Acid Dew Point Corrosion - Suitable Materials
Acid Dew Point Corrosion - Suitable Materials
(OP)
Hello
can anyone assist with suitable materials (cost effective) for stacks and ducting susceptible to acid dew point corrosion. The flue gases contain substantial amounts of SO2/SO3. Operating temperatures range from 80 degrees celcius to about 300 deg celcius depending on equipment.
Thanx
VN
can anyone assist with suitable materials (cost effective) for stacks and ducting susceptible to acid dew point corrosion. The flue gases contain substantial amounts of SO2/SO3. Operating temperatures range from 80 degrees celcius to about 300 deg celcius depending on equipment.
Thanx
VN





RE: Acid Dew Point Corrosion - Suitable Materials
RE: Acid Dew Point Corrosion - Suitable Materials
First, what is your fuel - what other gases are in the exhaust? Chlorides (probably)? Nitrates (almost certainly)? And what quantities of SO2 and SO3 are present?
The gas temperatures you listed are above the normal SO2, HCl and NOx condensation temperatures, but fall well within the SO3 condensation range - H2SO4 (sulfuric acid, the condensate of SO3) is extremely agressive. Alloy AL 29-4C may provide a measure of resistance to H2SO4, but the chimney industry tends towards Alloys such as C-276 for this service. In part this is because of the probability of also having to deal with Hydrochloric, Nitrous, Ntric and Sulfurous acids.
These other acids condense at temperatures below 50 to 60 degrees Celsius, which may occur on surfaces of your system that are not properly insulated and inside the top portion of the flue. Moreover, once you start condensing out sulfuric acid, the potential presence of these other acids can not be ignored. At the other end, the presence of Cl and SO2/SO3 in gases approaching 300 degrees Celsius can lead to high temperature corrosion of some ferrous metals. The gas make-up and the profile of operating temperatures is needed to make an intelligent materials selection.
Other options include thin corrosion-resistant metal cladding on the exposed surfaces and various non-metal protective linings that can be applied.
RE: Acid Dew Point Corrosion - Suitable Materials
Provided the paint is not damaged during erection, then this solution has proved to be very cost effective.
Stacks with this system have been in service without problems for 5-10 years.
Terry.
RE: Acid Dew Point Corrosion - Suitable Materials
Can you tell me what temperature range you operated at?
RE: Acid Dew Point Corrosion - Suitable Materials
50mm thick refractory was applied on ther inside of the stack and the acid resistant paint was used on the outside of the stack for the top 10m. The refractory acts as an insulating barrier to reduce the metal temperature to levels where CS has adequate strength.
From memory, stack metal temperatures were kept below 100°C - however I couldn't be 100% certain of this figure.
The paint we used was supplied by International Paints - they should be able to assist with information regarding temperatures etc.
RE: Acid Dew Point Corrosion - Suitable Materials
This system IS NOT SUITABLE for service below the acid dew point of the gases, which was VIN's original question.
A very specialized refractory concrete (talk to Sauereisen in Pittsburgh, PA or to ATO-Chem for specifics) over a membrane material is needed to protect the steel against acid attack. There are also several coatings, but now we have to be cautious about higher temperatures. Finally, we get back to the super stainless and high alloy materials, such as Hastelloy. Without precise service conditions, and maybe even then, we are stuck with these relatively expensive choices.