Sunday, June 7, 2009

[MW:2342] Re: Fw: [MW:2336] Re: corrosion

Hamed,
Generally speaking (and we all need to be careful when we talk in generalizations), if you have a small cathode (weld metal) and large anode (base metal) the corrosion issue is unlikely to be serious, UNLESS you have something else going on in the HAZ. Don't forget that it is easy to get the formation of unwanted carbides at grain boundaries in the HAZ, unless the carbon content is down below about 0.03% in both electrode and base metal. This is, of course, the classic problem when welding 304 and 316 stainless. What happens then is that the carbides form small, but very effective cathodes in the grain boundaries and you get intergranular corrosion ("weld decay") in part of the HAZ where the sensitisation temperature has been exceeded.
 
Have a look at "weld decay" and "knife-line attack" on any of the websites I've mentioned e.g. http://www.corrosion-doctors.org/Forms-intergranular/intergranular.htm, http://come.to/corrosion.
 
If you have any doubt, it's a good idea to do some tests. Without knowing your operating conditions, plant design etc., it would unwise of me to say "go ahead, no problem", because I don't know the answer to that. Remember also that the HAZ is likely to have all manner of residual stresses and a different grain structure from both weld and base metal, unless it is subject to PWHT. These alone are likely to make it more susceptible to corrosion. A simple way of thinking about it is to consider my approach to thermodynamics - nature, like me, is inclined to be lazy. It likes things to achieve a state of lowest energy and a tendency for entropy (chaos) to increase. The latter is reflected in the pile of stuff on my desk. Sometimes, it is happy with a metastable state - a bit like resting in a hammock. Stable, until someone gives you too much of a push, when you will achieve a lower state of energy, falling out and cursing loudly on the floor, and thus releasing energy.
 
Metals are almost all in a metastable condition and would rather return to their ore, or something similar (some kind of corrosion product). If you give something a bit of extra energy in the form of residual stress from welding, it is that much closer to being "tipped out of the hammock" and thus more inclined to corrode. On the other hand, you can "steady the hammock" by PWHT and restore the metastable condition. With a bit of luck! Of course, PWHT is not always possible, when you may need to ensure that you have effectively no carbon, or a vigorous carbide former other than one of your main alloying elements to mop up any segregated carbon.
 
When it comes to selecting electrodes, my advice is to talk to the electrode manufacturers and get their opinion. They know far better than I do and I have always found them to be most helpful. It's worth talking to more than one, though, and seeing how different their recommendations are. If they are significantly different (other than giving their own patent names/codes for their electrodes) it becomes worth seeking independent advice. I do not consider myself an expert in that field - one of my specialities is materials science and corrosion, which I teach at Robert Gordon University in Aberdeen, and on which I feel I can speak with some authority.
 
Regards,
Owen.
----- Original Message -----
Sent: Sunday, June 07, 2009 10:11 AM
Subject: [MW:2341] Fw: [MW:2336] Re: corrosion

Dear Owen
Thanks for your help & article that you share with us.At first I brang up the query about corrosion in HAZ & base metal in the joints that we use electrode E 8018C1 with more than 2% Ni for material (13MnNi 6-3) that contain0.8 % Ni. I read Bimetallic Corrosion & I found the example that weld seam is more cothodic than base metal. As you know in that case we have the cathode in small part compare big anode (base metal) .In our experince we have the base metal & HAZ which is more cathodic compare weld metal. My question is here also we have the partial corrosion in HAZ or not?
Also I want to know in your projects for selection of electrode for PQR this matter(Corrosion probability) also was checked or not?
 
REGARDS
H.Malekan

--- On Sat, 6/6/09, Owen Jenkins <owen@osjl.co.uk> wrote:

From: Owen Jenkins <owen@osjl.co.uk>
Subject: [MW:2336] Re: corrosion
To: materials-welding@googlegroups.com
Date: Saturday, June 6, 2009, 12:51 PM

Ali,
Go and read some of the references I sent a little while ago and stop challenging those who know better than you do. You are wrong. I have told you this before.
 
Owen.
----- Original Message -----
Sent: Saturday, June 06, 2009 12:55 PM
Subject: [MW:2335] Re: corrosion

Dear Keyvan
i think for low alloy steel & carbon steel ,the different chemical analysis in haz & weld isn't major to occur chemical cell.

 

From: kayvan alyari <k.alyari@gmail.com>
To: materials-welding@googlegroups.com
Sent: Saturday, June 6, 2009 3:17:06 PM
Subject: [MW:2334] Re: corrosion

Dear Friends,
 
The main cause is formation of electro chemical cell between HAZ & BM especially in interfaces!
 
Yours truely
Keyvan

 
On 6/6/09, Owen Jenkins <owen@osjl.co.uk> wrote:
If you'd like to know a bit more about corrosion associated with welding, you can download the following guide to bimetallic corrosion: http://www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf . The NPL website has a number of other useful downloads which may help you, as well as some illustrations of various forms of corrosion associated with welding. In some cases the HAZ is more susceptible to attack, in others it is the weld metal itself. You can also visit the following sites for more information:  Corrosion Clinic http://come.to/corrosion, Corrosion doctors http://www.corrosion-doctors.org/CorrDocs/CDs.htm, NASA's KSC Corrosion Technology Laboratory http://corrosion.ksc.nasa.gov/ .
 
A Google search would have turned up all that lot for you. Worth trying occasionally, as I've remarked before.
 
Regards,
Owen.
----- Original Message -----
Sent: Saturday, June 06, 2009 9:49 AM
Subject: [MW:2331] Re: corrosion

 
Hi,
It's about the grain sizes and intergranular changes because of welding i think
After welding you will see a lot of different sections that are affected by heat differently.
In most applications weld metals have fine grains due to recrystallization and tempering effects of the upper beads
[And so upper sections of the weld metal have bigger grains (because they are less tempered)]
In the fusion line there is a non-complete fusion section (grain structure is like a casting)
After that, in HAZ a highly heated (but no fusion) big grain size section comes near the fusion line
This section is the most risky one. In most cases if corrosion cracking happens it does it here.
This doesn't mean that weld metal behaves more noble than base metal i think. the corrosion is between the weld metal and HAZ
I didn't see anywhere anything certain about the causes of this type of corrosion.
--
Best Regards,

Erhan TURGUT

Ali Asghari yazmış:
hi
why corrosion occure preferentially in haz ,especially fine grain location?
why weld metal noble than base metal?
 
regard advance





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