Oxide growth on CrMoV

[kclim]

Does anyone have any good resources for above subject? In particular, information regarding growth kinetics and layer morphology at high (550C) temperature.

The query is in regards to steam piping at a power station, and oxide exofoliation. While a lot of data exists for the Cr Mo (no V) steel grades, I can't find much in the way for CrMoV (0.5 0.5 0.25) steel - I presume this is because it is not commonly used in the States.

Thanks in advance

 

[metengr]

kclim;
I don't believe you will see much difference (if at all) in oxidation rates between the CrMoV alloy you mention and a Cr-Mo of similar chemical composition w/o V. The V, which is a carbide former, provides added creep rupture strength for Cr-Mo alloys and should be tied up in the form of precipitates or carbides. The oxidation characteristics of Cr-Mo steels are affected by Cr and to a lesser extent Mo.

 

[davefitz]

15 yrs ago there were some chromizing processes developed which would reduce the rate of oxide scale formation inside heated pipes. These processes were abandoned after it was determined that the waste product from the process, and also any future sediment waste from boiler acid cleaning operations, would contain hexavalent chromium- very toxic and too expensive to treat properly at that time. See the movie "Erin Brockavich" for an understanding of the possible effects.

I understand that the push to develop ultrasupercritical steam processes has lead to revisiting these processes, since the advanced steam cycle would imply some ferritic steels would be heated to well over 1100F at the tube ID, and lead to excessive spalling of magnetite unless an effective chromizing treatment is applied to the bore. There are now available waste treatment processe that effectively convert hexavalent chrome to a less toxic valent chrome ( bioreactors), so if may be feasible now to treat the waste from the original chromizing process. The question of what to do with the later waset from acid cleaning ops is open.

 

[kclim]

metengr,

What you're saying makes sense. Perhaps I can ask a more specific question of you - how would the oxidation of the CrMoV material compare to say a P22 (2.25 Cr 1 Mo) material? We're finding that we replace a lot of CrMoV material with 2.25 Cr Mo.

As there is less Cr, do we expect the CrMoV material to form less coherent/thicker scales?

Regards

 

[metengr]

kclim;
I would expect that the oxidation rates or characteristics would be different between the lower CrMoV alloy and Grade 22 because of the higher chromium content in Grade 22, which does increase oxidation resistance. Keep in mind that the CrMoV alloys were developed to take advantage of increased allowable stresses, mainly because of the V addition, for the workhorse Cr-Mo alloys that have been around since the 1940’s. Replacing the CrMoV with Grade 22 does require a heavier wall thickness to compensate for the reduced allowable stresses in using the higher chrome, Grade 22.

Oxide exfoliation is more complex phenomenon, and is driven more by oxide thickness or layering upon exposure to temperature. We have large turbine steam chests and inner cylinders that are made of cast CrMoV materials and we also have Cr-Mo material piping, and I have seen similar problems with oxide exfoliation in either alloy. So, no to answer your second question (based on expierence). I can tell you this, oxide exfoliation in power plants is driven more by thermal cycling - especially during start-up and shutdown events.

 

[davefitz]

Regarding exfoiliation during startups and shutdowns, these had been espescially problematic on US utility boilers which generally do not have steam -to-reheater bypass systems. The spalled off oxides will erode the HP turbine blades; the large change in steam flow plus thermal shock that occurs during initial sync of the turbine will lead to a large influx of high velocity oxides hitting the first few rows of blades , and one can easily lose 5% of HP stage efficiency in the first year of operations from this.(SPE solid particle erosion)

If the oxide scale and exfoiliation occurs on thin wall reheater tubes, it can lead to a wall thickness less than desing and early tube failure due to creep- the combination of wall thinning (higher membrane stress) plus oxide buildup on the id of teh bue ( acts as an insulator- leads to high tube metal temps- 12 mil is typical buildup)

This is less of a problem on european or japanes plants, since they generally use a steam to reheater bypass system during startups. This avoids the large change in steam flow and wall temp during turbine sync, and any exfoiliation that does occur will be passed thru the bypass valve into the reheater and not the HP turbine.

To add to a prior reply, there is another recent innovation that can be used to remove hexavalent chromium from waste products of the chromizing process- SAMMS ( nanotechnology) self assembled monolayres on mesoporous structures.