Specifying Hot Dip Galvanized Steel
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For the vast majority of applications, hot dip galvanizing can be simply specified using the following reference: Hot dip galvanized to EN ISO : by a member of Galvanizer Association (Galco, Sperrin). This standard contains coating thickness requirements as shown in table below which will typically be sufficient to achieve acceptably long coating life.
Minimum coating thicknesses achieved by hot dip galvanizing to EN ISO (not centrifuged)
Microstructure of a typical hot dip galvanized coating
Thicker Coatings by Surface Roughening
For instances where an increased coating thickness is required to achieve a longer coating life, or where work will be exposed in a severe service environment, steelwork may be grit blasted prior to galvanizing. This produces a coarse profile so opening up more steel surface to react with molten zinc resulting in a thicker galvanized coating. Typically the following reference may be used: Grit blast to Sa2½ with G24 chilled angular iron grit before hot dip galvanizing to BS EN ISO : to achieve a nominal coating thickness by a member of Galvanizing Association (Galco, Sperrin). It should be noted that the term `nominal` implies a target to be aimed for but not guaranteed, the value of which can vary with the steel section thickness, with upper limits being indicated in table below. For steel section thickness well in excess of 6 mm, a coating thickness in excess of 140 μm might be achieved.
Nominal coating thicknesses on samples that are grit blasted prior to hot dip galvanizing
Microstructure of a thick coating obtained by grit blasting steel prior to galvanising
Use of a Reactive Steel
The silicon (and to a lesser extent the phosphorous) content of a steel can affect its reactivity so causing the galvanized coating to grow more rapidly during immersion in the galvanizing bath. Typically steel with a silicon content in excess of 0.25% may be reactive and in the very rare cases where prior grit blasting will not achieve a sufficiently thick coating the specification of a steel`s chemistry can enable a still thicker galvanized coating to be specified using the following reference: Use a high silicon steel hot dip galvanized to BS EN ISO : to achieve a nominal coating thickness of 200 μm, by a member of Galvanizers Association (Galco, Sperrin). The 200 μm requirement would only apply for heavier steel section thicknesses. For light gauge steelwork a more realistic figure might be circa 120 μm. Where such a specification is used the coating may be comprised fully of zinc-iron alloy such that it will be dull grey in appearance and might be more susceptible to mechanical or handling damage. As a result, increased care should be taken when handling such product. You should contact your steel stockholder to discuss material availability.
Microstructure of thick coating obtained on a reactive steel
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You can find more information about Hot Dip Galvanizing on Galvanizing Association website
Newbie question on controlling thickness of galvanized coating
Newbie question on controlling thickness of galvanized coating
arunmrao(Materials)
(OP)
19 Jul 14 14:41
This galvanized steel strip is to be placed in zinc anode casting , which the foundry produces .
Currently, process of galvanizing is outsourced and I do not have access to their process.
My question, what is the right process , I hear of Cold Galvanizing, Zinc paint spray etc to achieve this thickness and acceptable to oil and gas industry.. Can someone guide me please.
If HDG is the right process, what am I doing wrong to get such a thick deposit.
Thanks in anticipation and also tolerating an amateur question. ( This is not a hobby or student question.)
This is my first post here . I attempted to hot dip galvanize a small steel strip ( 150mm x 50mm x6mm thick) to achieve a coating thickness of 50 microns max. I ended up getting a real thick coat.This galvanized steel strip is to be placed in zinc anode casting , which the foundry produces .Currently, process of galvanizing is outsourced and I do not have access to their process.My question, what is the right process , I hear of Cold Galvanizing, Zinc paint spray etc to achieve this thickness and acceptable to oil and gas industry.. Can someone guide me please.If HDG is the right process, what am I doing wrong to get such a thick deposit.Thanks in anticipation and also tolerating an amateur question. ( This is not a hobby or student question.)
I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)
Replies continue below
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RE: Newbie question on controlling thickness of galvanized coating
hokie66(Structural)
19 Jul 14 17:25
The thickness of zinc applied in the hot dipped galvanizing process is largely dependent on the thickness of the steel to which it is applied. The heavier the steel, the more zinc.
If you need a coating of a specific thickness, hot dipping is not the way to go, but it does give the best corrosion protection. Depends on what you need.
RE: Newbie question on controlling thickness of galvanized coating
Compositepro(Chemical)
19 Jul 14 17:32
I'm not a experting galvanizing but do know about coating processes. In hot dip galvanizing you have a bath of molten zinc and the zinc will freeze instantly when cold metal is dipped into it. This frozen zinc will not adhere well to the metal and will be very thick. The metal must dwell in the molten bath until it heats to above the melting point of zinc. At this point the zinc becomes fluid again and can wet the surface of the metal for good adhesion. But the surface of the metal will be completely flooded with zinc. When you draw the metal out of the bath is when the coating thickness is established. The slower you withdraw the metal, the more time you give for the zinc to drain from the metal surface back to the bath by gravity. Higher bath temperature will heat the part hotter and lower the viscosity of the zinc. Lower viscosity and the longer drain time before the zinc cools enough to freeze will result in a thinner coating.
RE: Newbie question on controlling thickness of galvanized coating
Ron(Structural)
19 Jul 14 20:01
arunmrao....zinc can be electroplated, which allows much more control over thickness. Compositpro's explanation makes sense as well.
RE: Newbie question on controlling thickness of galvanized coating
arunmrao(Materials)
(OP)
20 Jul 14 02:58
I did experiment with raising the temperature or the dip time . Also, as the thickness is 6 mm, it has higher chilling capacity.
The question still remains,if HDG is the right method ( to obtain 50 microns deposit)?
What is Cold Galvanizing and how is it done?
Thanks,I did experiment with raising the temperature or the dip time . Also, as the thickness is 6 mm, it has higher chilling capacity.The question still remains,if HDG is the right method ( to obtain 50 microns deposit)?What is Cold Galvanizing and how is it done?
I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)
RE: Newbie question on controlling thickness of galvanized coating
hokie66(Structural)
20 Jul 14 03:21
http://www.gaa.com.au/index.php?page=coating-thick...
"Cold galvanizing" is essentially an applied coating (or paint), consisting mostly of zinc powder or flakes. Both organic and inorganic zinc primers are sometimes referred to a "cold galvanizing". Some manufacturers will claim the same performance as hot dipped galvanizing, but that is usually just sales talk.
A zinc thickness of 50 microns maximum will be almost impossible to attain. Refer to the attached."Cold galvanizing" is essentially an applied coating (or paint), consisting mostly of zinc powder or flakes. Both organic and inorganic zinc primers are sometimes referred to a "cold galvanizing". Some manufacturers will claim the same performance as hot dipped galvanizing, but that is usually just sales talk.
RE: Newbie question on controlling thickness of galvanized coating
swall(Materials)
The company is the world’s best Hot Dip Galvanized W-Beam Guardrail supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
20 Jul 14 10:50
Cold galvanizing is an imprecise term, but generally is some sort of high metallic zinc content paint.
RE: Newbie question on controlling thickness of galvanized coating
Ron(Structural)
20 Jul 14 11:33
Agree with hokie66...50 microns (2 mils) is thin by most standards; however, HDG is usually specified by coating weight per surface area rather than thickness.
I still think your better option for a consistent and thin coating is to use electroplating of the steel with zinc (electro galvanizing).
RE: Newbie question on controlling thickness of galvanized coating
arunmrao(Materials)
(OP)
20 Jul 14 11:51
Thanks all for your helpful advice. I shall communicate to the team tomorrow morning.
I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)
RE: Newbie question on controlling thickness of galvanized coating
CoryPad(Materials)
21 Jul 14 09:17
You mentioned 50 micrometers maximum - any lower limit? Electroplated zinc will be much thinner, something like 5 to 10 micrometers.
RE: Newbie question on controlling thickness of galvanized coating
mcgyvr(Mechanical)
21 Jul 14 10:48
You can also buy steel sheets/coil pre-galvanized (galvannealed..ASTM A653)
like
G90 (approx. .8 mils on each side)
G185 (approx. 3 mils on each side)
Typically HDG (ASTM A123) minimum is in excess of 50um.. Like 85 min or so.
RE: Newbie question on controlling thickness of galvanized coating
arunmrao(Materials)
(OP)
21 Jul 14 13:32
I was getting thickness in the range 90-95 .
corypad,
I do not have readily the number, though, I have been told that it is around 50-55 .
Sorry, as I am unable to provide any input beyond this.
mcgvyr,I was getting thickness in the range 90-95 .corypad,I do not have readily the number, though, I have been told that it is around 50-55 .Sorry, as I am unable to provide any input beyond this.
I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)
RE: Newbie question on controlling thickness of galvanized coating
TomDOT(Materials)
21 Jul 14 14:20
There is one method for reducing HDG thickness, used extensively by the coil coating industry for continuous galvanizing (sheet steel) - the sheet steel passes through air blades immediately after coming out of the bath, and the blades "wipe" off excess zinc while it is still liquid. Random example from Googling:
http://www.tradekorea.com/product/detail/P/C...
Alternately, can you just buy some 6mm thick galvanized sheet and shear it to your desired width? Or do you really need the edges galvanized as well? Galvanized sheet is a commodity, so I'm wondering if you really really need to re-invent the wheel. Call out ASTM A653 with your required coating thickness range.
You can get electroplate at 50 microns. Just because it's often only 5-10 doesn't mean that an electroplater can't do it pretty easily.There is one method for reducing HDG thickness, used extensively by the coil coating industry for continuous galvanizing (sheet steel) - the sheet steel passes through air blades immediately after coming out of the bath, and the blades "wipe" off excess zinc while it is still liquid. Random example from Googling:Alternately, can you just buy some 6mm thick galvanized sheet and shear it to your desired width? Or do you really need the edges galvanized as well? Galvanized sheet is a commodity, so I'm wondering if you really really need to re-invent the wheel. Call out ASTM A653 with your required coating thickness range.
RE: Newbie question on controlling thickness of galvanized coating
CoryPad(Materials)
21 Jul 14 17:13
TomDOT,
Do you know a supplier that routinely runs at 50-micrometer thickness? I have never seen anything that high. ASTM B633 tops out at 25 micrometers, and that isn't very common. I wonder what the time, and current, and hydrogen would be in a process used to produce 50 micrometers of zinc.
Thanks.
RE: Newbie question on controlling thickness of galvanized coating
TomDOT(Materials)
22 Jul 14 08:32
I'll ask the Signals guys if they have any contacts. My materials are much more likely to be hot-dipped, as we don't mind extra thickness (as long as the bond is maintained.)
Hm, Metallizing could be another option for the original poster.
RE: Newbie question on controlling thickness of galvanized coating
mcgyvr(Mechanical)
22 Jul 14 09:35
again... you can buy pre-galvanized sheets (To ASTM A653) in the thickness you desire..
G90/G185 are just 2 examples. There are many other thicknesses available.
RE: Newbie question on controlling thickness of galvanized coating
CoryPad(Materials)
22 Jul 14 15:10
mcgyvr,
There is no debate about obtaining coatings of this thickness using hot dip galvanizing, which is identified in ASTM A653. There is some question on who/where/what can provide coatings of this thickness via electroplating, which is outside ASTM A653.
RE: Newbie question on controlling thickness of galvanized coating
mcgyvr(Mechanical)
22 Jul 14 16:47
I'm sure any electroplater can do it if you asked them to.. But why?
Its just not cost effective at all.. Unlike at home "Bath Time" cost decent money. :)
RE: Newbie question on controlling thickness of galvanized coating
arunmrao(Materials)
(OP)
22 Jul 14 16:50
A clarification to some of those, who might not know. I am in Nigeria, where there is a great shortage of resources, apart from several other challenges. Hence every day, there is a need to reinvent the wheel to keep the machines and equipment running.
I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)
RE: Newbie question on controlling thickness of galvanized coating
TomDOT(Materials)
22 Jul 14 17:01
http://www.youtube.com/watch?v=3nyp7vtNUQQ
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