Material Corrosion Flashcards
Define corrosion in metals
Corrosion is defined as the destructive and unintentional attack of a metal ; it is electrochemical and ordinarily begins at the surface.
What is an electrochemical process?
Electrochemical processes are ones in which there is a chemical reaction in which there is a transfer of electrons from one chemical species to another.
What is the galvanic series?
A table that shows the relative reactivities of a number of metals and alloys. The alloys near the top are cathodic and unreactive, whereas those at the bottom are most anodic.
What is passivity?
A phenomenon where normally active metals lose their chemical reactivity and become extremely inert. This happens under particular environmental conditions due to the formation of a highly adherent and very thin oxide film on the metal surface, which serves as a protective barrier to prevent further corrosion.
What affect does coldworking have on the corrosion of a material.
Cold working or plastically deforming ductile metals is used to increase their strength; however, a cold-worked metal is more susceptible to corrosion than the same material in an annealed state.
What are the eight forms of metallic corrosion
Uniform Galvanic Crevice Pitting Intergranular Selective leaching Erosion-Corrosion Stress Corrosion
What is uniform attack
Uniform attack is the electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface as oxidation and reduction reactions occur randomly.
What is galvanic corrosion?
Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. Typically, the most reactive metal will corrode before the more inert one and thus protect it.
What is an electrolyte?
An electrolyte is typically a liquid which contains anions and cations which in solution is electrically neutral but when an electric potential is applied the charged ions moves to their respective electrodes and an electric current can flow.
What effects rate of galvanic attack?
The rate of galvanic attack depends on the relative anode-to-cathode surface areas that are exposed to the electrolyte, and the rate is related directly to the cathode-anode area ratio; that is, for a given cathode area, a smaller anode will corrode more rapidly than a larger one. The reason for this is that corrosion rate depends on current density.
What measures may be taken to reduce the effects of galvanic corrosion?
- If coupling of dissimilar metals is necessary, choose two that are close in the galvanic series
- Avoid an unfavorable anode-to-cathode surface area ratio; use a large anode area
- Electrically insulate dissimilar metals from each other
- Electrically connect a third, anodic metal to the other two (cathodic protection)
What is crevice corrosion?
A build up of ions or dissolved gases in a crevice region between two metal pieces can lead to electrochemical corrosion.
What are the conditions for crevice corrosion to occur?
For such a concentration cell to form, the crevice must be wide enough for the solution to penetrate, yet narrow enough for stagnancy; usually the width is several thousandths of an inch.
How may crevice corrosion be prevented?
Crevice corrosion may be prevented by using welded instead of riveted or bolted joints, using non- absorbing gaskets when possible, removing accumulated deposits frequently, and designing containment vessels to avoid stagnant areas and ensure complete drainage.
What is pitting?
Pitting has a similar corrosion mechanism to crevice corrosion but occurs on the top of a horizontal surface downwards.
