×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Alloy 20 / Acetic Acid - Method of attack?

Alloy 20 / Acetic Acid - Method of attack?

Alloy 20 / Acetic Acid - Method of attack?

(OP)
In reviewing the possibility of temporarily using an alloy 20 line for 85% acetic acid service, I noticed the corrosion rate was much higher than acceptable - 400 mpy at ambient temperature. This made me curious - what is the mechanism of attack on this alloy? Is it similar across other organic acids? A brief google search revealed very little in this regard.

RE: Alloy 20 / Acetic Acid - Method of attack?

This is from the MTI MS-2 Vol. 2: Formic, Acetic and Other Organic Acids.
I have never heard a good explanation as to why higher alloy grades perform worse in these acids, but I know that very clean alloys do better so it must be linked to localized grain boundary attack.

"The use of non-molybdenum-containing stainless steels is rarely reliable in actual practice
and the molybdenum-bearing grades are always preferred for both corrosion resistance
and product purity."

"The best-known of these alloys is alloy 20Cb-3 (UNS N08020). Laboratory tests indicate
a rate of less than 1 mpy (0.025 mm/y) up to at least 50% boiling acetic acid and <5 mpy
(<0.13 mm/y) in 99% acid. However, in laboratory hot-wall tests, alloy 20Cb-3 was more
severely attacked than type 316. Also, in plant tests, the relative merits vs. type 316L are
unpredictable. Field corrosion tests showed rates of 170 mpy (4.3 mm/y) for type 316L vs.
25 mpy (0.63 mm/y) for alloy 20Cb-3 in one distillation column while the performance was
reversed in the next still."

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Alloy 20 / Acetic Acid - Method of attack?

(OP)
Thanks, Ed. So I understand from the reference you provided that higher alloy stainless grades with moly are better suited to organic acid than, say, 304 grades of stainless. However, there does not seem to be a clear understanding of the mechanism of corrosion, and field results can vary widely based on service conditions.

If implemented, what would your recommended frequency of checks be to gauge initial corrosion rate in a pipe? Every month for the first couple months of service to gauge if fast corrosion is occurring, then set inspection rate based on initial data?

RE: Alloy 20 / Acetic Acid - Method of attack?

It is interesting, the highly alloyed SS grades (6%Mo) are no better than 316L in these acids.
You need to think about this, to start with you need to mark locations and take baseline readings either before or very early in service. The thickness variations will be large enough that future readings will rely on these baselines.
Since you could expect corrosion rates of hundreds of MPY setting the test interval of 3-4 months until you get a clear picture is a good idea. Be careful what locations you select as you may be much higher loss rates in some locations (near welds, elbows, outlet of valves).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Alloy 20 / Acetic Acid - Method of attack?

Quote (EdStainless)

"The use of non-molybdenum-containing stainless steels is rarely reliable in actual practice
and the molybdenum-bearing grades are always preferred for both corrosion resistance
and product purity."

This suggests that pitting is the mode of attack.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

RE: Alloy 20 / Acetic Acid - Method of attack?

IM, sort of, the trick is that higher Mo grades aren't much of an improvement over 316.
I have also done some work in Sulfuric where we got the best results with very clean 310.
Cr is your friend if you can prevent ANY carbide formation.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Alloy 20 / Acetic Acid - Method of attack?

You know this stuff better than I do, Ed, but there are situations where more Mo does make a difference. Resistance to naphthenic acid corrosion seems to require SS with a minimum of 3% Mo.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now
The Great Project Profitability Debate
A/E firms have a great opportunity to lead the world into the future, but the industry’s greatest asset—real-time data—is sitting wasted in clunky, archaic ERP platforms. Learn how real-time, fully interactive dashboards in a modern ERP allow you to unlock data that will shape the future of the world. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close