Chemical cleaning of mixed stainless-carbon steel

[itdepends]

I'm looking for some advice regarding the chemical cleaning of calcium carbonate scales from systems containining a mixture of 304/316 stainless steel and carbon steel.

From my reading online and ASTM A 380-06 (standard for cleaning, descaling and passivation of stainless steel) it appears that the best "officially sanctioned" solution is a inhibited mixture of hydroxyacetic acid and formic acid (Table A2.1, Part III of the code). However- I do not know if this mixture will be effective in dissolving the calcium.

Are there other options out there? I've found some references to citric acid for stainless steel but none for carbon steel- and the solubility of calcium citrate is very low. Sulpuric acid is also unsuitable (low solubility of calcium sulphate). Hydrochloric acid will attack the SS, nitric acid will be ok for calcium dissolution and the SS but I'm uncertain how it will go with the carbon steel (with corrosion inhibitors of course). I'd like to avoid nitric acide if possible anyway due to the requirement for a very short contact time (in the order of minutes).

Acetic acid seems to be a possibility- calcium acetate has a very good solubility- it won't attack the stainless steel. Can it be inhibited for use with carbon steel?

Sorry for the long rambling- I just wanted to put down where I'd gotten to so far.

Cheers.

 

[EdStainless]

Who are you going to hire to do the work?
Will you fill your system and then use a small pump to circulate?
How will you know when you have reached the end point?

There are companies that do chemical descaling and cleaning. They should run some lab tests for you and describe what they will do, and how you will know when they are done.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[itdepends]

We'll do the cleaning ourselves. Circulation will be provided by the existing plant pumps. This is a relatively small plant- so re routing piping to pump in/out will not be a problem. We will analyse the acid on an hourly basis- once the concentration stops dropping- thats our endpoint. I'd prefer to use an acid with low agression (like acetic) because it allows for longer cleaning times (and won't attack the stainless steel at all).

 

[EdStainless]

usually the acid strength is maintained by continous injection of a small amount of fresh acid. The end point is determined when the Ca stops increacing.
Make sure that you don't exceed the solubility limits. If so you will just transfer scale to the cooler sections of your system

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

I wouldn't recommend a home brew to do an effect job until you have a experience in use acidic cleaning media.
Here is a very good one that I've used on all sorts of mix and match metals.

http://www.rydlyme.com/home.html

 

[chemicalcleaning]

You could use warm sulfamic acid (7-10%) with inhibitor (rodine or similar). It's reactivity depends on temperature from being fairly slow at room temperature to being quite quick at 65C. It's only good for cleaning water scale and very light rusting. Also it's easy to handle.
As with all acids there'll quite a bit of gassing so the acid will need to be added slowly and carefully.
Have antifoam at the ready or you'll end up with a foam party!

 

[moltenmetal]

unclesyd: I read the MSDS for that Rydlime stuff, and it's 5-10% hydrochloric acid with other "proprietary non toxic additives". Hmmm...

 

[unclesyd]

moltenmetal,

I think Rydlyme is like all the other manufacturers of chemical cleaning compounds in that they will will not tell you anything that isn't required by a regulatory agency. I fought this when I first tried to use Rydlyme on site as the environmental people wanted a 100% analysis. I finally got got an internal agreement that we were able to use a chemical cleaning product if it meet the current and applicable regulatory requirements.

I think their additive package is derived from Tall Oil as it smells just like the settling ponds at a local stump factory.

In my use of the product I've never had any problems. I haven't seen any operator problems, like chemical dermatitis.

I've used Rydlyme on compressor housings, water jackets on pumps, and many mixed metal heat exchangers including SS.