Day 36: Introduction to Corrosion. Importance of Corrosion What is Corrosion? Some theory. The four things that are required for corrosion Types of Corrosion. Environmental Degradation of Materials. Materials are “attacked” by their operating environment.
Direct Costs of Corrosion:
Nearly 300 G$ in 1998. Clearly, they rise proportionally till today.
Between 3-5% of the Gross Domestic Product.
From Corrosion Doctors web site
This obvious, up front relatively uniform corrosion is relatively benign. We see it for a long time before it hurts us. Not all corrosion is so nice.
Atlantic Southeast 529, 8-21-1995
Lead wool added for balancing
The Safety Board concludes that one of the four blades from the left
engine propeller separated in flight because a fatigue crack that originated from multiple corrosion pits in the taper bore surface of the blade spar propagated toward the outside of the blade, around both sides of the taper bore, then reached critical size. (See Section 1.16.1.)
Results of investigations conducted in two previous propeller blade
failures in 1994, one in Brazil with this model blade and the other in Canada with a similar model blade, indicated that corrosion was produced when entrapped moisture reacted with residual chlorine in a bleached cork used to retain the lead wool in the taper bore hole of the propeller.
Point: Corrosion is subtle and very hard to detect
Note the current path. The salt bridge provides for ion exchange.
Any voltage, even if small will produce corrosion damage over time.
Clearly dissimilar metals will create a corrosion cell. The anodic metal will be damaged.
We are assuming that the Fe is surrounded by a weak acid in which H+ ions are abundant.
This acid is called an electrolyte. It provides a home for the dissolve Fe+2 ion.
Note that there has to be an internal movement of electrons through the Fe.
Surfaces near high O2 concentration are cathodic!
Steel screw in Mg
Steel screws and brass
Dissimilar metals, the damage occurs at the anode.
This is a concentration cell in action. Notice how the damage occurs in out of sight places.
Highly localized. Goes deep into the metal.
Chloride ions find their way into the pits, exacerbating the situation.
Failure is along grain boundaries.
Tiny cathode (steel)
Large area anode (zinc)
An example of a favorable area ratio. Bad deal: huge cathode, tiny anode
By imposing a voltage which causes electrons to flow towards the object to be protected, we make it less anodic and protect it from corrosion damage.
Free Radical oxidation of UHMWPE tibial implant. Could happen in vivo or in vitro. Vitamin E has been tried to deal with the free radicals.