follow-up, Molydisulfide lubricants and hi strength steel bolts (long)
02-20-2003, 08:31 AM
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Racer
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follow-up, Molydisulfide lubricants and hi strength steel bolts (long)
Some time ago in a thread about lubricating bolts I posted a caution about "Moly"something products. This is a follow up about Molybdenum-disulfide and Sulfide Stress Corrosion Cracking of high strength steel bolts (this is referred to as SSCC, SSC or SCC in various forums & publications). The text below was taken from the corrosion section of our company discussion forum. In summary:
1) Molydisulfide is a source of one element of the Sulfide Stress Corrosion environment.
2) Lubricants appied where "dry" torque is specified may result in overstressing bolts.
3) Add water to the above and the overstressed bolt is prone to cracking failure.
Please note crack growth rates in Stress Corrosion Cracking mechanisms are very high and that failure is usually catastrophic, there's precious little time to detect the problem by routine inspections. I prevent by not applying lubricants unless specified, preferential use of water repellant grease form, and challenging the need for the very low friction coefficients of these products.
Disclaimer: I am not an expert on this subject, I am stating opinions and passing on information that I think is useful to the Rennlist paticipants. IMHO, YMMV etc. etc.
Enjoy <img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />
Nol
quote"
Sulfide Stress Corrosion Cracking on Hi Strength steel.
From memory: Incident investigation following a failure of flange bolts on offshore facility in mid-1990's concluded that the cause of bolt failure could be attributed to the molydisulphide lubricant. In this instance molydisulphide combined with the bolt material in a moist environment allowed corrosion to be accelerated through the presence of sulphides. Apparently when molydisulphide is applied in paste form (grease base) then it is not so problematic because grease repels water. In this instance water had been able to penetrate because the molydisulphide was a spray application. I think there was also reference to the spray application resulting in over-tensioning of the bolts through overspray onto the bolt bearing surface and hence incorrect torque readings which contributed to overtensioning and the eventual failure. Also possibly the corrosion environment exascerbated by the lack of airflow in the boltholes - water wouldn't dry or oxygen deficiency or something but we're getting to the limits of my memory now.
Recommendation resulting from this was immediate discontinuation of the use of molydisulphide as a lubricant (not sure how far-reaching this was in terms of spray- or grease-application or marine environments)
"unquote
1) Molydisulfide is a source of one element of the Sulfide Stress Corrosion environment.
2) Lubricants appied where "dry" torque is specified may result in overstressing bolts.
3) Add water to the above and the overstressed bolt is prone to cracking failure.
Please note crack growth rates in Stress Corrosion Cracking mechanisms are very high and that failure is usually catastrophic, there's precious little time to detect the problem by routine inspections. I prevent by not applying lubricants unless specified, preferential use of water repellant grease form, and challenging the need for the very low friction coefficients of these products.
Disclaimer: I am not an expert on this subject, I am stating opinions and passing on information that I think is useful to the Rennlist paticipants. IMHO, YMMV etc. etc.
Enjoy <img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />
Nol
quote"
Sulfide Stress Corrosion Cracking on Hi Strength steel.
From memory: Incident investigation following a failure of flange bolts on offshore facility in mid-1990's concluded that the cause of bolt failure could be attributed to the molydisulphide lubricant. In this instance molydisulphide combined with the bolt material in a moist environment allowed corrosion to be accelerated through the presence of sulphides. Apparently when molydisulphide is applied in paste form (grease base) then it is not so problematic because grease repels water. In this instance water had been able to penetrate because the molydisulphide was a spray application. I think there was also reference to the spray application resulting in over-tensioning of the bolts through overspray onto the bolt bearing surface and hence incorrect torque readings which contributed to overtensioning and the eventual failure. Also possibly the corrosion environment exascerbated by the lack of airflow in the boltholes - water wouldn't dry or oxygen deficiency or something but we're getting to the limits of my memory now.
Recommendation resulting from this was immediate discontinuation of the use of molydisulphide as a lubricant (not sure how far-reaching this was in terms of spray- or grease-application or marine environments)
"unquote
