1. Types of surface treatment:
Surface treatment is the process of forming a covering layer on the surface of the workpiece by a certain method. Its purpose is to endow the surface of the product with beautiful anti-corrosion effects. The surface treatment methods are attributed to the following methods:
1. Electroplating: Immerse the part to be electroplated in an aqueous solution containing the metal compound to be deposited, and pass an electric current through the plating solution to precipitate and deposit the electroplated metal on the part. General electroplating includes galvanized, copper, nickel, chromium, copper-nickel alloy, etc., and sometimes blackening (blue), phosphating, etc. are also included.
2. Hot-dip galvanizing: It is done by immersing the carbon steel parts in a zinc-melting bath with a temperature of about 510°C. The result is that the iron-zinc alloy on the surface of the steel gradually becomes passive zinc on the outer surface of the product. Hot-dip aluminizing is a similar process.
3. Mechanical plating: The surface of the product is impacted by the particles of the plated metal, and the coating is cold welded to the surface of the product.
2. Quality control:
The quality of electroplating is measured primarily by its corrosion resistance, followed by appearance. Corrosion resistance is to imitate the working environment of the product, set it as the test condition, and perform a corrosion test on it. The quality of electroplating products is controlled by the following aspects:
1. Appearance:
The surface of the product is not allowed to have local uncoated, scorched, rough, dull, peeling, crusted conditions and obvious streaks, pinhole pitting, black slag plating, loose passivation film, cracking, peeling and serious damage are not allowed. Passivation marks.
2. Coating thickness:
The operating life of a fastener in a corrosive atmosphere is proportional to its coating thickness. The general recommended thickness of economical electroplating coating is 0.00015in ~ 0.0005in (4 ~ 12um).
Hot-dip galvanizing: the standard average thickness is 54 um (43 um for diameter ≤ 3/8), and the minimum thickness is 43 um (37 um for diameter ≤ 3/8).
3. Coating distribution:
With different deposition methods, the way the coating gathers on the fastener surface is also different. During electroplating, the coating metal is not uniformly deposited on the peripheral edge, and a thicker coating is obtained at the corners. In the threaded portion of the fastener, the thickest coating is located on the thread crest, gradually thinning along the thread flank, and the thinnest deposition is at the bottom of the thread, while hot-dip galvanizing is just the opposite, the thicker coating is deposited on the inside corners and On the bottom of the thread, mechanical plating tends to deposit the same metal as hot-dip plating, but is smoother and has a much more uniform thickness over the entire surface.
4. Hydrogen embrittlement:
During the processing and handling of fasteners, especially during the pickling and alkaline cleaning before plating and subsequent electroplating, the surface absorbs hydrogen atoms, and the deposited metal layer then traps hydrogen. When the fastener is tightened, the hydrogen swirls toward the most stressed parts, causing the pressure to build up beyond the strength of the base metal and producing microscopic surface cracks. Hydrogen is particularly active and quickly seeps into the newly formed fissures. This pressure-rupture-penetration cycle continues until the fastener breaks. Usually occurs within a few hours after the first stress application.
To eliminate the threat of hydrogen embrittlement, fasteners are heat baked as soon as possible after plating to allow hydrogen to seep out of the coating, typically at 375-4000F (176-190C) for 3-24 hours.
Since mechanical galvanizing is non-electrolyte, this virtually eliminates the threat of hydrogen embrittlement. In addition, due to engineering standards, it is forbidden to hot-dip galvanize fasteners with hardness higher than HRC35 (Imperial Gr8, metric 10.9 and above). Therefore, hydrogen embrittlement rarely occurs in hot-dip plated fasteners.
5. Adhesion:
Cut or pry off with a solid tip and considerable pressure. If, in front of the blade tip, the coating peels off in flakes or skins, exposing the base metal, the adhesion shall be considered insufficient.
