FI91889C - Procedure for galvanizing stainless steel - Google Patents

Description

91689

METHOD FOR GALVANIZING STAINLESS STEEL

The invention relates to a process for the unilateral and reciprocal electrolytic galvanizing of stainless steel 5, in which the stainless steel is coupled in the form of a stainless steel strip to a cathode in a galvanic cell, and to an aqueous solution of zinc salt having a pH of 1.0 to 2.5, preferably 1.5 to 2 .0, metallic zinc 10 is separated on the surface of the stainless steel strip.

Methods for electroplating steel strips have been known for many years and have been described in detail in the literature in many applications.

15 On the other hand, the electrolytic galvanizing of stainless steel strips is still very little known. The reason for this is that, as the companies have shown, only a very narrow range of conditions is available for such experiments.

20 For steel strips, the usual methods operate in the pH range from less than 1 to about 4 at temperatures of 40 to 70 ° C and current densities up to 200 A / dm2. This area must now be severely limited in the stainless steel method.

25 If an acid concentration with a pH of less than 1.5 is used, the stainless steel strip becomes corrosive to the acid and becomes opaque as a result, and raster electron micrographs show clear pitting. At pH values above 2.5, on the other hand, very poor zinc stability in stainless steel is achieved and the coating can be pulled off like a film. In addition, various alloy parts of stainless steel have the undesirable property of impairing durability.

Finally, due to the lower specific conductivity of stainless steel, which is only about one-fifth of that of steel, the separation current densities are also limited for economic reasons.

91889 2

ΕΡ-Α-0 269 208 describes a method for unilaterally coating a zinc layer or a zinc-nickel alloy on a steel body at a pH of less than 3.5. Track påållyståmåt-5 tomån side are protected levittåmållå protective layer which is Thereafter the påållystyksen again removed. The provision of a particularly permanent coating and / or a simple and nevertheless one-sided coating is not included in this publication.

The object of the invention is to provide a method for galvanizing stainless steel, in particular for the automotive industry, which solves the problems described above and, with clearly better adhesive strength of the zinc coating, prevents contact corrosion with galvanized sheet metal.

This object is solved by the described method according to the invention, in that the adhesive strength of the separated zinc layer is increased by the following heat treatment, which is preferably carried out at a temperature of 100 to 400 ° C, in particular at a temperature of 200 to 300 ° C, for 1 to 30 minutes. , in which case the stainless steel strip is heated to a heat source and kept at a temperature. The time duration of the heat treatment is related to the temperature to be used and is in the range of 30 minutes, preferably at the lower treatment temperatures of the limit range, and 1 minute, preferably at the upper treatment temperatures of the limit area. However, the values of the treatment duration may vary within the values still given according to the parameters selected in the electroplating.

According to a further feature of the invention, the coating is carried out at a constant current density. This measure has the advantage that the duration of the process can be substantially reduced, because pre-treatment with Mata lamb at current intensity is not necessary.

35 In addition, the size of the electroplating plant remains small due to the omission of the pre-treatment department.

In the method according to the invention, the currents of the current 91889 3 are from a single-sided coating between 25 and 200 A / dm2, preferably between 50 and 150 A / dm2, and a mole-based coating between 10 and 100 A / dm2, preferably between 10 and 100 A / dm2. / dm 2.

According to one feature of the process according to the invention, the zinc salt may also be zinc sulphate, the concentration of zinc ions released into the solution being 20 to 150 g / l, preferably 100 to 130 g / l.

The temperature of the electrolyte is 20 to 90 ° C, preferably 45 to 60 ° C.

However, the rising temperature values of the electrolyte are also associated with an increased incidence of pitting corrosion, especially at pH values which are close to the lower limit of said range.

According to a further feature of the method according to the invention, an insoluble anode consisting of lead, lead alloy or precious metal-coated titanium is used as the anode.

In the following, the details or advantages of the method according to the invention will be described in connection with use examples:

Example 1: 25 A ferritic cold-rolled stainless steel strip, grade AISI 409, was degreased on both sides of a bench annealing plant and cleaned, after which the side to be coated was pickled, rinsed and introduced into an electrolytic zinc coating. An insoluble noble metal anode on top of the titanium body material was used as the anode for the coating, and the electrolyte was an aqueous solution of zinc sulfate, with a concentration of zinc ions released in the solution of 35,125 g / l. At a current density of 70 A / dm2, an electrolyte temperature of 60 ° C and an electrolyte pH of 1.2, the noble metal strip was unilaterally coated with a 15 μτη: η thick zinc layer.

The final analysis of the produced material was performed according to the Erichsen test according to DIN 50101, part 1. This 5 consists of deforming the strip to a fracture, followed by gluing a transparent adhesive film over the deformed point and rapidly removing the film. The adhesion of the applied layer was evaluated at four levels:

Very good adhesion: no parts of the zinc layer 10 adhering to the adhesive tape

Decreased adhesion: less than 5% of the surface area of the zinc layer in the adhesive tape 15 Low adhesion: more than 5% of the surface area of the zinc layer in the adhesive tape

Virtually no adhesion: the zinc layer comes off almost 20 completely

The adhesion of the zinc layer was very good with the given parameters, however, on the uncoated side of the strip there was a weak spot absorption, which did not completely disappear when the electrolyte temperature was reduced to 50 ° C.

Similar results were obtained - under exactly similar conditions - also on a tape of AISI 430 quality-30 sat.

Example 2:

On stainless steel strip, the quality of AISI 410 Cb was observed under similar conditions as in Example 1, surprisingly with reduced adhesion of the zinc layer.

35

Example 3:

The galvanized strip of Example 2 was subjected to a heat treatment at 220 ° C for 20 minutes in an infrared oven. After that, the improvement of the adhesion of the zinc layer could be found to be very good by the Erichsen test.

5

Example 4:

Otherwise, under conditions analogous to Examples 1-3, the pH was raised to 1.7, which in both temperatures resulted in complete fading of the point absorption. Regardless of the quality of the stainless steel, the same results as in Examples 1-3 could be obtained for the adhesion of the zinc layer, i. for AISI 410 Cb, very good adhesion was obtained only after the thermal post-treatment according to Example 3.

15

Example 5:

A further increase in pH to 2.2 with otherwise identical parameters as in Examples 1 and 2 resulted in a decrease in the adhesion of the zinc-20 layer at both temperatures (60 ° C and 50 ° C). With grades AISI

409 and 430 had reduced adhesion and grade AISI

410 Cb only more weak.

Example 6: The strips of Example 5 were subjected to a thermal post-treatment at 300 ° C for 1 minute, at which point all grades could show very good adhesion of the zinc layer by the Erichsen test.

30 Example 7:

By raising the pH of the zinc electrolyte during the coating of the stainless steel, the zinc layer of the stainless steel strip of all grades could be completely pulled off like a film. There was virtually no infectivity at all.

Example 8: 91889 6

By heat treatment at 300 ° C for 30 minutes, an improvement in the adhesion of the zinc layer to very good adhesion could be achieved with the strips of Example 7.

5

Example 9:

The galvanizing plant was now converted to reciprocal use and stainless steel strips, grades AISI 409, 430 and 410 Cb, were also coated on both sides with a 7.5 μτα thick zinc layer after mutual identical pretreatment as in Example 1. The current density was 35 A / dm2 and the zinc ion concentration was 125 g / l. At both pH 1.2 and 1.7, the specimens prepared at 50 ° C and 60 ° C were flawless with excellent adhesion of the coating to grades AISI 409 and 430, while reduced adhesion was observed with AISI 410 Cb.

Example 10: The adhesion of the zinc layer with AISI 410 Cb stainless steel strip could be improved by thermal post-treatment at 220 ° C for 20 minutes - as in all infrared furnaces treated so far - with very good adhesion.

Example 11:

Otherwise, under identical conditions to Example 9 at pH 2.2 and electrolyte temperature of 50 to 50 ° C, identical results were also obtained for the adhesion of the zinc layer, with a further improvement in the adhesion of AISI 410 Cb after thermal treatment as in Example 9.

35

Example 12: At a temperature of 60 ° C, a low adhesion of AISI 409 and 430 was obtained for pH 91889 7 and even lower for AISI 410 Cb in the Erichsen test.

Example 13: 5 Thermal post-treatment at 300 ° C for 1 minute with all the strips and grades of Example 12 showed an improvement in the adhesion of the electrolytically coated zinc layer to a very good adhesion.

10

Example 14:

After raising the pH of the zinc electrolyte to 3.2, the same results for adhesion were obtained for both grades at both 50 ° C and 60 ° C as in Example 12.

Example 15:

With the thermal post-treatment according to Example 13, the same improvements in adhesion as in Example 13 could also be achieved for all grades of stainless steel.

Claims (8)

91889 PATENT CLAIM ET A process for the unilateral and double-sided electrolytic galvanizing of stainless steel, in which process the stainless steel is coupled in the form of a stainless steel strip to a cathode in a galvanic cell, and an aqueous solution of zinc salt having a pH of 1.0 to 2.5, preferably 1.5 to 2, 0, separating the metallic zinc on the surface of the stainless steel strip, characterized in that the adhesive strength of the separated zinc-10 layer is increased by a subsequent heat treatment known per se, preferably carried out at a temperature of 100 to 400 ° C, in particular at a temperature of 200 to 300 ° C, for 1 to 30 minutes. , in which case the stainless steel strip is heated to a heat source and kept at a temperature. Method according to Claim 1, characterized in that the coating is carried out at a constant current density. Method according to Claim 2, characterized in that the current densities are one-sided coating between 25 and 200 A / dm 2, preferably between 50 and 150 A / dm 2, and double-sided coating between 10 and 100 A / dm 2, preferably between 10 and 100 A / dm 2. 25 - 75 A / dm2. Process according to Claim 1, characterized in that the zinc salt is zinc sulphate, the concentration of zinc ions released into the solution being 20 to 150 g / l, preferably 100 to 130 g / l. Process according to Claim 1, characterized in that the electrolyte has a temperature of 20 to 90 ° C, preferably 45 to 60 ° C. Method according to Claim 1, characterized in that an insoluble anode consisting of lead, lead alloy or precious metal-coated titanium is used as the anode. 91 Sb9