David Pye's series on the heat treatment of exotic metals continues. Here are Parts 1 and 2.
 

Galvanic Corrosion

Galvanic corrosion, or dissimilar-metal corrosion, occurs when two different metals are located together in a corrosive electrolyte. A galvanic couple forms between the two metals, where one metal becomes the anode and the other the cathode. The anode, or sacrificial metal, corrodes and deteriorates faster than it would alone, while the cathode deteriorates more slowly than it would otherwise.

Three conditions must exist for galvanic corrosion to occur.

  • Electrochemically dissimilar metals must be present
  • The metals must be in electrical contact
  • The metals must be exposed to an electrolyte


Environmental Cracking

Environmental cracking is a corrosion process that can result from a combination of environmental conditions affecting the metal. Chemical, temperature and stress-related conditions can result in the following types of environmental corrosion:

  • Stress-corrosion cracking (SCC)
  • Corrosion fatigue
  • Hydrogen-induced cracking
  • Liquid-metal embrittlement


Flow-Assisted Corrosion (FAC)

Flow-assisted corrosion, or flow-accelerated corrosion, results when a protective layer of oxide on a metal surface is dissolved or removed by wind or water, exposing the underlying metal to further corroding and deterioration.


Intergranular Corrosion

Intergranular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. It often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. These boundaries can be more vulnerable to corrosion than the bulk of the metal.


De-alloying

De-alloying, or selective leaching, is the selective corrosion of a specific element in an alloy. The most common type of de-alloying is de-zincification of unstabilized brass. The result of corrosion in such cases is a deteriorated and porous copper.


Fretting Corrosion

Fretting corrosion occurs as a result of repeated wearing, weight and/or vibration on an uneven, rough surface. Corrosion, resulting in pits and grooves, occurs on the surface. Fretting corrosion is often found in rotation and impact machinery, bolted assemblies and bearings, as well as to surfaces exposed to vibration during transportation.
 

High-Temperature Corrosion

Fuels used in gas turbines, diesel engines and other machinery, which contain vanadium or sulfates, can (during combustion) form compounds with a low melting point. These compounds are very corrosive toward metal alloys normally resistant to high temperatures and corrosion, including stainless steel. High-temperature corrosion can also be caused by high-temperature oxidization, sulfidation and carbonization.

The procedure of cleaning and finishing prior to heat treatment (irrespective of the as-received surface condition) has a tendency be overlooked and, in some cases, not thoroughly applied. Sometimes the procedure is overlooked because of costs. In some cases (that the writer has been involved in with the use of city water), it is not often known if there are other contaminants in the city water that can affect the surface-cleaning quality. Sometimes there is a tendency to forget when handling components of titanium alloys to wear lint-free gloves. Handling the component by exposed hands and fingers will leave body oil on the surface of the component.