Surface treatment is a process of covering layer formed on the surface of the workpiece through a certain method, its purpose is assigned to the product surface appearance, corrosion effect, surface treatment methods are attributed to the following methods:
1. Electroplating: the plated component is dipped in an aqueous solution containing the deposited metal compound and the current is passed through the plating solution to precipitate the plating metal and deposit it on the component. General plating zinc, copper, nickel, chromium, nickel and copper alloy, sometimes cook (black phosphating and Bluing) including.
2 、 hot-dip galvanizing: the carbon steel component is immersed in a zinc bath with a temperature of about 510 DEG C. As a result, the iron zinc alloy on the surface of the steel gradually becomes the passivating zinc on the outer surface of the product. Hot dip aluminizing is a similar process.
3, mechanical plating: coating metal particles by the impact of the product surface, and the surface coating to the cold welding products.
The general use of screw plating, but used in electric power, highway and other outdoor six corners of the wood screws by hot dip galvanizing; electroplating cost per kilogram for 1.2 - 1.6 yuan, hot galvanizing is generally 2 - 2.5 yuan / kg, higher cost.
Effect of electroplating:
The quality of the plating is mainly measured by its corrosion resistance, followed by its appearance. The ability to resist corrosion is to mimic the working environment of a product and set it as a test condition for corrosion tests. The quality of electroplating products is controlled from the following aspects:
1, appearance:
The surface of the product is not allowed to have no local coating, burning, rough, dull, peeling, skin conditions and clear stripes, a pinhole on the black passivation film plating slag, osteoporosis, cracking, shedding and serious traces do not allow passivation.
2. Coating thickness:
The operating life of a fastener in a corrosive atmosphere is proportional to the thickness of its coating. Generally recommended economic plating thickness is 0.00015in ~ 0.0005 in (4 ~ 12um)
The average thickness of hot dip galvanizing: Standard 54 um (call 3/8 diameter less than or equal to 43 UM), the minimum thickness of 43 um (called 3/8 diameter less than or equal to 37 um).
3, coating distribution:
With different deposition methods, the surface of the coating on the fastener surface is also different. When plating, the coating metal is not uniformly deposited on the peripheral edge, and the thicker coating is obtained at the corner. In the threaded part of fasteners, the thickness of the coating is located on the top of thread, the thread along the side gradually thinning in the teeth at the bottom of the deposition of thin and hot dip galvanized opposite, deposition of thick, thread angle and bottom metal deposition tendency and thermal mechanical plating of the same dip, but more smooth and far more uniform thickness over the entire surface.
4, hydrogen embrittlement:
During the process of processing and processing, especially in the process of acid washing and alkali washing before plating and subsequent electroplating, the surface absorbs hydrogen atoms, deposits the metal deposits and then captures hydrogen. When the fastener is tightened, hydrogen toward the most stress concentrated part caused by increased pressure to turn enough, more than the strength of the base metal and tiny surface crack. Hydrogen is very active and soon seep into the new fissures. This pressure rupture - penetration cycle continues until the fastener breaks. It usually occurs within a few hours after the first stress application.
In order to eliminate the threat of hydrogen embrittlement, fasteners should be baked as quickly as possible after plating to allow hydrogen to seep out of the coating and bake usually at 375-4000F (176-190 C) for 3-24 hours.
Since mechanical galvanizing is non electrolyte, this virtually eliminates the threat of hydrogen embrittlement.
