Pitting, crevise, corrosion fatigue and erosion corrosion 4

[Shibuz]

Hi,
Can someone please help me with the following corrosion questions? I do have a presentation to make about these subjects:

1. Pitting: Can carbon steel pit? or is pitting just a phenomenon of stainless steel? Is Chloride ions the only thing that can cause pitting?

2. Crevise corrosion: since crevise corrosion is caused by the starvation of oxygen, do we have to have oxygenated water for crevise corrosion to occur? What if we are dealing with downhole (oil well) metallurgy? Usaully oxygen is not present downhole. So is it safe to assume that crevise corrsion will not occur downhole?

3. Corrosion Fatigue: Does corrosion fatigue start with a pit? In the presence of what? what mecanism is it?

4. Erosion Corrosion: Do you have to have some type of abrasives present for erosion corrosion to occur? Should there be a corrosiove species present for thsi to occor?

Thanks for the help.
S zach

 

[metengr]

If you have a presentation to make, I would suggest you conduct an internet search on each corrosion topic to better understand this material. There is plenty of technical information in the public domain on each corrosion mechanism.

If you need additional information, I would suggest reviewing several corrosion engineering text books. The one in particular by Fontanna called Corrosion Engineering is well written.

 

[Shibuz]

Thanks for the suggestion. I already did one presentation covering the basics of corrsoion. This is a second take. These were the questions posed during the first presentation.

 

[CESSNA1]

SHIBUZ: You may also want to contact the Natioal Association of Corrosion Engineers at

http://www.nace.org

Regards
Dave

 

[EdStainless]

In general all forms of corrosion require the breakdown of the passive film on the metal. Pitting, crevice and even SCC cannot start unless there is an active corrosion site on the surface. The difference between pitting and crevice is that the conditions are more severe in the crevice (lack of O, trapped impurities, build up of corrosion products).
In CF and erosion it may be mechanical conditions that break the passive film, or it may be corrosion.
Good books:
Fontana and Greene; Corrosion Engineering
Scully; Fundamentals of Corrosion


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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[CoryPad]

1. Carbon steel can pit.

2. Crevise corrosion should be minimal downhole.

3. Corrosion fatigue routinely starts with a pit, in the presence of a corrosive medium, by the mechanisms of material dissolution (corrosion) and plasticity and cracking (fatigue).

4. Abrasives are not necessary for erosion corrosion.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Shibuz]

Friends,
Thanks for the help.

1. I have read in this discussion groups itself that "carbon steel doesn't have a protective layer like stainless does. So it will corrode over the entire surface in the presence of chlorides etc. With SS, the chlorides chemically react with the chromium oxide passive layer, causing it to break down at localized spots - thus generating pits" If this is true, then is it still possible for carbon steel to have pitting corrosion?

2. is oxygenated water (or the presence of oxygen in some manner) necessary for crevise corrosion to occur? i also have read that crevise corrosion can occur because of acidity changes in the crevise? what do they mean by that?

3. thank you for explaining this.

4. One article about erosion corrosion read like this: "when erosion removes the protective film of corrosion products, coarrosion can occur at a faster rate". What is this "corrosion products" referred here?

Many many thanks for the help.

 

[EdStainless]

1. CS will pit in other environemnts. Anything that causes a local breakdown of the passive film will result in pitting.

2. For most materails, the less O present the more difficult it is to repaire the passive layer and the more likely that you will get corrosion. The fluid is hte crevice is isolated from the bulk. As the O is used up no more can reach that area. As the metal starts to corrode the metal ions in solution will lead to a lower pH. And then all hell breaks loose.

3. There are some CF cases where the corrosion only occurs at the high stress area at the tip of the crack. Sometimes it is synergy and sometimes just coincidence

4. The passive layer on the surface of a material is actually a corrosion product (in SS it is Cr oxide). It serves to protect the surface from further attack. If this surface is removed it will try to re-form. As is is removed material is lost. Some metals form soft films making them sensitive to erosion/corr (Cu or CSteel). Some form hard layers that are tougher (SS or Ti). If you have a hard dense layer and a strong material then you have good resistance (high alloy SS).

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[Shibuz]

Friends,
Thank you very much for the help.
S Zach