NDE Inspection of welds after galvanized.

[demsha]

Hello,

We have A 572 structural steel components of transmission towers welded then hot dip galvanized (Zinc coating). I would like to know which is the best NDE method to check weld cracks after glavanizing and why the method is good? Your responses will be greatly appreciated.

 

[SJones]

Please do not cross post.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[elyaas]

Hi demsha,

Check out this link

http://elsa.jrc.ec.europa.eu/publications/JRC56810.pdf

Regards

 

[gtaw]

Magnetic particle testing with dry magnetic particles works.

Best regards - Al

 

[phighfill]

gtaw

It depends upon how thick the coating of galvanize finishes up to be.

 

[Kiwi2671]

Can someone please explain how you can inspect something you cannot see ?.
If a weld is covered with galvanising how do you inspect it ?
The structures should be rejected, NCR raised, welds blasted, tested then re-galvanised.
Regards,
Kiwi

 

[gtaw]

The fabricators have been noticing cracks in the area of the access hole, copes, and blocks when subjecting some of the HSLA structural shapes to HDG. ASTM A992 is a good example of a base metal having this problem on a regular basis.

A magnetic particle test using an AC yoke and dry particle seems to be doing a pretty good job of detecting the cracks in the radius of the access holes and other thermal cut surfaces after HDG. Since there has been a problem identified, a number of fabrictors routinely perform the MT in the areas of concern.

I have not been asked to do UT, but I am confident pulsed echo UT with a 70 degree wedge would find the cracks as well, but MT seems to be working and it is less expensive.

Best regards - Al

 

[Kiwi2671]

Al,
I may be barking up the wrong tree here and it may be exactly what you are commenting on.
However, my understanding of the initial post is they have neglected to perform required NDT prior to galvanising and now are looking for an "out" so they don't have to remove the galvanising and perform the required tests,
Cheers,
Kiwi

 

[gtaw]

You may be correct in your assumption, but that wasn't my take on the subject.

Back to my comment; the members involved exhibited no evidence of cracking in the area of the access holes or the copes and blocks before HDG. However, after HDG some cracks were large enough to be seen. The MT was used to check all the steel after HDG and low and behold, cracks were identified using MT after HDG.

The sensitivity of MT is reduced as the thickness of the coating increases, but MT with an AC yoke seems to be working with the normal thickness of HDG coatings. Whether the test is sensitive enough to satisfy your needs you can easily validate the MT by putting different thicknesses of plastic film on a sample of steel to see if you can still detect the crack.

Best regards - Al

 

[BillPSU]

Kiwi you may want to scan through the Elsa document linked above. The hot galvanizing process appears to be causing cracking in fabricated components. There is a large I-beam which is split the length of the beam.

The part was not split like that before it was placed in the zinc tank.

Bill

 

[Rich2001]

LME or LMIE cracking of the steel and welds due to galvanizing can occur. Zinc is a paramagnetic material therefore magnet particle inspection can be utilized. For best results with this nde method utilize Dray power AC yoke method.

Here are a copy articles, though several years ago, that may be helpful:

http://www.aws.org/itrends/2005/10/023/

http://www.aws.org/itrends/2004/07/it0704-29.pdf

Another technique that works well is ACFM. ACFM is routinely used to inspect surfaces beneath coatings. One of the systems that I have used was the Lizard ACFM M8 NDT System.

Richard

http://www.6sproductivitycom.com