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Tin amount in galvanize?
What about the tin amount in the molten zinc?Some of European Galvanizers and also some of us say that Exceeding of tin amount can cause the microcracks and result in destruction of alloy coating.As an example,the Munchen Stadium Construction..
Can you represent any articles about Tin usage and also optimum percentage%(wt.)?
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Hakan Esen, Material Engineer(BSc.) |
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There seems a loose correlation between microcracking and Sn concentration in the melt. But these cracks have mostly been known to occur only where there was an unusual stress in the steel that was revealed by galvanizing, not caused by it.
Sn levels of less than 0.2% appear to have much lower risk, and 0.1% even better.
A new EU guideline seems imminent which will recommend lower Sn, and Bi, but allow marginally higher Pb.
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Geoff Crowley |
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Sirs:
From 1972 to early 2003, I had no questions about cracking of steel due to galvanizing. During 2003 I had many questions about cracking. The cracking first showed itself as wires used to hang the steel began breaking. The galvanizing workers refused to work because upon withdrawal from the molten zinc the wires would break and splash the molten zinc on the workers. At first "bad wires" were suspected. Then tin in the zinc was identified as a possible cause. Then stressed steel products were showing cracks. These included overhead highway sign structures, steel tube pilings that cracked along welds, square steel tubes that peeled like a banana, scaffolding, cracks along welds, a stadium roof structure in England, rectangular frames, etc.
In September of 2003, I received a 19 page scientific study of these cracks on a stressed steel bridge structure.
One of the conclusions of this study was: "The presence of tin at locations where the cracks had been propagating just prior to removal from the galvanizing bath proves conclusively that tin was the embrittling agent."
The steel cracking appears to start at about 0.3% tin and is extensive above 0.4% tin.
On Oct. 1 and Oct. 3, 2003, I sent two letters to ASTM recommending that the tin level in hot dip galvanizing be severly limited. As of today, I believe that ASTM has taken no action on this matter.
Regards,
- Hot Springs, South Dakota