Discussion on Electroplating and Rhodium Plating of Aluminum and Aluminum Alloys

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) â†’ Retreating the old coating â†’ Two cleaning â†’ Alkali etching â†’ Two cleanings â†’ De-ashing (light extraction) â†’ Two cleanings â†’ One sink zinc â†’ Two cleanings â†’ De-zincification â†’ Two-way cleaning â†’ Secondary zinc reduction â†’ Sulfuric acid neutralization, activation â†’ Cleaning â†’ Electroless nickel plating â†’ Two cleaning â†’ Activation â†’ Rhodium plating â†’ Two-step cleaning â†’ Passivation â†’ Two-step cleaning â†’ Blow-drying â†’ Aging â†’ Inspection database.

2 The main process description

2.1 Degreasing liquid composition and process conditions

Sodium hydroxide 40-609/L, 70-90 "c, 1-Zmin.

Heavy oil is first cleaned with organic solvents.

2.2 Retire old plating

The removal of the old coating is one of the key processes for the tinning of aluminum alloys.

2.2.1 Composition and operation conditions of the old fortified layer solution

Nitrate 1 0 to 150 stay L, room temperature, retired so far.

2.2.2 Solution composition and operating conditions of the old zinc layer

Sulfuric acid 10 to 150 stay L, room temperature, retreat so far.

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 Retreating Ni-P alloy solution composition and process conditions

Concentrated nitric acid 700 mL/L, sodium chloride 30 to 409/L, hexamethylenetetramine 6 to 109/L, urea 15 to 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.