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Knowledge of powder metallurgy electroplating

There are few researches on electroplating and surface treatment in the teaching and scientific research of powder metallurgy major, and the manufacturers of powder metallurgy products basically have no electroplating production line (due to environmental protection problems), but if the general electroplating plants do not grasp the characteristics of powder metallurgy products, it is difficult to plating powder parts, and the information in this aspect is difficult to find and has more technical value.I have collected these things with my heart, plus a little of my own experience.Hope to have some meaning to the netizens of the same industry. 
Because powder metallurgy products contain certain pores, direct plating can not achieve good surface quality.At the same time, the electroplating solution remaining in the pores of parts is difficult to remove, which causes internal corrosion.Secondly, in the electroplating process, when the product is transferred from one bath to another, the residual solution in the pores will pollute the liquid in the other bath.After electroplating, the residual liquid in the pores will seep out of the coating again, causing rust spots on the surface of the workpiece. 
according to the practical experience, for the parts with density greater than 6.8g/cm3, the electroplating process is basically similar to steel parts, but the current density is a little higher; For parts with low density, sealing treatment should be carried out to plug the pores before electroplating. 
I.sealing method  
1.Mechanical hole sealing method:
large finishing allowance is left for parts, and surface pores are closed by finishing.Other mechanical methods can also be used to deform the surface to close the pores, such as sandblasting, grinding and polishing. 
2.Fixed material plugging method:
immerse the parts in molten zinc stearate at 200 ℃ to infiltrate into the pores for sealing.After immersion, the excess zinc stearate on the surface should be removed and then electroplated
it is also a feasible and simple method to use vacuum impregnated resin and high soft point paraffin
3.Steam treatment:
steam treatment can form a layer of Fe3O4 oxide film on the surface of parts, which can plug the surface pores.Fe3O4 is a dense semiconductor and does not hinder electroplating.Parts after steam treatment do not need any cleaning treatment before electroplating
4.Passivation solution filling method:
immerse the parts in the solution of 2G/L potassium dichromate, 2G/L baking soda, and the rest is water to fill the pores with passivation solution.The temperature of passivation solution is 70-80 ℃.After electroplating, the parts are heated at 100-110 ℃ and the passivation solution is discharged from the pores by evaporation of water in the pores.This method is suitable for galvanizing
5.Organic silicide plugging method:
immerse the parts in carbon tetrachloride solution containing 4% organic silicide.The parts are preheated to 200 ℃ and then immersed in the above solution.After soaking, dry at 200 ℃.In this way, a layer of organic silicide film can be formed on the surface of parts, which can prevent the invasion of electroplating solution.This method is satisfactory for nickel and cadmium plating
2.Electroplating process of P/M products
here, I will introduce the electroplating process examples of P/M parts.These examples are not used to plug pores before electroplating
P/M copper/nickel/chromium plating process:(31 μ), The process flow is as follows:
powder metallurgy parts (drying at 180 ℃ for 2 hours) → chemical degreasing (2 minutes) → hydrochloric acid solution corrosion (1 minute) → sodium cyanide (10%) → sodium cyanide copper plating (as required) → acetic acid solution corrosion (20%, 24 hours) → zinc plating (as required) → chromium plating → hot water washing → drying
(my experience is:after the electroplating factory finishes the powder parts with this process, I come back to use the engine oil for a hot boiling at about 100 ℃ for 30 minutes, so as to better discharge the electrolyte)
The formula is as follows:
1.Chemical degreasing:
caustic soda 40-60g/L
sodium carbonate 30g/L
trisodium phosphate 10g/L
water:1L
temperature 70-90 ℃
2.Cyanide copper plating electrolyte:
cuprous cyanide 35g/L
sodium cyanide 8g/L
3.Acid nickel plating (bright):
nickel sulfide 300g/L,
sodium chloride 10-15g/L,
boric acid 35-50g/L,
magnesium sulfate 50-80g/L,
sodium dodecyl sulfonate 0.05-1.0g/l,
temperature 55-60 ℃
current density 2-4a/cm2
4.Chromium plating solution:
chromic acid 300g/L, sulfuric acid (specific gravity 1.84) 3.8g/l,
temperature 40-55 ℃, current density 25-50a/cm2
(3) the process of low tin bronze chromium and low tin zinc plating on P/M parts is as follows:
P/M parts → mounting fixture → preheating → anodic corrosion → chromium plating (or decorative chromium plating → low tin bronze plating → continuous hot and cold water washing → mounting fixture → chromium plating), and then hot and cold water washing → drying
the decorative chromium plating mentioned above is to coat a layer of low tin bronze before plating, which is better than hard chromium plating, because the adhesion of low tin bronze is stronger than chromium.After plating, the pores on the surface of iron-based powder metallurgy parts are filled to facilitate chromium plating
electrolyte formula:
1.Weak corrosion is to remove the oxide layer, and the formula is:100ml h2so4900ml water
2.Strong corrosion formula:300ml h2so4700ml water
3.The formula of chromium plating solution is:Cr2O3 200-250ml, h2so42-2.5g/l<4.Low tin bronze electrolyte formula:
CuCN 35-42g/L
na2sno3.3h2o 30-40g/L
NaCN 20-25g/L
NaOH 7-10g/L
in which the tin content is 10-12%, not more than 13%, the rest is copper<5.Passivation treatment
cr3o3 250-300g/L
H2SO4 6-12g/L
HNO3 20-26g/L
temperature:18-25 ℃
6, cyanide zinc plating:
ZnO 5-50g/L, NaCN 65-110g/L
NaOH 70-100g/L, Na2S 0.5-5g/L<(4) inspection methods for P/M parts after electroplating
in order to inspect the electroplating quality of P/M parts, the electroplating quality inspection must be carried out for some parts with high requirements.The inspection methods are:Damp heat test method or salt spray test method
  A) Hygrothermal test method:
test conditions:temperature 40 ± 2 ℃, relative humidity 95-98%, time 7 days
test results:it is qualified if there is no corrosion
  B) salt spray test:
test conditions:temperature 35 + 2 degrees C, relative humidity 95% + 2, spray regulation 1 times/15 minutes, 16 hours per cycle, 10 days.Br/>salt spray formula:
NaCl 50g/L
cacl2.2h2o 0.3g/l
pH value 3-3.2 (adjusted with glacial acetic acid)
test results:the appearance should not change
note:there is no special requirement for the baking of P/M parts.The key is not to soak in oil before baking.It is better to polish or polish the surface before baking.The baking process of P/M parts is the same as that of other metals.It's better to go to the unit with automatic paint baking production line.


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