Acid Dew Point Corrosion - H2SO4

[SMF1964]

Any suggestions on a benchtop recipe to mimic the acid concentration encountered in an oil-fired boiler's exhaust ductwork? We are upgrading materials and would like to run some immersion tests to see if there is a galvanic effect of the weld filler and DMW when we tie the new to the old material.

Don't tell me to eliminate the entire dissimilar metal weld - we were lucky enough to get the $ for a partial upgrade.

 

[metengr]

SMF1964;
I don't think an immersion test will be of any practical use because the pH of acid dew point corrosion is influenced by many variables in exhaust gas ducts at power plants. I have seen the pH range from 4 down to 1. The immersion test results using dilute sulfuric acid will not be meaningful, and will skew your data.

The most important point is that you have located the weld transition in the hot zone (safely above the dew point temperature) of the duct. This would reduce any changes of acid dew point corrosion and prevent any moisture formation at the DMW - (e.g., no moisture, no galvanic attack).

 

[SMF1964]

I recognize that the benchtop will not accurately reflect real conditions. The unit cycles daily and more often than not is actually not running, so the 'hot zone' you refer to is not a clearly defined region. In the ductwork at this plant, the corrosion is most signficant at the bottom half of the duct, and as a result this section has been repaired many times (wallpaper, patches, etc.) where as the top half of the duct is still original material. For this reason, we are only replacing the bottom section with carpenter 20. The concern is of a galvanic couple where the Car20 is joined to the carbon steel, so we'd like to set up a series of coupon tests of these two alloys welded with three different filler metals and see if there is any less aggressive galvanic attack at the fusion line/HAZ.

 

[metengr]

It sounds to me that the reason for the lower half section of the duct having the corrosion problem is because the ash settles out from gravity and/or gas slow stratification. When you have ash deposits on the duct surface, any moisture will result in corrosion. If the transition welds are located high enough along the duct wall away from the deposit build-up, you should still be ok.

Regarding the DMW, corrosion will occur if you have deposits that can absorb moisture.

If you want to evaluate corrosion affects, I would weld some DMW coupons that you can handle and obtain some of the ash from the duct. Generously spread the ash deposits over the entire coupon and use a spray mist to provide moisture. Monitor over time and see what happens on the surface of the coupons.

 

[hacksaw]

you should have such tests installed in your duct work. Few bench top testing arrangements will provide a realistic measure of what is going on.

 

[EdStainless]

If you are only trying to rank joining methods then you might want to use imersion in Green Death. It will at least tell you which is 'best'.

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Corrosion never sleeps, but it can be managed.

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

 

[SMF1964]

Green Death was my initial selection, but was concerned that the chlorides would be excessively aggressive. Since we don't have chlorides in our ash ... Never mind (the jury will disregard that last statement).