One might guess HDG is better, but cannot really answer such a vague question.
Better with respect to abrasion, appearance, bearing surface in a bolted connection, chromating, corrosion resistance (where & from what...?), cost, ease of removal prior to welding, embrittlement risk, field application, flexibility...?
Is the application a fastener, sheet or structural member?
“Cold galvanizing” often refers to application of a zinc-rich paint.
Perhaps you meant either mechanical plating or electroplating? If so, which? To what specification?
Seems like I have seen "galvanized" items that were actually electroplated- which would be a heavier coating than the "zinc coating" used on screws and stuff. I don't recall that they were called "cold galvanized" though.
In our work, we galvanize miscellaneous items, and use "cold galvanizing compound" to touch up miscellaneous field welds. It is indeed a paint product, with appearance similar to the galvanizing. It's not intended to replace galvanizing, just for those types of applications where you can't galvanize.
I see ads for "cold welding" compound...IE, epoxy cement, as well.
The coatings conform to MIL-P-21035B PAINT HIGH ZINC DUST CONTENT, GALVANIZING REPAIR, which makes them zinc-rich paints rather than “galvanizing” IMHO.
Better corrosion resistance than with MIL-P-21035B paint can be obtained with Dacromet®. This could be considered a “Warm Galvanizing” coating or more properly “Warm Galvalume,” since it also contains aluminum. From
“Our coating compositions are proprietary water based coating dispersions containing metal oxides, metallic zinc and aluminum flakes. The zinc and aluminum platelets align in multiple layers forming a metallic silver gray coating. Applied as a liquid material, the coating becomes totally inorganic after curing at 610° F/321°C.”
As the cure temperature is only 51oC below the Al-Zn eutectic, and especially with some proprietary additives present, I would expect some significant sintering during the cure.
In addition to the “cold galvanizing” field repairs of HDG mentioned by JStephen, there are also hot methods for applying molten zinc to a cleaned surface: Thermal spray is used for large areas and “hot stick” for small. The latter is essentially soldering: heating a fluxed area with a torch and then applying stick zinc alloy such as Galvaguard®. For more information, Industrial Galvanizers' website is very informative on HDG, and also compares the various types of zinc coatings:
There is a product that is supposed to be the equal of hot dip galvanizing in terms of corrosion protection. It is ZRC Cold Galvanizing Compound, made by ZRC. It seems to be more than just a high zinc dust paint because the binder that the zinc is in is conductive and does not electrically isolate the zinc particles. It has also been tested by UL. There is good info on their website.
Hot dipping provides a harder more abrasion resistant surface though. With hot dipping you have to be concerned about possible thermal distortions, like if you have a thin plate welded to something more massive. I have seen this result in the thin plate buckling and welds breaking. The American Galvanizers Association has some good material on their website regarding hot dipping, and welding to galvanized steel.
We have done some work that was to be zinc metalized. As I recall that was slow, with moderate equipment investment. If you had a lot of small areas to touch up, it might be just the thing. For large areas, it was a lot slower (and thus more expensive) than other coatings. And for small items, you can just hot dip them.
The jobs I work on often have hot-dipped galvanized embed plates. Althoug the company I work for doesn't supply the embeds, I do have one question. Is it Ok to galvanize the parts of the embed plate which are cast into the concrete.
I've been told that you should not galvanize anything that is embedded in concrete.
you can have galvanized rebar, so I don't think that a galvanized embed plate would be a problem. Maybe if the galvanized steel came in contact with black steel you could potentially have a dissimilar metals problem. If embeded galvanized steel does not come into contact with non-galvanized steel there shouldn't be a problem. I haven't heard that you shouldn't embed anything galvanized in concrete. The American Galvanizers Association says that even if galvanized and non-galv steel come in contact in concrete, as long as they are both in the passive condition there shouldn't be a problem.
If you were embeding something that was aluminized or had aluminum that would be a problem. The alkalinity of concrete attacks aluminum, and if you have aluminum in contact with embeddes steel in concrete you can get aggressive corrosion of the aluminum.
agree with bjb. there are bridges in the NE part of US constructed using galvanized reinforcing bars. not aware of restrictions using galvanized steel in concrete.
Hot-dipped galvanizing actually allows a metallurgical bond to occur between the steel and the zinc. Since it is a "post production" process, all parts are covered.
Cold galvanizing can be either a zinc-rich paint or an electroplating process. Electroplated zinc is a good process; however, the zinc layer is much thinner than with HDG.
Zinc-rich paints are nice coatings, but they are only coatings and don't offer the same type of long-term sacrificial corrosion protection that either HDG or electroplating will provide.
According to the manufacturer, ZRC is able to function as more than a coating, and is able to provide long-term sacrificial protection. What they claim is that the binder that the zinc particles are in is conductive, so that galvanic action is possible. They claim that the protection is equivalent to HDG, but the ZRC is less abrasion resistant. It has been used on the Peace Bridge connecting Buffalo, NY to Canada. I know that product manufacturers make all kinds of claims that may or may not be substantiated, but they do seem to have some technical info on their website to back up their claims. Personally, when I specify cold galvanizing ZRC is the only product I allow because I am comfortable with the technical backup they have. I use it for welded fabrications that are not symmetric or have thin plates welded to something heavier. The thermal distortions of HDG can cause problems with these kind of fabrications.
