Hydrogen embrittlment of high strength bars 2

[BigBar]

I am looking to use high strength bars, high carbon bars (835MPa yield +)for ground anchors with galvanised coating for protection but have come across the hurdle of "hydrogen embrittlment".

I am looking for ways of elevating this phenomenon and have learned ASTM A123-89 and BS 729-71 standards for galvanisation do not specify the pickling bath or temperature that the galvanisation is carried out.

By blast cleaning the bars as opposed to pickling, or by post-treatment detrimental effects can be avoided or at lease minimised. I would be most grateful for any comments or case history on these alternative methods.


Cheers Big Bars.

 

[PEinc]

High strength (>150 ksi ultimate) ground anchor tendon bars are not usually galvanized. Class I corrosion protection is provided by full encapsulation in a grouted, corrugated or deformed sheath, usually plastic, sometimes steel. Sometimes the bar tendons are epoxy coated instead of encapsulated.

In more than 20 years of designing and building tiedback walls, I have never used or seen used galvanized, high strength tendons. I have seen and used galvanized coatings for lower strength (<75 ksi yield) bar tendons used for soil nails in non-agressive soils.

Review information from the Post-Tensioning Institute's (PTI) Recommendations for Prestressed Rock and Soil Anchors. Various publications on ground anchors and anchored retaining walls from the Federal Highway Administration describe required corrosion protection for bar and strand tendons. Contact tendon suppliers such as Williams Form Engineering Corp. (

http://www.williamsform.com)

and Dywidag-Systems International (

http://www.dywidag-systems.com)

for available tendon protection details. Williams' Publication No. 999 states that &quot;High strength steel bars should always be mechanically cleaned never acid washed to avoid problems associated with hydrogen embrittlement.&quot; I am not aware of other suppliers who offer galvanized, high strength tieback tendons.

FHWA's publication, Tiebacks, FHWA/RD-82/047, page 88 states, &quot;In the past, prestressing steels were galvanized in an attempt to improve their corrosion resistance. Today,prestressing steels are not galvainzed because: 1) They may contact bare steel and cause a dissimilar metal corrosion system to develop. 2) Zinc coatings are sacrificial and atomic hydrogen is evolved as the galvanizing is consumed. Atomic hydrogen could cause hydrogen embrittlement of the tendon. 3) Galvanizing only provides protection until it is consumed. Galvanizing or cadmium plating, a similar sacrificial coating, should not be used for protection of tieback tendons.&quot; Although this publication is from 1984, newer, similar FHWA and PTI publications make no mention of galvanized tendons.

 

[PEinc]

P.S. PTI's Fifth Edition Post-Tensioning Manual states on page 285, &quot;Galvanizing of any anchor tendon component or cathodic protection is not recommended.&quot; and (Commentary note) &quot;Both methods are not considered to provide a reliable long term corrosion protection.&quot;