Dear Gordon,

The temp.in the internal of an exhaust stack is 50-60 degree C as the heat of flue gas is extracted in the economiser. Due to low temp. sulphuric acid formation takes place inside the chimeny, which corrodes and punctures the exhaust stack within 2 -3 years.

In case of economizer not being used the temp. of flue gas can sometimes go up to 400 degree centigrade.

4% furnace oil is being used.

Would a coating of aluminum thickness 175-200 microns withstand in the above atmosphere or which other coating would you suggest ? What could be the life expectation of the coating ?

Thanks in advance,

Raghav

Hi Raghav

to the Surface Engineering Forum.

Aluminium should prolong life, but by how much is very difficult to predict. Aluminium is good in mild acidic conditions and should have no problems with temperatures rising to around 400 C or so when the economiser is off. It really depends on how aggressive or acidic the environment gets. Usually, the worst of the corrosion in these structures is localised to where you have nooks and crannies or where the gas flow is turbulent and you get areas of low gas flow, which collect deposits and condensation. You could consider spraying the worst areas with something like inconel 625 and then cover the general area with aluminium.

Dear SEF Members

I'm flattered that some of you post your messages addressed to me personally, but we do have a community of members here and are exposed to the WWW. There will always be people more qualified or will have better knowledge or experience on certain subjects. I suggest, that we take care when addressing posts, so we do not put these people off from posting replies.

Hi Raghav
in your stack you are below dew point and will get highly concentrated sulfuric acid.
There are two ways to overcome the problem:
1. Reheat the flue gas above dew point
2. Line the stack with nickel alloy material like UNS 06059 thin plate.
No other material will resist the corrosion for economically long time.
Source for this material is ThyssenKrupp VDM, Altena, Germany
contact through http://www.thyssenkrupp.com
I am retired materials consultant and had to deal with such problems in the last year of my career.
best regards

Hi hapewoe

to the Surface Engineering Forum.

Thank you for your contribution.