Galvanizing Transfer Problem 3

[GStuartS]

I have a process that uses a heated (cartridge heater) metal (304 SS) foot to soften HDPE and make it flow through a fine mesh expanded metal. The heater foot (.375 x .75 contact area)is stationary and the 30 ga. galvanized expanded metal is pulled under the foot, heated (400F - 600F foot temp.) and the HDPE flows after heat is transfered (radiant & conduction)to it. The galvanized expanded metal is between the heater foot and the HDPE. My problem is a buildup of Zinc on the heater foot. It eventualy builds to the point where a nib is formed and the expanded metal is torn. I am having trouble finding information about a metal that would be resistant to bonding with the Zinc. My local metal sources have suggested a grey cast iron thinking its graphite component would help but I don't think the cast iron would wear well in this pressure (approx. 10 lbf) abrasion situation. This process runs 2-3 ft./min , 8 hrs/day so longevity is an important attribute. Thanks for any assistance.

 

[Rich2001]

Here are a couple problems.

The galvanizing may not be pure zinc but contain many low melting point elements, such as bismuth, lead and tin. These could form other intermetallics compounds and low melting eutectics aggravating your problem.

The intermetallic compounds can penetrate the grain boundaries of the steel (expanded metal) causing embrittlement of liquid metal assisted cracking. Examine the Carbon Equivalent in Zinc

for C<0.12%
CEZ = C+ Si/17 +Mn/7.5 + Cu/13 + Ni/17 + Cr/4.5 + Mo/3 +V/1.5 + Nb/2 + Ti/4.5 + 420B
where:
- all concentrations are in %
- valid for c<0.12%
- for B>0.0004% use a value of 0.004

If CEZ < 0.44% zinc assisted cracking will probally not be a problem.


For C>0.12% (medium Carbon Steels)
Slm400 = 227 - 320C - 10Si -76 Mn -50Cu - 30 Ni - 92Cr - 88Mo - 200V -220Nb +200Ti

If Slm400 > 0.42 cracking should not be a problem.

(Based on research by the Tomoe Corpoartion, Japan)

Watch for cracking of the 304 SS heater. Austentic stainless cracks in the present of zinc (solid metal embrittlement).

Why not use 304 stailess steel expanded metal?

 

[EdStainless]

You either need to get rid of the galvanizing, or change alloy for the foot. Why not Cr plated steel? or electroless Ni (Ni-P) plated?
Is wear an issue? Either of these platings will be a lot harder than the stainless.
I can think of other options, but they are more exotic.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[GStuartS]

Rich2001 and EdStainless, Thank for the help. This information gives me a couple of educated directions to explore.
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