Because of powder metallurgy products with certain pore, direct plating is short of good surface quality, at the same time because of the plating solution is very difficult to remove residual porosity in the parts and cause internal corrosion. Secondly, in the process of plating, when the products from one tank to another tank, remaining in the pore solution will another slot liquid pollution. After plating, porosity of the coating from residual liquid to make workpiece surface produce rusty spot.

According to practical experience, for parts density greater than 6.8 g/cm3, the plating process is similar to steel, only current density is bigger; Low density and for parts, the first hole sealing treatment, with clogged pore, before plating.

A hole, sealing method

1, mechanical sealing hole method:

Parts with larger finishing allowance, through finishing closed pore surface. Can also be used by other mechanical methods make the surface deformation and closed pore, such as sandblasting, grinding and polishing.

2, fixed material blockage method:

Dipping parts under 200 ℃ in molten zinc stearate, make its seep into the pore of hole sealing, should remove excess surface after leaching zinc stearate, and electroplating.

With vacuum impregnating resin, high soft point of paraffin wax is a feasible and simple method.

3, steam treatment method:

Steam treatment can form a layer on the surface of parts Fe3O4 oxide film, can jam pore surface. Fe3O4 is a density of semiconductor, it doesn't interfere with plating. After steam treatment parts, don't have any clean before plating processing.

4, passivation liquid filling method:

Impregnation will part on the 2 g/L potassium dichromate, 2 g/L calcined soda, for configuration solution of heavy water, the passivation liquid filled in the pore. Passivation solution temperature is 70-80 ℃. After plating, the parts in the heated to 100-110 ℃, borrow pore moisture evaporation will passivation solution out of the voids, which adapted to the galvanized.

5, organic silicide plugging method:

Dipping parts in carbon tetrachloride solution containing 4% organic silicide. Parts to preheat to 200 ℃, and then suddenly dipping into the above solution. Impregnated with again after drying under 200 ℃. This can be in parts surface forming a layer of organic silicide film, can have the effect of the plating solution out. This method has the satisfactory effect on the plating nickel, cadmium plated.

Second, the electroplating technology of powder metallurgy products

Here, the tube sheet is introduced under the powder metallurgy parts plating process instances, these instances are not using jam pore before plating.

Powder metallurgy copper/nickel plating, chrome plating process: (mu), the process flow is as follows:

Powder metallurgy parts (180 ℃ baking / 2 hours) - chemical to oil (2 minutes) - corrosion in hydrochloric acid solution (1 minute) and sodium cyanide (10%) and sodium cyanide copper plating corrosion (as required) - acetic acid solution (20%, 24 hours) - > galvanized (as required) to chromium plating, hot water washing and drying.

(my experience is: electroplating factory take the processing after plating the metal powder, I come back then have to be a hot cooked with oil, the temperature about 100 ℃ (boil), 30 minutes, the goal is to make better electrolyte discharge).

Formula is as follows:

1. The chemical to oil:

Caustic soda 40-60 g/l

30 g/l sodium carbonate

Trisodium phosphate 10 g/l

Water: 1 l

70-90 ℃ temperature.

2. The cyanide copper plating electrolyte:

Cuprous cyanide 35 g/l

8 g/l sodium cyanide (swim).

3. Acid nickel plating (light) :

Nickel sulfide 300 g/l,

10-15 g/l sodium chloride,

Boric acid 35 to 50 g/l,

Magnesium sulfate 50-80 - g/l,

0.05 1.0 g/l sodium dodecyl sulfate

Temperature 55 to 60 degrees

Current density of 2-4 a/cm2.

4. Chromium plating solution:

Proportion of chromic acid 300 g/l, sulfuric acid (1.84) 3.8 g/l,

40 to 55 degrees temperature, current density of 25 to 50 a/cm2.

Three, a factory of low tin bronze plating - chromium powder metallurgy parts, low - zinc, tin process is as follows:

To pack hanging fixture, preheating of powder metallurgy parts - anode corrosion - chrome plated (or decorative chromium plating - low tin bronze plating - continuous hot and cold water flushing to pack hanging fixture, chrome), and then hot, cold water washing and drying.

Above speak decorative chromium plating for plating a layer of low tin bronze plating before, this method is better than hard chromium plating, because low tin bronze adhesion is better than chrome, plating, on iron base powder metallurgy parts surface pore filling, and chrome plated.

The electrolyte formula:

1. Weak corrosion is to remove oxide layer, formula is: 100 ml - H2SO4, 900 ml water.

2. Strong corrosion formula is: 300 ml - H2SO4, 700 ml water.

3. The plating chromium plating solution formula is: the Cr2O3 MML, 200-250 - H2SO4 2 to 2.5 g/l.

4. Low tin bronze electrolyte formula:

CuCN 35 to 42 g/l

Na2SNO3. 3 h2o 30 to 40 g/l

NaCN 20-25 g/l

NaOH 7-10 g/l

With tin content of 10-12%, no more than 13%, more than for copper.

5. Passivation;

Cr3O3 250-300 - g/l

- H2SO4 for 6 to 12 g/l

HNO3-20 g/l

Temperature: 18 to 25 ℃

6, cyanide galvanization:

ZNO 5-50 g/l NaCN. 65-110 g/l

NaOH (70-100 g/l Na2S 0.5 5 g/l.

Four, powder metallurgy parts inspection method after plating

To test of powder plating quality, high demand for some parts, must be pick up check the plating quality and the pick up inspection method is: damp heat test or salt spray test.

A) hot and humid test method:

Test conditions: temperature 40 plus or minus 2 ℃, relative humidity 95-98%, the time is 7 days.

Test results: should be no corrosion for qualified.

B) the salt spray test:

Test conditions: temperature 35 plus or minus 2 ℃, relative humidity of 95% plus or minus 2, spray 1 times / 15 minutes, 16 hours per cycle, time for 10 days.

Salt spray formula:

NaCl 50 g/l

CaCl2.2 H2O 0.3 g/l

PH 3-3.2 (with glacial acetic acid to adjust)

Test results: the appearance should be no change.

Note: powder metallurgy parts of the lacquer that bake, no special requirements, the key is before immersion oil, it is best to surface finishing and polishing, and then to the lacquer that bake. Powder metallurgy parts with other metal paint the same painting process. Best to have automatic painting production line of the unit to make.