Unfinished galvanized pipe as architectural feature

A. Sir:

Pickled steel is very reactive, both toward oxygen in air, moisture, and perspiration on hands. Thus the "raw" pickled steel will become stained and corrode and visitors will get these oxides all over their hands. Perhaps a clear coat after pickling would do what you wish. Without a doubt galvanized would look the best and last a very, very long time.

Regards,

A. Dear Sir;

Few years ago, a customer of mine came with similar idea. Although we opposed, he insisted on using just pickled steel. For few days, they looked as he wanted but after some weeks with rain and moisture, very heavy rust formed on surface.

Another problem would be the reduction of strength after "corrosion pits" occur on the metal. So there would be heavy risk of failure if these pipes carry some loads.

I guess you want to have the finish of metals used in 50s and 60s ("brutal architecture" structures). What I know is that they used special kind of steel, which has some surface property like aluminium and titanium, in which oxide layer forms on the surface without any further advance.

In my opinion, a galvanized steel with some contrast spangle surface always looks great.

I wish you good luck in your project.

A. I suspect Chris is talking about spangle look rather than pickled.

Chris: Pickling is a term in galvanizing for the cleaning of the steel in acid. The steel comes out of the steel chemically clean (no oxides), but only stays like that an hour or so, then rapidly rusts.

But galvanized steel retaining the spangle look (large flat crystals of zinc on the surface) is possible to retain.
Handling the steel will reduce the effect, but a clear lacquer over spangled galv can retain it.

A. Dear Chris;

We call that shapes as "spangles". Recently as Pb addition was forbidden galvanized finish became more spangle-free. You might ask your supplier if their Pb, Sn and Bi concentrations are high. Over %0.5 Pb (or 0.1 Bi instead) with some Sn addition would give the that texture.


And also you must seek for "batch galvanized" pipes. That means the plant produces different kinds of products, not just pipes. Plants that only produces pipes would have spangle free texture.

To be sure I advice you to visit your supplier(s) and check for their products. No plant would adjust their zinc bath with your request, so you need to find the suitable one for you. You can try the galvanizers outside the EU or American region.

A. Sir:

The pipe on the left is likely done on an "automatic" pipe galvanizing kettle where each 20 foot pipe is galvanized every 6 seconds. The crystalline coating is likely due to 1% lead in the zinc (prime western zinc). The zinc could actually have bismuth and/or tin (and possibly nickel) to produce these same crystals in the zinc (spangles).

The pipe on the right is very characteristic of "flow coat" and consists of pure zinc and higher aluminum. The zinc coating is only on the outside of the pipe (likely painted on the inside). This coating is applied continuously as the pipe is being made and then is cut to length further down the line. "flow coat" zinc coatings are about only 1/4 the zinc thickness as "automatic" pipe galvanized coatings.

"Spangle" has nothing to due with quality.

Regards,

A. Hi Geoff. Although there may be times when a specific "look" is part of the requirements, such as was recently discussed in the letter which we appended your inquiry to, to my knowledge galvanizing is otherwise considered a functional finish rather than a decorative finish.

Unless the finish is obviously low quality, or damaged by serious wet storage stain, or in some way non-functional, I don't think the client is correct in objecting. I think you could take a look at ISO 1461 "Hot dip galvanized coatings on iron and steel articles - specifications and test methods", and consider this a reasonable standard to judge by, and see if you feel that the client has or doesn't have a valid complaint. Good luck.

Regards,

A. The question of un-met expectations often comes up with galvanizing. This is most often brought about because the finish is variable, and the variability is caused largely by the substrate being coated. Either the structure shape not allowing for vent/draining properly, or the chemistry of the steel, or the thickness. All can lead to surface irregularity and or colour variations, none of them in the control of the galvanizer.

Getting the customer to understand before processing is the key, but rarely easy to achieve.

A. Hi. I'm not a galvanizing expert like many of our readers are, and pictures can be deceiving, but to me this looks like it has a textured clear coat on top of the galvanizing rather than shotblasting/peening under it. If you have a sample of it, you may be able to tell from the conductivity of the surface or by scrapping it with the edge of good knife and seeing if you can remove any non-zinc material.

I've never heard of any specific finish called a "Tube finish", see no evidence of such a phrase in Google or Bing, and don't feel that your customer claiming he wanted a "Tube finish" has any validity.

Regards,

A. As said, photos can be hard to tell exactly, but this certainly looks like galvanizing. The slight spangle is visible. From the image there's nothing to directly suggest shotblasting, but the coating thickness will probably reveal if it had been blasted. Blasting is often used as a means to increase coating thickness in galvanizing.