First Stage

Fig. C-14. Typical Creep Curve

The stress rupture test is similar to the creep test except that the test is always carried out to failure. Higher stresses are used and therefore the creep rates are higher, with strains often approaching 50%. Due to the higher stress levels, stress rupture tests can often cause failures in 1000 h, or less. The higher stress levels, and greater creep levels, used in the stress rupture test cause microstructural changes (e.g., overaging) to occur in shorter times than they would in a normal creep test. A break in the stress rupture curve, as shown in Fig. C.15, is often indicative of a microstructural instability.

Creep becomes much more complicated when it is combined with cyclic loading and fatigue also becomes operative.

C.6 Corrosion10

Corrosion is the gradual degradation of a material due to the environment. In chemical corrosion, the material dissolves in a corrosive liquid until the material is consumed. In electrochemical corrosion, metal atoms are removed from the solid material due to an electrical circuit that is produced. In electrochemical corrosion, the anode, which is the metal (Ma) that corrodes, undergoes an oxidation reaction and gives up electrons to the circuit.

A reduction reaction, which is the reverse of the anode reaction, occurs at the cathode.

T = Temperature

T = Temperature

Log Rupture Time

Fig. C-15. Stress-Rupture Curves

In oxygen free liquids, such as acid pickling baths, the most common cathodic reaction evolves hydrogen.

Hydrogen produced in this manner is important in both hydrogen embrittle-ment and stress corrosion cracking.

For electrochemical corrosion to occur, there must be a liquid electrolyte (e.g., water) to allow an electrical circuit to transport the current (i.e., electrons).

Galvanic corrosion is a quite common type of attack that occurs when two metals of different compositions are electrically coupled in the presence of an electrolyte. The less noble, or more reactive, metal becomes the anode and corrodes, while the more inert metal, or the cathode, does not corrode. The galvanic series in seawater is shown in Fig. C.16. An example of galvanic




Magnesium Alloys

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