letter 6056

Galvannealed vs. Hot dip galvanized 

.

Hello ,

Can anyone tell me the difference between a hot dip galvanize steel from a galvannealed steel? Thanks.

Most likely, which is cheaper? Can they both be powder coated?

Sharon S
- Billerica, Massachusetts

Hot dipped galvanized steel is produced by immersing steel into a bath of molten zinc, resulting in the formation of a pure zinc coating on the steel surface. "Galvanneal" refers to steel with a zinc-iron alloy coating. This is produced by heating a hot dipped galvanized surface so that the zinc coating and the top layer of the steel surface essentially meld together to form a zinc-iron alloy. Galvanneal would most likely be more expensive because of the additional processing. Either surface can be powder coated but you should consult the powder coating manufacturer to determine what type of pretreatment is required.

Patrick Patton
- Westlake, Ohio

1. Both processes involve coating the object with molten Zinc in a batch or continuous process. The main difference is that with the Galvannealed steel, the object is then post-processed by heating in an oven to induce diffusion alloying, creating the characteristic Zinc-Iron (6-15%) coating. See ASTM A653 [link is to spec at TechStreet], A924 [link is to spec at TechStreet], and A902 [link is to spec at TechStreet].

2. Expect the Galvannealed steel price to reflect the cost of the extra processing (heat treatment).

3. Both can be effectively powder coated. Check with the steel and powder coating vendors and reference ASTM D2092 [link is to spec at TechStreet] for the most effective surface preparation techniques.

Ted Bellinger
- Merrifield, Virginia

Sharon,

I'm a metallurgical engineer working in a galvanizing plant and I have to admit that Patrick and Ted answered your question very well. Just to add a few details of my own, a galvanneal steel has better paintability and weldability than galvanized steel. However it is less resistant to corrosion since the coating is lighter and not as adhesive to steel (because of the diffusion of iron in the zinc coating).

Luc LeBlanc
galvanizing company - Windsor, Ontario, Canada

Dear Sir,

Kindly clarify as to what percentage extent is paintability and weldability increased with galvannealed process compared to galvanised sheet, and point out the technical differences of galvannealed to this time electro-galvanised metal sheet.

Thank you for your assistance

Warm regards,

JOOMRATEE Yasseen
- Mauritius

Hello, Joomratee. I've seen television commercials promising that some cosmetic or other will "reduce the appearance of fine lines by 78 percent" and I chuckle that it is silly to try to rigorously quantify such an obviously qualitative parameter.

Galvanneal is meant to be painted; galvanized is meant to be left unpainted (although it is possible to successfully paint if the chromic acid treatment is omitted) -- and I think that's the whole answer. To ask by what percentage it's more paintable, forces us to ask you whether you mean primary adhesion, secondary adhesion, or something else, and exactly what pretreatments and final rinses you will allow on each type of surface, etc., and exactly what formula you want us to use to correlate all this disparate data into a single index.

I guess my answer is that galvannealing improves paintability by 78 percent :-)

The zinc thickness of electrogalvanized sheet is far more controllable than the thickness of hot dip galvanized, but it is usual for the thickness to be far less -- perhaps 1/10 the thickness of hot dipping. Patient application of the search engine will reveal dozens of threads about that subject on this site. Good luck.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

I want to know what kind of steel sheets can be galvannealed. Is there any constraint in galvannealing all kinds of steel sheets. If so why?

Fanish Tiwari
- Chennai,India