DE2335555C3 - Process for the surface treatment of tin-plated steel with a solution containing chromium (VI) ions - Google Patents

Description

The 55 thus provided with a protective layer can be seen that by the inventive However, tin-plated steel sheets can extremely heat the chromate-treated tinplate Requirements not yet meet. to a temperature that is above the melting

From the magazine "Werkstoffe und Korrosion" ratur des tin lies, a passivation effect (1951, Issue 8, pp. 289 to 292) is known that will call the treated material an extra-tinned sheet for packaging purposes with the help of 60 properly high resistance to attack by chromates such as potassium dichromate, passivated sulfur is granted. In addition, according to the invention, the ability to obtain protection against corrosion-treated tinplate in this way has a greatly reduced tendency to attack. The protective effect on the solution of the tin under attack by fruit is produced by an electrochemical process, acids, an excellent adhesion for paints, the so-called anodic polarization. 65 and the like and an attractive gloss.
The pH of these chromate solutions is between Preferred embodiments of the invention

7.5 and 9.5. The passivation treatment is carried out with the heated detail hereinafter by way of examples to about 8O ⁰ C solutions by dipping explained.

When carrying out the method according to the invention is also known as a base plate cold-rolled sheet of low-carbon steel electrolytically cleaned, rinsed with water, with acid pickled and rinsed again with water, as is customary with conventional tin electroplating. It is It is common to electroplate the tin film immediately after application by a reflow treatment to undergo. In contrast, according to the invention, the melting treatment after Chromate treatment made. The chromate treatment can be performed under the following conditions, for example:

15th

Bath composition .. aqueous solution of 10 to 50 g of sodium dichromate per liter

20 pH 3.0 to 5.0

Temperature room temperature to

70 ⁰ C

Current density 0.1 to 10 A / dm ²

Treatment time 0.1 to 3 seconds

Polarity cathodic electrolysis

using tinplate as cathode sodium carbonate, sodium dichromate. The anodic Treatment with sodium carbonate results in the formation of an oxide film, but such an oxide film is Heating not converted into a strong sulfur-resistant film. It is believed that the presence a small amount of chromium oxide on the electroplated tin film is the cause of the formation is a surface layer that has high sulfur resistance

According to the invention it is also possible after heating the tinplate (the tinned iron sheet) subject to a so-called chemical treatment. The chemical treatment makes the Sulfur resistance is further improved and excessive dissolution of tin is suppressed. Under this chemical treatment is the usual cathodic treatment in an aqueous dichio-nate solution to understand immediately after the reflow treatment is applied.

There were comparative tests with respect to various properties of according to the invention Process treated tinplate and usual tinplate. These attempts included sulfur resistance, tin dissolution, porosity (surface pore), thiocyanate tests, the Dissolution of iron, the oxide film formation, the Löibarkeii, Expressed by the capillary rise, the paint adhesion and the surface gloss. the Results are given in Table 1 below. To carry out the tests, the The following eight types of test specimens a to h are used. The tin plating of the samples was 0.227 kg / Basic set. The melting treatment was carried out under the same conditions in each case.

35

The chromate treatment can also be carried out by immersing the tinplate in a chromic acid solution be performed.

After the chromate film has been formed, the tinplate (the tinned iron sheet) is heated to a temperature above the melting temperature of the tin, for example by resistance heating with an electric current passed through it, to a maximum of 400 ^° C., preferably to 280 to 350 ^{° C.} After the completion of the heating, the tinplate is cooled. Heating to a temperature above 400 ^° C. leads to an unacceptably poor deformability of the same. By a heating temperature of more than 28O ° C, the sulfur resistance is improved, but the moldability of the white sheet is prepared by heating to more than 350 ⁰ C impaired.

In the chromate treatment according to the invention, the tinplate can also be used as a cathode instead of Anode or alternately as a cathode and anode. In this case, the sulfur resistance becomes improved. However, because of the formation of an oxide layer before the reflow treatment, dei Tinplate geting shine. Accordingly, if a high gloss is desired, this is the last one mentioned chromate treatment not suitable.

The reason why the sulfur resistance is improved by the treatment of the present invention is not yet exactly known. However, on the basis of tests it was found that the treatment according to the invention achieves better sulfur resistance than anodic Treat the tinplate in a solution of

a) Tinned steel sheets were made by cathodic electrolysis in a sodium dichromate solution chromate treatment and then resistance heating to 290 "C by direct Application of an electric current subjected to a melting treatment.

b) When treated in the same ^dir: Nnten steel sheets cathodic chemical treatment was applied in a sodium dichromate solution having a concentration of 30 g / l at 45 C for one second, choosing the current density at 5 A / dm was maintained. ²

c) Tinned steel sheets treated in the same way as in a) were subjected to cathodic treatment in a sodium dichiomat solution having a concentration of 30 g / l at 45 ° C. for one second while the current density was kept at ^{1 A / dm s.}

d) Tin-plated steel sheets were Immersion diver in a chromic acid solution (30 g / l) at 8O ⁰ C a: subjected to chromate treatment, then wurdi reflow applied un <thereafter, the sheets were as far heritage commercially as in c).

e) Tinned steel sheets were subjected to anodic cathodic electrolysis (cathodic treatment; at 5 A / dm ² for a period of 1 second and anodic treatment at 1 A / dm ² for a:

5 ⁶

Period of 0.25 seconds) in a sodium solvent such as trichlorethylene and tetrachloro

dichromate solution (30 _pounds / l) of a chromate treatment carbon, purified. The cut edges of the test

was subjected to treatment, and then a patch was sealed by wax and

foam treatment by _{e guides.} then in d, e above-mentioned corrosive solution

* ^ε 5 (250 ecm) for 15 minutes at 20 to 25 C

f) Tinned steel sheets were you, ch anodic dips. The color of the solution after immersion

Electrolysis (1 A / dm », 1 second) in a Na- was determined by means of a colon meter. D.e Kon-

trium dichromate solution (30 g / l) at 45 ° C with a concentration of E.sen.onen, expressed as mg / dm

Subjected to chromate treatment, and then e.ne was separately prepared on the base

a melting treatment is carried out. «Calibration curve determined.

$) Tinned steel sheets were used in a melting

subject to treatment. Porosity (surface pore) test

h) Tinned steel sheets were melted 15

treatment, and then by rjj _e samples were placed in a] 0% ipe for 5 minutes

cathodic electrolysis (5 A / dm ^2, 1 second) is immersed in chromic acid solution of 90 ⁰ C. After

a sodium dichromate solution (30 g / l) at 45 ⁰ C thorough washing with water were the samples

chemical treatment carried out. For 49 minutes in distilled water with a

^{Immersed 20} pH of 4.5 at 95 ° C. After that

In each test, the following procedure was followed- the number of rust stains with the bare

^turns: eyes counted.

Sulfur resistance test ₂ ,

Fisen solution test (iron solution wide ISV)

The samples were placed in a for 20 seconds

Test solution immersed at 95 ° C, and the blackening,. ". . ,,. _XI ...

by sulfurization was visible to the naked eye definitely. The test solution contained 8 g 30 iron per liter in a corrosive solution among the

Potassium polysulfide and 2 g sodium hydroxide. The conditions under which the tin is somewhat stronger

Test results were determined as the iron is anodic in the following 5 hours. The corrosive

divided: the solution contained the following components:

35 NB · no blackening at all,

Sulfuric acid, 2.18 η 23 ecm

DB: barely noticeable blackening spots, ammonium rhodanate, 40 g / l 25 ecm

SB: weak blackening spots, 40 hydrogen peroxide solution, 3% · · · · 2 ecm

EB: slight blackening over the entire upper part. Total ... 50 ecm

surface,

HB: strong blackening. 45

Circular test samples with an effective surface area of 2581 mm ^{2 were} punched out in a 0.5% aqueous sodium carbonate

Thiocyanate test _{solution anodized for 30} seconds at 1.5 A,

_. ^. 50 washed with water and using

This test is used for the quantitative determination of acetone dried

the amount of iron ions present in a corrosive _{The test sample was put into the} above

Solution of the exposed steel surface by the _corroding n _d e solution containing 150 cc glass top flat pores of the tin layer dissolved out advertising _ge f _ä ß introduced, and with a thread-provided to, by conversion of the iron ions in red Th.o- _{55 The plastic} lid was rotated so that the test cyanate fui de analysis by means of a colon meter. _be together. To prevent a

The corrosive solution contained the following _leakage i _{ness was an} iron-free seal comparable ^elements: turns.

At the time of the colorimetric measurement were

Ammonium thiocyanate 20 g / l ⁶ °? ecm added to the hydrogen peroxide solution. the

P ... in / 1 above corrosive solution became 2 hours

acetic acid IU g / l _{] ang at 2} 6.7 ° C with 2581 mm ² surface area of the

Hydrogen peroxide solution (20 volume test sample contacted under pressure. Then

percent) 10 g / l use the colorimeter to determine the amount of

65 dissolved iron, expressed in μg / cm ^ϊ , determined. When the amount of iron so determined increases (large

Rectangular test specimens of ISV size were used, the tinned steel is subject to a stronger one X 8 cm made and with an organic corrosion.

Tin dissolution test

The samples were vertically immersed in 100 cc of a corrosive solution at 25 ^C C for a given period of time. The difference between the weight of the test sample before and after immersion was determined, and from this, the dissolution of tin in the solution, expressed in ppm, was calculated.

Condition a

Degassing the solution:

Heat degassing treatment in an argon gas atmosphere.

Corrosive solution:

0.4 ° / ₀ citric acid (pH 2.8), NO ₂ - N (see the following Table V below.): 100 ppm (0.61 g / l KNO _2).

Test sample dimension:

2.5cm x 10cm (total surface size 50cm ² ). Edge sealing of the test sample:

None.
Immersion time:

120 minutes.

Condition B

Degassing the solution:

None (the oxygen is dissolved in equilibrium with air).

Corrosive solution:

0.4 ° / _o strength Zitronensäuie (pH 3), NO ₂ -N (. See Table V): 20 ppm.

Test sample dimension :

3.0 χ 10 cm (total surface size 60 cm *). Edge sealing of the test sample:

Sealed with wax. Immersion time:

60 minutes.

Oxide layer

The thickness of the oxide or chromate layer was expressed by the amount of electricity required to reduce the oxide or chromate on the upper size unit. For this purpose, the electrolytic reduction method with an electrolyte made from a Ο, ΐη potassium chloride solution was used. A test sample having a given surface area was used as a cathode supported by a holder, and a constant current was allowed to flow through the cathode and a carbon rod anode. Characteristic stress-time curves were obtained, which were used as the basis for determining the thickness of the oxide layer.

Soldering wedge test (capillary rise)

This test is used to check whether solder has penetrated the solder joint on the side of a tinplate can can. Rectangular test specimens each 75 × 25 mm in size were produced. the The lengthwise direction of the rectangular test specimen coincided with the direction of movement of the steel during tin plating together. Each test sample was folded so that it formed a capillary section extending from the tinned surface of the sample was surrounded. The sample was to a depth of about 5 mm in a Immersed in palm oil, which acted as a flux. Then the test sample was made at a given temperature immersed in solder bath to a depth of 30 mm for one minute. After taking it out In the solder bath, the sample was quickly cooled with water.

The solder consisted of 70% lead and 30% tin and the temperature of the solder bath was 277 ^° C. The folded section was opened after the rapid cooling and the capillary rise above the upper surface of the solder bath was determined. A large value of the capillary rise means that the tinplate has good solderability.

Paint adhesion

In each test sample, a Epoxyphenolfirnis was applied and 10 minutes baked at 200 ⁰ C for. The painted surface was scratched in the form of a grid. The scratched sample was subjected to a spherical stretch deformation to a depth of 5 mm using an Erichsen deep-drawing test machine with a spherical punch (diameter 20 mm). Attempts were made to pull off the paint layer by means of an adhesive strip. Those test samples that could not be peeled off were rated "very good".

shine

The gloss of the tinplate samples treated by the method according to the invention was determined and in ° £, based on the gloss of the comparative sample from common tinplate, expressed.

As can be seen from Table I below, the treated by the process according to the invention, i. H. the chromate treatment immediately after the tin plating and the subsequent Tinned steel subjected to melt treatment sheet has a much higher sulfur resistance than conventional tinned steel sheet without one chemical treatment. In addition, the number of

609 647 in the

9 10

Surface pores, determined by the hot water when the tinplate is heated, only one test, after the treatment tin coating according to the invention, showed no improvement in welding to half to a third. resistance has been achieved, one can assume that The test value of the thiocyanate test can also be determined by the fact that the chromium oxides deposited on the tin layer by the chromium hand using the treatment method according to the invention are reduced to an extent when heated two thirds are reduced. Form a layer that is highly resistant to sulfur.

On the other hand, the tin dissolution, the solder The invention is illustrated by the following examples

availability (the capillary rise), the paint adhesion and explained in more detail the gloss of the tin-plated steel sheet is not impaired by the treatment according to the invention

When the chemical treatment, i.e. H. the cathodi- Example 1

see electrolytic treatment with a sodium di-

chromate solution, after the melting treatment in base sheets made of a cold-rolled, carbon-

the method according to the invention is used, poor steel were electrolytically cleaned, the sulfur resistance can thereby be greatly improved by rinsing with water, by pickling with acid those of a conventionally treated in a similar way and pretreated by rinsing with water, then tional tinplate are what is plated with tin (5.6 g / m ¹ ) and again with water from Table I. The chemical rinsed off. Then the tin treated in this way after the melting treatment is also plated steel by cathodic electrolysis in an effective manner in terms of reducing the number of surface pores containing 30 g of sodium dichromate per liter and drastically reducing the surface pores at 45 ^° C a current density of 5 A / dm * 0.5 Se thiocyanate test value to less than half. have long been subjected to chromate treatment. In addition, the chemical treatment can then be followed by electrical re-heating after the melting treatment by direct application of an electrical improvement in relation to the dissolution of the tin. excessive subjected, and afterwards it was completely eliminated by cathodic dissolution of the tin, which can be electrolysis in a sodium dichromate solution. Due to the chemical treatment after the uptake of 30 g / l at 45 ° C. at a current density of 5 A / dm ¹ melt treatment, the other properties of the tinned steel sheet are not influenced, in particular carried out The sulfur resistance of the special in this way is determined by the iron dissolution, the solder-treated tinplate was determined conventional

It should be noted that when given the tional way treated tinplate. Chromate treatment by anodic-cathodic elec- 35 trolysis or anodic electrolysis,

the sulfur resistance itself compared to that η -; _s _: _c j ₂

can be improved, which can be achieved by cathodic electrolytic chromate treatment. The anodic metal sheets made from a cold-rolled, carbon-cathodic electrolytic chromate treatment, were treated by electrolytic cleaning, treatment and anodic electrolytic chromate rinsing with water, acid pickling and rinsing treatment, however, lead to an increased thickness of the pretreated with water, then tin-plated oxide layer, and in the subsequent Aufschmdz · (5.6 g / m ») and rinsed off again with water. The treatment can then not achieve the desired gloss this way tin-plated steel was achieved by the tinned steel sheet. ₄₅ cathodic electrolysis in a solution that is per liter

Immediately after the _j ^ ™ g by electro- 30 g of sodium dichromate is contained, at 45 "C with a phttierung is on the by electroplating on- current density of 0.5 A / dm * 0.2 seconds of a tin applied without This was then subjected to a melting chromate treatment. Then, a chromate coating was subjected to a chromate coating and an electrical resist pickling was formed by direct application

Q mraige leads to a chemical by cathodic electrolysis in a needle solution with 30 g / l at 45 ° C at approximately 1 A / dm * for a period of

^^^^ JZ ^^ T ^ S ^^ ^ g ™ ** «*« »are Letasarttel-KonseTvefr

mdrtertatet ™ ^ and WqBbkcbc vcr- bushings nA an inner volume of lucern,

turns, «If only one threatens Eleküophuiauiy beginning with a diameter of 65.4 mm and a height of

^^^ S ^ SS ^ S ^^ !? "5 ⁷ ^ ^l « 2 ^cs ^ ^{; wkoaratk} »« ^ WeiBbfcxliea

^ 2 ^{; wkoaratk} »« ^ WeiBbfcxliea emer conventional chemical treatment ^TOte "? G? ™« fa »wading, were

r tin eκ »<» π # u · ηΐιίΐυ __ n ^ ut. .k. ir ». etektroplatrienmg mit Chromt bdudete »Weiß- mS ^ sl2S. ^^ Ver

! ίίΐ ^ y * £ ^ ⁰ J ^ y? * ^ * ⁵ ? * -. Μ «Tin cans were sold under V

CteilkD

! ίΞίΐ, ΐΐ ^ y? " * £ ^ ₁ J ^ y? * ^ * ⁵ ? *. Μ" Cans have been made using temperat ur des Zmns begende 1 ^ ", ^ tnr cmc Vw- S ₅ emer CysteincnlorkDösnng and emer citric acid

imle

d ^^ eaerangdCT ^ bnieWUesbuadq ^^ hislier fcn. The received yet exact revenge known Due to the fact that Tabdfcn IV and

gfS b ^ S ^^ ZmWmlöSnitesls imlewor fcn. The results obtained are in the ft * te Tbd i ^^

: Treatment of the 11th sample (d) 23 35 555 pH value temperature current ratui : density Treat ω Treatment) (h) 12th Thio porosity lungs time cyanate over tampering (without careful treatment) (g) Sulfur- 1 CsL ° C A / dm ¹ persistent (Upper Table I. 45 5.0 S. age) surfaces sample 4.5 45 5.0 1 mg / dm * pores / cm * 4.5 45 5.0 1 4.73 100-150 4.5 45 5.0 1 4.63 100-150 4.5 45 5.0 1 SB 4.01 50-100 3.0 45 5.0 1 Äbhche Zhmplattienmg (with chemical treatment) (h) DB 4.25 50-100 No. Chromate treatment before melting treatment 4.0 45 5.0 1 usual ziaapiaiaarimg (with chemical DB 4.25 30-50 1 Conc 5.0 45 5.0 1 DB 4.57 50-100 2 Tin plat- (a) tration 6.0 25th 5.0 1 DB 4.73 50-100 3 tion 4.5 35 5.0 1 DB 4.57 50-100 4th after 4.5 1 DB 4.17 50-100 5 invention cathodic g / I 45 5.0 DB 4.09 50-100 6th according to electrolytic 10 4.5 55 5.0 1 DB 7th proceedings Chromate 30th 4.5 65 5.0 1 DB 4.82 50-100 8th treated treatment 50 4.5 75 5.0 1 4.72 50-100 9 Tinplate I. 70 4.5 45 1.0 1 DB 5.46 30-50 10 30th 4.5 45 2.5 1 DB 5.54 30-50 Melting 30th 4.5 45 5.0 1 DB 4.20 30-50 11th treatment 30th 4.5 45 10.0 1 DB 4.01 30-50 12th 30th 4.5 30th 5.0 1 DB 4.09 30-50 13th 30th 4.5 45 5.0 1 DB 4.57 30-50 14th after Tin plating (b) 30th 4.5 30th 5.0 1 DB 2.31 0-5 15th invention
according to tion
i. 4.5 45 5.0 1 DB 2.31 0-5 16 proceedings T
cathodic 30th 4.5 1 NB 0.94 0-5 17th treated electrolytic 30th 45 0.25 NB 1.91 0-5 18th Tinplate or immersion 30th 4.5 45 0.5 0.2 NB 19th Chromate
treatment 30th 4.5 45 1.0 0.2 NB 3.60 0-5 20th 4th 30th 4.5 45 2.5 0.2 3.46 5-10 21 Melting (c)
treatment 30th 4.5 45 5.0 0.2 DB 3.13 0-5 22nd j. 30th 4.5 0.2 NB 2.85 0-5 chemical 30th 80 NB 2.50 0-5 23 treatment 30th 0.7 immersed NB 24 30th Zmnplattkrung - »- cathodic-anodic electrolytic (s) NB 4.32 20-30 25th comparison 50 Chromium treatment -> melting treatment 4.98 10-20 26th 50 Zina plating - »■ anodic electrolytic DB 27 Oif <Hiiall> elmndhing - * ■ Aufxchuiefalhehandlung NB 5.38 150 or 30th more 28 30th NB 6.60 150 or 29 30th more 30th HB 4.01 10-30 30th 30th 4.21 20-30 EB 31 30th HB 32 33

13th 14th

Table I (continued)

μg / cm ^ϊ

ISU in the tin dissolution of iron

mg

ppm

Oxide layer Capillary rise height of the solder

mc / cm ¹ mm

Paint adhesion gloss

Overall rating

1 - 2 - 3 - 4th - 5 - 6th - 7th - 8th - 9 - 10 0.30 11th 0.51 12th 0.48 13th - 14th - 15th - 16 - 17th - 18th - 19th 0.58 20th 0.51 21 0.51 22nd 0.29 23 - 24 - 25th - 26th - 27

28 -

29 - 30th - 31 0.55 32 0.55 33 0.55

14.6 146 0.96 12th 13.0 130 0.89 12th 13.3 133 0.85 13th 13.2 132 0.92 10 13.8 138 0.96 12th 14.0 140 1.06 12th 14.1 141 0.99 12th 14.3 143 0.85 10 13.7 137 0.99 12th 15.6 156 0.96 13th 12.5 125 0.92 12th 14.6 146 0.92 12th 14.4 144 0.89 11th 14.1 141 0.81 11th 15.0 150 0.98 12th 14.4 144 0.99 12th 10.9 109 0.81 12th 14.3 143 0.71 13th 9.0 90 0.39 14th 8.6 86 0.39 15th 8.8 88 0.35 10 8.4 84 0.32 8th 2.5 25th 1.27 13th 1.8 18th 0.97 12th 1.5 15th 0.54 12th 0.8 8th 0.64 12th 1.0 10 0.48 12th

2.3

B - -

1.03

*) NB: no blackening at all,

DBr barely noticeable blackening spots, SB: weak blackening spots, EB: weak blackening over the entire surface, HB: strong blackening.

- 1.60 16 - - 1.73 14th 15.3 153 1.66 14th 14.1 141 1.88 15th 5.5 55 1.00 12th

very good 99 pretty good very good 97 Well very good 98 Well very good 93 not good very good 102 Well very good 101 Well very good 102 Well very good 100 Well very good 93 Well very good 100 Well very good 99 Well very good 96 Well very good 92 pretty good very good 93 pretty good very good 100 Well very good 98 Well very good 99 Well very good 98 Well very good 96 Well very good 99 Well very good 96 Well very good 93 pretty good very good 101 Well very good 99 Well very good 99 Well very good 97 Well pretty much
bad 91 pretty much
bad pretty much
bad 100 pretty gt Well 62 bad Well 52 bad Well 100 bad Well 100 bad Well 100 bad

15th

Table Π Duration ccs Sulfur resistance of the test samples 50 "C 240 ° C heating on**·) Test sample*) permanent
Testing**)
S. without
Heat DB
SB NB
DB 28O ⁰ C 320 ⁰ C 20th
40 DB
SB HB NB
NB NB
NB A. 20th
40 B.

*) Test sample A: Immediately after the electroplating with tin (0.68 kg / basic set), the cathodic electrolytic Chromate treatment carried out in a sodium dichromate solution.

Test sample B: Sheets which only had a tin addition applied by electroplating (0.227 kg / basic set). * ♦) The sulfur resistance test was the same as that of Table I. **) The rating levels for sulfur resistance are the same as those in Table I.

Table III

treatment

Resistance to sulfur, according to the

Cysteine chloride method

Sodium Thiosulfate Process

Potassium Polysulfide Process

According to the invention tin plating -> ■ chromate normal normal normal

treatment by cathodic electrolysis

-> Melting treatment ■ -> chemical treatment

Conventional tinplate (0.227 kg / basic set) considerable considerable considerable

with chemical treatment, darkness, darkness

Table IV

Test in a cysteine chloride solution (2 g / l)

treatment

Tin dissolution (ppm)

immediately after storage at 38 ° C after the heat- over a period of sterilsaiion _χ ^ _q ^ _{2 weeks 3 weeks}

Corrosion on the inside of the can from sulfur

immediately after storage at 38 ° C after the heat for a period of sterilization

1 week

2 weeks 3 weeks

According to the invention 9 55 61 80 Conventional 21 103 128 is;

normal normal normal

considerable darkness

weakly colored

Table V

Tests in the citric acid solution

treatment

Corrosive solution

Tin dissolution (ppm)

immediately after 24 hours

after heat sterilization

Laeerung at 38 ⁰ C

Citric acid pH 3.0 according to the invention

Citric acid pH 3.0, + NO ₂ - N 5 ppm *) Citric acid pH 3.0, + NO ₂ - N 10 ppm Citric acid pH 3.0, + NO ₈ -N 20 ppm

Conventional citric acid pH 3.0

Citric acid pH 3.0, + NO ₂ - N 5 ppm Citric acid pH 3.0, + NO ₂ - N 10 ppm Citric acid pH 3.0, + NO ₂ - N 20 ppm * ■> NO.-N stands for the amount of nitrogen which was added in the form of potassium nitrite.

42 79 609 647/303 76 105 121 184 126 200 79 116 116 131 184 231 210 263

Claims (2)

1 2 or electrolytically, the electrolysis ♦ Λ taking place with cathodic connection of the patent claims to be protected: subject. It should be emphasized that this known passivation treatment does not go hand in hand with a heat treatment 1. Process for the surface treatment of From GB-PS 5 47 408 a process is known - ,. rolled steel for cans and the like, in which a chromate charge with intermediate softening the tinned Siahl immediately after the connection of an extremely thin intermediate layer, the Tin plating to increase the resistance to tin can consist of one to be protected Sulfur is applied through the formation of a protective layer io ferrous material. This between one Treatment with a chromium (V!) Ion layer, preferably made of cadmium or nickel containing aqueous solution is subjected, has a maximum thickness of 0.0025 mm and characterized in that the itself is not an actual protective layer, since it Chromate-treated tinned steel on a top- merely serves as a means to attach the chromate layer half of the melting temperature of tin is 15 there? Iron or tie the steel. At this temperature is heated. Known processes are the sheets to be protected 2. The method according to claim 1, characterized after electroplating in a chromate-containing records that the tinned steel is immersed after the bath and then at a temperature Heating a cathodic treatment in a sublayer aqueous solution below the melting temperature Dichromate solution is subjected. 20 metals heated. A method is already known from DT-OS 19 22 820, with the aid of which a primer for layers to be applied subsequently is produced on aluminum foils or sheets. This method provides a 25 beidseitigcs applying chromium trioxide, preferably in the form of a 0.5 to 10 ° / ₀ aqueous solution before, followed by hot air drying: in the Following the application of the chromate solution is heated the aluminum foils or sheets 30 temperatures of 250 to 500 ⁰ C. The oxide layer obtained after this treatment forms a recorded primer for paints, adhesives, printing inks, coatings and the like and at the same time improves the adhesive strength when The invention relates to a method for the effect of aggressive media and simultaneous single-surface treatment of tinned steel for the preservation effect of higher temperatures.
vendose and the like, in which the tinned steel The DT-AS 12 08 149 can also be seen that immediately after tinning to increase the chromate protective films by electrolytic means Resistance to sulfur can be applied by forming a tinplate. Protective layer of a treatment using 40 The invention is based on the object To create an aqueous solution containing chromium (Vl honing) of the type mentioned above, is subjected. with the help of tinplate can be produced, which Such a method is already known from GB-PS in its properties to the conventional 5 24 476. Tinplate produced with this known method show superior
a tin-plated steel sheet (tinplate) is used for the 45 This object is achieved according to the invention by avoiding discoloration under the influence of the fact that the chromate-treated tin-plated steel on sulfur, which is particularly undesirable in cans and the like above the melting temperature of the tin, a temperature that is subject to a chromate treatment is heated,
attracted to the formation of a thin oxide film according to an advantageous embodiment of the invention the tinplate leads. The chromate treatment consists of the tin-plated steel, after heating, by immersing the tin-plated steel sheet in cathodic treatment in an aqueous dichroic oxidizing solution, which is then subjected to a mat solution.
Temperature of 70 to 90 ⁰ C and 10% sodium der with the help of the method according to the invention or contains potassium chromate. achievable technical progress is primarily