Galvanized Pipe Supports - 450¦F? 2

[StressGuy]

The latest issue of the Grinnell pipe support catalog (now Anvil International) has something that I've not seen before. For pipe supports, specifically the clamps we all use for spring hanger assemblies, they indicate a maximum temperature of 450°F for galvanized hardware vs. 750°F for plain carbon steel.

The old Grinnell catalogs didn't have this and I've not been able to find such a limitation noted in any competitors catalogs.

Historically, I've always seen galvanized clamps used in services up to 750°F and typically alloy clamps used for higher temperatures.

Does anyone have any info on the basis of this 450°F limitation? Is this some recent discovery that is filtering out into the world or is it something that has been long known, but just not in our field?

I know this is more properly a metallurgy/corrosion question. However, I figured that with the specific application of pipe supports being the concern, I'd have a better chance finding the answer in this forum.

I'm also planning to contact Anvil and the other support vendors to see what they have to say, but I'd also like to get opinions that don't have a sales spin attached.



Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[Rich2001]

Above 450F the low melting temperature alloying elements used in galvanizing,i.e. Bismuth, Lead, Tin, and Aluminum, can segregate and embrittle the steel resulting in a catastrophic failure.

Additionally, at 750F the zinc can cause cracking due to near liquid metal embrittlement.

I am a firm believer that all galvanized items utilized to support critical loads be magnetic particle inspected to find cracks under the galvanizing. Cracking can occurs in the galvanizing process due to hydrogen embrittlement due to the acid bath, SCC due to the Casutic bath or Liquid Metal in the galvaning bath. Unless the galvanizer has good process control problems can occur.

 

[Hookem]

Be CAREFUL. I learned that the galvanizing turns cathodic above about 160 to 170 degF, sacrificing the pipe to which it is in contact with. Not desireable!!!!!! Check it out.

 

[MJCronin]

HOOKEM,

Where did you learn this ?

What is your reference ?

I consider it irresponsible to post rumors and vague opinions.

MJC

 

[StressGuy]

I'll have to search for a reference, but I believe the issue of reversal occurs in the range of 170F-180F, not above.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[kenvlach]

Two issues have been raised here. They are not related IMHO.

1) The 450^oF limit mentioned by stressguy is due to SMIE (Solid Metal Induced Embrittlement).
“ASTM Specifications permit the galvanizing of ASTM A325 bolts but not ASTM A490 bolts. Similarly, the application of zinc to ASTM A490 bolts by metallizing or mechanical coating is not permitted because the effect of mechanical galvanizing on embrittlement and delayed cracking [my emphasis] of ASTM A490 bolts has not been fully investigated to date.” – Specification for Structural Joints Using ASTM A325 or A490 Bolts p. 7, Research Council on Structural Connections (June 23, 2000).

http://www.boltcouncil.org/download/RCSC Specification 6-23-2000.pdf

-- Note that even (room temperature) mechanical plating is prohibited for A490 bolts. The potential for embrittlement, whether SMIE or LME (Liquid Metal Embrittlement), increases with strength and stress levels of steels, e.g., for A490 bolts rather than low-carbon sheet steel. So, I agree with Rich2001's recommendation for testing of all critical items. See
Metal Embrittlement in galvanized bolts
thread330-48335; has some nice photos of broken bolts.

SMIE is described in detail in ASM Handbook, vol. 13 Corrosion, pp. 184-187 (1987) and ASM Handbook, vol. 13A Corrosion, pp. 393-397 (Oct. 2003). [virtually identical on this topic: the 2003 edn. has 4 more ref. & photos, but the figures & tables are the same]. SMIE is similar to LME, and the slower rate of attack is primarily due to the effect of temperature upon the solid-state diffusion within the substrate metal. In general, SMIE has been noted for temperatures beginning at 0.75 T_melt (absolute temperature scale) of the embrittler. However, the effect is likely merely delayed at lower temperatures, hence the precaution taken by ASTM w.r.t. A490 bolts.
SMIE embrittlers reported for steel are Bi, Cd, Ga, Hg, In, K, Li, Na, Pb, Sb, Sn and Zn. Al isn’t listed as an SMIE embrittler for steel (although reported for LME).
“Unlike galvanized sheet that is somewhat prone to embrittlement when exposed to temperatures greater than 200^oC (400^oF) where the zinc diffuses into the steel sheet, type 1 aluminized low-carbon sheet steel performs well at elevated temperatures...type 1 aluminized can be used for applications up to about 650^oC (1200^oF).” – ibid., p. 792 (2003).

2) Reversal of polarity of zinc vs. steel in aqueous environments!?
Ulick R. Evans, The Corrosion and Oxidation of Metals, p. 207 (1960) says this reversal has long been known and gives references to detailed studies.
“The cause of the reversal of polarity is uncertain, but it is likely that zinc hydroxide formed on the zinc surface, changes to zinc oxide when the temperature is raised; zinc oxide has an appreciable electronic conductivity which increases with rise of temperature, so that the cathodic reaction (reduction of oxygen or liberation of hydrogen) can proceed appreciably on the hot zinc surface, the iron functioning as an anode.”
-- What a sentence! Explains why the corrosion of zinc increases, but not really why steel should become anodic w.r.t. zinc.

H. H. Uhlig and R. W. Revie, in Corrosion and Corrosion Control, 3rd Edn., pp. 239-240 (1985):
“In many aerated hot waters, reversal of polarity between zinc and iron occurs at temperatures of about 60^oC (140^oF) or above.”
This reversal is favored by high concentrations of carbonates and nitrates, and inhibited by chlorides and sulfates. It is apparently due to the formation of ZnO rather than Zn(OH)₂, the ZnO acting as a semiconductor in aerated waters.
“It [ZnO] can therefore perform in aerated waters as an oxygen electrode whose potential, like mill scale on steel, is noble to both zinc and iron. Accordingly, in deaerated hot or cold waters in which an oxygen electrode does not function because oxygen is not present, zinc is always anodic to iron, but this is not necessarily true in aerated waters.”

Mars G. Fontana, Corrosion Engineering, 3rd Ed., p. 45 (1986), gives the reversal as occurring in domestic waters at 180^oF.

-- I’m haven’t seen the original studies, but the above explanations haven’t convinced me that zinc per se becomes cathodic to iron. Rather, the observed anodic behavior of iron and steel at higher temperatures seems due to the cathodic behavior of zinc oxide.