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Fastener Corrosion

Plan For, Shop For & Fasten Confidently with Tanner

Interested in learning more technical information about fasteners? Please check out our Fastener Technical Data Center here. In our Fastener Technical Data Center, you will find a centralized location of essential technical data about a wide variety of aspects all relating to fasteners.

Corrosion is the wearing away or alteration of a metal by galvanic (electrochemical) reaction or by direct chemical attack. An example is the rusting of iron or steel.

Different Types of Corrosion

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 attack is directly related to the amount of water vapor, sulfur and chlorine compounds present.

On contact of steel and copper in moisture, atoms of iron divide

  1. Positive particles of metal dissolve in the moisture, absorbing oxygen andhydrogen, becoming ferrous ions.
  2. Negative charged electrons flow through steel to copper into the moisture where they combine with oxygen and water, becoming hydroxyl ions.
  3. Hydroxyl ions combine with ferrous ions, producing iron oxide (rust), the corrosion product.

Galvanic Corrosion (Electromechanical)

All metals have a specific relative electrical potential.When metals of different electrical potential, such as steel and copper, are in contact in the presence of moisture (electrolyte), a low energy electric current flows from the metal having the higher position in the galvanic series to the one having the lower position.

This is called “galvanic” action. One result is that corrosion of the metal having the higher position (steel in this example) is accelerated. Corrosion may be thought of as a by-product, something akin to the forming of ash when wood burns.

Actually, the mechanism is an anode reaction, a cathode reaction, the conduction of electrons through the metal from anode to cathode, and the conduction of ions through the electrolyte solution. Corrosion occurs in the anode area, while the cathode area is protected.

It is important to know from which of two metals current will flow. A guide is provided by the arrangement of metals and alloys set forth in the galvanic series chart.

Many different types of corrosion have been identified. Most are electrochemical in nature. Thus, crevice or cell corrosion, stress or fatigue corrosion, deposit and impingement attack and intergranular corrosion are all forms of galvanic corrosion caused by localized galvanic cells of different potentials.

Direct Attack

Select the material most likely to resist the corrosive environment to which the fastener will be subjected.

Galvanic Attack

  1. If possible, use the same or similar metals in an assembly, especially where an electrolyte may be present.
  2. When dissimilar metals are used together in the presence of an electrolyte, separate them with a dielectric material such as insulation, paint or coating.
  3. Avoid combinations where the area of the less noble material is relatively small. The current density is greater when the current flows from the small area to the large than in the reverse situation. Typically, the fastener will be small compared to the rest of the assembly. The fastener alloy, if not the same as the material being joined, should be lower in the galvanic series.
  4. The galvanic process can be used to advantage by coupling the part to be protected to pieces of less noble metal which are not functional and can thus corrode sacrificially.

Avoid Irregular Stresses

As a general rule in using mechanical fasteners, avoid irregular stresses in design. Even high stresses in bolted assemblies do not necessarily impair corrosion-resistance as long as they are uniform.

Corrosion Diagram

Corrosion Diagram

Galvanic Series Chart

Galvanic Series Chart

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