Does hot dip galvanizing weaken steel?

That's what I said in my first paragraph: "The hot dip galvanizing is causing your problem...". Dr Cook, what are you disagreeing with?

If an amount of cold-work exceeding some threshold level is done prior to galvanizing, then the subsequent galvanizing will cause a problem in that cold-worked material.

Possible solutions are (1) reduce the amount of cold work if the application permits (that is, use a larger bend radius), or (2) bend hot so that no cold-working is done - subsequent galvanizing will then not damage the bend zone.

Or (3) galvanize first then bend, but be aware that the silicon content of the steel is important, to avoid a brittle zinc layer that might crack when bent.

Bill,

I found your technology on bending quite interesting and this may well be exactly what is causing the problem.

I was thinking that the galvanizer of these brackets may be using more than 0.3% tin (Sn) in his zinc which has caused many, many cracking problems in Europe and North America. Unfortunately I could not get information about the zinc in the kettle to verify my thinking.

Galvanizers have found that tin in their zinc reduces zinc usage, perhaps by lowing the freezing point of the zinc allowing better zinc drainage back into the kettle. Unfortunately this tin content, above about 0.3% causes cracks in stressed steel and MANY failures have been reported including:

1)Hanging wires breaking in galvanizing plants in the zinc.
2)Overhead highway signs cracking.
3)Stressed steel bridges failing.
4)Pipe pilings splitting sometimes like a banana being peeled.
5)A stadium roof structure cracking and needing braces.
6)Rectangular welded structures cracking and breaking.
7)Scaffolding cracking and failing.

One company in a patent recommended 0.5% tin and since has lowered this recommendation to 0.2%. Another company recommended above 1% tin and is contained in their patent. The second company has been successfully sued.

Tin causing the cracking has been carefully studied and PROVEN to be the problem. In any case of failure the failed galvanized product can be tested tin can be proven to be the cause.

About eight years ago I became aware of the problem and sent two letters to ASTM recommending a limit on the use of tin in hot dip galvanizing. My understanding is that no action was taken by ASTM because "Cook lacked sufficient proof." I have both dated letters and I have the full scientific paper which proves that tin is the problem.

Regards,

Dr Cook, thanks for your extra information.

I agree entirely with you that tin in the zinc would be damaging. Steel and molten tin make a very unhappy partnership. Even when the tin is in dilute solution in another molten metal, intergranular penetration by the tin can effectively form microcracks in the steel surface which can then easily propagate when lightly stressed. (The successful use of tinned steel sheet for cans is another story - highly polished steel, very soft steel, different steel microstructure, low service stress, etc etc - and tinplate was made by hot-dip tinning for very many years before electrolytic plating)

But surely the use of tin in the kettle would be unusual? It's not a constituent of zinc blocks from the zinc refinery, so would have to be a deliberate addition. Was it just for the purposes represented by the patents you mentioned?

The information that I quoted from Onesteel in fact deals primarily with the temperature cycle imposed on the steel by the galvanizing process, rather than details of the zinc itself. Similar degradation of the steel properties would occur if a furnace was used to impose the same temperature cycle as occurs in the kettle, on the same amount of work-hardening, in the complete absence of zinc or tin or any other metal.

So there are evidently two distinct paths, each of which leads to premature service failure - excessive cold work followed by galvanizing which imposes a damaging temperature cycle on the bulk steel, or excessive tin in the zinc which causes degradation of the steel surface.

I sympathize with your attempts to get something new into an ASTM spec - it's bad enough trying to get an editorial correction made of an obvious error!