(Material Selection/Corrosion) Bimetallic/Galvanic corrosion compatibility

[mzamzam]

I am a QA/QC engineer in a refinery company but I am not a corrosion specialist. I have a puzzling point about the material used in building some of our towers which were constructed by Chiyoda Corp. The Towers shell and internal rings are made of carbon steel (I can not remember the exact JIS material designation but I think it is equivalent to A285 or A516 Gr.C). The trays, which have a direct contact to the rings, and some internal components are made from SS410 (ASTM A240 410 equivalent).

I searched many times for any codes that can explain the compatibility for the usage of these two different alloys together. In some cases we would prefer to use the austenitic stainless steel series (3xx). I know that the SS316 is more noble than 410 and i think that it will cause/produce a higher galvanic corrosion.

During my searches, I found the two famous codes MIT-STD-889B and NASA-STD-6012. In MIT main chart/table it prohibited both the (ferritic/martensitic & c. steel) coupling and (austenitic & c. steel) one for industrial services. In NASA (ferritic/martenistic & c. steel) is an accepted couple (potential difference between 12%cr and Iron doesn't exceed 0.25V) for space industry (but it does not work in same condition we have in towers "high press. and temp."). I could not find any ASME, ASTM or petroleum related codes that talking about that.
I also read some convincing articles that say that the potential difference, or anodic index difference, is not an enough factor to make a compatible couple. I also saw Wikipedia page

http://en.wikipedia.org/wiki/Galvanic_corrosion#Anodic_index

where the editor said that the reference is corrosion engineering handbook. I opened the handbook and could not find such anodic index diff. values (0.15, 0.25 & 0.5V) mentioned within!!!

It is very confusing for me. Can I find any NACE, ASME, ASTM, EN, DIN or JIS for?
What are the factors the material selectors and designers use/consider in such case? I think Chiyoda engineers were having a strong reference while making such couple.

Thanks in advance for any help.

 

[EdStainless]

These are decisions left up to the designer, there is not Code (legally binding) or Standard (commonly accepted reference) that governs this.
What is the conductivity of your fluid at working temp and pressure?
The risk of galvanic attack is proportional to the actual potential difference. In a high conductivity environment (seawater) you have 0.25V potential, but if the conductivity is lower the potential will be lower. And the risk of corrosion will be lower.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[mzamzam]

One of these tower which has a high trays corrosion rate and had a suggestion to be replaced with 3xx grade is a Debutanizer. This corrosion occur mainly due to existence of chlorine (and sulfur) in natural gas liquid (electrolyte). I thought NGLs has a lower conductivity than sea water but I also was thinking about the direct contact between the debutanizer Trays & Rings. The ring will have a higher galvanic corrosion risk than the shell. Replacement of the trays in shutdown is an ordinary maintenance process (there no welding, just disassembly). The ring is welded to the shell and will be much harder to repair and will have more precautions before any repair.

 

[EdStainless]

I would not move to 3xx stainless for the trays. Since the rings would be difficult and expensive to replace or repair I would stay with lower alloy materials for the trays ans replace them as needed. In the long run this is a lot less expensive.
You may want to assemble some dissimilar metal crevice corrosion coupons and galvanic couple coupons to place at various locations in the tower.
Once you can quantify the risk you can make better decisions.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube