Hydrogen embrittlment of high strength bars 1

[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.

 

[PEinc]

See thread274-75131

 

[TVP]

PEinc's response in the geotechnical forum was quite good. I only want to provide some additional information regarding standards, etc. ASTM B 850 or ISO DIS 9588 are good references for this subject. The requirements for embrittlement relief of steels in these standards are based on tensile strength/hardness, not yield strength. SAE/USCAR-5 is another good reference. Again, the requirements are based on hardness. If acid cleaning is used with subsequent electroplating, then a standard treatment is 4 hours at 200 + 5 C is required. Higher strength levels require additional time at temperature.

 

[PEinc]

Thank you, TVP.

BigBar,
If you are using the bars as ground anchors, you should be trying to follow the Post-Tensioning Institute's recommendations - the "Bible" for ground anchors. If you are designing a tiedback highway retaining wall, then you should also be following FHWA and AASHTO recommendations.

There have been very few cases of anchor failures. Where there were failures, some (long ago) were caused by hydrogen embrittlement. Others were caused by improper or lack of protection near the anchor heads. Others were caused by anchors which were subjected to unplanned-for bending in addition to their tension design load.

Most corrosion details for anchors are similar from project to project. There are several common suppliers who also provide the corrosion protection materials such as trumpets, anchorhead caps, sheaths, special grease, electrical isolation pads (for under the bearing plates), misc. coatings, etc. Talk to the suppliers and check out PTI, FHWA, AASHTO.

 

[BigBar]

First of all thankyou for all the responses I have received to my original inquiry. However the resident engineer has specified a galvanised high strength bar. The bars are passive nails and will not be stressed unless movement occurs within the surrounding ground. I am looking for alternative techniques for pickling the steel before galvanising such as abrasive cleaning or post treatment. If anyone has come across this before I would be most grateful to learn the standards or methods for galvanisation unde these circumstances.

Cheers Big Bars

 

[PEinc]

I have used galvanized soil nails from Williams Form Engineering several times. The nails are usually Grade 75 (75 ksi yield). Call John Lohmeyer in Phoenixville, PA, USA at 610-415-9910 or jlohmeyer@williamsform.com. He can answer your questions and possibly supply the galvanized bars in either grade 75 or 150.