Is the weld done after galvanization of steel? What is the type of weld (e.g. fillet, butt)? What is the application for the galvanized steel?

You can also check the links below.
http://www.aws.org/itrends/2005/10/023/
http://www.galvanizeit.org/resources/files/AGA%20PDFs/T_WHDG_02.pdf

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Is MPI an effective technique, apart from RT, to check galvanized steel welds (due to diamagnetic properties of Zn)?

No. The surface needs to be cleaned of zinc to allow for a proper inspection of welds.

You mention both MT and RT. These are complementary techniques - MT for surface-breaking defects and RT for buried or far-surface defects.

ASTM E-709 states that "Thin non-conductive coatings, such as paint in the order of 1 or 2 mils (0.02 to 0.05mm), will not normally interfere with the formation of indications, but they must be removed at all points where electrical contact is to be made."

While the presence of a galvanised coating is not ideal, depending on the thickness of the galvanised coating, I do not see that it would totally invalidate the test or preclude the detection of surface-breaking defects. The item is galvanised for a reason, removal of the coating would potentially be more deleterious to the item than a reduced-sensitivitiy MPI test. If the legs of the yoke are firmly magnetically attached to the test item when energised then I would say the test is feasible. If they pull off very easily then re-think.

Nigel Armstrong
Karachaganak Petroleum
Kazakhstan

P.S. I should have added you could investigate the use of an ACFM technique.

http://www.tscinspectionsystems.com/

Nigel Armstrong
Karachaganak Petroleum
Kazakhstan