The way I understand gammaman's question is that the steel "shrinks", i.e., it gets smaller, or to put it in another way, it diminishes its size, by 2-3 mm as a result of galvanizing. He doesn't say that the steel surface preparation eats away 2-3 mm. What he says is that the steel is 2-3 mm smaller because the galvanizing process has done that.
First of all, gammaman doesn't mention what kind of steel he's talking about: pipe? plate? shapes? cable? All of them may be galvanized according to the application they were designed to .
Secondly, he doesn't mention what the original dimensions were: were the 2-3 mm eaten away from a piece of pipe 30 feet long? Or was it a 2 inches long nipple?
That said, this is the first time I heard that a piece of steel shrinks due to galvanizing. There's nothing in the galvanizing process that could produce that result. In the eletrolytic process, a D.C. current flows from the zinc anode to the piece acting as the cathod. As a result, the zinc is dissolved and deposited onto the piece. In the hot dip process, the piece is submerged into a bath of molten zinc. As a result, the zinc in contact with the colder piece solidifies and sticks onto the surface.
Giovanni S. Crisi
Sao Paulo - Brazil
Steel is soluable in molten zink, so in theory a part left in the molten zink long enough would have some of the parent metal disolved off of it, and be smaller. Between surface preperation and disolved material, there could be a measurable difference, but I think mils [thousanths of an inch] not millimeters.
when a plate is bead blasted to Nearwhite finish the surface prep dip before the hot dip can be many hours shorter, if the plate still has millscale on it the prep time is much longer (the angular profile produced during surface prep is the only material removal i know of but this is only in the mils). I have found plate machined to dimention always comes back larger when measured due to the coating thickness.Shrinking of material is something ive never heard of.
I have seen plate wellded together (without a vent hole) Blow apart in the tank.
Just sittin' here eatin' my lunch with my copy of ASTM A123 open in front of me (hey, I am an engineer, whadya expect?). For what it's worth, nowhere in A123 does in mention shrinkage of steel from galvanizing. They have ranges on everything else, but nada on the shrinkage.
Bob G.
Hi, gammaman1 here again. OK, the steel is structural steel, mild carbon, JIS SS400, but still basically carbon steel. The vendor says is shrinks in length, I say they cut too long (2-3 mm over) and this they say is to compensate for shrinkage due to hot dip galvanize. I have been all over the net and can find nothing on this.
So Giovanni has got it, I think, and thanks to all for your comments.
How long are the parts? I would be interested to hear if the parts actually do shrink or not.
I really, really, really. . . . . . . . really don't think galvanizing shrinks steel anymore than I think RT releases isotopes or whatever into the atmosphere!!!! I just couldn't resist. . . . .
When heat treating tool steel, the parts usually grow a few thousanths of an inch from the process. If overheated some tool steels can actually shrink. Galvanizing temperature is below critical, so I wouldn't expect a dimensional change from galvanizing either.
FWIW, the galvanizing bath is around 850*F to keep the zinc molten when dipping the cooler mass into the bath.
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