Materials - Test 7 Flashcards

Environmental Degradation

degredation of one or more material properties as a result of exposure to its surrounding environment

Corrosion

An electrochemical attack of a material normally resulting in oxidation of that metal - it's trying to return the material to its original, low energy state.

What are the key elements for corrosion to proceed?

1. Water/moisture is present to act as a medium for ion conduction and ionic transport (electrolyte)

2. Reduction in energy — thermodynamic driving force

3. Source of oxidant (usually oxygen, sometimes sulphur)

4. Mass transport of oxidant to the metal surface

Uniform Attack

1. Oxidation and reduction occur uniformly.

2. Volume change in oxide displaces the oxide layer from the material - exposing the metal underneath.

Crevice Corrosion

Occurs between two pieces of a metal where access to oxygen is restricted

Intergranular Corrosion

Occurs along grain boundaries where special phases exists at around a very high temperatures.

Galvanic Corrosion

It is an electrochemical process where one metal corrodes in preference to another metal it is in contact with through an electrolyte. More reactive metal = anode, and less reactive is the cathode.

Pitting Corrosion

Downward propagation of small pits and holes in metal (occurs in weird environments like high salt)

Stress Corrosion

Stress and corrosion work together at crack tips - accelerating crack propagation.

Erosion-Corrosion

The gradual wearing away of a metal surface by a combination of both corrosion and abrasion from an impinging water stream

Selective Leaching

Preferred corrosion of an element in an alloy

Self Formation of Protective Oxide

The oxide layer forms upon initial exposure to oxygen. It will adhere to the surface and dense enough to limit oxygen transport. It happens in aluminium alloy, stainless steel (Cr addition), and titanium

Physical Barrier - Paint

Coating the surface with nonreactive material inhibits contact of water and oxygen with the surface or metal.

Sacrificial or cathodic protection

More chemically active layer than given metal. The sacrificial coating acts as the anode and corrodes while the main metal stays intact. Choose more active materials in the Galvanic and EMG series.

Allotropes

A type of phase where the solid itself has different structural forms like FCC, BCC and HCP.

Binary Isomorphous

Two elements who will only have one type of a solid phase.

Eutectic System

a mixture of elements with the same chemical composition that solidifies at a lower temperature than any other composition of the same elements.

Eutectic Point

Where two liquidus lines will coincide. The lowest temperature at which the liquid phase is stable at a given pressure and the easiest melting temperature.

Eutectoid Point

Two solid phases turning into a totally different third phase.

Pearlite

A matter in mollusks and pearls, mechanically, is between soft, ductile ferrite and the brittle, hard cementite

Annealing

raising the temp to convert the sample to a uniform phase and reduce the effects of cold work

Normalizing/Solutionizing

getting rid of all defects or making a uniform “solution”