stress corrsion cracking

[map]

dear sir!
i am a student of mechanical engineering.
i am working on a project about gas pipeline failures.
i want to have full information about stress corrosion cracking as how it happens ?
what is its range?
and everything about it
plz help me
i ll be grateful to u
urs faithfully
muzammil arshad [sig][/sig]

 

[MT]

Stress corrosion cracking is very complex problem, because it involve stress, corrosion environment and material. It is almost impossible to write the whole explanation only in few lines, so I'll give you a list of references:

Russel H. Jones, Stress-Corrosion Cracking, ASM international, this book will give you a whole explanation about subject.
You can also see articles of the same author and articles of R.W.Staehle, this will give you a detail explanation of Stress-Corrosion Cracking

In gas pipelines is very common problem also Hydrogen embrittlement, which is one of the stress corrosion cracking mechanisms, but you can find a good literature under this name too, you just look the articles of Alexander Robert Troiano.

 

[CC9513493]

Dear Collegue
You can consult more about this subject in a Specifications NACE MR 0175 & Looking in the complete set of RP Nace.

Regards

Jorge@tecnicontrol.com.co

 

[Shuneed]

Could anybody can explain, why corrosion takes place in metal / steel?

In marine environment the first layer (scale) of metal/steel peel off so quickly, why?
I'll be grateful, thanx

 

[AlanD]

You like to ask the tough questions Shuneed

Metal because of its electron configuration has partially filled shells, which means that that a metal has an over abundance of electrons for it to be chemically stable (this is also the reason why metals are good electrical conductors). On the other hand oxygen as a single atom has a shortage of electrons in its partially filled outer shell, which it wishes to fill.

When the metal comes in contact with oxygen, the oxygen's affinity for for the spare electrons of the metal means that the two atoms will combine to form a metal oxide. In some metals this oxide will result in a layer that protects the metal from further corrosion, as long as the conditions remain favourable, typically the chromium oxide layer on stainless steels or the artificially grown anodized layer on aluminium. When conditions are not favourable, the metal will continue to oxidize and in some cases the oxide layer breaks away from the surface of the metal, exposing fresh metal to corrosion to occur on.

Iron oxides form a particular case, with respect to the oxide formed, as there are 2 distinct types and an intermediate oxide formed FeO, Fe₂O₃ and the Fe₃O₄. Fe₂O₃ is the porous red form of iron oxide. This porousity is important as it permits the transportation of oxygen ions to the un-oxidised iron. FeO has a low porosity and from memory the iron ions and the oxygen ions must migrate through the existing oxide layer for further corrosion to occur. This results in oxygen starved oxide forming, this black oxide form is coincidentally well bonded to the iron base metal.

In the marine environment you described, it is the formation of the Fe₂O₃ oxide forming combined with the actions of NaCl and other salts that result in the peeling away effect you have noticed with steels. Although this is not exclusive to steel, many metal oxides layers will not be adversely effected by being in a marine environment.

I hope this helps