Hydrogen Embrittlement on metals.

Though the subject is quit huge to discuss, I would like add few morepoints though not all.

1) The purity level of the Hydrogen is also categorised as less than 99% ormore than 99%.
2) The pressure is one important element(hardly addressed during designphase by material engineers), for less than 99% purity which means amixture, the partial pressure should be >1bar for the hydrogen to have itsshow.
3) Limits are present for Carbon monoxide and Oxygen and other elements aswell, which induces the phenomenon described by you(Owen).
4) Whether the system is under cyclic condition or not which applies to PSA and likes.
5) Materials are defined by their yield and tensile strengths limitations.Take note the equipment material prescription varies differently frompiping material for the same conditions.
6) Not the least though, Hydrogen embrittlement should not be equated withHydrogen induced cracking.

Coming to the point of temperature, linked to materials, considering carbonsteel as the material of constr., for eg. A well known refinerylicensor prescribes the temp. limitation at 260 deg.C, A German company holding numerous technologies in the subject of Hydrogen limits the use of Carbon steel to 200 deg.C, A UK Engg. company having H2 licenses, limits it to 240 deg.C. As I had seen lab test reports andfound H2 effects above 200 deg.C, such variation of 200 to 260 is quit abig gap under standard test procedures.

Let me know the thoughts of our groups on the temperature factor, what could be the reasoning behind such broader limits.

Some useful links for members interested further study.
http://www.uni-saarland.de/fak8/wwm/research/phd_barnoush/hydrogen.pdf
Some site generalises the subject which cannot be put into practice or inyour engg. Take caution, in your understanding of the subject.
http://octane.nmt.edu/waterquality/corrosion/hdamage.htm
http://www.corrosionist.com/Corrosion_Control_Offshore_Oil_Gas.htm
http://hrc.nevada.edu/qa/SIP/SIP-UNR-018R0D0.pdfhttp://www.hydrogen.energy.gov/pdfs/progress05/v_a_4_adams.pdf

With regards,
Kannan Sundaram

"Owen Jenkins" <owen@osjl.co.uk> Sent by: Tomaterials-welding@googleg <materials-welding@googlegroups.com>roups.com cc
Subject21/04/2009 20:49 [MW:2002]
Re: effect of H on notch strengthPlease respond to materials-welding@googleg roups.com

Ali,The name is Owen, not oven. An oven is something in which you heat food, oroccasionally heat-treat metals!

For hydrogen embrittlement to occur, you must have monatomic hydrogendiffusing through the metal and gathering in grain boundaries. Monatoms ofhydrogen are small enough to diffuse through grains of steel. Theembrittlement occurs as a result of monatomic hydrogen atoms diffusing intothe grain boundaries, reacting to form molecules of hydrogen gas, whichthen force the grains apart. Diffusion and reaction rates both slow withdecreasing temperature, hence the effect of hydrogen embrittlementdecreases below a certain temperature. The effect of the ductile-brittletransition then becomes more important with decreasing temperature. It isalso worth remembering that, at constant pressure, the volume of a givenmass of hydrogen will decrease with temperature, or, at constant volume,the pressure will decrease with decreasing temperature, hence the drivingforce pushing the grains apart will be reduced. If it is too cold for thehydrogen atoms to diffuse through the steel, HE can't occur.

The monatomic hydrogen is commonly formed as a result of acid/metalcorrosion reactions (e.g. pickling steel prior to painting) or otherreactions in which hydrogen is liberated as a cathodic reaction (e.g.fusion welding where water is present or there are hydrogen compounds inthe flux). The hydroxonium (or hydrogen) ions are reduced to monatomichydrogen by electrons from the dissolving (corroding) metal or the weldingcurrent flow. The monatomic gas can diffuse through metal grains - thediatomic gas molecules can't.

2H+ + e- ® 2H.2H. ® H2

Some steels are much more prone to hydrogen embrittlement than others. HSLAand high strength carbon steels tend to be more problematic than others.Hydrogen embrittlement can be prevented by suitable heating to anappropriate temperature after the process which generated the hydrogen (oneof the functions of post-weld heat treatment), to allow the hydrogen todiffuse out of the steel. If you have a properly qualified weld procedure,the PWHT should be designed to mitigate the effects of hydrogenembrittlement.

When HE is suspected in cases of failure, there is never any sign of thehydrogen - it has all long gone, so it is usually inferred from thecircumstances and the nature of the cracks.

Regards,
Owen