Analyzing corrosion on a surface 1

[EngineerDave]

What in your opinion would be the best test to determine the source of corrosion on a part.

My initial thoughts are

1) Determine the type of metal that the products is constructed of, from this there should be a database of chemicals that corrode it.

2) Analyze the deposits on the surface by SEM/EDS, perhaps this will give some clue as to the nature of the corrosion and thus the corrosive agent.

I would appreciate any suggestions on this you might have.

 

[mcguire]

You are right on those two, know the metal and identify the corrosion products. I would then say understand the environment which led to the corrosion. The three should mesh together into a clear picture.
If the results aren't as one would predict then either the metal isn't as it should be or the environment was different than expected.

 

[stanweld]

To emphasize mcquire's statements, analyze the environment and the service history of the item in the environment and any changes in the environment.

 

[kenvlach]

Agree in principle, but for analyzing corrosion products, especially thin, multilayered ones, supplement the SEM/EDS with more sensitive methods, such as Auger Electron Spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Can thus determine valence states, analyze light elements and do composition depth profiling. SIMS (secondary ion mass spectrometry) would also be useful due to high sensitivity and ability to detect H. Such methods are useful in analyzing anodic oxidation products on Cu, which is a Cu₂O+CuO, Cu(OH)₂ bilayer.

Also, newer techniques such as AFM (atomic force microscopy) and STM (scanning tunneling microscopy), can be used to study corrosion in situ, e.g.,
'Mechanistic Studies of the Corrosion of 2024 Aluminum Alloy in Nitrate Solutions,' J. Electrochem. Soc., Vol. 150, No. 8, pp. B396-B404 (August 2003).
"...local dissolution of the matrix has been found to occur around the Cu- and Mg-rich particles and has been quantified by using atomic force microscopy. Secondary ion mass spectroscopy and ultraviolet spectroscopy helped elucidate the mechanism of dissolution of the intermetallics."
Always good to experimentally replicate observed results.

 

[JimMetalsCeramics]

Your approach is correct. Know the environment that the part is exposed to, the base metal and the reaction products.