Hot Moist Hydrogen Chloride - Rate of Attack on Mild Steel 1

[awolokay]

Does anyone have or know of any data for rate of attack of hot moist hydrogen chloride gas on mild steel?

Conditions are pressure up to 100 psig; temperature up to 300 °F; mass fraction HCl up to 0.98 with balance water vapour and traces of air.

We are considering temporary use of mild steel for a reactor emergency depressurization line. Under normal conditions, the line is isolated from the reactor by a rupture disk assembly. In twenty years of operation, there has never been an emergency depressurization.

In the event that a rupture were to occur, it is estimated that the depressurization would occur in about 3 minutes. The question is, would a mild steel line stand up to the hydrogen chloride for this time - if necessary, adding a suitable corrosion allowance. A second issue would be generation of hydrogen and whether this would be signficant.

If the line were ever used, we would be prepared to replace it after a single use.

 

[cheute79]

I can't find any data at my finger tips, but some years ago I worked with a system to recover wet 1,1,2-trichloroethane. The scrubber tower was built of Hastelloy C, but was corroding away at a constant rate. The process was used only a few days per month, and the solvent was expected to be changed with a couple years. Getting a new scrubber of Hastelloy was very costly, so I looked into using some thick walled (I think it was 3/8" or 1/2") CS, and keeping it around in the event it was needed, figuring it would last for at least 6 months - it was an insurance policy to keep the plant running. Anyway, the trichlor formed about a 2 mol % solution (just like you have). I'm sure that it will hold up. I also remember I had data on the corrosion rate, and the corrosion mechanism in CS was more uniform than the pitting of Hastelloy or SS. The project was 4 employers and about 14 years ago, so I probably don't have the data anymore.

 

[SJones]

At room temperature, the NACE Cor-Sur database quotes greater than 50 mils per year (1.25 mm/year). The dissolution of steel in HCl is an activation controlled process and, therefore, the rate can be expected to increase rapidly with temperature to possibly somewhere in the region of several hundred millimetres per year. For 3 minutes exposure duration, a carbon steel pipe with a 1 to 3 mm corrosion allowance should hold up and, if it definitely will be junked after the event, seems a reasonable approach.



Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[awolokay]

Cheute79:

Thanks for your quick reply - and it was useful to know that you had thought along similar lines.

I'm not sure though, if perhaps there was a misunderstanding - the gas stream is almost 100% HCl, not 2%. Even so, I'm inclined to think it will hold up under these conditions for the very short time involved, but I'd feel more comfortable if I had something to back up that feeling!

 

[awolokay]

Steve:

Many thanks for your response - that's really helpful and gives me at least a basis for evaluating this proposal.

And yes, it will be junked if there is an event or even before. We are moving equipment from an existing plant where the lines are PTFE-lined steel; once the existing plant is shut down, the PTFE-lined pipe can be reused - the possible use of the carbon steel is only a stop-gap measure for about 6 months.