Galvanised Steelwork - Strange Pattern

[Matt7128]

Hi

Recently came across the pattern shown in the attached photo on an inspection of galvanised steelwork. Its not something that I have seen before and am curious as to whether anyone else has seen something similar and knows what the cause might be? It is a visual discoloration only and does not feel uneven to touch.

As a guess I would say that it was due to inadequate surface cleaning prior to galvanising but could not say for definate.

I compared it against the common visual defects listed on the American Galvanizers Association website :

http://www.galvanizeit.org/inspection-course/finish-visual-defects/a-c/

but could not find a match.

Anyones views/opinions are very much appreciated.

Thanks
Matt

 

[chicopee]

I stopped downloading your attachment after 4.04 MB.

 

[berkshire]

Matt,
Even though you had a high resolution picture, for some reason it would not zoom in very much.
However from what I could see, it appeared that the spelter was too cool. Either the tank was cool or the galvanizer did not let it stay in the tank long enough.
A better picture would help.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Matt7128]

Thanks for the quick responses guys. Berkshire you make an interesting point, I noticed that the effect does not appear on the flanges at all, could the differance in thickness between the web and flanges been a factor??

Chicopee, I tried reducing the photo and reattaching so that its a bit easier to view. The file size is down to under 0.5MB so hopefully that should help. The photo in question is the web of a 250PFC but the effect is also present on the web of 250UB's that were galvanised in the same batch.

Any thoughts on whether the issue could impact on the long term protection of the member and whether it is grounds for rejection?

Thanks
Matt

 

[berkshire]

Matt,
Have you tried a knife test in the area where the spangling appears to be separating?
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[chicopee]

Looks like a spangle pattern that you would see on a hot dip galvanized metal sheet. Do the patterns show dendrites?

 

[Compositepro]

The pattern is similar to what is created by convection currents when you place a hot object in a cold liquid, or vice versa.
Cooler fluid falls and is replaced by hot fluid moving in the opposite direction. Because the flows interfere with one another once a flow cell in one direction is started it tends to be fairly stable. I would guess this would show-up mainly on horizontal surfaces in the bath and would vanish if the metal were allowed to warm-up fully.

 

[Matt7128]

Thanks for your continual help guys.

Berkshire - tried a stout knife test today and there was no flaking/issues with the adherance of the coating to the steel. The surface seems relatively smooth and the pattern seems to be chemically ingrained rather than physical in nature.

Chicopee - could not see any dendrites visable, but could be present at a scale too small to see. Would you expect them to be clearly visable to the naked eye if there?

Compositepro - you make an interesting point and I can see the logic. I know that the galavnisers are busy at the moment and suspected that the pattern could be due to them shortening the amount of time the steel spent in the bath, which would support your theory. If you are correct I would assume that there would be no long term adverse affects due to that phenomenom?

Many Thanks.
Matt

 

[berkshire]

Matt,
I figured the spelter was too cool in my first post, however if you have good adherance, it was hot enough.
Unless anybody has problems with the visual appearance from a cosmetic viewpoint, I would say you are good to go.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[chicopee]

You should be able to see the dendrites with the naked eye; since you indicated that you did not see the dendrites, try a magnifying glass or a low power microscope.