Corrosion...

I've been meaning to post on this subject for awhile. I think that most people think their aluminum or even Titanium bike is impervious to the elements.  It's pretty natural to think that aluminum doesn't corrode and you've probably been told that titanium doesn't either.  Let me first preface this by saying that I am by no means an expert in the area of metallurgical properties.  This being said, I'd like to just touch upon the effects that the elements (rain, road salt, and even sweat) have on your frame.  Here's some eye-opening info...

Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases.

Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes, a crack, or it can extend across a wide area to produce general wastage.  Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations....

It continues...

Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidized, some more easily than others...

Here are a few photos of a Colnago that I recently worked on.  Notice the corrosion that's occurring under the paint...

This type of problem is more common than you think.  Consider yourself warned... Thanks for reading. -John

Comments

[uber]

freakin colnago's!

[Fritz]

"All metals exhibit a tendency to be oxidized..." -- true, but with Aluminum the oxidized layer forms a protective barrier that prevents oxygen and water from penetrating deeper into the metal. There are actually two layers forms -- a thin outside, permeable layer and an inner layer that's a little thicker and impermeable.

[John Satory]

Thanks for your comment Fritz.
This is true. From what info I found, aluminum does form a barrier oxide film when the surface is abraded and then exposed to air. The natural film can be visualized as the result of a dynamic equilibrium between opposing forces-those tending to form compact barrier layer and those tending to break it down. If the destructive forces are absent, as in dry air, the natural film will consist only of the barrier layer and will form rapidly. If the destructive forces are too strong, as in wet conditions with or without the presence of salt, the oxide will be hydrated faster than it is formed, and little barrier will remain.
All of this being said, most bikes especially in my neck of the woods are exposed to some pretty nasty conditions...some more than others. Most of the aluminum corrosion that I see is minor and isn't really a cause for concern, but others are... Thanks again for your comment.

[KWillets]

Ewwwww...Aluminum and Steel. Take a look here:

http://www.ocean.udel.edu/seagrant/publications/corrosion.html

Notice on the table how aluminum is just below zinc, the usual sacrificial metal.