DE1521045C - Process to improve the corrosion resistance of cathodically chromated iron or steel surfaces - Google Patents

Description

ι 2

The invention relates to a method for improving (2) alkylamines, such as methylamine, ethylamine, n-Pro-

tion of the corrosion resistance of cathodic pylimine, isopropylamine, n-butylamine, sec-butyl-

chromated iron or steel surfaces. The eramine, n-amylamine, dimethylamine, diethylamine,

inventive method is characterized by di-n-propylamine, di-isopropylamine, trimethyl

net that the chromated metal surface before the 5 amine and triethylamine;

To dry for 1 to 60 seconds at 30 to 7O ⁰ C with (3) polyalkylene polyamines such as ethylene diamine, Diwäßrigen solution is treated, the ethylenediamine 1 to 100 g / liter, diethylenetriamine and propylene of an alkanolamine, alkylamine, Polyalkylenpoly- diamine;

amine, hydrazine or one of its water-soluble (4) hydrazine and its water-soluble alkyl derivatives,

Alkyl derivatives, an alkali salt of an aminocarbon io such as symmetrical and asymmetrical dimen-

acid, an aromatic amine or an aromatic hydrazine;

tables heterocyclic nitrogenous base or (5) chelating agents such as lithium, sodium, ICa-

contains a mixture thereof. lium, rubidium and cesium salts of the following

According to the process according to the invention, aminocarboxylic acids are obtained: Ethylenediamine tetraacetic acid

one chromated metal surfaces, with high grain 15 re, nitrilotriacetic acid, cyclohexanediamine tetra-

corrosion resistance and excellent acetic acid, hydroxyethyliminodiacetic acid, hy-

like for paints. hydroxyethylethylenediaminetetraacetic acid, diethy-

During cathodic chromating, 6-valerine triamine pentaacetic acid, di- (hydroxyethyl) glycine,

term chromium ions, derived from chromic acid, chromic acid, ethoxydiamine tetraacetic acid, glycol ether diamine

mates or dichromates in the treatment solution 20 tetraacetic acid and ethylenediamine-di-o-hydroxy-

abbiten, electrolytically reduced, whereby on · the phenylacetic acid;

Metal surface has a protective coating made of a hydra- (6) aromatic amines, such as aniline, benzylamine and

tized chromium oxide complex, mainly p-phenylenediamine;

Contains trivalent chromium ions. (7) aromatic heterocyclic nitrogen-containing ba

The analysis of the protective coating shows that it is 25 sen, such as pyridine,-picoline, β-picoline, a, y-L tjdin

contains no other metals except chromium and does not contain and «,« '- lutidine.

consists of metallic chrome. This water-soluble nitrogenous compound was also established, that the protective coating contains traces of an anion either on its own or as a mixture, which were originally used in the chromating solution as. Additive was present. 30 Treatment with the solution of nitrogen

If the still moist protective coating made of hydra-containing compound has also proven to be effective for

tized chromium oxide at a suitable temperature to improve the paint adhesion mechanically

is dried, the anion is processed into the protective coating, cathodically / h ^ omatierien metal coating

incorporated as part of the chromium oxide complex. Proven in damp areas

In contrast, the anion is not firmly bound in the tem- perature state. 35 That which comes from mechanical processing

During cathodic chromating, oil adsorbed by and from the chromate layer can be absorbed Electrolytes, made mainly from chromic acid, because of the affinity between oil and hydrated Chromates or di: chromates originating from hexavalent chromium oxide in the subsequent degreasing only ti? e. Contains chromium ions, various additives, difficult to remove. Incomplete degreasing results in: Boric acid, phosphoric acid, sulfuric acid, disulfone-40 therefore give poor paint adhesion. This acid, nitric acid, acetic acid, their corresponding deficiency can, however, be caused by the salts according to the invention and halogen compounds are added. action, since the adsorbed stick-

Of these additives, strongly acidic and substance-containing compounds are used as a suitable barrier;

Corrosive anions such as sulfate ions, nitrate ions act for oil adsorption. The inventive

or Chloratioren, in the electrolytic reduction 45 process is carried out under the following conditions-

of hexavalent chromium ions included and managed by:

the film of hydrated chromium oxide adsorbs _L concentration of water

The adsorbed anions are water soluble and soluble nitrogenous

under the influence of moisture from the compound 1 to 100 g / liter

Fil.n dissolved out, causing corrosion of the metal 5 ° ₂ temperature 30 to 70 ° C

surfaces is accelerated. i Dive time ./. '.'. '.'. '.'. '.'. '.'. '. 1 to 60 seconds

Additives that deliver strongly acidic anions are

necessary to form a particularly good film. In general, a higher concentration results

however, have a detrimental effect on corrosion resistance. However, if the concentration

persistence. Therefore, the acidic anions must tration the water-soluble nitrogenous compounds.

from the fertilizer formed by cathodic chromating exceeds 100 g / liter, the film becomes out

Film can be removed. hydrated chromium oxide yellow, and the corrosion

To remove the adsorbed, strongly acidic resistance and the paint adhesion are not

'Anions and to improve the corrosion resistance more improved. If the concentration is less than

stäiuligkcit and the adhesion for paints to 60 | g / liter, the treatment time increases to less than

the film has an instant immersion of the cheap values on it.

methodically chromate metal immediately after the higher temperatures result in a greater

Chrom'iti'jnini; in an aqueous solution of little, but the evaporation increases

sttns liier the mentioned water-soluble, basic loss of water and the decomposition of the water-soluble

Acting nitrogenous compounds as specific nitrogenous compounds. Hei Teinpe-

soihIi.ts proven effective. temperatures below 30 "C, the procedure requires a 1111-

(1) Alkanolamines, such as monoethanolamine, diethanoi- desirable for a long time.

amine and triethanolamine; If the dive time exceeds 60 seconds,

The film formed begins to dissolve, while with a dip time of less than 1 second the Procedure practically cannot be carried out with success.

The treatment is carried out immediately before the drying of the formed by cathodic chromating Films carried out as the dry film acts as a solid, tight barrier against the attack of nitrogenous Connection behaves.

Iron and steel sheets treated according to the invention show no rust formation when tested outdoors (2 months in winter and 15 days in summer) and also with salt spray devices (for 48 hours). Furthermore, they withstand the tests in a chamber at a relative humidity of 90% at 40 ^° C. for one week without discoloration. None of the usual chemical surface treatments has so far achieved such a high level of corrosion resistance.

After applying a film of about 10 microns Thickness of modified alkyd resin or modified epoxy resin paint on a steel sheet treated according to the invention, the painted sheet with a Draw ratio of 2.2 deep-drawn to a cup and stretch-deformed to a screw cup on the cup side. When tested with an adhesive strip, the "coated" side of the bowl shows no Paint adhesion. Even after immersing the bowl for one hour in a boiling 3% sodium chloride solution, the test is carried out with the adhesive strip no loss of paint adhesion on the side of the bowl was found.

The metal surfaces treated according to the invention show good temperature resistance up to 300 ^° C. and just as good weldability as the metals themselves.

example 1

A 0.25 mm thick sheet of cold-rolled mild steel with a low carbon content, so-called black plate, is ^{cathodically cleaned for 20 seconds at a current density of 4 A / dm 2} and at 70 ^° C. in 7% sodium hydroxide solution, then rinsed with water for 10 seconds Pickled at room temperature in 7%> ger sulfuric acid and rinsed again with water. Immediately thereafter, it is cathodically treated using a lead-antimony alloy anode under the conditions given below in a chromating electrolyte of the composition given below, then immediately rinsed with water and immersed in the solution according to the invention under the conditions given below. It is then immediately rinsed again with water and dried.

(1) The electrolyte and cathodic chromating conditions.

Chromic acid 50 g / liter

Ethyl alcohol 0.2 g / liter

Sulfuric acid 0.2 g / liter

Fluosilicic acid 0.3 g / liter

Temperature 50 ⁰ C

Current density 20 A / dm ³

Time 10 seconds

(2) Treatment conditions according to the invention

Monoethylamine 18 g / liter

Temperature 70 ⁰ C

Dive time 30 seconds

A transparent, bluish-purple color is formed. Movie. The thus-treated black plate shows only very "small traces of rust after it has been subjected to a Salzsprühtesi 'with a 5 ° / _o sodium chloride solution for 48 St.ünden at 35 ° C. After one week, the condensation water at a relative humidity of 9O ° / o at 40 ^° C. it shows no traces of rust and no discoloration.

B e i s ρ i e 1 2

^V

A 0.5 mm thick sheet of cold rolled mild steel with a low carbon content becomes the same, Pretreatment and cathodic chromating as described in Example 1. Then it will be it immediately rinsed with water and immediately afterwards in the solution according to the invention under the following conditions mentioned, rinsed again immediately with water and dried.

Disodium salt of ethylenediamine

tetraacetic acid 1 g / liter

Temperature 60 ⁰ C

Dive time 30 seconds

The film formed is bluish purple and transparent. ^{If about 30 mg / dm 2 of} machine oil are applied to the steel sheet treated in this way, the sheet is degreased with trichlorethylene vapor for a period of 2 minutes after 24 hours and then coated with an approximately 20 micron thick layer of a melamine resin-modified acrylic resin stoving varnish, so no decrease in paint adhesion is found in a cross-cut test. Even if the sheet coated with the stoving lacquer is immersed in boiling 3% sodium chloride solution for 1 hour, the lacquer film shows no loss of adhesion.

For example, 3

A sheet of cold-rolled mild steel with low The carbon content is subjected to the same pretreatment described in Example 1, then immediately using a lead-antimony alloy anode among those given below Treated conditions in a chromating electrolyte, rinsed again with water and immediately immersed in the solution according to the invention under the conditions given below, again rinsed with water and dried.

(1) The electrolyte and cathodic chromating conditions.

Chromic acid 60 g / liter

Phenol-2,4-disulfonic acid 0.6 g / liter

Temperature 50 ° C

Current density 20 A / dm ²

Time 20 seconds

(2) Treatment conditions according to the invention

Triethanolamine 100 g / liter

Temperature 7O ⁰ C

Dive time 2 seconds

The film formed is bluish yellow and transparent. The black plate treated in this way shows almost no pink traces when subjected to the same salt spray test is subjected according to Example I for 48 hours. Even if the treated steel sheet with an approximately 20 micron thick layer of one with a melamine

coated with resin modified alkyd resin and is subjected to the same salt spray test according to Example 1 for 150 hours, none are found Bubbles in the paint layer.

Example 4

A black plate made from cold rolled mild steel with low carbon content becomes the same, Pretreatment described in Example 1 and immediately thereafter the same as described in Example 3 subjected to cathodic chromating. Then it is rinsed with water, immediately in the inventive Solution immersed in the following conditions, rinsed again with water, and dried.

Ethylenediamine 45 g / liter

Temperature 50 ⁰ C

Dive time 60 seconds

Rinsed with water and immersed in the inventive solution under the following conditions, then is it again immediately rinsed with water and dried.

(1) The electrolyte and cathodic chromating conditions

Sodium dichromate 500 g / liter

Chromium sulfate, 10 g / liter

Ammonium sulfate 20 g / liter

ίο temperature 50 ° C

Current density 25 A / dm ²

Time 10 seconds

(2) Treatment conditions according to the invention

Hydrazine 50 g / liter

. Diethylenetriamine 40 g / liter

Temperature 60 ⁰ C

Dive time 10 seconds

The film formed is bluish yellow and translucent. The treated steel sheet has the same corrosion resistance like that of example 1. If the treated steel sheet is about 10 microns thick Layer of a white lacquer modified with an alkyd resin and coated with a draw ratio as of 2.2 is deep-drawn to form a cup, no decrease in paint adhesion is found on the cup side. ·

B e i s ρ i e 1 5

A black plate made of cold rolled mild steel with low carbon content becomes the same, im Subjected to the pretreatment described in Example 1 and then immediately using an anode a lead-antimony alloy and among those below mentioned conditions in the chromating electrolyte the following composition cathodically treated. Immediately afterwards it becomes with The formed film shows the same corrosion resistance as that in Example 1 and the same adhesiveness the lacquer layer as in example 2.

Claims (1)

Claim: "' Process for improving the corrosion resistance of cathodically chromated iron or steel surfaces, characterized in that the cathodically chromated metal surface is treated for 1 to 60 seconds at 30 to 70 ° C with an aqueous solution containing VinsiiOO g / liter of an alkanolamine, alkylamine, Polyalkylenepolyamine, hydrazine or one of its water-soluble alkyl derivatives, an alkali salt of an aminocarboxylic acid, an aromatic amine or an aromatic heterocyclic nitrogen-containing base or a mixture thereof.