Mill scale are formed during rolling operation is above steel in galvanic list. hence it is cathodic to steel & if remain on steel will cause for electrochemical corrosion. Whenever electrolyte is available , corrosion will start by completing corrosion triangle i.e Mill scale act as cathode & Steel as Anode ( corrode sacrificially)
Once it removed by any surface preparation method , it will not reoccur
Please find attached document for more information.
Regards
Bharat B. Gole
From: pgoswami@quickclic.net
To: materials-welding@googlegroups.com
Subject: [MW:8052] RE: 8051] effects of scales on corrosion rate
Date: Wed, 3 Nov 2010 09:08:34 -0400
Hello Asad,
These scales would be harmful.There may not be issues during fabrication, but definitely during service,such as:-
- Loose/soft scales may may get removed and clog the piping/valves and strainers downstream
- Hard scales would stick to the vessel surface.If the operating media is corrosive, which any process fluid is always to certain degree, then it would ideal sites for crevice/pitting corrosion.
If required it would be advisable to get your client involved and influence the fabricator to get the scales removed completely.
Thanks
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of asad azmi
Sent: Wednesday, November 03, 2010 6:34 AM
To: materials-welding@googlegroups.com
Subject: [MW:8051] effects of scales on corrosion rate
Dear All,
Greetings.
During inspection of the internal surface of the CS vessel before despatch, heavy scales some loose and some hard (adhered to surface) were observed on the 33mm thick dished head. The Vessel is PWHTed. Dished head is HOT FORMED.
pls refer attachment for your reference.
Contarctor is not willing to remove the scales completely. whether the scales which are adhered to surface are also need to be removed.
Pls suggest whether the vessel can be accepted with scales. what is the effect of scales on the metal surface. is it going to affect service requirement.
Thanks and Regards
Asad Azmi
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