EP0346740B1 - Alcaline aqueous bath for the galvanic deposition of zinc-iron alloys - Google Patents

Description

The invention relates to an alkaline aqueous bath for the galvanic deposition of zinc-iron alloys containing as essential components a zincate and an iron compound.

Electrolytes for the deposition of zinc alloys have long been known. Their technical usability is limited, not least because of their alloy composition, which is very dependent on the current density, above all to coil galvanization.

Recently, attempts have been made to introduce acidic electrolytes from which corrosion-resistant alloys of zinc with nickel, cobalt, iron or chromium are deposited. Despite relatively good to very good corrosion data, the range of applications for such electrolytes remained remarkably limited. The reasons for this are in particular the instability of the electrolytes due to the high salt concentrations and the problematic alloy composition, which is dependent on the current density. The same also applies to previously known zinc iron baths, which have the disadvantage that zinc iron could not be used as the end surface, since both red rust and white rust form very quickly due to the iron content.

One reason for this inadequate corrosion behavior of the previous zinc-iron coatings may be that they were deposited from baths which contained different, but generally unfavorable chelating agents. For example, the following chelated images are mentioned in DE-DS 3 506 709:
Hydroxy carboxylic acids, amino alcohol, polyamine, aminocarboxylic acid and others

In addition, iron salts are used in this bath, which lead to an accumulation of unwanted foreign anions.

The object of the invention is to create a bath of the type described above, which enables deposition of highly corrosion-resistant zinc-iron alloys independently of the current density, while avoiding disruptive foreign anions.

This object is achieved by an alkaline aqueous bath according to the characterizing part of the patent claim.

Further refinements of the invention can be found in the characterizing parts of the subclaims.

The bath according to the invention enables the deposition of almost constant zinc-iron alloy coatings with exceptionally high corrosion resistance in an outstanding manner.

The current density independence is particularly surprising and of great technical importance for the implementation of the process.

The absence of interfering foreign ions and other complexing agents or chelating agents are further advantages.

Of the zincates, the sodium zincate can be used with particular advantage, but if desired the other alkali zincates can also be used.

In addition to alkali hydroxide, mixtures of alkali hydroxides with alkali carbonates can also be obtained in the bath.

According to the process, bonds with iron can be those with polyhydroxydehydes, such as monosaccharides, disaccharides, trisaccharides and starch degradation products.

The iron saccharate to be used according to the invention is known per se and can be prepared by processes known per se, for example by reacting iron (II) chloride, soda, sucrose and sodium hydroxide solution.

It is particularly advantageous to use the sugar or saccharides in excess in the bath.

Temperatur

: 25 °C

Stromdichte

: 1 - 4 A/dm²

pH-Wert

: > 13

The bath according to the invention is used in a manner known per se under the following process conditions:

temperature

: 25 ° C

Current density

: 1 - 4 A / dm²

PH value

:> 13

Iron-steel are particularly suitable as substrates for the zinc-iron alloy coatings to be deposited.

The basic composition of the bath according to the invention is as follows:
120 g / liter alkali hydroxide
10 g / liter zinc oxide
30 g / liter alkali carbonate

The alloy coatings deposited from the bath according to the invention can be provided with chromate cover layers in a manner known per se by means of suitable chromating solutions.

The following examples serve to illustrate the invention.

example 1

The following table shows the alloy composition (proportion Fe) at different current densities, depending on the zinc and iron concentration in the bath.

In addition to the components listed in the table, the baths used had the following composition:
120 g / liter NaOH
30 g / liter Na₂CO₃
10 g / liter sodium zincate
8 g / liter gloss additive

The results showed the unusual standard of the baths according to the invention, namely relatively low iron concentration in the electrolyte and the nevertheless unusually uniform iron content in the coating at different current densities.

For comparison, the corresponding data of a commercial alkaline zinc-iron bath are given:

Example 2

8 µm thick Zn-Fe coatings were deposited from a bath of the composition given in Example I in the table. These contained 0.5% Fe.

These coatings were passivated by immersing them in a conventional chromating solution:

Some of the samples were tempered for 1 hour at 120 ° C after chromating (special requirement of the automotive industry), the rest were at 60 - 80 ° C for approx. 15 min. dried long.

• a) bis zu deutlich beginnender Oberflächenkorrosion (Weißrost);
• b) bis zum Auftreten von Rotrost.

After a storage period of at least 1 week, the samples were subjected to the salt spray test in accordance with DIN 50021 SS tested,

• a) up to clearly beginning surface corrosion (white rust);
• b) until the appearance of red rust.

In parallel, samples made from alkaline zinc electrolytes are tested for comparison. The results are shown in the table below.

In this comparison it must be noted that the chromated zinc samples used for the comparison already represent an unusually high standard. Nevertheless, even the annealed Zn-Fe chromate coatings behaved more consistently.

The corrosion protection of approx. 1000 hours salt spray test for Zn-Fe chromated achieved with the untempered samples proves that the bath according to the invention achieves corrosion protection values that were previously only achievable with a special zinc-nickel process from acid baths, which, however has the disadvantages shown in the table.

Claims (8)

1. Alkaline aqueous bath for the electrodeposition of zinciron alloys containing as the main constituent a zincate and an iron compound, characterized in that the iron compound is present in the form of a combination of iron with a polyhydroxy aldehyde.
2. Alkaline aqueous bath in accordance with Claim 1, characterized in that the iron compound is present in the form of a combination of iron with a monosaccharide, disaccharide or trisaccharide.
3. Aqueous alkaline bath in accordance with Claim 1, characterized in that the zincate is present in the form of iron saccharate.
4. Aqueous alkaline bath in accordance with Claim 1, characterized in that the zincate is present in the form of sodium zincate.
5. Alkaline aqueous bath in accordance with Claim 1, characterized in that zinc is present in concentrations of 1 to 40 g/liter, preferably 4 to 12 g/liters, alkalihydroxide in concentrations of 60 to 200 g/liter, preferably 80 to 140 g/liter, iron in concentrations of 0.001 to 10 g/liter, preferably 0.05 to 4 g/liter and saccharose (sugar) in concentrations of 1 to 100 g/liter, preferably 30 to 60 g/liter.
6. Alkaline aqueous bath in accordance with Claim 1, characterized in that it contains brighteners and levelling agents.
7. Method for electrodeposition of zinc-iron alloys, characterized in that alkaline aqueous baths in accordance with Claims 1 to 6 are used.
8. Method in accordance with Claim 7, characterized in that the same is implemented at temperatures of 0 to 60°C and with current densities of 0.1 to 10 A/dm².