DE2008900B2 - PROCESS FOR COATING STEEL OBJECTS - Google Patents

Description

The invention relates to a method of coating steel objects with one of lead or one at least 80 wt .-% lead-containing lead alloy consisting of an intermediate layer and a top layer made of a tin alloy.

With the German patent 5 79 732 it has already been proposed, Eiszellein, which alternately the Are exposed to the influence of aqueous sodium chloride solution, fresh water and moist air and therefore subject to particularly strong rust attack, to be provided with a metallic protective coating. To do this, on top of the cells a layer of lead and then another layer of tin or a zinc-free layer Tin alloy applied.

It is also known from German patent specification 6 63 791 for coating metal objects with a permanent, corrosion-resistant coating of silvery appearance, these objects through a molten bath of 25.4% by weight of cadmium, 3.7% by weight of zinc and 70.9% by weight of tin. This ternary alloy already melts at 164 ° C, so that work is carried out at such low temperatures can be that, for example, steel objects without impairing their mechanical properties can be coated.

The object of the invention is to provide a method for coating steel objects with a multilayer Specify coating that leads to objects with increased corrosion resistance.

Starting from a method for coating steel objects with one made of lead or one lead alloy containing at least 80% by weight of lead existing intermediate layer and a top layer made of a tin alloy, is that according to the invention This object is achieved in that the object is first copper-plated, then leaded and then in a molten bath of an alloy of 63 to 73% by weight tin, 22 to 32% by weight cadmium, 0 up to 10% by weight zinc, antimony and / or bismuth is immersed and this alloy in a layer thickness from 5 to 20 μπι is applied.

According to an advantageous embodiment of the method according to the invention, the temperature of the molten bath for applying the top layer a little below the melting temperature of the intermediate layer held.

The intermediate layer consists of lead or a lead alloy and preferably has a melting point between 230 and 328 ° C.

To carry out the method according to the invention, the steel objects are first after customary known method copper-plated and then leaded Die with the intermediate layer of lead or the Lead alloy coated objects are then melted from the constituents of the Cover layer immersed or passed through this melt. Excess melt can be wiped off or be stripped off. The temperature of the melt is preferably a little below the melting point the intermediate layer held. This creates a thin layer of an alloy with good corrosion resistance formed on the previously applied lead layer. On the other hand, it is used at lower temperatures worked, less good results and lower corrosion resistance are obtained.

Steel pipes can be inside or outside or on both sides using the method according to the invention Coatings are provided.

Examples

Each 1 m long steel pipe with a wall thickness of 0.7 mm and an outer diameter of 8 mm were initially coated with a 3 μm thick copper coating Mistake. The intermediate layer was then applied to the copper-plated pipes and then the pipes provided with the intermediate layer passed through a melt containing cadmium and tin. Details such as the composition and melting points of the respective layers as well as the layer thicknesses can be found in Table 1 below. Samples 4 and 5 without an intermediate layer or without top layer are listed for comparison purposes.

Tabel

sample Intermediate layer Enamel thickness Top layer Enamel temperature Total- Point Point the melt thickness of the composition ⁰ C μηι composition ⁰ C ⁰ C Coating 280 8th 158 230 Wt% Weight o / o μιτι 1 Pb-Sn 300 7th Sn-Cd-Zn 140 250 11th 80:20 68: 27: 5 2 Pb-Ag Sn-Cd-Zn 10 97.5: 2.5 64: 27: 9

continuation Intermediate layer
composition
Weight o / o Enamel
Point
⁰ C thickness
μπι Top layer
composition
Wt% Enamel
Point
° C temperature
the melt
C. total
thickness of the
Coating
μηη sample Pb-Sb-Sn
80: 14: 6
Pb-Sn
80:20 260
280 OO OO I Sn-Cd
72:28
Sn-Cd-Zn
68: 27: 5 172
158 220
230 13th
8th
3 3
4th
5

To test the corrosion resistance, the samples were sprayed with salt water. Both pipe ends are tightly sealed with a synthetic resin and the outer wall of the pipes is then degreased. At a distance of 10 mm each, the pipes were hung at an angle of 30 ° in a closed chamber containing slaze water. The volume of the chamber was 0.34 m ³ . The tubes were sprayed with a solution of 5% sodium chloride of greater than 99.5% purity in distilled water. ^{At 35 °} C., the salt solution had a density of 1.020 and a pH value of 6.9. A device with two nozzle heads was used for spraying, and the solution was sprayed ^{under a pressure of 1 kgf / cm 2.} 3.81 saline was sprayed over the course of 24 hours.

The course and extent of rust formation as the action of the Saline solution. The results of these tests are shown in Table 2.

During the spraying, small white spots were first observed on the surface of the tubes. the The number of these spots gradually increased until these spots covered the entire surface. On the mi! white spots covered surface then formed black spots, which gradually turned into red Rust stains passed over. These red spots of rust changed shape and covered them over time entire surface of the steel pipes. Table 2 shows the progress of the roasting, starting from Formation of the white spots at the beginning through the formation of the black spots to the emergence of the red rust spots.

Tabel 2 the spots after 336 hours 672 hours 1176 hours 1680 hours 2018 hours sample Ar; and number 168 hours total Surface 4-6 2-5 3-6 4-8 48 hours Vj d. Surface white bottle black bottle red bottle red bottle red bottle 1 1-2 white bottle total Surface 2-4 2-4 3-7 6-8 white bottle Vi d. Surface white bottle black bottle red bottle red bottle red bottle 2 1-2 white bottle total Surface 2-6 1-5 4-8 5-9 white bottle V ₃ d. Surface white bottle black bottle red bottle red bottle red bottle 3 2-3 white bottle ⁴ / d. 5 Surface white bottle 2/5 d. Surface red bottle 4th 3-8 red bottle 5-8 red bottle 2-5 red bottle 5 total Surface red bottle white bottle

Claims (2)

Patent claims: 1. A method of coating steel objects with one of lead or at least 80 Wt .-% lead-containing lead alloy consisting of an intermediate layer and a top layer a tin alloy, characterized in that that the object is first copper-plated, then leaded and then in a molten bath an alloy of 63 to 73% by weight tin, 22 to 32% by weight cadmium, 0 to 10% by weight zinc, Antimony and / or bismuth is immersed and this alloy in a layer thickness of 5 to 20 μπι is applied. 2. The method according to claim 1, characterized in that the Tempere door of the weld pool to Application of the top layer kept a little below the melting temperature of the intermediate layer will.