How does GL-5 affect steel?

[RubbinIsRacin]

Could someone please explain how GL-5 rated oil affects gear surfaces?

I have a disagreement with someone who insists that it will cause pitting in spur or helical gears in lightly loaded applications. I believe this person is mixed up and must be thinking about possible effects on yellow metal.

But I wanted to ask just to make sure I haven't missed something or if there's a situation where GL-5 would have some deleterious effect on ferrous material, like maybe upside down on the moon or something.

 

[dnewton3]

GL-5 is in nearly every (but not all) differential made in the last 15 years, as factory fill. Probably one of the most widely used hypoid gear lubricants of all time. It's also in quite a lot of manual gear boxes too.

If pitting steel were a giant issue with GL-5, I'd think it would have been abandoned by now.

 

[RubbinIsRacin]

I know it goes in all hypoid gear applications. I don't believe there's any problem at all. I just wanted to make sure I didn't miss something for an unusual application.

 

[mechtech2]

If the bottle says GL4 AND GL5, it is buffered concerning the sulfur content, and corrosion is not a big issue.
But even straight GL4 has about 1/3 the sulfur of a GL5. GL4 is not void of this additive, it just has less. Modern fluids are almost always buffered, though.
Also, it takes heat and moisture to enable the reaction.

But you are right - it is the possible effects on brass in the tranny that can be a problem, NOT with steel. It should actually HELP steel parts last longer.

 

[Kestas]

We have seen some occasions where activated sulfur from EP additives can attack and micropit the wear surface. We've seen this in bearings. But as mechtech mentioned, the key is heat, which can activate the sulfur and lead to damage.

Many modern lubes are buffered to the eliminate this problem. No longer does sulfur in the lube necessarily mean that it will damage steel, or even brass.

 

[bruce381]

Micro pitting is normally due to a "chunk" of metal being squished in the contact zone and causeing a small deformation /crack which will propagate into a micro pit then a spall IMHO due to debris NOT sulfur additives.
bruce

 

[Kestas]

I know the difference between micropitting from corrosion and micropitting from mechanical spalling. There's a world of difference between the two under the microscope. The micropitting I've seen from EP attack is similar to corrosion, is not visible to the naked eye, and is only seen at 1000X magnification. EDS data from the pits confirmed the concentrated presence of EP additive, thus making additive the culprit.

 

[bruce381]

No argument from me just stating what I remembered let me ask then does the corrosion pit propagate as a debris started micro pit? And is the corriosn debris the starting point?
bruce

 

[Tempest]

This is a good link to what is happening with Ep additives:
http://www.engineersedge.com/lubrication/extreme_pressure_additives.htm

 

[Kestas]

Bruce, what I have seen before and documented is cracks emanating from corrosion pits, which then leads to spalling and further advance failure mechanisms. I don't think you get "debris" from a corrosion pit, the material simply goes into solution.

 

[bruce381]

Originally Posted By: Kestas
Bruce, what I have seen before and documented is cracks emanating from corrosion pits, which then leads to spalling and further advance failure mechanisms. I don't think you get "debris" from a corrosion pit, the material simply goes into solution.


My understanding has been that from a corrosion site, corrosion debris will eventually form and further weaken and stress the base metal with micro cracks going to macro to a spall to a pit, I guess we agree somewhat on the end result though.

bruce

 

[Kestas]

Corrosion debris doesn't really stress the base metal directly. But iron oxide can degrade the grease itself and chemically change its lubricating properties, rendering it much less effective as a lube. This can give rise to other mechanisms of surface damage.