 b. c. c. (211) conditions A) 135.80 b. c. c. (222) Chromium deposition 44.25 b. c. c. (110) under the conditions 68.46 H. c. p. (110) conditions A), Aftercare 81.50 b. c. c. (211) under the conditions 134.90 b. c. c. (222) conditions B)

example 1

Cold-rolled steel sheets were treated according to the following steps, whereby steel sheets were obtained, which had a high corrosion resistance and were very easy to color. The stages were: degreasing, Rinsing, pickling, rinsing, main electrolytic treatment, rinsing, post-treatment, rinsing, drying.

*) 2 Θ is an angle between the refracted ray and the incoming ray. In the crystal structure column, bcc denotes a body-centered cubic lattice and hcp denotes a hexagonal close-packed lattice.

Examples of the invention are explained below:

. Treatment conditions:
Composition of the aqueous
Treatment solution:

CrO,

SO ₁ ² -

Cr "+

Bath temperature

Current density

Treatment time

2. Post-treatment conditions

aqueous treatment solution

Bath temperature

Current density

Treatment time

45 g / l
0.3 g / l
0.5 g / l
50 ⁰ C
40 A / dm ² (cathodic treatment) 3 seconds

1 N-Na ₈ SO ₄ 50 ⁰ C

10 A / dm ² (cathodic treatment) 0.5, 1 and

2 seconds

3. Appearance, anti-corrosion and colorability of the treated steel sheets:

Tabel 2 Appearance Post-treatment conditions Time Layer weight in mg / dm ¹ 0.57 No. Current density metallic chromate chrome 0.35 pale brown no after 1.5 0.35 1 treatment 0.5 sec. 0.32 metallic luster 10 A / dm * ISek. 1.5 2 metallic luster 10 A / dm ² 2 sec. 1.5 3 metallic luster 10 A / dm ² 1.5 4th

Amount of rust in% by spraying with salt water, test (JIS Z 2371)

2 hours.

4h

6 hours 24 hours

1 0.5 2.5 5 25th 2 0 0 0 3 3 0 0 0 1 4th 0 0 0 2

4. Adhesiveness of the paint:

Good results were achieved in the tensile test and in the Dupont notched impact test, as well as in the determination the adhesive strength according to the so-called cross-cut method, in which in the surface by means of a razor and a stencil cut a hundred small squares in a checkerboard fashion; by applying and tearing off an adhesive strip it is determined what percentage of the The surface remains stuck or comes off with the adhesive tape.

Example 2

The same tests were carried out as in Example 1, with only the aftertreatment conditions were changed after point 2 as follows:

Post-treatment conditions:

aqueous treatment solution as in the

following

Tabel
specified

Current density 30 A / dm ²

(cathodic treatment)

Treatment time 0.5 seconds
Temperature 5O ⁰ C

S. 6th

Table 3 1. Conditions for chromium deposition:

~: ~~~ Bath composition:

No. Addition concentration (N) p _f Q 250 g / l

1 NaF 1 ⁵ Cr ³⁺ 1.0 g / l

* ⁽ ^ i ^{) iS} ° ⁴ J siomdSite ::::::::. '::;.':;:;"!! 40A / dm *

^wau treatment time see table 4

", ¹⁰ 2. Conditions of cathodic post-treatment:

The test pieces held up excellently at “,. "-Η,. · _Α

Salzwassersprühversuch and gave excellent bath composition Lös _Un? From

Results regarding the adhesion for paint, lN-Na.SO

they showed no interference color when viewed and _,. -tn ^ r ^{2 4}

had good light reflection. 15 bath temperature 20 C

Current density 1 see table 4

Example treatment time / _below

The corrosion resistance of the steel sheets according to Cold-rolled steel sheets with thicknesses of 0.12 to the above-mentioned conditions according to point]. gal-

1.2 mm were treated as follows: 20 were vanishly chrome-plated, but they were not reworked.

Treatment stages: have been dealt with, and those of the steel sheets,

Degreasing, rinsing, pickling, rinsing, galvanic chromium-plating and then under the conditions

chrome ¹ ), rinsing, cathodic aftertreatment ² ), _after point 2. have been aftertreated in

Rinse, dry. Table 4 compared.

Table 4

Chromium Deposition Conditions

Current density
in A / dm ¹

Time

in seconds

Post-treatment-related

Current density in A / dm ^s

in seconds

Salt water spray attempt in 2 hours

4h

6h

24 hours

I. 40 1 no follow-up treatment 10 50 60 85 2 40 3 no follow-up treatment 0.5 30th 35 75 3 4G 5 no follow-up treatment 0.5 7th 15th 45 la 40 1 20 1 2 7th 7th 30th Ib 40 1 20 3 1 5 5 50 2a 40 3 20 1 0 2 3 30th 2 B 40 3 20 3 0.5 3 5 30th 3a 40 5 20 1 - 3 5 35 3b 40 5 20 3 0.5 2 2 30th

Comment:

1. The salt water spray test (JIS Z 2371) gives the amount of rust in%.

2. In the case of chromium deposition at 40 A / dm ¹ , the chrome layer thickness was about 0.01 μm in 1 second, about 0.04 μm in 3 seconds and about 0.07 μm in 5 seconds.

The products obtained according to the invention can mainly be used for the production of

later used with a thermosetting resin paint cans. As a result of their high cohesion too

be painted. However, since they are already very polyvinyl chloride, melamine and polyester resins

Have high corrosion resistance, but can they also be called layered? Base sheets for

can be used well in practice without a final coat of paint. 50 of these resins can be used.

Claims (3)

1771 claims: 1. Method for the catholic aftertreatment of an iron or steel product, which is with a Layer of metallic chrome or a multiple layer of metallic chrome and chromate is coated, characterized in that that the coated iron or steel product in an aqueous solution of a sulfate, ι » Sulphites, pyrosulphates, halogen salts, halogenoxy acid salts or nitrates of an alkali metal, alkaline earth metal or ammonium is post-treated. 2. The method according to claim 1, characterized in that the aqueous solution has a temperature of more than about 20 ⁰ C and a pH of more than 2 and the cathodic aftertreatment is carried out for more than 0.5 seconds. ^ao 3. The method according to claim 1 or 2, characterized in that the aqueous solution contains less than 100 g / l of the dissolved substance and is operated with a current density of less than 50 A / dm ^s . "5