De-rating the strength of bolts due to galvanizing 2

[LarryDavid]

Hi there,

What are peoples thoughts on de-rating the tensile strength of bolts which have been galvanised?

I am designing multiple flanges which will be assembled using AS4291 / AS1110 Grade 8.8 bolts which will be galvanized. I realise that hydrogen embrittlement generally occurs in bolts with a tensile strength above 1000 MPa, so is it necessary to de-rate bolts with a lower strength than this? Can the galvanising process reduce the strength in any other way?

Thankyou

 

[SJones]

It's not so much the galvanising as the pickling beforehand and the corrosion reaction in service. I don't have the AS bolting spec to hand but assume that it is the same as ISO 898-1 with a maximum hardness of HRC 34 above 16 mm. A popular threshold for embrittlement risk assessment purposes has been set at > HRC 35; therefore, it would imply that there should be no need to derate for the purposes of avoiding HE. As a comparison, plating, which generates far more hydrogen, is considered acceptable below this hardness threshold level too. Just ensure that the bolts are hardness tested at an appropriate frequency.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[hokie66]

Grade 8.8 bolts do not need to be derated if galvanized.

 

[CoryPad]

Besides the hydrogen embrittlement issue, which is widely agreed to be a problem for property classes 10.9 and 12.9 fasteners, there is the temperature exposure issue as well. The minimum tempering temperature specified in ISO 898-1 is 425 [°]C for property classes 8.8 and 10.9, and it is 380 [°]C for property class 12.9. A typical molten zinc bath temperature for hot-dip galvanizing is 450 [°]C, so it is possible that the hot-dip galvanizing process can temper high-strength parts to a lower strength than is desired. This usually is not a problem for property class 8.8 fasteners since they are usually tempered at closer to 500 [°]C.

 

[Tmoose]

what is the operating temperature? I think The sacrificial protection of galvanizing reverses at some relatively low temperature,like around 150 F, at least around some glycol based coolants.

I was thinking That may be why so many of the MIL specs are based on fancy paint jobs.

 

[CoryPad]

Tmoose is correct, there can be a reversal between iron and zinc.

Volume 13A, Corrosion: Fundamentals, Testing, and Protection -> Aqueous Corrosion -> Temperature and Heat Transfer

“Fluid temperature changes can affect the polarity in galvanic corrosion. The corrosion potential of the anode might be more sensitive to temperature than that of the cathode. The anode potential can actually become noble with respect to that of the cathode (Ref 17). An example is the iron-zinc couple, the polarity of which can reverse as temperature increases. Iron actually protects the zinc. The temperature of this reversal is as low as 60 °C (140 °F), but there is some dependence of temperature on constituents.”

 

[LarryDavid]

Thanks for the comments guys,

Correct - the mechanical properties (AS4291 are identical to ISO 898-1)

These flanges are being installed on cement lined pipelines, maximum operating temperature of the water is 50degC so I dont expect the bolts to even reach that.