Galvannealed vs. Hot Dip Galvanized

A. 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.

A. 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, ASTM A924, and ASTM A902.

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 for the most effective surface preparation techniques.

A. 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).

A. Hello, Joomratee. Galvanneal is designed to be painted; galvanizing and zinc plating are designed to be left unpainted (although it is possible to successfully paint them). So galvanneal is more paintable, and I think that's the whole answer. "By what percentage?" would force you to tell us precisely what spec and what method you would use to measure adhesion and under exactly what circumstances (aging time, corners vs. flats, how you intend to assign percentages, and what formula you want used to correlate all this data into a single index.

The zinc thickness of electrogalvanized (zinc electroplated) sheet is far more controllable than the thickness of hot dip galvanized, but it is usual for the thickness to be far less -- perhaps 1/4 the thickness of hot dipping, probably less. Patient application of the search engine will reveal dozens of threads about that subject on this site. Good luck!

A. To answer your question I need to ask a few.

Typically galvanizing is done in a line with coil which can then be sheeted. There are certainly other galvanizing processes with an actual dipping that could be done to coat sheets of steel.

I have yet to hear of anyone willing to put a galvanneal product on a sheet; this would involve laying the sheets down to run them thru an annealing cycle. Typically the annealing is done within seconds of the galv coating being applied. What type of application are you looking at?

A. Hi Mehul. I don't have a definitive answer for you, but I think galvanneal, due to the additional annealing, probably has more iron on the surface for more reliable adhesion.

Regards,

A. The difference between galvanizing and galvannealing is nothing except what we do do force iron to diffuse to zinc coating. This function is post annealing after zinc coating normally to 500 °C in a furnace about 30 meters long. In this treatment the iron will distribute to zinc coating and the brittle phase will become more homogenized; this will cause galvanized sheet to have more weld-ability especially for automotive applications.
Each way that it can diffuse iron to zinc coating will use like reducing Al in zinc bath and Si in steel substrate.

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A. Hi Darryl. Please remember that people can have very different finishes in mind when they use words like "gunstock color" or "dark gray slate", so there is a fair probability that what you furnish may not be what the customer had in mind. But you could buy galvanized sheet (available in large quantity from a steel warehouse or small quantity from a hardware store). You could form it with a sheetmetal brake, and then you could try a patina for zinc. Good luck.

Regards,

A. Hi Tim. You are implying something but I don't know what :-(
Are you saying that you only currently offer this product in galvanized, and don't yet do powder coating, but you want to, and you want to build a pretreatment line for preparing the material for powder coating? It may be possible to powder coat without any pretreatment to 'spike your waste water'. But remember that galvanneal is brittle; you don't powder coat it until all forming and bending is done.

Regards,