the surface treatment of the hardware fasteners

I. Surface treatment types:

Surface treatment is the process of forming a cover layer on the surface of the workpiece through certain methods, whose purpose is to give the beautiful surface and anti-corrosion effect.. The surface treatment method is attributed to the following methods:

1. Electroplating: immerse the parts receiving electroplating in the aqueous solution containing the deposited metal compound, and flow through the plating liquid, the electroplating metal is precipitated and deposited on the parts.General electroplating has galvanized, copper, nickel, chromium, copper and nickel alloy, sometimes boiled black (blue), phosphorization is also included.

2. Hot dip galvanizing: done by soaking a dissolved zinc with a temperature of about 510℃.The result is that the iron-zinc alloy on the surface of the steel piece gradually becomes the blunt zinc on the outer surface of the product.Hot-dip aluminum plating is a similar process.

3. Mechanical plating: impact the surface of the product through the particles of the coating metal, and cold weld the coating to the surface of the product.

II. Operation process

(I). Galvanized (blue and white, colorful, black)

1. Pretreatment: hot degrease tank (5 tanks),electrolytic degrease tank (3 tanks ) ,rust removal tank (4 tanks)

2. Electroplating: electroplating tank (20 tanks) (ammonium chloride, zinc oxide, gloss, softener solution).

3. Post-treatment: dissolution tank (1 tank) (blue and white / colorful / black)

(II) Boil it in black:

1. Pretreatment: thermal grease tank (4 tanks), rust removal tank (4 tanks)

2. Boil black: boiled black (5 tanks) (alkali, sodium nitrite solution)

3. Post-treatment: antirust oil (1 tank)

(Ⅲ) Phosphorization:

1. Pretreatment: thermal degrease tank ( 1 tank) ,rust removal tank (1 tank) ,electrolytic degrease tank ( 1 tank),

Surface treatment (1 tank)

2. Phosphating: Phosphating (forming skin membrane)

3. Post-treatment: immersed in antirust oil (2 tanks)

(IV) Hot-dip galvanized:

1. Pre-treatment: degrease tank (1 tank) ,rust removal tank (1 tank) ,FLUX tank (1 tank) drying

2. Hot dip galvanizing: hot dip zinc tank

3.Post-treatment:centrifugal treatment,ammonia chloride ,clear water cooling.

II. Quality control:

The quality of electroplating is mainly measured by its corrosion resistance, followed by the appearance.Corrosion resistance is to imitate the product working environment, set as test conditions, and corrosion test.The quality of Electroplating Products is controlled from the following aspects:

1. Appearance:

Product surface does not allow local no coating, burning, rough, gray, peeling, skin condition, and obvious stripes, do not allow pinhole pinpoint, black slag, passivation film loose, cracking, shedding and serious passivation traces.

2. Coating thickness:

The working life of the fastener in the corrosive atmosphere is directly proportional to its coating thickness.The generally recommended economic electroplating coating thickness should be 0.00015 i n to 0.0005 i n (4 to 12 u m).

Hot dip galvanizing: the standard average thickness is 54 um and the minimum thickness is 43 um.

3. Coating distribution:

Different deposition methods used the accumulate on the surface of the fastener differently.The plating metal is not uniformly deposited on the peripheral edge, and a thick coating is obtained at the corner.In the thread part of the fastener, the thickest plating is located on the top of the thread, gradually thinner along the thread side, the thinnest deposition at the bottom, and the hot dip galvanizing is just the opposite, the thicker plating deposits in the corner and thread bottom, mechanical plating metal deposition tendency is the same as the hot dip, but is smoother and much more uniform on the whole surface.

4. Hydrogen crunchy:

During processing and treatment process, especially during pickling and alkali washing before plating, the surface absorbs hydrogen atoms, and the deposited metal plating then captures the hydrogen.When the fastener is tightened, the hydrogen shifts toward the most concentrated stress, causing increased pressure to exceed the strength of the matrix metal and minor surface rupture.Hydrogen is particularly active and quickly seeps into the newly formed cleft.This pressure-burst-infiltration cycle continues until the fastener breaks.It usually occurs within a few hours after the first stress application.

To eliminate the threat of hydrogen brittlement, fasteners are baked as quickly as possible after plating to bleed hydrogen from the plating, with baking usually lasting 3-24 hours at 375-4000F (176-190℃).

Since mechanical galvanizing is non-electrolyte, this actually eliminates the threat of hydrogen fragility, while we galvanizing uses the electrochemical method.In addition, because the engineering standard prohibited the hardness higher than HRC35 fasteners (British Gr8, metric grade 10.9 or above) hot dip galvanizing.So hot-dip fasteners are rarely become hydrogen crunchy.

5. Adhesion:

Cut off or pry down with a solid sharp tip and considerable pressure.If the coating is peeled above the tip so that the base metal is exposed, the adhesion is considered insufficient.