Corrosion Flashcards
Dry corrosion
Dry corrosion occurs through chemical reactions of metals or alloys with gases, in furnaces at high temperatures. A principal cause is a reaction of the metal with oxygen and other molecules in flue gases.
Wet corrosion
Wet corrosion occurs when a metal is placed into a fluid, usually an electrolyte. An electrolyte is a solution containing ions which carry electric charges.
Uniform attack
If a metal is placed in an electrolyte, some parts will become anodic while others become cathodic. The locations of cathode and anode will continually change, resulting in uniform corrosion.
Galvanic attack
This type of corrosion comes in three forms.
Galvanic corrosion occurs when dissimilar metals are placed together in the presence of a corrosive environment. Different metals have a greater affinity to corrode than one another, and so one becomes cathodic while the other becomes anodic.
Concentration cells occur when there is a difference in concentration of the electrolyte. Differentials in electrolyte cause one part to become anodic while the other becomes cathodic, causing corrosion.
Stress cells are the result of high residual stress in parts of a metal object. Areas of high stress tend to become anodic while the lower stress areas become cathodic.
Methods of protection
Cathodic Protection - Here, the object in question is made to become the cathode and hence protecting from corroding.
Sacrificial anodes - blocks of metal that are more anodic than the material used in the structure are attached, such that the original structure becomes the cathode and hence is the receiver of electrons rather than the loser. These find extensive use in boat hulls to stop the hull corroding in such a harsh environment.
Impressed current cathodic protection (ICCP) systems use a current to reverse the standard electrical flow associated with corrosion - i.e., the cathode becomes the anode, and the anode becomes the cathode.
