Protection of galvanized parts welded later? 7

[IJR]

Pals

This is a standard problem and there is probably a solution somewhere

You have hot dipped galvanized a part and shipped it. It is then supposed to be welded to another hot-dipped galvanized part.

The welding messes up the protective coating specified.

How do you go around this problem? Do zinc based metallic looking paints do the job?

respects
ijr

 

[kelowna]

My approach is to minimize or eliminate field weld of galvanized parts. Where that is not possible, I require field touch with zinc rich primer, surface tolerant, applied by brush, not sprayed. Zinga has a good product.

It is a future maintenance issue, but something you can not do much about and zinc rich primer is quite tough, easy to apply, blends not too badly and it is compatible with the galvanizing.

 

[AidanMc]

Does it have to be welded? When I took a welding course, this was actually the specific application they taught us brazing for.

Brazing is stronger than solder, but not quite as good as a weld. The temp is low enough though that it doesn't mess up the galvanized coating on the parts. Just a thought and perhaps another option.

Aidan McAllister
Metallurgical Engineer
Automotive Enthusiast

 

[MiketheEngineer]

Welding on galvanized metals releases some dangerous fumes -so be careful.

 

[HgTX]

Galvanized coating should be removed before welding.

For repair to the coating, there are three options I can think of, in what I think is descending order of cost. You get what you pay for.

1. Metallizing
2. Solder rods
3. Paint ("cold galvanizing").

With the paint, you want to make sure that it will provide a dry film with minimum 94% zinc content. Beware--often you will find a zinc % on the can that is not the zinc content of the paint but rather the purity of the zinc dust ingredient. (Kind of like "contains 100% pure aloe vera!" where aloe vera is only the 3rd ingredient in the goo.)

Hg



Eng-Tips policies: faq731-376

 

[rmw]

Be sure to have some milk handy for the welder when he/she gets sick from the fumes. I can't remember why it works (I once knew) but I sure know it does.

rmw

 

[Adalius]

rmw -

Milk is/was recommended because the calcium in it bonds with heavy metals quite readily.

However, having welded zinc for years and having dealt several times with metal fume fever, I would really appreciate it if everyone would try to break themselves from the habit of suggesting milk as a solution.

Proper medical attention should be acquired in the event of metal fume fever, even a slight case, as the effects can be horrendous and in some cases cause permanent damage.

Milk certainly does make you feel a little bit better, but is not a solution. The calcium in milk, by weight, is only enough to bond with, if memory serves, roughly 1-5% of the zinc inhaled in a typical situation. This still leaves 95% of the problem or more in your system.

 

[SJones]

Incorporating zinc into the weld can also be detrimental and it should be removed from the joint area. The usual restoration treatment would be an organic or inorganic zinc rich paint with a minimum metallic zinc content of >80%

Steve Jones
Materials & Corrosion Engineer

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