Finishing a bikeframe

  Paints

To protect the frame against corrosion, a finishing layer will be applied; usually the frame is spayed with paint. Before finishing, the frames must be prepared as well as possible. With steel factory frames, it is common to blast and phosphate the frames before applying the primer. This removes the remnants of the flux. This stuff is crystal clear; but it also attracts moisture. If residues remain under the lacquer, they will swell through it like pustules. Phosphatizing is really nothing more than hanging the frame in a large container with diluted phosphoric acid. The phosphate layer that forms on the steel, provides good adhesion for the primer.

An old-fashioned lacquer is a resin (= binding agent) and a pigment (= colorant) dissolved in light hydrocarbons (= turpentine). The solvent evaporates, resin and dye remain. The resin reacts with oxygen, so that after curing it no longer dissolves in white spirit. In the past, natural resins were used, nowadays mostly alkyd resin (= polyester). In aerosol form, in addition to slow-drying alkyd resins, also fast-drying acrylate and nitrocellulose lacquers are available. An annoying phenomenon with multilayer spraying is that the solvent from the new layer often affects the undercoat; try it out before you start. Water-based coatings are a modern trend. Here the resin (usually acrylic) is mixed with an emulsifier (= soap) and water mixed into a dispersion (= slurry). The water disappears through drying and the resin remains and hardens.

The highest quality paints consist of two components. Certainly aluminum should be sprayed with 2-component lacquer; it is also used for reinforced plastics. After mixing, these lacquers must be processed quickly. “Thermoset” networks are created in the paint (epoxy or polyurethane). A coarse-mesh network is flexible and impact-resistant, and a fine-mesh network is hard, but certainly not impact-resistant! Polyurethane is usually chosen as the top layer, because it is insensitive to UV light.

Two-component lacquer can be used longer when the components only become active at a higher temperature, and the sprayed frame is dryed in an oven. Such an oven process is also used in another modern painting technique: powder coatings. Electrostatically (positively) charged powders are sprayed onto a conductive substrate (frame) and heated at 200 ° C.

Unfortunately, these 2 methods are only suitable for manufacturers. Do-it-your self painters will often still be using solvent based lacquers. Make sure the surface is free of grease, dust and oxide. Use a primer; it contains metal molecules and ions, which provide better adhesion to the substrate. When spraying, wear a dust mask and remember that explosive vapors can be generated: be careful with electric sparks! You can also use a paintbrush.

Galvanic processes

Unfortunately, these are also only suitable for manufacturers. After blasting, polishing and degreasing, steel can be nickel plated or chrome plated by an electrolytic process. The frame is the cathode (negatively charged). The positive nickel or chromium ions are attracted to the cathode, absorb and deposit electrons. With nickel plating, a power source is not always necessary, because there are nickel baths in which the nickel chemically deposits on the steel. An additional advantage of this process is the very even layer thickness. Nickel plating is often used as an adhesive layer for chrome plating. Although chrome layers are decorative and shiny, the pipe manufacturers actually advise against it. Chrome is very hard and hairline cracks quickly occur, which can lead to corrosion under the chrome layer. The chrome layer then easily peels off; certainly thin-walled pipe loses so much strength. Chrome-plated spokes are also not recommended for this reason. The chrome plating itself is done in baths with chromic and sulfuric acids; these baths are very polluting for the environment (see the video at the bottom of the page).

Aluminum is in principle corrosion resistant, because the oxide forms a sealing layer, but with the stronger alloys this property has been lost. Galvanic processes for the protection of aluminum are commonly referred to as anodizing. Because aluminum is not used as a cathode, but as an anode in the process, we also speak of anodizing. First, the dirt is removed and the part is hung in an acid bath to remove the oxides; then it enters a bath with sodium hydroxide solution (5%) where it is etched. Then it enters a bath with approximately 15% sulfuric acid as an anode. A thick layer of oxides is now formed that is slightly porous. We can then hang the part in a container of dye; the porous layer absorbs the dye, and must then be sealed. This is done in a container with nickel acetate or nickel fluoride. If we don't use color, we can seal the layer in a container of boiling water. Here too, this will result in a tightly sealing oxide layer. Hard anodizing, as is often the case with rims, is done with alternating current in baths with the active ingredient oxalic acid (3%), at 20-30 ºC. The color is gray to black.

Magnesium is very sensitive to corrosion, but a new anodizing process (Keronite) makes this material very useful. Known applications include fork legs for telescopic suspension.

Titanium and stainless steel are corrosion resistant; nevertheless, clear coat is often sprayed over the stickers.