Corrosion of Carbon Steel

Usually carbon steel will poision the stainless steel making it rusty. This is why whenever you have any stainless steel equipment fabricated you want to keep it away from where carbon steel is being worked. Just the airborne particles from grinding settling on the stainless plate is enough to poision the stainless and give it rusty spots later on. Its more of an appearance issue rather than a structural or corrosion resistance problem.

When the two materials are in contact with each other the difference is nobility is not great enough to cause any significant corrosion problems in normal service. Sometimes stainless steel vessels are fabricated with carbon steel jackets to reduce their cost and no problems result. Stainless steel pipe can also have carbon steel backing flanges for the same reasons. Seen this done many times without ill effects as long as the CS does not come into contact with the process fluids inside the pipe.

Thank you for the information. The application I am dealing with is a stainless product being threaded into a carbon nipple on a pipeline. It doesnt look like there should be any issues.

healybj8 gave a good answer, but threads areintimate contact. i'm not optimistic what is fluid?

milo

i too agree with healybj8. i wish to say my experience with these two material. bi-metallic correction is a big problem in process plants where there is a product flow through two different materials. we use shell and tube exchangers where ss and cs were combinely used to minimise manufacturing costs. but through the cooling water flow, corrosion of tube sheet (the plate which is welded to no of ss tubes) takes very rapidly than any other exchangers which is made of one single material. but for these cases we use maganesium blocks fixed to the end covers which will act as sacrificing blocks and reduces the corrosion act. and moreover we use belzona compound painted on the surface of the tube sheet so as to minimise the contact of two different materials with the product flow.

regards.

r.babu

Please confirm: do you use Manganese block or do you use Magnesium block?

Please confirm: do you use Manganese block or do you use Magnesium block?

i am sure ...it is magnesium blocks.

Hi Mag:

Use Magnesium for a sacrificial anode in fresh water and zinc in salt water.

Cheers

mechtech

thanks mechtech. you are absolutely right.

regards,

r.babu

Dear ramanathanbabu, you said:

"moreover we use Belzona compound painted on the surface of the tube sheet so as to minimise the contact of two different materials with the product flow"

I think Belzona compounds in that case of shell-and-tube heat exchangers is not to minimize the contact (because the contact already happened between SS tubes and CS tubesheet), but Belzona (or any similar product) is to prevent the chance of locating the electrolyte between the dissimilar metals in which the circle of formation of a galvanic cell is un-existing. So, where no galvanic cell formatted, where no corrosion due to locating of dissimilar metals occurs.

Note. In a lot of cases, we use a CS tubesheet cladded with the same SS material of tubes. Cladded may be carried out at mill or by using weld deposit before drilling the tubesheet. And cladding is differ from lining or painting.

in the context of:

"all paints are not alike" (paints being for cosmetic/aesthetic purposes)...and,

"all protective coatings are not alike" (coatings being either inhibitive, sacrificial, or barrier coatings to help prevent corrosion)...and,

"all liner materials are not alike" (liners being chemically compounded for immersion types of service)...

...which of the multitudinous "Belzona compounds" might you be referring to...?

BELZONA R METAL. sorry i dont remember it properly. but i am sure it spells metal after its trade name.

regards,

r.babu

this is a typical situation where you can have galvanic corrosion. The fact that the two materials are in contact is the first condition to have galvanic corrosion then is necessary to have an electrical path between the two materials (water or other electrolyte that carry the electrical current).

The differences in potential of the two materials is also a function of the fluid (in seawater is different from fresh water..).

The severity of the galvanic corrosion is a function of many factors:

-difference in potential

-area effect (ratio of the two are exposed)

-conductivity of the fluid..

so, coupling SS and CS is a potential for galvanic corrosion then you need to have the other conditions..

galvanic corrosion

S.

corrosion control & corrosion prevention

"is a stainless product being threaded into a carbon nipple on a pipeline"

Dear Shads 302,

Considering the valuable information from Strider6 (thankyou for the links!):

there will be corrosion in your situation if your pipeline is transporting electrically conducting material,

but the area of your stainless steel part is small and the carbon steel area is much larger, so the carbon steel (which is less nobler) will corrode only slowly as the effected area at which the material is affected is large.

The corrosion current is following the possible pathways that are allowed by the resistance of the fluid and the resulting current density is transforming the metal (at the less noble side) into ions that go into solution or form hydroxides.

So I would try what you suggested and look after 1 month, after 3 months and after 1 year to judge about necessary replacement intervals of the pipeline - not likely to be a problem.

RHABE

This type of corrosion is called corrosion of dissimilar metals. Locating any dissimilar metals in contact with an electrolyte helps in constructing a galvanic cell. To the extent of considering the rust of ferrous materials such as carbon steel as a dissimilar metal to the base metal (when a such broken or removing has occured to the rust), which constructs a galvanic cell with the base metal itself in presence of electrolyte, which propagate corrosion.

Strider6 (post 5) is correct (according to NACE, National Association of Corrosion Engineers, International) in referring to the possibility here as "Galvanic Corrosion".

Also, the descriptive-reference to "area effect" is on-the-money (also in RHABE's description); the correct term for this is the "Anode:Cathode Ratio".

The reference to the "difference in potential" being of primary concern is correct, and while it is true that the composition of the electrolyte ("fluid") makes a big difference, I believe it helpful to emphasize the fact that it is the concentration of positive metal ions in the electrolyte that exerts the greatest influence on corrosion. The H⁺ ions in acids (H being categorized as metallic) are what enable acids to accellerate corrosion to such phenomenal speeds that it appears as though the acid is "eating" the metal! [The exact same decomposition would occur, tho more slowly, in saltwater, and slower yet in 'fresh' water.]

SS to Carbon steel is very useful for cladding purposes (as already mentioned) because the construction processes preclude electrolytes from getting between the two... and (where this is done), it is usually necessary for economics of the project.

SS to CS should be avoided wherever possible, as it simply INVITES corrosion problems (SS being very cathodic with respect to CS).

I think all the aforementioned mechanisms of corrosion are at work in the typical industrial environment. The degree to which they occur is the key question, and that is sometimes difficult to predict. Small anode, big cathode--big problem. Been there done that. Don't want to see the re-run.

I have seen many examples of shell and tube heat exchangers with SS tubeside construction and CS shellside construction that did not show corrosion at the shell-tubesheet weld even after many years. I supose poor management of CTW would cause problems on the inside (that would be observable to only the most determined souls) and salts on the outside could cause the same from rainwater.

Given the high cost of SS today, one should evaluate the environment carefully and make the best decision possible. There are many S&T HX with CS shell and SS tubes that have had a long life. Likewise SS vessels with CS jackets.

i rate this a good answer.

milo

Yes!