BLACK GALVANIZED STEEL
Description
Galvanic zinc plating with black chromatization is an alternative to oxidation. Unlike chemical oxidation it provides better protection of steel from corrosion, while at the same time being inferior in this parameter to rainbow and olive zinc. The use of a coating with a thickness of less than 6 microns is ineffective, because in this case it does not provide the required degree of protection. The thickness of the zinc does not in any way affect the wear resistance of the black outer chromate layer (black zinc is actually a two-layer coating). The thickness of the black chromate film is, as a rule, 0.25-1 microns, the content of hexavalent chromium in it ranges from 80 to 400 mg/m2. The durability of black zinc without oiling in 5% salt spray does not exceed 20-60 hours. With oiling this parameter increases significantly. The corrosion rate of non-passivated zinc depends on pH:
Black passivation film has a higher transient resistance than colorless or iridescent film.
You can order black galvanized steel according to GOST 9.305-84 by phone and email indicated in the section "CONTACTS".
Characteristics
|
Designation (example) |
Zn.black |
|
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:
- Black galvanizing is significantly superior in protective properties to chemical oxide coating
- The coating allows you to obtain a deep black color on the treated steel surface, comparable to the color after chemical oxidation. Especially with additional oiling.
- The black zinc coating produced at our enterprise is ductile, withstands bending and flaring, thanks to the use of unique shine-forming additives in the electrolyte. It also has a high uniformity in thickness, which is confirmed by metallographic studies:
A microphotograph of a cross-section of the coating is shown below:
Disadvantages of galvanizing:
- Without oiling the black chromate film, the color depth, its protective and mechanical properties are reduced.
- Electrochemical galvanizing causes some loss of steel ductility due to hydrogenation. Steel with a tensile strength higher than 1380 MPa cannot be galvanized.
- Zinc coating has increased brittleness at temperatures above 250°; C and below minus 70° C.
- Black zinc coating has low chemical resistance to products formed during the aging of organic materials.
Read also articles
Galvanizing mechanism
Description of the galvanic galvanizing process. Electrolytes.
Hydrogenation and removal of hydrogenation of steel after galvanizing
The problem of hydrogenation during galvanizing. How to dehydrate?
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