Reason for Stress Corrosion Cracking

[HardMetal]

Why does the application of stress to a part produce a different corrosion mechanism other than just the type of corrosion that would occur without the application of that stress? Does thermodynamics or kinetics play a role in the process? Is it a matter of changing the fracture toughness properties (KI becomes KISCC)?

Can you please provide any sources for information that I could investigate?

Thanks.

Paul

 

[metengr]

Yes, to most of the above reasons. Stress corrosion cracking (SCC) occurs in various materials because three conditions exist simultaneously; tensile stress, corrosive environment and susceptible material. All three of these service conditions must be present simultaneously, otherwise SCC will not occur in materials.

The theory of SCC damage mechanisms in materials is rather complex because of the wide range of variables and types of SCC damage mechanisms – intergranular and intragranular. I would suggest performing an internet search on SCC of metals, because there are many technical papers in the public domain that discuss this very subject.

 

[HardMetal]

Thanks for your help, Metengr. I've done internet searches and I can't find anything specifically describing how stersses affect the role played by thermodynamics or kinetics. Do you know of any sources?

 

[metengr]

HardMetal;
I am by no means an expert in SCC damage mechanism theory. However, having been around the block a few times, my view of this is as follows; the role of thermodynamics and kinetics affects material susceptibility to SCC given certain heat treatment conditions and alloy compositions. The tensile stress is what aids in the initiation and propagation of SCC.

 

[NickE]

Also to add: Streses present in the matrix from precipitates can also aid corrosion. (Ie small coherent precipitates strain the lattice) Thus the lower corrosion resistance of 17-7H900 vs 17-7H1150. (Althought the commonly referenced NASA study on SCC says that H900 with the heat tint still on the parts is reccommended for ClSCC service.) Go figure -- I have no clue as to why though.

I always thought stress accelerated corrosion rate. But I have no idea where I got that from.

 

[SJones]

A number of papers from this search result will be downloadable for free:

http://search.nace.org/query.html?st=11&charset=iso-8859-1&qt=SCC

Happy surfing.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[swall]

Another characteristic of stress corrosion cracking (scc)compared to corrosion is that scc can occur even if the observed corrosion is relatively minor.

 

[swall]

NickE--If you used Van Vlack's materials science book at Michigan Tech, you could have gotten the idea of residual stress leading to accelerated corrosion from Figure 12-19 on page 346. The classic bent nail sketch, with areas of higher corrosion pointed out. BUT--many authors, including Uhlig, dispute this hypothesis. Uhlig even provides some free energy data to support his contention that (residual) stress does not lead to accelerated corrosion. And, with scc, we now know that shot peening can reduce the tendency for scc because it provides a surface compressive stress that offsets the tensile stresses that can cause scc.

 

[NickE]

Swall- nope we didnt use VanVlack, We used Askeland and Reed-Hill,Abbaschian... Hmmm, I'll do some looking on holiday...

 

[edrush]

Any chemical reaction which is not spontaneous has an energy hump or energy barrier that must be overcome (I can't remember what the hump is called, activation energy?). The way I look at it, the stress provides some of the energy required to get over the barrier. Mechanical energy provids some of what I usually think of as thermal energy. You have to break the existing bond before a new one can form, and the mechanical energy helps break this bond.

Since stress is usually concentrated at a crack tip, the corrosion is also concentrated at the crack tip and may not be obvious.

 

[rconner]

I think there are many committees composed of a great many experts in ASTM and other interest groups that look at many reportedly at many different terminology corrosion aspects, e.g. stress/strain corrosion/cracking, hydrogen/embrittlement effects and susceptibility, and/or various "environment-assisted" cracking/fatigue for various metallic and non-metallic materials etc. I guess you could read some resulting standards such as ASTM E1681, G129, G142 (and perhaps other letters of the alphabet!) as well as many texts and manuals that talk about these subjects, but I suspect some folks in the end could be pretty much confused by some of the explanations and/or technical jargon offered. In the real world, I’m not sure it’s very easy to separate or fingerprint all these terminologies/behaviors, as there are all sorts of materials and forms/structures of same, environments, nooks and crannies/notches, temperatures, and stress levels etc., and even combinations of all this that can interact with each other out there.

I think you will eventually find some basic corrosion references that will tell you that strained/stressed areas of structures are “anodic to” or have “more electronegative potential” for corrosion activity relative to lesser stressed/strained/worked areas of structures. This probably at least makes sense to most, at least to those with a basic understanding of galvanic corrosion (where all factors exist to result in same corrosion preferentially occurs at the anode, where electrons are stripped off/dissolution occurs by chemical reactions etc., not the cathode of a galvanic cell), and this can even be visibly demonstrated with various colored chemical reaction indicators. However, some who are very inquisitive (or perhaps some dense like me?) might ask further, “Why is the stressed area anodic, or why are the electrons easier to strip off/reactions with the material more likely to occur at the stressed/strained location?” I think it is at this deeper depth of questioning where you may find murkier and perhaps even some contradictory explanations.

All this being said, many years ago I heard an old professor (who I think got his Ph. D. in physical metallurgy at Rensselaer about a half century ago) once tell our class in referring to a tensile testing machine pulling a sample, “Spacing between atoms increases with elongation.” As I did not remember reading this anywhere in the text or study materials, I wrote (t)his comment down in my notes. Is it possible, in probably my too much sort of Simpleton's (and certainly not expert) thinking, that atoms or metal structures/grains etc. are sort of more accessible to reactants(ions), or being penetrated/wedged even further apart with stuff like hydrogen atoms etc. when they are “spaced” further apart or their motion is somehow affected by particularly tensile stress? To take this simplistic explanation further, at some point perhaps an increasing spacings could break bonds enough to become a “crack” (see interesting discussion of cracking at a quite micro level at

http://www.chemsoc.org/chembytes/ezine/2003/trebin_apr03.htm)