Acid Dew Point Corrosion - Suitable Materials

[VN]

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

 

[jerryf]

We use AL29-4C for stacks on condensing boilers. Heat-Fab Inc. produces the vent stacks and they are very resistant to acid corrosion up to 500 deg F. We have been using them for over 10 years without trouble.

 

[dickwilber]

The acid corrosion of duct and flue surfaces has long plagued builders of combustion systems. The solutions can be quite complex - the information you have included is too sparse for a comprehensive reply.

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.

 

[TerryStam]

In the past we have used a combination of refractory lining and acid / temperature resistant paint on fired heater stacks as high as 30m. In these cases, the corrosive agents were SOx and NOx.

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.

 

[dickwilber]

Terry, you say a combination of paint and refractory - I assume the paint is an undercoating and the refractory goes over it?

Can you tell me what temperature range you operated at?

 

[TerryStam]

Dick,

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.

 

[dickwilber]

It appears that you are operating at temperatures well above the acid dew point, which is often, depending upon sulfur content, humidity, etc., above 300 degrees F (150 degrees C). The gunite (refractory concrete) provides an insulating barrier between the hot gases and the metal stack. It also, if high alumina cement (Lumnite here in the US, Ciment Fondu in Canada and Europe) was used, is resistant to mild acid conditions. As a bonus the refractory concrete adds mass, damping the stack vibrations caused by vortex shedding. A gunite lined stack in these service conditions usually, depending on actual temperatures, temperature fluctuations, and ambient conditions, will provide about 20 years service before needing extensive repair.

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.