BDCS Notes - Steel Corrosion

Corrosion: The destruction of a material by electrochemical reaction to the environment (or the destruction that can be detected by rust formation). Can result in lowering weight limits of structures, costly steel replacement, and/or collapse of a structure.

Corrosion requires four elements:

1. An Anode – The electrode where corrosion occurs.
2. A Cathode – The other electrode needed to form a corrosion cell.
3. A Conductor – A metallic pathway for electrons to flow.
4. An Electrolyte – A liquid that can support the flow of electrons.

Steel, being a heterogeneous material, contains anodes and cathodes. Steel is also an electrical conductor. Therefore, steel contains three of the four elements needed for corrosion, while moisture provides the fourth element (an electrolyte).

Salt, from deicing or a marine environment, accelerates corrosion of steel bridges and reinforcing steel in concrete.

Methods for Corrosion Resistance

Three mechanisms by which coatings provide corrosion protection:

1. Barrier Coatings – Isolate the steel from the moisture. Low water and oxygen permeability.

2. Inhabitive Primer Coatings - Contain passivating pigments. Low-solubility pigments that migrate to steel surface when moisture passes through the film to passivate steel surface.

3. Sacrificial Primers (Cathodic Protection) – Contain pigments (such as zinc), which gives up electrons to the steel, becomes the anode, and corrodes to protect the steel.

BDCS Notes - Aluminum Testing and Properties

Aluminum Testing and Properties

Tests for aluminum are similar to those for steel. They typically include stress-strain tensile tests to determine elastic modulus, yield strength, ultimate strength, and percent elongation.

In contrast to steel, aluminum alloys do not display an upper and lower yield point. Instead, the stress-strain curve is linear up to the proportional limit and then is a smooth curve up to the ultimate strength. Yield strength is still based on the .2% offset method.

The modulus of elasticity for aluminum is 69 GPa (10,000 ksi).

The coefficient of thermal expansion for aluminum is .000023/ degree C. This is twice that of steel and concrete. Therefore, joints between aluminum and steel or concrete must be designed to accommodate differential movement.

Obviously, strengths of aluminum are considerably affected by temperature. At temperatures above 150 C, tensile strengths are reduced considerably.

Welding and Fastening

Aluminum pieces are joined either by welding or fastening. Welding requires that tough oxide coating on aluminum be broken and kept from reforming during welding, so arc welding helps shield oxygen from reentering the atmosphere.

Two kinds of arc welding are:
Gas Metal Arc Welding (GMAW): Filler wire serves as the electrode.
Gas Tungsten Arc Welding (GTAW): Tungsten electrode and a separate filler wire.

Fastening: Either aluminum or steel can be used. When steel bolts are used, they must be galvanized, aluminized, cadmium plated, or made of stainless steel to prevent galvanic corrosion.

Corrosion: Aluminum develops a thin oxidation layer immediately upon exposure to the atmosphere. A tough oxide film protects the surface from further oxidation. Alloying elements alter the corrosion resistance of the aluminum.

Airplane Metal: Give extra protection by painting or cladding with a thin coat of a corrosion-resistant alloy.

Galvanic Corrosion: Occurs when aluminum is in contact with any of several metals in the presence of an electrical conductor, such as water. Best protection method is to break the path of the galvanic cell by painting, using an insulator, or keeping dissimilar metals dry.