Aluminum and aluminum alloy plating is more difficult, because aluminum has a strong affinity for oxygen, the surface is easily oxidized; aluminum is amphoteric metal, is not stable in acid, alkali; aluminum potential is very negative, easy to be immersed in the plating solution Erosion and displacement of the metal being plated; Aluminum's expansion coefficient is larger than most metals, so the coating is easy to air bubbles, fall off, affecting the binding force.
The key to the electroplating of aluminum and aluminum alloys is the special pre-plating treatment that removes the natural oxide film and prevents its re-formation, as well as preventing the displacement reaction when the aluminum parts are immersed in the electrolyte. As a naval overhauled aircraft factory, we often encounter aluminum and aluminum alloy plating during the repair process. Through the experiment, the better process conditions were explored. The surface of the aluminum alloy plating blessing parts not only has a good gloss, but also has good bonding and corrosion resistance and meets the requirements for navigation marks. This article describes the plating process for the repair of old aluminum alloy parts.
1 Process flow
Degreasing (degreasing) → Retreating the old coating → Two cleaning → Alkali etching → Two cleanings → De-ashing (light extraction) → Two cleanings → One sink zinc → Two cleanings → De-zincification → Two-way cleaning → Secondary zinc reduction → Sulfuric acid neutralization, activation → Cleaning → Electroless nickel plating → Two cleaning → Activation → Rhodium plating → Two-step cleaning → Passivation → Two-step cleaning → Blow-drying → Aging → Inspection database.
2 Description of main processes 2.1 Composition of degreasing solution and process conditions Sodium hydroxide 40-609/L, 70-90°C, 1-Zmin.
Heavy oil is first cleaned with organic solvents.
2.2 Withdrawal of the old coating Removal of the old coating is one of the key processes for the tinning of aluminum alloys.
2.2.1 The composition and operation conditions of the old forgotten layer solution shall be withdrawn. The amount of nitrate shall be 10 to 150 and leave at room temperature.
2.2.2 Solution composition and operating conditions for removing the old zinc layer Sulfuric acid 10 to 150 and leave L at room temperature.
2.2.3 Remove the old oxide film solution composition and operating conditions Sodium hydroxide 40 609/L, 50 to 90 "C, retired.
2.24 Retreat of Ni-P alloy solution composition and process conditions Concentrated nitric acid 700mL / L, 30-409/L sodium chloride, hexamethylenetetramine 6-109 / L, urea 15-ZoglL, room temperature.
(1) The pre-plating parts should be dried or blown with cold air to prevent the water content from reducing the concentration of nitric acid, resulting in corrosion of the substrate.
(2) The deplated parts should be cleaned quickly so as not to produce a black oxide film on the surface after the deplating.
Aluminum alloy electroless nickel plating is generally used to remove concentrated nitric acid. Although concentrated nitric acid has strong oxidizing ability, concentrated nitric acid has a slower corrosion rate to the Ni-P alloy layer, and a large amount of No 2 gas is generated when the concentrated nitric acid is removed, which greatly pollutes the environment. After the electroless Ni-P alloy layer of precision parts is deplated, the amount of concentrated nitric acid on the base metal is often excessive, especially for bolts. In order to overcome the drawbacks of the over-etching of the electroless nickel plating layer on the bolts, we have passed hundreds of tests to screen out the above-mentioned nickel removal process. When the Ni-P alloy layer is withdrawn, the corrosion of the substrate is very small, and it is not easy to be over-etched, and the gas during the deposition is also less, the quality of the plating can be guaranteed, and the working environment is improved.
2.3 Alkali etching Sodium hydroxide 40 to 6og/L, 70 to 90 "c, 1 to 2 and n.
2.4 Deashing (light)
After the surface of aluminum and aluminum alloys are alkali-etched, a layer of grey or black ash is attached to the surface of the parts, and is also not cleaned in flowing cold or hot water, and can be removed in the following solutions.
Chromium drunk 10 9 several, sulfuric acid 10mIJL, room temperature, except net.
The ash on the surface of aluminum alloy products containing silicon or manganese (especially cast aluminum with higher silicon content) should be removed in the following solution: V (nitric acid): V (hydrofluoric acid) 2:1 mixed acid (Because the reaction is fierce, add a certain amount of corrosion inhibitor), dip 55 or so, can be removed.
2.5 Zinc dipping
Zinc dipping liquid formulation and process conditions:
Sodium hydroxide 40 to 509/L, zinc oxide 10 to 150 stay L, potassium sodium tartrate 11 to 20 years old L, ferric chloride 1 to 29, hydrofluoric acid lg/L, room temperature, (30±5) 5 .
Zinc leaching is a very critical process. A good zinc immersion layer can bond well with the substrate and the subsequent nickel layer, and can mask defects on the surface of the substrate and help to obtain a good coating.
We use alkaline zinc dipping solution. It is only necessary to dip a layer of zinc on the surface of the workpiece to allow the difference in color. However, it must be ensured that the surface of the substrate is completely impregnated with a zinc layer. Otherwise, the next step cannot be performed. After immersing zinc, do not rinse with water.
2.6 Zinc removal
When zinc is first immersed, the aluminum oxide film is first dissolved, zinc is deposited on the surface of the aluminum workpiece, and it is inevitable that there is still a small amount of oxide film remaining, the deposition layer is not uniform, and the bonding force is poor. After the nitric acid was removed, the surface of the aluminum workpiece was further purified. The zinc is then submerged for a second time in the zinc dipping solution to obtain a dense, uniform and strong zinc layer.
Dezincification fluid composition and operating conditions:
Nitric acid somuL with a mass fraction of 65%, normal temperature, (5 ± 5) 5, and the zinc layer must be completely removed.
2.7 Neutralization and Activation Solution Composition and Process Conditions:
Sulfuric acid 0 .7 to 1.09/L, room temperature, 5 to 10,.
2.8 Electroless Nickel Plating
Different from general electroplating, electroless nickel plating is performed by autocatalytic reaction on the surface; these are based on proper pretreatment, and poor precoating results in poor adhesion, rough coating, and reduced corrosion resistance of the coating. . In addition, the electroless nickel plating solution is more sensitive, and the residual solution and residue brought into the pretreatment will affect the life of the plating solution, resulting in deterioration of the plating solution.
Bath formulation and process conditions:
NiSO4·7H20.21g/L, NaH2PO2·H202.49/L, lactic acid (88%) 30mL/L, C2H5C00H.2g/L, lead ion 1mg/L, pH 4.5±0.5, temperature (95% of soil 2) ). C, (24 ± 8) and n, the deposition rate is 17 μm/h, and the mass fraction of phosphorus in the coating is 8% to 9%.
In the process of electroless nickel plating, the surface of the electroless nickel plating layer allows the nickel layer to be black, but no color difference is allowed and uniform color is required.
2.9 Tin Plating Bath Composition and Process Conditions:
CdO.40 to 45g/L, NaCN.11O to 140g/L, Na2SO4·10H2O.40 to 60g/L, NiS04·7H2O.0.5 to 1.5g/L, brightener 5 to 8mL/L, 15 to 30°C, 1.0-2.5A/dm2, 3-6v, 15-
In 60 min, the anode material is Cd-1 and Cd-2.
2.9.1 Reversible electroplating (1) After electroplating for 5 minutes, reversing plating is allowed to be performed, tK:tA=10:1.
(2) At the beginning, give a small current and gradually rise to normal current within 1 to 5 minutes; 1 to 3 minutes before the slot, sometimes to increase the current density 0.5 to 1.0 times to avoid impact pinhole and bubble phenomenon plating.
3 Inspection According to the aerospace industry standard HB5036:92 inspection side.
3.1 Appearance Visual inspection is carried out with natural scattered light or white transmitted light. Under special circumstances, a 3- to 5-fold magnifying glass is used for inspection. The passivated tantalum coating is gold-yellow with a rainbow, and the passivation film is complete and uniform in color.
3.2 Thickness The thickness of the plating layer is 15-25μm.
3.3 Cohesion Force specimens of 7Omm x 25mm fire (1 to 2) mm in size and surface roughness to be clipped, clamped with pliers, and repeatedly bent 180 "(bend to each side 90 degrees) until fractured; or parts (samples) ) Iteratively bent 180. Until the part (specimen) breaks, the coating should not be separated from the base metal. When examined with a 5x magnifier, cracks occur in the base metal or coating, but no peeling or peeling, the coating binding force Qualified.Testing aluminum alloy electroplating parts is very good.
3.4 Corrosion resistance The rhodium plated layer is subjected to a salt spray corrosion test in accordance with GB6458, and the corrosion resistance has met the requirements of HB5362. The time for the white corrosion product of tin coating yellow passivation salt spray test is greater than % h; the time for the red corrosion product salt spray test is greater than 360 h.
3.5 Hardness The Vickers hardness of the coating is 475, which is higher than the hardness of the aluminum alloy 120.
4 Conclusion
With the development of miniaturization and high motorization of military equipment in China, more and more aluminum alloy materials are used in airborne equipment products. Through trials and investigations, a complete set of aluminum and aluminum alloy rhodium-plating repair process was explored. The product quality fully met the requirements of the military and was well received by the users. In particular, there has been a new breakthrough in the nickel-removal process. The process is characterized by small pollution, low gas, and good over-corrosion protection, and plays an important role in the repair of aluminum and aluminum alloy parts on aircraft.
Magnetic Drive Centrifugal Pump
Magnetic Drive Centrifugal Pump,Magnetic Drive Pump,Centrifugal Pump,Mag Drive Centrifugal Pump
Hengshui Yuanhan Trading Co.,Ltd , https://www.hengshuiyuanhanpump.com