Corrosion in salt drying

[Bob511]

Hi,

I have been asked by one of my friends who work in maintenance to give him an advice on the corrosion of SS inside their dryer. They work in the salt business and they dry the wet salt inside a direct fired dryer. It seems that there is some kind of acid that eveolves during the drying process. I do not know what kind of acids that could evolve from drying salt. The only thing i can think of is HCl because they have some 0.05% of MgCl2 in their salt. Maybe this mag chloride is hydrtated and while drying it undergoes hydrolysis and HCl will evolve. They have some small percntage of magnesium and sodium sulfate too. The SS 316 is being eaten up within few years because of this alleged acid gas and now they are thinking of using some expensive alloy.

Does anybody have a suggestion as to what kind of acid (or anything else) that is being produced in the drying process that will cause the SS to fail tha fast?

Thank you.

 

[Bob511]

i forgot to add that the dryer uses natural gas.

 

[unclesyd]

Is all the damage due to plain corrosion with no discernible cracking?

Is the burner flame always oxidizing?

They should be able to sample the exit gas and see specifically what is the corrodent. This information will be needed prior to changing material

FYI,
A hot solution of MgCl₂ will crack SS faster than any other material.

 

[Bob511]

thanks unclesyd for taking the time to answer.

There is some cracking but the main damage is because of plain corrosion. I advised him to check if the flame is oxidizing and to sample the exit gas for acids and other stuff. What I know is the MgCl2.2H2O will hydrolyse when dehydrated and HCl will evolve. They have some percentage of crystallized mag chloride that crystallizes along sodium chloride. I do not know of any other salts that will produce acids while being dried under the conditions they have inside their dryer.

 

[EdStainless]

So is the corrosion pitting? Does it favor the welds and HAZ?
Any signs of wear or erosion?
My thought is that the wet MgCl2*6H2O is driving the corrosion.
The most economical move would be to build using 2205 duplex. They could reduce the wall thickness a lot because the alloy is twice as strong as 316. This unit should not be any more expensive than one in 2205 (should cost a little less) and they will get longer life. It wont last forever, but they should see a good improvement.
The next step up in alloys would be to grades like the 6% Mo super-austenitics. There they would be talking 5 times the price. I am not sure that even if they lasted forever that it would be worth it.

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Plymouth Tube

 

[ApC2Kp]

Bob511,
The possibility of wear and erosion, as EdStainless has raised in his question, could be understood better if there was more information on the type of dryer. You had mentioned it being a natural gas direct fired dryer. Would it be similar to a rotary kiln with the solids circulating inside? Is the corrosion / erosion / wear more severe in the wet end or the dry end?
The abrasive properties of sodium chloride were used by a prison inmate to cut through a pair of the steel bars of his cell. He was working through a third steel bar when he was discovered. The cut surfaces of the steel bars were mirror smooth from the table salt and thread used to abrade through the steel bars.