An example is the rusting of iron, or steel. Direct Attack Corrosion. Atmospheric corrosion is an example of direct chemical attack. Present in the atmosphere are oxygen, carbon dioxide, water vapor, sulfur, and chlorine compounds. The severity of the attack is directly related to the amount of water vapor, sulfur and chlorine compounds present.
All metals have specific relative electrical potential.
When metals of different electrical potenial such as steel and copper are in contact in the presence of moisture, a low energy electric current flows from the metal having the higher position in the galvanic series. To help prevent direct attack corrosion. Select the material most likey to resist the corrosive environment to which the fastener will be subjected. To help prevent galvanic corrosion. If possible use the same, or similar metals in an assembly, especially where an electrolyte may be present.