RAINBOW GALVANIZING CAST IRON
Description
Protecting cast iron products from corrosion is the main task of galvanizing with rainbow chroming. A feature of cast iron is the presence of a casting crust on the surface, because It is most often formed by casting. Depending on the method and quality of casting, as well as on the contamination and oxidation of the parts, the quality of the coating will also change. The deposition of zinc from alkaline baths on cast iron is also difficult due to the low overvoltage of hydrogen evolution on its surface. Those. It is easier for hydrogen to form on cast iron than zinc, which greatly increases hydrogenation. The problem is solved by using modern organic additives to the electrolyte. However, even in their presence, zinc may not be deposited on a porous or damaged casting skin. This is often observed when restoring the coating on used cast iron products. The properties of zinc coating on cast iron are similar to those on steel. The durability of yellow zinc in 5% salt spray is 200-300 hours. The corrosion rate of non-passivated zinc depends on pH:
You can order galvanizing of cast iron with rainbow chromating according to GOST 9.305-84 by phone and email indicated in the section "CONTACTS".
Characteristics
|
Designation (examples) |
Zn |
|
Thickness |
6-50 microns (optimal, greater thickness is possible) |
|
Microhardness |
490-1180 MPa |
|
Electrical resistivity at 18° C |
5.75⋅10-8 Ohm⋅m |
|
Permissible operating temperature |
300° C |
Advantages:
- Zinc protects cast iron from corrosion electrochemically and mechanically up to 70°C. When this temperature is exceeded, the electrochemical anodic protection disappears. The anodic nature of the zinc coating protection allows you to protect cast iron products even if the coating is damaged.
- Prevents contact corrosion of cast iron when mating with parts made of aluminum and its alloys.
- Ensures easy screwing of threaded cast iron parts.
- Provides average anti-corrosion characteristics - better than with colorless chromium plating, but worse than with olive chrome plating.
- Iridescent chromate film on zinc coating has the property of “self-healing”, i.e. capable of restoring its properties in case of minor mechanical damage, such as scratches.
- The zinc coating can withstand bending and flaring, but the base metal itself (cast iron) is fragile and these operations may not pass.
Disadvantages of galvanizing:
- Chromate film on zinc is mechanically relatively weak. Loses its decorative appearance subject to periodic mechanical impact.
- Electrochemical galvanizing causes a loss of ductility of cast iron due to hydrogenation. It manifests itself more strongly than for ordinary steel.
- Has increased brittleness at temperatures above 250°; C and below minus 70° S.
- Has low chemical resistance to the effects of products released during aging of organic materials
Read also articles
Galvanizing mechanism
Description of the galvanic galvanizing process. Electrolytes.
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