Sacrificial anode holders corroding in oil tank 1

[Bain89]

Hi experts - CP is not really my area, however we have noticed a problem with our zinc anodes in an oil storage tank. The holder that hold the zinc anodes are corroding, which we cannot explain. The zinc would have the lowest corrosion potential so should protect the carbon steel holders, right?

Any ideas would be most helpful.

Thanks

 

[cap1a79]

Is it hot storage tank which is operating above 60 c?
And let me know whether the corroded area is the interface between two different metals.

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[Bain89]

Hi Lee - thanks for the reply. It is below 60 degrees, probably closer to 25 C. Yes the corroded area is the carbon steel holder that holds the zinc anode and connects it to the carbon steel oil tank shell (which is coated).

Thanks

 

[cap1a79]

Bain89..Refer to below.

1) There is polarity reverse which could occur generally at above 50-60 C.
2) This phenomenon could happen mostly at above 50 or 60 C. But it could occur even at room temperature depending on specific condition. (ex., 600 ppm of HCO3 with 73 ppm of No3) referring the below reference.
Reference from "Corrosion and Electrochemistry of Zinc, Table 7.7"

https://books.google.com.om/books?i...onepage&q=polarity reversal zinc nace&f=false

3) In this time, what I would suggest is to check "whether oxygen was properly controlled". (<-- The polarity reverse does not occur at all in case the system is oxygen-free condition.

4) Refer to below link as well. Good luck.

http://events.nace.org/seminars/ccc_series/2009_SEPT/presentations/mills.pdf

https://corrosion-doctors.org/CP/an-zinc.htm

Lee SiHyoung,
WorleyParsons Oman Engineering,

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[Dhurjati Sen]

@Bain89 Why not put a star for cap1a79?

Or probably could you not grasp what he said?

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[Bain89]

cap1a79, sorry for the delay I was on a business trip. Thanks a lot for these tips and the link to the text, very interesting, I did not know temperature could affect the polarity to this extent. I guess there are some different corrosion product layers that are affecting the corrosion potential.

We have some fluid sampling data we are waiting for, will keep you updated if it agrees with the table on the text for a polarity switch.

Cheers,
Bain

 

[Bain89]

Hi Dhurjati Sen, no I was on a business trip and don't check the forum daily. And even though I specialize in fractography and my PhD was in CO2 corrosion, which won the top prize from the German Society for Corrosion Prevention for outstanding scientific research, yes I could still follow his post, so please keep your condescending comments to yourself and try to contribute in a more positive way.

 

[MJCronin]

Is there a way to verify the polarity reversal with some kind of meter ?

MJCronin
Sr. Process Engineer

 

[cap1a79]

I believe genrally reference electrode to measure the relative potential of the steel will work for the case. In other words, if the steel is properly protected (with sacrificing zinc), it will be expressed in reference electrode once installed into the vessel or tank.


Lee SiHyoung,
WorleyParsons Oman Engineering,