At 4% Cr, you do not have enough to create a protective layer of chrome oxide, such as you would find with the higher austentitic chrome alloys like 310 and 316.  This is especially true if some of that 4% is tied up as chrome carbides.  However, the bigger question about your room temperature exposure to SO2/SO3 is the moisture content of the gas.  If your conditions are below the dew point of water, then you can create a thin film of H2SO3 or H2SO4 on the metal surface.  This would be corrosive through a simple dissolution reaction where the acid attacks the iron-chrome matrix.  However, if your gas is dry, then the gases would not be particularly corrosive at room temperature.

Chrome Steel - how much chrome? Are You sure that chrome is free in steel structure? If chrome connect with oxygen, carbon, other, 12% may be not enough to change the voltage on corrosive polarization curves.

Chrome is nearly 4% Chrome is in carbide form or in substitutional solid solution form in steel. it is an alloy. Chrome and Iron are not different, chrome is not free. It is exposed to low level of oxygen. Carbon is in steel only as carbide in steel matrix.

At 4% Cr, you do not have enough to create a protective layer of chrome oxide, such as you would find with the higher austentitic chrome alloys like 310 and 316.  This is especially true if some of that 4% is tied up as chrome carbides.  However, the bigger question about your room temperature exposure to SO2/SO3 is the moisture content of the gas.  If your conditions are below the dew point of water, then you can create a thin film of H2SO3 or H2SO4 on the metal surface.  This would be corrosive through a simple dissolution reaction where the acid attacks the iron-chrome matrix.  However, if your gas is dry, then the gases would not be particularly corrosive at room temperature.

Sir, Stephen N Smith

CO2 Gas in dry condition and H2SO3/H2SO4 film formation is there on the sample as minimum water is added in the Autoclave. When i observed the Micro structure in SEM i found corrosion product diffused in sub surface in 4% chrome steel which is having annealed structure but other chrome steel with minimal chrome >1 % and higher chrome more than 10% chrome just had corroded product externally without diffusion. I tried to observe under optical microscope but could not observe it. However Mo nearly 1% is present in the 4% Chrome steel. Does anneal micro structure have some effect of corrosion product diffusion and some literature studies at high temperature say that sulphur and carbon have antagonism where carbide stability is disturbed due to Sulphur product on the surface.

The annealing process alone does not explain what you are describing.  However, it sounds like you are getting the development of large amounts of Cr carbides during the annealing, which reduces both effective the Cr and C content of your metal.  Since I cannot see either your SEM or metallographic images nor do I know very much about the other samples, it is very difficult to know exactly what you are observing.  Is it possible that what you see in the SEM is that the corrosion product exists where the ferrite grains were prior to exposure to your test?  We frequently see the growth of corrosion product that retains the physical boundaries of the ferrite grains as well as the structure of any intermetallic particles, such as Fe3C, MnS and Cr carbines.