Newbie question on controlling thickness of galvanized coating 1

[arunmrao]

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)

 

[hokie66]

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.

 

[Compositepro]

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.

 

[Ron]

arunmrao....zinc can be electroplated, which allows much more control over thickness. Compositpro's explanation makes sense as well.

 

[arunmrao]

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)

 

[hokie66]

A zinc thickness of 50 microns maximum will be almost impossible to attain. Refer to the attached.

http://www.gaa.com.au/index.php?page=coating-thickness-and-factors-influencing-thickness

"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.

 

[swall]

Cold galvanizing is an imprecise term, but generally is some sort of high metallic zinc content paint.

 

[Ron]

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).

 

[arunmrao]

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)

 

[CoryPad]

You mentioned 50 micrometers maximum - any lower limit? Electroplated zinc will be much thinner, something like 5 to 10 micrometers.

 

[mcgyvr]

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.

 

[arunmrao]

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)

 

[TomDOT]

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:

http://www.tradekorea.com/product/detail/P274853/CGL-(Continuous-Galvanizing-Line).html#.U81XpLFVNVI

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.

 

[CoryPad]

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.

 

[TomDOT]

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.

 

[mcgyvr]

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.

 

[CoryPad]

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.

 

[mcgyvr]

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.

 

[arunmrao]

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)

 

[TomDOT]

http://www.youtube.com/watch?v=3nyp7vtNUQQ