Hot dipped galvanizing 2

[asixth]

I had a steel fabricator today tell me that a cold-formed tube section needs to have a "blow hole" provided before it can be hot-dippped galvanized, otherwise the built up air pressure may cause a fully sealed section to rupture. Is this correct, or was the fabricator leading me on?

 

[hokie66]

He is correct. Venting is required. Closed sections are galvanized both inside and out. Locations of the vents should be as required by the galvanizer.

 

[SlideRuleEra]

The steel fabricator is right... but for the wrong reason. The section would not rupture, but a trapped air bubble will keep the molten zinc from coating all surfaces. As hokie66 stated, vents are necessary to allow all air to escape.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

They have standard sized holes with standard plugs available.

Also good to provide a drain to keep ice from busting the tubing is outside.

Dik

 

[apsix]

The primary reason for venting is to galvanise both external and internal surfaces, but there is a risk of explosion without vents if moisture is trapped within.

It also helps if the items don't float!

 

[JStephen]

I understood that the galvanizers would normally drill vent holes as required. Little explosions underneath molten metal are evidently unpleasant.

 

[asixth]

Just a quick follow up question guys...

What increase in mass can I expect with a hot dipped galvanized finish and what additional wall thickness will result?

 

[civeng80]

standard procedure to blow holes where air can be trapped in hot dip galvanizing.

 

[apsix]

Typical heavy duty galvanising thickness is 600 g/m2.
Zinc density is 7,140 kg/m^3.

 

[hokie66]

asixth,

The thickness of galvanizing varies with the thickness of the steel. The 600 g/m2 noted by apsix would be typical for steel about 6 mm thick, but the zinc deposition increases with steel thickness. It can easily be 1000 g/m2 for rolled sections. And with a tube, you have both outside and inside zinc. Suggest you review information published by Industrial Galvanizers, or call a local galvanizer for advice.

 

[Paperandpencil]

We had a 2 hour talk in our office yesterday given by a technical rep from the UK hot dip galvanizers association. Venting in tubular sections essential or they can explode. A typical 1 metre length of sealed tube will apparently displace 7 tonnes of molten zinc from the bath when it explodes, which is not good for the people on the shop floor. The thickness of zinc that will be deposited is related to the section thickness, surface preparation, the silicon content and the quality of the steel. Apparently galvanizers recently noted funny things coming out of their baths, it turns out it was poor quality imported steel, Turkey?. The paperwork said it complied with the euro standards, but subsequent testing showed no good, too many old cars in the mix?. Apparently some buildings had to be taken down. Makes you want to galvanize everything, the zinc crystals cannot lie.

 

[SlideRuleEra]

Another reason for venting (which I had not considered until today). Steel is only slightly heavier than molten zinc. Too many air pockets and the steel section will not sink. See page too of this link from "Industrial Galvanizers Corporation Pty Ltd, Galvanizing Design Manual"

http://www.corp.indgalv.com.au/technical/pdf/galman1.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[CTW]

Straight from the AGA.

http://www.galvanizeit.org/aga/designing-fabricating/design-considerations/venting-drainage

"In the hot-dip galvanizing process, steel is completely coated with corrosion-inhibiting zinc, which forms a highly abrasion-resistant metallurgical bond with the base steel. In order to ensure that all interior and exterior surfaces are protected from corrosion, entire steel fabrications are lowered into and raised out of cleaning solutions, flux solutions, and molten zinc metal. In order to facilitate interior and exterior cleaning and coating, it is necessary to provide holes in fabrications to be galvanized.

The primary reason for vent and drain holes is to allow air to be evacuated from within and around the fabrication, allowing it to be completely immersed in the cleaning solutions and molten zinc and for the excess zinc and solutions to drain out and away from the part.

The secondary reason is that if fabrications to be galvanized are not properly vented, cleaning solutions or rinse waters trapped in overlapping or contacting surfaces flash to steam. The resulting pressure increase (up to 3600 psi [25MPa]) can rupture the fabrication. Additionally, trapped moisture that flashes to steam can result in localized uncoated surfaces.

Because items being galvanized are immersed in and withdrawn from all cleaning solutions and molten zinc at an angle, vent holes should be located at the highest point and drain holes at the lowest point as mounted during the galvanizing process."

 

[asixth]

So if vents are provided for tubular sections, how is waterproofing dealt with...Will moisture collect in the inside of the tube if it is provided in an external environment?

 

[apsix]

Probably best if the vent hole is located to act as a drainage hole once the tube is erected.