Corrosion is the deterioration of a metal because of a reaction with its environment. The products of corrosion vary widely, from the flaky red dust of iron to the relatively benign oxide over aluminum. In general, corrosion should be avoided or minimized where possible.
There are many types of corrosion, but a particular type called galvanic corrosion is most important for inserts and fasteners. Galvanic corrosion occurs when dissimilar metals are in contact in the presence of an electrolytic solution. All metals exhibit different degrees of "activity" or "nobility" and can be arranged in a galvanic series of increasing activity. Gold and platinum are most noble, while zinc and magnesium are most active. The most common electrolytic solution encountered is ordinary water. Seawater or salt spray is more damaging because of high concentrations of dissolved salts.
The best way to preclude galvanic corrosion is to use similar potential metals and eliminate the electrolyte conductor.
The active stainless steel CoilThread® Insert are not passivated. This minimizes the possibility of galvanic corrosion occurring when they are installed in aluminum or magnesium parent materials.
Some additional precautions for reducing galvanic corrosion are:
- Isolate the fasteners from the electrolyte. This can be done through gasketing or sealing.
- Specify cadmium plated inserts. The cadmium plate provides a sacrificial barrier against corrosion. In addition, the cadmium plate has lubricating properties that minimize galling when stainless steel screws are used.
- Apply corrosion inhibiting pastes or compounds to the screw. These include zinc chromate primer
(MIL-P-8585) and strontium chromate primer (MIL-P-23377).
Note: Pastes applied to the CoilThread insert can become trapped between the wire and the hole and can cause loss of proper tolerance. It is therefore recommended to apply the paste only to the screw, not the insert. If Zinc chromate primer is applied to the tapped hole, it should be thinned and applied sparingly. The insert should be installed while the primer is still wet.
- Specify a dry film lubricant such as molybdenum disulphide on the inserts. This provides a secondary barrier against corrosion.
- Where practical, or when it will not interfere with the completed assembly, the external joint should be coated with a suitable paint.